Two neuropharmacological types of rabbit ON-alpha ganglion cells express GABAC receptors

Complex inhibitory microcircuitry regulates retinal signaling near visual threshold

Neuronal microcircuits, small, localized signaling motifs involving two or more neurons, underlie signal processing and computation in the brain. Compartmentalized signaling within a neuron may enable it to participate in multiple, independent microcircuits. Each A17 amacrine cell in the mammalian retina contains within its dendrites hundreds of synaptic feedback microcircuits that operate independently to modulate feedforward signaling in the inner retina. Each of these microcircuits comprises a small (<1 μm) synaptic varicosity that typically receives one excitatory synapse from a presynaptic rod bipolar cell (RBC) and returns two reciprocal inhibitory synapses back onto the same RBC terminal. Feedback inhibition from the A17 sculpts the feedforward signal from the RBC to the AII, a critical component of the circuitry mediating night vision. Here, we show that the two inhibitory synapses from the A17 to the RBC express kinetically distinct populations of GABA receptors: rapidly activating GABA(A)Rs are enriched at one synapse while more slowly activating GABA(C)Rs are enriched at the other. Anatomical and electrophysiological data suggest that macromolecular complexes of voltage-gated (Cav) channels and Ca(2+)-activated K(+) channels help to regulate GABA release from A17 varicosities and limit GABA(C)R activation under certain conditions. Finally, we find that selective elimination of A17-mediated feedback inhibition reduces the signal to noise ratio of responses to dim flashes recorded in the feedforward pathway (i.e., the AII amacrine cell). We conclude that A17-mediated feedback inhibition improves the signal to noise ratio of RBC-AII transmission near visual threshold, thereby improving visual sensitivity at night.

GABAergic neurotransmission and retinal ganglion cell function

Ganglion cells are the output retinal neurons that convey visual information to the brain. There are ~20 different types of ganglion cells, each encoding a specific aspect of the visual scene as spatial and temporal contrast, orientation, direction of movement, presence of looming stimuli; etc. Ganglion cell functioning depends on the intrinsic properties of ganglion cell's membrane as well as on the excitatory and inhibitory inputs that these cells receive from other retinal neurons. GABA is one of the most abundant inhibitory neurotransmitters in the retina. How it modulates the activity of different types of ganglion cells and what is its significance in extracting the basic features from visual scene are questions with fundamental importance in visual neuroscience. The present review summarizes current data concerning the types of membrane receptors that mediate GABA action in proximal retina; the effects of GABA and its antagonists on the ganglion cell light-evoked postsynaptic potentials and spike discharges; the action of GABAergic agents on centre-surround organization of the receptive fields and feature related ganglion cell activity. Special emphasis is put on the GABA action regarding the ON-OFF and sustained-transient ganglion cell dichotomy in both nonmammalian and mammalian retina.

Type 2 wide-field amacrine cells in TH::GFP mice show a homogenous synapse distribution and contact small ganglion cells

In vertebrate retinas, wide-field amacrine cells represent a diverse class of interneurons, important for the extraction of selective features, like motion or objects, from the visual scene. Most types of wide-field amacrine cells lack dedicated output processes, whereas some types spatially segregate outputs from inputs. In the tyrosine hydroxylase (TH)::green fluorescent protein (GFP) mouse line, two types of GFP-expressing wide-field amacrine cells have been described: dopaminergic type 1 and γ-aminobutyric acid-ergic type 2 cells (TH2). TH2 cells possess short and long radial processes stratifying in the middle of the inner plexiform layer, where they collect excitatory and inhibitory inputs from bipolar cells and other amacrine cells, respectively. Although it was shown that these inputs lead to ON-OFF light responses, their spatial distribution along TH2 cell processes is unknown. Also, the postsynaptic targets of TH2 cells have not been identified so far. Here, we analysed the synapse distribution of these cells in TH::GFP mice and show that they form a weakly coupled network. Electrical synapses (made of connexin36) and chemical (excitatory and inhibitory) synapses are uniformly distributed along TH2 dendrites, independent of dendrite length or distance from soma. Moreover, we reveal that TH2 cells contact at least two types of small ganglion cells; one of them is the W3 cell, a ganglion cell sensitive to object motion. Contacts were often associated with markers of inhibitory synapses. Thus, TH2 wide-field amacrine cells likely provide postsynaptic inhibition to W3 ganglion cells and may contribute to object-motion detection in the mouse retina.

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.

