MELATONIN ANALOGUES INDUCE PIGMENT GRANULE CONDENSATION IN ISOLATED XENOPUS LEAVIS MELANOPHORES IN TISSUE CULTURE

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Abstract
2-Iodomelatonin, a ligand which has recently been used to identify melatonin-binding sites in the brain, produced condensation of pigment granules when added to isolated Xenopus laevis melanophores in culture. Melatonin (EC 50 = 5.7 × 10−13 mol/l), 2-iodomelatonin (EC 50 = 3.4 × 10−12 mol/l) and also 2-chloromelatonin (EC 50 = 2.9 × 10−13 mol/l) were all potent agonists in this test. Melatonin analogues in which the side-chain was conformationally restricted by linkage to the 2-position of the indole ring were inactive (EC 50 > 10−6 mol/l). The remarkable sensitivity and selectivity of this pigment condensation response suggests it will be useful in future studies of melatonin agonists and antagonists.