Featured Review Cellscience Reviews Vol.2 No.3 ISSN 1742-8130

Retinal GABA receptors and visual processing: a model system for presynaptic inhibition

Erika D. Eggers, Tomomi Ichinose, Botir T. Sagdullaev & Peter D. Lukasiewicz

Introduction

The retina is an ideal system for studying GABA (γ-aminobutyric acid) mediated inhibition in the CNS. Recent work shows that there are a variety of retinal GABA receptors with distinct functional properties, suggesting that these receptors play unique roles in retinal signal processing. Inhibitory signaling is mediated by the ionotropic GABAA and GABAC receptors, and the G-protein-coupled GABAB receptors. The readers are referred to a review by Slaughter (1995) for information on retinal GABAB receptors. Inhibitory signaling in the vertebrate retina underlies several essential mechanisms of visual information processing, including the center-surround receptive field organization of retinal neurons, and the motion and direction sensitivity of some retinal neurons. Our article will review recent advances in our understanding of ionotropic GABA receptors, focusing on presynaptic GABAC receptors, which are highly enriched in the retina. We compare these findings with those in other parts of the CNS to illustrate the common mechanisms used to mediate different types of GABAergic inhibitory signaling.

GABA and the Retina

The retina is a well characterized, layered structure that is ideal for studying GABA-mediated inhibition (Figure 1). The retina can be isolated intact and can be activated by light, its natural stimulus. Responses to these stimuli can be recorded, in vitro, from morphologically-identified neurons that are part of well described circuits (Masland, 2001). Using the retina, we can study how GABAergic inhibition contributes to physiologically relevant signaling in an intact CNS circuit. The signaling pathway consisting of photoreceptors, bipolar cells and ganglion cells is the most direct route by which visual information flows to the brain. In the first synaptic layer, the outer plexiform layer (OPL), horizontal cells modulate the transmission between photoreceptors and bipolar cells (Figure 1). In the second synaptic layer, the inner plexiform layer (IPL), amacrine cells modulate transmission between bipolar cells and ganglion cells (Figure 1). GABA mediated signaling occurs in both synaptic layers.

Figure 1. The retina is a well-organized, laminar structure composed of five distinct classes of neurons. Photoreceptors (PR) are activated by light and synapse onto bipolar cells (BC) and horizontal cells (HC) in the outer plexiform layer (OPL). Bipolar cells then synapse onto ganglion cells (GC) and amacrine cells (AC) in the inner plexiform layer (IPL). The vertical, excitatory pathway (yellow) consisting of photoreceptors, bipolar cells and ganglion cells is modulated by two lateral, inhibitory pathways (red). These inhibitory signaling pathways are mediated by horizontal cells (HC) in the outer retina and amacrine cells (AC) in the inner retina.

GABA receptor diversity in the retina

The retina is perhaps the best place in the CNS to study the roles of GABAA and GABAC receptors because both classes are found in abundance. These two classes of ionotropic GABA receptors share some similarities; they both are composed of five subunits and gate chloride channels. However, GABAA and GABAC receptors are molecularly distinct and possess different functional properties. GABAA receptors are heteromeric complexes, comprised of different combinations of the following subunits, α, β, γ, δ and π. Expression studies suggest that only limited subunit combinations form functional receptors in the CNS (Mehta & Ticku, 1999). GABAC receptors, by contrast, are most likely comprised of heteromeric combinations of ρ1 and ρ2 subunits (Enz et al., 1995; Zhang et al., 1995; Yeh et al., 1996). Recent findings suggest that ρ1 subunits are required for the expression of GABAC receptors in vivo (McCall et al., 2002) .

While studies of both recombinant and retinal GABAC receptors indicate that they possess ρ subunits (Amin & Weiss, 1994; Enz et al., 1995), the subunit composition of native GABAC receptors remains unknown. Given that both GABAA and GABAC receptor subunits are present on bipolar cells, do these subunits coassemble to form GABAC receptors? Most evidence suggests that ρ subunits do not coassemble with GABAA receptor subunits. Shimada et al. (1992) failed to find any evidence that ρ1 subunits were coassembled with α, β, or γ2 GABAA receptor subunits. In addition, co-immunoprecipitation assays failed to show evidence for coassembly of ρ1 subunits and α and β subunits (Hackam et al., 1998). On the other hand, several studies suggest that heterologously expressed ρ and γ2 GABAA receptor subunits form receptors with properties similar to native GABAC receptors (Qian & Ripps, 1999; Pan et al., 2000; Qian & Pan, 2002), but these expressed receptors differed from native receptors since they were potentiated by barbiturates. Immunolabelling studies in retina (Koulen et al., 1998; Haverkamp & Wässle, 2000) found no evidence for colocalization of GABAA receptor subunits with ρ subunits, suggesting that these subunits did not combine to form GABAC receptors in the retina.

Physiological properties of GABAA and GABAC receptors

Although GABAA and GABAC receptors both gate chloride channels, these two classes of receptors have distinct biophysical characteristics that confer unique functional properties. GABAC receptors are about ten-fold more sensitive to GABA than the most common types of GABAA receptors (Feigenspan & Bormann, 1994). In addition, GABAC receptors open and close more slowly than the typical GABAA receptor and mediate prolonged current responses (Amin & Weiss, 1994; Qian & Dowling, 1995; Chang & Weiss, 1999). Using recombinant ρ1 GABAC receptors, Chang and Weiss (1999) attributed the slow current onset to the slow association rate of GABA to a restricted access binding site. The slow decay of the current response mediated by the recombinant GABAC receptors was attributed to the open receptor pore hindering agonist unbinding. The slow native GABAC receptor-mediated response component in rod bipolar cells was eliminated in ρ1 subunit knockout mice (McCall et al., 2002, Figure 2). The briefer GABA responses in knockout mice were mediated by the remaining GABAA receptors. Thus compared to conventional GABAA receptors, GABAC receptors are activated by lower concentrations of GABA and mediate more prolonged inhibitory signals. GABAA and GABAC receptors also display unique pharmacological signatures. GABAC receptors, unlike GABAA receptors, are neither antagonized by bicuculline nor potentiated by barbiturates or benzodiazepines. Conversely, GABAC receptors, but not GABAA receptors are antagonized by TPMPA ((1,2,5,6-Tetrahydropyridin-4-yl) methylphosphinic acid).