Inhibitory mechanisms that generate centre and surround properties in ON and OFF brisk-sustained ganglion cells in the rabbit retina

Lateral inhibition produces the centre-surround organization of retinal receptive fields, in which inhibition driven by the mean luminance enhances the sensitivity of ganglion cells to spatial and temporal contrast. Surround inhibition is generated in both synaptic layers; however, the synaptic mechanisms within the inner plexiform layer are not well characterized within specific classes of retinal ganglion cell. Here, we compared the synaptic circuits generating concentric centre-surround receptive fields in ON and OFF brisk-sustained ganglion cells (BSGCs) in the rabbit retina. We first characterized the synaptic inputs to the centre of ON BSGCs, for comparison with previous results from OFF BSGCs. Similar to wide-field ganglion cells, the spatial extent of the excitatory centre and inhibitory surround was larger for the ON than the OFF BSGCs. The results indicate that the surrounds of ON and OFF BSGCs are generated in both the outer and the inner plexiform layers. The inner plexiform layer surround inhibition comprised GABAergic suppression of excitatory inputs from bipolar cells. However, ON and OFF BSGCs displayed notable differences. Surround suppression of excitatory inputs was weaker in ON than OFF BSGCs, and was mediated largely by GABA(C) receptors in ON BSGCs, and by both GABA(A) and GABA(C) receptors in OFF BSGCs. Large ON pathway-mediated glycinergic inputs to ON and OFF BSGCs also showed surround suppression, while much smaller GABAergic inputs showed weak, if any, spatial tuning. Unlike OFF BSGCs, which receive strong glycinergic crossover inhibition from the ON pathway, the ON BSGCs do not receive crossover inhibition from the OFF pathway. We compare and discuss possible roles for glycinergic inhibition in the two cell types.

Receptor targets of amacrine cells

Amacrine cells are a morphologically and functionally diverse group of inhibitory interneurons. Morphologically, they have been divided into approximately 30 types. Although this diversity is probably important to the fine structure and function of the retinal circuit, the amacrine cells have been more generally divided into two subclasses. Glycinergic narrow-field amacrine cells have dendrites that ramify close to their somas, cross the sublaminae of the inner plexiform layer, and create cross talk between its parallel ON and OFF pathways. GABAergic wide-field amacrine cells have dendrites that stretch long distances from their soma but ramify narrowly within an inner plexiform layer sublamina. These wide-field cells are thought to mediate inhibition within a sublamina and thus within the ON or OFF pathway. The postsynaptic targets of all amacrine cell types include bipolar, ganglion, and other amacrine cells. Almost all amacrine cells use GABA or glycine as their primary neurotransmitter, and their postsynaptic receptor targets include the most common GABA(A), GABA(C), and glycine subunit receptor configurations. This review addresses the diversity of amacrine cells, the postsynaptic receptors on their target cells in the inner plexiform layer of the retina, and some of the inhibitory mechanisms that arise as a result. When possible, the effects of GABAergic and glycinergic inputs on the visually evoked responses of their postsynaptic targets are discussed.

Functional expression of ionotropic glutamate receptors in the rabbit retinal ganglion cells

It has been known that retinal ganglion cells (RGCs) with distinct morphologies have different physiological properties. It was hypothesized that different functions of RGCs may in part result from various expressions of N-methyl-d-aspartate (NMDA), α-amino-3-hydroxyl-5-methyl-isoxazole-4-propinoic acid (AMPA), and kainic acid (KA) receptors on their dendrites. In the present study, we aimed to characterize the functional expression of AMPA and NMDA receptors of morphologically identified RGCs in the wholemount rabbit retina. The agmatine (AGB) activation assay was used to reveal functional expression of ionotropic glutamate receptors after the RGCs were targeted by injecting Neurobiotin. To examine the excitability of these glutamate receptors in an agonist specific manner, the lower concentrations of AMPA (2 μM) and NMDA (100 μM) were chosen to examine G7 (ON-OFF direction selective ganglion cells) and G11 (alpha ganglion cells) types of RGCs. We found that less than 40% of G7 type RGCs had salient AGB activation when incubated with 2 μM AMPA or 100 μM NMDA. The G11 type RGCs also showed similar activation frequencies, except that all of the OFF subtype examined had no AGB permeation under the same AMPA concentration. These results suggest that RGCs with large somata (G7 and G11 types) may express various heterogeneous functional ionotropic glutamate receptors, thus in part rendering their functional diversity.

