Intracellular Calcium Is Regulated by Different Pathways in Horizontal Cells of the Mouse Retina

Hypoxia increases intracellular calcium in glutamate-activated horizontal cells of goldfish retina via mitochondrial KATP channels and intracellular stores

Central neurons of the common goldfish (Carassius auratus) are exceptional in their capacity to survive Ca2+-induced excitotoxicity and cell death during hypoxia. Horizontal cells (HCs) are inhibitory interneurons of the retina that are tonically depolarized by the neurotransmitter, glutamate, yet preserve intracellular Ca2+ homeostasis. In HCs isolated from goldfish, and in the absence of glutamatergic input, intracellular Ca2+ concentration ([Ca2+]i) is protected from prolonged exposure to hypoxia by mitochondrial ATP-dependent K+ (mKATP) channel activity. In the present study, we investigated the effects of hypoxia upon [Ca2+]i in isolated HCs during tonic activation by glutamate to better predict the effects of hypoxia in the active retina. Dynamic changes in [Ca2+]i were measured using the ratiometric Ca2+ indicator, Fura-2. Application of 100 μM glutamate during hypoxia (PO2 = 25 mmHg) produced a 1.3-fold greater rise in [Ca2+]i compared to the same glutamate stimulus during normoxia. The hypoxia-dependent increase in [Ca2+]i was abolished by application of 5-hydroxydecanoic acid, which renders mKATP channels inactive. Extracellular Ca2+ did not contribute to the elevated [Ca2+]i observed during hypoxia, as the effect persisted in Ca2+-free solution and during application of verapamil, an L-type Ca2+ channel blocker. By contrast, inhibition of the mitochondrial Ca2+ uniporter or ryanodine receptors (with ruthenium red or ryanodine, respectively) abolished the hypoxia-dependent rise in [Ca2+]i. This study reports an mKATP-dependent rise in [Ca2+]i during hypoxia in HCs activated by glutamate, and suggests roles for the mitochondria and intracellular Ca2+ stores in regulating this mechanism.

Robust expression of the TRPC1 channel associated with photoreceptor loss in the rat retina

Baseline intracellular calcium levels are significantly higher in neuronal and glial cells of rat retinas with retinitis pigmentosa (RP). Although this situation could initiate multiple detrimental pathways that lead to cell death, we considered the possibility of TRPC1 being involved in maintaining calcium homeostasis in the retina by acting as a component of store-operated calcium (SOC) channels with special relevance during photoreceptor degeneration. In this study, we examined by Western blot the expression of TRPC1 in healthy control rat retinas (Sprague-Dawley, SD) and retinas with RP (P23H-1 rats). We also analyzed its specific cellular distribution by immunofluorescence to recognize changes during neurodegeneration and to determine whether its presence is consistent with high basal calcium levels and cellular survival in degenerating retinas. We found that TRPC1 immunostaining was widely distributed across the retina in both rat strains, SD and P23H, and its expression levels significantly increased in the retinas with advanced degeneration compared to the age-control SD rats. In the outer retina, TRPC1 immunoreactivity was distributed in pigment epithelium cells, the photoreceptor inner segments of older animals, and the outer plexiform layer. In the inner retina, TRPC1 labeling was detected in horizontal cells, specific somata of bipolar and amacrine cells, and cellular processes in all the strata of the inner plexiform layer. Somata and processes were also highly immunoreactive in the ganglion cell layer and astrocytes in the nerve fiber layer in all animals. In the P23H rat retinas, the TRPC1 distribution pattern changed according to advancing photoreceptor degeneration and the gliosis reaction, with TRPC1 immunoreactive Müller cells mainly in advanced stages of disease. The cellular TRPC1 immunoreactivity found in this work suggests different mechanisms of activation of these channels depending on the cell type. Furthermore, the results support the idea that photoreceptor loss due to RP is associated with robust TRPC1 protein expression in the rat inner retina and raise the possibility of TRPC1 channels contributing to maintain high basal calcium levels during neurodegeneration and/or maintenance processes of the inner retina.

Calcium Channels in Retinal Function and Disease

Voltage-gated Ca2+ (Cav) channels play pivotal roles in regulating gene transcription, neuronal excitability, and neurotransmitter release. To meet the spatial and temporal demands of visual signaling, Cav channels exhibit unusual properties in the retina compared to their counterparts in other areas of the nervous system. In this article, we review current concepts regarding the specific subtypes of Cav channels expressed in the retina, their intrinsic properties and forms of modulation, and how their dysregulation could lead to retinal disease.

Form-deprivation myopia downregulates calcium levels in retinal horizontal cells in mice

The process of eye axis lengthening in myopic eyes is regulated by multiple mechanisms in the retina, and horizontal cells (HCs) are an essential interneuron in the visual regulatory system. Wherein intracellular Ca²⁺ plays an important role in the events involved in the regulatory role of HCs in the retinal neural network. It is unknown if intracellular Ca²⁺ regulation in HCs mediates changes in the retinal neural network during myopia progression. We describe here a novel calcium fluorescence indicator system that monitors HCs' intracellular Ca²⁺ levels during form-deprivation myopia (FDM) in mice. AAV injection of GCaMP6s, as a protein calcium sensor, into a Gja10-Cre mouse monitored the changes in Ca²⁺signaling in HC that accompany FDM progression in mice. An alternative Gja10-Cre/Ai96-GCaMP6s mouse model was created by cross mating Gja10-Cre with Ai96 mice. Immunofluorescence imaging and live imaging of the retinal cells verified the identity of these animal models. Changes in retinal horizontal cellular Ca²⁺ levels were resolved during FDM development. The numbers of GCaMP6s and the proportion of HCs were tracked based on profiling changes in GCaMP6s⁺calbindin⁺/calbindin⁺ coimmunostaining patterns. They significantly decreased more after either two days (P < 0.01) or two weeks (P < 0.001) in form deprived eyes than in the untreated fellow eyes. These decreases in their proportion reached significance only in the retinal central region rather than also in the retinal periphery. A novel approach employing a GCaMP6s mouse model was developed that may ultimately clarify if HCs mediate Ca²⁺ signals that contribute to controlling FDM progression in mice. The results indicate so far that FDM progression is associated with declines in HC Ca²⁺ signaling activity.

