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Pasquaré SJ, Chamorro-Aguirre E, Gaveglio VL. The endocannabinoid system in the visual process. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY 2022. [DOI: 10.1016/j.jpap.2022.100159] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
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Thoreson WB. Transmission at rod and cone ribbon synapses in the retina. Pflugers Arch 2021; 473:1469-1491. [PMID: 33779813 DOI: 10.1007/s00424-021-02548-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2020] [Revised: 02/23/2021] [Accepted: 02/25/2021] [Indexed: 12/29/2022]
Abstract
Light-evoked voltage responses of rod and cone photoreceptor cells in the vertebrate retina must be converted to a train of synaptic vesicle release events for transmission to downstream neurons. This review discusses the processes, proteins, and structures that shape this critical early step in vision, focusing on studies from salamander retina with comparisons to other experimental animals. Many mechanisms are conserved across species. In cones, glutamate release is confined to ribbon release sites although rods are also capable of release at non-ribbon sites. The role of non-ribbon release in rods remains unclear. Release from synaptic ribbons in rods and cones involves at least three vesicle pools: a readily releasable pool (RRP) matching the number of membrane-associated vesicles along the ribbon base, a ribbon reserve pool matching the number of additional vesicles on the ribbon, and an enormous cytoplasmic reserve. Vesicle release increases in parallel with Ca2+ channel activity. While the opening of only a few Ca2+ channels beneath each ribbon can trigger fusion of a single vesicle, sustained release rates in darkness are governed by the rate at which the RRP can be replenished. The number of vacant release sites, their functional status, and the rate of vesicle delivery in turn govern replenishment. Along with an overview of the mechanisms of exocytosis and endocytosis, we consider specific properties of ribbon-associated proteins and pose a number of remaining questions about this first synapse in the visual system.
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Affiliation(s)
- Wallace B Thoreson
- Truhlsen Eye Institute, Departments of Ophthalmology & Visual Sciences and Pharmacology & Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE, 68198, USA.
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Sensing through Non-Sensing Ocular Ion Channels. Int J Mol Sci 2020; 21:ijms21186925. [PMID: 32967234 PMCID: PMC7554890 DOI: 10.3390/ijms21186925] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2020] [Revised: 09/17/2020] [Accepted: 09/18/2020] [Indexed: 12/12/2022] Open
Abstract
Ion channels are membrane-spanning integral proteins expressed in multiple organs, including the eye. In the eye, ion channels are involved in various physiological processes, like signal transmission and visual processing. A wide range of mutations have been reported in the corresponding genes and their interacting subunit coding genes, which contribute significantly to an array of blindness, termed ocular channelopathies. These mutations result in either a loss- or gain-of channel functions affecting the structure, assembly, trafficking, and localization of channel proteins. A dominant-negative effect is caused in a few channels formed by the assembly of several subunits that exist as homo- or heteromeric proteins. Here, we review the role of different mutations in switching a “sensing” ion channel to “non-sensing,” leading to ocular channelopathies like Leber’s congenital amaurosis 16 (LCA16), cone dystrophy, congenital stationary night blindness (CSNB), achromatopsia, bestrophinopathies, retinitis pigmentosa, etc. We also discuss the various in vitro and in vivo disease models available to investigate the impact of mutations on channel properties, to dissect the disease mechanism, and understand the pathophysiology. Innovating the potential pharmacological and therapeutic approaches and their efficient delivery to the eye for reversing a “non-sensing” channel to “sensing” would be life-changing.
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Wen X, Thoreson WB. Contributions of glutamate transporters and Ca 2+-activated Cl - currents to feedback from horizontal cells to cone photoreceptors. Exp Eye Res 2019; 189:107847. [PMID: 31628905 DOI: 10.1016/j.exer.2019.107847] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/08/2019] [Revised: 09/26/2019] [Accepted: 10/15/2019] [Indexed: 02/05/2023]
Abstract
Lateral inhibitory feedback from horizontal cells (HCs) to cones establishes center-surround receptive fields and color opponency in the retina. When HCs hyperpolarize to light, inhibitory feedback to cones increases activation of cone Ca2+ currents (ICa) that can in turn activate additional currents. We recorded simultaneously from cones and HCs to analyze cone currents activated by HC feedback in salamander retina. Depolarization-activated inward tail currents in cones were inhibited by CaCCinh-A01 that inhibits both Ano1 and Ano2 Ca2+-activated Cl- currents (ICl(Ca)). An Ano1-selective inhibitor Ani9 was less effective suggesting that Ano2 is the predominant ICl(Ca) subtype in cones. CaCCinh-A01 inhibited feedback currents more strongly when intracellular Ca2+ in cones was buffered with 0.05 mM EGTA compared to stronger buffering with 5 mM EGTA. By contrast, blocking glutamate transporter anion currents (ICl(Glu)) with TBOA had stronger inhibitory effects on cone feedback currents when Ca2+ buffering was strong. Inward feedback currents ran down at rates intermediate between rundown of glutamate release and ICl(Ca), consistent with contributions to feedback from both ICl(Ca) and ICl(Glu). These results suggest that Cl- channels coupled to glutamate transporters help to speed inward feedback currents initiated by local changes in intracellular [Ca2+] close to synaptic ribbons of cones whereas Ano2 Ca2+-activated Cl- channels contribute to slower components of feedback regulated by spatially extensive changes in intracellular [Ca2+].
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Affiliation(s)
- Xiangyi Wen
- Department of Ophthalmology, West China Hospital, Sichuan University, Chengdu, Sichuan, 610041, China; Department of Optometry and Visual Science, West China Hospital, Sichuan University, Chengdu, Sichuan, 610041, China; Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, University of Nebraska Medical Center, Omaha, NE, USA; Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE, USA
| | - Wallace B Thoreson
- Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, University of Nebraska Medical Center, Omaha, NE, USA; Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE, USA.
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Van Hook MJ, Nawy S, Thoreson WB. Voltage- and calcium-gated ion channels of neurons in the vertebrate retina. Prog Retin Eye Res 2019; 72:100760. [PMID: 31078724 PMCID: PMC6739185 DOI: 10.1016/j.preteyeres.2019.05.001] [Citation(s) in RCA: 45] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2019] [Revised: 04/25/2019] [Accepted: 05/01/2019] [Indexed: 02/06/2023]
Abstract
In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.
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Affiliation(s)
- Matthew J Van Hook
- Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, University of Nebraska Medical Center, Omaha, NE, USA
| | - Scott Nawy
- Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, University of Nebraska Medical Center, Omaha, NE, USA; Department Pharmacology & Experimental Neuroscience(2), University of Nebraska Medical Center, Omaha, NE, USA
| | - Wallace B Thoreson
- Truhlsen Eye Institute, Department of Ophthalmology & Visual Sciences, University of Nebraska Medical Center, Omaha, NE, USA; Department Pharmacology & Experimental Neuroscience(2), University of Nebraska Medical Center, Omaha, NE, USA.
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Lee JR, Lee JY, Kim HJ, Hahn MJ, Kang JS, Cho H. The inhibition of chloride intracellular channel 1 enhances Ca 2+ and reactive oxygen species signaling in A549 human lung cancer cells. Exp Mol Med 2019; 51:1-11. [PMID: 31316050 PMCID: PMC6802611 DOI: 10.1038/s12276-019-0279-2] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2018] [Revised: 02/22/2019] [Accepted: 03/18/2019] [Indexed: 01/22/2023] Open
Abstract
Chloride intracellular channel 1 (CLIC1) is a promising therapeutic target in cancer due to its intrinsic characteristics; it is overexpressed in specific tumor types and its localization changes from cytosolic to surface membrane depending on activities and cell cycle progression. Ca2+ and reactive oxygen species (ROS) are critical signaling molecules that modulate diverse cellular functions, including cell death. In this study, we investigated the function of CLIC1 in Ca2+ and ROS signaling in A549 human lung cancer cells. Depletion of CLIC1 via shRNAs in A549 cells increased DNA double-strand breaks both under control conditions and under treatment with the putative anticancer agent chelerythrine, accompanied by a concomitant increase in the p-JNK level. CLIC1 knockdown greatly increased basal ROS levels, an effect prevented by BAPTA-AM, an intracellular calcium chelator. Intracellular Ca2+ measurements clearly showed that CLIC1 knockdown significantly increased chelerythrine-induced Ca2+ signaling as well as the basal Ca2+ level in A549 cells compared to these levels in control cells. Suppression of extracellular Ca2+ restored the basal Ca2+ level in CLIC1-knockdown A549 cells relative to that in control cells, implying that CLIC1 regulates [Ca2+]i through Ca2+ entry across the plasma membrane. Consistent with this finding, the L-type Ca2+ channel (LTCC) blocker nifedipine reduced the basal Ca2+ level in CLIC1 knockdown cells to that in control cells. Taken together, our results demonstrate that CLIC1 knockdown induces an increase in the intracellular Ca2+ level via LTCC, which then triggers excessive ROS production and consequent JNK activation. Thus, CLIC1 is a key regulator of Ca2+ signaling in the control of cancer cell survival.
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Affiliation(s)
- Jae-Rin Lee
- 0000 0001 2181 989Xgrid.264381.aDepartment of Molecular Cell Biology, Sungkyunkwan University, Suwon, Korea ,0000 0001 2181 989Xgrid.264381.aSingle Cell Network Research Center, Sungkyunkwan University, Suwon, Korea
| | - Jong-Yoon Lee
- 0000 0001 2181 989Xgrid.264381.aSingle Cell Network Research Center, Sungkyunkwan University, Suwon, Korea ,0000 0001 2181 989Xgrid.264381.aDepartment of Physiology, Sungkyunkwan University, Suwon, Korea
| | - Hyun-Ji Kim
- 0000 0001 2181 989Xgrid.264381.aSingle Cell Network Research Center, Sungkyunkwan University, Suwon, Korea ,0000 0001 2181 989Xgrid.264381.aDepartment of Physiology, Sungkyunkwan University, Suwon, Korea
| | - Myong-Joon Hahn
- 0000 0001 2181 989Xgrid.264381.aDepartment of Molecular Cell Biology, Sungkyunkwan University, Suwon, Korea
| | - Jong-Sun Kang
- 0000 0001 2181 989Xgrid.264381.aDepartment of Molecular Cell Biology, Sungkyunkwan University, Suwon, Korea ,0000 0001 2181 989Xgrid.264381.aSingle Cell Network Research Center, Sungkyunkwan University, Suwon, Korea
| | - Hana Cho
- 0000 0001 2181 989Xgrid.264381.aSingle Cell Network Research Center, Sungkyunkwan University, Suwon, Korea ,0000 0001 2181 989Xgrid.264381.aDepartment of Physiology, Sungkyunkwan University, Suwon, Korea
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Pangrsic T, Singer JH, Koschak A. Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear. Physiol Rev 2019; 98:2063-2096. [PMID: 30067155 DOI: 10.1152/physrev.00030.2017] [Citation(s) in RCA: 62] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Calcium influx through voltage-gated Ca (CaV) channels is the first step in synaptic transmission. This review concerns CaV channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the CaV channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of CaV channels at presynaptic, ribbon-type active zones, because the spatial relationship between CaV channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, CaV1.3, and CaV1.4 channels or their protein modulatory elements.
