Electrophysiological Properties of Mouse Horizontal Cell GABAAReceptors

Origin of Retinal Oscillatory Potentials in the Mouse, a Tool to Specifically Locate Retinal Damage

To determine the origin of oscillatory potentials (OPs), binocular electroretinogram (ERG) recordings were performed under light and dark adaptation on adult healthy C57BL/6J mice. In the experimental group, 1 μL of PBS was injected into the left eye, while the right eye was injected with 1 μL of PBS containing different agents: APB, GABA, Bicuculline, TPMPA, Glutamate, DNQX, Glycine, Strychnine, or HEPES. The OP response depends on the type of photoreceptors involved, showing their maximum response amplitude in the ERG induced by mixed rod/cone stimulation. The oscillatory components of the OPs were affected by the injected agents, with some drugs inducing the complete abolition of oscillations (APB, GABA, Glutamate, or DNQX), whereas other drugs merely reduced the oscillatory amplitudes (Bicuculline, Glycine, Strychnine, or HEPES) or did not even affect the oscillations (TPMPA). Assuming that rod bipolar cells (RBC) express metabotropic Glutamate receptors, GABA_A, GABA_C, and Glycine receptors and that they release glutamate mainly on Glycinergic AII amacrine cells and GABAergic A17 amacrine cells, which are differently affected by the mentioned drugs, we propose that RBC-AII/A17 reciprocal synapses are responsible for the OP generation in the ERG recordings in the mice. We conclude that the reciprocal synapses between RBC and AII/A17 are the basis of the ERG OP oscillations of the light response, and this fact must be taken into consideration in any ERG test that shows a decrease in the OPs' amplitude.

Retinal horizontal cells use different synaptic sites for global feedforward and local feedback signaling

In the outer plexiform layer (OPL) of the mammalian retina, cone photoreceptors (cones) provide input to more than a dozen types of cone bipolar cells (CBCs). In the mouse, this transmission is modulated by a single horizontal cell (HC) type. HCs perform global signaling within their laterally coupled network but also provide local, cone-specific feedback. However, it is unknown how HCs provide local feedback to cones at the same time as global forward signaling to CBCs and where the underlying synapses are located. To assess how HCs simultaneously perform different modes of signaling, we reconstructed the dendritic trees of five HCs as well as cone axon terminals and CBC dendrites in a serial block-face electron microscopy volume and analyzed their connectivity. In addition to the fine HC dendritic tips invaginating cone axon terminals, we also identified "bulbs," short segments of increased dendritic diameter on the primary dendrites of HCs. These bulbs are in an OPL stratum well below the cone axon terminal base and make contacts with other HCs and CBCs. Our results from immunolabeling, electron microscopy, and glutamate imaging suggest that HC bulbs represent GABAergic synapses that do not receive any direct photoreceptor input. Together, our data suggest the existence of two synaptic strata in the mouse OPL, spatially separating cone-specific feedback and feedforward signaling to CBCs. A biophysical model of a HC dendritic branch and voltage imaging support the hypothesis that this spatial arrangement of synaptic contacts allows for simultaneous local feedback and global feedforward signaling by HCs.

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.

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.

In Vivo Study on Mechanism Underlying Increased Pharmacological Effects of Phenobarbital in Rats with Glycerol-Induced Acute Renal Failure

The mechanism underlying the increased pharmacological effects of phenobarbital in rats with glycerol-induced acute renal failure (ARF) was examined. In the experiments, a surgical cannula was inserted in the lateral ventricle of the rats for phenobarbital infusion, and the ARF induction was performed by intramuscular administration of 50% glycerol. The onset time of anesthesia by phenobarbital was determined with the tail flick method. In addition, cerebral microsomes were prepared from excised cerebral cortices of sham and ARF rats, and the cerebral expression of the γ-aminobutyric acid (GABA)A receptor and two cation-chloride transporters, KCC2 and NKCC1, was evaluated by Western blotting, as their functions are involved in the anesthetic effects of phenobarbital. When phenobarbital was infused in the ventricle, anesthesia was induced 2.2-times faster in ARF rats than in sham rats, and there was no detectable increase in the cerebral expression of the GABAA receptor in ARF rats. It was additionally noted that the cerebral expression of KCC2 decreased, whereas that of NKCC1 was unaltered in ARF rats. These findings indicated that the anesthetic effects of phenobarbital are potentiated in ARF rats, probably due to imbalanced cerebral expression of KCC2 and NKCC1, suggesting that altered cation-chloride handling in nerve cells is associated.

