The effect of picrotoxin on the absolute and increment retinal sensitivity.

The aim of this study was to investigate the influence of the GABAergic inhibitory neurones on the absolute and increment retinal sensitivity, determined be measuring b-wave threshold of frog electroretinogram (ERG). We found that GABAergic blockade by picrotoxin (PT -noncompetitive GABA antagonist) had no apparent effect on the absolute (dark adapted) b-wave threshold (12 microV criterion amplitude) and slightly, but significantly lowered its increment threshold under both levels of background illumination (0.25 lx and 200 lx), used in this study. A similar effect to that observed in light-adapted eyes was also seen in dark-adapted eyes when suprathreshold stimulus intensities, giving 25 microV b-wave amplitude were presented. Another observation was marked increase of the d-wave amplitude after PT both in dark and light-adapted eyes. We suggest that the different photoreceptor input to the GABAergic interneurones and neurones, responsible for b-wave generation is the cause for the different PT effect on the absolute (rod mediated) and increment (predominantly cone mediated) b-wave threshold of frog ERG. The possible role of the GABAergic inhibitory neurones in the process of light adaptation is discussed.