A multiscale continuum model of the vertebrate outer retina: The temporal dynamics of background-induced flicker enhancement

The retina is a part of the central nervous system that is accessible, well documented, and studied by researchers spanning the clinical, experimental, and theoretical sciences. Here, we mathematically model the subcircuits of the outer plexiform layer of the retina on two spatial scales: that of an individual synapse and that of the scale of the receptive field (hundreds to thousands of synapses). To this end we formulate a continuum spine model (a partial differential equation system) that incorporates the horizontal cell syncytium and its numerous processes (spines) within cone pedicles. With this multiscale modeling approach, detailed biophysical mechanisms at the synaptic level are retained while scaling up to the receptive field level. As an example of its utility, the model is applied to study background-induced flicker enhancement in which the onset of a dim background enhances the center flicker response of horizontal cells. Simulation results, in comparison with flicker enhancement data for square, slit, and disk test regions, suggest that feedback mechanisms that are voltage-axis modulators of cone calcium channels (for example, ephaptic and/or pH feedback) are robust in capturing the temporal dynamics of background-induced flicker enhancement. The value and potential of this continuum spine approach is that it provides a framework for mathematically modeling the input-output properties of the entire receptive field of the outer retina while implementing the latest models for transmission mechanisms at the synaptic level.

Synaptic Scaffolds, Ion Channels and Polyamines in Mouse Photoreceptor Synapses: Anatomy of a Signaling Complex

Synaptic signaling complexes are held together by scaffold proteins, each of which is selectively capable of interacting with a number of other proteins. In previous studies of rabbit retina, we found Synapse-Associated Protein-102 (SAP102) and Channel Associated Protein of Synapse-110 (Chapsyn110) selectively localized in the tips of horizontal cell processes at contacts with rod and cone photoreceptors, along with several interacting ion channels. We have examined the equivalent suites of proteins in mouse retina and found similarities and differences. In the mouse retina we identified Chapsyn110 as the scaffold selectively localized in the tips of horizontal cells contacting photoreceptors, with Sap102 more diffusely present. As in rabbit, the inward rectifier potassium channel Kir2.1 was present with Chapsyn110 on the tips of horizontal cell dendrites within photoreceptor invaginations, where it could provide a hyperpolarization-activated current that could contribute to ephaptic signaling in the photoreceptor synapses. Pannexin 1 and Pannexin 2, thought to play a role in ephaptic and/or pH mediated signaling, were present in the outer plexiform layer, but likely not in the horizontal cells. Polyamines regulate many ion channels and control the degree of rectification of Kir2.1 by imposing a voltage-dependent block. During the day polyamine immunolabeling was unexpectedly high in photoreceptor terminals compared to other areas of the retina. This content was significantly lower at night, when polyamine content was predominantly in Müller glia, indicating daily rhythms of polyamine content. Both rod and cone terminals displayed the same rhythm. While polyamine content was not prominent in horizontal cells, if polyamines are released, they may regulate the activity of Kir2.1 channels located in the tips of HCs. The rhythmic change in polyamine content of photoreceptor terminals suggests that a daily rhythm tunes the behavior of suites of ion channels within the photoreceptor synapses.

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.

Horizontal Cell Feedback to Cone Photoreceptors in Mammalian Retina: Novel Insights From the GABA-pH Hybrid Model

How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears to be mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control photoreceptor synaptic gain and enhance key aspects of temporal and spatial center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, the next key task in visual processing is the transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via horizontal cell feedback, which acts as a thermostat to keep the synaptic transmission in an optimal range during changes to light patterns and intensities. Novel findings of a recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse are reviewed. This novel inter-neuronal messaging system carries feedback signals using two separate, but interwoven regulated systems. The complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, has obscured it’s being defined. The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential, a function of stimulus intensity and size, regulates inhibition of photoreceptor voltage-gated Ca²⁺ channels, and strong artificial pH buffering eliminates this action. This review compares and contrasts models of how these foundations are linked, focusing on a recent report in mammals that shows tonic horizontal cell release of GABA activating Cl⁻ and HCO₃⁻ permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated HCO₃⁻ efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization in darkness and contributing to cleft acidification via NHE-mediated H⁺ efflux. This model challenges interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.

Analysis of tetrodotoxin-sensitive sodium and low voltage-activated calcium channels in developing mouse retinal horizontal cells

Expression patterns of voltage-gated ion channels determine the spatio-temporal dynamics of ion currents that supply excitable neurons in developing tissue with proper electrophysiological properties. The purpose of the study was to identify fast cationic inward currents in mouse retinal horizontal cells (HCs) and describe their biophysical properties at different developmental stages. We also aimed to reveal their physiological role in shaping light responses (LRs) in adult HCs. HCs were recorded in horizontal slices of wild-type mouse retina at postnatal stages ranging from p8 through p60. Voltage-dependent inward currents were isolated with appropriate voltage protocols and blockers specific for sodium and T-type calcium channels. LRs were evoked with full-field flashes (130 μW/cm²). Transient and steady inward currents were identified at all developmental stages. Transient currents were mediated by T-type calcium and TTX-sensitive sodium channels, whereas steady currents were blocked by cadmium, indicating the presence of high voltage-activated calcium channels. Activation and steady-state inactivation kinetics of T-type calcium channels revealed a contribution to the resting membrane potential during postnatal development. Additionally, both sodium and T-type calcium channels had an impact on HC LRs at light offset in adult animals. Our results showed that the voltage-dependent inward currents of postnatally developing mouse HCs consist of T-type calcium, TTX-sensitive sodium, and high voltage-activated calcium channels, and that transient ionic currents contributed to light-evoked responses of adult HCs, suggesting a role in HC information processing.

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

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.

Novel hybrid action of GABA mediates inhibitory feedback in the mammalian retina

The stream of visual information sent from photoreceptors to second-order bipolar cells is intercepted by laterally interacting horizontal cells that generate feedback to optimize and improve the efficiency of signal transmission. The mechanisms underlying the regulation of graded photoreceptor synaptic output in this nonspiking network have remained elusive. Here, we analyze with patch clamp recording the novel mechanisms by which horizontal cells control pH in the synaptic cleft to modulate photoreceptor neurotransmitter release. First, we show that mammalian horizontal cells respond to their own GABA release and that the results of this autaptic action affect cone voltage-gated Ca2+ channel (CaV channel) gating through changes in pH. As a proof-of-principle, we demonstrate that chemogenetic manipulation of horizontal cells with exogenous anion channel expression mimics GABA-mediated cone CaV channel inhibition. Activation of these GABA receptor anion channels can depolarize horizontal cells and increase cleft acidity via Na+/H+ exchanger (NHE) proton extrusion, which results in inhibition of cone CaV channels. This action is effectively counteracted when horizontal cells are sufficiently hyperpolarized by increased GABA receptor (GABAR)-mediated HCO3- efflux, alkalinizing the cleft and disinhibiting cone CaV channels. This demonstrates how hybrid actions of GABA operate in parallel to effect voltage-dependent pH changes, a novel mechanism for regulating synaptic output.

