The role of GABA in the outer plexiform layer of the turtle retina has been examined by intracellular recordings from L- and C-type horizontal cells in the isolated retina preparation.
GABA (1–5 mM) slightly depolarized the L-type horizontal cells, reduced the amplitude of their photoresponses, and slowed down the rate of hyperpolarization during the ON component of the photoresponse. These effects could not be replicated by either muscimol or baclofen. When synaptic transmission from the photoreceptors had been blocked by either kynurenic acid or cobalt ions, GABA depolarized L-type horizontal cells and augmented the remaining photoresponses. Neither muscimol nor baclofen exerted any effect on L-type horizontal cells under these conditions. Nipecotic acid, a competitive inhibitor of the GABA-uptake system, induced effects on turtle L-type horizontal cells which were similar to those exerted by GABA. Thus, the complex GABA effect on turtle L-type horizontal cells seems to represent the summation of at least two actions; an indirect one mediated by the red cones via GABAa-type receptors and a direct one which probably reflects the activation of an electrogenic GABA-uptake system.
GABA (1–5 mM) induced a transient depolarization in C-type horizontal cells but eliminated color opponency in only three cells out of seven studied. This observation is inconsistent with the notion that the only neural mechanism responsible for the chromatic properties of C-type horizontal cells in the turtle retina is a GABAergic negative feedback from the L-type horizontal cells onto the green ones.