Ontogeny of mechanisms which regulate light/dark differences in retinal dopamine synthesis

The ability of bicuculline, a GABA antagonist, to enhance dopamine (DA) synthesis in retinas of rats 1, 4, 7, 15 and 60 days after eye opening was assessed and compared to the time course of postnatal development of the light-induced increase in DA synthesis. The accumulation of dihydroxyphenylalanine (DOPA) following administration of the L-aromatic amino acid decarboxylase inhibitor, NSD 1015, was used to estimate DA synthesis. In dark-adapted rats, neither bicuculline nor light enhanced DOPA accumulation 1 day after eye opening, but on the remaining days either treatment significantly augmented DA synthesis, and by day 15 the effects were as great as those observed in adult retinas. At each time point, the magnitude of the drug effect on DA synthesis in the dark was similar to that observed following light exposure. These results suggest that an endogenous GABAergic input to the DA neurons appears at the same time as the acquisition of the dopaminergic response to light. The effect of bicuculline treatment on DA synthesis in light-exposed animals was also assessed. At 4 and 7 days the drug significantly enhanced DOPA accumulation over that produced by exposure to light alone, but on later days bicuculline exerted no such additive effect. These data imply that early in the maturation of the light response mechanisms other than removal of an inhibitory GABAergic tone may be partially responsible for excitation of the DA neurons.