Melatonin analogues induce pigment granule condensation in isolated Xenopus laevis melanophores in tissue culture.

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 (EC50 = 5.7 x 10(-13) mol/l), 2-iodomelatonin (EC50 = 3.4 x 10(-12) mol/l) and also 2-chloromelatonin (EC50 = 2.9 x 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 (EC50 greater than 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.