Characterization of a retinal melatonin receptor

Melatonin (5-methoxy-N-acetyltryptamine) at picomolar concentrations (IC₅₀, 40 pM) inhibited the calcium-dependent release of [³H] dopamine elicited at 3 Hz (2min, 20 mA, 2msec) from rabbit retina through activation of a site possessing the pharmacological and functional characteristics of a receptor. The effect of melatonin shows biological specificity as this hormone does not modify [³H]dopamine release from striatum or olfactory tubercle. This paper describes the effects of small modifications of the melatonin structure on the inhibition of calcium-dependent release of [³H]dopamine from retina. The more active melatonin analogs were those possessing a 5-methoxy group on carbon 5 of the indole nucleus and an N-acetyl group on the same position as in melatonin. The potencies of 5-methoxy indoles compounds was as follows (IC₅₀): melatonin (40pM) = 6-chloromelatonin (40 pM) > 6-hydroxymelatonin (1.6nM) ≥ 6-methoxymelatonin (2 nM) > 5-methoxytryptamine (63 nM) > 5-methoxy-N,N-di-methyltryptamine (200nM) >> 5-methoxytryptophol (4μM). The structure activity relationships of melatonin and related indoles indicated that the efficacy of melatonin is determined by the moiety substituted on carbon 5 (i.e., 5-methoxy group), whereas the affinity for the receptor is determined primarily by the moiety substituted on carbon 3 (i.e., ethyl N-acetyl group) of the indole nucleus. N-acetyltryptamine competitively antagonized the inhibitory effect of melatonin in the chicken retina and appears to be a partial agonist in the rabbit retina. The present results show that the rabbit or chicken retina preloaded in vitro with [³H] dopamine can be used to screen the effect of chemical modifications in the melatonin molecule that may lead to the discovery of potent and selective melatonin receptor agonists and antagonists with therapeutic potential.