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Affiliation(s)
- Tina Pangrsic
- Synaptic Physiology of Mammalian Vestibular Hair Cells Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen and Auditory Neuroscience Group, Max Planck Institute of Experimental Medicine , Göttingen, Germany ; Department of Biology, University of Maryland , College Park, Maryland ; and Pharmacology and Toxicology, Institute of Pharmacy, University of Innsbruck , Innsbruck , Austria
| | - Joshua H Singer
- Synaptic Physiology of Mammalian Vestibular Hair Cells Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen and Auditory Neuroscience Group, Max Planck Institute of Experimental Medicine , Göttingen, Germany ; Department of Biology, University of Maryland , College Park, Maryland ; and Pharmacology and Toxicology, Institute of Pharmacy, University of Innsbruck , Innsbruck , Austria
| | - Alexandra Koschak
- Synaptic Physiology of Mammalian Vestibular Hair Cells Group, Institute for Auditory Neuroscience and InnerEarLab, University Medical Center Göttingen and Auditory Neuroscience Group, Max Planck Institute of Experimental Medicine , Göttingen, Germany ; Department of Biology, University of Maryland , College Park, Maryland ; and Pharmacology and Toxicology, Institute of Pharmacy, University of Innsbruck , Innsbruck , Austria
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Restoration of vision after de novo genesis of rod photoreceptors in mammalian retinas. Nature 2018; 560:484-488. [PMID: 30111842 PMCID: PMC6107416 DOI: 10.1038/s41586-018-0425-3] [Citation(s) in RCA: 166] [Impact Index Per Article: 27.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2017] [Accepted: 07/03/2018] [Indexed: 02/07/2023]
Abstract
In zebrafish, Müller glial cells (MGs) are a source of retinal stem cells that can replenish damaged retinal neurons and restore vision1. In mammals, however, MGs lack regenerative capability as they do not spontaneously re-enter the cell cycle to generate a population of stem/progenitor cells that differentiate into retinal neurons. The regenerative machinery may exist in the mammalian retina, however, as retinal injury can stimulate MG proliferation followed by limited neurogenesis2–7. The fundamental question remains whether MG-derived regeneration can be exploited to restore vision in mammalian retinas. Previously, we showed that gene transfer of β-catenin stimulates MG proliferation in the absence of injury in mouse retinas8. Here, we report that following gene transfer of β-catenin, cell-cycle-reactivated MGs can be reprogrammed into rod photoreceptors via a subsequent gene transfer of transcription factors that are essential for rod cell fate specification and determination. MG-derived rods restored visual responses in Gnat1rd17:Gnat2cpfl3 double mutant mice, a model of congenital blindness9,10, throughout the visual pathway from the retina to the primary visual cortex. Together, our results provide evidence of vision restoration after de novo MG-derived genesis of rod photoreceptors in mammalian retinas.
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Kerov V, Laird JG, Joiner ML, Knecht S, Soh D, Hagen J, Gardner SH, Gutierrez W, Yoshimatsu T, Bhattarai S, Puthussery T, Artemyev NO, Drack AV, Wong RO, Baker SA, Lee A. α 2δ-4 Is Required for the Molecular and Structural Organization of Rod and Cone Photoreceptor Synapses. J Neurosci 2018; 38:6145-6160. [PMID: 29875267 PMCID: PMC6031576 DOI: 10.1523/jneurosci.3818-16.2018] [Citation(s) in RCA: 50] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2016] [Revised: 05/10/2018] [Accepted: 05/31/2018] [Indexed: 11/21/2022] Open
Abstract
α2δ-4 is an auxiliary subunit of voltage-gated Cav1.4 L-type channels that regulate the development and mature exocytotic function of the photoreceptor ribbon synapse. In humans, mutations in the CACNA2D4 gene encoding α2δ-4 cause heterogeneous forms of vision impairment in humans, the underlying pathogenic mechanisms of which remain unclear. To investigate the retinal function of α2δ-4, we used genome editing to generate an α2δ-4 knock-out (α2δ-4 KO) mouse. In male and female α2δ-4 KO mice, rod spherules lack ribbons and other synaptic hallmarks early in development. Although the molecular organization of cone synapses is less affected than rod synapses, horizontal and cone bipolar processes extend abnormally in the outer nuclear layer in α2δ-4 KO retina. In reconstructions of α2δ-4 KO cone pedicles by serial block face scanning electron microscopy, ribbons appear normal, except that less than one-third show the expected triadic organization of processes at ribbon sites. The severity of the synaptic defects in α2δ-4 KO mice correlates with a progressive loss of Cav1.4 channels, first in terminals of rods and later cones. Despite the absence of b-waves in electroretinograms, visually guided behavior is evident in α2δ-4 KO mice and better under photopic than scotopic conditions. We conclude that α2δ-4 plays an essential role in maintaining the structural and functional integrity of rod and cone synapses, the disruption of which may contribute to visual impairment in humans with CACNA2D4 mutations.SIGNIFICANCE STATEMENT In the retina, visual information is first communicated by the synapse formed between photoreceptors and second-order neurons. The mechanisms that regulate the structural integrity of this synapse are poorly understood. Here we demonstrate a role for α2δ-4, a subunit of voltage-gated Ca2+ channels, in organizing the structure and function of photoreceptor synapses. We find that presynaptic Ca2+ channels are progressively lost and that rod and cone synapses are disrupted in mice that lack α2δ-4. Our results suggest that alterations in presynaptic Ca2+ signaling and photoreceptor synapse structure may contribute to vision impairment in humans with mutations in the CACNA2D4 gene encoding α2δ-4.
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Affiliation(s)
- Vasily Kerov
- Department of Molecular Physiology and Biophysics
| | | | | | - Sharmon Knecht
- Department of Biological Structure, University of Washington, Seattle, Washington 98195, and
| | - Daniel Soh
- Department of Molecular Physiology and Biophysics
| | | | | | | | - Takeshi Yoshimatsu
- Department of Biological Structure, University of Washington, Seattle, Washington 98195, and
| | - Sajag Bhattarai
- Department of Ophthalmology and Institute for Vision Research
| | - Teresa Puthussery
- Casey Eye Institute, Oregon Health & Science University, Portland, Oregon 97239
| | | | - Arlene V Drack
- Department of Ophthalmology and Institute for Vision Research
| | - Rachel O Wong
- Department of Biological Structure, University of Washington, Seattle, Washington 98195, and
| | - Sheila A Baker
- Department of Biochemistry,
- Department of Ophthalmology and Institute for Vision Research
| | - Amy Lee
- Department of Molecular Physiology and Biophysics,
- Department of Otolaryngology-Head and Neck Surgery
- Department of Neurology, University of Iowa, Iowa City, Iowa 52242
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Van Hook MJ, Thoreson WB. Endogenous calcium buffering at photoreceptor synaptic terminals in salamander retina. Synapse 2014; 68:518-28. [PMID: 25049035 DOI: 10.1002/syn.21768] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2014] [Revised: 06/09/2014] [Accepted: 07/02/2014] [Indexed: 11/12/2022]
Abstract
Calcium operates by several mechanisms to regulate glutamate release at rod and cone synaptic terminals. In addition to serving as the exocytotic trigger, Ca2+ accelerates replenishment of vesicles in cones and triggers Ca2+-induced Ca2+ release (CICR) in rods. Ca2+ thereby amplifies sustained exocytosis, enabling photoreceptor synapses to encode constant and changing light. A complete picture of the role of Ca2+ in regulating synaptic transmission requires an understanding of the endogenous Ca2+ handling mechanisms at the synapse. We therefore used the "added buffer" approach to measure the endogenous Ca2+ binding ratio (κendo ) and extrusion rate constant (γ) in synaptic terminals of photoreceptors in retinal slices from tiger salamander. We found that κendo was similar in both cell types-∼25 and 50 in rods and cones, respectively. Using measurements of the decay time constants of Ca2+ transients, we found that γ was also similar, with values of ∼100 s(-1) and 160 s(-1) in rods and cones, respectively. The measurements of κendo differ considerably from measurements in retinal bipolar cells, another ribbon-bearing class of retinal neurons, but are comparable to similar measurements at other conventional synapses. The values of γ are slower than at other synapses, suggesting that Ca2+ ions linger longer in photoreceptor terminals, supporting sustained exocytosis, CICR, and Ca2+ -dependent ribbon replenishment. The mechanisms of endogenous Ca2+ handling in photoreceptors are thus well-suited for supporting tonic neurotransmission. Similarities between rod and cone Ca2+ handling suggest that neither buffering nor extrusion underlie differences in synaptic transmission kinetics.
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Affiliation(s)
- Matthew J Van Hook
- Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska, 68198
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Mercer AJ, Thoreson WB. Tracking quantum dot-tagged calcium channels at vertebrate photoreceptor synapses: retinal slices and dissociated cells. CURRENT PROTOCOLS IN NEUROSCIENCE 2013; Chapter 2:Unit 2.18. [PMID: 23315944 PMCID: PMC3707139 DOI: 10.1002/0471142301.ns0218s62] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
At synapses in the central nervous system, precisely localized assemblies of presynaptic proteins, neurotransmitter-filled vesicles, and postsynaptic receptors are required to communicate messages between neurons. Our understanding of synaptic function has been significantly advanced using electrophysiological methods, but the dynamic spatial behavior and real-time organization of synapses remains poorly understood. In this unit, we describe a method for labeling individual presynaptic calcium channels with photostable quantum dots for single-particle tracking analysis. We have used this technique to examine the mobility of L-type calcium channels in the presynaptic membrane of rod and cone photoreceptors in the retina. These channels control release of glutamate-filled synaptic vesicles at the ribbon synapses in photoreceptor terminals. This technique offers the advantage of providing a real-time biophysical readout of ion channel mobility and can be manipulated by pharmacological or electrophysiological methods. For example, the combination of electrophysiological and single-particle tracking experiments has revealed that fusion of nearby vesicles influences calcium channel mobility and changes in channel mobility can influence release. These approaches can also be readily adapted to examine membrane proteins in other systems.
