GABAC receptors in the vertebrate retina

I4AA-Sensitive chloride current contributes to the center light responses of bipolar cells in the tiger salamander retina

Light-evoked currents in depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) were recorded under voltage-clamp conditions in living retinal slices of the larval tiger salamander. Responses to illumination at the center of the DBCs' and HBCs' receptive fields were mediated by two postsynaptic currents: DeltaI(C), a glutamate-gated cation current with a reversal potential near 0 mV, and DeltaI(Cl), a chloride current with a reversal potential near -60 mV. In DBCs DeltaI(C) was suppressed by L-2-amino-4-phosphonobutyric acid (L-AP4), and in HBCs it was suppressed by 6,7-dinitroquinoxaline-2,3-dione (DNQX). In both DBCs and HBCs DeltaI(Cl) was suppressed by imidazole-4-acetic acid (I4AA), a GABA receptor agonist and GABA(C) receptor antagonist. In all DBCs and HBCs examined, 10 microM I4AA eliminated DeltaI(Cl) and the light-evoked current became predominately mediated by DeltaI(C). The addition of 20 microM L-AP4 to the DBCs or 50 microM DNQX to HBCs completely abolished DeltaI(C). Focal application of glutamate at the inner plexiform layer elicited chloride currents in bipolar cells by depolarizing amacrine cells that release GABA at synapses on bipolar cell axon terminals, and such glutamate-induced chloride currents in DBCs and HBCs could be reversibly blocked by 10 microM I4AA. These experiments suggest that the light-evoked, I4AA-sensitive chloride currents (DeltaI(Cl)) in DBCs and HBCs are mediated by narrow field GABAergic amacrine cells that activate GABA(C) receptors on bipolar cell axon terminals. Picrotoxin (200 microM) or (1,2,5,6-tetrahydropyridine-4yl) methyphosphinic acid (TPMPA) (2 other GABA(C) receptor antagonists) did not block (but enhanced and broadened) the light-evoked DeltaI(Cl), although they decreased the chloride current induced by puff application of GABA or glutamate. The light response of narrow field amacrine cells were not affected by I4AA, but were substantially enhanced and broadened by picrotoxin. These results suggest that there are at least two types of GABA(C) receptors in bipolar cells: one exhibits stronger I4AA sensitivity than the other, but both can be partially blocked by picrotoxin. The GABA receptors in narrow field amacrine cells are I4AA insensitive and picrotoxin sensitive. The light-evoked DeltaI(Cl) in bipolar cells are mediated by the more strongly I4AA-sensitive GABA(C) receptors. Picrotoxin, although acting as a partial GABA(C) receptor antagonist in bipolar cells, does not suppress DeltaI(Cl) because its presynaptic effects on amacrine cell light responses override its antagonistic postsynaptic actions.

Influence of light and neural circuitry on tyrosine hydroxylase phosphorylation in the rat retina

Light has been shown to increase dopamine synthesis and release in vertebrate retinas, but the retinal circuits mediating the light signal are unknown. We utilized three antibodies which recognize phosphorylated forms of tyrosine hydroxylase (TH) at serines 19, 31, and 40 to study the effects of light and neuroactive drugs on TH phosphorylation in the rat retina. Phosphorylated TH and total TH immunoreactivities were co-localized exclusively in retinal neurons whose shape and location are characteristic of dopaminergic interplexiform cells. Phosphorylated TH was weak to absent in darkness, but light strongly stimulated phosphorylation in all the three serine residues. Light-induced phosphorylation of TH induction by light was uniformly blocked by a combination of NMDA and AMPA glutamate receptor antagonists. In darkness, the combination of NMDA+AMPA induced phosphosphorylation of TH at serines 19 and 40 but it was weak at serine 31. A GABA(A) antagonist had the same effect. An agonist of depolarizing (ON) bipolar cells, L-(+)-2-amino-4-phosphonobutyric acid, did not prevent light-induced phosphorylated TH formation. Carbachol, a non-specific cholinergic agonist, selectively induced phosphorylation of TH at serine 31 in darkness, an effect which was blocked by the nicotinic antagonist, d-tubocurarine. These results show that retinal circuits involving glutamatergic, GABAergic and cholinergic synapses influence phospho-TH formation at different serine residues in this enzyme. Gamma amino butyric acid (GABA) and glutamate influence TH phosphorylation at serines 19 and 40, whereas cholinergic inputs affect its phosphorylation at serine 31.

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.

Suppressive actions of betaxolol on ionic currents in retinal ganglion cells may explain its neuroprotective effects

Betaxolol, a beta 1-selective adrenoceptor antagonist, is widely used in the treatment of glaucoma. In addition to its ocular hypotensive effects, betaxolol has been suggested to act as a retinal neuroprotective agent (Osborne et al., 1997). To investigate possible mechanisms underlying the neuroprotective effects, we tested the actions of betaxolol on ion channels and calcium signaling in isolated retinal ganglion cells. Betaxolol (50 microM) reduced by about 20% the high-voltage-activated (HVA) Ca channel currents in ganglion cells isolated from tiger salamander retina. In contrast, the beta 1-adrenoceptor antagonists propranolol (10 microM) and timolol (50 microM) had no inhibitory actions on HVA Ca channel currents. The L-type Ca channel antagonist, nisoldipine, blocked the HVA Ca channel current partially and the remaining current was not inhibited by betaxolol. Outward current was inhibited in the presence of betaxolol. Both iberiotoxin (IBTX; 10 nM), a selective inhibitor of large-conductance Ca-activated K channels, and Cd2+ (100 microM), which suppresses Ca-activated K channels subsequent to its block of Ca channels, reduced outward current and the remaining current was not blocked significantly with betaxolol. In the presence of betaxolol, Na channel currents were reduced by about 20%, as were currents evoked by glutamate (10 mM) and GABA (1 mM). Current clamp recordings from isolated ganglion cells showed that betaxolol had several effects on excitability: spike height decreased, repetitive spike activity was suppressed, spike width increased and hyperpolarization following spikes was reduced. Calcium imaging in isolated rat retinal ganglion cells revealed that betaxolol inhibited glutamate-induced increases in [Ca2+]i. These results suggest that betaxolol has a diversity of suppressive actions on ganglion cell ion channels and that, as a consequence, one of the net actions of the drug is to reduce Ca2+ influx. The subsequent reduction in [Ca2+]i may contribute to the apparent neuroprotective actions of betaxolol in promoting ganglion cell survival following ischemic insult, as may occur in glaucoma and retinal disease.

