Changes in rhythmicities of ambulatory and drinking activities after hypophysectomy in rats

Effects of hypophysectomy on circadian rhythms of ambulatory and drinking activities in adult male rats were automatically recorded and investigated with a Gundai-type ambulodrinkometer. The rats were maintained under LL conditions following an LD condition. When free-running rhythms had stabilized, the rats were hypophysectomized. The rhythmicities of both activities became indistinct over the 2-4 weeks following surgery, and then gradually recovered. At this time phase shifts (phase advance) were observed in the hypophysectomized rats, while these changes were not observed in sham-operated rats. All rats entrained to lighting when kept under LD conditions again. These results suggest that the pituitary does not play an essential role in maintaining circadian rhythms, but does function as an important subordinal oscillator.