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Anastassov I, Shen W, Ripps H, Chappell RL. Zinc modulation of calcium activity at the photoreceptor terminal: a calcium imaging study. Exp Eye Res 2013; 112:37-44. [PMID: 23619034 DOI: 10.1016/j.exer.2013.04.011] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2013] [Revised: 04/12/2013] [Accepted: 04/13/2013] [Indexed: 01/02/2023]
Abstract
There is abundant experimental evidence that zinc ions (Zn(2+)) are present in the synaptic vesicles of vertebrate photoreceptors, and that they are co-released with glutamate. Here we show that increasing the concentration of extracellular zinc (2 μM-2 mM) suppresses the entry of calcium into the synaptic terminals of isolated salamander double cones. The resultant dose-dependent curve was fit by an inverse Hill equation having an IC50 of 38 μM, and Hill coefficient of 1.1. Because there is currently no reliable way to measure the concentration of extracellular zinc, it is not known whether the zinc released under normal circumstances is of physiological significance. In an attempt to circumvent this problem we used zinc chelators to reduce the available pool of endogenous zinc. This enabled us to determine how the absence of zinc affected calcium entry. We found that when intra- or extra-cellular zinc was chelated by 250 μM of membrane-permeable TPEN or 500 μM of membrane-impermeable histidine, there was a significant rise in the depolarization-induced intracellular calcium level within photoreceptor terminals. This increase in internal [Ca(2+)] will undoubtedly lead to a concomitant increase in glutamate release. In addition, we found that blocking the L-type calcium channels that are expressed on the synaptic terminals of photoreceptors with 50 μM nicardipine or 100 μM verapamil abolished the effects of zinc chelation. These findings are a good indication that, when released in vivo, the zinc concentration is sufficient to suppress voltage-gated calcium channels, and reduce the rate of glutamate release from photoreceptor terminals.
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Affiliation(s)
- Ivan Anastassov
- Department of Biological Sciences, Hunter College and the Graduate Center, CUNY, 695 Park Avenue, New York, NY 10065, USA.
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Thoreson WB, Mangel SC. Lateral interactions in the outer retina. Prog Retin Eye Res 2012; 31:407-41. [PMID: 22580106 PMCID: PMC3401171 DOI: 10.1016/j.preteyeres.2012.04.003] [Citation(s) in RCA: 136] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2012] [Revised: 03/05/2012] [Accepted: 03/09/2012] [Indexed: 10/28/2022]
Abstract
Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.
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Affiliation(s)
- Wallace B. Thoreson
- Departments of Ophthalmology & Visual Sciences and Pharmacology & Experimental Neuroscience, University of Nebraska Medical Center, Omaha, NE 68198 USA
| | - Stuart C. Mangel
- Department of Neuroscience, The Ohio State University College of Medicine, Columbus, OH 43210 USA
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Cowley KC, Zaporozhets E, Schmidt BJ. Propriospinal transmission of the locomotor command signal in the neonatal rat. Ann N Y Acad Sci 2010; 1198:42-53. [PMID: 20536919 DOI: 10.1111/j.1749-6632.2009.05421.x] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
Long direct bulbospinal projections are known to convey descending activation of locomotor networks. Less is understood about the role, if any, of propriospinal mechanisms in this function. Here we review our recent studies on propriospinal neurons in the in vitro neonatal rat brainstem-spinal cord preparation. Neurochemical suppression of synaptic activity in the cervicothoracic spinal cord blocked locomotor-like activity, suggesting synaptic relays make a critical contribution to descending transmission of the locomotor signal. Staggered contralateral hemisections in the cervicothoracic region, intended to eliminate all long direct bulbospinal transmission, failed to suppress locomotion, suggesting the propriospinal system alone is sufficient. Midsagittal lesion experiments showed that locomotor-related commissural components are required for rhythm generation in response to electrical stimulation of the brainstem and are redundantly distributed. No single segment was essential, although a bi-directional gradient was noted, centered on the thoracolumbar junction. These results strongly favor a role for propriospinal mechanisms in the activation of locomotion and suggest that propriospinal neurons are a logical target for interventions to restore locomotor function after spinal cord injury.