Analysis of Feedback Signaling from Horizontal Cells to Photoreceptors in Mice

Genetic manipulation of horizontal cells using a Connexin57-iCre mouse (Cx57-iCre) line combined with calcium imaging is proving to be a valuable method to study horizontal cell feedback inhibition onto photoreceptor terminals. While it is accepted that horizontal cells provide lateral inhibitory feedback to photoreceptors, the cellular mechanisms that underlie this feedback inhibition remain only partially elucidated. Feedback inhibition of photoreceptors acts via modulation of their voltage-gated calcium channels at their synaptic terminal. Calcium imaging of photoreceptors in retinal slices, therefore, reflects the impact of inhibitory feedback from horizontal cells. The development of a Cx57-iCre mouse line permits genetic manipulation of horizontal cells. In wild-type mouse retina, depolarization of horizontal cells by kainate provokes a decrease in photoreceptor Ca²⁺_i, whereas hyperpolarization by NBQX elicits an increase in photoreceptor Ca²⁺_i. These responses indicate increased feedback inhibition occurred when horizontal cells are depolarized, and decreased feedback inhibition, when hyperpolarized. This system was used to test the role of GABA release from horizontal cells in feedback inhibition by the selective elimination of VGAT/VIAAT, the inhibitory amino acid transmitter transporter that loads GABA into the synaptic vesicles of horizontal cells. Combined with calcium imaging of photoreceptors in retinal slices, the knockout of specific proteins, e.g., VGAT, provides a robust technique to test the role of GABA in feedback inhibition by horizontal cells.

Single-Cell Dissociation and Characterization in the Murine Retina and Optic Nerve

Recent technological advances have extended the range of analytic tools to very small samples. It is now possible to assay the transcriptome, and in some cases even the proteome, of single cells reliably. This allows addressing novel questions, such as the genotype/phenotype relationships of single neurons, heterogeneity within individual cells of the same type, or the basis of differential vulnerability to injury. An important prerequisite for these kinds of studies is the ability to isolate well-defined individual cells without contamination by adjacent tissue. In the retina and optic nerve, cells of different types and functions are closely intermingled, limiting the use of standard methods such as laser capture microdissection. Here, we describe a simple method to isolate morphologically intact cells from the retina and the optic nerve and discuss considerations in recognizing and isolating different cell types after dissociation.

Local Signals in Mouse Horizontal Cell Dendrites

The mouse retina contains a single type of horizontal cell, a GABAergic interneuron that samples from all cone photoreceptors within reach and modulates their glutamatergic output via parallel feedback mechanisms. Because horizontal cells form an electrically coupled network, they have been implicated in global signal processing, such as large-scale contrast enhancement. Recently, it has been proposed that horizontal cells can also act locally at the level of individual cone photoreceptors. To test this possibility physiologically, we used two-photon microscopy to record light stimulus-evoked Ca²⁺ signals in cone axon terminals and horizontal cell dendrites as well as glutamate release in the outer plexiform layer. By selectively stimulating the two mouse cone opsins with green and UV light, we assessed whether signals from individual cones remain isolated within horizontal cell dendritic tips or whether they spread across the dendritic arbor. Consistent with the mouse's opsin expression gradient, we found that the Ca²⁺ signals recorded from dendrites of dorsal horizontal cells were dominated by M-opsin and those of ventral horizontal cells by S-opsin activation. The signals measured in neighboring horizontal cell dendritic tips varied markedly in their chromatic preference, arguing against global processing. Rather, our experimental data and results from biophysically realistic modeling support the idea that horizontal cells can process cone input locally, extending the classical view of horizontal cell function. Pharmacologically removing horizontal cells from the circuitry reduced the sensitivity of the cone signal to low frequencies, suggesting that local horizontal cell feedback shapes the temporal properties of cone output.

How do horizontal cells 'talk' to cone photoreceptors? Different levels of complexity at the cone-horizontal cell synapse

The first synapse of the retina plays a fundamental role in the visual system. Due to its importance, it is critical that it encodes information from the outside world with the greatest accuracy and precision possible. Cone photoreceptor axon terminals contain many individual synaptic sites, each represented by a presynaptic structure called a 'ribbon'. These synapses are both highly sophisticated and conserved. Each ribbon relays the light signal to one ON cone bipolar cell and several OFF cone bipolar cells, while two dendritic processes from a GABAergic interneuron, the horizontal cell, modulate the cone output via parallel feedback mechanisms. The presence of these three partners within a single synapse has raised numerous questions, and its anatomical and functional complexity is still only partially understood. However, the understanding of this synapse has recently evolved, as a consequence of progress in understanding dendritic signal processing and its role in facilitating global versus local signalling. Indeed, for the downstream retinal network, dendritic processing in horizontal cells may be essential, as they must support important functional operations such as contrast enhancement, which requires spatial averaging of the photoreceptor array, while at the same time preserving accurate spatial information. Here, we review recent progress made towards a better understanding of the cone synapse, with an emphasis on horizontal cell function, and discuss why such complexity might be necessary for early visual processing.

Calcium-activated BKCa channels govern dynamic membrane depolarizations of horizontal cells in rodent retina

KEY POINTS

Large conductance, Ca²⁺ -activated K⁺ (BK_Ca ) channels play important roles in mammalian retinal neurons, including photoreceptors, bipolar cells, amacrine cells and ganglion cells, but they have not been identified in horizontal cells. BK_Ca channel blockers paxilline and iberiotoxin, as well as Ca²⁺ free solutions and divalent cation Ca_v channel blockers, eliminate the outwardly rectifying current, while NS1619 enhances it. In symmetrical 150 mm K⁺ , single channels had a conductance close to 250 pS, within the range of all known BK_Ca channels. In current clamped horizontal cells, BK_Ca channels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BK_Ca channel activation puts a ceiling on horizontal cell depolarization and regulates the temporal responsivity of the cells.