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Affiliation(s)
- Aaron J Mercer
- Departments of Molecular and Integrative Physiology, University Of Michigan, Ann Arbor, USA
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Abstract
Photoreceptors are exquisitely adapted to transform light stimuli into electrical signals that modulate neurotransmitter release. These cells are organized into several compartments including the unique outer segment (OS). Its whole function is to absorb light and transduce this signal into a change of membrane potential. Another compartment is the inner segment where much of metabolism and regulation of membrane potential takes place and that connects the OS and synapse. The synapse is the compartment where changes in membrane potentials are relayed to other neurons in the retina via release of neurotransmitter. The composition of the plasma membrane surrounding these compartments varies to accommodate their specific functions. In this chapter, we discuss the organization of the plasma membrane emphasizing the protein composition of each region as it relates to visual signaling. We also point out examples where mutations in these proteins cause visual impairment.
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Affiliation(s)
- Sheila A Baker
- Department of Biochemistry, Carver College of Medicine, University of Iowa, Iowa City, Iowa, USA.
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The dynamic architecture of photoreceptor ribbon synapses: cytoskeletal, extracellular matrix, and intramembrane proteins. Vis Neurosci 2012; 28:453-71. [PMID: 22192503 DOI: 10.1017/s0952523811000356] [Citation(s) in RCA: 59] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Rod and cone photoreceptors possess ribbon synapses that assist in the transmission of graded light responses to second-order bipolar and horizontal cells of the vertebrate retina. Proper functioning of the synapse requires the juxtaposition of presynaptic release sites immediately adjacent to postsynaptic receptors. In this review, we focus on the synaptic, cytoskeletal, and extracellular matrix proteins that help to organize photoreceptor ribbon synapses in the outer plexiform layer. We examine the proteins that foster the clustering of release proteins, calcium channels, and synaptic vesicles in the presynaptic terminals of photoreceptors adjacent to their postsynaptic contacts. Although many proteins interact with one another in the presynaptic terminal and synaptic cleft, these protein-protein interactions do not create a static and immutable structure. Instead, photoreceptor ribbon synapses are remarkably dynamic, exhibiting structural changes on both rapid and slow time scales.
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Kim JC, Son MJ, Woo SH. Suppression of L-type Ca2+ current by fluid pressure in rat ventricular myocytes: Possible role of Cl−–OH− exchange. Biochem Biophys Res Commun 2011; 413:17-23. [DOI: 10.1016/j.bbrc.2011.08.026] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2011] [Accepted: 08/07/2011] [Indexed: 10/17/2022]
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Bartoletti TM, Jackman SL, Babai N, Mercer AJ, Kramer RH, Thoreson WB. Release from the cone ribbon synapse under bright light conditions can be controlled by the opening of only a few Ca(2+) channels. J Neurophysiol 2011; 106:2922-35. [PMID: 21880934 DOI: 10.1152/jn.00634.2011] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Light hyperpolarizes cone photoreceptors, causing synaptic voltage-gated Ca(2+) channels to open infrequently. To understand neurotransmission under these conditions, we determined the number of L-type Ca(2+) channel openings necessary for vesicle fusion at the cone ribbon synapse. Ca(2+) currents (I(Ca)) were activated in voltage-clamped cones, and excitatory postsynaptic currents (EPSCs) were recorded from horizontal cells in the salamander retina slice preparation. Ca(2+) channel number and single-channel current amplitude were calculated by mean-variance analysis of I(Ca). Two different comparisons-one comparing average numbers of release events to average I(Ca) amplitude and the other involving deconvolution of both EPSCs and simultaneously recorded cone I(Ca)-suggested that fewer than three Ca(2+) channel openings accompanied fusion of each vesicle at the peak of release during the first few milliseconds of stimulation. Opening fewer Ca(2+) channels did not enhance fusion efficiency, suggesting that few unnecessary channel openings occurred during strong depolarization. We simulated release at the cone synapse, using empirically determined synaptic dimensions, vesicle pool size, Ca(2+) dependence of release, Ca(2+) channel number, and Ca(2+) channel properties. The model replicated observations when a barrier was added to slow Ca(2+) diffusion. Consistent with the presence of a diffusion barrier, dialyzing cones with diffusible Ca(2+) buffers did not affect release efficiency. The tight clustering of Ca(2+) channels, along with a high-Ca(2+) affinity release mechanism and diffusion barrier, promotes a linear coupling between Ca(2+) influx and vesicle fusion. This may improve detection of small light decrements when cones are hyperpolarized by bright light.
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Affiliation(s)
- Theodore M Bartoletti
- Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska 68198-5840, USA
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Abstract
Cones release glutamate-filled vesicles continuously in darkness, and changing illumination modulates this release. Because sustained release in darkness is governed by vesicle replenishment rates, we analyzed how cone membrane potential regulates replenishment. Synaptic release from cones was measured by recording postsynaptic currents in Ambystoma tigrinum horizontal or OFF bipolar cells evoked by depolarization of simultaneously voltage-clamped cones. We measured replenishment after attaining a steady state between vesicle release and replenishment using trains of test pulses. Increasing Ca(2+) currents (I(Ca)) by changing the test step from -30 to -10 mV increased replenishment. Lengthening -30 mV test pulses to match the Ca(2+) influx during 25 ms test pulses to -10 mV produced similar replenishment rates. Reducing Ca(2+) driving force by using test steps to +30 mV slowed replenishment. Using UV flashes to reverse inhibition of I(Ca) by nifedipine accelerated replenishment. Increasing [Ca(2+)](i) by flash photolysis of caged Ca(2+) also accelerated replenishment. Replenishment, but not the initial burst of release, was enhanced by using an intracellular Ca(2+) buffer of 0.5 mm EGTA rather than 5 mm EGTA, and diminished by 1 mm BAPTA. This suggests that although release and replenishment exhibited similar Ca(2+) dependencies, release sites are <200 nm from Ca(2+) channels but replenishment sites are >200 nm away. Membrane potential thus regulates replenishment by controlling Ca(2+) influx, principally by effects on replenishment mechanisms but also by altering releasable pool size. This in turn provides a mechanism for converting changes in light intensity into changes in sustained release at the cone ribbon synapse.
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Mercer AJ, Rabl K, Riccardi GE, Brecha NC, Stella SL, Thoreson WB. Location of release sites and calcium-activated chloride channels relative to calcium channels at the photoreceptor ribbon synapse. J Neurophysiol 2010; 105:321-35. [PMID: 21084687 DOI: 10.1152/jn.00332.2010] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Vesicle release from photoreceptor ribbon synapses is regulated by L-type Ca(2+) channels, which are in turn regulated by Cl(-) moving through calcium-activated chloride [Cl(Ca)] channels. We assessed the proximity of Ca(2+) channels to release sites and Cl(Ca) channels in synaptic terminals of salamander photoreceptors by comparing fast (BAPTA) and slow (EGTA) intracellular Ca(2+) buffers. BAPTA did not fully block synaptic release, indicating some release sites are <100 nm from Ca(2+) channels. Comparing Cl(Ca) currents with predicted Ca(2+) diffusion profiles suggested that Cl(Ca) and Ca(2+) channels average a few hundred nanometers apart, but the inability of BAPTA to block Cl(Ca) currents completely suggested some channels are much closer together. Diffuse immunolabeling of terminals with an antibody to the putative Cl(Ca) channel TMEM16A supports the idea that Cl(Ca) channels are dispersed throughout the presynaptic terminal, in contrast with clustering of Ca(2+) channels near ribbons. Cl(Ca) currents evoked by intracellular calcium ion concentration ([Ca(2+)](i)) elevation through flash photolysis of DM-nitrophen exhibited EC(50) values of 556 and 377 nM with Hill slopes of 1.8 and 2.4 in rods and cones, respectively. These relationships were used to estimate average submembrane [Ca(2+)](i) in photoreceptor terminals. Consistent with control of exocytosis by [Ca(2+)] nanodomains near Ca(2+) channels, average submembrane [Ca(2+)](i) remained below the vesicle release threshold (∼ 400 nM) over much of the physiological voltage range for cones. Positioning Ca(2+) channels near release sites may improve fidelity in converting voltage changes to synaptic release. A diffuse distribution of Cl(Ca) channels may allow Ca(2+) influx at one site to influence relatively distant Ca(2+) channels.
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Affiliation(s)
- A J Mercer
- University of Nebraska Medical Center, Department of Ophthalmology and Visual Sciences, 4050 Durham Research Center, Omaha, NE 68198-5840, USA
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18
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Krizaj D, Mercer AJ, Thoreson WB, Barabas P. Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors. Am J Physiol Cell Physiol 2010; 300:C187-97. [PMID: 20881233 DOI: 10.1152/ajpcell.00264.2010] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pH(i)) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca²(+)), in salamander photoreceptor inner segments loaded with Ca²(+) and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ∼0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH₄Cl) depolarized photoreceptors and stimulated Ca²(+) release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca²(+)](i). Alkalinization-evoked [Ca²(+)](i) decreases were independent of voltage-operated and store-operated Ca²(+) entry, plasma membrane Ca²(+) extrusion, and Ca²(+) sequestration into internal stores. The [Ca²(+)](i)-suppressive effects of alkalinization were antagonized by the fast Ca²(+) buffer BAPTA, suggesting that pH(i) directly regulates Ca²(+) binding to internal anionic sites. In summary, this data suggest that endogenously produced protons continually modulate the membrane potential, release from Ca²(+) stores, and intracellular Ca²(+) buffering in rod and cone inner segments.
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Affiliation(s)
- David Krizaj
- Department of Ophthalmology, Moran Eye Center, University of Utah School of Medicine, Salt Lake City, 84132, USA.
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19
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Duncan G, Rabl K, Gemp I, Heidelberger R, Thoreson WB. Quantitative analysis of synaptic release at the photoreceptor synapse. Biophys J 2010; 98:2102-10. [PMID: 20483317 DOI: 10.1016/j.bpj.2010.02.003] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2009] [Revised: 01/11/2010] [Accepted: 02/02/2010] [Indexed: 02/02/2023] Open
Abstract
Exocytosis from the rod photoreceptor is stimulated by submicromolar Ca(2+) and exhibits an unusually shallow dependence on presynaptic Ca(2+). To provide a quantitative description of the photoreceptor Ca(2+) sensor for exocytosis, we tested a family of conventional and allosteric computational models describing the final Ca(2+)-binding steps leading to exocytosis. Simulations were fit to two measures of release, evoked by flash-photolysis of caged Ca(2+): exocytotic capacitance changes from individual rods and postsynaptic currents of second-order neurons. The best simulations supported the occupancy of only two Ca(2+) binding sites on the rod Ca(2+) sensor rather than the typical four or five. For most models, the on-rates for Ca(2+) binding and maximal fusion rate were comparable to those of other neurons. However, the off-rates for Ca(2+) unbinding were unexpectedly slow. In addition to contributing to the high-affinity of the photoreceptor Ca(2+) sensor, slow Ca(2+) unbinding may support the fusion of vesicles located at a distance from Ca(2+) channels. In addition, partial sensor occupancy due to slow unbinding may contribute to the linearization of the first synapse in vision.