Figure 2. Retinal GABAC receptors mediate prolonged GABA responses in rod bipolar cells. Shown are GABA currents in response to focal, puff applications of GABA (100 µM) to the axon terminals of rod bipolar cells in retinas from WT and GABACR null mice. Responses from WT mice are long lasting and are mediated mainly by GABAC receptors, and to a lesser extent, by GABAA receptors. Responses from mice that lack GABAC receptors are much briefer and are mediated exclusively by GABAA receptors. Reprinted with permission in modified form from McCall et al, 2002, Journal of Neuroscience. © 2002 by the Society for Neuroscience.

Retinal locations of GABAC and GABAA receptors

A diversity of GABAA receptors is found in the retina. GABAA receptors are found on all types of retinal neurons and they mediate both pre- and postsynaptic inhibition. Because GABAA receptors are located in both the IPL and OPL, they can, in principle, modulate signaling between photoreceptors and bipolar cells, as well as signaling between bipolar and ganglion cells. In situ hybridization and immunocytochemical studies suggest there are a variety of GABAA receptors in the retina arising from different subunit combinations (Greferath et al., 1995; Wassle et al., 1998; Zhang et al., 2003). In addition to distinct distributions of GABAA receptor subunits on different retinal interneurons, there are also different assortments of GABAA receptor subunits on a single neuron (Wassle et al., 1998), suggesting that there are distinct GABAA receptors on individual retinal neurons. Based on functional studies of heterologously expressed GABAA receptor subunits (reviewed by Mehta & Ticku, 1999) these findings suggest that retinal GABAA receptors may have unique signaling roles. Although these studies strongly suggest a variety of GABAA receptor-mediated functions in the retina, it is not known whether they have distinct roles because subunit-specific physiological and pharmacological studies have not been performed. The determination of their functional roles must await the development of subunit-specific pharmacological agents.

The distribution of GABAC receptors in the retina is more limited than GABAA receptors. GABAC receptors only mediate presynaptic inhibition. Immunohistochemical studies demonstrate that GABAC receptors are most abundant in the IPL, specifically on bipolar cell terminals, at contacts made by amacrine cells (Enz et al., 1996; Koulen et al., 1997; Fletcher et al., 1998). By contrast, immunolabeling in the OPL is generally weaker and more diffuse (Haverkamp et al., 2000). Although some studies have shown that GABAC receptors are also present on cone photoreceptor terminals in pig and mouse retina (Picaud et al., 1998; Pattnaik et al., 2000), where they might play a role in modulating glutamate release, electrophysiological studies suggest that GABA does not play a major role in controlling glutamate release from cone photoreceptors (Verweij et al., 1996; Kamermans et al., 2001). Work by Kamermans and colleagues (2002) suggests that GABA may be involved in slow modulation of this synapse and does not act as a traditional, fast inhibitory transmitter.

The anatomical evidence for GABAC receptor localization is confirmed by electrophysiological recordings from bipolar cells showing that the largest GABAC receptor-mediated responses are at the bipolar cell terminals (Lukasiewicz et al., 1994; Matthews et al., 1994; Wu & Maple, 1998; Shields et al., 2000). GABA responses in mammalian rod bipolar cells are relatively insensitive to blockade by bicuculline, indicating that they are mediated primarily by GABAC receptors (Euler & Wässle, 1998; Shields et al., 2000; McCall et al., 2002). Electrophysiological studies also indicate that a type of horizontal cell in some species of fish has GABAC receptors (Qian & Dowling, 1993; Dong et al., 1994). In most vertebrates, however, horizontal cells do not express GABAC receptors (Stockton & Slaughter, 1991; Blanco et al., 1996; Koulen et al., 1997). Because subunit specific antibodies do not exist for different GABAC ρ subunits, and as there are no pharmacological agents available that are specific for individual ρ subunits, our ability to study different subtypes of GABAC receptors is limited. As the majority of the GABAC and GABAA receptors are present in the inner retina, where the function of GABAergic synaptic transmission has been most extensively studied, we will focus our further discussion of the function roles of GABA receptors on IPL information processing.

GABA-mediated inhibition and IPL information processing

Amacrine cells mediate lateral and feedback inhibitory signaling in the inner plexiform layer. Amacrine cells comprise the most diverse class of retinal interneurons (MacNeil & Masland, 1998). Approximately one half of all amacrine cells are GABAergic and this population is composed of morphologically diverse subtypes (Pourcho & Goebel, 1983) that serve different functions, as described below. Amacrine cells synapse onto bipolar cell terminals, ganglion cell and amacrine cell dendrites, which are all located in the IPL. Reciprocal synapses occur between amacrine cell processes and bipolar cell terminals that modulate bipolar cell output (Dowling & Boycott, 1966). Amacrine cells are activated by bipolar cell glutamate release and, in turn, release GABA back onto the bipolar cell terminal. Lateral amacrine cell processes also make synaptic contacts onto bipolar cell terminals and ganglion cell dendrites and mediate lateral inhibition, as described below.

GABA receptor modulation of bipolar cell output

The predominant role of GABAC receptors in the retina is to mediate presynaptic inhibition of the bipolar cell output (Lukasiewicz et al., 2004). GABAC receptors mediate a significant fraction of GABA-evoked responses at bipolar cell terminals (Lukasiewicz et al., 1994; Matthews et al., 1994; Pan & Lipton, 1995), which, as stated above, are characterized by slow activation and deactivation kinetics and by high GABA sensitivity. Synaptic activation of these receptors by light (Eggers & Lukasiewicz, 2006) or other stimuli (Hartveit, 1997; Lukasiewicz & Shields, 1998; Shen & Slaughter, 2001) confirms that GABAC receptors mediate a significant fraction of presynaptic inhibition at bipolar cell terminals, although GABAA receptors also supply presynaptic inhibition to bipolar cells. GABA receptors on bipolar cell axon terminals play a number of roles in visual processing. GABAC receptors have been shown to control the excitatory output of bipolar cells onto ganglion cells (Lukasiewicz & Werblin, 1994; Dong & Werblin, 1998) and amacrine cells (Bloomfield & Xin, 2000; Matsui et al., 2001).

Presynaptic GABAC receptor-mediated inhibition can shape the temporal properties of the glutamate signal from bipolar cell to ganglion cells. Dong & Werblin (1998) suggest that GABAC receptor-mediated inhibitory feedback gives rise, in part, to transient responses in ganglion cells, although this may not be the primary mechanism of transient response formation (Bieda & Copenhagen, 2000). Freed et al (2003) reported that the blockade of GABAC receptors decreased the correlated bursting of spontaneous excitatory postsynaptic currents (EPSCs) and spikes in ganglion cells and amacrine cells, suggesting a temporal processing role for GABAC receptors.

