Selective MT 2 Melatonin Receptor Antagonist Modulates Circadian Activity via Inhibition of the Endogenous Melatonin Signal in the East Bound Jet Lag Model

Abstract

Activation of melatonin receptors facilitates re-entrainment of circadian activity rhythms following an advance of the dark onset. In an eastbound jet lag model, administration of exogenous melatonin, at the new dark onset, accelerates re-entrainment via activation of the MT1 receptor (Dubocovich et al., 2005). Conversely, melatonin proficient mice (C3H/HeN) with a targeted deletion of the MT2 receptor (MT2KO), but not the MT1 receptor (MT1KO), experience a deceleration in re-entrainment, probably due to impairment of the endogenous melatonin facilitated signaling mediated by activation of the MT2 receptor (Pfeffer et al., 2012). To further elucidate the functional modulatory role of melatonin receptors on the synchronization of the circadian clock, we assessed locomotor activity rhythms in C3H/HeN mice in response to pharmacological intervention to determine the ability of the MT2 selective antagonist, ATBT-23 (hMT1/hMT2 Ki=190), to decelerate re-entrainment by blocking the MT2 receptor, thus mimicking the MT2KO. Male C3H/HeN mice in a 12/12 L/D cycle were subjected to an eastbound jet lag paradigm in which Vehicle (30% ethanol/saline, s.c) or ATBT-23 (1 mg/kg in Vehicle, s.c) were administered on three consecutive days at the new dark onset following an abrupt 6-hour advance of the L/D cycle. The number of days to re-entrain and re-entrainment rates (hrs/day) were analyzed by two-way ANOVA (Tukey's post hoc test) and a mixed effect two-way repeated measure ANOVA (Sidak's post hoc test), respectively. WT (7.54±0.33 days, n=28) and MT1KO (7.36±0.54 days, n=14) mice treated with vehicle did not show a significant difference in the number of days necessary to attain re-entrainment. However, MT2KO (11.92±0.40 days, n=13) mice, treated with vehicle, took significantly longer to re-entrain when compared to WT and MT1KO mice (p<0.0001). This deceleration of re-entrainment in the MT2KO was reproduced by ATBT-23, which decelerated the rate of re-entrainment in both WT (p<0.0001, n=25) and MT1KO (p<0.01, n=12) males, increasing the number of days to re-entrain to 9.76±0.46 (p<0.01) and 10.00±0.63 (p<0.05) days, respectively. ATBT-23 (n=14) did not significantly affect the number of days to re-entrain in MT2KO mice, suggesting deceleration of re-entrainment is not mediated by the MT1 receptor. The present study suggests that ATBT-23 decelerates re-entrainment of circadian activity rhythms by blocking the potent acceleration of re-entrainment by endogenous melatonin via action on the MT2 receptor. Therefore, selective MT2 melatonin receptor antagonists can potentially be used to delay advanced circadian rhythms, as observed in depression and advance sleep phase disorders, when administered at proper circadian times.