ABSTRACT

Large conductance, calcium-activated potassium (BK_Ca ) channels have numerous roles in neurons including the regulation of membrane excitability, intracellular [Ca²⁺ ] regulation, and neurotransmitter release. In the retina, they have been identified in photoreceptors, bipolar cells, amacrine cells and ganglion cells, but have not been conclusively identified in mammalian horizontal cells. We found that outward current recorded between -30 and +60 mV is carried primarily in BK_Ca channels in isolated horizontal cells of rats and mice. Whole-cell outward currents were maximal at +50 mV and declined at membrane potentials positive to this value. This current was eliminated by the selective BK_Ca channel blocker paxilline (100 nm), iberiotoxin (10 μm), Ca²⁺ free solutions and divalent cation Ca_v channel blockers. It was activated by the BK_Ca channel activator NS1619 (30 μm). Single channel recordings revealed the conductance of the channels to be 244 ± 11 pS (n = 17; symmetrical 150 mm K⁺ ) with open probability being both voltage- and Ca²⁺ -dependent. The channels showed fast activation kinetics in response to Ca²⁺ influx and inactivation gating that could be modified by intracellular protease treatment, which suggests β subunit involvement. Under current clamp, block of BK_Ca current increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BK_Ca current activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BK_Ca current in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.

Calcium dynamics and regulation in horizontal cells of the vertebrate retina: lessons from teleosts

Horizontal cells (HCs) are inhibitory interneurons of the vertebrate retina. Unlike typical neurons, HCs are chronically depolarized in the dark, leading to a constant influx of Ca2+ Therefore, mechanisms of Ca2+ homeostasis in HCs must differ from neurons elsewhere in the central nervous system, which undergo excitotoxicity when they are chronically depolarized or stressed with Ca2+ HCs are especially well characterized in teleost fish and have been used to unlock mysteries of the vertebrate retina for over one century. More recently, mammalian models of the retina have been increasingly informative for HC physiology. We draw from both teleost and mammalian models in this review, using a comparative approach to examine what is known about Ca2+ pathways in vertebrate HCs. We begin with a survey of Ca2+-permeable ion channels, exchangers, and pumps and summarize Ca2+ influx and efflux pathways, buffering, and intracellular stores. This includes evidence for Ca2+-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptors and for voltage-gated Ca2+ channels. Special attention is given to interactions between ion channels, to differences among species, and in which subtypes of HCs these channels have been found. We then discuss a number of unresolved issues pertaining to Ca2+ dynamics in HCs, including a potential role for Ca2+ in feedback to photoreceptors, the role for Ca2+-induced Ca2+ release, and the properties and functions of Ca2+-based action potentials. This review aims to highlight the unique Ca2+ dynamics in HCs, as these are inextricably tied to retinal function.

Dopamine D1 receptor modulation of calcium channel currents in horizontal cells of mouse retina

Horizontal cells form the first laterally interacting network of inhibitory interneurons in the retina. Dopamine released onto horizontal cells under photic and circadian control modulates horizontal cell function. Using isolated, identified horizontal cells from a connexin-57-iCre × ROSA26-tdTomato transgenic mouse line, we investigated dopaminergic modulation of calcium channel currents (ICa) with whole cell patch-clamp techniques. Dopamine (10 μM) blocked 27% of steady-state ICa, an action blunted to 9% in the presence of the L-type Ca channel blocker verapamil (50 μM). The dopamine type 1 receptor (D1R) agonist SKF38393 (20 μM) inhibited ICa by 24%. The D1R antagonist SCH23390 (20 μM) reduced dopamine and SKF38393 inhibition. Dopamine slowed ICa activation, blocking ICa by 38% early in a voltage step. Enhanced early inhibition of ICa was eliminated by applying voltage prepulses to +120 mV for 100 ms, increasing ICa by 31% and 11% for early and steady-state currents, respectively. Voltage-dependent facilitation of ICa and block of dopamine inhibition after preincubation with a Gβγ-blocking peptide suggested involvement of Gβγ proteins in the D1R-mediated modulation. When the G protein activator guanosine 5'-O-(3-thiotriphosphate) (GTPγS) was added intracellularly, ICa was smaller and showed the same slowed kinetics seen during D1R activation. With GTPγS in the pipette, additional block of ICa by dopamine was only 6%. Strong depolarizing voltage prepulses restored the GTPγS-reduced early ICa amplitude by 36% and steady-state ICa amplitude by 3%. These results suggest that dopaminergic inhibition of ICa via D1Rs is primarily mediated through the action of Gβγ proteins in horizontal cells.

Targeted Deletion of Vesicular GABA Transporter from Retinal Horizontal Cells Eliminates Feedback Modulation of Photoreceptor Calcium Channels

The cellular mechanisms underlying feedback signaling from horizontal cells to photoreceptors, which are important for the formation of receptive field surrounds of early visual neurons, remain unsettled. Mammalian horizontal cells express a complement of synaptic proteins that are necessary and sufficient for calcium-dependent exocytosis of inhibitory neurotransmitters at their contacts with photoreceptor terminals, suggesting that they are capable of releasing GABA via vesicular release. To test whether horizontal cell vesicular release is involved in feedback signaling, we perturbed inhibitory neurotransmission in these cells by targeted deletion of the vesicular GABA transporter (VGAT), the protein responsible for the uptake of inhibitory transmitter by synaptic vesicles. To manipulate horizontal cells selectively, an iCre mouse line with Cre recombinase expression controlled by connexin57 (Cx57) regulatory elements was generated. In Cx57-iCre mouse retina, only horizontal cells expressed Cre protein, and its expression occurred in all retinal regions. After crossing with a VGAT^flox/flox mouse line, VGAT was selectively eliminated from horizontal cells, which was confirmed immunohistochemically. Voltage-gated ion channel currents in horizontal cells of Cx57-VGAT^−/− mice were the same as Cx57-VGAT^+/+ controls, as were the cell responses to the ionotropic glutamate receptor agonist kainate, but the response to the GABA_A receptor agonist muscimol in Cx57-VGAT^−/− mice was larger. In contrast, the feedback inhibition of photoreceptor calcium channels, which in control animals is induced by horizontal cell depolarization, was completely absent in Cx57-VGAT^−/− mice. The results suggest that vesicular release of GABA from horizontal cells is required for feedback inhibition of photoreceptors.

Genetic dissection of horizontal cell inhibitory signaling in mice in complete darkness in vivo

PURPOSE

To test the hypothesis that horizontal cell (HC) inhibitory signaling controls the degree to which rod cell membranes are depolarized as measured by the extent to which L-type calcium channels (LTCCs) are open in complete darkness in the mouse retina in vivo.