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Affiliation(s)
- Gabriel Duncan
- Department of Neurobiology and Anatomy, University of Texas Health Science Center at Houston, Houston, Texas, USA
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20
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Babai N, Kanevsky N, Dascal N, Rozanski GJ, Singh DP, Fatma N, Thoreson WB. Anion-sensitive regions of L-type CaV1.2 calcium channels expressed in HEK293 cells. PLoS One 2010; 5:e8602. [PMID: 20066046 PMCID: PMC2798859 DOI: 10.1371/journal.pone.0008602] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2009] [Accepted: 12/09/2009] [Indexed: 11/21/2022] Open
Abstract
L-type calcium currents (ICa) are influenced by changes in extracellular chloride, but sites of anion effects have not been identified. Our experiments showed that CaV1.2 currents expressed in HEK293 cells are strongly inhibited by replacing extracellular chloride with gluconate or perchlorate. Variance-mean analysis of ICa and cell-attached patch single channel recordings indicate that gluconate-induced inhibition is due to intracellular anion effects on Ca2+ channel open probability, not conductance. Inhibition of CaV1.2 currents produced by replacing chloride with gluconate was reduced from ∼75%–80% to ∼50% by omitting β subunits but unaffected by omitting α2δ subunits. Similarly, gluconate inhibition was reduced to ∼50% by deleting an α1 subunit N-terminal region of 15 residues critical for β subunit interactions regulating open probability. Omitting β subunits with this mutant α1 subunit did not further diminish inhibition. Gluconate inhibition was unchanged with expression of different β subunits. Truncating the C terminus at AA1665 reduced gluconate inhibition from ∼75%–80% to ∼50% whereas truncating it at AA1700 had no effect. Neutralizing arginines at AA1696 and 1697 by replacement with glutamines reduced gluconate inhibition to ∼60% indicating these residues are particularly important for anion effects. Expressing CaV1.2 channels that lacked both N and C termini reduced gluconate inhibition to ∼25% consistent with additive interactions between the two tail regions. Our results suggest that modest changes in intracellular anion concentration can produce significant effects on CaV1.2 currents mediated by changes in channel open probability involving β subunit interactions with the N terminus and a short C terminal region.
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Affiliation(s)
- Norbert Babai
- Department of Ophthalmology & Visual Science, University of Nebraska Medical Center, Omaha, Nebraska, United States of America
| | - Nataly Kanevsky
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat Aviv, Israel
| | - Nathan Dascal
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Ramat Aviv, Israel
| | - George J. Rozanski
- Department of Cellular & Integrative Physiology, University of Nebraska Medical Center, Omaha, Nebraska, United States of America
| | - Dhirendra P. Singh
- Department of Ophthalmology & Visual Science, University of Nebraska Medical Center, Omaha, Nebraska, United States of America
| | - Nigar Fatma
- Department of Ophthalmology & Visual Science, University of Nebraska Medical Center, Omaha, Nebraska, United States of America
| | - Wallace B. Thoreson
- Department of Ophthalmology & Visual Science, University of Nebraska Medical Center, Omaha, Nebraska, United States of America
- * E-mail:
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21
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Bartoletti TM, Babai N, Thoreson WB. Vesicle pool size at the salamander cone ribbon synapse. J Neurophysiol 2009; 103:419-23. [PMID: 19923246 DOI: 10.1152/jn.00718.2009] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Cone light responses are transmitted to postsynaptic neurons by changes in the rate of synaptic vesicle release. Vesicle pool size at the cone synapse constrains the amount of release and can thus shape contrast detection. We measured the number of vesicles in the rapidly releasable and reserve pools at cone ribbon synapses by performing simultaneous whole cell recording from cones and horizontal or off bipolar cells in the salamander retinal slice preparation. We found that properties of spontaneously occurring miniature excitatory postsynaptic currents (mEPSCs) are representative of mEPSCs evoked by depolarizing presynaptic stimulation. Strong, brief depolarization of the cone stimulated release of the entire rapidly releasable pool (RRP) of vesicles. Comparing charge transfer of the EPSC with mEPSC charge transfer, we determined that the fast component of the EPSC reflects release of approximately 40 vesicles. Comparing EPSCs with simultaneous presynaptic capacitance measurements, we found that horizontal cell EPSCs constitute 14% of the total number of vesicles released from a cone terminal. Using a fluorescent ribeye-binding peptide, we counted approximately 13 ribbons per cone. Together, these results suggest each cone contacts a single horizontal cell at approximately 2 ribbons. The size of discrete components in the EPSC amplitude histogram also suggested approximately 2 ribbon contacts per cell pair. We therefore conclude there are approximately 20 vesicles per ribbon in the RRP, similar to the number of vesicles contacting the plasma membrane at the ribbon base. EPSCs evoked by lengthy depolarization suggest a reserve pool of approximately 90 vesicles per ribbon, similar to the number of additional docking sites further up the ribbon.
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Affiliation(s)
- Theodore M Bartoletti
- Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, 4050 Durham Research Center, Omaha, NE 68198-5840, USA
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22
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Stella SL, Hu WD, Brecha NC. Adenosine suppresses exocytosis from cone terminals of the salamander retina. Neuroreport 2009; 20:923-9. [PMID: 19491713 DOI: 10.1097/wnr.0b013e32832ca4b0] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
In the retina, adenosine is released in the dark and has been shown to inhibit Ca2+ influx through voltage-gated Ca2+ channels in cones. Therefore, we tested whether adenosine can inhibit exocytosis from isolated cone photoreceptors. Simultaneous measurements of membrane exocytosis and Ca2+ were made from cones using the activity-dependent dye, Synaptored-C2, and the Ca2+ indicator dye, Fluo-4. Adenosine suppressed exocytosis in cones, indicating that transmitter release is also reduced from cone terminals, and further supports an inhibitory mechanism for modulating transmitter release onto second-order neurons. Furthermore, this raises the possibility that adenosine might be neuroprotective for photoreceptors and second-order neurons by suppressing Ca2+ levels in cones and reducing exocytosis of L-glutamate, respectively.
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Affiliation(s)
- Salvatore L Stella
- Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, California 90095-1763, USA.
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23
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Szikra T, Barabas P, Bartoletti TM, Huang W, Akopian A, Thoreson WB, Krizaj D. Calcium homeostasis and cone signaling are regulated by interactions between calcium stores and plasma membrane ion channels. PLoS One 2009; 4:e6723. [PMID: 19696927 PMCID: PMC2725299 DOI: 10.1371/journal.pone.0006723] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2009] [Accepted: 07/27/2009] [Indexed: 12/13/2022] Open
Abstract
Calcium is a messenger ion that controls all aspects of cone photoreceptor function, including synaptic release. The dynamic range of the cone output extends beyond the activation threshold for voltage-operated calcium entry, suggesting another calcium influx mechanism operates in cones hyperpolarized by light. We have used optical imaging and whole-cell voltage clamp to measure the contribution of store-operated Ca2+ entry (SOCE) to Ca2+ homeostasis and its role in regulation of neurotransmission at cone synapses. Mn2+ quenching of Fura-2 revealed sustained divalent cation entry in hyperpolarized cones. Ca2+ influx into cone inner segments was potentiated by hyperpolarization, facilitated by depletion of intracellular Ca2+ stores, unaffected by pharmacological manipulation of voltage-operated or cyclic nucleotide-gated Ca2+ channels and suppressed by lanthanides, 2-APB, MRS 1845 and SKF 96365. However, cation influx through store-operated channels crossed the threshold for activation of voltage-operated Ca2+ entry in a subset of cones, indicating that the operating range of inner segment signals is set by interactions between store- and voltage-operated Ca2+ channels. Exposure to MRS 1845 resulted in ∼40% reduction of light-evoked postsynaptic currents in photopic horizontal cells without affecting the light responses or voltage-operated Ca2+ currents in simultaneously recorded cones. The spatial pattern of store-operated calcium entry in cones matched immunolocalization of the store-operated sensor STIM1. These findings show that store-operated channels regulate spatial and temporal properties of Ca2+ homeostasis in vertebrate cones and demonstrate their role in generation of sustained excitatory signals across the first retinal synapse.
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Affiliation(s)
- Tamas Szikra
- Department of Ophthalmology, University of California San Francisco (UCSF) School of Medicine, San Francisco, California, United States of America
| | - Peter Barabas
- Department of Ophthalmology & Visual Sciences, Moran Eye Center, University of Utah School of Medicine, Salt Lake City, Utah, United States of America
| | - Theodore M. Bartoletti
- Department of Ophthalmology & Visual Sciences, University of Utah School of Medicine, Salt Lake City, Utah, United States of America
| | - Wei Huang
- Department of Ophthalmology & Visual Sciences, Moran Eye Center, University of Utah School of Medicine, Salt Lake City, Utah, United States of America
| | - Abram Akopian
- Department of Ophthalmology, New York University Medical Center, New York, New York, United States of America
| | - Wallace B. Thoreson
- Department of Ophthalmology & Visual Sciences, University of Utah School of Medicine, Salt Lake City, Utah, United States of America
- Pharmacology & Experimental Neurosciences Univ. of Nebraska Medical Center, Omaha, Nebraska, United States of America
| | - David Krizaj
- Department of Ophthalmology & Visual Sciences, Moran Eye Center, University of Utah School of Medicine, Salt Lake City, Utah, United States of America
- Department of Physiology, University of Utah School of Medicine, Salt Lake City, Utah, United States of America
- * E-mail:
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24
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Babai N, Thoreson WB. Horizontal cell feedback regulates calcium currents and intracellular calcium levels in rod photoreceptors of salamander and mouse retina. J Physiol 2009; 587:2353-64. [PMID: 19332495 PMCID: PMC2697303 DOI: 10.1113/jphysiol.2009.169656] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2009] [Accepted: 03/26/2009] [Indexed: 11/08/2022] Open
Abstract
We tested whether horizontal cells (HCs) provide feedback that regulates the Ca(2+) current (I(Ca)) of rods in salamander and mouse retinas. In both species, hyperpolarizing HCs by puffing a glutamate antagonist, 6,7-dinitro-quinoxaline-2,3-dione (DNQX), onto HC processes caused a negative shift in the voltage dependence of rod I(Ca) and increased its peak amplitude. Conversely, depolarizing HCs by puffing kainic acid (KA) into the outer plexiform layer (OPL) caused a positive voltage shift and decreased rod I(Ca.) Experiments on salamander retina showed that these effects were blocked by addition of the pH buffer, Hepes. Intracellular calcium concentration ([Ca(2+)](i)) was examined in rods by confocal microscopy after loading salamander and mouse retinal slices with Fluo-4. Rods were depolarized to near the dark resting potential by bath application of high K(+) solutions. Hyperpolarizing HCs with 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo[f]quinoxaline (NBQX) enhanced high K(+)-evoked Ca(2+) increases whereas depolarizing HCs with KA inhibited Ca(2+) increases. In both species these effects of NBQX and KA were blocked by addition of Hepes. Thus, like HC feedback in cones, changes in HC membrane potential modulate rod I(Ca) thereby regulating rod [Ca(2+)](i) at physiological voltages, in both mouse and salamander retinas. By countering the reduced synaptic output that accompanies hyperpolarization of rods to light, HC feedback will subtract spatially averaged luminance levels from the responses of individual rods to local changes. The finding that HC to rod feedback is present in both amphibian and mammalian species shows that this mechanism is highly conserved across vertebrate retinas.
