Vol. 304, Issue 3, 931-939, March 2003

Short-Term Exposure to Melatonin Differentially Affects the Functional Sensitivity and Trafficking of the hMT₁ and hMT₂ Melatonin Receptors

M. J. Gerdin , M. I. Masana , D. Ren, R. J. Miller and M. L. Dubocovich

Departments of Molecular Pharmacology and Biological Chemistry (M.J.G., M.I.M., D.R., R.J.M., M.L.D.) and Psychiatry and Behavioral Sciences (M.L.D.), Northwestern University Institute for Neuroscience (M.J.G., M.I.M., R.J.M., M.L.D.), Northwestern Drug Discovery Program (M.J.G., M.I.M., R.J.M., M.L.D.), Feinberg School of Medicine, Northwestern University, Chicago, Illinois

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-[¹²⁵I]iodomelatonin binding, cAMP functional assays, and confocal microscopy, we demonstrated robust differences in specific 2-[¹²⁵I]iodomelatonin binding, receptor desensitization, and cellular trafficking of hMT₁ and hMT₂ melatonin receptors expressed in Chinese hamster ovary (CHO) cells after short (10-min) exposure to melatonin. Exposure to melatonin decreased specific 2-[¹²⁵I]iodomelatonin binding to CHO-MT₂ cells (70.3 ± 7.6%, n = 3) compared with vehicle controls. The robust decreases in specific binding to the hMT₂ 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-[¹²⁵I]iodomelatonin binding (34.2 ± 13.0%, n = 5) without either desensitization or receptor internalization. We conclude that differential regulation of the hMT₁ and hMT₂ melatonin receptors by the hormone melatonin could underlie temporally regulated signal transduction events mediated by the hormone in vivo.