Melatonin is a potent modulator of dopamine release in the retina

Melatonin, a hormone originally discovered in the pineal gland¹, has also been found in the retina of several vertebrate species^2-5. The enzyme system for melatonin synthesis also exists in the retina ^2,3,6-8, where the activity of one such enzyme, (serotonin N-acetyltransferase) varies with changes in light intensity in a circadian pattern^3,8. As the activity of dopamine containing amacrine neurones of the retina is influenced by changes in illumination^9-12 it was of interest to determine the effect of melatonin and its precursors, serotonin and N-acetylserotonin, on the release of ³H-dopamine from rabbit retina. I report here that picomolar concentrations of melatonin (IC₅₀ 9pM) selectively inhibited the calcium-dependent release of ³H-dopamine from rabbit retina, but not from striatum. Melatonin, was 1,000 times more potent than its precursor N-acetylserotonin in inhibiting the release of ³H-dopamine in retina, while the putative neurotransmitter serotonin¹³, was inactive. It is suggested that the light-dependent production of melatonin could play a physiological role in modulating the activity of dopamine-containing neurones in the retina.