GABA sensitivity of spectrally classified horizontal cells in goldfish retina.

We studied the GABA sensitivity of horizontal cells in the isolated goldfish retina. After the glutamatergic input to the horizontal cells was blocked with DNQX, GABA depolarized the monophasic and biphasic horizontal cells. The pharmacology of these GABA-induced depolarizations was tested with the GABA receptor antagonists bicuculline-methiodide and picrotoxin, the GABA transporter agonist nipecotic acid, and the GABA transporter antagonist SKF 89976-A. The GABA-induced responses of monophasic horizontal cells consisted of two components; one with characteristics of GABA-gated chloride channels, and one with characteristics of GABA transporters. In biphasic horizontal cells, we only found evidence for GABA-gated chloride channels. The results show that monophasic horizontal cells in goldfish contain the two components of a positive feedback loop (GABA transporters and GABA-gated chloride channels), as described in salamander. Furthermore, our results indicate that the monophasic horizontal cells may project directly to the biphasic horizontal cells, via an excitatory GABAergic pathway. We propose that the function of these GABAergic systems in horizontal cells is to abolish cone dominance in bipolar cells surround in the dark-adapted retina.