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.

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.

Zinc as Allosteric Ion Channel Modulator: Ionotropic Receptors as Metalloproteins

Zinc is an essential metal to life. This transition metal is a structural component of many proteins and is actively involved in the catalytic activity of cell enzymes. In either case, these zinc-containing proteins are metalloproteins. However, the amino acid residues that serve as ligands for metal coordination are not necessarily the same in structural proteins compared to enzymes. While crystals of structural proteins that bind zinc reveal a higher preference for cysteine sulfhydryls rather than histidine imidazole rings, catalytic enzymes reveal the opposite, i.e., a greater preference for the histidines over cysteines for catalysis, plus the influence of carboxylic acids. Based on this paradigm, we reviewed the putative ligands of zinc in ionotropic receptors, where zinc has been described as an allosteric modulator of channel receptors. Although these receptors do not strictly qualify as metalloproteins since they do not normally bind zinc in structural domains, they do transitorily bind zinc at allosteric sites, modifying transiently the receptor channel's ion permeability. The present contribution summarizes current information showing that zinc allosteric modulation of receptor channels occurs by the preferential metal coordination to imidazole rings as well as to the sulfhydryl groups of cysteine in addition to the carboxyl group of acid residues, as with enzymes and catalysis. It is remarkable that most channels, either voltage-sensitive or transmitter-gated receptor channels, are susceptible to zinc modulation either as positive or negative regulators.

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.

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.

Generation of recombinant antibodies to rat GABAA receptor subunits by affinity selection on synthetic peptides

The abundance and physiological importance of GABAA receptors in the central nervous system make this neurotransmitter receptor an attractive target for localizing diagnostic and therapeutic biomolecules. GABAA receptors are expressed within the retina and mediate synaptic signaling at multiple stages of the visual process. To generate monoclonal affinity reagents that can specifically recognize GABAA receptor subunits, we screened two bacteriophage M13 libraries, which displayed human scFvs, by affinity selection with synthetic peptides predicted to correspond to extracellular regions of the rat α1 and β2 GABAA subunits. We isolated three anti-β2 and one anti-α1 subunit specific scFvs. Fluorescence polarization measurements revealed all four scFvs to have low micromolar affinities with their cognate peptide targets. The scFvs were capable of detecting fully folded GABAA receptors heterologously expressed by Xenopus laevis oocytes, while preserving ligand-gated channel activity. Moreover, A10, the anti-α1 subunit-specific scFv, was capable of detecting native GABAA receptors in the mouse retina, as observed by immunofluorescence staining. In order to improve their apparent affinity via avidity, we dimerized the A10 scFv by fusing it to the Fc portion of the IgG. The resulting scFv-Fc construct had a Kd of ∼26 nM, which corresponds to an approximately 135-fold improvement in binding, and a lower detection limit in dot blots, compared to the monomeric scFv. These results strongly support the use of peptides as targets for generating affinity reagents to membrane proteins and encourage investigation of molecular conjugates that use scFvs as anchoring components to localize reagents of interest at GABAA receptors of retina and other neural tissues, for studies of receptor activation and subunit structure.