METHODS

Dark-adapted wild-type (wt), CACNA1F (Ca(v)1.4(-/-)), arrestin-1 (Arr1(-/-)), and CACNA1D (Ca(v)1.3(-/-)) C57Bl/6 mice were studied. Manganese-enhanced MRI (MEMRI) evaluated the extent that rod LTCCs are open as an index of loss of HC inhibitory signaling. Subgroups were pretreated with D-cis-diltiazem (DIL) at a dose that specifically antagonizes Ca(v)1.2 channels in vivo.

RESULTS

Knockout mice predicted to have impaired HC inhibitory signaling (Ca(v)1.4(-/-) or Arr1(-/-)) exhibited greater than normal rod manganese uptake; inner retinal uptake was also supernormal. Genetically knocking out a closely associated gene not expected to impact HC inhibitory signaling (CACNA1D) did not generate this phenotype. The Arr1(-/-) mice exhibited the largest rod uptake of manganese. Manganese-enhanced MRI of DIL-treated Arr1(-/-) mice suggested a greater number of operant LTCC subtypes (i.e., Ca(v)1.2, 1.3, and 1.4) in rods and inner retina than that in DIL-treated Ca(v)1.4(-/-) mice (i.e., Ca(v)1.3). The Ca(v)1.3(-/-) + DIL-treated mice exhibited evidence for a compensatory contribution from Ca(v)1.2 LTCCs.

CONCLUSIONS

The data suggest that loss of HC inhibitory signaling is the proximate cause leading to maximally open LTCCs in rods, and possibly inner retinal cells, in mice in total darkness in vivo, regardless of compensatory changes in LTCC subtype manifested in the mutant mice.

MRI of rod cell compartment-specific function in disease and treatment in vivo

Rod cell oxidative stress is a major pathogenic factor in retinal disease, such as diabetic retinopathy (DR) and retinitis pigmentosa (RP). Personalized, non-destructive, and targeted treatment for these diseases remains elusive since current imaging methods cannot analytically measure treatment efficacy against rod cell compartment-specific oxidative stress in vivo. Over the last decade, novel MRI-based approaches that address this technology gap have been developed. This review summarizes progress in the development of MRI since 2006 that enables earlier evaluation of the impact of disease on rod cell compartment-specific function and the efficacy of anti-oxidant treatment than is currently possible with other methods. Most of the new assays of rod cell compartment-specific function are based on endogenous contrast mechanisms, and this is expected to facilitate their translation into patients with DR and RP, and other oxidative stress-based retinal diseases.

Functional properties of spontaneous excitatory currents and encoding of light/dark transitions in horizontal cells of the mouse retina

As all visual information is represented in the spatio-temporal dynamics of transmitter release from photoreceptors and the combined postsynaptic responses of second-order neurons, appropriate synaptic transfer functions are fundamental for a meaningful perception of the visual world. The functional contribution of horizontal cells to gain control and organization of bipolar and ganglion cell receptive fields can only be evaluated with an in-depth understanding of signal processing in horizontal cells. Therefore, a horizontal slice preparation of the mouse retina was established to record from horizontal cell bodies with their dendritic fields intact and receiving functional synaptic input from cone photoreceptors. Horizontal cell bodies showed spontaneous excitatory currents (spEPSCs) of monophasic and more complex multi-peak waveforms. spEPSCs were induced by quantal release of glutamate from presynaptic cones with a unitary amplitude of 3 pA. Non-stationary noise analysis revealed that spEPSCs with a monoexponential decay were mediated by 7-8 glutamate receptors with a single-channel amplitude of 1.55 pA. Responses to photopic full-field illumination were characterized by reduction of a tonic inward current or hyperpolarization, inhibition of spEPSCs, followed by a fast and transient inward current at light offset. The response to periodic dark/light transitions of different frequencies was dependent on the adaptational status of the cell with a limiting frequency of 10 Hz. Both on and off components of the light response were mediated by AMPA and kainate receptors. Detailed analysis of horizontal cell synaptic physiology is a prerequisite for understanding signal coding and processing at the photoreceptor ribbon synapse.

Expression and cellular localization of the voltage-gated calcium channel α2δ3 in the rodent retina

High-voltage-activated calcium channels are hetero-oligomeric protein complexes that mediate multiple cellular processes, including the influx of extracellular Ca(2+), neurotransmitter release, gene transcription, and synaptic plasticity. These channels consist of a primary α(1) pore-forming subunit, which is associated with an extracellular α(2)δ subunit and an intracellular β auxiliary subunit, which alter the gating properties and trafficking of the calcium channel. The cellular localization of the α(2)δ(3) subunit in the mouse and rat retina is unknown. In this study using RT-PCR, a single band at ∼ 305 bp corresponding to the predicted size of the α(2)δ(3) subunit fragment was found in mouse and rat retina and brain homogenates. Western blotting of rodent retina and brain homogenates showed a single 123-kDa band. Immunohistochemistry with an affinity-purified antibody to the α(2)δ(3) subunit revealed immunoreactive cell bodies in the ganglion cell layer and inner nuclear layer and immunoreactive processes in the inner plexiform layer and the outer plexiform layer. α(2)δ(3) immunoreactivity was localized to multiple cell types, including ganglion, amacrine, and bipolar cells and photoreceptors, but not horizontal cells. The expression of the α(2)δ(3) calcium channel subunit to multiple cell types suggests that this subunit participates widely in Ca-channel-mediated signaling in the retina.

Differential regulation of cone calcium signals by different horizontal cell feedback mechanisms in the mouse retina

Controlling neurotransmitter release by modulating the presynaptic calcium level is a key mechanism to ensure reliable signal transmission from one neuron to the next. In this study, we investigated how the glutamatergic output of cone photoreceptors (cones) in the mouse retina is shaped by different feedback mechanisms from postsynaptic GABAergic horizontal cells (HCs) using a combination of two-photon calcium imaging and pharmacology at the level of individual cone axon terminals. We provide evidence that hemichannel-mediated (putative ephaptic) feedback sets the cone output gain by defining the basal calcium level, a mechanism that may be crucial for adapting cones to the ambient light level. In contrast, pH-mediated feedback did not modulate the cone basal calcium level but affected the size and shape of light-evoked cone calcium signals in a contrast-dependent way: low-contrast light responses were amplified, whereas high-contrast light responses were reduced. Finally, we provide functional evidence that GABA shapes light-evoked calcium signals in cones. Because we could not localize ionotropic GABA receptors on cone axon terminals using electron microscopy, we suggest that GABA may act through GABA autoreceptors on HCs, thereby possibly modulating hemichannel- and/or pH-mediated feedback. Together, our results suggest that at the cone synapse, hemichannel-mediated (ephaptic) and pH-mediated feedback fulfill distinct functions to adjust the output of cones to changing ambient light levels and stimulus contrasts and that the efficacy of these feedback mechanisms is likely modulated by GABA release in the outer retina.

