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Evidence from toxicological studies suggested that an ionotropic GABA receptor of novel pharmacology (picrotoxin-insensitive, bicuculline-sensitive) exists in the chick embryo retina. In this report, we provide direct morphological and electrophysiological evidence for the existence of such an iGABA receptor. Chick embryo retinas (14-16 days old) incubated in the presence of kainic acid showed pronounced histopathology in all retinal layers. Maximal protection from this toxicity required a combination of bicuculline and picrotoxin. Individual application of the antagonists indicated that a picrotoxin-insensitive, bicuculline-sensitive GABA receptor is likely to be present on ganglion and amacrine, but not bipolar, cells. GABA currents in embryonic and mature chicken retinal neurons were measured by whole cell patch clamp. GABA was puffed at the dendritic processes in the IPL. Picrotoxin (500 microM, in the bath) eliminated all (>95%) the GABA current in the majority of ganglion and amacrine cells tested, but many cells possessed a substantial picrotoxin-insensitive component. This current was eliminated by bicuculline (200 microM). This current was not a transporter-associated current, since it was not altered by GABA transport blockers or sodium removal. The current-voltage relation was linear and reversed near E(Cl), as expected for a ligand-gated chloride current. Both pentobarbital and lorazepam enhanced the picrotoxin-insensitive current. We conclude that chicken retinal ganglion and amacrine cells express a GABA receptor that is GABA-A-like, in that it can be blocked by bicuculline, and positively modulated by barbiturates and benzodiazepines, but is insensitive to the noncompetitive blocker picrotoxin. Understanding the molecular properties of this receptor will be important for understanding both physiological GABA neurotransmission and the pathology of GABA receptor overactivation.

Metabotropic glutamate receptors (mGluRs) are located in both plexiform layers in the retina and may modulate transmission between photoreceptors and bipolar cells and between bipolar and ganglion cells. We investigated whether mGluR activation modulates excitatory synaptic input to bipolar cells and ganglion cells in the salamander retinal slice preparation. The group III mGluR agonist L-2-amino-4-phosphonobutyric acid (AP4) inhibited monosynaptic excitatory postsynaptic currents (EPSCs) in ganglion cells evoked by electrical stimuli, whereas group I and group II agonists had no significant effect. AP4 reduced the frequency but not the amplitude of ganglion cell miniature EPSCs, suggesting a presynaptic action at bipolar cell terminals. AP4 also reduced ganglion cell EPSCs evoked by the offset of a light stimulus, suggesting that group III mGluRs modulate release from OFF bipolar cells. Comparison of light-evoked EPSCs in OFF bipolar cells and ganglion cells indicated that AP4 reduced ganglion cell EPSCs by acting primarily at bipolar cell terminals, and to a lesser extent at photoreceptor terminals. The group II/III mGluR antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) blocked the effect of AP4 at bipolar cell terminals, consistent with localization of group III mGluRs at these sites. However, CPPG did not increase EPSCs at light offset, indicating that activation of group III mGluRs by synaptic glutamate does not play a large role in modulating transmission from bipolar cells to ganglion cells.

Amacrine cells that respond transiently to maintained illumination are thought to mediate transient inhibitory input to ganglion cells. The excitation of these transient amacrine cells is thought to be limited by inhibitory feedback to bipolar cells. We investigated the possibility that desensitizing AMPA and/or kainate (KA) receptors on amacrine cells might also limit the duration of amacrine cell excitation. To determine how these receptors might affect amacrine cell input and output, we made whole-cell recordings from amacrine and ganglion cells in the salamander retinal slice. The specific AMPA receptor antagonist GYKI-53655 blocked non-NMDA receptor-mediated amacrine cell excitatory postsynaptic currents (EPSCs) and kainate puff-elicited currents, indicating that AMPA, and not KA, receptors mediated the responses. Cyclothiazide, an agent that reduces AMPA receptor desensitization, increased the amplitude and duration of amacrine cell EPSCs. To measure the output of transient amacrine cells, we recorded glycinergic inhibitory postsynaptic currents (IPSCs) from ganglion cells, and found that these were also enhanced by cyclothiazide. Thus, prolongation of amacrine cell AMPA receptor activation enhanced amacrine cell output. Current responses elicited by puffing glycine onto ganglion cell dendrites were not affected by cyclothiazide, indicating that the enhancement of glycinergic IPSCs was not due to a direct effect on glycine receptors. These data suggest that rapid AMPA receptor desensitization and/or deactivation limits glycinergic amacrine cell excitation and the resulting inhibitory synaptic output.