Methamphetamine‐induced, suprachiasmatic nucleus‐independent circadian rhythms of activity and <i>mPer</i> gene expression in the striatum of the mouse

Abstract

While the suprachiasmatic nucleus (SCN) coordinates the majority of daily rhythms, some circadian patterns of expression are controlled from outside of the SCN. These include responses to daily methamphetamine (MAP) injection, or daily restricted feeding. The mechanisms underlying these SCN-independent circadian rhythms are unknown. A circadian oscillation in the expression of mPer1 and/or mPer2, mouse period, in the SCN is considered necessary to generate an SCN-dependent circadian rhythm. Therefore, in this experiment, we examined the association between mPer gene expression and the MAP-induced, SCN-independent circadian rhythm. Acute injection of MAP caused an elevation of mPer1, mBmal1, and mNpas2 gene expression in the striatum and mPer1 in the liver. Daily MAP injection at a fixed time for 6 days shifted the rhythmic mPer1 and mPer2 expression in the striatum from a nocturnal to a diurnal rhythm, but failed to affect that in the SCN. Although lesion of the SCN 'flattened'mPer gene oscillation in the striatum and liver, daily MAP injection caused both behavioural and mPer gene expression rhythms. Daily MAP injection at variable injection intervals (12-36 h) for 6 days, however, failed to produce mPer gene rhythm in the striatum. Daily repeated MAP signals may strengthen the oscillatory force of SCN-independent circadian behavioural and molecular rhythms. The present results suggest that daily oscillation of mPer genes outside the SCN is closely associated with the regulation of SCN-independent rhythms. Thus, the present experiment highlights strongly the important role of clock gene expression, in the brain, that underlies the circadian behavioural rhythm.