Expression of Pannexin1 in the outer plexiform layer of the mouse retina and physiological impact of its knockout

Pannexin1 (Panx1) belongs to a class of vertebrate proteins that exhibits sequence homology to innexins, the invertebrate gap junction proteins, and which also shares topological similarities with vertebrate gap junction proteins, the connexins. Unlike gap junctional channels, Panx1 forms single-membrane channels, whose functional role in neuronal circuits is still unsettled. We therefore investigated the subcellular distribution of Panx1 in the mouse retina of wildtype and Panx1-null mice by reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and electron microscopy. Use of Panx1-deficient mice served as a model to assess the physiological role of Panx1 by electroretinographic recordings and also to ensure the specificity of the anti-Panx1 antibody labeling. Expression of Panx1 was found in type 3a OFF bipolar cells and in dendrites and axonal processes of horizontal cells. Panx1 was also found in horizontal cell dendrites representing the lateral elements of the triad synapse at cone and rod terminals. In vivo electroretinography of Panx1 knockout mice indicated an increased a- and b-wave compared to Panx1 wildtype mice under scotopic conditions. The effect on the b-wave was confirmed by in vitro electroretinograms from the inner retina. These results suggest that Panx1 channels serve as sinks for extracellular current flow making them possible candidates for the mediation of feedback from horizontal cells to photoreceptors.

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.

Regulation of synaptic transmission at the photoreceptor terminal: a novel role for the cation-chloride co-transporter NKCC1

The Na(+)-K(+)-2Cl(-) co-transporter type 1 (NKCC1) is localized primarily throughout the outer plexiform layer (OPL) of the distal retina, a synaptic lamina that is comprised of the axon terminals of photoreceptors and the dendrites of horizontal and bipolar cells. Although known to play a key role in development, signal transmission and the gating of sensory signals in other regions of the retina and in the CNS, the contribution of NKCC1 to synaptic transmission within the OPL is largely unknown. In the present study, we investigated the function of NKCC1 at the photoreceptor-horizontal cell synapse by recording the electrical responses of photoreceptors and horizontal cells before and after blocking the activity of the transporter with bumetanide (BMN). Because NKCC1 co-transports 1 Na(+), 1 K(+) and 2 Cl(-), it is electroneutral and its activation had little effect on membrane conductance. However, recordings from postsynaptic horizontal cells revealed that inhibiting NKCC1 with BMN greatly increased glutamate release from both rod and cone terminals. In addition, we found that NKCC1 directly regulates Ca(2+)-dependent exocytosis at the photoreceptor synapse, raising the possibility that NKCC1 serves to suppress bulk release of glutamate vesicles from photoreceptor terminals in the dark and at light offset. Interestingly, NKCC1 gene and protein expressions were upregulated by light, which we attribute to the light-induced release of dopamine acting on D1-like receptors. In sum, our study reveals a new role for NKCC1 in the regulation of synaptic transmission in photoreceptors.

A new cross-correlation algorithm for the analysis of "in vitro" neuronal network activity aimed at pharmacological studies

Modern drug discovery for Central Nervous System pathologies has recently focused its attention to in vitro neuronal networks as models for the study of neuronal activities. Micro Electrode Arrays (MEAs), a widely recognized tool for pharmacological investigations, enable the simultaneous study of the spiking activity of discrete regions of a neuronal culture, providing an insight into the dynamics of networks. Taking advantage of MEAs features and making the most of the cross-correlation analysis to assess internal parameters of a neuronal system, we provide an efficient method for the evaluation of comprehensive neuronal network activity. We developed an intra network burst correlation algorithm, we evaluated its sensitivity and we explored its potential use in pharmacological studies. Our results demonstrate the high sensitivity of this algorithm and the efficacy of this methodology in pharmacological dose-response studies, with the advantage of analyzing the effect of drugs on the comprehensive correlative properties of integrated neuronal networks.