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Affiliation(s)
- Norbert Babai
- Department of Ophthalmology and Visual Science, University of Nebraska Medical Center, Omaha, NE 68198-5840, USA
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25
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Szikra T, Cusato K, Thoreson WB, Barabas P, Bartoletti TM, Krizaj D. Depletion of calcium stores regulates calcium influx and signal transmission in rod photoreceptors. J Physiol 2008; 586:4859-75. [PMID: 18755743 DOI: 10.1113/jphysiol.2008.160051] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Tonic synapses are specialized for sustained calcium entry and transmitter release, allowing them to operate in a graded fashion over a wide dynamic range. We identified a novel plasma membrane calcium entry mechanism that extends the range of rod photoreceptor signalling into light-adapted conditions. The mechanism, which shares molecular and physiological characteristics with store-operated calcium entry (SOCE), is required to maintain baseline [Ca(2+)](i) in rod inner segments and synaptic terminals. Sustained Ca(2+) entry into rod cytosol is augmented by store depletion, blocked by La(3+) and Gd(3+) and suppressed by organic antagonists MRS-1845 and SKF-96365. Store depletion and the subsequent Ca(2+) influx directly stimulated exocytosis in terminals of light-adapted rods loaded with the activity-dependent dye FM1-43. Moreover, SOCE blockers suppressed rod-mediated synaptic inputs to horizontal cells without affecting presynaptic voltage-operated Ca(2+) entry. Silencing of TRPC1 expression with small interference RNA disrupted SOCE in rods, but had no effect on cone Ca(2+) signalling. Rods were immunopositive for TRPC1 whereas cone inner segments immunostained with TRPC6 channel antibodies. Thus, SOCE modulates Ca(2+) homeostasis and light-evoked neurotransmission at the rod photoreceptor synapse mediated by TRPC1.
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Affiliation(s)
- Tamas Szikra
- Department of Ophthalmology, UCSF School of Medicine, San Francisco, CA 94143, USA
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26
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Cadetti L, Bartoletti TM, Thoreson WB. Quantal mEPSCs and residual glutamate: how horizontal cell responses are shaped at the photoreceptor ribbon synapse. Eur J Neurosci 2008; 27:2575-86. [PMID: 18547244 DOI: 10.1111/j.1460-9568.2008.06226.x] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
At the photoreceptor ribbon synapse, glutamate released from vesicles at different positions along the ribbon reaches the same postsynaptic receptors. Thus, vesicles may not exert entirely independent effects. We examined whether responses of salamander retinal horizontal cells evoked by light or direct depolarization during paired recordings could be predicted by summation of individual miniature excitatory postsynaptic currents (mEPSCs). For EPSCs evoked by depolarization of rods or cones, linear convolution of mEPSCs with photoreceptor release functions predicted EPSC waveforms and changes caused by inhibiting glutamate receptor desensitization. A low-affinity glutamate antagonist, kynurenic acid (KynA), preferentially reduced later components of rod-driven EPSCs, suggesting lower levels of glutamate are present during the later sustained component of the EPSC. A glutamate-scavenging enzyme, glutamic-pyruvic transaminase, did not inhibit mEPSCs or the initial component of rod-driven EPSCs, but reduced later components of the EPSC. Inhibiting glutamate uptake with a low concentration of DL-threo-beta-benzoyloxyaspartate (TBOA) also did not alter mEPSCs or the initial component of rod-driven EPSCs, but enhanced later components of the EPSC. Low concentrations of TBOA and KynA did not affect the kinetics of fast cone-driven EPSCs. Under both rod- and cone-dominated conditions, light-evoked currents (LECs) were enhanced considerably by TBOA. LECs were more strongly inhibited than EPSCs by KynA, suggesting the presence of lower glutamate levels. Collectively, these results indicate that the initial EPSC component can be largely predicted from a linear sum of individual mEPSCs, but with sustained release, residual amounts of glutamate from multiple vesicles pool together, influencing LECs and later components of EPSCs.
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Affiliation(s)
- Lucia Cadetti
- Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Durham Research Center Room 4050, Omaha, NE 68198-5840, USA
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27
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Abstract
Retinal horizontal cells (HCs) provide negative feedback to cones, but, largely because annular illumination fails to evoke a depolarizing response in rods, it is widely believed that there is no feedback from HCs to rods. However, feedback from HCs to cones involves small changes in the calcium current (I(Ca)) that do not always generate detectable depolarizing responses. We therefore recorded I(Ca) directly from rods to test whether they were modulated by feedback from HCs. To circumvent problems presented by overlapping receptive fields of HCs and rods, we manipulated the membrane potential of voltage-clamped HCs while simultaneously recording from rods in a salamander retinal slice preparation. Like HC feedback in cones, hyperpolarizing HCs from -14 to -54, -84, and -104 mV increased the amplitude of I(Ca) recorded from synaptically connected rods and caused hyperpolarizing shifts in I(Ca) voltage dependence. These effects were blocked by supplementing the bicarbonate-buffered saline solution with HEPES. In rods lacking light-responsive outer segments, hyperpolarizing neighboring HCs with light caused a negative activation shift and increased the amplitude of I(Ca). These changes in I(Ca) were blocked by HEPES and by inhibiting HC light responses with a glutamate antagonist, indicating that they were caused by HC feedback. These results show that rods, like cones, receive negative feedback from HCs that regulates the amplitude and voltage dependence of I(Ca). HC-to-rod feedback counters light-evoked decreases in synaptic output and thus shapes the transmission of rod responses to downstream visual neurons.
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28
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Thoreson WB. Kinetics of synaptic transmission at ribbon synapses of rods and cones. Mol Neurobiol 2007; 36:205-23. [PMID: 17955196 PMCID: PMC2474471 DOI: 10.1007/s12035-007-0019-9] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2007] [Accepted: 05/18/2007] [Indexed: 11/24/2022]
Abstract
The ribbon synapse is a specialized structure that allows photoreceptors to sustain the continuous release of vesicles for hours upon hours and years upon years but also respond rapidly to momentary changes in illumination. Light responses of cones are faster than those of rods and, mirroring this difference, synaptic transmission from cones is also faster than transmission from rods. This review evaluates the various factors that regulate synaptic kinetics and contribute to kinetic differences between rod and cone synapses. Presynaptically, the release of glutamate-laden synaptic vesicles is regulated by properties of the synaptic proteins involved in exocytosis, influx of calcium through calcium channels, calcium release from intracellular stores, diffusion of calcium to the release site, calcium buffering, and extrusion of calcium from the cytoplasm. The rate of vesicle replenishment also limits the ability of the synapse to follow changes in release. Post-synaptic factors include properties of glutamate receptors, dynamics of glutamate diffusion through the cleft, and glutamate uptake by glutamate transporters. Thus, multiple synaptic mechanisms help to shape the responses of second-order horizontal and bipolar cells.
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Affiliation(s)
- Wallace B Thoreson
- Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, 4th floor, Durham Research Center, 985840 Nebraska Medical Center, Omaha, NE 68198-5840, USA.
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29
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Stella SL, Hu WD, Vila A, Brecha NC. Adenosine inhibits voltage-dependent Ca2+ influx in cone photoreceptor terminals of the tiger salamander retina. J Neurosci Res 2007; 85:1126-37. [PMID: 17304584 PMCID: PMC3737423 DOI: 10.1002/jnr.21210] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Endogenous adenosine has already been shown to inhibit transmitter release from the rod synapse by suppressing Ca(2+) influx through voltage-gated Ca(2+) channels. However, it is not clear how adenosine modulates the cone synapse. Cone photoreceptors, like rod photoreceptors, also possess L-type Ca(2+) channels that regulate the release of L-glutamate. To assess the impact of adenosine on Ca(2+) influx though voltage-gated Ca(2+) channels in cone terminals, whole-cell perforated-patch clamp recording and Ca(2+) imaging with fluo-4 were used on isolated cones and salamander retinal slices. Synaptic markers (VAMP and piccolo) and activity-dependent dye labeling revealed that tiger salamander cone terminals contain a broad, vesicle-filled cytoplasmic extension at the base of the somatic compartment, which is unlike rod terminals that contain one or more thin axons, each terminating in a large bulbous synaptic terminal. The spatiotemporal Ca(2+) responses of the cone terminals do not differ significantly from the Ca(2+) responses of the soma or inner segment like that observed in rods. Whole-cell recording of cone I(Ca) and Ca(2+) imaging of synaptic terminals in cones demonstrate that adenosine inhibited both I(Ca) and the depolarization-evoked Ca(2+) increase in cone terminals in a dose-dependent manner from 1 to 50 muM. These results indicate that, as in rods, adenosine's ability to suppress voltage-dependent Ca(2+) channels at the cone synapse will limit the amount of L-glutamate released. Therefore, adenosine has an inhibitory effect on L-glutamate release at the first synapse, which likely favors elevated adenosine levels in the dark or during dark-adapted conditions.
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Affiliation(s)
- Salvatore L Stella
- Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, California, USA.
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30
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Cadetti L, Bryson EJ, Ciccone CA, Rabl K, Thoreson WB. Calcium-induced calcium release in rod photoreceptor terminals boosts synaptic transmission during maintained depolarization. Eur J Neurosci 2006; 23:2983-90. [PMID: 16819987 PMCID: PMC2474468 DOI: 10.1111/j.1460-9568.2006.04845.x] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
We examined the contribution of calcium-induced calcium release (CICR) to synaptic transmission from rod photoreceptor terminals. Whole-cell recording and confocal calcium imaging experiments were conducted on rods with intact synaptic terminals in a retinal slice preparation from salamander. Low concentrations of ryanodine stimulated calcium increases in rod terminals, consistent with the presence of ryanodine receptors. Application of strong depolarizing steps (-70 to -10 mV) exceeding 200 ms or longer in duration evoked a wave of calcium that spread across the synaptic terminals of voltage-clamped rods. This secondary calcium increase was blocked by high concentrations of ryanodine, indicating it was due to CICR. Ryanodine (50 microm) had no significant effect on rod calcium current (I(ca)) although it slightly diminished rod light-evoked voltage responses. Bath application of 50 microm ryanodine strongly inhibited light-evoked currents in horizontal cells. Whether applied extracellularly or delivered into the rod cell through the patch pipette, ryanodine (50 microm) also inhibited excitatory post-synaptic currents (EPSCs) evoked in horizontal cells by depolarizing steps applied to rods. Ryanodine caused a preferential reduction in the later portions of EPSCs evoked by depolarizing steps of 200 ms or longer. These results indicate that CICR enhances calcium increases in rod terminals evoked by sustained depolarization, which in turn acts to boost synaptic exocytosis from rods.