GABAC receptor-mediated inhibition contributes to the center-surround receptive field organization of ganglion cells. Illumination of the receptive field center depolarizes ON center ganglion cells and hyperpolarizes OFF center ganglion cells (Werblin & Dowling, 1969). Receptive field surround illumination antagonizes the response to center illumination, hyperpolarizing ON center ganglion cells and depolarizing OFF center ganglion cells. The classical center-surround receptive field organization of ganglion cells reflects spatial signal processing and is usually attributed to lateral interactions between horizontal cells and photoreceptors at the OPL (Baylor et al., 1971). However, work by Cook & McReynolds (1998) demonstrated that lateral interactions between amacrine cells and bipolar cells and ganglion cells at the IPL accounts for a significant amount of ganglion cell surround inhibition. A large component of this surround input is mediated by GABAC receptors (Bloomfield & Xin, 2000; Flores-Herr et al., 2001; Ichinose & Lukasiewicz, 2005), indicating that lateral inhibitory signals from amacrine cells to bipolar cells mediate a portion of the ganglion cell surround. This is illustrated in Figure 3, which shows that inhibition of ganglion cells, mediated by a dim surround illumination, is blocked by GABAC receptor antagonists (Ichinose & Lukasiewicz, 2005). Postsynaptic GABAA receptors on ganglion cells and amacrine cells also mediate a component of the surround input measured in amacrine (Bloomfield & Xin, 2000) and ganglion cells (Cook & McReynolds, 1998; Flores-Herr et al., 2001; Ichinose & Lukasiewicz, 2005).

Figure 3. Lateral inhibition mediated by GABAC receptors located in the IPL contributes to the inhibitory, receptive field surround of ganglion cells. A. Dim surround illumination decreases the response of ganglion cells to a spot of light. Surround inhibition occurred in the presence of the GABAA receptor blocker bicuculline, indicating that it was mediated by GABAC receptors. B. Dim surround inhibition was eliminated when GABAC receptors are blocked with the addition of picrotoxin and I4AA (imidazole-4-acetic acid), non-specific GABA receptor antagonists. Reprinted with permission in modified form from Ichinose and Lukasiewicz, 2005, Journal of Physiology. © 2005, The Physiological Society.

Temporal tuning of inhibition by GABA receptors

GABAA receptors are often found with GABAC receptors on bipolar cell terminals, but the relative numbers of these two classes of receptor vary with bipolar cell type (Euler & Wässle, 1998; Shields et al., 2000). Because bipolar cell GABAA and GABAC receptors have different kinetic properties, the time course of GABA currents in different bipolar cell types depends on the relative proportions of GABAA and GABAC receptors. Rod bipolar cells, which possess predominantly GABAC receptors, had long-lasting responses, whereas OFF cone bipolar cells, which had a larger fraction of GABAA receptors, had significantly briefer responses, as illustrated in Figure 4 (Shields et al., 2000). The temporal characteristics of inhibition may be matched to the temporal characteristics of excitation in rod and cone bipolar cells. Rod mediated responses are slower than cone mediated responses, attributable to longer synaptic transfer times at rod synapses than at cone synapses (Schnapf & Copenhagen, 1982). Also, cone-mediated synaptic events in ON cone bipolar cells are longer than those in OFF cone bipolar cells (Ashmore & Copenhagen, 1980), attributable to the presence of metabotropic glutamate receptors on ON cone bipolar cell dendrites (Slaughter & Miller, 1981; Nawy & Jahr, 1990) and AMPA and/or kainate receptors on OFF cone bipolar cell dendrites (DeVries, 2000). GABAergic inhibition at the bipolar cell terminals appears to be matched to the kinetics of the excitatory responses; rod bipolar cells have the slowest excitatory responses and the largest complement of GABAC receptors, whereas OFF cone bipolar cells have the fastest excitatory responses and possess a larger complement of GABAA receptors.

Figure 4. GABAA and GABAC receptors temporally tune GABA responses to different bipolar cell types in the retina. Current responses to puff applications of GABA (200 µM), applied to the axon terminals of bipolar cells in slices from ferret retinas (GABA currents were inward because ECl for these experiments was -2 mV and bipolar cells were voltage clamped to -70 mV). Current responses in rod bipolar cells (red trace) were prolonged and mediated largely by GABAC receptors. By contrast, current responses from OFF cone bipolar cells (black trace) were briefer and mediated by a larger fraction of GABAA receptors. Reprinted with permission in modified form from Shields et al, 2000, Journal of Neuroscience. © 2000 by the Society for Neuroscience.

While GABA receptor kinetics may shape synaptic responses, it is not known how significant this role is because the time course of inhibition is attributed to both receptor kinetics and the time course of GABA release. Recent work addresses this issue and shows that GABAA and GABAC receptor properties are important factors in determining the timecourse of light-evoked inhibitory postsynaptic currents (IPSCs) in rod bipolar cells (Eggers & Lukasiewicz, 2006). Elimination of slowly responding GABAC receptors shortened the time course of light-evoked IPSCs, confirming that receptor properties affect light-evoked IPSC time course (Figure 5, Lukasiewicz et al., 2004). These findings suggest that light-evoked synaptic inputs to rod bipolar cells are temporally tuned by postsynaptic GABA receptor properties.

Figure 5. GABAC receptors prolong light-evoked, synaptic inhibition in rod bipolar cells. Brief light flashes (10 ms, indicated by yellow bar below response traces) were used to evoke inhibitory currents in rod bipolar cells from WT and GABACR null mice. Light-evoked inhibition recorded in WT mice was larger and more prolonged (black trace) compared to inhibition recorded from mice that lacked GABAC receptors (red trace).

GABAC receptors may mediate spillover transmission

GABAC receptors have a higher affinity for GABA than GABAA receptors and thus are more likely to be activated by spillover transmission from neighboring synapses. When GABA uptake was blocked in the retina, GABAC, but not GABAA receptor-mediated synaptic responses were enhanced (Ichinose & Lukasiewicz, 2002, Figure 6), demonstrating that GABAC receptors were more sensitive to spillover transmission. When GABA uptake was reduced, the enhanced GABAC receptor-mediated inhibition reduced ganglion cell sensitivity to illumination (Ichinose & Lukasiewicz, 2002), similar to the reductions in ganglion cell light sensitivity produced by surround illumination (Sakmann & Creutzfeldt, 1969; Thibos & Werblin, 1978). These results suggest that GABA transporters limit the extent of light-evoked inhibitory transmission at the inner retina by restricting spillover activation of GABAC receptors. An alternative explanation to enhanced spillover upon uptake blockade is suggested by the findings of Chang and Weiss (1999), demonstrating that the binding site of recombinant GABAC receptors may have more restricted access than the binding site on GABAA receptors. In this scheme, GABAC receptors may be synaptically localized and the blockade of GABA uptake enhances GABAC receptor-mediated responses by increasing the concentration of GABA in the synapse, countering the restricted binding site access.

