Participation of the GABAergic system in the action of 2-amino-4-phosphonobutyrate on the OFF responses of frog retinal ganglion cells.

Perfusion of dark adapted frog eyecups with the ON pathway blocker 2-amino-4-phosphonobutyrate (APB) not only abolished the ganglion cells (GCs)' ON responses and the ERG b-wave, but it markedly potentiated the OFF responses of all ON-OFF and phasic OFF GCs and the d-wave amplitude of a simultaneously recorded ERG as well. The blockade of GABA(A) and GABA(C) receptors by picrotoxin eliminated this potentiating effect in 24 out of 41 GCs, although in the rest of the cells it did not produce any change in the APB effect. On the other hand, the d-wave potentiation was preserved during the GABAergic blockade in all experiments. Our results indicate that GABAergic transmission is involved in the inhibition exerted by the ON upon the OFF channel in part of the ON-OFF and phasic OFF GCs in the frog retina. The tonic OFF GCs probably do not receive an inhibitory input from the ON channel, because their light responses were not altered either by APB alone or by APB during blockade of GABA(A) and GABA(C) receptors.