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Affiliation(s)
- Kristine C Cowley
- Department of Physiology, Faculty of Medicine, University of Manitoba, Winnipeg, Canada
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Zaporozhets E, Cowley KC, Schmidt BJ. Propriospinal neurons contribute to bulbospinal transmission of the locomotor command signal in the neonatal rat spinal cord. J Physiol 2006; 572:443-58. [PMID: 16469789 PMCID: PMC1779678 DOI: 10.1113/jphysiol.2005.102376] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2005] [Accepted: 02/02/2006] [Indexed: 11/08/2022] Open
Abstract
This study examines whether propriospinal transmission contributes to descending propagation of the brainstem locomotor command signal in the in vitro neonatal rat spinal cord. Using double bath partitions, synaptic transmission was suppressed in the cervicothoracic region while monitoring locomotor-like activity on lumbar ventral roots evoked by either chemical or electrical stimulation of the brainstem. Locomotor-like activity induced by electrical stimulation was more stable (cycle period coefficient of variation (CV) 11.7 +/- 6.1%) than the rhythm induced by chemical stimulation (CV 31.3 +/- 6.4%). Ca(2+)-free bath solution, elevated Mg(2+) ion concentration, excitatory amino acid receptor antagonists (AP5 and/or CNQX), and the muscarinic receptor antagonist, atropine, were used in attempts to block synaptic transmission. Each of these manipulations, except muscarinic receptor blockade, was capable of blocking locomotor-like activity induced by brainstem stimulation. However, locomotor-like activity induced by higher intensity electrical stimulation of the brainstem (1.2-5 times threshold) was relatively refractory to synaptic suppression using AP5 and CNQX, and Ca(2+)-free solution was more effective if combined with high Mg(2+) (15 mm) or EGTA. Enhancement of neuronal excitation in the cervicothoracic region, using Mg(2+)-free bath solution, facilitated brainstem activation of locomotor-like activity in the lumbar cord, consistent with a propriospinal mechanism of locomotor signal propagation. Blockade of brainstem-induced locomotor-like activity was related to the number of cervicothoracic segments exposed to synaptic suppression, being most effective if five or more segments were included. These results provide direct evidence that propriospinal pathways contribute to bulbospinal activation of the locomotor network in the in vitro neonatal rat brainstem-spinal cord preparation, and suggest that a propriospinal system is recruited in parallel with long direct projections that activate the locomotor network.
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CADETTI L, THORESON WB, PICCOLINO M. Pre- and post-synaptic effects of manipulating surface charge with divalent cations at the photoreceptor synapse. Neuroscience 2005; 129:791-801. [PMID: 15541900 PMCID: PMC1383428 DOI: 10.1016/j.neuroscience.2004.08.043] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/26/2004] [Indexed: 11/25/2022]
Abstract
Persistence of horizontal cell (HC) light responses in extracellular solutions containing low Ca2+ plus divalent cations to block Ca2+ currents (ICa) has been attributed to Ca2+-independent neurotransmission. Using a retinal slice preparation to record both ICa and light responses, we demonstrate that persistence of HC responses in low [Ca2+]o can instead be explained by a paradoxical increase of Ca2+ influx into photoreceptor terminals arising from surface charge-mediated shifts in ICa activation. Consistent with this explanation, application of Zn2+ or Ni2+ caused a hyperpolarizing block of HC light responses that was relieved by lowering [Ca2+]o. The same concentrations of Zn2+ and Ni2+ reduced the amplitude of ICa at the rod dark potential and this reduction was relieved by a hyperpolarizing shift in voltage dependence induced by lowering [Ca2+]o. Block of ICa by Mg2+, which has weak surface charge effects, was not relieved by low [Ca2+]o. Recovery of HC responses in low [Ca2+]o was assisted by enhancement of rod light responses. To bypass light stimulation, OFF bipolar cells were stimulated by steps to -40 mV applied to presynaptic rods during simultaneous paired recordings. Consistent with surface charge theory, the post-synaptic current was inhibited by Zn2+ and this inhibition was relieved by lowering [Ca2+]o. Nominally divalent-free media produced inversion of HC light responses even though rod light responses remained hyperpolarizing; HC response inversion can be explained by surface charge-mediated shifts in ICa. In summary, HC light responses modifications induced by low divalent cation solutions can be explained by effects on photoreceptor light responses and membrane surface charge without necessitating Ca2+-independent neurotransmission. Furthermore, these results suggest that surface charge effects accompanying physiological changing divalent cation levels in the synaptic cleft may provide a means for modulating synaptic output from photoreceptors.