Synaptic remodeling generates synchronous oscillations in the degenerated outer mouse retina

During neuronal degenerative diseases, neuronal microcircuits undergo severe structural alterations, leading to remodeling of synaptic connectivity. The functional consequences of such remodeling are mostly unknown. For instance, in mutant rd1 mouse retina, a common model for Retinitis Pigmentosa, rod bipolar cells (RBCs) establish contacts with remnant cone photoreceptors (cones) as a consequence of rod photoreceptor cell death and the resulting lack of presynaptic input. To assess the functional connectivity in the remodeled, light-insensitive outer rd1 retina, we recorded spontaneous population activity in retinal wholemounts using Ca(2+) imaging and identified the participating cell types. Focusing on cones, RBCs and horizontal cells (HCs), we found that these cell types display spontaneous oscillatory activity and form synchronously active clusters. Overall activity was modulated by GABAergic inhibition from interneurons such as HCs and/or possibly interplexiform cells. Many of the activity clusters comprised both cones and RBCs. Opposite to what is expected from the intact (wild-type) cone-ON bipolar cell pathway, cone and RBC activity was positively correlated and, at least partially, mediated by glutamate transporters expressed on RBCs. Deletion of gap junctional coupling between cones reduced the number of clusters, indicating that electrical cone coupling plays a crucial role for generating the observed synchronized oscillations. In conclusion, degeneration-induced synaptic remodeling of the rd1 retina results in a complex self-sustained outer retinal oscillatory network, that complements (and potentially modulates) the recently described inner retinal oscillatory network consisting of amacrine, bipolar and ganglion cells.

Cell-specific cre recombinase expression allows selective ablation of glutamate receptors from mouse horizontal cells

In the mouse retina, horizontal cells form an electrically coupled network and provide feedback signals to photoreceptors and feedforward signals to bipolar cells. Thereby, horizontal cells contribute to gain control at the first visual synapse and to the antagonistic organization of bipolar and ganglion cell receptive fields. However, the nature of horizontal cell output remains a matter of debate, just as the exact contribution of horizontal cells to center-surround antagonism. To facilitate studying horizontal cell function, we developed a knockin mouse line which allows ablating genes exclusively in horizontal cells. This knockin line expresses a Cre recombinase under the promoter of connexin57 (Cx57), a gap junction protein only expressed in horizontal cells. Consistently, in Cx57+/Cre mice, Cre recombinase is expressed in almost all horizontal cells (>99%) and no other retinal neurons. To test Cre activity, we crossbred Cx57+/Cre mice with a mouse line in which exon 11 of the coding sequence for the ionotropic glutamate receptor subunit GluA4 was flanked by two loxP sites (GluA4fl/fl). In GluA4fl/fl:Cx57+/Cre mice, GluA4 immunoreactivity was significantly reduced (∼50%) in the outer retina where horizontal cells receive photoreceptor inputs, confirming the functionality of the Cre/loxP system. Whole-cell patch-clamp recordings from isolated horizontal cell somata showed a reduction of glutamate-induced inward currents by ∼75%, suggesting that the GluA4 subunit plays a major role in mediating photoreceptor inputs. The persistent current in GluA4-deficient cells is mostly driven by AMPA and to a very small extent by kainate receptors as revealed by application of the AMPA receptor antagonist GYKI52466 and concanavalin A, a potentiator of kainate receptor-mediated currents. In summary, the Cx57+/Cre mouse line provides a versatile tool for studying horizontal cell function. GluA4fl/fl:Cx57+/Cre mice, in which horizontal cells receive less excitatory input, can thus be used to analyze the contribution of horizontal cells to retinal processing.

Calcium channels in rat horizontal cells regulate feedback inhibition of photoreceptors through an unconventional GABA- and pH-sensitive mechanism

Horizontal cells send inhibitory feedback to photoreceptors, helping form antagonistic receptive fields in the retina, but the neurotransmitter and the mechanisms underlying this signalling are not known. Since the proteins responsible for conventional Ca(2+)-dependent release of GABAergic synaptic vesicles are present in mammalian horizontal cells, we investigated this conventional mechanism as the means by which horizontal cells inhibit photoreceptors. Using Ca(2+) imaging in rat retinal slices, we confirm that horizontal cell depolarization with kainate inhibits and horizontal cell hyperpolarization with NBQX disinhibits the Ca(2+) signals produced by pH-sensitive activation of voltage-gated calcium channels (Ca channels) in photoreceptors. We show that while 100 μm Co(2+) reduces photoreceptor Ca(2+) signals, it disinhibits them at 10 μm, an effect reminiscent of earlier studies where low [Co(2+)] eliminated feedback. The low [Co(2+)] disinhibition is pH sensitive. We localized L-, N- and P/Q-type Ca channels in rat horizontal cells, and showed that both the N-type Ca channel blocker -conotoxin GVIA and the P/Q-type Ca channel blocker -agatoxin IVA increased Ca(2+) signals in photoreceptors in a pH-sensitive manner. Pronounced actions of GABAergic agents on feedback signals to photoreceptors were observed, and are pH sensitive, but are inconsistent with direct inhibition by GABA of photoreceptor [Ca(2+)]. Patch-clamp studies revealed that GABA activates a conductance having high bicarbonate permeability in isolated horizontal cells, suggesting that the commonality of pH sensitivity throughout the results could arise from a GABA autofeedback action in horizontal cells. This could change cleft pH with concomitant inhibitory influences on photoreceptor Ca channels.

