Retinal 5-hydroxytryptamine levels were reduced by local injection of a-fluoromethyldopa, an inhibitor of aromatic L-amino acid decarboxylase, or reserpine, indicating that this amine is both synthesized and stored in this tissue. The retinae of light-adapted chicks have higher levels of 5-hydroxytryptamine and its major metabolite, 5-hydroxyindoleacetic acid, than dark-adapted animals. The turnover of 5-hydroxytryptamine appears to be greater in the light after inhibition of synthesis. Conversely, the rate of synthesis of 5-hydroxytryptamine appears to be greater in the dark. The possible interaction of dopaminergic neurons and the 5-hydroxytryptamine-containing cells and the effect of a-fluoromethyldopa on this relationship are discussed. The data lend support to the notion that 5-hydroxytryptamine in chick retina may have a neurotransmitter function.Much evidence has been accumulated to indicate that the biogenic amines, norepinephrine, dopamine, and 5-hydroxytryptamine (5-HT),3 may be the neurotransmitters for some neurons in the central nervous system. In the vertebrate retina, dopamine has been identified as the possible transmitter for a class of amacrine cells with cell bodies in the inner nuclear layer (Ehinger, 1976). Sufficient 5-HT to justify a possible neuronal role for this amine has been measured only in chick (Suzuki et al., 1975) and bovine retina (Thomas and Redburn, 1979).

The effects of an intravitreal injection of alpha-fluoromethyldopa, an irreversible mechanism-based inactivator of aromatic-L-amino acid decarboxylase, on the retinal dopamine content of light-adapted chicks and rabbits have been examined. A single administration of 10 nmol of alpha-fluoromethyldopa totally inactivates aromatic L-amino acid decarboxylase within 2 hr in vivo in rabbits. By 4 to 6 hr, the level of dihydroxyphenylalanine increased 7-fold and the levels of dopamine and dihydroxyphenylacetic acid fell by 90%. With an injection of 50 nmol, similar results were observed in chicks. The levels of dihyroxyphenylacetic acid began to fall soon after injection when significant (30 to 40%) amounts of dopamine were still present. These results are discussed in relation to the use of tissue dihydroxyphenylacetic acid levels to indicate the level of dopaminergic neuronal activity or dopamine synthesis. Recovery by the retinae of both species was shown by the return of aromatic L-amino acid decarboxylase activity and the resynthesis of dopamine.