Figure 6. Blockade of GABA uptake increases spillover transmission and enhances GABAC receptor-mediated responses. Light-evoked, inhibitory synaptic currents (black trace) recorded from bipolar cells in salamander retina (duration of light stimulation indicated by yellow bar below responses). Light-evoked inhibitory currents were potentiated (grey trace) after GABA uptake was blocked with NO-711. This potentiation did not dependent on GABAA receptors because is still occurred in the presence of bicuculline. Reprinted with permission in modified form from Ichinose and Lukasiewicz, 2002, Journal of Neuroscience. © 2002 by the Society for Neuroscience.

GABA receptors in other parts of the CNS play similar roles to those in the retina

Presynaptic inhibition modulates neurotransmission in many areas of the CNS

In other parts of the CNS, GABA receptors play similar roles to those described above in the retina. The presynaptic modulation of neurotransmitter release by ionotropic GABA receptors is one parallel. GABAA receptors, present on GABAergic neuron terminals in the suprachiasmatic nucleus (Belenky et al., 2003) and in the hippocampus (Axmacher & Draguhn, 2004) decrease GABA release. In the adult spinal cord, presynaptic GABAA receptors depolarize glycinergic terminals, due to a relatively positive ECl in the axon terminal, and decrease action potential-mediated release, most likely due to inactivation of voltage-gated Ca²⁺ or Na⁺ channels (Jang et al., 2002). GABAA receptors are also presynaptically located on the terminals of cerebellar interneurons. Early in development, when ECl is more positive than the resting potential, GABA released from these interneurons feedbacks onto GABAA autoreceptors (Mejia-Gervacio & Marty, 2005) to further enhance GABA release (Figure 7). In a similar manner, presynaptic GABAA receptors on the Calyx of Held increase glutamate release in early development and this positive feedback function is taken over by glycine later in development (Turecek & Trussell, 2002).

Figure 7. GABAA receptors on molecular layer neurons in the cerebellum mediate an afterdepolarization. GABAA autoreceptors are activated by GABA release, and this GABA feedback signal causes an afterdepolarization after spiking. This afterdepolarization is mediated by GABAA receptors because it can be blocked by bicuculline. Reprinted with permission in modified form from Mejia-Gervacio & Marty, 2005, Journal of Physiology. © 2005, The Physiological Society.

While GABAC receptors are primarily expressed in the retina, they are also expressed in the superior colliculus, on GABAergic terminals (Boller & Schmidt, 2003). Although GABAC receptors in the superior colliculus neurons are not as extensively studied as in the retina, they have been shown to decrease the paired-pulse depression of GABAergic IPSCs, consistent with their roles as autoreceptors (Kirischuk et al., 2003, Figure 8). While presynaptic ionotropic GABA receptors exist outside the retina, in many cases they function as autoreceptors, modulating GABA release. By contrast, presynaptic GABA receptors on retinal bipolar cells are innervated by GABAergic amacrine interneurons, which limit glutamate release, resulting in the modulation of the transmission of visual information.

Figure 8. Presynaptic GABAC receptors decrease synaptic depression in the superior colliculus. The amplitude of the second IPSC evoked by a pair of stimuli is reduced, indicating that paired pulse depression occurred at this synapse. Paired pulse depression increased (dotted trace) after GABAC receptors were blocked by the addition of the GABAC receptor antagonist TPMPA. These data suggest that GABAC receptors normally limit paired pulse depression. Reprinted with permission in modified form from Kirischuk et al, 2003 European Journal of Neuroscience, © 2003, Federation of European Neuroscience Societies.

GABAA receptors can also mediate tonic inhibition

GABA release in the retina is mediated by graded potential release as well as TTX-sensitive spiking mechanisms (Cook & McReynolds, 1998; Shields & Lukasiewicz, 2003). In other parts of the central nervous system, at low spiking frequencies, GABA is most often released in spike-dependent fashion, giving rise to pulsatile, synchronous release (Lu & Trussell, 2000). By contrast, interneurons in the hippocampus show prolonged, asynchronous GABA release (Hefft & Jonas, 2005), producing more extended activation of GABAA receptors that yields sustained inhibition. Further evidence for GABAA receptor-mediated tonic inhibition is put forth by Mitchell and Silver (2003), who found that tonic, shunting GABAA receptor-mediated inhibition of cerebellar granule cells reduced excitation and altered the excitatory transmission input-output relationship.

Spillover and tonic neurotransmission are usually associated with the activation of high affinity receptors, such as retinal GABAC receptors, that are activated by much lower concentrations of GABA than are present at the synapse (Ichinose & Lukasiewicz, 2002). In other parts of the nervous system, comparable spillover and tonic inhibitory transmission is mediated by different subtypes of GABAA receptors. Similar to GABAC receptors, δ and α6/α4 containing GABAA receptors desensitize slowly and have a higher affinity for GABA than the more typical α1βγ GABAA receptors (Saxena & Macdonald, 1996; Brown et al., 2002). In cerebellar granule (Nusser et al., 1998) and dentate gyrus cells (Nusser & Mody, 2002; Wei et al., 2003) δ and α6 (cerebellum) or α4 (dentate gyrus) containing GABAA receptors are located extrasynaptically. These GABAA receptors are activated by spillover (Wei et al., 2003) in the dentate gyrus and mediate tonic GABAA currents in cerebellar granule cells (Brickley et al., 1996; Brickley et al., 2001). Brickley et al. (2001) demonstrated that extrasynaptic GABAA receptors in the cerebellum contain an α6 subunit by knocking out the α6 subunit. This eliminated only the extrasynaptic receptors, as the α6 knockout mice showed no tonic currents, but synaptic currents were unchanged (Figure 9).

Figure 9. Tonic, but not phasic, GABA receptor-mediated currents in cerebellar granule cells are decreased when the α6 GABAA receptor subunit is knocked out. A. In WT mice, both phasic and tonic GABAA receptor-mediated currents are blocked by the application of SR95531. B. In mice that lack α6δ GABAA receptor subunits, the tonic current was eliminated, but the phasic current remained unchanged. These data indicate that tonic currents were mediated by α6δ-containing GABAA receptors. Adapted by permission from Macmillan Publishers Ltd: Nature, Brickley et al, 2001.

