Participation of GABA in the sensitivity control of the OFF-response in the turtle ERG.

: The intensity-response--V/log I, function of the d-wave in the turtle ERG was investigated before and after blockade of the GABAergic transmission by 0.4 mmol/l picrotoxin (PT) under conditions of two adapting background illuminations--0.01 and 10 lx. After PT the d-wave V/log I curves showed a shift to the left along the intensity axis under both adapting conditions, revealing an increase in the gain and the contrast sensitivity of the retinal elements involved in the d-wave generation. These changes were similar to the changes in the b-wave V/log I function described under similar conditions. The relative increase of the b- and d- wave amplitude had approximately equal values at both background levels at higher stimulus intensities. In the range of low stimulus intensity, however, at background illumination of 0.01 lx the relative increase of the d-wave was considerably greater than that of the b-wave. The experimental data obtained were best fitted by the Naka-Rushton equation. The participation of the GABAergic neurons on the d-wave sensitivity control is discussed.