Role of the pineal gland in circadian organization of diurnal ground squirrels.

Golden-mantled ground squirrels, Spermophilus lateralis, pinealectomized (pinx) or sham-pinx at 70 days of age, were maintained in a LD 10:14 photoperiod; phase angles of activity onset were 1.6 +/- 0.3 and 1.9 +/- 0.3 hr in advance of light onset, respectively, and did not differ significantly between the groups. Rates of phase shifting of the activity rhythm after a 6 hr phase advance in the LD cycle also were comparable for the two groups and stable re-entrainment was achieved in 11 days. The period of the free-running activity rhythm in constant light did not differ between the groups. As demonstrated previously in nocturnal rodents, the pineal gland exerts little if any influence on generation or entrainment of the ground squirrel diurnal circadian activity rhythm. The rodent pineal is neither a master circadian oscillator nor a significant component of the transduction process by which light entrains the circadian activity rhythm.