GABAERGIC AND GLYCINERGIC IPSCS IN GANGLION-CELLS OF RAT RETINAL SLICES

GABAergic and glycinergic IPSCs were studied in identified retinal gan glion cells (RGCs) of light-adapted rat retinal slices, using whole-ce ll recording techniques. GABAergic IPSCs were blocked specifically by SR95531 (3 mu M) and bicuculline (3 mu M) and glycinergic IPSCs by str ychnine (0.3 mu M) From 37 RGCs studied, 25 showed exclusively GABAerg ic IPSCs, 6 presented only glycinergic IPSCs, and 6 showed both. This distribution may result from differences in amacrine cells input rathe r than from receptor heterogeneity, because both GABA and glycine elic ited Cl--selective currents in all RGCs tested. TTX markedly reduced G ABAergic IPSCs frequency, whereas glycinergic IPSCs were unaffected. C a2+-free media, with or without high Mg2+, blocked TTX-resistant GABAe rgic and glycinergic IPSCs. These results suggest that GABAergic IPSCs in RGCs can be elicited either by Na+-dependent action potentials or by local Ca2+ influx in medium or large dendritic field GABAergic amac rine cells, whereas glycinergic IPSCs are generated by action potentia l-independent Ca2+ influx in narrow field glycinergic amacrine cells. Both types of IPSCs had fast rise times and biexponential decays, but glycinergic IPSC decay was significantly slower than that of GABAergic IPSCs. An elementary conductance of 54 pS for the glycine-gated chann els was estimated from single-channel events, clearly detected in the falling phase of glycinergic IPSCs, and from responses to exogenous gl ycine.