Reentrainment of the spontaneous locomotor activity rhythm to a daylight reversal in C57BL/6 and C3H/He mice: implication of melatonin.

Abstract The adaptation of the locomotor activity rhythm to a daylight reversal was previously found to be faster in C57BL/6 mice, which present a low level of melatonin, than in C3H/He mice, which exhibit a large nocturnal melatonin peak. Because pinealectomy has been shown to accelerate resynchronisation time in rats after a daylight reversal, we investigated the involvement of melatonin in the resynchronisation rate of locomotor activity rhythm in C57BL/6 and C3H/He strains. We first tested the effects of melatonin, administered at zeitgeber time (ZT) 20 (with ZT0 corresponding to light onset) for the 3 days preceding the daylight reversal, on the reentrainment of locomotor activity rhythm in both strains. Second, the effects of S-22153, a melatonin receptor antagonist, on the reentrainment of locomotor activity rhythm in C3H/He mice were examined. S-22153 was administered for the 3 days preceding the daylight reversal either at ZT12 or at ZT20, i.e., when endogenous melatonin levels are respectively low and high. Melatonin significantly delayed the resynchronisation of locomotor activity rhythm in C57BL/6 mice without affecting this parameter in C3H/He mice. S-22153 significantly accelerated the resynchronisation in C3H/He mice when administered at ZT20, but had no effect when administered at ZT12. These results support the hypothesis that the differences between C3H/He and C57BL/6 in the reentrainment of their locomotor activity rhythm depend, at least in part, on the interstrain differences in melatonin synthesis.