GABA enhances short wavelength-sensitive cone input and reduces red cone input to carp L-type horizontal cells

Light responses of cone-driven horizontal cells were recorded intracellular ly in the isolated superfused carp retina and the effects of gamma-aminobut yric acid (GABA) on signals from red-sensitive (R-) and short-wavelength-se nsitive (S-) cones (green cones and/or blue cones) were studied. In the pre sence of a bright red (694 nm) background light, which substantially suppre ssed signal from R-cones, the responses of L-type horizontal cells (L-HCs) to 532-nm flashes, predominantly driven by the S-cone input, were potentiat ed by application of GABA. In contrast, the responses of these cells to 694 -nm flashes driven by the R-cone input, were suppressed, when signal from S -cones was suppressed by a bright 532-nm background light. Both the effects could be reversed by coapplication of bicuculline, suggesting the involvem ent of GABA(A) receptors. It was unlikely that the potentiation by GABA of the S-cone driven responses of the L-HCs was mediated by actions of GABA on the cone photoreceptors. The dual action of GABA persisted in the dopamine -depleted retina, indicating no involvement of the dopaminergic interplexif orm cells. We speculate that this dual action may be partially due to diffe rential modulation by GABA of different postsynaptic mechanisms respectivel y mediating signal transfer from R-cones and S-cones to L-HCs. (C) 2000 Els evier Science Inc.