Effects of GABA and glycine on the distal cells of the cyprinid retina.

Two putative retinal neurotransmitters, gamma-aminobutyric acid (GABA) and glycine, were applied to the isolated carp and goldfish retinas while intracellular recordings from horizontal cells and cones were made. At relatively low concentrations (0.1-1 mM), GABA consistently hyperpolarized dark-adapted H1 or L-type cone horizontal cells and cone photoreceptors, reduced light evoked responses and suppressed the on-transients. The effects of GABA on H1 horizontal cells were abolished when Co2+ was applied to the retina, indicating that GABA exerts its effects on the horizontal cells via the receptors. Thus GABA is likely to be involved in the feedback synapse between H1 horizontal cells and the cones. Low concentrations of glycine (0.1-1 mM), hyperpolarized a number of H2 or C-type horizontal cells and selectively abolished their depolarizing responses to red light; thus glycine may be involved in synaptic pathways which mediate or modulate the depolarizing responses to red flashes in C-type horizontal cells.