GABA antagonists enhance dopamine turnover in the rat retina in vivo

Previous results provided evidence for an inhibitory GABAergic influence on the dopamine neurons of the rat retina, without proving that endogenous GABA physiologically regulates the activity of these cells. We injected picrotoxinin intraocularly to dark-adapted rats and measured retinal dopamine turnover. Dopamine was analyzed radioenzymatically, and the decline in dopamine after alpha-methyl-p-tyrosine was used as an index of dopamine turnover. Picrotoxinin significantly stimulated dopamine turnover (P less than 0.05). In similar experiments with light-exposed rats picrotoxinin slightly enhanced dopamine turnover beyond that produced by light alone. Intraocular bicuculline methiodide produced similar results in dark-adapted and also in light-exposed rats. These data suggest that endogenous GABA tonically inhibits the activity of the retinal dopamine neurons in the dark and that there may be some remaining GABA tone on these cells in the light-exposed rat. Atropine pretreatment did not affect the picrotoxinin-induced activation of retinal dopamine turnover which negates the involvement of a cholinergic interneuron.