GABA-like immunoreactivity in the cat retina: electron microscopy

Profile of miR-23 Expression and Possible Role in Regulation of Glutamic Acid Decarboxylase during Postnatal Retinal Development

As neurotransmitter, GABA is fundamental for physiological processes in the developing retina. Its synthesis enzymes are present during retinal development, although the molecular regulatory mechanisms behind the changes in expression are not entirely understood. In this study, we revealed the expression patterns of glutamic acid decarboxylase 67(GAD67) and its coding gene (GAD1) and its potential miRNA-dependent regulation during the first three postnatal weeks in rat retina. To gain insight into the molecular mechanisms, miRNA-sequencing supported by RT-qPCR and in situ hybridization were carried out. GAD1 expression shows an increasing tendency, peaking at P15. From the in silico-predicted GAD1 targeting miRNAs, only miR-23 showed similar expression patterns, which is a known regulator of GAD1 expression. For further investigation, we made an in situ hybridization investigation where both GAD67 and miR-23 also showed lower expression before P7, with the intensity of expression gradually increasing until P21. Horizontal cells at P7, amacrine cells at P15 and P21, and some cells in the ganglion cell layer at several time points were double labelled with miR-23 and GAD67. Our results highlight the complexity of these regulatory networks and the possible role of miR-23 in the regulation of GABA synthesizing enzyme expression during postnatal retina development.

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.

Glycinergic and GABAergic interneurons shift the location and differentially alter the size of ganglion cell receptive field centers in the mammalian retina

By using the multi-electrode array (MEA) recording technique in conjunction with white-noise checkerboard stimuli and reverse correlation methods, we studied modulatory actions of glycinergic and GABAergic interneurons on spatiotemporal profiles of ganglion cells (GCs) in dark-adapted mouse retinas. We found that application of 2 µM strychnine decreased receptive field center radii of GCs by a mean value of 11%, and shifted the GC receptive field (RF) centers by a mean distance of 28.3 µm. On the other hand, 200 µM picrotoxin + 100 µM bicuculline + 50 µM TPMPA increased GC receptive field center radii by a mean value of 19%, and shifted the GC RF centers by a mean distance of 53.7 µm. Glycinergic neurons in the mouse retina are narrow-field amacrine cells that have been shown to mediate ON-OFF crossover inhibitory synapses within the RGs' RF center, therefore they may increase the size and shift the location of GC RF center by synergistic addition to bipolar cell inputs to GCs. GABAergic neurons are wide-field amacrine cells and horizontal cells that are known to mediate antagonistic surround responses of GCs, and thus they decrease the GCs' RF center size. Our results suggest that a major global function of glycinergic and GABAergic interneurons in the mammalian retina is to provide the flexibility for adjusting the size and location of GCs' RF centers. The apparent shifts of GC RF centers suggest that the synergistic addition by GlyACs and the surround inhibition by GABAergic interneurons are not spatially symmetrical within GC RFs.

The Synaptic and Morphological Basis of Orientation Selectivity in a Polyaxonal Amacrine Cell of the Rabbit Retina

Much of the computational power of the retina derives from the activity of amacrine cells, a large and diverse group of GABAergic and glycinergic inhibitory interneurons. Here, we identify an ON-type orientation-selective, wide-field, polyaxonal amacrine cell (PAC) in the rabbit retina and demonstrate how its orientation selectivity arises from the structure of the dendritic arbor and the pattern of excitatory and inhibitory inputs. Excitation from ON bipolar cells and inhibition arising from the OFF pathway converge to generate a quasi-linear integration of visual signals in the receptive field center. This serves to suppress responses to high spatial frequencies, thereby improving sensitivity to larger objects and enhancing orientation selectivity. Inhibition also regulates the magnitude and time course of excitatory inputs to this PAC through serial inhibitory connections onto the presynaptic terminals of ON bipolar cells. This presynaptic inhibition is driven by graded potentials within local microcircuits, similar in extent to the size of single bipolar cell receptive fields. Additional presynaptic inhibition is generated by spiking amacrine cells on a larger spatial scale covering several hundred microns. The orientation selectivity of this PAC may be a substrate for the inhibition that mediates orientation selectivity in some types of ganglion cells. Significance statement: The retina comprises numerous excitatory and inhibitory circuits that encode specific features in the visual scene, such as orientation, contrast, or motion. Here, we identify a wide-field inhibitory neuron that responds to visual stimuli of a particular orientation, a feature selectivity that is primarily due to the elongated shape of the dendritic arbor. Integration of convergent excitatory and inhibitory inputs from the ON and OFF visual pathways suppress responses to small objects and fine textures, thus enhancing selectivity for larger objects. Feedback inhibition regulates the strength and speed of excitation on both local and wide-field spatial scales. This study demonstrates how different synaptic inputs are regulated to tune a neuron to respond to specific features in the visual scene.

Neurotransmission plays contrasting roles in the maturation of inhibitory synapses on axons and dendrites of retinal bipolar cells

Neuronal output is modulated by inhibition onto both dendrites and axons. It is unknown whether inhibitory synapses at these two cellular compartments of an individual neuron are regulated coordinately or separately during in vivo development. Because neurotransmission influences synapse maturation and circuit development, we determined how loss of inhibition affects the expression of diverse types of inhibitory receptors on the axon and dendrites of mouse retinal bipolar cells. We found that axonal GABA but not glycine receptor expression depends on neurotransmission. Importantly, axonal and dendritic GABAA receptors comprise distinct subunit compositions that are regulated differentially by GABA release: Axonal GABAA receptors are down-regulated but dendritic receptors are up-regulated in the absence of inhibition. The homeostatic increase in GABAA receptors on bipolar cell dendrites is pathway-specific: Cone but not rod bipolar cell dendrites maintain an up-regulation of receptors in the transmission deficient mutants. Furthermore, the bipolar cell GABAA receptor alterations are a consequence of impaired vesicular GABA release from amacrine but not horizontal interneurons. Thus, inhibitory neurotransmission regulates in vivo postsynaptic maturation of inhibitory synapses with contrasting modes of action specific to synapse type and location.

Drift-diffusion simulation of the ephaptic effect in the triad synapse of the retina

Experimental evidence suggests the existence of a negative feedback pathway between horizontal cells and cone photoreceptors in the outer plexiform layer of the retina that modulates the flow of calcium ions into the synaptic terminals of cones. However, the underlying mechanism for this feedback is controversial and there are currently three competing hypotheses: the ephaptic hypothesis, the pH hypothesis, and the GABA hypothesis. The goal of this investigation is to demonstrate the ephaptic hypothesis by means of detailed numerical simulations. The drift-diffusion (Poisson-Nernst-Planck) model with membrane boundary current equations is applied to a realistic two-dimensional cross-section of the triad synapse in the goldfish retina to verify the existence of strictly electrical feedback, as predicted by the ephaptic hypothesis. The effect on electrical feedback from the behavior of the bipolar cell membrane potential is also explored. The computed steady-state cone calcium transmembrane current-voltage curves for several cases are presented and compared with experimental data on goldfish. The results provide convincing evidence that an ephaptic mechanism can produce the feedback effect seen in experiments. The model and numerical methods presented here can be applied to any neuronal circuit where dendritic spines are invaginated in presynaptic terminals or boutons.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

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.

The GABAergic system in the retina of neonate and adult Octodon degus, studied by immunohistochemistry and electroretinography

In the vertebrate retina, gamma-aminobutyric acid (GABA) mediates inhibitory processes that shape the visual response and is also thought to have neurotrophic functions during retinal development. To investigate the role of GABAergic signaling at the beginning of visual experience, we used immunohistochemistry to compare the distribution of GABA, the two isoforms of glutamic acid decarboxylase GAD65/67, and the GABA receptor types A, B, and C, in neonate versus adult Octodon degus, a native South American rodent with diurnal-crepuscular activity and a high cone-to-rod ratio. In parallel, we used electroretinography to evaluate retinal functionality and to test the contribution of fast GABAergic transmission to light responses at both developmental stages. Neonate O. degus opened their eyes on postnatal day (P)0 and displayed an adult-like retinal morphology at this time. GABA, its biosynthetic sources, and receptors had a similar cellular distribution in neonates and adults, but labeling of the outer plexiform layer and of certain amacrine and ganglion cells was more conspicuous at P0. In neonates, retinal sensitivity was 10 times lower than in adults, responses to ultraviolet light could not be detected, and oscillatory potentials were reduced or absent. Blockade of GABA(A/C) receptors by bicuculline and TPMPA had no noticeable effect in neonates, while it significantly altered the electroretinogram response in adults.

