Glycinergic input to carp retinal ganglion cells may be mediated by glycine receptors with homologous kinetics

Abstract

Current responses of carp retinal ganglion cells (RGCs) retrogradely labeled and freshly dissociated to rapid application of glycine were recorded by whole-cell patch clamp techniques and effects of glycine antagonists on these responses were analyzed. The current response to maintained application of glycine at a concentration higher than 30 μM exhibited desensitization, which was well fitted to a monoexponential function. Strychnine (1 μM), a glycine receptor antagonist, completely blocked the response to 100 μM glycine. Strychnine at a concentration range between 10 and 200 nM suppressed the response to 100 μM glycine in a dose-dependent manner, and only a slow-activated and sustained current eventually remained in the presence of 200 nM strychnine. Power spectral density (PSD) analysis revealed no changes in the density–frequency dependence caused by strychnine. It was further shown that dissociation of strychnine from glycine receptors was rather slow. Moreover, Zn 2+ exerted similar dual action on this sustained response and the response in Ringer's: potentiating and reducing them at low and high concentrations of Zn 2+, respectively. 5,7-Dichlorokynurenic acid (DCKA, 500 μM), a selective blocker of the glycine recognition site at the NMDA receptor, partially reduced the glycine response, but without changing its kinetics. These results suggest that glycinergic input to carp ganglion cells may be mediated by strychnine-sensitive glycine receptors with homologous kinetics, and slow dissociation of strychnine from glycine receptors may partially account for the changes in glycine response kinetics occurring in the presence of strychnine.