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Affiliation(s)
- L. CADETTI
- Departments of Ophthalmology, University of Nebraska Medical Center, 985540 Nebraska Medical Center, Omaha, NE 68198, USA
- Department of Biology, University of Ferrara, 44100 Ferrara, Italy
| | - W. B. THORESON
- Departments of Ophthalmology, University of Nebraska Medical Center, 985540 Nebraska Medical Center, Omaha, NE 68198, USA
- Department of Pharmacology, University of Nebraska Medical Center, Omaha, NE 68198, USA
- *Correspondence to: W. B. Thoreson, Ophthalmology Department, University of Nebraska Medical Center, 985540 Nebraska Medical Center, Omaha, NE 68198–5540, USA. Tel: +1-402-559-2019; fax: +1-402-559-5514. E-mail address: (W. B. Thoreson)
| | - M. PICCOLINO
- Department of Biology, University of Ferrara, 44100 Ferrara, Italy
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Fahrenfort I, Sjoerdsma T, Ripps H, Kamermans M. Cobalt ions inhibit negative feedback in the outer retina by blocking hemichannels on horizontal cells. Vis Neurosci 2005; 21:501-11. [PMID: 15579217 DOI: 10.1017/s095252380421402x] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2003] [Indexed: 11/06/2022]
Abstract
In goldfish, negative feedback from horizontal cells to cones shifts the activation function of the Ca2+ current of the cones to more negative potentials. This shift increases the amount of Ca2+ flowing into the cones, resulting in an increase in glutamate release. The increased glutamate release forms the basis of the feedback-mediated responses in second-order neurons, such as the surround-induced responses of bipolar cells and the spectral coding of horizontal cells. Low concentrations of Co2+ block these feedback-mediated responses in turtle retina. The mechanism by which this is accomplished is unknown. We studied the effects of Co2+ on the cone/horizontal network of goldfish retina and found that Co2+ greatly reduced the feedback-mediated responses in both cones and horizontal cells in a GABA-independent way. The reduction of the feedback-mediated responses is accompanied by a small shift of the Ca2+ current of the cones to positive potentials. We have previously shown that hemichannels on the tips of the horizontal cell dendrites are involved in the modulation of the Ca2+ current in cones. Both the absence of this Co2+-induced shift of the Ca2+ current in the absence of a hemichannel conductance and the sensitivity of Cx26 hemichannels to low concentrations of Co2+ are consistent with a role for hemichannels in negative feedback from horizontal cells to cones.
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Affiliation(s)
- I Fahrenfort
- Research Unit Retinal Signal Processing, The Netherlands Ophthalmic Research Institute, Amsterdam, The Netherlands
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Fahrenfort I, Klooster J, Sjoerdsma T, Kamermans M. The involvement of glutamate-gated channels in negative feedback from horizontal cells to cones. PROGRESS IN BRAIN RESEARCH 2005; 147:219-29. [PMID: 15581709 DOI: 10.1016/s0079-6123(04)47017-4] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Photoreceptors are the light sensitive cells in the retina. They project to horizontal cells and bipolar cells via a glutamatergic feed forward pathway. Horizontal cells are strongly electrically coupled and integrate in that way the input from the photoreceptors. Horizontal cells feedback to cones negatively. The combined signal from the photoreceptors and the horizontal cells is sent to the bipolar cells. The feedback pathway from horizontal cells to cones is thought to form the basis for the center/surround organization of bipolar cells. The nature of the feedback pathway is an issue of intense debate. It was thought for a long time that this feedback pathway was GABAergic, because cones have GABA-receptors and horizontal cells release GABA via a GABA-transporter working in the reversed direction. However, recently we showed in goldfish that horizontal cells feed back to cones via an alternative mechanism. In goldfish, negative feedback from horizontal cells to cones shifts the calcium current of the cone to more negative potentials. This feedback pathway is independent of GABA, since feedback cannot be blocked by either saturating concentrations of PTX, the GABA-transporter blocker SKF89976A, or application of GABA. The mechanism of negative feedback from horizontal cells to cones involves hemichannels located at the tips of the invaginating horizontal cells, just opposite to the calcium channels of the cones. Current flowing through these hemichannels changes the extracellular potential deep in the synaptic cleft and in that way modulates the calcium current of the cones. Such a modulation of the extracellular potential is called ephaptic. If negative feedback from horizontal cells to cones is indeed ephaptic, other channels present in the synapse should also be able to act as a current source, i.e., should also be able to change the output of the cone. We showed that glutamate-gated channels present at the tips of the horizontal cell dendrites can also mediate feedback responses. Surprisingly, although the glutamate-gated conductance of the horizontal cells is eight times the hemichannel conductance, glutamate-gated channels are not the major current source in negative feedback from horizontal cells to cones. In this chapter we present evidence that this is due to the more focal localization of the hemichannels, compared to a diffuse and extrasynaptic localization of the glutamate-gated channels.