Are α4/6δ containing GABAA receptors comparable to the GABAC receptors in the retina? GABAA receptors with α6βδ subunits have an EC50 values of ~0.5 µM (Saxena & Macdonald, 1996) and slow desensitization constants of ~ 5 seconds (Brown et al., 2002). However, these receptors have a shorter burst duration and mean open time than non-δ containing GABAA receptors (Fisher & Macdonald, 1997), suggesting that they may not mediate more prolonged GABA responses, as GABAC receptors do. Recombinant and native GABAC receptors have a similar high affinity to GABA, with an EC50 value of ~ 1 µM (Qian & Dowling, 1993; Chang & Weiss, 1999) and show a virtual absence of desensitization during prolonged GABA application. GABAC receptors also show slow current decay constants, attributable to very slow unbinding kinetics of the recombinant ρ1 GABAC receptors when the channel is open (Chang & Weiss, 1999). While the α4/6βδ subtypes of the GABAA receptor are primarily extrasynaptic and mediate spillover/tonic GABA responses, GABAC receptors are synaptic (Koulen et al., 1998) and mediate slow inhibitory synaptic responses that modulate tonic, graded glutamate release from bipolar cells. Because they have a high affinity for GABA, GABAC receptors at neighboring synapses may be also be activated by spillover.

Temporally tuning inhibition with distinct GABA receptors

In addition to shaping inhibition by receptor location, GABA receptors with distinct kinetic properties can also temporally tune inhibition. In the retina, the time course of inhibition in different types of bipolar cells is tuned by the relative proportions of GABAA and GABAC receptors (Shields et al., 2000; Eggers & Lukasiewicz, 2006). In the hippocampus, spontaneous IPSCs in different interneurons have distinct time courses, suggesting that they possess different types of GABAA receptor subunits (Hajos & Mody, 1997). Additionally, the decay time of GABAerigc IPSCs in many regions of the CNS decreases with development because GABAA receptor subunits switch from α2 or α5 in early development to α1 containing GABAA receptors in adults (Brickley et al., 1996; Dunning et al., 1999; Okada et al., 2000, Figure 10). Another example of how different types of receptors temporally tune inhibition comes from the spinal cord and brainstem, where the relative activation of glycine and GABAA receptors, with distinct kinetics, determines the timecourse of IPSCs (Jonas et al., 1998; O'Brien & Berger, 1999). In the retina the kinetics of GABAC receptor-mediated inhibition are matched to the time course of bipolar cell excitation, as noted above. During development, GABAA receptor kinetics temporally tune inhibition in a similar way; the timecourse of inhibition decreases in parallel with the decreases in the time course of excitation that occur during development (Takahashi, 2005).

Figure 10. The decay time of GABAergic IPSCs changes over development in the thalamus. A. The decay time constant and rise-time of evoked IPSCs decrease from postnatal day 5 to postnatal day 31. B. Over this same time period, the expression of GABAA α subunits changes from predominantly α2 to predominantly α1. Reprinted with permission in modified form from Okada et al, 2000, Journal of Neuroscience. © 2000 by the Society for Neuroscience.

Modulation of excitatory signaling

Just as GABAC receptor-mediated currents modulate the temporal and spatial properties of excitatory signaling in the retina, tonic GABAA receptor-mediated currents (Brickley et al., 1996), attributed to spillover (Hamann et al., 2002), decrease the excitability of cerebellar granule cells. This spillover current is significant, mediating the largest amount of inhibitory charge transfer (97%) (Hamann et al., 2002). Spillover transmission by distant, extrasynaptic GABA receptors enhances the influence of inhibitory signaling by increasing its spatial extent. This tonic, spillover inhibition reduces spontaneous firing in granule cells by preventing spontaneous excitatory postsynaptic potentials from eliciting action potentials, preserving a high signal-to-noise ratio for the encoding of sensory inputs (Chadderton et al., 2004). GABAA receptors can also play a role in temporal tuning of excitatory responses, similar to the role GABAC receptors play in the retina, making excitatory responses more transient. In hippocampal pyramidal cells, GABAA receptor-mediated inhibition decreases the variation in the timing of spiking (Pouille & Scanziani, 2001). This would have the effect, on average, of making signaling from pyramidal cells more transient (Figure 11).

Figure 11. GABAA receptor-mediated inhibition makes spiking in hippocampal pyramidal cells more transient. Current responses to the stimulation of two separate Shaeffer collaterals recorded from hippocampal pyramidal cells in cell-attached patch mode. In control conditions, a single spike always occurred at the same time over four trials when the 2 Schaeffer collaterals were simultaneously stimulated (left). When inhibition was blocked by the application of bicuculline (right), spiking was no longer synchronized with the stimuli and occurred over a range of times for each of the four stimuli. Reprinted with permission in modified form from Pouille and Scanziani, 2001, Science. © 2001, AAAS.

Summary

Ionotropic GABA receptors in the retina and in other parts of the CNS play similar roles. In the retina, slowly responding GABAC receptors affect the timing and magnitude of transmission from bipolar cells onto ganglion cells, the outputs of the retina. In other parts of the CNS, presynaptic GABAA receptors modulate release either by autoreceptor-mediated inhibition or by heterosynaptic inhibition. Spillover and tonic inhibition are found in both the retina and other parts of the CNS, but they are mediated by different types of ionotropic GABA receptors, GABAC receptors in retina and δ and α6/α4 containing GABAA receptors in the brain The retina is an ideal place to study ionotropic GABAergic inhibition because it can be activated with natural stimuli, the physiological functions of inhibition in sensory processing can be determined, and its inhibitory mechanisms are similar to those found in other parts of the CNS.

Acknowledgements

This work was supported by T32 EY13360 and F32 EY15629 (EDE), EY08922 (PDL), EY-02687 (core grant to Dept. of Ophthalmology), the M.R. Bauer Foundation (PDL) and Research to Prevent Blindness.

References

AMIN, J. & WEISS, D. S. (1994). Homomeric rho 1 GABA channels: activation properties and domains. Receptors and Channels 2, 227-236.

ASHMORE, J. F. & COPENHAGEN, D. R. (1980). Different postsynaptic events in two types of retinal bipolar cell. Nature 288, 84-86.

AXMACHER, N. & DRAGUHN, A. (2004). Inhibition of GABA release by presynaptic ionotropic GABA receptors in hippocampal CA3. NeuroReport 15, 329-334.

BAYLOR, D. A., FOURTES, M. G. F. & O'BRYAN, P. M. (1971). Receptive fields of cones in the retina of the turtle. Journal of Physiology 214, 265-294.