Role of neurotransmitter receptors in mediating light-evoked responses in retinal interplexiform cells

Interplexiform (IP) cells are a long-neglected group of retinal neurons the function of which is yet to be determined. Anatomical study indicates that IP cells are located in the inner nuclear layer, juxtaposed with the third-order neurons. However, the synaptic transmission of IP cells in the inner retina is poorly understood. Using whole cell patch-clamp and pharmacological techniques, we extensively studied synaptic receptors in IP cells. The IP cells in amphibian retinal slices were identified by electrical and morphological properties with voltage-clamp recording and Lucifer yellow dialysis. We find that light-evoked excitatory postsynaptic currents (L-EPSCs) are mediated by AMPA and N-methyl-d-aspartate receptors in IP cells. Although both receptors contributed to the amplitude and kinetics of L-EPSCs, AMPA receptor desensitization substantially shaped L-EPSCs in the neurons, similar to those found in the third-order neurons. The light-evoked inhibitory postsynaptic currents (L-IPSCs) in IP cells were primarily mediated by strychnine-sensitive glycine receptors with a small component of GABA(C) receptors. GABA(C) receptor rho2 subunits were detected in IP cells with single-cell RT-PCR assays. Expression of GABA(C) receptors is one of the special features for IP cells, distinct from most of the third-order neurons that depend on GABA(A) and glycine receptors to relay the inhibitory signals. However, GABA(A) receptors in IP cells acted like nonsynaptic receptors that were activated by exogenous GABA application. Furthermore, L-IPSCs in IP cells were inhibited by the serial inhibitions between amacrine cells in the inner retina. In addition, application of neurotransmitters on the axon terminals of IP cells had no significant current generated in the cells, indicating that the synaptic inputs of IP cells are mainly from the inner retina. This study demonstrates the important role that light signals are encoded by both experiment of inhibitory receptors in IP cells.

Interneuron circuits tune inhibition in retinal bipolar cells

While connections between inhibitory interneurons are common circuit elements, it has been difficult to define their signal processing roles because of the inability to activate these circuits using natural stimuli. We overcame this limitation by studying connections between inhibitory amacrine cells in the retina. These interneurons form spatially extensive inhibitory networks that shape signaling between bipolar cell relay neurons to ganglion cell output neurons. We investigated how amacrine cell networks modulate these retinal signals by selectively activating the networks with spatially defined light stimuli. The roles of amacrine cell networks were assessed by recording their inhibitory synaptic outputs in bipolar cells that suppress bipolar cell output to ganglion cells. When the amacrine cell network was activated by large light stimuli, the inhibitory connections between amacrine cells unexpectedly depressed bipolar cell inhibition. Bipolar cell inhibition elicited by smaller light stimuli or electrically activated feedback inhibition was not suppressed because these stimuli did not activate the connections between amacrine cells. Thus the activation of amacrine cell circuits with large light stimuli can shape the spatial sensitivity of the retina by limiting the spatial extent of bipolar cell inhibition. Because inner retinal inhibition contributes to ganglion cell surround inhibition, in part, by controlling input from bipolar cells, these connections may refine the spatial properties of the retinal output. This functional role of interneuron connections may be repeated throughout the CNS.

Glycine receptors of A-type ganglion cells of the mouse retina

A-type ganglion cells of the mouse retina represent the visual channel that transfers temporal changes of the outside world very fast and with high fidelity. In this study we combined anatomical and physiological methods in order to study the glycinergic, inhibitory input of A-type ganglion cells. Immunocytochemical studies were performed in a transgenic mouse line whose ganglion cells express green fluorescent protein (GFP). The cells were double labeled for GFP and the four alpha subunits of the glycine receptor (GlyR). It was found that most of the glycinergic input of A-type cells is through fast, alpha1-expressing synapses. Whole-cell currents were recorded from A-type ganglion cells in retinal whole mounts. The response to exogenous application of glycine and spontaneous inhibitory postsynaptic currents (sIPSCs) were measured. By comparing glycinergic currents recorded in wildtype mice and in mice with specific deletions of GlyRalpha subunits (Glra1spd-ot, Glra2-/-, Glra3-/-), the subunit composition of GlyRs of A-type ganglion cells could be further defined. Glycinergic sIPSCs of A-type ganglion cells have fast kinetics (decay time constant tau = 3.9 +/- 2.5 ms, mean +/- SD). Glycinergic sIPSCs recorded in Glra2-/- and Glra3-/- mice did not differ from those of wildtype mice. However, the number of glycinergic sIPSCs was significantly reduced in Glra1spd-ot mice and the remaining sIPSCs had slower kinetics than in wildtype mice. The results show that A-type ganglion cells receive preferentially kinetically fast glycinergic inputs, mediated by GlyRs composed of alpha1 and beta subunits.