Ubiquilin 1 interacts with Orai1 to regulate calcium mobilization

Store-operated calcium entry (SOCE) channels composed of Stim and Orai proteins play a critical role in diverse biological processes. Upon endoplasmic reticulum (ER)-mediated calcium (Ca(2+)) depletion, Stim proteins oligomerize with Orai to initiate Ca(2+) influx across the plasma membrane. The ubiquitin-like (UBL) and ubiquitin-associated (UBA) domains of ubiquilin 1 are involved in the degradation of presenilin and polyglutamine proteins. Through screening of Orai1 interaction partner(s) that might have an effect on SOCE, ubiquilin 1 was identified as a target of Orai1. However, the UBL and UBA domains of ubiquilin 1 were dispensable for this interaction. Additionally, ubiquilin 1 and Orai1 colocalized in the cytosolic compartment. Ubiquilin 1 increased the ubiquitination of Orai1, resulting in the formation of a high-molecular-weight form. MG132, a proteasome inhibitor, failed to block the degradation of Orai1, whereas bafilomycin A, a lysosome inhibitor, prevented Orai1 degradation. Confocal microscopy studies demonstrated that a fraction of Orai1 colocalized with ubiquilin 1 and the autophagosomal marker LC3. Because Orai1 is a constituent of SOCE, we determined the effect of ubiquilin 1 on Orai1-mediated Ca(2+) influx. As we expected, intracellular Ca(2+) mobilization, a process normally potentiated by Orai1, was downregulated by ubiquilin 1. Taken together, these findings suggest that ubiquilin 1 downregulates intracellular Ca(2+) mobilization and its downstream signaling by promoting the ubiquitination and lysosomal degradation of Orai1.

Histamine receptors of cones and horizontal cells in Old World monkey retinas

In primates the retina receives input from histaminergic neurons in the posterior hypothalamus that are active during the day. In order to understand how this input contributes to information processing in Old World monkey retinas, we have been localizing histamine receptors (HR) and studying the effects of histamine on the neurons that express them. Previously, we localized HR3 to the tips of ON bipolar cell dendrites and showed that histamine hyperpolarizes the cells via this receptor. We raised antisera against synthetic peptides corresponding to an extracellular domain of HR1 between the 4th and 5th transmembrane domains and to an intracellular domain near the carboxyl terminus of HR2. Using these, we localized HR1 to horizontal cells and a small number of amacrine cells and localized HR2 to puncta closely associated with synaptic ribbons inside cone pedicles. Consistent with this, HR1 mRNA was detected in horizontal cell perikarya and primary dendrites and HR2 mRNA was found in cone inner segments. We studied the effect of 5 μM exogenous histamine on primate cones in macaque retinal slices. Histamine reduced I(h) at moderately hyperpolarized potentials, but not the maximal current. This would be expected to increase the operating range of cones and conserve ATP in bright, ambient light. Thus, all three major targets of histamine are in the outer plexiform layer, but the retinopetal axons containing histamine terminate in the inner plexiform layer. Taken together, the findings in these three studies suggest that histamine acts primarily via volume transmission in primate retina.

Two separate Ni(2+) -sensitive voltage-gated Ca(2+) channels modulate transretinal signalling in the isolated murine retina

PURPOSE

Light-evoked responses from vertebrate retinas were recorded as an electroretinogram (ERG). The b-wave is the most prominent component of the ERG, and in the bovine retina its NiCl(2) -sensitive component was attributed to reciprocal signalling by pharmacoresistant R-type voltage-gated Ca(2+) channels, which similar to other voltage-dependent Ca(2+) channels trigger and control neurotransmitter release. The murine retina has the great advantage that the effect of gene inactivation for Ni(2+) -sensitive Ca(2+) channels can be analysed to prove or disprove that any of these Ca(2+) channels is involved in retinal signalling.

METHODS

Superfused retinas from different murine genotypes lacking either one or both highly Ni(2+) -sensitive voltage-gated Ca(2+) channels were used to record their ex vivo ERGs.

RESULTS

The isolated retinas from mice lacking Ca(v)2.3 R-type or Ca(v)3.2 T-type or both voltage-gated Ca(2+) channels were superfused with a NiCl(2) (15 μm) containing nutrient solution. The change in the b-wave amplitude and implicit time, caused by NiCl(2), was calculated as a difference spectrum and compared to data from control animals. From the results, it can be deduced that Ca(v)2.3 contributes rather to a later component in the b-wave response, while in the absence of Ca(v)3.2 the gain of Ni(2+) -mediated increase in the b-wave amplitude is significantly increased, probably due to a loss of reciprocal inhibition to photoreceptors. Thus, each of the Ni(2+)-sensitive Ca(2+) channels contributes to specific features of the b-wave response.

CONCLUSION

Both high-affinity Ni(2+)-sensitive Ca(2+) channels contribute to transretinal signalling. Based on the results from the double knockout mice, additional targets for NiCl(2) must contribute to transretinal signalling, which will be most important for the structurally similar physiologically more important heavy metal cation Zn(2+).

Immunocytochemical evidence for SNARE protein-dependent transmitter release from guinea pig horizontal cells

Horizontal cells are lateral interneurons that participate in visual processing in the outer retina but the cellular mechanisms underlying transmitter release from these cells are not fully understood. In non-mammalian horizontal cells, GABA release has been shown to occur by a non-vesicular mechanism. However, recent evidence in mammalian horizontal cells favors a vesicular mechanism as they lack plasmalemmal GABA transporters and some soluble NSF attachment protein receptor (SNARE) core proteins have been identified in rodent horizontal cells. Moreover, immunoreactivity for GABA and the molecular machinery to synthesize GABA have been found in guinea pig horizontal cells, suggesting that if components of the SNARE complex are expressed they could contribute to the vesicular release of GABA. In this study we investigated whether these vesicular and synaptic proteins are expressed by guinea pig horizontal cells using immunohistochemistry with well-characterized antibodies to evaluate their cellular distribution. Components of synaptic vesicles including vesicular GABA transporter, synapsin I and synaptic vesicle protein 2A were localized to horizontal cell processes and endings, along with the SNARE core complex proteins, syntaxin-1a, syntaxin-4 and synaptosomal-associated protein 25 (SNAP-25). Complexin I/II, a cytosolic protein that stabilizes the activated SNARE fusion core, strongly immunostained horizontal cell soma and processes. In addition, the vesicular Ca(2+)-sensor, synaptotagmin-2, which is essential for Ca(2+)-mediated vesicular release, was also localized to horizontal cell processes and somata. These morphological findings from guinea pig horizontal cells suggest that mammalian horizontal cells have the capacity to utilize a regulated Ca(2+)-dependent vesicular pathway to release neurotransmitter, and that this mechanism may be shared among many mammalian species.

