The effect of blockade of GABAergic synapses by picrotoxin on the b- and d-wave of frog ERG was investigated under scotopic (0.002 lx), mesopic (2 lx) and photopic (200 lx) background illumination (Ib). Diffuse white stimuli with two levels of contrast (0.5 and 2.5) were used with each Ib. The aim was to compare the effects of picrotoxin at different background levels, but same stimulus contrast. We found that picrotoxin markedly increased the amplitude and rate of rise of the leading edge of the b- and d-wave with each Ib. This effect was most pronounced at mesopic Ib, smaller at photopic Ib and least pronounced at scotopic Ib. It was relatively stronger on the d-wave than on the b-wave amplitude under scotopic and mesopic conditions. Under photopic conditions, the difference between the picrotoxin effect on the b- and d-wave was much smaller. The possible neuronal mechanisms of the above described picrotoxin effects are discussed.

Department of Physiology Medico Biological Institute Medical Academy Sofia Bulgaria