BELENKY, M. A., SAGIV, N., FRITSCHY, J. M. & YAROM, Y. (2003). Presynaptic and postsynaptic GABAA receptors in rat suprachiasmatic nucleus. Neuroscience 118, 909-923.

BIEDA, M. C. & COPENHAGEN, D. R. (2000). Inhibition is not required for the production of transient spiking responses from retinal ganglion cells. Visual Neuroscience 17, 243-254.

BLANCO, R., VAQUERO, C. F. & P, D. L. V. (1996). The effects of GABA and glycine on horizontal cells of the rabbit retina. Vision Research 36, 3987-3995.

BLOOMFIELD, S. A. & XIN, D. (2000). Surround inhibition of mammalian AII amacrine cells is generated in the proximal retina. Journal of Physiology 523.3, 771-783.

BOLLER, M. & SCHMIDT, M. (2003). GABAC receptors in the rat superior colliculus and pretectum participate in synaptic neurotransmission. J Neurophysiol 89, 2035-2045.

BRICKLEY, S. G., CULL-CANDY, S. G. & FARRANT, M. (1996). Development of a tonic form of synaptic inhibition in rat cerebellar granule cells resulting from persistent activation of GABAA receptors. J Physiol 497 ( Pt 3), 753-759.

BRICKLEY, S. G., REVILLA, V., CULL-CANDY, S. G., WISDEN, W. & FARRANT, M. (2001). Adaptive regulation of neuronal excitability by a voltage-independent potassium conductance. Nature 409, 88-92.

BROWN, N., KERBY, J., BONNERT, T. P., WHITING, P. J. & WAFFORD, K. A. (2002). Pharmacological characterization of a novel cell line expressing human alpha(4)beta(3)delta GABA(A) receptors. Br J Pharmacol 136, 965-974.

CHADDERTON, P., MARGRIE, T. W. & HAUSSER, M. (2004). Integration of quanta in cerebellar granule cells during sensory processing. Nature 428, 856-860.

CHANG, Y. & WEISS, D. S. (1999). Channel opening locks agonist onto the GABAC receptor. Nature Neuroscience 2, 219-225.

COOK, P. B., LUKASIEWICZ, P. D. & MCREYNOLDS, J. S. (1998). Action potentials are required for the lateral transmission of glycinergic transient inhibition in the amphibian retina. The Journal of Neuroscience 18, 2301-2308.

COOK, P. B. & MCREYNOLDS, J. S. (1998). Lateral inhibition in the inner retina is important for spatial tuning of ganglion cells. Nature Neuroscience 1, 714-719.

DEVRIES, S. H. (2000). Bipolar cells use kainate and AMPA receptors to filter visual information into separate channels. Neuron 28, 847-856.

DONG, C. & WERBLIN, F. S. (1998). Temporal contrast enhancement via GABAC feedback at bipolar terminals in the tiger salamander retina. Journal of Neurophysiology 79, 2171-2180.

DONG, C.-J., PICAUD, S. A. & WERBLIN, F. S. (1994). GABA transporters and GABAc-like receptors on catfish cone- but not rod-driven horizontal cells. The Journal of Neuroscience 14, 2648-2658.

DOWLING, J. E. & BOYCOTT, B. B. (1966). Organization of the primate retina: electron microscopy. Proceedings of the Royal Society London, Series B 166, 80-111.

DUNNING, D. D., HOOVER, C. L., SOLTESZ, I., SMITH, M. A. & O'DOWD, D. K. (1999). GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons. J Neurophysiol 82, 3286-3297.

EGGERS, E. D. & LUKASIEWICZ, P. D. (2006). GABAA, GABAC and glycine receptor-mediated inhibition differentially affects light-evoked signaling from mouse retinal rod bipolar cells. Journal of Physiology In press.

ENZ, R., BRANDSTÄTTER, J. H., HARTVIET, E., WÄSSLE, H. & BORMANN, J. (1995). Expression of GABA receptors p1 and p2 subunits in the retina and brain of the rat. European Journal of Neuroscience 7, 1495-1501.

ENZ, R., BRANDSTÄTTER, J. H., WÄSSLE, H. & BORMANN, J. (1996). Immunocytochemical Localization of the GABAc Recptor rho subunits in the mammalian retina. The Journal of Neuroscience 16, 4479-4490.

EULER, T. & WÄSSLE, H. (1998). Different contributions of GABAA and GABAC receptors to rod and cone bipolar cells in a rat retinal slice preparation. Journal of Neurophysiology 79, 1384-1395.

FEIGENSPAN, A. & BORMANN, J. (1994). Differential pharmacology of GABA-A and GABA-C receptors on rat retinal bipolar cells. European Journal of Pharmacology 288, 97-104.

FISHER, J. L. & MACDONALD, R. L. (1997). Single channel properties of recombinant GABAA receptors containing gamma 2 or delta subtypes expressed with alpha 1 and beta 3 subtypes in mouse L929 cells. J Physiol 505 ( Pt 2), 283-297.

FLETCHER, E. L., KOULEN, P. & WÄSSLE, H. (1998). GABAA and GABAC receptors on mammalian rod bipolar cells. The Journal of Comparative Neurology 396, 351-365.

FLORES-HERR, N., PROTTI, D. A. & WÄSSLE, H. (2001). Synaptic currents generating the inhibitory surround of ganglion cells in the mammalian retina. The Journal of Neuroscience 21, 4852-4863.

FREED, M. A., SMITH, R. G. & STERLING, P. (2003). Timing of quantal release from the retinal bipolar terminal is regulated by a feedback circuit. Neuron 38, 89-101.

GREFERATH, U., GRUNERT, U., FRITSCHY, J. M., STEPHENSON, A., MOHLER, H. & WÄSSLE, H. (1995). GABAA receptor subunits have differential distributions in the rat retina: in situ hybridization and immunohistochemistry. The Journal of Comparative Neurology 353, 553-571.

HACKAM, A. S., WANG, T. L., GUGGINO, W. B. & CUTTING, G. R. (1998). Sequences in the amino termini of GABA ρ and GABAA subunits specify their selective interaction in vitro. J Neurochem 70, 40-46.

HAJOS, N. & MODY, I. (1997). Synaptic communication among hippocampal interneurons: properties of spontaneous IPSCs in morphologically identified cells. J Neurosci 17, 8427-8442.

HAMANN, M., ROSSI, D. J. & ATTWELL, D. (2002). Tonic and spillover inhibition of granule cells control information flow through cerebellar cortex. Neuron 33, 625-633.

HARTVEIT, E. (1997). Functional organization of cone bipolar cells in the rat retina. Journal of Neurophysiology 77, 1716-1730.