CONCLUSION

In spite of modest differences regarding retinal morphology and GABAergic expression, overall light response properties and GABAergic signaling are undeveloped in neonate O. degus compared to adults, suggesting that full retinal functionality requires a period of neural refinement under visual experience.

Neuronal and glial cell expression of angiotensin II type 1 (AT1) and type 2 (AT2) receptors in the rat retina

The bio-active peptide, angiotensin II (Ang II), has been suggested to exert a neuromodulatory effect on inner retinal neurons. In this study, we examined the distribution of angiotensin receptors (ATRs) in the developing and mature rat retina and optic nerve using immunofluorescence immunocytochemistry. Double-labeling experiments were performed with established markers to identify different retinal cell populations. In adult retinae, ATRs were observed on neurons involved in "ON" pathways of neurotransmission. Angiotensin II type 1 receptors (AT(1)Rs) were expressed by a sub-population of "ON" cone bipolar cells that also labeled for G alpha(0) and islet-1. Extra-neuronal expression of AT(1)Rs was evident on retinal astrocytes, Müller cells and blood vessels. Immunoreactivity for the angiotensin II type 2 receptor (AT(2)R) was observed on conventional and displaced GABAergic amacrine cells. Co-localization studies showed that AT(2)R-expressing amacrine cells constituted at least two separate sub-populations. Cell counts revealed that all wide-field amacrine cells expressing protein kinase C-alpha were also AT(2)R-positive; a further subset of amacrine cells expressing AT(2)Rs and stratifying in sublamina "b" of the inner plexiform layer (IPL) was identified. Developmental expression of AT(1)Rs was dynamic, involving multiple inner neuronal classes. At postnatal day 8 (P8), AT(1)R immunoreactivity was observed on putative ganglion cells. The characteristic bipolar cell labeling observed in adults was not evident until P13. In contrast, AT(2)Rs were detected as early as P2 and localized specifically to amacrine cells from this age onward. These data provide further evidence for the potential role of angiotensin II in the modulation of retinal neurons and glia. The differential pattern of expression of these receptors across these cell types is similar to that observed in the brain and suggests that a similar functional role for Ang II may also exist within the retina.

Diverse mechanisms underlie glycinergic feedback transmission onto rod bipolar cells in rat retina

Synaptic inhibition shapes visual signaling in the inner retina, but the physiology of most amacrine cells, the interneurons that mediate this inhibition, is poorly understood. Discerning the function of most individual amacrine cell types is a daunting task, because few molecular or morphological markers specifically distinguish between approximately two dozen different amacrine cell types. Here, we examine a functional subset of amacrine cells by pharmacologically isolating glycinergic inhibition and evoking feedback IPSCs in a single cell type, the rod bipolar cell (RBC), with brief glutamate applications in the inner plexiform layer. We find that glycinergic amacrine cells innervating RBCs receive excitatory inputs from ON and OFF bipolar cells primarily via NMDA receptors (NMDARs) and Ca2+-impermeable AMPA-type glutamate receptors. Glycine release from amacrine cells is triggered by Ca2+ influx through both voltage-gated Ca2+ (Ca(v)) channels and NMDARs. These intracellular Ca2+signals are amplified by Ca2+-induced Ca2+ release via both ryanodine and IP3 receptors, which are activated independently by Ca2+ influx through Ca(v) channels and NMDARs, respectively. Glycinergic feedback signaling depends strongly, although not completely, on voltage-gated Na+ channels, and the spatial extent of feedback inhibition is expanded by gap junction connections between glycinergic amacrine cells. These results indicate that a diversity of mechanisms underlie glycinergic feedback inhibition onto RBCs, yet they highlight several physiological themes that appear to distinguish amacrine cell function.

Relative contributions of rod and cone bipolar cell inputs to AII amacrine cell light responses in the mouse retina

AII amacrine cells (AIIACs) are crucial relay stations for rod-mediated signals in the mammalian retina and they receive synaptic inputs from depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) as well as from other amacrine cells. Using whole-cell voltage-clamp technique in conjunction with pharmacological tools, we found that the light-evoked current response of AIIACs in the mouse retina is almost completely mediated by two DBC synaptic inputs: a 6,7-dinitro-quinoxaline-2,3-dione (DNQX)-resistant component mediated by cone DBCs (DBC(C)s) through an electrical synapse, and a DNQX-sensitive component mediated by rod DBCs (DBC(R)s). This scheme is supported by AIIAC current responses recorded from two knockout mice. The dynamic range of the AIIAC light response in the Bhlhb4-/- mouse (which lacks DBC(R)s) resembles that of the DNQX-resistant component, and that of the connexin36 (Cx36)-/- mouse resembles the DNQX-sensitive component. By comparing the light responses of the DBC(C)s with the DNQX-resistant AIIAC component, and light responses of the DBC(R)s with the DNQX-sensitive AIIAC component, we obtained the input-output relations of the DBC(C)-->AIIAC electrical synapse and the DBC(R)-->AIIAC chemical synapse. Similar to other glutamatergic chemical synapses in the retina, the DBC(R)-->AIIAC synapse is non-linear. Its highest voltage gain (approximately 5) is found near the dark membrane potential, and it saturates for presynaptic signals larger than 5.5 mV. The DBC(C)-->AIIAC electrical synapse is approximately linear (voltage gain of 0.92), consistent with the linear junctional conductance found in retinal electrical synapses. Moreover, relative DBC(R) and DBC(C) contributions to the AIIAC response at various light intensity levels are determined.

Changes in Localization of Amino Acids in the Detached Cat Retina

PURPOSE

To investigate the distribution of amino acids (glutamate, aspartate, glutamine, GABA, glycine) in detached retinas with minimum postmortem artifact and to clarify the relation between amino acid distribution and histopathological change in the outer portion of detached retinas.

METHODS

Unilateral retinal detachment was produced in cats by injecting 0.25% sodium hyaluronate into the subretinal space using a glass micropipet. The eyes were fixed by perfusion for 10 min, 1, 3, 6 and 24 h, 2, 3 and 7 days after detachment and then examined under conventional light- and electron-microscopic immunocytochemistry.

RESULTS

For glutamate, aspartate and glutamine, the inner segments and perikarya of the photoreceptor cells, which were not immunopositive in the normal retinas, showed various degrees of immunoreactivity immediately after retinal detachment. Photoreceptor cells with the strong immunoreactivity developed necrosis. The staining pattern of GABA and glycine scarcely changed during the course of retinal detachment.

CONCLUSIONS

Excess intracellular glutamate, aspartate and glutamine in photoreceptor cells may cause a part of neuronal death after retinal detachment.

Effect of neurosteroids on the retinal gabaergic system and electroretinographic activity in the golden hamster

It has been established that neurosteroids can either inhibit or enhance GABA(A) receptor activity. Although GABA is the main inhibitory neurotransmitter in the mammalian retina, the effects of neurosteroids on retinal GABAergic activity have not been investigated. The aim of this work was to study the neurochemical and electroretinographic effects of neurosteroids in the golden hamster. On one hand, pregnenolone sulfate inhibited and allotetrahydrodeoxycorticosterone increased GABA-induced [36Cl]- uptake in neurosynaptosomes. On the other hand, in whole retinas, pregnenolone sulfate increased, whereas allotetrahydrodeoxycorticosterone decreased high potassium-induced [3H]GABA release. The effect of both neurosteroids on GABA release was Ca2+-dependent, as in its absence release was not altered. The intravitreal injection of pregnenolone sulfate or vigabatrin (an irreversible inhibitor of GABA degradation) significantly decreased scotopic b-wave amplitude, whereas the opposite effect was evident when bicuculline or allotetrahydrodeoxycorticosterone were injected. A protein with a molecular weight close to that of hamster adrenal cytochrome P450 side-chain cleavage (P450scc) was detected in the hamster retina. P450scc-like immunoreactivity was localized in the inner nuclear and the ganglion cell layers. These results indicate that neurosteroids significantly modulate retinal GABAergic neurotransmission and electroretinographic activity. In addition, the selective localization of P450scc suggests that neurosteroid biosynthesis might occur only in some layers of the hamster retina.

