Picrotoxin effects on the frog electroretinogram under different background illumination

The effect of blockade of the GABA-ergic synapses in the frog retina by picrotoxin (PT) on ERG was studied under two conditions of background illumination (Ib): scotopic Ib with illumination level below 0.01 lx and mesopic Ib with illumination level of 2 lx. Under scotopic Ib PT produced a greater relative increase in the d-wave amplitude as compared to the b-wave. The substitution of scotopic Ib by mesopic one in untreated retinas produced a greater decrease in the b-wave amplitude than in the d-wave amplitude. The presentation of the mesopic Ib in PT treated retinas potentiated to nearly the same extent the PT effect on both b- and d-wave. So the initial shift between the amplitudes of the b- and d-wave produced by the mesopic Ib was not significantly altered by PT. The results obtained show that the GABA-ergic neurons in the mixed frog retina participate in the gain control of the ON and OFF channels in a wider range of background illumination. Their relative significance in this control seems to be greater under mesopic Ib than under scotopic Ib. We suggest that the effect of mesopic Ib on the sensitivity of the ERG b- and d-wave is complex and involves some other mechanisms in addition to the GABA-ergic one.