Light-evoked responses of bipolar cells in a mammalian retina

We recorded light-evoked responses from rod and cone bipolar cells using patch-clamp techniques in a slice preparation of the rat retina. Rod bipolar cells responded to light with a sustained depolarization (ON response) followed at light offset by a slight hyperpolarization. ON and OFF cone bipolar cells were encountered, both with diverse temporal properties. The responses of rod bipolar cells were composed primarily of two components, a nonspecific cation current and a chloride current. The chloride current was reduced greatly in axotomized cells and could be suppressed by coapplication of the GABA(A) antagonist bicuculline and the GABA(C) antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid. This suggests that it largely reflects feedback from GABAergic amacrine cells. The response latency of intact rod bipolar cells was shorter than that of the axotomized cells, and the sensitivity curve covered more than twice the dynamic range. Application of the GABA receptor antagonists partially mimicked the effects of axotomy. These findings suggest that functional properties of the axon terminal system-notably synaptic feedback from amacrine cells-play an important role in defining the response properties of mammalian bipolar cells.

GABAρ1/GABA A α1 receptor chimeras to study receptor desensitization

γ-Aminobutyrate type C (GABA C ) receptors are ligand-gated ion channels that are expressed preponderantly in the vertebrate retina and are characterized, among other things, by a very low rate of desensitization and resistance to the specific GABA A antagonist bicuculline. To examine which structural elements determine the nondesensitizing character of the human homomeric ρ1 receptor, we used a combination of gene chimeras and electrophysiology of receptors expressed in Xenopus oocytes. Two chimeric genes were constructed, made up of portions of the ρ1-subunit and of the α1-subunit of the GABA A receptor. When expressed in Xenopus oocytes, one chimeric gene (ρ1/α1) formed functional homooligomeric receptors that were fully resistant to bicuculline and were blocked by the specific GABA C antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid and by zinc. Moreover, these chimeric receptors had a fast-desensitizing component, even faster than that of heterooligomeric GABA A receptors, in striking contrast to the almost nil desensitization of wild-type ρ1 (wt ρ1) receptors. To see whether the fast-desensitizing characteristic of the chimera was determined by the amino acids forming the ion channels, we replaced the second transmembrane segment (TM2) of ρ1 by that of the α1-subunit of GABA A . Although the α1-subunit forms fast-desensitizing receptors when coexpressed with other GABA A subunits, the sole transfer of the α1TM2 segment to ρ1 was not sufficient to form desensitizing receptors. All this suggests that the slow-desensitizing trait of ρ1 receptors is determined by a combination of several interacting domains along the molecule.

GABArho 1/GABAAalpha 1 receptor chimeras to study receptor desensitization

gamma-Aminobutyrate type C (GABA(C)) receptors are ligand-gated ion channels that are expressed preponderantly in the vertebrate retina and are characterized, among other things, by a very low rate of desensitization and resistance to the specific GABA(A) antagonist bicuculline. To examine which structural elements determine the nondesensitizing character of the human homomeric rho1 receptor, we used a combination of gene chimeras and electrophysiology of receptors expressed in Xenopus oocytes. Two chimeric genes were constructed, made up of portions of the rho1-subunit and of the alpha1-subunit of the GABA(A) receptor. When expressed in Xenopus oocytes, one chimeric gene (rho1/alpha1) formed functional homooligomeric receptors that were fully resistant to bicuculline and were blocked by the specific GABA(C) antagonist (1,2,5, 6-tetrahydropyridine-4-yl)methylphosphinic acid and by zinc. Moreover, these chimeric receptors had a fast-desensitizing component, even faster than that of heterooligomeric GABA(A) receptors, in striking contrast to the almost nil desensitization of wild-type rho1 (wt rho1) receptors. To see whether the fast-desensitizing characteristic of the chimera was determined by the amino acids forming the ion channels, we replaced the second transmembrane segment (TM2) of rho1 by that of the alpha1-subunit of GABA(A). Although the alpha1-subunit forms fast-desensitizing receptors when coexpressed with other GABA(A) subunits, the sole transfer of the alpha1TM2 segment to rho1 was not sufficient to form desensitizing receptors. All this suggests that the slow-desensitizing trait of rho1 receptors is determined by a combination of several interacting domains along the molecule.

GABA receptor rho1 subunit interacts with a novel splice variant of the glycine transporter, GLYT-1

Ionotropic gamma-aminobutyric acid (GABA(A) and GABA(C)) receptors mediate fast synaptic inhibition in the central nervous system. GABA(C) receptors are expressed predominantly in the retina on bipolar cell axon terminals, and are thought to mediate feedback inhibition from GABAergic amacrine cells. Utilizing the yeast two-hybrid system, we previously identified MAP1B as a binding partner of the GABA(C) receptor rho1 subunit. Here we describe the isolation of an additional rho1 interacting protein: a novel C-terminal variant of the glycine transporter GLYT-1. We show that GLYT-1 exists as four alternatively spliced mRNAs which encode proteins expressing one of two possible intracellullar N- and C-terminal domains. Variants containing the novel C terminus efficiently transport glycine when expressed in COS cells, but with unusual kinetics. We have confirmed the interaction between the novel C terminus and rho1 subunit and demonstrated binding in heterologous cells. This interaction may be crucial for the integration of GABAergic and glycinergic neurotransmission in the retina.