Synaptic organization of GABAergic amacrine cells in the salamander retina

The synaptic organization of GABA-immunoreactive (GABA-IR) amacrine cells in the inner plexiform layer (IPL) of salamander retina was studied with the use of postembedding immuno-electron microscopy. A total of 457 GABA-IR amacrine synapses, with identified postsynaptic elements, were analyzed on photomontages of electron micrographs covering 3,618 microm2 of the IPL. GABA-IR amacrine synapses were distributed throughout the IPL, with a small peak at the proximal margin of sublamina a. The majority of the output targets (81%) were GABA(-) neurons. Most of the contacts were simple synapses with one postsynaptic element identified as a process of an amacrine cell (55%), bipolar cell (19%) or ganglion cell (26%), and serial synapses were very rare. Of the 89 postsynaptic bipolar terminals, 63% participated in a reciprocal feedback synapse with the same presynaptic GABA-IR amacrine profile. There appeared to be no preference between GABA-IR amacrine contacts with rod- or cone-dominated bipolar cells (9.1% vs. 8.9%) or in the total number of amacrine synapses in sublaminas a and b (52% vs. 47%). The preponderance of amacrine cell input to bipolar cells in the OFF layer was derived from GABA-IR cells. These findings provide ultrastructural support to the existing physiological studies regarding the functional roles of the GABAergic amacrine cells in this species. Our results have added to the data base demonstrating that, in contrast to mammals, GABA-IR amacrine cells in amphibians and other nonmammals contact other amacrine cells more frequently, suggesting greater involvement of GABAergic amacrine cells in modulating lateral inhibitory pathways.

Identification and characterization of an aquaporin 1 immunoreactive amacrine-type cell of the mouse retina

Using immunocytochemistry, a type of amacrine cell that is immunoreactive for aquaporin 1 was identified in the mouse retina. AQP1 immunoreactivity was found in photoreceptor cells of the outer nuclear layer (ONL) and in a distinct type of amacrine cells of the inner nuclear layer (INL). AQP1-immunoreactive (IR) amacrine cell somata were located in the INL and their processes extended through strata 3 and 4 of the inner plexiform layer (IPL) with thin varicosities. The density of the AQP1-IR amacrine cells increased from 100/mm(2) in the peripheral retina to 350/mm(2) in the central retina. The AQP1-IR amacrine cells comprise 0.5% of the total amacrine cells. The AQP1-IR amacrine cell bodies formed a regular mosaic, which suggested that they represent a single type of amacrine cell. Double labeling with AQP1 and glycine, gamma-aminobutyric acid (GABA) or GAD(65) antiserum demonstrated that the AQP1-IR amacrine cells expressed GABA or GAD(65) but not glycine. Their synaptic input was primarily from other amacrine cell processes. They also received synaptic inputs from a few cone bipolar cells. The primary synaptic targets were ganglion cells, followed by other amacrine cells and cone bipolar cells. In addition, gap junctions between an AQP1-IR amacrine process and another amacrine process were rarely observed. In summary, a GABAergic amacrine cell type labeled by an antibody against AQP1 was identified in the mouse retina and was found to play a possible role in transferring a certain type of visual information from other amacrine or a few cone bipolar cells primarily to ganglion cells.

Evidence that certain retinal bipolar cells use both glutamate and GABA

Retinal bipolar neurons release the excitatory transmitter, glutamate. However, certain bipolar cells contain GABA, raising the question whether a neuron might release both transmitters and, if so, what function might the inhibitory transmitter play in a particular circuit? Here we identify a subset of cone bipolar cells in cat retina that contain glutamate, plus its vesicular transporter (VGLUT1), and GABA, plus its synthetic enzyme (GAD(65)) and its vesicular transporter (VGAT). These cells are negative for a marker of ON bipolar cells and restrict their axons to the OFF strata of the inner synaptic layer. They do not colocalize with the neurokinin 3 receptor that stains a type (or two) of OFF bipolar cells. By "targeted injection," we identified two types of OFF bipolar cell with the machinery to make and package both transmitters. One of these types costratifies with a dopamine plexus.

Morphological analysis of disabled-1-immunoreactive amacrine cells in the guinea pig retina

Disabled-1 (Dab1) is an adapter molecule in a signaling pathway, stimulated by reelin, that controls cell positioning in the developing brain. It localizes to selected neurons in the nervous system, including the retina, and Dab1-like immunoreactivity is present in AII amacrine cells in the mouse retina. This study was conducted to characterize Dab1-labeled cells in the guinea pig retina in detail using immunocytochemistry, quantitative analysis, and electron microscopy. Dab1 immunoreactivity is present in a class of amacrine cell bodies located in the inner nuclear layer adjacent to the inner plexiform layer (IPL). These cells give rise to processes that ramify the entire depth of the IPL. Double-labeling experiments demonstrated that these amacrine cells make contacts with the axon terminals of rod bipolar cells and that their processes make contacts with each other via connexin 36 in sublamina b of the IPL. In addition, all Dab1-labeled amacrine cells showed glycine transporter 1 immunoreactivity, indicating that they are glycinergic. The density of Dab1-labeled AII amacrine cells decreased from about 3,750 cells/mm(2) in the central retina to 1,725 cells/mm(2) in the peripheral retina. Dab1-labeled amacrine cells receive synaptic inputs from the axon terminals of rod bipolar cells in stratum 5 of the IPL. From these morphological features, Dab1-labeled cells of the guinea pig retina resemble the AII amacrine cells described in other mammalian species. Thus, the rod pathway of the guinea pig retina follows the general mammalian scheme and Dab1 antisera can be used to identify AII amacrine cells in the mammalian retina.

High-affinity GABA uptake in retinal glial (Müller) cells of the guinea pig: electrophysiological characterization, immunohistochemical localization, and modeling of efficiency

Glial cells may act as important modulators of neuronal information processing, in particular, via fast uptake of neuronally released transmitters. Here, we characterize the electrogenic gamma-aminobutyric acid (GABA) transporters present in the plasma membranes of Müller (glial) cells of the guinea pig retina and present an estimate of their functional efficiency. The GABA-evoked whole-cell currents are voltage-dependent, with increasing amplitudes and decreasing affinity constants at more negative membrane potentials. The transmembranal GABA transport is concentration-dependent, with near-maximal currents at 100 microM GABA, and is dependent on extracellular sodium and chloride ions; the stoichiometry is 1 GABA/2 Na(+)/1 Cl(-). Immunohistochemical labeling and whole-cell voltage-clamp records reveal that Müller cells express both GAT-1 and GAT-3 (but not GAT-2), and that the transporter proteins are expressed predominantly at plasma membrane sites that, in situ, are localized in the outer retina where GABA uptake is performed exclusively by Müller cells. When extracellular GABA enters the cell interior, it evokes, via activation of the GABA transaminase, an NAD(P)H fluorescence signal selectively in the distal region of the Müller cells where their mitochondria are located. Using our experimental data, we simulated the GABA clearance from the extracellular space surrounding one Müller cell; these estimates show that a pulse of 100 microM extracellular GABA is fully cleared after 70 ms. It is suggested that Müller cells may be involved in the regulation of GABAergic transmission within the retina by providing a fast termination of GABAergic signaling via their highly efficient GABA uptake.