Antagonists of ionotropic gamma-aminobutyric acid receptors impair the NiCl2-mediated stimulation of the electroretinogram b-wave amplitude from the isolated superfused vertebrate retina

PURPOSE

NiCl(2) (15 microM) stimulates the electroretinogram (ERG) b-wave amplitude of vertebrate retina up to 1.5-fold through its blocking of E/R-type voltage-gated Ca(2+) channels. Assuming that such an increase is mediated by blocking the release of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) via ionotropic GABA receptors, we tested the effect of both GABA itself and GABA-receptor antagonists such as (-)bicuculline (1.51-fold increase) and (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA; 1.46-fold increase) on the b-wave amplitude.

METHODS

Recording of the transretinal potentials from the isolated bovine retina.

RESULTS

GABA (100 microM) reduced the b-wave amplitude only when NiCl(2) (15 microM) was applied first. Each antagonist applied on its own stimulated the b-wave amplitude only partially: subsequent NiCl(2) superfusion caused a small but additional increase, leading to a 1.69- and a 1.88-fold total increase of the amplitude by Ni(2+) plus (-)bicuculline or Ni(2+) plus TPMPA, respectively. Only the application of both antagonists in combination, before superfusing low NiCl(2) (15 microM), completely prevented subsequent stimulation by NiCl(2) with a similar 1.90-fold total increase of b-wave amplitude. Those retina segments that did not respond to NiCl(2) could not be stimulated by (-)bicuculline and vice versa.

CONCLUSION

The stimulatory effect of NiCl(2) on the ERG b-wave amplitude is mainly, but not only, mediated by a NiCl(2)-sensitive, Ca(v)2.3-triggered GABA release acting through ionotropic GABA-A and GABA-C receptors.

ZO-1 and the spatial organization of gap junctions and glutamate receptors in the outer plexiform layer of the mammalian retina

Information processing in the retina starts at the first synaptic layer, where photoreceptors and second-order neurons exhibit a complex architecture of glutamatergic and electrical synapses. To investigate the composition of this highly organized synaptic network, we determined the spatial relationship of zonula occludens-1 (ZO-1) with different connexins (Cx) and glutamate receptor (GluR) subunits in the outer plexiform layer (OPL) of rabbit, mouse, and monkey retinas. ZO-1 is well known as an intracellular component of tight and adherens junctions, but also interacts with various connexins at gap junctions. We found ZO-1 closely associated with Cx50 on dendrites of A-type horizontal cells in rabbit, and with Cx57 at dendro-dendritic gap junctions of mouse horizontal cells. The spatial arrangement of ZO-1 at the giant gap-junctional plaques in rabbit was particularly striking. ZO-1 formed a clear margin around the large Cx50 plaques instead of being colocalized with the connexin staining. Our finding suggests the involvement of ZO-1 in the composition of tight or adherens junctions around gap-junctional plaques instead of interacting with connexins directly. Furthermore, gap junctions were found to be clustered in close proximity to GluRs at the level of desmosome-like junctions, where horizontal cell dendrites converge before invaginating the cone pedicle. Based on this distinct spatial organization of gap junctions and GluRs, it is tempting to speculate that glutamate released from the photoreceptors may play a role in modulating the conductance of electrical synapses in the OPL.

Ether-à-gogo-related gene (erg1) potassium channels shape the dark response of horizontal cells in the mammalian retina

Postsynaptic to photoreceptors, horizontal cells face prolonged exposure to glutamate in the dark. Therefore, efficient hyperpolarizing mechanisms are crucial to keep horizontal cells within an operating range and to reduce glutamate-induced excitotoxicity. Combining electrophysiology, single-cell reverse transcriptase polymerase chain reaction, and immunocytochemistry, we found that horizontal cell bodies but not their axon terminals express the ether-à-gogo-related gene isoform 1 (erg1) K(+) channel. Erg1-mediated outward currents displayed voltage-dependent activation and C-type inactivation. Recovery from inactivation involved a transient open state. Gating of erg1 channels kept the voltage response to glutamate brief and at physiological amplitudes. With erg1 channels blocked, the response of horizontal cells to the onset of darkness was significantly enhanced. These results indicate a functional dichotomy between horizontal cell bodies and axon terminals in the processing of photoreceptor signals. The dark response thus reflects a finely tuned balance determined by the successive gating of ionotropic glutamate receptors and erg1 channels.

Robust syntaxin-4 immunoreactivity in mammalian horizontal cell processes

Horizontal cells mediate inhibitory feed-forward and feedback communication in the outer retina; however, mechanisms that underlie transmitter release from mammalian horizontal cells are poorly understood. Toward determining whether the molecular machinery for exocytosis is present in horizontal cells, we investigated the localization of syntaxin-4, a SNARE protein involved in targeting vesicles to the plasma membrane, in mouse, rat, and rabbit retinae using immunocytochemistry. We report robust expression of syntaxin-4 in the outer plexiform layer of all three species. Syntaxin-4 occurred in processes and tips of horizontal cells, with regularly spaced, thicker sandwich-like structures along the processes. Double labeling with syntaxin-4 and calbindin antibodies, a horizontal cell marker, demonstrated syntaxin-4 localization to horizontal cell processes; whereas, double labeling with PKC antibodies, a rod bipolar cell (RBC) marker, showed a lack of co-localization, with syntaxin-4 immunolabeling occurring just distal to RBC dendritic tips. Syntaxin-4 immunolabeling occurred within VGLUT-1-immunoreactive photoreceptor terminals and underneath synaptic ribbons, labeled by CtBP2/RIBEYE antibodies, consistent with localization in invaginating horizontal cell tips at photoreceptor triad synapses. Vertical sections of retina immunostained for syntaxin-4 and peanut agglutinin (PNA) established that the prominent patches of syntaxin-4 immunoreactivity were adjacent to the base of cone pedicles. Horizontal sections through the OPL indicate a one-to-one co-localization of syntaxin-4 densities at likely all cone pedicles, with syntaxin-4 immunoreactivity interdigitating with PNA labeling. Pre-embedding immuno-electron microscopy confirmed the subcellular localization of syntaxin-4 labeling to lateral elements at both rod and cone triad synapses. Finally, co-localization with SNAP-25, a possible binding partner of syntaxin-4, indicated co-expression of these SNARE proteins in the same subcellular compartment of the horizontal cell. Taken together, the strong expression of these two SNARE proteins in the processes and endings of horizontal cells at rod and cone terminals suggests that horizontal cell axons and dendrites are likely sites of exocytotic activity.