Expression of alpha 7 nicotinic acetylcholine receptors by bipolar, amacrine, and ganglion cells of the rabbit retina

Cholinergic agents affect the light responses of many ganglion cells (GCs) in the mammalian retina by activating nicotinic acetylcholine receptors (nAChRs). Whereas retinal neurons that express beta2 subunit-containing nAChRs have been characterized in the rabbit retina, expression patterns of other nAChR subtypes remain unclear. Therefore, we evaluated the expression of alpha7 nAChRs in retinal neurons by means of single-, double-, and triple-label immunohistochemistry. Our data demonstrate that, in the rabbit retina, several types of bipolar cells, amacrine cells, and cells in the GC layer express alpha7 nAChRs. At least three different populations of cone bipolar cells exhibited alpha7 labeling, whereas glycine-immunoreactive amacrine cells comprised the majority of alpha7-positive amacrine cells. Some GABAergic amacrine cells also displayed alpha7 immunoreactivity; alpha7 labeling was never detected in rod bipolar cells or rod amacrine cells (AII amacrine cells). Our data suggest that activation of alpha7 nAChRs by acetylcholine (ACh) or choline may affect glutamate release from several types of cone bipolar cells, modulating GC responses. ACh-induced excitation of inhibitory amacrine cells might cause either inhibition or disinhibition of other amacrine and GC circuits. Finally, ACh may act on alpha7 nAChRs expressed by GCs themselves.

Presynaptic Inhibition Modulates Spillover, Creating Distinct Dynamic Response Ranges of Sensory Output

Sensory information is thought to be modulated by presynaptic inhibition. Although this form of inhibition is a well-studied phenomenon, it is still unclear what role it plays in shaping sensory signals in intact circuits. By visually stimulating the retinas of transgenic mice lacking GABAc receptor-mediated presynaptic inhibition, we found that this inhibition regulated the dynamic range of ganglion cell (GC) output to the brain. Presynaptic inhibition acted differentially upon two major retinal pathways; its elimination affected GC responses to increments, but not decrements, in light intensity across the visual scene. The GC dynamic response ranges were different because presynaptic inhibition limited glutamate release from ON, but not OFF, bipolar cells, which modulate the extent of glutamate spillover and activation of perisynaptic NMDA receptors at ON GCs. Our results establish a role for presynaptic inhibitory control of spillover in determining sensory output in the CNS.

Glycine- and GABA-activated inhibitory currents on axon terminals of rabbit cone bipolar cells

Glycine- and GABA-activated currents were examined in the axon terminals of 12 types of rabbit cone bipolar cells. In the superfused retinal slice, a cell was voltage clamped at 0 mV in the presence of cobalt; then glycine or GABA was puffed onto the axon terminal. Types CBa1, CBa2, and a few CBa1-2 cells demonstrated larger glycine-activated currents than GABA-activated ones. However, some OFF cells (CBa2(n), CBa1-2(n), CBa1(w)), most CBa1-2, and most ON cells (CBb3, CBb3-4, CBb3(n), and CBb4) displayed larger GABA-activated currents. The ON cell, CBb5, possessed only a GABA-activated current. The predominance of glycinergic currents in CBa1, CBa2, and a few CBa1-2 cells suggests a major input from the glycinergic AII amacrine cell and thus a key role for these cells in the rod bipolar pathway. Certain OFF cells (most CBa1-2) expressed larger GABA-activated currents. All types expressed both GABA(A) and GABAC currents about equally, although most OFF types (CBa1, CB a2(n), CBa1-2, and CBa2(n)) displayed a slightly greater GABA(A) component.

Compartmental localization of gamma-aminobutyric acid type B receptors in the cholinergic circuitry of the rabbit retina

Although many effects of gamma-aminobutyric acid (GABA) on retinal function have been attributed to GABA(A) and GABA(C) receptors, specific retinal functions have also been shown to be mediated by GABA(B) receptors, including facilitation of light-evoked acetylcholine release from the rabbit retina (Neal and Cunningham [1995] J. Physiol. 482:363-372). To explain the results of a rich set of experiments, Neal and Cunningham proposed a model for this facilitation. In this model, GABA(B) receptor-mediated inhibition of glycinergic cells would reduce their own inhibition of cholinergic cells. In turn, muscarinic input from the latter to the glycinergic cells would complete a negative-feedback circuitry. In this study, we have used immunohistochemical techniques to test elements of this model. We report that glycinergic amacrine cells are GABA(B) receptor negative. In contrast, our data reveal the localization of GABA(B) receptors on cholinergic/GABAergic starburst amacrine cells. High-resolution localization of GABA(B) receptors on starburst amacrine cells shows that they are discretely localized to a limited population of its varicosities, the majority of likely synaptic-release sites being devoid of detectable levels of GABA(B) receptors. Finally, we identify a glycinergic cell that is a potential muscarinic receptor-bearing target of GABA(B)-modulated acetylcholine release. This target is the DAPI-3 cell. We propose, based on these data, a modification of the Neal and Cunningham model in which GABA(B) receptors are on starburst, not glycinergic amacrine cells.