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Affiliation(s)
- Lucia Cadetti
- Department of Ophthalmology & Visual Science, University of Nebraska Medical Center, Omaha, 68198-5840, USA
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Abstract
Synaptic depression produced by repetitive stimulation is likely to be particularly important in shaping responses of second-order retinal neurons at the tonically active photoreceptor synapse. We analyzed the time course and mechanisms of synaptic depression at rod and cone synapses using paired-pulse protocols involving two complementary measurements of exocytosis: (1) paired whole-cell recordings of the postsynaptic current (PSC) in second-order retinal neurons and (2) capacitance measurements of vesicular membrane fusion in rods and cones. PSCs in ON bipolar, OFF bipolar, and horizontal cells evoked by stimulation of either rods or cones recovered from paired-pulse depression (PPD) at rates similar to the recovery of exocytotic capacitance changes in rods and cones. Correlation between presynaptic and postsynaptic measures of recovery from PPD suggests that 80-90% of the depression at these synapses is presynaptic in origin. Consistent with a predominantly presynaptic mechanism, inhibiting desensitization of postsynaptic glutamate receptors had little effect on PPD. The depression of exocytotic capacitance changes exceeded depression of the presynaptic calcium current, suggesting that it is primarily caused by a depletion of synaptic vesicles. In support of this idea, limiting Ca2+ influx by using weaker depolarizing stimuli promoted faster recovery from PPD. Although cones exhibit much faster exocytotic kinetics than rods, exocytotic capacitance changes recovered from PPD at similar rates in both cell types. Thus, depression of release is not likely to contribute to differences in the kinetics of transmission from rods and cones.
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Abstract
Cone-driven responses of second-order retinal neurons are considerably faster than rod-driven responses. We examined whether differences in the kinetics of synaptic transmitter release from rods and cones may contribute to differences in postsynaptic response kinetics. Exocytosis from rods and cones was triggered by membrane depolarization and monitored in two ways: (1) by measuring EPSCs evoked in second-order neurons by depolarizing steps applied to presynaptic rods or cones during simultaneous paired whole-cell recordings or (2) by direct measurements of exocytotic increases in membrane capacitance. The kinetics of release was assessed by varying the length of the depolarizing test step. Both measures of release revealed two kinetic components to the increase in exocytosis as a function of the duration of a step depolarization. In addition to slow sustained components in both cell types, the initial fast component of exocytosis had a time constant of <5 ms in cones, >10-fold faster than that of rods. Rod/cone differences in the kinetics of release were substantiated by a linear correlation between depolarization-evoked capacitance increases and EPSC charge transfer. Experiments on isolated rods indicate that the slower kinetics of exocytosis from rods was not a result of rod-rod coupling. The initial rapid release of vesicles from cones can shape the postsynaptic response and may contribute to the faster responses of cone-driven cells observed at light offset.
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Affiliation(s)
- Katalin Rabl
- Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska 68198-5840, USA
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Abstract
For over 30 years, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and G protein signaling. Recently, photoreceptors have become an equally attractive model for studying many facets of neuronal cell biology. The primary goal of this review is to illustrate this rapidly growing trend. We will highlight the areas of active research in photoreceptor biology that reveal how different specialized compartments of the cell cooperate in fulfilling its overall function: converting photon absorption into changes in neurotransmitter release. The same trend brings us closer to understanding how defects in photoreceptor signaling can lead to cell death and retinal degeneration.
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Affiliation(s)
- Marie E Burns
- Center for Neuroscience and Department of Psychiatry and Behavioral Sciences, University of California, Davis, Davis, California 95616, USA.
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Cadetti L, Thoreson WB. Feedback effects of horizontal cell membrane potential on cone calcium currents studied with simultaneous recordings. J Neurophysiol 2005; 95:1992-5. [PMID: 16371457 PMCID: PMC2474467 DOI: 10.1152/jn.01042.2005] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Horizontal cell (HC) to cone feedback helps establish the center-surround arrangement of visual receptive fields. It has been shown that HC activity influences cone synaptic output by altering the amplitude and voltage dependence of the calcium current (ICa) in cones. In this study, we obtained voltage-clamp recordings simultaneously from cones and HCs to directly control the membrane potential of HCs and thereby measure the influence of HC membrane potential changes on ICa in adjacent cones. Directly hyperpolarizing voltage clamped HCs produced a negative activation shift and increased the amplitude of ICa in cones. Both of these effects were abolished by enhancing extracellular pH buffering capacity with HEPES. In contrast, addition of the gap junction blocker, carbenoxolone, did not significantly alter the shifts or amplitude changes in cone ICa produced by changes in HC membrane potential. These results support the hypothesis that changes in the HC membrane potential alter the voltage dependence and amplitude of cone ICa by altering extracellular pH levels at the synapse.
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Affiliation(s)
- Lucia Cadetti
- Dept. of Ophthalmology, Univ. of Nebraska Medical Center, Durham Research Center, Omaha, NE 68198-5840, USA
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Bekar LK, Loewen ME, Forsyth GW, Walz W. Chloride concentration affects Kv channel voltage-gating kinetics: Importance of experimental anion concentrations. Brain Res Bull 2005; 67:142-6. [PMID: 16140173 DOI: 10.1016/j.brainresbull.2005.06.023] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2005] [Revised: 06/06/2005] [Accepted: 06/11/2005] [Indexed: 10/25/2022]
Abstract
Chloride concentration has been shown to have a dramatic impact on protein folding and subsequent tertiary conformation [K.D. Collins, Ions from the Hofmeister series and osmolytes: effects on proteins in solution and in the crystallization process, Methods 34 (2004) 300-311; I. Jelesarov, E. Durr, R.M. Thomas, H.R. Bosshard, Salt effects on hydrophobic interaction and charge screening in the folding of a negatively charged peptide to a coiled coil (leucine zipper), Biochemistry 37 (1998) 7539-7550]. As it is known that Kv channel gating is linked to the stability of the cytoplasmic T1 multimerization domain conformation [D.L. Minor, Y.F. Lin, B.C. Mobley, A. Avelar, Y.N. Jan, L.Y. Jan, J.M. Berger, The polar T1 interface is linked to conformational changes that open the voltage-gated potassium channel, Cell 102 (2000) 657-670; B.A. Yi, D.L. Minor Jr., Y.F. Lin, Y.N. Jan, L.Y. Jan, Controlling potassium channel activities: interplay between the membrane and intracellular factors, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 11016-11023] and that intracellular chloride concentration has been linked to Kv channel kinetics [L.K. Bekar, W. Walz, Intracellular chloride modulates A-type potassium currents in astrocytes, Glia 39 (2002) 207-216; W.B. Thoreson, S.L. Stella, Anion modulation of calcium current voltage dependence and amplitude in salamander rods, Biochim. Biophys. Acta 1464 (2000) 142-150], the objective of the present study was to address how chloride concentration changes affect Kv channel kinetics more closely in an isolated expression system. Initially, no significant chloride concentration-dependent effects on channel steady-state gating kinetics were observed. Only after disruption of the cytoskeleton with cytochalasin-D did we see significant chloride concentration-dependent shifts in gating kinetics. This suggests that the shift in gating kinetics is mediated through effects of intracellular chloride concentration on cytoplasmic domain tertiary conformation as cytoskeletal interaction appears to mask the effect. Furthermore, as cytoskeletal disruption only impacts channel gating kinetics at low physiological intracellular chloride concentrations, these studies highlight the importance of paying close attention to anion concentrations used under experimental conditions.
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Affiliation(s)
- L K Bekar
- Department of Physiology, University of Saskatchewan, Rm B39 Health Sciences Building, Saskatoon, SK, Canada S7N 5E5.
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Schein S, Ahmad KM. A clockwork hypothesis: synaptic release by rod photoreceptors must be regular. Biophys J 2005; 89:3931-49. [PMID: 16169984 PMCID: PMC1366960 DOI: 10.1529/biophysj.105.070623] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2005] [Accepted: 09/07/2005] [Indexed: 11/18/2022] Open
Abstract
We can see at light intensities much lower than an average of one photon per rod photoreceptor, demonstrating that rods must be able to transmit a signal after absorption of a single photon. However, activation of one rhodopsin molecule (Rh*) hyperpolarizes a mammalian rod by just 1 mV. Based on the properties of the voltage-dependent Ca2+ channel and data on [Ca2+] in the rod synaptic terminal, the 1 mV hyperpolarization should reduce the rate of release of quanta of neurotransmitter by only 20%. If quantal release were Poisson, the distributions of quantal count in the dark and in response to one Rh* would overlap greatly. Depending on the threshold quantal count, the overlap would generate too frequent false positives in the dark, too few true positives in response to one Rh*, or both. Therefore, quantal release must be regular, giving narrower distributions of quantal count that overlap less. We model regular release as an Erlang process, essentially a mechanism that counts many Poisson events before release of a quantum of neurotransmitter. The combination of appropriately narrow distributions of quantal count and a suitable threshold can give few false positives and appropriate (e.g., 35%) efficiency for one Rh*.
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Affiliation(s)
- Stan Schein
- Department of Psychology, and Brain Research Institute, University of California, Los Angeles, Los Angeles, California 90095-1563, USA.
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Abstract
The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.
