Regulation by the pituitary gland of circadian rhythms in the hamster

Locomotor activity of male hamsters was recorded during long-term exposure to constant light (LL), constant darkness (DD) and during entrainment (modification of a circadian rhythm) to a 14 h light : 10 h darkness photoperiod (14L : 10D). In LL the period of the activity cycle was substantially longer in hypophysectomized than in control animals. This difference persisted during tests in DD. Although hypophysectomy reduced the duration of the active phase in some hamsters, overall the difference between the groups was not significant. The phase angle of onset of activity in 14L : 10D was not affected by hypophysectomy. Hypophysectomized female hamsters tested in DD had activity rhythms whose periods were longer than those of control animals; they were also significantly less active than corresponding controls during the first 4 h of the subjective night but the duration of the active phase did not differ significantly between the groups. These results suggest that hormones of the pituitary-gonadal axis modulate the period of circadian oscillation.