GABA-mediated negative feedback from horizontal cells to cones in carp retina.

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Abstract
In the isolated, perfused retina of the carp, properties of the feedback pathway from horizontal cells to cones were investigated by means of electrophysiological and neuropharmacological methods. When horizontal cells were hyperpolarized by illumination with an annulus, a depolarizing synaptic potential was produced in cones at the center of the annulus, suggesting that horizontal cells receive inputs from cones and exert a negative feedback to cones. A hyperpolarizing response (IPSP [inhibitory postsynaptic potential]) was detected in cones when a transient depolarization (EPSP [excitatory post-synaptic potential]) was induced in horizontal cells by application of a transretinal current pulse; this activated the release of transmitter from the photoreceptor terminals. The IPSP was abolished when the retina was perfused with a GABA-containing Ringer solution, because of desensitization of the feedback synapse. GABA also hyperpolarized the cone membrane, indicating the presence of a GABA-sensitive site in the cone. Apparently, the GABA-mediated negative feedback operates from horizontal cells to cones in the dark, and ceases its function in the light.