Glycinergic and GABAergic control of intensity-response function of frog ERG waves under different conditions of...

The effect of glycinergic blockade by strychnine and GABAergic blockade by picrotoxin on the intensity-response function and time course of ERG b- and d-wave was investigated in dark and light adapted frog eyecups as well as in chromatically adapted eyecups, in which the responses were predominantly mediated by one photoreceptor type. Both of the blockers markedly increased the maximal response amplitude and thus increased the contrast gain of the mechanisms generating ERG waves. Strychnine but not picrotoxin narrowed the dynamic range of the b-wave in all eyecups. The glycine and GABAergic control on retinal sensitivity however, depended on photoreceptor input. The b-wave sensitivity was increased to the greatest extent when green rods mediated the response; it was slightly increased when the response was mediated by red cones and was not changed at all when red rods mediated the b-wave. On the other hand the d-wave sensitivity was enhanced regardless of photoreceptor input so that it became equal (dark) or even greater (light adapted eyes) than that of the b-wave. Strychnine shortened but picrotoxin increased the implicit time of the b-wave and both the blockers markedly slowed the d-wave time course. Thus the blockers considerably diminished or fully eliminated the initial difference in time course between the ON and OFF response. The glycine- and GABAergic blockade did not principally alter the light adaptation process expressed in decreasing retinal sensitivity, narrowing the dynamic range and speeding the time course of the responses with increasing background illumination.