HAVERKAMP, S., GRUNERT, U. & WÄSSLE, H. (2000). The cone pedicle, a complex synapse in the retina. Neuron 27, 85-95.

HAVERKAMP, S. & WÄSSLE, H. (2000). Immunocytochemical analysis of the mouse retina. The Journal of Comparative Neurology 424, 1-23.

HEFFT, S. & JONAS, P. (2005). Asynchronous GABA release generates long-lasting inhibition at a hippocampal interneuron-principal neuron synapse. Nat Neurosci 8, 1319-1328.

ICHINOSE, T. & LUKASIEWICZ, P. D. (2002). GABA transporters regulate inhibition in the retina by limiting GABAC receptor activation. J Neurosci 22, 3285-3292.

ICHINOSE, T. & LUKASIEWICZ, P. D. (2005). Inner and outer retinal pathways both contribute to surround inhibition of salamander ganglion cells. J Physiol 565, 517-535.

JANG, I. S., JEONG, H. J., KATSURABAYASHI, S. & AKAIKE, N. (2002). Functional roles of presynaptic GABA(A) receptors on glycinergic nerve terminals in the rat spinal cord. J Physiol 541, 423-434.

JONAS, P., BISCHOFBERGER, J. & SANDKUHLER, J. (1998). Corelease of two fast neurotransmitters at a central synapse. Science 281, 419-424.

KAMERMANS, M., FAHRENFORT, I., SCHULTZ, K., JANSSEN-BIENHOLD, U., SJOERDSMA, T. & WEILER, R. (2001). Hemichannel-mediated inhibition in the outer retina. Science 292, 1178-1180.

KAMERMANS, M., FAHRENFORT, I. & SJOERDSMA, T. (2002). GABAergic modulation of ephaptic feedback in the outer retina. Investigative Ophthalmology and Visual Science, ARVO Abstract.

KIRISCHUK, S., AKYELI, J., IOSUB, R. & GRANTYN, R. (2003). Pre- and postsynaptic contribution of GABAC receptors to GABAergic synaptic transmission in rat collicular slices and cultures. Eur J Neurosci 18, 752-758.

KOULEN, P., BRANDSTÄTTER, J. H., ENZ, R., BORMANN, J. & WÄSSLE, H. (1998). Synaptic clustering of GABAC receptor ρ-subunits in rat retina. European Journal of Neuroscience 10, 115-127.

KOULEN, P., BRANDSTÄTTER, J. H., KROGER, S., ENZ, R., BORMANN, J. & WÄSSLE, H. (1997). Immunocytochemical localization of the GABAC receptor ? subunits in the cat, goldfish and chicken retina. The Journal of Comparative Neurology 380, 520-532.

LU, T. & TRUSSELL, L. O. (2000). Inhibitory transmission mediated by asynchronous transmitter release. Neuron 26, 683-694.

LUKASIEWICZ, P. & SHIELDS, C. (1998). Different combinations of GABAA and GABAC receptors confer distinct temporal properties to retinal synaptic responses. Journal of Neurophysiology 79, 3157-3167.

LUKASIEWICZ, P. D., EGGERS, E. D., SAGDULLAEV, B. T. & MCCALL, M. A. (2004). GABAC receptor-mediated inhibition in the retina. Vision Research 44, 3289-3296.

LUKASIEWICZ, P. D., MAPLE, B. R. & WERBLIN, F. S. (1994). A novel GABA receptor on bipolar cell terminals in the tiger salamander retina. The Journal of Neuroscience 14, 1202-1212.

LUKASIEWICZ, P. D. & WERBLIN, F. S. (1994). A novel GABA receptor modulates synaptic transmission from bipolar to ganglion and amacrine cells in the tiger salamander retina. The Journal of Neuroscience 14, 1213-1223.

MACNEIL, M. A. & MASLAND, R. H. (1998). Extreme diversity among amacrine cells: implications for functions. Neuron 20, 971-982.

MASLAND, R. H. (2001). The fundamental plan of the retina. In Nat Neurosci, vol. 4, pp. 877-886.

MATSUI, K., HASEGAWA, J. & TACHIBANA, M. (2001). Modulation of excitatory synaptic transmission by GABA(C) receptor-mediated feedback in the mouse inner retina. J Neurophysiol 86, 2285-2298.

MATTHEWS, G., AYOUB, G. S. & HEIDELBURGER, R. (1994). Presynaptic inhibition by GABA is mediated via two distinct GABA receptors with novel pharmacology. The Journal of Neuroscience 14, 1079-1090.

MCCALL, M. A., LUKASIEWICZ, P. D., GREGG, R. G. & PEACHEY, N. S. (2002). Elimination of the ρ1 subunit abolishes GABAC receptor expression and alters visual processing in the mouse retina. The Journal of Neuroscience 22, 4163-4174.

MEHTA, A. K. & TICKU, M. J. (1999). An update on GABAA receptors. Brain Research Reviews 29, 196-217.

MEJIA-GERVACIO, S. & MARTY, A. (2005). Control of interneuron firing pattern by axonal autoreceptors in the juvenile rat cerebellum. J Physiol.

MITCHELL, S. J. & SILVER, R. A. (2003). Shunting inhibition modulates neuronal gain during synaptic excitation. Neuron 38, 433-445.

NAWY, S. & JAHR, C. E. (1990). Suppression by glutamate of cGMP-activated conductance in retinal bipolar cells. Nature 346, 269-271.

NUSSER, Z. & MODY, I. (2002). Selective modulation of tonic and phasic inhibitions in dentate gyrus granule cells. J Neurophysiol 87, 2624-2628.

NUSSER, Z., SIEGHART, W. & SOMOGYI, P. (1998). Segregation of different GABA{-A} receptors to synaptic and extrasynaptic membranes of cerebellar granule cells. The Journal of Neuroscience 18, 1693-1703.

O'BRIEN, J. A. & BERGER, A. J. (1999). Cotransmission of GABA and glycine to brain stem motoneurons. J Neurophysiol 82, 1638-1641.

OKADA, M., ONODERA, K., VAN RENTERGHEM, C., SIEGHART, W. & TAKAHASHI, T. (2000). Functional correlation of GABA(A) receptor alpha subunits expression with the properties of IPSCs in the developing thalamus. J Neurosci 20, 2202-2208.

PAN, Z.-H. & LIPTON, S. A. (1995). Multiple GABA receptor subtypes mediate inhibition of calcium influx at rat retinal bipolar cell terminals. The Journal of Neuroscience 15, 2668-2679.