GABA transporters regulate inhibition in the retina by limiting GABA(C) receptor activation

Inhibition is mediated by two classes of ionotropic receptors in the retina, GABA(A) and GABA(C) receptors. We used the GABA transport blocker NO-711 to examine the role of GABA transporters in shaping synaptic responses mediated by these two receptors in the salamander retinal slice preparation. Focal applications (puffs) of GABA onto GABA(C) receptors on bipolar cells terminals or GABA(A) receptors on ganglion cells elicited currents that were enhanced by NO-711, demonstrating the presence of transporters in the inner plexiform layer (IPL). IPSCs were evoked in bipolar and ganglion cells by puffing kainate into the IPL. NO-711 enhanced the IPSCs only in bipolar cells, suggesting that, when GABA uptake was blocked, the GABA(C) receptors were more strongly activated by spillover transmission than the GABA(A) receptors on ganglion cells. NO-711 enhanced the light-evoked IPSCs mediated by GABA(C) receptors on bipolar cell axon terminals, which resulted in reduced transmission between bipolar and ganglion cells. NO-711 also shifted the intensity-response relationship of the ganglion cell, reducing its sensitivity to light. Surround illumination has been shown by others to produce similar shifts in ganglion cell light sensitivity. Our results show that GABA transporters limit the extent of inhibitory transmission at the inner retina during light-evoked signal processing.

Vesicular gamma-aminobutyric acid transporter expression in amacrine and horizontal cells

The vesicular gamma-aminobutyric acid (GABA) transporter (VGAT), which transports the inhibitory amino acid transmitters GABA and glycine, is localized to synaptic vesicles in axon terminals. The localization of VGAT immunoreactivity to mouse and rat retina was evaluated with light and electron microscopy by using well-characterized VGAT antibodies. Specific VGAT immunoreactivity was localized to numerous varicose processes in all laminae of the inner plexiform layer (IPL) and to the outer plexiform layer (OPL). Amacrine cell somata characterized by weak VGAT immunoreactivity in the cytoplasm were located in the ganglion cell layer and proximal inner nuclear layer (INL) adjacent to the IPL. In rat retina, VGAT-immunoreactive cell bodies also contained GABA, glycine, or parvalbumin (PV) immunoreactivity, suggesting vesicular uptake of GABA or glycine by these cells. A few varicose VGAT-immunoreactive processes entered the OPL from the IPL. VGAT immunoreactivity in the OPL was predominantly localized to horizontal cell processes. VGAT and calcium binding protein-28K immunoreactivities (CaBP; a marker for horizontal cells) were colocalized in processes and terminals distributed to the OPL. Furthermore, VGAT immunoreactivity overlapped or was immediately adjacent to postsynaptic density-95 (PSD-95) immunoreactivity, which is prominent in photoreceptor terminals. Preembedding immunoelectron microscopy of mouse and rat retinae showed that VGAT immunoreactivity was localized to horizontal cell processes and their terminals. Immunoreactivity was distributed throughout the cytoplasm of the horizontal cell processes. Taken together, these findings demonstrate VGAT immunoreactivity in both amacrine and horizontal cell processes, suggesting these cells contain vesicles that accumulate GABA and glycine, possibly for vesicular release.

Differential expression of GAD(65) and GAD(67) during the development of the rat retina

The development of synthetic enzymes in the GABAergic system (GAD(67) and GAD(65)) of the rat retina was analyzed from birth to the 4th postnatal week by the reverse transcriptase polymerase chain reaction (RT-PCR) and by immunohistochemistry. As previously observed for GABA, immunoreactive GAD(67) profiles are seen clearly in the inner retinal layers at birth. At the end of the 1st week of postnatal life, immunolabeling is detected in amacrine and/or ganglion cells and in horizontal cells. GAD(67) immunoreactivity is transiently expressed in horizontal cells and disappears during the 3rd postnatal week. GAD(65) however does not develop until the 5th postnatal day. Immunolabeling is detected in the processes layering the inner plexiform layer (IPL) before being detected in the amacrine and/or ganglion cell bodies. The appearance of transcripts for GAD coincided with the appearance of the proteins. A transient form of mRNA transcripts of the GAD(67) gene containing an extra exon (ES-exon) is also observed which disappears progressively from birth to the 4th postnatal week. This form synthesizes a truncated, enzymatically inactive protein, which could participate in the regulation of GABA synthesis from glutamate present at high levels during retinogenesis.

Characterization of the spontaneous synaptic activity of amacrine cells in the mouse retina

Amacrine cells are a heterogeneous class of interneurons that modulate the transfer of the light signals through the retina. In addition to ionotropic glutamate receptors, amacrine cells express two types of inhibitory receptors, GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs). To characterize the functional contribution of these different receptors, spontaneous postsynaptic currents (sPSCs) were recorded with the whole cell configuration of the patch-clamp technique in acutely isolated slices of the adult mouse retina. All amacrine cells investigated (n = 47) showed spontaneous synaptic activity. In six amacrine cells, spontaneous excitatory postsynaptic currents could be identified by their sensitivity to kynurenic acid. They were characterized by small amplitudes [mean: -13.7 +/- 1.5 (SE) pA] and rapid decay kinetics (mean tau: 1.35 +/- 0.16 ms). In contrast, the reversal potential of sPSCs characterized by slow decay kinetics (amplitude-weighted time constant, tau(w), >4 ms) was dependent on the intracellular Cl(-) concentration (n = 7), indicating that they were spontaneous inhibitory postsynaptic currents (sIPSCs). In 14 of 34 amacrine cells sIPSCs were blocked by bicuculline (10 microM), indicating that they were mediated by GABA(A)Rs. Only four amacrine cells showed glycinergic sIPSCs that were inhibited by strychnine (1 microM). In one amacrine cell, sIPSCs mediated by GABA(A)Rs and GlyRs were found simultaneously. GABAergic sIPSCs could be subdivided into one group best fit by a monoexponential decay function and another biexponentially decaying group. The mean amplitude of GABAergic sIPSCs (-42.1 +/- 5.8 pA) was not significantly different from that of glycinergic sIPSCs (-28.0 +/- 8.5 pA). However, GlyRs (mean T10/90: 2.4 +/- 0.08 ms) activated significantly slower than GABA(A)Rs (mean T10/90: 1.2 +/- 0.03 ms). In addition, the decay kinetics of monoexponentially decaying GABA(A)Rs (mean tau(w): 20.3 +/- 0.50), biexponentially decaying GABA(A)Rs (mean tau(w): 30.7 +/- 0.95), and GlyRs (mean tau(w) = 25.3 +/- 1.94) were significantly different. These differences in the activation and decay kinetics of sIPSCs indicate that amacrine cells of the mouse retina express at least three types of functionally different inhibitory receptors: GlyRs and possibly two subtypes of GABA(A)Rs.

Circadian activity of the GABAergic system in the golden hamster retina

Daily changes in gamma-aminobutyric acid (GABA) turnover rate were studied in the golden hamster retina. This parameter showed significant variations throughout the light-dark cycle, with minimal values during the day. Retinal glutamic acid decarboxylase (GAD) activity was higher at midnight than at noon. Moreover, [3H]GABA binding significantly varied throughout the 24-h cycle, with maximal values during the day. Saturation studies performed at 12:00 and 24:00 h indicated that the maximal concentration of [3H]GABA binding sites (Bmax) was significantly higher at noon, whereas the dissociation constant (Kd) remained unchanged. High K+-induced GABA release was significantly higher at midnight than at midday. Daily variations in retinal GABA turnover rate, GABA release, and in its specific binding persisted in golden hamsters exposed to constant darkness. In summary, these results support the idea of a circadian clock-controlled GABAergic activity in the hamster retina.

Synaptic currents generating the inhibitory surround of ganglion cells in the mammalian retina

The receptive field (RF) of retinal ganglion cells (RGCs) consists of an excitatory central region, the RF center, and an inhibitory peripheral region, the RF surround. It is still unknown in detail which inhibitory interneurons (horizontal or amacrine cells) and which inhibitory circuits (presynaptic or postsynaptic) generate the RF surround. To study surround inhibition, light-evoked whole-cell currents were recorded from RGCs of the isolated, intact rabbit retina. The RFs were stimulated with light or dark spots of increasing diameters and with annular light stimuli. Direct inhibitory currents could be isolated by voltage clamping ganglion cells close to the Na(+)/K(+) reversal potential. They mostly represent an input from GABAergic amacrine cells that contribute to the inhibitory surround of ganglion cells. This direct inhibitory input and its physiological function were also investigated by recording light-evoked action potentials of RGCs in the current-clamp mode and by changing the intracellular Cl(-) concentration. The excitatory input of the ganglion cells could be isolated by voltage clamping ganglion cells at the Cl(-) reversal potential. Large light spots and annular light stimuli caused a strong attenuation of the excitatory input. Both GABA(A) receptors and GABA(C) receptors contributed to this inhibition, and picrotoxinin was able to completely block it. Together, these results show that the RF surround of retinal ganglion cells is mediated by a combination of direct inhibitory synapses and presynaptic surround inhibition.

