Intracellular recordings were made from amacrine cells in the isolated, superfused carp retina, and the effects of γ-aminobutyric acid (GABA) on sustained and transient ON signals of these cells were studied. Exogenous GABA application partially suppressed the sustained response of ON amacrine cells, which could be completely reversed by picrotoxin (PTX), a chloride channel blocker, and by bicuculline (BCC), a specific GABA_A receptor antagonist. On the other hand, suppression by GABA of the ON response which was predominantly driven by rod signals in a certain portion of transient ON-OFF amacrine cells was completely blocked by PTX, but not by BCC, indicating that GABA_C receptors may be involved in the effect. These results suggest that GABA_A and GABA_C receptors may be respectively involved in mediating the transmission of sustained and transient signals in the carp inner retina.
