Role of MT1 melatonin receptors in methamphetamine-induced locomotor sensitization in C57BL/6 mice

Rationale Melatonin modifies physiological and behavioral responses to psychostimulants, with the MT 1 and MT 2 melatonin receptors specifically implicated in facilitating methamphetamine (METH)-induced sensitization in melatonin-proficient mice. Objective The objective of the study is to assess differences in locomotor sensitization after a single dose of methamphetamine in low-melatonin-expressing C57BL/6 wild-type and MT 1 receptor knockout (MT 1 KO) mice, comparing with melatonin-expressing C3H/HeN mice. Methods Mice received a vehicle or methamphetamine (1.2 mg/kg, i.p.) pretreatment (day 1) during the light (ZT5-9) or dark (ZT 19–21) periods in novel test arenas. Locomotor sensitization was assessed by methamphetamine challenge after an eight-day abstinence (day 9). TH protein expression was evaluated by immunofluorescence and Western blot analysis. Results Methamphetamine pretreatment induced statistically significant locomotor sensitization upon challenge after eight-day abstinence in C3H and C57 wild-type mice during the light period. The magnitude of sensitization in C57 mice was diminished in the dark period and completely abrogated in MT 1 KO mice. No differences were observed in tyrosine hydroxylase immunoreactivity in the mesolimbic dopamine system. Additional exposures to the test arenas after methamphetamine pretreatment (nights 2–6) enhanced sensitization. Conclusions Deletion of the MT 1 melatonin receptor abolishes sensitization induced by a single METH pretreatment. The magnitude of sensitization is also altered by time of day and contextual cues. We conclude that the MT 1 melatonin receptor is emerging as a novel target of therapeutic intervention for drug abuse disorders.