Selective MT 2 melatonin receptor antagonists block melatonin‐mediated phase advances of circadian rhythms

This study demonstrates the involvement of the MT 2 (Mel 1b ) melatonin receptor in mediating phase advances of circadian activity rhythms by melatonin. In situ hybridization histochemistry with digoxigenin‐labeled oligonucleotide probes revealed for the first time the expression of mt 1 and MT 2 melatonin receptor mRNA within the suprachiasmatic nucleus of the C3H/HeN mouse. Melatonin (0.9 to 30 μg/mouse, s.c.) administration during 3 days at the end of the subjective day (CT 10) to C3H/HeN mice kept in constant dark phase advanced circadian rhythms of wheel running activity in a dose‐dependent manner [EC 50 = 0.72 μg/mouse; 0.98 ± 0.08 h ( n = 15) maximal advance at 9 μg/mouse]. Neither the selective MT 2 melatonin receptor antagonists 4P‐ADOT and 4P‐PDOT (90 μ/mouse, s.c.) nor luzindole (300 μg/mouse, s.c.), which shows 25‐fold higher affinity for the MT 2 than the mt 1 subtype, affected the phase of circadian activity rhythms when given alone at CT 10. All three antagonists, however, shifted to the right the dose‐response curve to melatonin, as they significantly reduced the phase shifting effects of 0.9 and 3 mg melatonin. This is the first study to demonstrate that melatonin phase advances circadian rhythms by activation of a membrane‐bound melatonin receptor and strongly suggests that this effect is mediated through the MT 2 melatonin receptor subtype within the circadian timing system. We conclude that the MT 2 melatonin receptor subtype is a novel therapeutic target for the development of subtype‐selective analogs for the treatment of circadian sleep and mood‐related disorders.—Dubocovich, M. L., Yun, K., Al‐Ghoul, W. M., Benloucif, S., Masana, M. I. Selective MT 2 melatonin receptor antagonists block melatonin‐mediated phase advances of circadian rhythms. FASEB J. 12, 1211–1220 (1998)