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.