MT-2 -: Primary role for melatonin MT 2 receptors in the regulation of anhedonia and circadian temperature rhythm.

Circadian rhythms disturbance is widely observable in patients with major depression (MD) and is also associated with depression vulnerability. Of them, disturbed melatonin secretion rhythm is particularly relevant to MD and is strongly phase-locked to core body temperature (CBT) rhythm. Here we aim to study the specific role of each melatonin receptor (MT 1 and MT 2 ) subtype in melatonin regulation of circadian CBT and its possible relationship with depressive-like behaviors. MT 1 -/- , MT 2 -/- and WT (C57BL/6) mice were used.  Anhedonia, using the sucrose intake test, circadian CBT, environmental place preference (EPP) conditioning and vulnerability to chronic social defeat stress (CSDS) procedure were studied. Moreover, the antidepressant effects of reboxetine (15 mg/kg/day, i.p.) for three weeks or ketamine (15 mg/kg i.p. every four days, 4 doses in total) were studied. Further, exposure to ultra-mild stress induced by individual housing for several weeks was also studied in these mice. MT 2 -/- mice showed anhedonia and lower CBT compared to WT and MT 1 -/-. In addition, while reward exposure raised nocturnal CBT in WT this increase did not take place in MT 2 -/- mice. Further, MT 2 -/- mice showed an enhanced vulnerability to stress-induced anhedonia and social avoidance as well as an impaired acquisition of novelty seeking behavior. Both reboxetine and ketamine reverted anhedonia and induced a clear anti-helpless behavior in the tail suspension test (TST). Reboxetine raised CBT in mice and reverted ultra-mild stress-induced anhedonia. Our findings show a primary role for MT 2 receptors in the regulation of circadian CBT as well as anhedonia and suggest that these receptors could be involved in depressive disorders associated to disturbed melatonin function.