Distribution of GABA and glycine receptors on bipolar and ganglion cells in the mammalian retina

The amino acids GABA and glycine mediate synaptic transmission via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of GABA and glycine receptors. In the present article, the distribution of GABA and glycine receptors on identified bipolar and ganglion cell types in the mammalian retina is reviewed. Immunofluorescence obtained with antibodies against GABA and glycine receptors is punctate. Electron microscopy shows that the puncta represent a cluster of receptors at synaptic sites. Bipolar cell types were identified with immunohistochemical markers. Double immunofluorescence with subunit-specific antibodies was used to analyze the distribution of receptor clusters on bipolar axon terminals. The OFF cone bipolar cells seem to be dominated by glycinergic input, whereas the ON cone bipolar and rod bipolar cells are dominated by GABAergic input. Ganglion cells were intracellularly injected with Neurobiotin, visualized with Streptavidin coupled to FITC, and subsequently stained with subunit specific antibodies. The distribution and density of receptor clusters containing the alpha1 subunit of the GABA(A) receptor and the alpha1 subunit of the glycine receptor, respectively, were analyzed on midget and parasol cells in the marmoset (a New World monkey). Both GABA(A) and glycine receptors are distributed uniformly along the dendrites of ON and OFF types of parasol and midget ganglion cells, indicating that functional differences between these subtypes of ganglion cells are not determined by GABA or glycinergic input.

Distinct ionotropic GABA receptors mediate presynaptic and postsynaptic inhibition in retinal bipolar cells

Ionotropic GABA receptors can mediate presynaptic and postsynaptic inhibition. We assessed the contributions of GABA(A) and GABA(C) receptors to inhibition at the dendrites and axon terminals of ferret retinal bipolar cells by recording currents evoked by focal application of GABA in the retinal slice. Currents elicited at the dendrites were mediated predominantly by GABA(A) receptors, whereas responses evoked at the terminals had GABA(A) and GABA(C) components. The ratio of GABA(C) to GABA(A) (GABA(C):GABA(A)) was highest in rod bipolar cell terminals and variable among cone bipolars, but generally was lower in OFF than in ON classes. Our results also suggest that the GABA(C):GABA(A) could influence the time course of responses. Currents evoked at the terminals decayed slowly in cell types for which the GABA(C):GABA(A) was high, but decayed relatively rapidly in cells for which this ratio was low. Immunohistochemical studies corroborated our physiological results. GABA(A) beta2/3 subunit immunoreactivity was intense in the outer and inner plexiform layers (OPL and IPL, respectively). GABA(C) rho subunit labeling was weak in the OPL but strong in the IPL in which puncta colocalized with terminals of rod bipolars immunoreactive for protein kinase C and of cone bipolars immunoreactive for calbindin or recoverin. These data demonstrate that GABA(A) receptors mediate GABAergic inhibition on bipolar cell dendrites in the OPL, that GABA(A) and GABA(C) receptors mediate inhibition on axon terminals in the IPL, and that the GABA(C):GABA(A) on the terminals may tune the response characteristics of the bipolar cell.

Transporter-mediated GABA release induced by excitatory amino acid agonist is associated with GAD-67 but not GAD-65 immunoreactive cells of the primate retina

The release of GABA from amacrine and interplexiform cells after exposure to excitatory amino acids (EAAs) agonists was investigated by immunohistochemistry. Cebus monkey retinas were treated in vitro with 50 microM kainate (KA) or 5 mM L-Glutamate (L-Glu), for 30 min at 37 degrees C. The effects of the EAAs were measured by detecting immunocytochemically the GABA remaining in the tissue after stimulation. L-Glu and KA reduced the number of GABA-immunoreactive perikarya in the innermost part of the inner nuclear layer by approximately 60% and 80%, respectively, as compared to controls. The cell processes in the inner plexiform layer (IPL) were restricted to only three defined bands in the strata 1, 3 and 5, as compared to an intense and homogeneous labeling in the IPL of the untreated retinas. The effect of KA was inhibited by 100 microM CNQX, 100 microM NNC-711, or when Na(+) was replaced by choline. The release of GABA was Ca(2+)-independent, suggesting the mobilization of GABA from the cytoplasmic pool of this neurotransmitter. At least two subsets of retinal neurons including amacrine and interplexiform cells retained GABA-immunoreactivity after stimulation with EAAs, as revealed by glutamic acid decarboxylase (GAD) immunocytochemistry. Our results suggest that non-NMDA receptor activation by KA and glutamate are associated with the efflux of GABA from cells of the inner retina (amacrine and interplexiform cells). The data also show that cells containing GAD-67 released GABA via its transporter, while cells containing exclusively GAD-65 apparently did not release the neurotransmitter by the reversal of the transporter.

Corelease of two functionally opposite neurotransmitters by retinal amacrine cells: experimental evidence and functional significance

The Dale's law postulates that a neuron releases the same neurotransmitter from all its branches. In the case of multiple neurotransmitters it would require all transmitters to be released from all branches. The retinal cholinergic amacrine cells contain and release gamma-aminobutyric (GABA) and, therefore, if GABA and acetylcholine (ACh) are released at the same sites, this could mean that amacrine cells simultaneously excite and inhibit postsynaptic cells. Conversely, if the two neurotransmitters are released at different synapses, or if their release is regulated in a distinct manner, they may play different physiological roles. Recent studies carried out in cultured cholinergic amacrine-like neurons showed that Ca(2+)-dependent release of ACh and GABA have a different sensitivity to membrane depolarization, to the effect of blockers of voltage gated Ca(2+) channels (VGCC) and to the effect of presynaptic A(1) adenosine receptors. Therefore, it is proposed that in retinal amacrine cells the Ca(2+)-dependent release of ACh and GABA occurs at distinct cellular locations. The possible nature of these release sites and the physiological significance of this model are discussed in this review.

Differential expression of the presynaptic cytomatrix protein bassoon among ribbon synapses in the mammalian retina

Bassoon is a 420-kDa presynaptic protein which is highly concentrated at the active zones of nerve terminals of conventional synapses, both excitatory glutamatergic and inhibitory GABAergic, in rat brain. It is thought to be involved in the organization of the cytomatrix at the site of neurotransmitter release. In the retina, there are two structurally and functionally distinct types of synapses: ribbon and conventional synapses. Antibodies against bassoon were applied to sections of rat and rabbit retina. Strong punctate immunofluorescence was found in the outer and inner plexiform layers. Using pre- and post-embedding immunostaining and electron microscopy, bassoon was localized in the outer plexiform layer at ribbon synapses formed by rods and cones but was absent from basal synaptic contacts formed by cones. In the inner plexiform layer a different picture emerged. As in the brain, bassoon was found at conventional inhibitory GABAergic synapses, made by amacrine cells, but it was absent from the bipolar cell ribbon synapses. These data demonstrate differences in the molecular composition of the presynaptic apparatuses of outer and inner plexiform layer ribbon synapses. Thus, differential equipment with cytomatrix proteins may account for the functional differences observed between the two types of ribbon synapses in the retina.

Retinal ganglion cell response properties in the transcorneal electrically evoked response of the visual system

To identify the retinal origin of a cortical evoked potential elicited by transcorneal electrical stimulation of the visual system (EER), the response properties of retinal ganglion cells (RGCs) of cats to transcorneal electrical stimuli were studied. The discharge latency of RGCs to transcorneal stimulation had two peaks with a high temporal resolution. The latency of early components of the EER is associated with the discharge latency of RGCs. Some RGCs showed prominent oscillatory discharges after transcorneal stimulation. Discharges of ON-bipolar cells responding to transcorneal stimulation were significantly inhibited by intravitreal injection of DL-2-amino-4-phosphonobutyrate (APB), which blocks the ON-pathway. These findings indicate that the EER has far-field potentials that might relate to oscillatory discharges of RGCs, and that ON bipolar cells and their related synaptic sites are involved in transcorneal electrical stimuli. The far-field potentials of the EER may have clinical applications, similar to those of somatosensoric evoked potentials and auditory brain stem potentials.