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Affiliation(s)
- Ruth Heidelberger
- Department of Neurobiology & Anatomy, University of Texas Health Science Center at Houston, Houston, TX 77030 USA
| | - Wallace B. Thoreson
- Department of Ophthalmology & Visual Sciences and Department of Pharmacology, University of Nebraska Medical Center, Omaha, NE 68198, USA
| | - Paul Witkovsky
- Department of Ophthalmology and Department of Physiology & Neuroscience, New York University School of Medicine, New York, NY 10016, USA
- *Corresponding author. Tel: +1 212 263 6488; fax: +1 212 263 7602. E-mail address: (P. Witkovsky)
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Cadetti L, Tranchina D, Thoreson WB. A comparison of release kinetics and glutamate receptor properties in shaping rod-cone differences in EPSC kinetics in the salamander retina. J Physiol 2005; 569:773-88. [PMID: 16223761 PMCID: PMC1383429 DOI: 10.1113/jphysiol.2005.096545] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023] Open
Abstract
Synaptic transmission from cones is faster than transmission from rods. Using paired simultaneous recordings from photoreceptors and second-order neurones in the salamander retina, we studied the contributions of rod-cone differences in glutamate receptor properties and synaptic release rates to shaping postsynaptic responses. Depolarizing steps evoked sustained calcium currents in rods and cones that in turn produced transient excitatory postsynaptic currents (EPSCs) in horizontal and OFF bipolar cells. Cone-driven EPSCs rose and decayed faster than rod-driven EPSCs, even when comparing inputs from a rod and cone onto the same postsynaptic neurone. Thus, rod-cone differences in EPSCs reflect properties of individual rod and cone synapses. Experiments with selective AMPA and KA agonists and antagonists showed that rods and cones both contact pharmacologically similar AMPA receptors. Spontaneous miniature EPSCs (mEPSCs) exhibited unimodal distributions of amplitude and half-amplitude time width and there were no rod-cone differences in mEPSC properties. To examine how release kinetics shape the EPSC, we convolved mEPSC waveforms with empirically determined release rate functions for rods and cones. The predicted EPSC waveform closely matched the actual EPSC evoked by cones, supporting a quantal release model at the photoreceptor synapse. Convolution with the rod release function also produced a good match in rod-driven cells, although the actual EPSC was often somewhat slower than the predicted EPSC, a discrepancy partly explained by rod-rod coupling. Rod-cone differences in the rates of exocytosis are thus a major factor in producing faster cone-driven responses in second-order retinal neurones.
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Affiliation(s)
- Lucia Cadetti
- Department of Ophthalmology, University of Nebraska Medical Center, Durham Research Center, Omaha, NE 68198-5840, USA
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Thoreson WB, Bryson EJ. Chloride equilibrium potential in salamander cones. BMC Neurosci 2004; 5:53. [PMID: 15579212 PMCID: PMC539262 DOI: 10.1186/1471-2202-5-53] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2004] [Accepted: 12/05/2004] [Indexed: 11/10/2022] Open
Abstract
BACKGROUND GABAergic inhibition and effects of intracellular chloride ions on calcium channel activity have been proposed to regulate neurotransmission from photoreceptors. To assess the impact of these and other chloride-dependent mechanisms on release from cones, the chloride equilibrium potential (ECl) was determined in red-sensitive, large single cones from the tiger salamander retinal slice. RESULTS Whole cell recordings were done using gramicidin perforated patch techniques to maintain endogenous Cl- levels. Membrane potentials were corrected for liquid junction potentials. Cone resting potentials were found to average -46 mV. To measure ECl, we applied long depolarizing steps to activate the calcium-activated chloride current (ICl(Ca)) and then determined the reversal potential for the current component that was inhibited by the Cl- channel blocker, niflumic acid. With this method, ECl was found to average -46 mV. In a complementary approach, we used a Cl-sensitive dye, MEQ, to measure the Cl- flux produced by depolarization with elevated concentrations of K+. The membrane potentials produced by the various high K+ solutions were measured in separate current clamp experiments. Consistent with electrophysiological experiments, MEQ fluorescence measurements indicated that ECl was below -36 mV. CONCLUSIONS The results of this study indicate that ECl is close to the dark resting potential. This will minimize the impact of chloride-dependent presynaptic mechanisms in cone terminals involving GABAa receptors, glutamate transporters and ICl(Ca).
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Affiliation(s)
- Wallace B Thoreson
- Departments of Ophthalmology and Pharmacology, University of Nebraska Medical Center, Omaha, NE, USA
| | - Eric J Bryson
- Departments of Ophthalmology and Pharmacology, University of Nebraska Medical Center, Omaha, NE, USA
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Fan SF, Yazulla S. Inhibitory interaction of cannabinoid CB1 receptor and dopamine D2 receptor agonists on voltage-gated currents of goldfish cones. Vis Neurosci 2004; 21:69-77. [PMID: 15137583 DOI: 10.1017/s0952523804041070] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Dopamine is a light-adaptive signal that desensitizes the retina, while cannabinoids reportedly increase photosensitivity. The presynaptic membrane of goldfish retinal cones has dopamine D2 receptors and cannabinoid CB1 receptors. This work focused on whether dopamine D2 receptor agonist quinpirole and cannabinoid CB1 receptor agonist WIN 55212-2 (WIN) interacted to modulate voltage-dependent membrane currents of cones. A conventional patch-clamp method was used to record depolarization evoked whole-cell outward currents (Iout) and an inward calcium current (ICa) from the inner segment of cones in goldfish retinal slices. WIN had biphasic actions: low concentrations (<1 μM) increased the currentsviaGs, while higher concentrations (>1 μM) decreased the currentsviaGi/Go. Neither dopamine nor the D2 agonist quinpirole (1–20 μM) had a significant effect on eitherIoutorICa. Quinpirole at 50 μM had a mild suppressive (∼20%) effect onIout. However, quinpirole (<10 μM) completely blocked the enhancement of both currents seen with 0.7 μM WIN. The effect of quinpirole was blocked by sulpiride and by pertussis toxin, indicating that quinpirole was actingviaa D2 receptor-Gi/o coupled mechanism. The suppressive action of 50 μM quinpirole (∼20%) was not additive with the suppressive effect of 3 μM WIN (∼40%). D2 agonistsviaGi/o oppose the action of low concentrations of CB1 agonists actingviaGs to modulate cone membrane currents, suggesting a role in shaping the cone light response and/or sensitivity to changes in ambient light conditions. The nonadditive effect of high concentrations of WIN and quinpirole suggests that both decrease membrane currentsviathe same transduction pathway, Gi/Go protein kinase A (PKA).
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Affiliation(s)
- Shih-Fang Fan
- Department of Neurobiology and Behavior, Stony Brook University, Stony Brook, NY 11794-5230, USA
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Thoreson WB, Tranchina D, Witkovsky P. Kinetics of synaptic transfer from rods and cones to horizontal cells in the salamander retina. Neuroscience 2004; 122:785-98. [PMID: 14622921 DOI: 10.1016/j.neuroscience.2003.08.012] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
We examined synaptic transmission between rods or cones and horizontal cells, using perforated patch recording techniques in salamander retinal slices. Experimental conditions were established under which horizontal cells received nearly pure rod or pure cone input. The response-intensity relation for both photoreceptors and horizontal cells was described by a Michaelis-Menten function with an exponent close to 1. A dynamic model was developed for the transduction from photoreceptor voltage to postsynaptic current. The basic model assumes that: (i) photoreceptor light-evoked voltage controls Ca2+ entry according to a Boltzmann relation; (ii) the rate of glutamate release depends linearly on the voltage-gated Ca2+ current (ICa) in the synaptic terminal; (iii) glutamate concentration in the synaptic cleft reflects the balance of release and reuptake in which reuptake obeys first order kinetics; (iv) the binding of glutamate to its receptor and channel gating are fast compared with glutamate kinetics in the synaptic cleft. The good fit to the model confirms that these are the key features of synaptic transmission from rods and cones. The model accommodated changes in kinetics induced by the glutamate uptake blocker, dihydrokainate. The match between model and response was not improved by including an estimate of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor desensitization or by making glutamate uptake voltage dependent.
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Affiliation(s)
- W B Thoreson
- Department of Ophthalmology, 985540 University of Nebraska Medical Center, Omaha, NE 68198-5540, USA.
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Rabl K, Bryson EJ, Thoreson WB. Activation of glutamate transporters in rods inhibits presynaptic calcium currents. Vis Neurosci 2004; 20:557-66. [PMID: 14977334 DOI: 10.1017/s0952523803205095] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
We found that L-glutamate (L-Glu) inhibits L-type Ca2+currents (ICa) in rod photoreceptors. This inhibition was studied in isolated rods or rods in retinal slices from tiger salamander using perforated patch whole cell recordings and Cl−-imaging techniques. Application of L-Glu inhibitedICaby ∼20% at 0.1 mM and ∼35% at 1 mM. L-Glu also produced an inward current that reversed aroundECl. The metabotropic glutamate receptor (mGluR) agonists t-ADA (Group I), DCG-IV (Group II), and L-AP4 (Group III) had no effect onICa. However, the glutamate transport inhibitor, TBOA (0.1 mM), prevented L-Glu from inhibitingICa. D-aspartate (D-Asp), a glutamate transporter substrate, also inhibitedICawith significantly more inhibition at 1 mM than 0.1 mM. Using Cl−imaging, L-Glu (0.1–1 mM) and D-Asp (0.1–1 mM) were found to stimulate a Cl−efflux from terminals of isolated rods whereas the ionotropic glutamate receptor agonists NMDA, AMPA, and kainate and the mGluR agonist, 1S,3R-ACPD, did not. Glutamate-evoked Cl−effluxes were blocked by the glutamate transport inhibitors TBOA and DHKA. Cl−efflux inhibits Ca2+channel activity in rod terminals (Thoreson et al. (2000),Visual Neuroscience17, 197). Consistent with the possibility that glutamate-evoked Cl−efflux may play a role in the inhibition, reducing intraterminal Cl−prevented L-Glu from inhibitingICa. In summary, the results indicate that activation of glutamate transporters inhibitsICain rods possibly as a consequence of Cl−efflux. The neurotransmitter L-Glu released from rod terminals might thus provide a negative feedback signal to inhibit further L-Glu release.
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Affiliation(s)
- Katalin Rabl
- Department of Ophthalmology, University of Nebraska Medical Center, Omaha 68198-5540, USA
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Abstract
Center-surround antagonism is a hallmark feature of the receptive fields of sensory neurons. In retinas of lower vertebrates, surround antagonism derives in part from inhibition of cone photoreceptors by horizontal cells. Using whole-cell patch recording methods, we found that light-evoked responses of cones in macaque monkey were antagonized when surrounding cones were illuminated. The spatial and spectral properties of this antagonism indicate that it results from inhibition by horizontal cells. It has been suggested that horizontal cell inhibition is mediated by the neurotransmitter GABA. The inhibition observed here, however, was inconsistent with a GABA-gated chloride conductance mechanism. Instead, surround illumination evoked an increase in calcium conductance and calcium-activated chloride conductance in cones. We expect that these conductances modulate neurotransmitter release at the cone synapse and increase visual sensitivity to spatial contrast.
