gamma-Aminobutyric acid: a neurotransmitter candidate for cone horizontal cells of the catfish retina.

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Abstract
In the catfish [Ictalurus punctatus] retina, horizontal cells that receive inputs exclusively from red-sensitive cones are the only neurons that accumulate exogenous GABA under these experimental conditions. When isolated eyecups are perfused with bicuculline methochloride, an antagonist of postsynaptic GABA receptors, responses of cone photoreceptors to a field of light (3 mm in diameter) become much slower and approach those to a small spot of light (0.3 mm). In addition, bicuculline methochloride decreases the frequency responses of cone horizontal cells to a field of light. In the catfish retina, feedback synapses from cone horizontal cells to cones are chemically mediated and may use GABA as a neurotransmitter. These results confirm the hypothesis that, in the catfish retina, a function of the negative feedback is to improve the frequency responses of the system.