Effect of putative melatonin receptor antagonists on melatonin-induced pigment aggregation in isolated Xenopus laevis melanophores.

The ability of putative melatonin receptor antagonists to antagonise melatonin-induced aggregation of pigment granules in cultured neural crest Xenopus laevis melanophores was examined. Neither ML 23 (N-(2,4-dinitrophenyl)-5-methoxytryptamine) nor 6-methoxy-2-benzoxazolinone showed agonist or antagonist activity. N-Acetyltryptamine and N-butanoyltryptamine were partial agonists; both compounds aggregated pigment granules in some cells, but also reversed melatonin-induced pigment agreggation in a fraction of the cells tested. In contrast, 2-benzyl N-acetyltryptamine (luzindole) did not show agonist activity (upto 10 microM) but did reverse the aggregating action of melatonin at 1 and 10 microM. Pretreatment of melanophores with luzindole shifted the melatonin concentration-response curve to the right.