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Thoreson WB, Bryson EJ, Rabl K. Reciprocal interactions between calcium and chloride in rod photoreceptors. J Neurophysiol 2003; 90:1747-53. [PMID: 12724369 DOI: 10.1152/jn.00932.2002] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
This study used imaging and electrophysiological techniques in salamander retinal slices to correlate Ca2+ and Cl- levels in rods and thus test the hypothesis of a feedback interaction between Ca2+- and Ca2+-activated Cl- channels whereby Cl- efflux through Cl- channels can inhibit Ca2+ channels. Increasing [K+]o levels produced a concentration-dependent depolarization of rods accompanied by increases in [Ca2+]i measured with Fura-2. The voltage dependence of increases in [Ca2+]i was compared with the voltage dependence of the calcium current (ICa). [Cl-]i was measured with the dye, MEQ. Depolarization with high K+ to membrane potentials below -20 mV reduced [Cl-]i; larger depolarizations increased [Cl-]i. The Na/K/Cl cotransport inhibitor, bumetanide, shifted the apparent Cl- equilibrium potential (ECl) to more negative potentials, suggesting that this cotransporter helps establish a relatively depolarized ECl. MEQ fluorescence changes evoked by high K+ were inhibited by niflumic acid (0.1 mM), NPPB (2 microM), or replacement of Ca2+ with Ba2+, suggesting that depolarization-evoked Cl- changes result partly from stimulation of Ca2+-activated Cl- channels. Replacing >/=12 mM [Cl-]o with CH3SO4- produced a significant reduction in [Cl-]i. [Ca2+]i increases evoked by 20 or 50 mM K+ were also significantly inhibited by replacing >/=12 mM [Cl-]o with CH3SO4-. Thus modest depolarization can evoke increases in [Ca2+]i that lead to reductions in [Cl-]i, and conversely, reductions in [Cl-]i inhibit depolarization-evoked [Ca2+]i increases. These findings support the hypothesis that feedback interactions between Ca2+- and Ca2+-activated Cl- channels may contribute to the regulation of presynaptic Ca2+ currents involved in synaptic transmission from rod photoreceptors.
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Affiliation(s)
- Wallace B Thoreson
- Department of Ophthalmology, University of Nebraska Medical Center, Omaha, Nebraska 68198, USA.
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Barnes S, Kelly MEM. Calcium channels at the photoreceptor synapse. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2003; 514:465-76. [PMID: 12596939 DOI: 10.1007/978-1-4615-0121-3_28] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
Presynaptic Ca2+ channels mediate early stages of visual information processing in photoreceptors by facilitating the release of neurotransmitter and by receiving modulatory input that alters transmission. Two types of L-type Ca2+ channels, composed of alpha1F and alpha1D subunits and having similar biophysical andpharmacological properties, appear to form the principle voltage-dependent Ca2+ influx pathways in rods and cones, respectively. The role played by these channels in neurotransmitter release at these graded potential, non-spiking synapses, has been well described. The channels mediate sustained glutamate release in darkness where the cells rest at potentials near -40 mV, and signal increases in light intensity as the cells hyperpolarize negative to this value. Synaptic modulation and integration mediated by these channels has not yet been as fully described but appears to involve GABA, nitric oxide (NO), glutamate, and dopamine. Ca2+ permeable cyclic nucleotide gated (CNG) channels appear to have supporting roles at the photoreceptor output synapse and may transduce NO signals from other cells by either directly permitting Ca2+ influx or by providing depolarizing influences that gate voltage dependent Ca2+ channels.
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Affiliation(s)
- Steven Barnes
- Department of Physiology & Biophysics, Dalhousie University, Halifax, Nova Scotia, B3H 4H7.
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Stella SL, Bryson EJ, Cadetti L, Thoreson WB. Endogenous adenosine reduces glutamatergic output from rods through activation of A2-like adenosine receptors. J Neurophysiol 2003; 90:165-74. [PMID: 12843308 DOI: 10.1152/jn.00671.2002] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Adenosine is released from retina in darkness; photoreceptors possess A2 adenosine receptors, and A2 agonists inhibit L-type Ca2+ currents (ICa) in rods. We therefore investigated whether A2 agonists inhibit rod inputs into second-order neurons and whether selective antagonists to A1, A2A, or A3 receptors prevent Ca2+ influx through rod ICa. [Ca2+]i changes in rods were assessed with fura-2. ICa in rods and light responses of rods and second-order neurons were recorded using perforated patch-clamp techniques in the aquatic tiger salamander retinal slice preparation. Consistent with earlier results using the A2 agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA), the A2A agonist CGS-21680 significantly inhibited ICa and depolarization-evoked [Ca2+]i increases in rods. The A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and A2A antagonist, ZM-241385, but not the A3 antagonist, VUF-5574, inhibited effects of adenosine on Ca2+ influx in rods. DPCPX and ZM-241385 also inhibited effects of CGS-21680, suggesting they both act at A2A receptors. Both A2 agonists, CGS-21680 and DPMA, reduced light-evoked currents in second-order neurons but not light-evoked voltage responses of rods, suggesting that activation of A2 receptors inhibits transmitter release from rods. The inhibitory effects of CGS-21680 on both depolarization-evoked Ca2+ influx and light-evoked currents in second-order neurons were antagonized by ZM-241385. By itself, ZM-241385 enhanced the light-evoked currents in second-order neurons, suggesting that endogenous levels of adenosine inhibit transmitter release from rods. The effects of these drugs suggest that endogenous adenosine activates an A2-like adenosine receptor on rods leading to inhibition of ICa, which in turn inhibits l-glutamate release from rod photoreceptors.
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Affiliation(s)
- Salvatore L Stella
- Department of Pharmacology and Department of Ophthalmology, University of Nebraska Medical Center, Omaha, Nebraska 68198-5540, USA.
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Abstract
The ability to see at night relies on the transduction of single photons by the rod photoreceptors and transmission of the resulting signals through the retina. Using paired patch-clamp recordings, we investigated the properties of the first stage of neural processing of the rod light responses: signal transfer from rods to bipolar and horizontal cells. Bypassing the relatively slow phototransduction process and directly modulating the rod voltage or current allowed us to characterize signal transfer over a wide range of temporal frequencies. We found that the rod to second-order cell synapse acts as a bandpass filter, preferentially transmitting signals with frequencies between 1.5 and 4 Hz while attenuating higher and lower frequency inputs. The similarity of the responses in different types of postsynaptic cell and the properties of miniature EPSCs (mEPSCs) recorded in OFF bipolar cells suggest that most of the bandpass filtering is mediated presynaptically. Modeling of the network of electrically coupled rod photoreceptors suggests that spread of the signal through the network contributed to the observed high-pass filtering but not to the low-pass filtering. Attenuation of low temporal frequencies at the first retinal synapse sharpens the temporal resolution of the light response; attenuation of high temporal frequencies removes voltage noise in the rod that threatens to swamp the light response.
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48
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Rabl K, Thoreson WB. Calcium-dependent inactivation and depletion of synaptic cleft calcium ions combine to regulate rod calcium currents under physiological conditions. Eur J Neurosci 2002; 16:2070-7. [PMID: 12473074 DOI: 10.1046/j.1460-9568.2002.02277.x] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
L-type Ca2+ currents (I(Ca)) in rod photoreceptors exhibit Ca2+-dependent inactivation. Perforated-patch whole-cell recordings were obtained from isolated rods of the tiger salamander using 1.8 mm Ca2+ in the bathing medium to determine the extent of Ca2+-dependent inactivation of I(Ca) with physiological [Ca2+] and endogenous buffering. I(Ca) was measured with voltage ramps applied before and after 5-s steps to -40, -30, -20, or -10 mV. Long depolarizing steps in isolated rods produced inactivation of I(Ca) ranging from 15% at -40 mV to > 80% at -10 mV. Because, in addition to Ca2+-dependent inactivation, depletion of synaptic cleft Ca2+ accompanying activation of I(Ca) can reduce presynaptic I(Ca) at calycal synapses, we investigated whether a similar mechanism worked at the invaginating rod synapse. Rods from retinal slices with intact synapses were compared with isolated rods in which synaptic cleft depletion is absent. I(Ca) was more strongly depressed by depolarization of rods in retinal slices, with ICa reduced by 47% following voltage steps to -40 mV. The depression of currents by depolarization was also greater for rods from retinal slices than isolated rods when Ca2+ was replaced with Ba2+ to reduce Ca2+-dependent inactivation. The stronger depolarization-evoked inhibition of I(Ca) in retinal slices compared to isolated rods probably reflects depletion of synaptic cleft Ca2+ arising from sustained Ca2+ influx. Inactivation of I(Ca) exhibited slow onset and recovery. These findings suggest that Ca2+-dependent inactivation and depletion of synaptic cleft Ca2+ may combine to regulate I(Ca) in response to light-evoked changes in rod membrane potential.
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Affiliation(s)
- Katalin Rabl
- Department of Ophthalmology, University of Nebraska Medical Center, 985540 Nebraska Medical Center, Omaha, NE 68198-5540, USA
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49
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Stella SL, Bryson EJ, Thoreson WB. A2 adenosine receptors inhibit calcium influx through L-type calcium channels in rod photoreceptors of the salamander retina. J Neurophysiol 2002; 87:351-60. [PMID: 11784755 DOI: 10.1152/jn.00010.2001] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Presynaptic inhibition is a major mechanism for regulating synaptic transmission in the CNS and adenosine inhibits Ca(2+) currents (I(Ca)) to reduce transmitter release at several synapses. Rod photoreceptors possess L-type Ca(2+) channels that regulate the release of L-glutamate. In the retina, adenosine is released in the dark when L-glutamate release is maximal. We tested whether adenosine inhibits I(Ca) and intracellular Ca(2+) increases in rod photoreceptors in retinal slice and isolated cell preparations. Adenosine inhibited both I(Ca) and the [Ca(2+)]i increase evoked by depolarization in a dose-dependent manner with approximately 25% inhibition at 50 microM. An A2-selective agonist, (N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine) (DPMA), but not the A1- or A3-selective agonists, (R)-N(6)-(1-methyl-2-phenylethyl)adenosine and N(6)-2-(4-aminophenyl)ethyladenosine, also inhibited I(Ca) and depolarization-induced [Ca(2+)]i increases. An inhibitor of protein kinase A (PKA), Rp-cAMPS, blocked the effects of DPMA on both I(Ca) and the depolarization-evoked [Ca(2+)]i increase in rods. The results suggest that activation of A2 receptors stimulates PKA to inhibit L-type Ca(2+) channels in rods resulting in a decreased Ca(2+) influx that should suppress glutamate release.
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Affiliation(s)
- Salvatore L Stella
- Department of Pharmacology and Department of Ophthalmology, University of Nebraska Medical Center, Omaha, Nebraska 68198-5540, USA
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50
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Abstract
A proton pump acidifies synaptic vesicles and provides the electrochemical gradient for transmitter uptake. Although external protons can modulate membrane voltage- and ligand-gated conductances, the fate of the protons released when vesicles fuse with the plasma membrane is unclear. In the dark, the glutamate-laden vesicles of cone photoreceptors fuse continuously with the plasma membrane. I now show that vesicular protons feed back to block the nearby calcium channels that mediate release. This local proton-mediated feedback is a novel mechanism through which neurons may regulate the release of transmitter.
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Affiliation(s)
- S H DeVries
- Department of Ophthalmology and Visual Science, Houston Health Science Center, University of Texas, Houston, TX 77030, USA.
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