Reciprocal synaptic interactions between rod bipolar cells and amacrine cells in the rat retina

Reciprocal synaptic transmission between rod bipolar cells and presumed A17 amacrine cells was studied by whole cell voltage-clamp recording of rod bipolar cells in a rat retinal slice preparation. Depolarization of a rod bipolar cell evoked two identifiable types of Ca2+ current, a T-type current that activated at about -70 mV and a current with L-type pharmacology that activated at about -50 mV. Depolarization to greater than or equal to -50 mV also evoked an increase in the frequency of postsynaptic currents (PSCs). The PSCs reversed at approximately ECl (the chloride equilibrium potential), followed changes in ECl, and were blocked by gamma-aminobutyric acidA (GABAA) and GABAC receptor antagonists and thus were identified as GABAergic inhibitory PSCs (IPSCs). Bipolar cells with cut axons displayed the T-type current but lacked an L-type current and depolarization-evoked IPSCs. Thus L-type Ca2+ channels are placed strategically at the axon terminals to mediate transmitter release from rod bipolar cells. The IPSCs were blocked by the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that non-NMDA receptors mediate the feed-forward bipolar-to-amacrine excitation. The NMDA receptor antagonist 3-((RS)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid had no consistent effect on the depolarization-evoked IPSCs, indicating that activation of NMDA receptors is not essential for the feedforward excitation. Tetrodotoxin (a blocker of voltage-gated Na+ channels) reversibly suppressed the reciprocal response in some cells but not in others, indicating that graded potentials are sufficient for transmitter release from A17 amacrine cells, but suggesting that voltage-gated Na+ channels, under some conditions, can contribute to transmitter release.

Interactions of inhibition and excitation in the light-evoked currents of X type retinal ganglion cells

The excitatory and inhibitory conductances driving the light-evoked currents (LECs) of cat and ferret ON- and OFF-center X ganglion cells were examined in sliced and isolated retina preparations using center spot stimulation in tetrodotoxin (TTX)-containing Ringer. ON-center X ganglion cells showed an increase in an excitatory conductance reversed positive to +20 mV during the spot stimulus. At spot offset, a transient inhibitory conductance was activated on many cells that reversed near ECl. OFF-center X ganglion cells showed increases in a sustained inhibitory conductance that reversed near ECl during spot stimulation. At spot offset, an excitatory conductance was activated that reversed positive to +20 mV. The light-evoked current kinetics of ON- and OFF-center X cells to spot stimulation did not significantly differ in form from their Y cell counterparts in TTX Ringer. When inhibition was blocked, current-voltage relations of the light-evoked excitatory postsynaptic currents (EPSCs) of both ON- and OFF-X cells were L-shaped and reversed near 0 mV. The EPSCs averaged between 300 and 500 pA at -80 mV. The metabotropic glutamate receptor agonist 2-amino-4-phosphonobutyric acid (APB), was used to block ON-center bipolar cell function. The LECs of ON-X ganglion cells were totally blocked in APB at all holding potentials. APB caused prominent reductions in the dark holding current and synaptic noise of ON-X cells. In contrast, the LECs of OFF-X ganglion cells remained in APB. An increase in the dark holding current was observed. The excitatory amino acid receptor antagonist combination of D-amino-5-phosphono-pentanoic acid (D-AP5) and 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo-(F)-quinoxalinedione (NBQX) was used to block ionotropic glutamate receptor retinal neurotransmission. The LECs of all ON-X ganglion cells were totally blocked, and their holding currents were reduced similar to the actions of APB. For OFF-X ganglion cells, the antagonist combination always blocked the excitatory current at light-OFF; however, in many cells, the inhibitory current at light-ON remained. ON-center X ganglion cells receive active excitation during center illumination, and a transient inhibition at light-OFF. In contrast OFF-center X ganglion cells experience a sustained active inhibition during center illumination, and a shorter increase in excitation at light-offset. Cone bipolar cells provide a resting level of glutamate release on X ganglion cells on which their light-evoked currents are superimposed [corrected].

Analysis of excitatory and inhibitory spontaneous synaptic activity in mouse retinal ganglion cells

Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) were identified and characterized with whole cell and perforated patch voltage-clamp recordings in adult mouse retinal ganglion cells. Pharmacological dissection revealed that all cells were driven by spontaneous synaptic inputs mediated by glutamate and gamma-aminobutyric acid-A (GABAA) receptors. One-half (7/14) of the cells also received glycinergic spontaneous synaptic inputs. Both GABAA and glycine receptor-mediated sIPSCs had rise times (10-90%) of < 1 ms. The decay times of the GABAA receptor-mediated sIPSCs were comparable with those of the glycine receptor-mediated sIPSCs. The average decay time constant for monoexponentially fitted sIPSCs was 63.2 +/- 74.1 ms (mean +/- SD, n = 3278). Glutamate receptor-mediated sEPSCs had an average rise time of 0.50 +/- 0.20 ms (n = 109) and an average monoexponential decay time constant of 5.9 +/- 8.6 ms (n = 2705). Slightly more than two-thirds of the spontaneous synaptic events were monoexponential (68% for sIPSCs and 76% for sEPSCs). The remainder of the events was biexponential. The amplitudes of the spontaneous synaptic events were not correlated with rise times, suggesting that the electrotonic filtering properties of the neurons and/or differences in the spatial location of synaptic inputs could not account for the difference between the decay time constants of the glutamate and GABAA/glycine receptor-mediated spontaneous synaptic events. The amplitudes of sEPSCs were similar to those recorded in tetrodotoxin (TTX), consistent with the events measured in control saline being the response to the release of a single quantum of transmitter. The range of the sIPSC amplitudes in control saline was wider than that recorded in TTX, consistent with some sIPSCs being evoked by presynaptic spikes having an average quantal size greater than one. The rates of sIPSCs and sEPSCs were determined under equivalent conditions by recording with perforated patch electrodes at potentials at which both types of event could be identified. Two groups of ganglion cell were observed; one group had an average sEPSCs/sIPSCs frequency ratio of 0.96 +/- 0.77 (n = 28) and another group had an average ratio of 6.63 +/- 0.82 (n = 7). These findings suggest that a subset of cells is driven much more strongly by excitatory synaptic inputs. We propose that this subset of cells could be OFF ganglion cells, consistent with the higher frequency of spontaneous action potentials found in OFF ganglion cells in other studies.

Different combinations of GABAA and GABAC receptors confer distinct temporal properties to retinal synaptic responses

This study addresses how gamma-aminobutyric acid-A(GABAA) and GABAC receptors confer distinct temporal properties to neuronal synaptic responses. The retina is a model system for the study of postsynaptic contributions to synaptic responses because GABAergic amacrine cells synapse onto neurons, which have different combinations of GABAA and GABAC receptors. It is not known, however, how GABAA versus GABAC receptors influence the time course of retinal synaptic responses or what proportion of inhibitory input is mediated by each receptor type. We examined the time courses of synaptic responses mediated by GABA receptors in ganglion and bipolar cells by recording currents evoked by activating amacrine cells with a stimulating electrode in the salamander retinal slice. The pharmacologically isolated, GABAergic synaptic currents were long-lasting in bipolar cells and relatively brief in ganglion cells. The receptors that mediated these temporally distinct synaptic responses exhibited different pharmacological properties. In ganglion cells, GABAergic synaptic currents were abolished by the GABAA receptor antagonists bicuculline or SR95531. In bipolar cells, the GABAC receptor antagonist 3-aminopropyl[methyl]phosphonic acid (3-APMPA) largely blocked GABAergic synaptic responses; the remaining response was blocked by bicuculline or SR95531. The GABAA receptor component of the bipolar cell response was relatively brief compared with the GABAC receptor component. Puffing GABA onto ganglion cell dendrites or bipolar cell terminals yielded similar pharmacological and kinetic results, indicating that transmitter release differences did not determine the response time courses. Moreover, the GABAC receptors on bipolar cells may be different from those reported in rat or fish retina because imidazole-4-acetic acid (I4AA), which acts as an antagonist in these preparations, acts as an agonist in salamander. Our data show that the prolonged synaptic responses in bipolar cells were mediated predominantly by GABAC receptors, whereas transient synaptic responses in ganglion cells were mediated by GABAA receptors.

