Participation of the GABAergic system of the turtle retina in the light adaptation process

A comparative study was made of the ERG b- and d-wave intensity-response functions before and after GABAergic blockade by means of 0.4 mmol l-1 picrotoxin. A wide range of background intensities, including part of the high photopic range, were used. The intensity-response functions of both the ERG waves fitted well to the Michaelis-Menten equation (V/Vmax = In/(In + sigma n). A sigma-value decrease and a Vmax and n increase were observed after picrotoxin treatment. The analysis of the intensity-response functions shows that, under a wide range of backgrounds, the GABAergic neurons influence the ON- and OFF-response in the distal retina in a similar way. They decrease the gain and the contrast gain of the b- and d-wave generating neuronal mechanisms and widen the intensity span of their responses under given background illumination. The GABAergic system is involved also in response 'scaling' in the ON- and OFF-channels in the distal retina. A very important effect in this respect seems to be the equalization of the range of the responses to increment stimuli under different backgrounds.