Evidence of metabotropic glutamate receptor subtypes found on catfish horizontal and bipolar retinal neurons

We have used electrophysiological, pharmacological and immunological techni ques to determine which classes of metabotropic glutamate receptors exist o n cone horizontal cells in the catfish retina. Patch-clamp recordings in ac utely dissociated cone horizontal cells provide evidence that group I and I II metabotropic glutamate receptors exist, and are linked to modulation of a voltage-gated calcium current. Group II metabotropic glutamate receptor a gonists did not affect the calcium current. Immunocytochemical techniques w ere used to study the localization of metabotropic glutamate receptor subty pes found in the catfish retina. Antibodies raised against group I(metabotr opic glutamate receptor la, metabotropic glutamate receptor 5), group II (m etabotropic glutamate receptor 2/3) and group In (metabotropic glutamate re ceptor 6) metabotropic glutamate receptor subtypes were used to label acute ly dissociated horizontal, bipolar and Muller cells. Results from immunosta ining provide evidence that cone horizontal cells express group I (metabotr opic glutamate receptor 1 alpha, metabotropic glutamate receptor 5) and gro up III (metabotropic glutamate receptor 6), but not group Ii (metabotropic glutamate receptor 2/3) receptor subtypes, consistent with our electrophysi ological results. Cane horizontal cells exposed to anti-metabotropic glutam ate receptor 1 alpha, 5 or 6 antibodies all demonstrated diffuse overall st aining, with patches of dark immunostaining found on both dendritic process es and cell somata. In catfish bipolar cells, all four of the anti-metabotr opic glutamate receptor antibodies stained the processes and cell bodies of bipolar cells homogeneously. There was no evidence for a group of bipolar cells that did not stain with the anti-metabotropic glutamate receptor anti bodies, although the densest immunostaining occurred when bipolar cells wer e incubated with the anti-metabotropic glutamate receptor 6 antibody. Mulle r cells did not show immunostaining against any anti-metabotropic glutamate receptor antibody. Our non-immune controls confirmed that immunostaining w as specific for the antigen, and immunoblots were performed to demonstrate the specificity of the antibodies in catfish retina. These results support the hypothesis that group I and In metabotropic gluta mate receptor subtypes are found on catfish horizontal cells, and group I, II and III metabotropic glutamate receptor subtypes are expressed on catfis h bipolar cells. The metabotropic glutamate receptors on catfish cone horiz ontal cells act to modulate the voltage-gated sustained calcium current fou nd on these cells. (C) 1999 IBRO. Published by Elsevier Science Ltd.