Melatonin-mediated regulation of human MT1 melatonin receptors expressed in mammalian cells

In mammals, the pineal hormone melatonin activates G protein-coupled MT1 and MT2 melatonin receptors. Acute exposure of recombinant MT1 and MT2 melatonin receptors to supraphysiological concentrations of melatonin differentially regulates these two receptors with the MT2, but not the MT1, exhibiting rapid desensitization and internalization. In the present study, we sought to determine whether prolonged exposure to supraphysiological and physiological concentrations of melatonin desensitized and/ or internalized the MT1 melatonin receptor. Using a Chinese hamster ovary (CHO) cell line stably expressing MT1-FLAG or transiently expressing MT1-green fluorescent protein (GFP) melatonin receptors, we found that prolonged exposure (8 h) to supraphysiological concentrations of melatonin (100 nM) significantly increased the number of MT1 melatonin receptors and decreased the affinity (K-i) of melatonin for competition for 2-[I-125]iodomelatonin. A similar treatment also desensitized the MT1 melatonin receptor-mediated stimulation of [S-35]GTPgammaS binding, but did not internalize the receptor. In contrast, prolonged exposure to a concentration of melatonin mimicking nocturnal levels (400 pM) did not affect the number of MT1 melatonin receptors, the affinity for melatonin, or the functional sensitivity of the receptor. We conclude that in vivo endogenous melatonin does not significantly affect the functional sensitivity of MT1 melatonin receptors, however, exogenous melatonin taken therapeutically at doses above physiological levels could desensitize the receptor thereby affecting physiological responses mediated following activation of MT1 melatonin receptors. (C) 2004 Elsevier Inc. All rights reserved.