Neurochemistry of the mammalian cone 'synaptic complex'

The cone 'synaptic complex' is a unique structure in which a single presynaptic axon secretes glutamate onto processes of bipolar cells (both ON and OFF) and horizontal cells. In turn, the horizontal cell processes antagonize cone and bipolar responses to glutamate (probably by GABA). What still remains largely unknown is the molecular identity of the postsynaptic receptors and their exact locations. We identified several subunits of the glutamate receptor and the GABAA receptor expressed at the cone synaptic complex and localized them ultrastructurally. Glutamate receptors: (i) Invaginating (probably ON) bipolar dendrites in the monkey and rat express the metabotropic glutamate receptor, mGluR6. The stain is intense on the dendritic membrane where it first enters the invagination, and weak at the tip nearest to the ribbon. The cone membrane is electron-dense where it apposes the intense stain for mGluR6. Surprisingly, invaginating bipolar dendrites in the cat also express the AMPA receptor subunits, GluR2/3 and GluR4. (ii) Dendrites forming basal contacts in the cat (probably OFF) express the AMPA subunits GluR2/3, GluR4, and also the kainate subunit, GluR6/7. The stain is especially intense at the dendritic tips in apposition to electron-dense regions of cone membrane. (iii) Horizontal cells in the cat express the AMPA subunits GluR2/3, GluR4 and the kainate subunit, GluR6/7. The stain is strongest in the cytosol of somas and primary dendrites, but is also present in the invaginating terminals where it localizes to the membrane subjacent to the ribbon. GABAA receptors: (i) ON and OFF bipolar dendrites in the monkey express the alpha 1 and beta 2/3 subunits. The stain is localized to the bipolar cell membrane in apposition to horizontal cell processes. (ii) Cones did not express the GABAA subunits tested by immunocytochemistry, but beta 3 mRNA was amplified by RT-PCR from rat photoreceptors.

CONCLUSIONS

(i) mGluR6 receptors concentrate on dendrites at the base of the invagination rather than at the apex. This implies that receptors at both 'invaginating' and 'basal' contacts lie roughly equidistant from the release sites and should therefore receive similar spatiotemporal concentrations of glutamate. (ii) The 'cone' membrane is electron-dense opposite to the receptor sites on both ON and OFF bipolar cells. This suggests a special role for this region in synaptic transmission. Possibly, these densities signify a transporter that would regulate glutamate concentration at sites remote (> 200 nm) from the locus of vesicle release.

Glycine and GABA receptors in the mammalian retina

Molecular cloning has introduced an unexpected diversity of neurotransmitter receptors. In this study we review the types, the localization and possible synaptic function of the inhibitory neurotransmitter receptors in the mammalian retina. Glycine receptors (GlyRs) and their localization in the mammalian retina were analyzed immunocytochemically. Specific antibodies against the alpha 1 subunit of the GlyR (mAb2b) and against all subunits of the GlyR (mAb4a) were used. Both antibodies produced a punctate immunofluorescence, which was shown by electron microscopy to represent clustering of GlyRs at synaptic sites. Synapses expressing the alpha 1 subunit of the GlyR were found on ganglion cell dendrites and on bipolar cell axons. GlyRs were also investigated in the oscillator mutant mouse. The complete loss of the alpha 1 subunit was compensated for by an apparent upregulation of the other subunits of the GlyR. GABAA receptors (GABAARs) and their retinal distribution were studied with specific antibodies that recognize the alpha 1, alpha 2, alpha 3, beta 1, beta 2, beta 3, gamma 2 and delta subunits. Most antibodies produced a punctate immunofluorescence in the inner plexiform layer (IPL) which was shown by electron microscopy to represent synaptic clustering of GABAARs. The density of puncta varied across the IPL and different subunits were found in characteristic strata. This stratification pattern was analyzed with respect to the ramification of cholinergic amacrine cells. Using intracellular injection with Lucifer yellow followed by immunofluorescence, we found that GABAARs composed of different subunits were expressed by the same ganglion cell, however, they were clustered at different synaptic sites. The distribution of GABAC receptors was studied in the mouse and in the rabbit retina using an antiserum that recognizes the rho 1, rho 2 and rho 3 subunits. GABAC receptors were found to be clustered at postsynaptic sites. Most, if not all of the synapses were found on rod and cone bipolar axon terminals. In conclusion we find a great diversity of glycine and GABA receptors in the mammalian retina, which might match the plethora of morphological types of amacrine cells. This may also point to subtle differences in synaptic function still to be elucidated.

Presynaptic and postsynaptic localization of GABA(B) receptors in neurons of the rat retina

The recently cloned GABA(B) receptors were localized in rat retina using specific antisera. Immunolabelling was detected in the inner and outer plexiform layers (IPL, OPL), and in a number of cells in the inner nuclear layer and the ganglion cell layer. Double-labelling experiments for GABA (gamma-aminobutyric acid) and GABA(B) receptors, respectively, demonstrated a co-localization in horizontal cells and amacrine cells. Electron microscopy showed that GABA(B) receptors of the OPL were localized presynaptically in horizontal cell processes invaginating into photoreceptor terminals. In the IPL, GABA(B) receptors were present presynaptically in amacrine cells, as well as postsynaptically in amacrine and ganglion cells. The postnatal development of GABA(B) receptors was also studied, and immunoreactivity was observed well before morphological and synaptic differentiation of retinal neurons. The present results suggest a presynaptic (autoreceptor) as well as postsynaptic role for GABA(B) receptors. In addition, the extrasynaptic localization of GABA(B) receptors could indicate a paracrine function of GABA in the retina.

Different contributions of GABAA and GABAC receptors to rod and cone bipolar cells in a rat retinal slice preparation

Whole cell currents were recorded from rod and cone bipolar cells in a slice preparation of the rat retina. Use of the gramicidin D perforated-patch technique prevented loss of intracellular compounds. The recorded cells were identified morphologically by injection with Lucifer yellow. During the recordings, the cells were isolated synaptically by extracellular cobalt. To distinguish the gamma-aminobutyric acid (GABA) receptors pharmacologically, the GABAA receptor antagonist, bicuculline, and the GABAC receptor antagonist, 3-aminopropyl(methyl)phosphinic acid, were used. In all bipolar cells tested, application of GABA induced postsynaptic chloride currents that hyperpolarized the cells from their resting potential of about -40 mV. GABA was applied at different concentrations to allow for the different affinity of GABA at GABAA and GABAC receptors. At a GABA concentration of 25 microM, in the case of rod bipolar cells, approximately 70% of the current was found to be mediated by GABAC receptors. In the case of cone bipolar cells, only approximately 20% of the current was mediated by GABAC receptors. Furthermore, this GABAC-mediated fraction varied among the different morphological types of cone bipolar cells, supporting the hypothesis of distinct functional roles for the different types of cone bipolar cells. There is evidence that the efficacy of GABAC receptors is modulated by glutamate through metabotropic glutamate receptors. We tested this hypothesis by applying agonists of metabotropic glutamate receptors (mGluR)1/5 to rod bipolar cells. The specific agonist (+/-)-trans-azetidine-2, 4-dicarboxylic acid and the potent mGluR agonist quisqualic acid reduced the amplitude of the GABAC responses by 10-30%. This suggests a functional role for the modulation of GABAC receptors by the metabotropic glutamate receptors mGluR1/5.