Identification of 70 amino acids important for GABA(C) receptor rho1 subunit assembly

gamma-Aminobutyric acid (GABA) is the most important inhibitory neurotransmitter in the mammalian central nervous system and gates at least three subclasses of receptors, termed GABA(A), GABA(B) and GABA(C). Accumulating evidence indicates that GABA(C) receptors are composed exclusively of rho subunits. The N-terminal half of the rho subunits has been shown to mediate formation of homo- and heterooligomeric GABA(C) receptors. In this study, we searched for specific sequences within the N-terminus of the rho1 subunit involved in the assembly process. Assembly sequences were localized to a 128-amino acid region by deletion of progressively larger regions of a chimeric rho1beta1 subunit previously shown to disrupt rho1 and rho2 assembly. To confirm this observation, a series of GABA(A) receptor beta subunit chimeras containing different regions of the rho1 N-terminus were tested for interference with rho1 and rho2 subunit assembly into functional GABA receptors. Transfer of 70 residues within the 128 amino acid region to the beta1 subunit created a chimera that disrupted rho1, but not rho2, assembly into functional receptors. These observations refine the location of signals involved in rho1 subunit assembly, and suggest that different signals exist for the formation of rho1 homooligomeric and rho1/rho2 heterooligomeric GABA(C) receptors.

Excitotoxic cell death dependent on inhibitory receptor activation

Although excitotoxic cell death is usually considered a Ca(2+)-dependent process, in certain neuronal systems there is strong evidence that excitotoxic cell death is independent of Ca2+ and is instead remarkably dependent on extracellular Cl-. We have shown (in isolated chick embryo retina) that at least some of the lethal Cl- entry is through GABA and glycine receptors. Here we show that when all the GABA and glycine receptors are blocked by using an appropriate cocktail of inhibitors, agonist-induced excitotoxic cell death can be completely prevented. To determine if ligand-gated Cl- channels contribute to excitotoxic cell death in other neurons, we examined KA-induced cell death in cultured rat cerebellar granule cells. GABA receptor blockade with either a competitive or noncompetitive antagonist provides complete neuroprotection. KA stimulates Cl- uptake by the granule cells, and this is blocked by the GABA antagonists. Granule cell cultures take up [3H]GABA and release it in response to KA treatment. A subpopulation of neurons in the cultures is shown to have GAD and high concentrations of GABA, and this presumably is the source of the GABA that leads to receptor activation and lethal Cl- entry. Finally, we show that retinal cell death due to 1 h of simulated ischemia (combined oxygen and glucose deprivation) is completely prevented by blocking the inhibitory receptors. These results indicate that, paradoxically, excitotoxic cell death is completely dependent on activation of inhibitory receptors, in at least some neuronal systems, and this pathological process may contribute to disease.

Distribution of GABAA and glycine receptors in the mammalian retina

1. GABA and glycine mediate synaptic inhibition via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of receptors for these two neurotransmitters. In the present paper, the distribution of GABAA and glycine receptors in the mammalian retina is reviewed. 2. In situ hybridization, immunocytochemistry with subunit-specific antibodies and single cell injection were used to analyse the localization of receptor subunits. Specific subunits are expressed in characteristic strata of the inner plexi-form layer, suggesting that different functional circuits involve specific subtypes of neurotransmitter receptors. 3. Different cell types express different combinations of receptor subunits and an individual neuron can express several receptor isoforms at distinct post-synaptic sites.

GABA(A) and GABA(C) receptors have contrasting effects on excitability in superior colliculus

We have recently found that GABA(C) receptor subunit transcripts are expressed in the superficial layers of rat superior colliculus (SC). In the present study we used immunocytochemistry to demonstrate the presence of GABA(C) receptors in rat SC at protein level. We also investigated in acute rat brain slices the effect of GABA(A) and GABA(C) receptor agonists and antagonists on stimulus-evoked extracellular field potentials in SC. Electrical stimulation of the SC optic layer induced a biphasic, early and late, potential in the adjacent superficial layer. The late component was completely inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione or CoCl(2), indicating that it was generated by postsynaptic activation. Muscimol, a potent GABA(A) and GABA(C) receptor agonist, strongly attenuated this postsynaptic potential at concentrations >10 microM. In contrast, the GABA(C) receptor agonist cis-aminocrotonic acid, as well as muscimol at lower concentrations (0.1-1 microM) increased the postsynaptic potential. This increase was blocked by (1,2,5, 6-tetrahydropyridine-4-yl)methylphosphinic acid, a novel competitive antagonist of GABA(C) receptors. Our findings demonstrate the presence of functional GABA(C) receptors in SC and suggest a disinhibitory role of these receptors in SC neuronal circuitry.