Mammalian retinal horizontal cells are unconventional GABAergic neurons

Lateral interactions at the first retinal synapse have been initially proposed to involve GABA by transporter-mediated release from horizontal cells, onto GABA(A) receptors expressed on cone photoreceptor terminals and/or bipolar cell dendrites. However, in the mammalian retina, horizontal cells do not seem to contain GABA systematically or to express membrane GABA transporters. We here report that mouse retinal horizontal cells express GAD65 and/or GAD67 mRNA, and were weakly but consistently immunostained for GAD65/67. While GABA was readily detected after intracardiac perfusion, it was lost during classical preparation for histology or electrophysiology. It could not be restored by incubation in a GABA-containing medium, confirming the absence of membrane GABA transporters in these cells. However, GABA was synthesized de novo from glutamate or glutamine, upon addition of pyridoxal 5'-phosphate, a cofactor of GAD65/67. Mouse horizontal cells are thus atypical GABAergic neurons, with no functional GABA uptake, but a glutamate and/or glutamine transport system allowing GABA synthesis, probably depending physiologically from glutamate released by photoreceptors. Our results suggest that the role of GABA in lateral inhibition may have been underestimated, at least in mammals, and that tissue pre-incubation with glutamine and pyridoxal 5'-phosphate should yield a more precise estimate of outer retinal processing.

Assembly of the outer retina in the absence of GABA synthesis in horizontal cells

BACKGROUND

The inhibitory neurotransmitter gamma-amino-butyric acid (GABA) not only modulates excitability in the mature nervous system but also regulates neuronal differentiation and circuit development. Horizontal cells, a subset of interneurons in the outer retina, are transiently GABAergic during the period of cone photoreceptor synaptogenesis. In rodents, both horizontal cells and cone axonal terminals express GABAA receptors. To explore the possibility that transient GABA expression in mouse neonatal horizontal cells influences the structural development of synaptic connectivity in the outer retina, we examined a mutant in which expression of GAD67, the major synthesizing enzyme for GABA, is selectively knocked out in the retina.

RESULTS

Immunocytochemistry and electron microscopy revealed that the assembly of triad synapses involving cone axonal pedicles and the dendrites of horizontal and bipolar cells is unaffected in the mutant retina. Moreover, loss of GABA synthesis in the outer retina did not perturb the spatial distributions and cell densities of cones and horizontal cells. However, there were some structural alterations at the cellular level: the average size of horizontal cell dendritic clusters was larger in the mutant, and there was also a small but significant increase in cone photoreceptor pedicle area. Moreover, metabotropic glutamate receptor 6 (mGluR6) receptors on the dendrites of ON bipolar cells occupied a slightly larger proportion of the cone pedicle in the mutant.

CONCLUSIONS

Together, our analysis shows that transient GABA synthesis in horizontal cells is not critical for synapse assembly and axonal and dendritic lamination in the outer retina. However, pre- and postsynaptic structures are somewhat enlarged in the absence of GABA in the developing outer retina, providing for a modest increase in potential contact area between cone photoreceptors and their targets. These findings differ from previous results in which pharmacological blockade of GABAA receptors in the neonatal rabbit retina caused a reduction in cone numbers and led to a grossly disorganized outer retina.

Guinea pig horizontal cells express GABA, the GABA-synthesizing enzyme GAD 65, and the GABA vesicular transporter

Gamma-aminobutyric acid (GABA) is likely expressed in horizontal cells of all species, although conflicting physiological findings have led to considerable controversy regarding its role as a transmitter in the outer retina. This study has evaluated key components of the GABA system in the outer retina of guinea pig, an emerging retinal model system. The presence of GABA, its rate-limiting synthetic enzyme glutamic acid decarboxylase (GAD(65) and GAD(67) isoforms), the plasma membrane GABA transporters (GAT-1 and GAT-3), and the vesicular GABA transporter (VGAT) was evaluated by using immunohistochemistry with well-characterized antibodies. The presence of GAD(65) mRNA was also evaluated by using laser capture microdissection and reverse transcriptase-polymerase chain reaction. Specific GABA, GAD(65), and VGAT immunostaining was localized to horizontal cell bodies, as well as to their processes and tips in the outer plexiform layer. Furthermore, immunostaining of retinal whole mounts and acutely dissociated retinas showed GAD(65) and VGAT immunoreactivity in both A-type and B-type horizontal cells. However, these cells did not contain GAD(67), GAT-1, or GAT-3 immunoreactivity. GAD(65) mRNA was detected in horizontal cells, and sequencing of the amplified GAD(65) fragment showed approximately 85% identity with other mammalian GAD(65) mRNAs. These studies demonstrate the presence of GABA, GAD(65), and VGAT in horizontal cells of the guinea pig retina, and support the idea that GABA is synthesized from GAD(65), taken up into synaptic vesicles by VGAT, and likely released by a vesicular mechanism from horizontal cells.

