Feeding-entrained circadian rhythms in hypophysectomized rats with suprachiasmatic nucleus lesions

Several pituitary hormones are important in the regulation of metabolism, and their release appears to be controlled by a circadian clock. Consequently, they may be involved in feeding-entrained circadian rhythms. Hypophysectomized (Hypox) and sham-operated male Sprague-Dawley rats had access to food for 4 h each day. Food-anticipatory activity (FAA) and core body temperature (T(b)) were monitored. Both groups entrained to the daily meal with an increase in activity in the 4 h preceding meal access and quickly reentrained after an 8-h phase advance of food access. FAA was not disrupted in either group after suprachiasmatic lesions were added. Core T(b) increased in the sham-operated subjects before mealtime, but Hypox rats failed to show this effect. Rather, T(b) declined during anticipation and throughout the food access period. Respiratory quotient (RQ), an indirect measure of metabolic rate, was measured for 24 h in some subjects. Sham-operated rats showed a dramatic downturn in RQ 1 h before mealtime, whereas Hypox rats showed a steadily decreasing RQ throughout the day. The results show that the pituitary hormones are not necessary for FAA and that in Hypox rats the anticipatory rise in T(b) and changes in RQ become dissociated from anticipatory behavior, indicating that these functions are separate outputs of the food-entrainable circadian oscillator.