Localisation of the GABA(C) receptors at the axon terminal of the rod bipolar cells of the mouse retina

In the vertebrate retina, the rod bipolar cells make reciprocal synapses with amacrine cells at the axon terminal. Amacrine cells may perform a fine control of the transmitter release from rod bipolar cells by means of GABAergic synapses acting on different types of GABA receptors. To clarify this possibility GABA-induced currents were recorded by the patch-clamp whole cell method in rod bipolar cells enzymatically dissociated from the mouse retina. All cells tested showed a desensitising chloride-sensitive GABA-induced current. When GABA 30 microM was applied in presence of 100 microM biccuculine, a blocker of the GABA(A) receptors, a slow-desensitising component of the current still remains. This current was blocked when GABA 30 microM was applied in presence of 100 microM 3-aminopropylphosphonic acid, an antagonist of the GABA(C) receptors. The current mediated by GABA(C) receptors showed an EC50 of less that 5 microM; the ionic current through the GABA(A) receptor showed an EC50 of ca. 30 microM. Two pieces of evidence demonstrated that the GABA(C)-mediated current was localised at the axon terminal of rod bipolar cells: (1) cells lacking the axon terminal only showed the biccuculine-sensitive GABA-induced current; and (2) after mechanical section of the axon terminal, bipolar cells lost the slow-desensitising component of the GABA-induced current. We conclude that the rod bipolar cells express two types of ionotropic GABA receptors, and that the high sensitive GABA(C) receptors are mainly localised at the level of the axon terminal and therefore may contribute to the modulation of the transmitter release from the rod bipolar cell.

Role of GABAA and GABAC receptors in the biphasic GABA responses in neurons of the rat major pelvic ganglia

The role of gamma-aminobutyric acid-A (GABAA) and GABAC receptors in the GABA-induced biphasic response in neurons of the rat major pelvic ganglia (MPG) were examined in vitro. Application of GABA (100 microM) to MPG neurons produced a biphasic response, an initial depolarization (GABAd) followed by a hyperpolarization (GABAh). The input resistance of the MPG neurons was decreased during the GABAd, whereas it was increased during the GABAh. The GABAd could be further separated into the early component (early GABAd) with a duration of 27 +/- 5 s (mean +/- SE; n = 11) and the late component (late GABAd) with a duration of 109 +/- 11 s (n = 11). The duration of the GABAh was 516 +/- 64 s (n = 11). The effects of GABA (5-500 microM) in producing the depolarization and the hyperpolarization were concentration-dependent. GABA (5-30 microM) induced only late depolarizations. The early component of the depolarization appeared when the concentration of GABA was >50 microM. Muscimol produced only early depolarizing responses. Baclofen (100 microM) had no effect on the membrane potential and input resistance of MPG neurons. Bicuculline (60 microM) blocked the early GABAd but not the late GABAd and the GABAh. Application of picrotoxin (100 microM) with bicuculline (60 microM) blocked both the late GABAd and the GABAh. CGP55845A (3 microM), a selective GABAB receptor antagonist, did not affect the GABA-induced responses. cis-4-Aminocrotonic acid (CACA, 1 mM) and trans-4-aminocrotonic acid (TACA, 1 mM), selective GABAC receptor agonists, produced late biphasic responses in the MPG neurons. The duration of the CACA responses was almost the same as those of the late GABAd and GABAh obtained in the presence of bicuculline. Imidazole-4-acetic acid (I4AA, 100 microM), a GABAC receptor antagonist, depressed the late GABAd and the GABAh but not the early GABAd. I4AA (100 microM) and picrotoxin (100 microM) also suppressed the biphasic response to CACA. The early GABAd and the late GABAd were reversed in polarity at -32 +/- 3 mV (n = 7) and -38 +/- 2 mV (n = 4), respectively, in the Krebs solution. The reversal potential of the GABAh was -34 +/- 2 mV (n = 4) in the Krebs solution. The reversal potentials of the late GABAd and the GABAh shifted to -20 +/- 3 mV (n = 5) and -22 +/- 3 mV (n = 5), respectively, in 85 mM Cl- solution. These results indicate that the late GABA(d) and the GABAh are mediated predominantly by bicuculline-insensitive, picrotoxin-sensitive GABA receptors, GABAC (or GABAAOr) receptors, in neurons of the rat MPG.

Zn2+ differentially modulates kinetics of GABA(C) vs GABA(A) receptors in carp retinal bipolar cells

GABA(C) and GABA(A) receptors co-exist in retinal bipolar cells. In the present study the effects of zinc on the kinetics of currents mediated by GABA(C) and GABA(A) receptors were investigated in isolated carp bipolar cells, using whole-cell patch-clamp technique. We observed for the first time that zinc exerted opposite effects on kinetics of the GABA(C) and GABA(A) responses: zinc significantly slowed down activation and desensitization of the GABA(C) response, but accelerated those of the GABA(A) response; zinc dramatically accelerated deactivation of the GABA(C) response, whereas it had no apparent effect on deactivation of the GABA(A) response. These results suggest that zinc may be functionally important in regulating retinal signal transmission.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Subpopulations of neurons in the rat neostriatum display GABABR1 receptor immunoreactivity

Immunoreactivity for gamma aminobutyric acid BR1 receptor (GABABR1) was detected in the neuropilar elements as well as in the perikarya of neurons in the neostriatum. Many of the GABABR1-immunoreactive perikarya were medium-sized with a thin rim of cytoplasm. They resembled the morphology of medium spiny neurons, the projection neurons of the neostriatum. In addition, some GABABR1-immunoreactive neurons were densely labeled and were of medium to large in size. These neurons were characterized by double immunofluorescence using their neurochemicals as markers. Over 90% of the parvalbumin- and choline acetyltransferase-immunoreactive neurons and about 80% of the nitric oxide synthase-immunoreactive neurons displayed GABABR1 immunoreactivity. The present results show for the first time that the major four subpopulations of striatal neurons express GABABR1 receptor and may have a functional implication in the GABA neurotransmission in the microcircuitry of the neostriatum.