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.

Connexin57 is expressed in dendro-dendritic and axo-axonal gap junctions of mouse horizontal cells and its distribution is modulated by light

Mouse horizontal cells are coupled by gap junctions composed of connexin57. These gap junctions are regulated by ambient light via multiple neuromodulators including dopamine. In order to analyze the distribution and structure of horizontal cell gap junctions in the mouse retina, and examine the effects of light adaptation on gap junction density, we developed antibodies that detect mouse retinal connexin57. Using immunohistochemistry in retinal slices, flat-mounted retinas, and dissociated retinal cells, we showed that connexin57 is expressed in the dendrites and axon terminal processes of mouse horizontal cells. No staining was found in retinas of connexin57-deficient mice. Significantly more connexin57-positive puncta were found in the distal than in the proximal outer plexiform layer, indicating a higher level of expression in axon terminal processes than in the dendrites. We also examined the gap junctions using immunoelectron microscopy and showed that connexin57 does not form hemichannels in the horizontal cell dendritic tips. Light adaptation resulted in a significant increase in the number of connexin57-immunoreactive plaques in the outer plexiform layer, consistent with previously reported effects of light adaptation on connexin57 expression in the mouse retina. This study shows for the first time the detailed location of connexin57 expression within mouse horizontal cells, and provides the first ultrastructural data on mouse horizontal cell gap junctions.

GABA agonists fail to protect the retina from ischemia-reperfusion injury

The purpose of this study was to test the hypothesis that ischemia/reperfusion injury in the rat retina may be ameliorated by reducing retinal metabolism with either hypothermia or inhibitory GABA agonists. The intraocular pressure of each right eye in rats was raised to 130 mm Hg for 60 min with the left eye serving as normal control. The rats were divided into four groups in terms of drug and hypothermia treatment: (1) Untreated ischemia, (2) Hypothermia, (3) Baclofen/midazolam and (4) Baclofen/muscimol. Electroretinogram was recorded before ischemia and again after 10-day reperfusion. Histological analysis with H&E staining and cell counts was performed. Untreated ischemia/reperfusion resulted in severely reduced ERG responses. The ERG b-wave was reduced from 423+/-144 microV to 130+/-91 microV (mean+/-SD, n=5). With hypothermia the ERG b-wave was reduced from 499+/-80 microV to 237+/-111 microV (n=4). With combinations of baclofen and midazolam the ERG b-wave was reduced from 432+/-96 microV to 104+/-67 microV (n=7). In baclofen/muscimol treated eyes the ERG b-wave went from 426+/-101 microV to 148+/-118 microV (n=6). The histological tissue damage was severe in untreated ischemia and the baclofen/midazolam and baclofen/muscimol groups, but less severe in the hypothermia group. The GABA agonists do not provide any protection in our ischemia/reperfusion model. Our results are consistent with earlier reports that hypothermia may be helpful in ischemic conditions in the retina.

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.