PAN, Z.-H., ZHANG, D., ZHANG, X. & LIPTON, S. A. (2000). Evidence for coassembly of mutant GABAC ρ1 with GABAA γ2S, glycine α1 and glycine α2 receptor subunits. European Journal of Neuroscience 12, 3137-3145.

PATTNAIK, B., JELLALI, A., SAHEL, J., DREYFUS, H. & PICAUD, S. (2000). GABAC receptors are localized with microtubule-associated protein 1B in mammalian cone photoreceptors. J Neurosci 20, 6789-6796.

PICAUD, S., PATTNAIK, B., HICKS, D., FORSTER, V., FONTAINE, V., SAHEL, J. & DREYFUS, H. (1998). GABA-A and GABA-C receptors in adult porcine cones: evidence form a photoreceptor-glia co-culture model. Journal of Physiology 513.1, 33-42.

POUILLE, F. & SCANZIANI, M. (2001). Enforcement of temporal fidelity in pyramidal cells by somatic feed-forward inhibition. Science 293, 1159-1163.

POURCHO, R. G. & GOEBEL, D. J. (1983). Neuronal subpopulations in cat retina which accumulate the GABA agonist, (3H)muscimol: a combined Golgi and autoradiographic study. J Comp Neurol 219, 25-35.

QIAN, H. & DOWLING, J. E. (1993). Novel GABA responses from rod-driven retinal horizontal cells. Nature 361, 162-164.

QIAN, H. & DOWLING, J. E. (1995). GABAa and GABAc receptors on hybrid bass retinal bipolar cells. Journal of Neurophysiology 74, 1920-1928.

QIAN, H. & PAN, Y. (2002). Co-assembly of GABA rho subunits with the GABA(A) receptor gamma(2) subunit cloned from white perch retina. Brain Res Mol Brain Res 103, 62-70.

QIAN, H. & RIPPS, H. (1999). Response kinetics and pharmacological properties of heteromeric receptors formed by coassembly of GABA ?- and ?2-subunits. Proceedings of the Royal Society 266, 2343-2450.

SAKMANN, B. & CREUTZFELDT, O. D. (1969). Scotopic and mesopic light adaptation in the cat's retina. Pflugers Arch 313, 168-185.

SAXENA, N. C. & MACDONALD, R. L. (1996). Properties of putative cerebellar gamma-aminobutyric acid A receptor isoforms. Mol Pharmacol 49, 567-579.

SCHNAPF, J. L. & COPENHAGEN, D. R. (1982). Differences in the kinetics of rod and cone synaptic transmission. Nature 296, 862-864.

SHEN, W. & SLAUGHTER, M. M. (2001). Multireceptor GABAergic regulation of synaptic communication in amphibian retina. Journal of Physiology 530, 55-67.

SHIELDS, C. R. & LUKASIEWICZ, P. D. (2003). Spike-dependent GABA inputs to bipolar cell axon terminals contribute to lateral inhibition of retinal ganglion cells. J Neurophysiol 89, 2449-2458.

SHIELDS, C. R., TRAN, M. N., WONG, R. O. & LUKASIEWICZ, P. D. (2000). Distinct ionotropic GABA receptors mediate presynaptic and postsynaptic inhibition in retinal bipolar cells. J Neurosci 20, 2673-2682.

SHIMADA, S., CUTTING, G. & UHL, G. R. (1992). γ-Aminobutyric acid A or C receptor? γ- Aminobutyric acid ρ1 receptor RNA induces bicuculline-,barbiturate-, and benzodiazepine-insensitive γ-aminobutyric acid responses in Xenopus oocytes. Molecular Pharmacology 41, 683-687.

SLAUGHTER, M. M. (1995). GABAB receptors in the vertebrate retina. Progress in Retinal and Eye Research 14, 293-312.

SLAUGHTER, M. M. & MILLER, R. F. (1981). 2-Amino-4-Phosphonobutyric Acid: A New Pharmacological Tool for Retina Research. Science 211, 182-185.

STOCKTON, R. A. & SLAUGHTER, M. M. (1991). Depolarizing actions of GABA and glycine on amphibian retinal horizontal cells. Journal of Neurophysiology 65, 680-692.

TAKAHASHI, T. (2005). Postsynaptic receptor mechanisms underlying developmental speeding of synaptic transmission. Neurosci Res 53, 229-240.

THIBOS, L. N. & WERBLIN, F. S. (1978). The response properties of the steady antagonistic surround in the mudpuppy retina. Journal of Physiology 278, 79-99.

TURECEK, R. & TRUSSELL, L. O. (2002). Reciprocal developmental regulation of presynaptic ionotropic receptors. Proc Natl Acad Sci U S A 99, 13884-13889.

VERWEIJ, J., KAMERMANS, M. & SPEKREIJSE, H. (1996). Horizontal cells feed back to cones by shifting the cone calcium-current activation range. Vision Research 36, 3943-3953.

WASSLE, H., KOULEN, P., BRANDSTATTER, J. H., FLETCHER, E. L. & BECKER, C. M. (1998). Glycine and GABA receptors in the mammalian retina. Vision Res 38, 1411-1430.

WEI, W., ZHANG, N., PENG, Z., HOUSER, C. R. & MODY, I. (2003). Perisynaptic localization of delta subunit-containing GABA(A) receptors and their activation by GABA spillover in the mouse dentate gyrus. J Neurosci 23, 10650-10661.

WERBLIN, F. S. & DOWLING, J. E. (1969). Organization of the retina of mudpuppy, Necuturus maculosis. II. Intracellular recording. Journal of Neurophysiology 32, 331-355.

WU, S. M. & MAPLE, B. R. (1998). Amino acid neurotransmitters in the retina: a functional overview. Vision Research 38, 1371-1384.

YEH, H. H., GRIGORENKO, E. V. & VERUKI, M. L. (1996). Correlation Between a Bicuculline-resistant Response to GABA and GABA-A Receptor p1 Subunit Expression in Single Rat Retinal Bipolar Cells. Visual Neuroscience 13, 283-292.

ZHANG, D., PAN, Z.-H., ZHANG, X., BRIDEAU, A. D. & LIPTON, S. A. (1995). Cloning of a gamma-aminobutyric acid type C receptor subunit in rat retina with a methionine residue critical for picrotoxin channel block. Proceedings of the National Academy of Sciences of the United States of America 92, 11756-11760.

ZHANG, J., DE BLAS, A. L., MIRALLES, C. P. & YANG, C. Y. (2003). Localization of GABAA receptor subunits alpha 1, alpha 3, beta 1, beta 2/3, gamma 1, and gamma 2 in the salamander retina. J Comp Neurol 459, 440-453.