Multiple types of spontaneous excitatory synaptic currents in salamander retinal ganglion cells

Spontaneous and light-evoked excitatory postsynaptic currents (sEPSCs and leEPSCs) in retinal ganglion cells of the larval tiger salamander were recorded under voltage clamp conditions from living retinal slices. sEPSCs were isolated from the spontaneous inhibitory postsynaptic currents (sIPSCs) by application of 100 M picrotoxin+1 microM strychnine. In addition to the previously reported sEPSCs [K. Matsui, N. Hosoi, M. Tachibana, Excitatory synaptic transmission in the inner retina: pair recordings of bipolar cells and neurons of the ganglion cell layer, J. Neurosci. 18 (1998) 4500-4510; W.R. Taylor, E. Chen, D.R. Copenhagen, Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells, J. Physiol. 486 (1995) 207-221] [which are equivalent to our fast AMPA receptor-mediated sEPSCs (fAMPAsEPSCs)], we found another type of AMPA receptor-mediated sEPSC with slower rise and decay time courses and larger peak amplitudes (sAMPAsEPSCs), and the NMDA receptor-mediated sEPSCs (NMDAsEPSCs) in ON-OFF ganglion cells. The frequency of all three types of sEPSCs is greatly reduced by cobalt (with zero calcium) and increased by hyperosmotic solution, suggesting that these events are mediated by calcium-dependent exocytosis of glutamatergic synaptic vesicles. The amplitude histograms of sEPSCs do not show multiple peaks, suggesting that larger events are not discrete multiples of elementary events, or quanta, of similar neurotransmitter contents, as in the neuromuscular junction [P. Fatt, B. Katz, Spontaneous subthreshold activity at motor nerve endings, J. Physiol. 117 (1952) 109-128]. The average I-V relations of the fAMPAsEPSCs and sAMPAsEPSCs were outward rectified with reversal potentials at -12.2 mV and -10.8 mV, and that of the NMDAsEPSCs was N-shaped with a reversal potential at -5.8 mV. The average conductance increase associated with a single fAMPAsEPSC, a single sAMPAsEPSC, and a single NMDAsEPSC were 163. 26+/-51.02 pS, 233.33+/-163.64 pS, and 37.5+/-50.0 pS at -110 mV; 241.67+/-22.92 pS, 444.90+/-469.94 pS, and 25.93+/-70.37 pS at -60 mV; and 440.48+/-183.33 pS, 1,192.68+/-651.22 pS, and 517.71+/-238. 24 pS at +30 mV, respectively. The average frequency of the three sEPSCs at +30 mV were 15 Hz, 3.7 Hz and 3.6 Hz, respectively. The rise time (time to peak) of fAMPAsEPSCs was 1.5+/-1.05 ms and the decay time could be fitted with a single exponential with an average time constant of 3.4+/-4.1 ms. The rise and decay time course of the sAMPAsEPSCs and NMDAsEPSCs were much slower and sawtooth-shaped, and each 'sawtooth' had time course and amplitude similar to those of individual fAMPAsEPSCs. We propose that each fAMPAsEPSC is mediated by single or synchronized multiples of glutamatergic synaptic vesicles from bipolar cells, and each sAMPAsEPSC or NMDAsEPSC is mediated by larger clusters of synaptic vesicles triggered by spontaneous calcium spikes in bipolar cell axon terminals [J. Burrone, L. Lagnado, Electrical resonance and calcium influx in the synaptic terminal of depolarizing bipolar cells from the goldfish retina, J. Physiol. 505 (1997) 571-584; D. Zenisek, G. Matthews, Calcium action potentials in retinal bipolar neurons, Vis. Neurosci. 15 (1998) 69-75].

Multitude of ion channels in the regulation of transmitter release

The presynaptic nerve terminal is of key importance in communication in the nervous system. Its primary role is to release transmitter quanta on the arrival of an appropriate stimulus. The structural basis of these transmitter quanta are the synaptic vesicles that fuse with the surface membrane of the nerve terminal, to release their content of neurotransmitter molecules and other vesicular components. We subdivide the control of quantal release into two major classes: the processes that take place before the fusion of the synaptic vesicle with the surface membrane (the pre-fusion control) and the processes that occur after the fusion of the vesicle (the post-fusion control). The pre-fusion control is the main determinant of transmitter release. It is achieved by a wide variety of cellular components, among them the ion channels. There are reports of several hundred different ion channel molecules at the surface membrane of the nerve terminal, that for convenience can be grouped into eight major categories. They are the voltage-dependent calcium channels, the potassium channels, the calcium-gated potassium channels, the sodium channels, the chloride channels, the non-selective channels, the ligand gated channels and the stretch-activated channels. There are several categories of intracellular channels in the mitochondria, endoplasmic reticulum and the synaptic vesicles. We speculate that the vesicle channels may be of an importance in the post-fusion control of transmitter release.

The protein MAP-1B links GABA(C) receptors to the cytoskeleton at retinal synapses

The ionotropic type-A and type-C receptors for the neurotransmitter gamma-aminobutyric acid (GABA(A) and GABA(C) receptors) are the principal sites of fast synaptic inhibition in the central nervous system, but it is not known how these receptors are localized at GABA-dependent synapses. GABA(C) receptors, which are composed of rho-subunits, are expressed almost exclusively in the retina of adult vertebrates, where they are enriched on bipolar cell axon terminals. Here we show that the microtubule-associated protein 1B (MAP-1B) specifically interacts with the GABA(C) rho1 subunit but not with GABA(A) receptor subunits. Furthermore, GABA(C) receptors and MAP-1B co-localize at postsynaptic sites on bipolar cell axon terminals. Co-expression of MAP-1B and the rho1 subunit in COS cells results in a dramatic redistribution of the rho1 subunit. Our observations suggest a novel mechanism for localizing ionotropic GABA receptors to synaptic sites. This mechanism, which is specific for GABA(C) but not GABA(A) receptors, may allow these receptor subtypes, which have distinct physiological and pharmacological properties, to be differentially localized at inhibitory synapses.