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Affiliation(s)
- I Fahrenfort
- Research Unit Retinal Signal Processing, The Netherlands Ophthalmic Research Institute, Amsterdam, The Netherlands
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Burgo A, Carmignoto G, Pizzo P, Pozzan T, Fasolato C. Paradoxical Ca2+ rises induced by low external Ca2+ in rat hippocampal neurones. J Physiol 2003; 549:537-52. [PMID: 12692181 PMCID: PMC2342954 DOI: 10.1113/jphysiol.2003.041871] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Confocal Ca2+ imaging of rat hippocampal slices shows a paradoxical effect of acute reductions of the [Ca2+]o. Upon slice perfusion with low-Ca2+ media, a prompt intracellular Ca2+ rise selectively occurs in neurones. This response is observed only in slices challenged with agonists of group I metabotropic glutamate or M1 muscarinic receptors. In contrast, the intracellular Ca2+ level of non-stimulated neurones is insensitive to reductions of [Ca2+]o. The phenomenon is observed in 20-25 % of cultured cortical neurones. Evidence is provided demonstrating that: (1) this paradoxical response is not due to a non-specific decrease in divalent cation concentration but it is selectively activated by a reduction in [Ca2+]o, being maximal with [Ca2+]o between 0.25 and 0.5 mM; (2) upon maximal stimulation, 70-90 % of CA1-CA3 pyramidal neurones sense a reduction in [Ca2+]o; a weaker response is observed in neurones from the neocortex, whereas neurones from the dentate gyrus and granule cells from the cerebellum fail to respond; (3) conditions that elicit paradoxical Ca2+ responses cause depolarisation and increase the firing rate of hippocampal neurones; (4) paradoxical Ca2+ rises depend, primarily, on Ca2+ influx through L-type voltage-operated Ca2+ channels and to a lesser extent on release from intracellular Ca2+ stores. Inhibition of phospholipase C or protein kinase C failed to suppress the neuronal response, whereas a selective inhibitor of the Src-family of tyrosine kinases abolishes the paradoxical neuronal Ca2+ rise. A model is presented to explain how this response is elicited by contemporaneous reduction of the [Ca2+]o and metabotropic receptor stimulation; implications for the pathophysiology of the CNS are also discussed.
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Affiliation(s)
- Andrea Burgo
- Department of Biomedical Sciences, University of Padua, Via G. Colombo 3, 35121 Padua, Italy
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Abstract
Most synapses rely on regulated exocytosis for determining the concentration of transmitter in the synaptic cleft. However, this mechanism may not be universal. Several synapses in the retina appear to use a synaptic machinery in which transmitter transporters play an essential role. Two types of transport-mediated synapses have been proposed. These synapses have been best observed in horizontal cells and cones of nonmammalian retinas. Horizontal cells use a transporter to mediate a bidirectional shuttle, whose balance point is set by ion concentrations and voltage. Nonmammalian cones combine exocytosis and the activity of a transporter. Because exocytosis is voltage independent over most of a cone's physiological voltage range, a voltage-dependent transporter determines the concentration of transmitter in the synaptic cleft. These two synapses may be models for transport-mediated synapses that operate in other parts of the brain.