GABA-gated Cl- channels in the rat retina

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian retina, and its physiological action is well established. GABA receptors have been localized immunocytochemically in the retina of different mammalian species, and all major retinal cell types have been found to express GABAA receptor subunits. Recently, a new type of GABA receptor with pharmacological and electrophysiological properties different from the known GABAA and GABAB receptors, has been described. These GABAC receptors are found predominantly in the vertebrate retina. This review concentrates on the electrophysiological characterization of GABA receptors expressed by amacrine and bipolar cells of the rat retina. We recorded GABA-induced currents from cultured neonatal amacrine and bipolar cells as well as from isolated bipolar cells of adult animals. While amacrine cells contain a homogeneous population of GABAA receptors, bipolar cells exhibit both GABAA and GABAC responses. Although both receptors gate chloride-selective ion channels, their biophysical and pharmacological properties differ markedly. These functional differences and the cellular distribution of GABAA and GABAC receptors suggest that they have different inhibitory functions in the rat retina.

GABA receptors of bipolar cells from the skate retina: actions of zinc on GABA-mediated membrane currents

GABA receptors of bipolar cells from the skate retina: actions of zinc on GABA-mediated membrane currents. J. Neurophysiol. 78: 2402-2412, 1997. gamma-Aminobutyric acid (GABA)-induced currents were recorded from isolated bipolar cells of the skate retina using perforated patch-clamp methodology. Pharmacological analysis of the responses, using selective agonists and antagonists of the major classes of GABA receptor, revealed the presence of both GABAA and GABAC receptors at both the dendrites and axon terminals of the bipolar cells. The two receptor types showed very different reactions to zinc, a divalent metallic cation that was detected in the synaptic terminal region of skate photoreceptors. Currents mediated by the activation of GABAC receptors were down-regulated by zinc, a feature that is typical of the action of zinc on GABAC receptors. On the other hand, the effects of zinc on GABAA receptor-mediated activity was highly dependent on zinc concentration. Unlike the GABAA receptors on other neurons, responses mediated by activation of the GABAA receptor of skate bipolar cells were significantly enhanced by zinc concentrations in the range of 0. 1-100 mu M; at higher concentrations of zinc (>100 mu M), response amplitudes were suppressed below control levels. The enhancement of GABAA receptor activity on skate bipolar cells showed little voltage dependence, suggesting that zinc is acting on the extracellular domain of the GABAA receptor. In the presence of 10 mu M zinc, the dose-response curve for 4,5,6, 7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; a GABAA agonist that suppresses GABAC-activated currents) was shifted to the left of the curve obtained in the absence of zinc, but without a significant change in the response maximum. This finding indicates that the enhancing effect of zinc is due primarily to its ability to increase the sensitivity of the GABAA receptor. The novel enhancement of neuronal GABAA receptor activity by zinc, observed previously in the GABAA-mediated responses of skate Müller (glial) cells, may reflect the presence of a unique subtype of GABAA receptor on the bipolar and Müller cells of the skate retina.

GABAergic and glycinergic IPSCs in ganglion cells of rat retinal slices

GABAergic and glycinergic IPSCs were studied in identified retinal ganglion cells (RGCs) of light-adapted rat retinal slices, using whole-cell recording techniques. GABAergic IPSCs were blocked specifically by SR95531 (3 microM) and bicuculline (3 microM) and glycinergic IPSCs by strychnine (0.3 microM). From 37 RGCs studied, 25 showed exclusively GABAergic IPSCs, 6 presented only glycinergic IPSCs, and 6 showed both. This distribution may result from differences in amacrine cells input rather than from receptor heterogeneity, because both GABA and glycine elicited Cl--selective currents in all RGCs tested. TTX markedly reduced GABAergic IPSCs frequency, whereas glycinergic IPSCs were unaffected. Ca2+-free media, with or without high Mg2+, blocked TTX-resistant GABAergic and glycinergic IPSCs. These results suggest that GABAergic IPSCs in RGCs can be elicited either by Na+-dependent action potentials or by local Ca2+ influx in medium or large dendritic field GABAergic amacrine cells, whereas glycinergic IPSCs are generated by action potential-independent Ca2+ influx in narrow field glycinergic amacrine cells. Both types of IPSCs had fast rise times and biexponential decays, but glycinergic IPSC decay was significantly slower than that of GABAergic IPSCs. An elementary conductance of 54 pS for the glycine-gated channels was estimated from single-channel events, clearly detected in the falling phase of glycinergic IPSCs, and from responses to exogenous glycine.

Hepatic retinopathia. Changes in retinal function

In patients suffering from hepatic failure, the brain is subject to defined morphological and functional changes known as hepatic encephalopathia (HE). The morphological changes are dominated by glial cells (Alzheimer-type II astrocytes). It has recently been possible to demonstrate, that the retinal glia (Müller) cells undergo similar morphological changes. The present study was carried out in order to reveal if these Müller cell changes cause any characteristic functional deficits. We examined 11 patients with different stages of HE due to liver cirrhosis. Six patients were at stage 0 or 1 (group I) and five at stage 2 or 3 (group II). They underwent ophthalmological routine examination, colour vision testing and standard ERG recording. None of the patients reported impaired vision, in daylight or at night. There were no fundus abnormalities except very mild changes of the pigment epithelium and abnormal reflexes of the inner limiting membrane, especially in the higher HE stages. The number of confusions in the colour arrangement test increased with the higher stages of HE, preferably in the tritan axis. The scotopic a- and b-waves of the electroretinogram (ERG) were almost unchanged in group I and significantly decreased and delayed in group II. The photopic ERG b-wave amplitudes were changed in a similar fashion. Oscillatory potentials proved to be most sensitive to hepatotoxic changes. Their latencies were significantly delayed even in group I. Amplitudes were decreased significantly only in group II. Patients suffering from hepatic failure and accompanying HE display functional abnormalities of the retina. These are best demonstrated by the ERG, and correlate well with the degree of HE. A hypothesis is presented that relates the observed functional changes to altered neurotransmitter levels and impaired retinal glial-neuronal interaction, due to Müller cell damage caused by elevated ammonia levels.

Serial inhibitory synapses in retina

Whole-cell voltage clamp in the retinal slice and intracellular current clamp in the intact retina were used to study inhibitory interactions in the inner plexiform layer. Picrotoxin or strychnine reduced inhibitory, light-evoked currents in a majority of ganglion cells. However, in nearly a third of the ganglion cells, each of these antagonists enhanced the inhibitory synaptic current. All inhibitory current was blocked by the addition of the other antagonist. This indicates a cross-inhibition between GABAergic and glycinergic feedforward pathways. Blocking of GABAARs with SR95531 shortened the time course of both excitatory and inhibitory synaptic currents in ganglion cells. Application of picrotoxin, which blocked both GABAARs and GABACRs, produced the opposite effect. Recordings in the intact retina indicated that the light responses of ON bipolar cells, sustained ON, and transient ON-OFF third-order neurons were all made more transient by SR95531 and made more sustained by picrotoxin. The data suggest that a GABAC feedback pathway to bipolar cells makes light responses more phasic and that this feedback is inhibited through a GABAAR pathway. Consequently, the balance between GABAAR and GABACR inhibition regulates the time course of inputs to ganglion cells.

Time course of appearance of GABA and GABA receptors during retinal regeneration in the adult newt

Appearance and maturation of the GABA (gamma-aminobutyric acid) system during newt retinal regeneration were studied by electrophysiological, immunohistochemical, and biochemical techniques. (1) Responses to GABA appeared in neurons dissociated from regenerating retinae before the segregation of the plexiform layers; whereas (2) GABA immunoreactivity appeared at sites of the presumptive horizontal cell and amacrine cell layers at the beginning of the segregation of these layers. During subsequent regeneration, GABA-immunoreactive cells at the amacrine cell layer increased in number and extended lateral processes, forming a GABA-immunoreactive inner plexiform layer. Also GABA immunoreactivity increased in the region of the outer plexiform layer, but not their somata which showed decreased GABA immunoreactivity. (3) GABA synthesis in the retina increased significantly at the beginning of the segregation of the plexiform layers. These results suggest that the increase of GABA synthesis during retinal regeneration correlates well with the development of GABA-immunoreactive cells and that functional GABA receptors appear earlier than increased GABA synthesis.