GABAC receptor rho subunits are heterogeneously expressed in the human CNS and form homo- and heterooligomers with distinct physical properties

In the central nervous system, receptors for gamma-aminobutyric acid (GABA) are responsible for inhibitory neurotransmission. Anatomical and electrophysiological studies indicate that GABAC receptors are composed of rho subunits. While the rho 1 subunit of various species forms homooligomeric receptors with GABAC-like properties, molecular cloning has identified additional rho subunits whose functional role is unclear. By RT-PCR, we demonstrated that rho 1 expression is primarily restricted to the retina, whereas the rho 2 subunit was present in all brain regions tested. Transfection of HEK-293 cells with rho 2 cDNA resulted in GABA-gated whole-cell currents that differed from those mediated by the rho 1 subunit in two respects: maximal amplitude (rho 1:rho 2 approximately 4:1) and inactivation time course (rho 1:rho 2 approximately 2:1). Cotransfection of rho 1 and rho 2 cDNA in a 1:1 ratio generated whole-cell currents with large amplitudes characteristic of rho 1 but more rapid inactivation typical for rho 2. This observation suggested formation of heterooligomeric GABAC receptors with distinct features. Therefore, we tested the assembly of rho 1 and rho 2 subunits by cotransfecting rho 2 cDNA together with a chimeric rho 1 beta 1 subunit, known to interfere with rho 1 assembly in a dominant-negative fashion. Reduction of rho 2 generated currents correlated with the ratio of chimeric to rho 2 cDNA. Secondly, we determined that the picrotoxinin sensitivity of cells transfected with various ratios of rho 1 and rho 2 cDNA differed from that expected of a pure mixture of homooligomeric receptors. The latter two observations support the idea that rho 1 and rho 2 subunits form heterooligomeric GABAC receptors in mammalian cells. Together, our results indicate that the presence of both rho subunits enables the formation of heterooligomeric receptors with physical properties distinct from homooligomers, thus increasing the diversity of GABAC receptors in the CNS.

Two distinct nicotinic receptors, one pharmacologically similar to the vertebrate alpha7-containing receptor, mediate Cl currents in aplysia neurons

Ionotropic, nicotinic receptors have previously been shown to mediate both inhibitory (Cl-dependent) and excitatory (cationic) cholinergic responses in Aplysia neurons. We have used fast perfusion methods of agonist and antagonist application to reevaluate the effects on these receptors of a wide variety of cholinergic compounds, including a number of recently isolated and/or synthesized alpha toxins [alpha-conotoxin (alphaCTx)] from Conus snails. These toxins have been shown in previous studies to discriminate between the many types of nicotinic receptors now known to be expressed in vertebrate muscle, neuroendocrine, and neuronal cells. One of these toxins (alphaCTx ImI from the worm-eating snail Conus imperialis) revealed that two kinetically and pharmacologically distinct elements underlie the ACh-induced Cl-dependent response in Aplysia neurons: one element is a rapidly desensitizing current that is blocked by the toxin; the other is a slowly desensitizing current that is unaffected by the toxin. The two kinetically defined elements were also found to be differentially sensitive to different agonists. Finally, the proportion of the rapidly desensitizing element to the sustained element was found to be cell-specific. These observations led to the conclusion that two distinct nicotinic receptors mediate Cl currents in Aplysia neurons. The receptor mediating the rapidly desensitizing Cl-dependent response shows a strong pharmacological resemblance to the vertebrate alpha-bungarotoxin-sensitive, alpha7-containing receptor, which is permeable to calcium and mediates a rapidly desensitizing excitatory response.

Calcium released from intracellular stores inhibits GABAA-mediated currents in ganglion cells of the turtle retina

We studied spiking neurons isolated from turtle retina by the whole cell version of the patch clamp. The studied cells had perikaryal diameters > 15 microns and fired multiple spikes in response to depolarizing current steps, indicating they were ganglion cells. In symmetrical [Cl-], currents elicited by puffs of 100 microM gamma-aminobutyric acid (GABA) were inward at a holding potential of -80 mV. All of the GABA-evoked current was blocked by SR95331 (20 microM), indicating that it was mediated by a GABAA receptor. The GABA-evoked currents were unaltered by eliciting a transmembrane calcium current either just before or during the response to GABA. On the other hand caffeine (10 mM), which induces Ca2+ release from intracellular stores, inhibited the GABA-evoked current on average by 30%. The caffeine effect was blocked by introducing the calcium buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into the cell but was unaffected by replacing [Ca2+]o with equimolar cobalt. Thapsigargin (10 microM), an inhibitor of intracellular calcium pumps, and ryanodine (20 microM), which depletes intracellular calcium stores, both markedly reduced a caffeine-induced inhibition of the GABA-evoked current. Another activator of intracellular calcium release, inositol trisphosphate (IP3; 50 microM), also progressively reduced the GABA-induced current when introduced into the cell. Dibutyryl adenosine 3'5'-cyclic monophosphate (cAMP; 0.5 mM), a membrane-permeable analogue of cAMP, did not reduce GABA-evoked currents, suggesting that cAMP-dependent kinases are not involved in suppressing GABAA currents, whereas calmidazolium (30 microM) and cyclosporin A (20 microM), which inhibit Ca/calmodulin-dependent phosphatases, did reduce the caffeine-induced inhibition of the GABA-evoked current. Alkaline phosphatase (150 micrograms/ml) and calcineurin (300 micrograms/ml) had a similar action to caffeine or IP3. Antibodies directed against the ryanodine receptor or the IP3 receptor reacted with the great majority of neurons in the ganglion cell layer. We found that these two antibodies colocalized in large ganglion cells. In summary, intracellular calcium plays a role in reducing the currents elicited by GABA, acting through GABAA receptors. The modulatory action of calcium on GABA responses appears to work through one or more Ca-dependent phosphatases.

