Effective thermoregulation in primates depends upon internal circadian synchronization

Abstract

1. 1. Squirrel monkeys display a prominent circadian rhythm in body temperature ( T co). When entrained to 24-hr light-dark (LD) cycles, T co is defended against mild 6-hr cold exposures at all circadian phases. In constant light (LL), however, the animals' ability to defend T co against similar cold exposures is impaired. 2. 2. We have examined the potential roles of internal circadian synchronization and other factors such as LL or chair-restraint in determining the animals' ability to thermoregulate in the cold. 3. 3. Constant light and/or chair-restraint were ruled out as being major contributions to this thermoregulatory response in experiments where an alternate circadian synchronizer (24-hr cycles of food availability) was supplied to animals in LL. These animals were able to maintain T co at the same levels as comparable controls when they were cold exposed at three different circadian phases. 4. 4. The role of internal circadian synchronization was examined using an adrenalectomized squirrel monkey preparation. These animals, when supplied with replacement cortisol at 24-hr intervals, have the same circadian thermoregulatory characteristics as intact control animals, and when synchronized by LD cycles can maintain T co during cold exposure. 5. 5. In LL, the adrenalectomized animals, with 24-hr cortisol replacement, become internally desynchronized since a portion of the circadian system is entrained by cortisol and the rest free-runs. In this state, the cold exposures produce significant decreases in T co. 6. 6. The data indicate that for effective thermoregulation to occur, proper temporal synchronization of the circadian timekeeping system of the squirrel monkey is essential.