Spatial and temporal distribution patterns of Na-K-2Cl cotransporter in adult and developing mouse retinas

The Na-K-2Cl cotransporter (NKCC) is a Cl(-) uptake transporter that is responsible for maintaining a Cl(-) equilibrium potential positive to the resting potential in neurons. If NKCC is active, GABA and glycine can depolarize neurons. In view of the abundance of GABAergic and glycinergic synapses in retina, we undertook a series of studies using immunocytochemical techniques to determine the distribution of NKCC in retinas of both developing and adult mice. We found NKCC antibody (T4) labeling present in retinas from wild-type mice, but not in NKCC1-deficient mice, suggesting that the NKCC1 subtype is a major Cl(-) uptake transporter in mouse retina. Strong labeling of NKCC1 was present in horizontal cells and rod-bipolar dendrites in adult mice. Interestingly, we also found that a diffuse labeling pattern was present in photoreceptor terminals. However, NKCC1 was barely detectable in the inner retina of adult mice. Using an antibody against K-Cl cotransporter 2 (KCC2), we found that KCC2, a transporter that extrudes Cl(-), was primarily expressed in the inner retina. The expression of NKCC1 in developing mouse retinas was studied from postnatal day (P) 1 to P21, NKCC1 labeling first appeared in the dendrites of horizontal and rod-bipolar cells as early as P7, followed by photoreceptor terminals between P10-P14; with expression gradually increasing concomitantly with the growth of synaptic terminals and dendrites throughout retinal development. In the inner retina, NKCC1 labeling was initially observed in the inner plexiform layer at P1, but labeling diminished after P5. The developmental increase in NKCC expression only occurred in the outer retina. Our results suggest that the distal synapses and synaptogenesis in mouse retinas undergo a unique process with a high intracellular Cl(-) presence due to NKCC1 expression.

Glycine receptor subunit composition alters the action of GABA antagonists

GABA receptor antagonists produce an unexpectedly significant inhibition of native glycine receptors in retina and in alpha1 or alpha2 homomeric glycine receptors (GlyRs) expressed in HEK 293 cells. In this study we evaluate this phenomenon in heteromeric glycine receptors, formed by mixing alpha1, alpha2, and beta subunits. Picrotoxinin, picrotin, SR95531, and bicuculline are all more effective antagonists at GlyRs containing alpha2 subunits than alpha1 subunits. Inclusion of beta subunits reduces the inhibitory potency of picrotoxinin and picrotin but increases the potency of SR95531 and bicuculline. As a result of these two factors, bicuculline is particularly poor at discriminating GABA and glycine receptors. Picrotin, which has been reported to be inactive at GABA receptors, blocks glycine currents in retina and in HEK293 cells, suggesting its utility as a selective glycine antagonist. However, picrotin is a more potent inhibitor of GABA than glycine in retinal neurons. We also tested if GABA and glycine receptor subunits can combine to form functional receptors. If GABAAR gamma2S subunits are co-expressed with GlyR alpha subunits, the mixed receptor is glycine-sensitive and GABA-insensitive. But the mixed receptor exhibits a non-competitive picrotoxinin inhibition that is not observed in the homomeric GlyRs. This suggests that glycine and GABA subunits can co-assemble to form functional glycine receptors.

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.

Horizontal cell receptive fields are reduced in connexin57-deficient mice

Horizontal cells are coupled by gap junctions; the extensive coupling of the horizontal cells is reflected in their large receptive fields, which extend far beyond the dendritic arbor of the individual cell. In the mouse retina, horizontal cells express connexin57 (Cx57). Tracer coupling of horizontal cells is impaired in Cx57-deficient mice, which suggests that the receptive fields of Cx57-deficient horizontal cells might be similarly reduced. To test this hypothesis we measured the receptive fields of horizontal cells from wildtype and Cx57-deficient mice. First, we examined the synaptic connections between horizontal cells and photoreceptors: no major morphological alterations were found. Moreover, horizontal cell spacing and dendritic field size were unaffected by Cx57 deletion. We used intracellular recordings to characterize horizontal cell receptive fields. Length constants were computed for each cell using the cell's responses to concentric light spots of increasing diameter. The length constant was dependent on the intensity of the stimulus: increasing stimulus intensity reduced the length constant. Deletion of Cx57 significantly reduced horizontal cell receptive field size. Dark resting potentials were strongly depolarized and response amplitudes reduced in Cx57-deficient horizontal cells compared to the wildtype, suggesting an altered input resistance. This was confirmed by patch-clamp recordings from dissociated horizontal cells; mean input resistance of Cx57-deficient horizontal cells was 27% lower than that of wildtype cells. These data thus provide the first quantification of mouse horizontal cell receptive field size and confirm the unique role of Cx57 in horizontal cell coupling and physiology.