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Affiliation(s)
- E A Schwartz
- Department of Neurobiology, Pharmacology, and Physiology, University of Chicago, Chicago, Illinois 60637, USA.
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Dmitriev A, Pignatelli A, Piccolino M. Resistance of retinal extracellular space to Ca2+ level decrease: implications for the synaptic effects of divalent cations. J Neurophysiol 1999; 82:283-9. [PMID: 10400957 DOI: 10.1152/jn.1999.82.1.283] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Ion-sensitive microelectrodes were used to measure the variations of [Ca2+]o induced by application of low Ca2+ media in the superfused eyecup preparation of the Pseudemys turtle. The aim of the experiments was to evaluate the possibility, suggested by previous studies, that in the deep, sclerad, layers of the retina [Ca2+]o may remain high enough to sustain chemical synaptic transmission even after prolonged application of low-Ca2+ saline. It was found that, at depths of 100-200 micron from the vitreal surface, [Ca2+ ]o did not fall below 1 mM even after application for periods of 30-60 min of nominally Ca2+-free media, and it was >0.3 mM after 30-min application of media containing EGTA and with a Ca2+ concentration of 1 nM. Previous studies in isolated salamander photoreceptors have shown that a reduction of [Ca2+ ]o to 0.3-1.0 mM may result in a paradoxical increase of Ca2+ influx into synaptic terminals due to the reduced screening of negative charge on the external face of the plasma membrane. On the basis of these results, the persistence or enhancement of synaptic transmission from photoreceptors to horizontal cells observed in various retinas treated with low-Ca2+ media may be accounted for within the classical Ca2+-dependent theory of synaptic transmission without invoking a Ca2+-independent mechanism.
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Affiliation(s)
- A Dmitriev
- Dipartimento di Biologia, Sezione di Fisiologia Generale, Università di Ferrara, 44100 Ferrara, Italy
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Kamermans M, Spekreijse H. The feedback pathway from horizontal cells to cones. A mini review with a look ahead. Vision Res 1999; 39:2449-68. [PMID: 10396615 DOI: 10.1016/s0042-6989(99)00043-7] [Citation(s) in RCA: 93] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The feedback pathway from HCs to cones forms the basis of the surround responses of the bipolar cells and is essential for the spectral opponency of horizontal cells. The nature of this feedback pathway is an issue of debate. Three hypothesis are presented in literature: (1) a GABAA-ergic feedback pathway; (2) a GABA-independent feedback pathway that modulates the Ca-current in cones; and (3) an electrical feedback pathway. In this review the evidence for the various pathways will be discussed. The conclusion is that the available evidence favors the hypothesis that feedback modulates the Ca-current in the cones in a GABA independent way. An alternative role of GABA in the outer plexiform layer is discussed and finally the functional consequences of the negative feedback pathway from horizontal cells to cones are presented.
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Affiliation(s)
- M Kamermans
- Graduate School of Neurosciences Amsterdam, The Netherlands Ophthalmic Research Institute, Amsterdam, The Netherlands.
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Schmidt KF. Divalent cations modulate glutamate receptors in retinal horizontal cells of the perch (Perca fluviatilis). Neurosci Lett 1999; 262:109-12. [PMID: 10203243 DOI: 10.1016/s0304-3940(99)00072-5] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Divalent cations had two effects on concentration-response relations of glutamate induced membrane currents recorded from retinal horizontal cells. The first effect was a reduction of maximum currents. Barium, magnesium, cobalt, nickel and an increased calcium concentration caused reductions of maximum currents between 14% and 70%. The second effect of divalent cations was related to the dopamine dependent modulation of glutamate receptors in horizontal cells. The dopamine dependent enhancement of glutamate gated currents requires the presence of divalent cations besides calcium in the extracellular solution. Without such divalent cations application of dopamine caused no increase of the maximum currents induced by glutamate, and only a slight shift of the half maximal saturation concentration was observed. Addition of magnesium or barium cations in millimolar concentration was sufficient to completely restore the dopamine dependent modulation.
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Affiliation(s)
- K F Schmidt
- Physiologisches Institut, Justus-Liebig-Universität, Giessen, Germany.
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