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

Abstract

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.

Effects of picrotoxin on light adapted frog electroretinogram are not due entirely to its action in proximal retina

“…This suggestion is supported by the fact that Herrmann et al (2011) did not obtain any significant effect of the exogenously applied GABA on the operational range of the b-wave responses in wild type mice, although GABA by itself increased their amplitude. All results shown in the present work are in full agreement with our previous results obtained in frog retina, where we have demonstrated that picrotoxin does not alter the dynamic range of the b-wave V-log I function, although it increases the amplitude and slows the time course of the b-and d-waves under photopic conditions of light adaptation (Kupenova, Popova, & Vitanova, 2008;Popova, 2000).…”

“…This concentration was chosen among the other tested because it had maximal effect on both the b-and d-wave amplitude (Popova, 2000). The activity of proximal neurons was blocked using N-methyl-D-aspartate (NMDA -Sigma), dissolved in Ringer solution to a concentration of 1 mM.…”

“…The authors concluded that picrotoxininduced reduction in the b-wave results from inactivation of the GABA C receptors of ON bipolar cells. However, many other data including those obtained in our laboratory indicate that the blockade of ionotropic GABA receptors by picrotoxin leads to an increase of the b-wave amplitude (frog: Arnarsson & Eysteinsson, 1997;Belcheva & Kupenova, 1980;Belcheva & Vitanova, 1974;Bonaventure, Wioland, & Jardon, 1986;DeVries & Friedman, 1978;Katz et al, 1991;Kupenova, Popova, & Vitanova, 2008;Popova, 1989Popova, , 2000Popova et al, 1986;turtle: Belcheva & Vitanova, 1978;Kupenova et al, 1991Kupenova et al, , 1997Vitanova et al, 1997;fish: Chappell & Rosenstein, 1996;Lewis et al, 2011). The effect is evident in conditions of both dark and light adaptation, indicating that it does not depend on the type of the photoreceptor input.…”

“…The effects of picrotoxin on the d-wave resembled that on the b-wave. Although Bonaventure, Wioland, and Jardon (1986) did not find any effect of picrotoxin on the d-wave, other authors reported that picrotoxin enhances the amplitude and delays the time course of the d-wave (Arnarsson & Eysteinsson, 1997;Belcheva & Kupenova, 1980;Belcheva & Vitanova, 1974Kupenova et al, 1991Kupenova et al, , 1997Lewis et al, 2011;Popova, 1989Popova, , 2000Popova et al, 1986;Vitanova et al, 1997;Xu & Karwoski, 1995). Picrotoxin revealed a prominent OFF component in all rod retina of skate (Chappell & Rosenstein, 1996) and in dark adapted toad retina (Katz et al, 1991).…”

“…Picrotoxin slows the time course of the b-wave. Its latency, implicit time and decay phase are significantly delayed during the GABAergic blockade (Belcheva & Vitanova, 1974Popova, 2000;Vitanova et al, 2001). The effect on the latency is considerably smaller compared to the effect on the implicit time (Popova, 2000).…”