GABAA and GABAC receptors on mammalian rod bipolar cells

Rod bipolar (RB) cells of mammalian retinae receive synapses from different gamma-aminobutyric acid (GABAergic) amacrine cells in the inner plexiform layer (IPL). We addressed the question whether RB cells of the rabbit and of the rat retina express different types of GABA receptors at these synapses. RB cells were immunolabeled in vertical sections of rat retinae with an antibody against protein kinase C (PKC). The sections were double-labeled for the alpha 1, alpha 2, alpha 3, or gamma 2 subunits of the GABAA receptor. Punctate immunofluorescence, which represents synaptic localization, was found for all four subunits. Many of the alpha 1-, alpha 3-, or gamma 2-immunoreactive puncta coincided with the axon terminals of the PKC-immunolabeled RB cells. Sections and wholemounts of rabbit retinae were also double labeled for PKC and the rho subunits of the GABAC receptor. Rabbit RB cells were decorated by many rho-immunoreactive puncta, which were shown by electron microscopy to represent synaptic localization. Previous work from our laboratory has shown that the alpha 1, alpha 2, alpha 3, and rho subunits are not found within the same synapse but are expressed at different synaptic sites. Taken together, these results suggest that RB cells of mammalian retinae express at least three different types of GABA receptors at synaptic sites in the IPL: GABAC receptors, GABAA receptors containing the alpha 1 subunit, and GABAA receptors containing the alpha 3 subunit.

A major locus for autosomal recessive retinitis pigmentosa on 6q, determined by homozygosity mapping of chromosomal regions that contain gamma-aminobutyric acid-receptor clusters

Retinitis pigmentosa (RP) is the most common inherited retinal dystrophy, with extensive allelic and nonallelic genetic heterogeneity. Autosomal recessive RP (arRP) is the most common form of RP worldwide, with at least nine loci known and accountable for approximately 10%-15% of all cases. Gamma-aminobutyric acid (GABA) is the major inhibitory transmitter in the CNS. Different GABA receptors are expressed in all retinal layers, and inhibition mediated by GABA receptors in the human retina could be related to RP. We have selected chromosomal regions containing genes that encode the different subunits of the GABA receptors, for homozygosity mapping in inbred families affected by arRP. We identify a new locus for arRP, on chromosome 6, between markers D6S257 and D6S1644. Our data suggest that 10%-20% of Spanish families affected by typical arRP could have linkage to this new locus. This region contains subunits GABRR1 and GABRR2 of the GABA-C receptor, which is the effector of lateral inhibition at the retina.

Differential localization of GABA(A) receptor alpha and beta subunits in the hamster retina and relationship with glutamic acid decarboxylase immunoreactivity

In order to determine the cellular localization of GABA(A)alpha and beta subunits in the hamster retina, single and double immunocytochemistry was performed in perfuse-fixed hamster retina using commercially-available antibodies against the two receptor subunits and glutamic acid decarboxylase. Strong GABA(A)beta immunoreactivity was found in two strata of the inner plexiform layer and in perikarya of amacrine cells and bipolar cells in the inner nuclear layer. In contrast, no GABA(A)alpha immunoreactivity was detected. All but a few of the GABA(A)beta-immunoreactive amacrine cells were found not to display glutamic acid decarboxylase immunoreactivity. The present results indicate that there is a differential localization of GABA(A)alpha and beta subunits in different neuronal subpopulations in the hamster retina.

Response to change is facilitated by a three-neuron disinhibitory pathway in the tiger salamander retina

Most retinal ganglion cells respond only transiently, for approximately 150 msec at the onset and termination of a light flash. The responses are transient because it has been shown that bipolar-to-ganglion cell transmission is truncated after 150 msec by a feedback inhibition to bipolar cell terminals. The feedback inhibition itself must be delayed by approximately 150 msec to allow the initial bipolar-ganglion cell transmission. This study identifies a three-component serial synaptic pathway from glycinergic amacrine cells to GABAergic amacrine cells to bipolar cell terminals as one source of this delay. We used perforated and whole-cell patch-clamp recordings to measure the timing of light responses in amacrine, bipolar, and ganglion cells under control and glycine and GABA receptor-blocked conditions. Our results suggest that, after a light flash, a population of glycinergic amacrine cells responds first, inhibiting a population of GABAergic amacrine cells for approximately 150 msec. The GABAergic amacrine cells feed back to bipolar terminals, but only after the 150 msec delay, allowing the bipolar terminals to excite ganglion cells for the first 150 msec. Blocking the glycinergic amacrine cell activity with strychnine allows the GABAergic system to become active earlier. GABAergic amacrine cells then inhibit release from bipolar cells earlier. Under these conditions, the ganglion cell response to change would be decreased.

