LIGHT-INDUCED SUPPRESSION OF THE RAT CIRCADIAN SYSTEM

In a constant environment, circadian rhythms persist with slightly alt ered period lengths. Results of studies with continuous light exposure are less clear, because of short exposure durations and single-variab le monitoring. This study sought to characterize properties of the osc illator(s) controlling the rat's circadian system by monitoring both b ody temperature and locomotor activity. We observed that prolonged exp osure of male Sprague-Dawley rats to continuous light (LL) systematica lly induced complete suppression of body temperature and locomotor act ivity circadian rhythms and their replacement by ultradian rhythms. Th is was preceded by a transient loss of coupling between both functions . Continuous darkness (DD) restored circadian synchronization of tempe rature and activity circadian rhythms within 1 wk. The absence of circ adian rhythms in LL coincided with a mean sixfold decrease in plasma m elatonin and a marked dampening but no abolition of its circadian rhyt hmicity. Restoration of temperature and activity circadian rhythms in DD was associated with normalization of melatonin rhythm. These result s demonstrated a transient internal desynchronization of two simultane ously monitored functions in the rat and suggested the existence of tw o or more circadian oscillators. Such a hypothesis was further strengt hened by the observation of a circadian rhythm in melatonin, despite c omplete suppression of body temperature and locomotor activity rhythms . This rat model should be useful for investigating the physiology of the circadian timing system as well as to identify agents and schedule s having specific pharmacological actions on this system.