Endogenous rhythm of general locomotor activity and its control by the suprachiasmatic nucleus in the mink(Mustela vison)

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Abstract
The present study of the circadian rhythm of locomotor activity during the summer in minks reared in a natural photoperiod showed that under laboratory conditions, this species has an essentially nocturnal behavior. But the relatively extensive activity that occurred in the light phase, as also described in the wild mink, suggests that the photoperiod is not the only factor controlling the circadian cycle. In animals exposed to constant light, the locomotor activity was concentrated in a single phase that shifted forward each day, showing that the period of endogenous rhythm is longer than 24 h under these conditions. An abrupt increase in this period of circadian rhythm was observed in one of the animals exposed to constant light. The rhythm of locomotor activity was also studied after bilateral lesion of the suprachiasmatic nuclei in animals reared in a natural photoperiod or in constant light. In the first case, the main nocturnal phase of locomotor activity disappeared and was replaced by brief alternating phases of activity and dormancy. An analogous result was first obtained with the minks exposed to constant light, but the different phases eventually merged into a single phase with a circadian shift forward.