The cytoplasmic domain of NrCAM binds to PDZ domains of synapse-associated proteins SAP90/PSD95 and SAP97

NrCAM, a member of the L1 family of cell adhesion molecules, serves important functions during the development of the nervous system, e.g. in adhesion-dependent processes such as neurite outgrowth and axonal pathfinding. Complex homo- and heterophilic binding and several extracellular ligands of NrCAM have been described, but less is known about intracellular interaction partners. The cytoplasmic carboxy-terminus of NrCAM contains a typical sequence motif for binding to PDZ domains, making interactions with PDZ domain-containing scaffolding proteins quite conceivable. In this study, we identified specific interactions of the intracellular domain of NrCAM with class I PDZ domains of the membrane-associated guanylate kinases SAP90/PSD95 and SAP97. In contrast to NrCAM, the intracellular domains of the other mammalian L1 family molecules, e.g. L1, CHL1 and Neurofascin, did not interact with these PDZ domains. In transfected COS-7 cells, NrCAM-mediated recruitment of SAP97 to the plasma membrane was dependent on the PDZ binding motif. We show that NrCAM and SAP97 are colocalized, e.g. within photoreceptor terminals of the mammalian retina. In summary, our results confirm a functional PDZ domain binding motif at the carboxy-terminus of NrCAM and support potential functions of NrCAM during the assembly of highly organized protein complexes at the cell membrane.

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

Horizontal cells modulate the output of the photoreceptor to bipolar cell synapse, thereby providing the first level of lateral information processing in the vertebrate retina. Because horizontal cells do not generate sodium-based action potentials, calcium is likely to play an important role for graded potential changes as well as for intracellular events involved in the modulatory role of horizontal cells within the retinal network. Therefore we wanted to determine how the activation of glutamate receptors, voltage-gated calcium channels, and release of calcium from internal stores shape the calcium signal in horizontal cells. All horizontal cells responded to depolarizing voltage steps with sustained inward currents, which activated at around –20 mV, reached a peak amplitude of –79.1 pA at 5 mV, and reversed sign at around 66 mV. The current was insensitive to tetrodotoxin, and it was partially blocked by the L-type channel antagonists verapamil and nifedipine. The N-type channel blocker ω-conotoxin GVIA induced an additional reduction of current amplitudes. Calcium influx through ionotropic glutamate receptors was mediated by both AMPA and kainate but not by N-methyl-d-aspartate receptors. Two agonists at group I metabotropic glutamate receptor, trans-1-amino-1,3-cyclopentanedicarboxylic acid and quisqualate, had no effect. However, intracellular calcium was increased by caffeine, indicating release of calcium from internal stores via ryanodine receptors. These data show that intracellular calcium in horizontal cells is regulated by voltage-dependent L- and N-type calcium channels, ionotropic AMPA and kainate receptors, and release of calcium from internal stores after activation of ryanodine receptors.

