Short-term exposure to melatonin differentially affects the functional sensitivity and trafficking of the hMT1 and hMT2 melatonin receptors

The hormone melatonin mediates a variety of physiological functions in mammals through activation of pharmacologically distinct MT1 and MT2 G protein-coupled melatonin receptors. We therefore sought to investigate how the receptors were regulated in response to short melatonin exposure. Using 2-[I-125]iodomelatonin binding, cAMP functional assays, and confocal microscopy, we demonstrated robust differences in specific 2-[I-125] iodomelatonin binding, receptor desensitization, and cellular trafficking of hMT(1) and hMT(2) melatonin receptors expressed in Chinese hamster ovary (CHO) cells after short (10-min) exposure to melatonin. Exposure to melatonin decreased specific 2-[I-125] iodomelatonin binding to CHO-MT2 cells (70.3 +/- 7.6%, n = 3) compared with vehicle controls. The robust decreases in specific binding to the hMT(2) melatonin receptors correlated both with the observed functional desensitization of melatonin to inhibit forskolin-stimulated cAMP formation in CHO-MT2 cells pretreated with 10 nM melatonin (EC50 of 159.8 +/- 17.8 nM, n = 3, p < 0.05) versus vehicle (EC50 of 6.0 +/- 1.2 nM, n = 3), and with the arrestin-dependent internalization of the receptor. In contrast, short exposure of CHO-MT1 cells to melatonin induced a small decrease in specific 2-[I-125] iodomelatonin binding (34.2 +/- 13.0%, n = 5) without either desensitization or receptor internalization. We conclude that differential regulation of the hMT(1) and hMT(2) melatonin receptors by the hormone melatonin could underlie temporally regulated signal transduction events mediated by the hormone in vivo.