Molecular composition of GABAC receptors

In the central nervous system inhibitory neurotransmission is primarily achieved through activation of receptors for gamma-aminobutyric acid (GABA). Three types of GABA receptors have been identified on the basis of their pharmacology and electrophysiology. The predominant type, termed GABAA and a recently identified type, GABAC, have integral chloride channels, whereas GABAB receptors couple to separate K+ or Ca2+ channels via G-proteins. By analogy to nicotinic acetylcholine receptors, native GABAA receptors are believed to be heterooligomers of five subunits, drawn from five classes (alpha, beta, gamma, delta, epsilon/chi). An additional class, called rho, is often categorized with GABAA receptor subunits due to a high degree of sequence similarity. However, rho subunits are capable of forming functional homooligomeric and heterooligomeric receptors, whereas GABAA receptors only express efficiently as heterooligomers. Intriguingly, the pharmacological properties of receptors formed from rho subunits are very similar to those exhibited by GABAC receptors and rho subunits and GABAC responses have been colocalized to the same retina cells, indicating that rho subunits are the sole components of GABAC receptors. In contrast, the propensity of GABAA receptor and rho subunits to form multimeric structures and their coexistence in retinal cells suggests that GABAC receptors might be heterooligomers of rho and GABAA receptor subunits. This review will summarize our current understanding of the molecular composition of GABAC receptors based upon studies of rho subunit assembly.

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.

Amino acid neurotransmitters in the retina: a functional overview

Physiological and pharmacological mechanisms of glutamatergic, GABAergic and glycinergic synapses in the tiger salamander retina were studied. We used immunocytochemical and autoradiographic methods to study localizations of these neurotransmitters and their uptake transporters; and electrophysiological methods (intracellular, extracellular and whole cell patch electrode recordings) to study the light responses, miniature postsynaptic currents and neurotransmitter-induced postsynaptic currents in various retinal neurons. Our results are consistent with the following scheme: Glutamate is used by the photoreceptor and bipolar cell output synapses and the release of glutamate is largely mediated by calcium-dependent vesicular processes. The postsynaptic glutamate receptors in DBCs are L-AP4 receptors, in HBCs, HCs and ganglion cells are the kainate/AMPA and NMDA receptors. Subpopulations of HCs make GABAergic synapses on cones and gate chloride condunctance through GABAA receptors. GABAergic HCs do not make feedforward synapses on bipolar cell dendrites and the neurotransmitter identity of the HCs making feedforward synapses is unknown. Subpopulations of amacrine cells make GABAergic synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and GABA gates chloride conductances in theses cells. Glycinergic amacrine cells make synapses on bipolar cell synaptic terminals, other amacrine cells and ganglion cells and glycine opens postsynaptic chloride channels. Glycinergic interplexiform cells make synapses on bipolar cells in the outer retina and glycine released from these cells open chloride channels in bipolar cell dendrites.

Ca2+-independent excitotoxic neurodegeneration in isolated retina, an intact neural net: a role for Cl- and inhibitory transmitters

Rapidly triggered excitotoxic cell death is widely thought to be due to excessive influx of extracellular Ca2+, primarily through the N-methyl-D-aspartate subtype of glutamate receptor. By devising conditions that permit the maintenance of isolated retina in the absence of Ca2+, it has become technically feasible to test the dependence of excitotoxic neurodegeneration in this intact neural system on extracellular Ca2+. Using biochemical, Ca2+ imaging, and electrophysiological techniques, we found that (1) rapidly triggered excitotoxic cell death in this system occurs independently of both extracellular Ca2+ and increases in intracellular Ca2+; (2) this cell death is highly dependent on extracellular Cl-; and (3) lethal Cl- entry occurs by multiple paths, but a significant fraction occurs through pathologically activated gamma-aminobutyric acid and glycine receptors. These results emphasize the importance of Ca2+-independent mechanisms and the role that local transmitter circuitry plays in excitotoxic cell death.

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.

GABAC receptors on ferret retinal bipolar cells: a diversity of subtypes in mammals?

The GABAC receptor subtypes on bipolar cells of rats and cold-blooded vertebrates differ in their pharmacological properties and probably have different molecular compositions. With the exception of the rat, native GABAC receptors in mammals had not been studied. In ferret, whole-cell, voltage-clamp recordings were made from bipolar cells in the retinal slice preparation to determine which subtype of GABAC receptor predominated. Puff-evoked GABA currents in bipolar cells were partially reduced by the GABAA receptor antagonist bicuculline, indicating that both GABAA and GABAC receptors mediated the responses. By contrast, GABA currents of ganglion cells were always completely blocked by bicuculline, indicating that GABAA receptors predominated on these cells. Small-amplitude GABA currents of bipolar cells evoked by short-duration puffs were less sensitive to bicuculline than large-amplitude currents evoked by long-duration puffs. This indicates that GABAC receptors mediated proportionately more of the small-amplitude, puff-evoked responses and GABAA receptors mediated more of the large-amplitude, puff-evoked responses. In bipolar cells, the bicuculline-resistant component of the GABA current was entirely blocked by 3-APMPA (3-aminopropyl-(methyl)phosphonic acid), a GABAC receptor antagonist. Picrotoxin, which is relatively ineffective at rat GABAC receptors, completely blocked GABA currents in ferret bipolar cells, indicating that GABAC receptors on ferret bipolar cells resemble those in lower vertebrates rather than those in the rat retina. These results suggest that there may be a diversity of GABAC receptor subtypes on mammalian bipolar cells.

Desensitizing GABAC receptors on carp retinal bipolar cells

Bicuculline- and baclofen-insensitive GABA receptors (GABAC receptors) on bipolar cells acutely dissociated from carp retina were investigated with using the whole-cell patch-clamp recording technique. The currents of these cells mediated by GABAC receptors showed striking desensitization, even at low concentrations of GABA. Both the time constant tau of the GABAC current decay and the extent of desensitization were significantly different from that of GABAC receptors previously observed in other retinas and elsewhere in the CNS, suggesting that the GABAC receptors of carp bipolar cells might be distinct in intracellular mechanisms and subunit composition.