A local GABAergic system within rat trigeminal ganglion cells

We investigated the GABAergic system within the Sprague-Dawley rat (2-3-weeks old) trigeminal ganglion (TG). Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed expression of glutamate decarboxylase (GAD) 65 and GAD67 mRNAs and mRNAs encoding GABA(A) receptor subunits alpha1-6, beta1-3, gamma1-3, and delta. In situ hybridization revealed that GAD65 and GAD67 mRNAs were expressed in neuronal cell bodies but not satellite cells. Immunohistochemical analysis showed that only GAD65 was expressed in all neuronal cell bodies, and approximately 70% of all neurons exhibited GABA immunoreactivity. Satellite cells were strongly immunopositive for GABA. GABA(A) receptor alpha1, alpha5, beta2/3 and gamma1/2/3 subunit immunoreactivities were observed in the majority of neurons, but no immunoreactivity for alpha2 was observed. Two types of cells were identified in TG based on cell size and morphology, type A and B. The percentage of cells expressing alpha3, alpha4, alpha6, and delta subunits appeared to be dependent on cell size, as delta and alpha6 expression were only observed in small (B-type) neurons. In whole-cell patch clamp experiments, GABA application induced inward Cl- currents in all neurons examined. The EC50 for GABA varied from 5.3 to 240 microm, and the Hill Coefficient (nH) varied between 0.98 and 2.6 at -60 mV. We found that GABA was released from TG cells by increasing extracellular K+ concentration to 100 mm. We speculate that GABA acts as a nonsynaptically released diffusible neurotransmitter, which may modulate somatic inhibition of neurons within the TG.

Synaptic transmission at retinal ribbon synapses

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.

Cellular distribution and subcellular localization of molecular components of vesicular transmitter release in horizontal cells of rabbit retina

The mechanism underlying transmitter release from retinal horizontal cells is poorly understood. We investigated the possibility of vesicular transmitter release from mammalian horizontal cells by examining the expression of synaptic proteins that participate in vesicular transmitter release at chemical synapses. Using immunocytochemistry, we evaluated the cellular and subcellular distribution of complexin I/II, syntaxin-1, and synapsin I in rabbit retina. Strong labeling for complexin I/II, proteins that regulate a late step in vesicular transmitter release, was found in both synaptic layers of the retina, and in somata of A- and B-type horizontal cells, of gamma-aminobutyric acid (GABA)- and glycinergic amacrine cells, and of ganglion cells. Immunoelectron microscopy demonstrated the presence of complexin I/II in horizontal cell processes postsynaptic to rod and cone ribbon synapses. Syntaxin-1, a core protein of the soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) complex known to bind to complexin, and synapsin I, a synaptic vesicle-associated protein involved in the Ca(2+)-dependent recruitment of synaptic vesicles for transmitter release, were also present in the horizontal cells and their processes at photoreceptor synapses. Photoreceptors and bipolar cells did not express any of these proteins at their axon terminals. The presence of complexin I/II, syntaxin-1, and synapsin I in rabbit horizontal cell processes and tips suggests that a vesicular mechanism may underlie transmitter release from mammalian horizontal cells.

Effects of GABA receptor antagonists on retinal glycine receptors and on homomeric glycine receptor alpha subunits

Glycinergic and GABAergic inhibition are juxtaposed at one retinal synaptic layer yet likely perform different functions. These functions have usually been evaluated using receptor antagonists. In examining retinal glycine receptors, we were surprised to find that commonly used concentrations of GABA antagonists blocked significant fractions of the glycine current. In retinal amacrine and ganglion cells, the competitive GABAA receptor antagonists (bicuculline and SR95531) were also competitive GlyR antagonists. Picrotoxinin produced a noncompetitive inhibition of retinal GlyRs. [1,2,5,6-tetrahydropyridine-4-yl] methylphosphinic acid, the GABACR antagonist, did not inhibit glycine receptors. All three GABAA receptor antagonists were competitive inhibitors of homomeric alpha1 or alpha2 GlyRs expressed in human embryonic kidney cells (HEK293) cells. Interestingly, bicuculline was much more effective at alpha2 GlyRs and might be used to separate glycine receptor subtypes. Thus commonly used concentrations of GABA antagonists do not unambiguously differentiate GABA and glycine pathways. Picrotoxinin inhibition of GABAC receptors requires two amino acids in the second transmembrane region (TM2): 2' serine and 6' threonine. Although TM2 regions in GABA and glycine receptors are highly homologous, neither 2' serine nor 6' threonine is essential for picrotoxinin sensitivity in glycine receptors.