Circadian rhythm of body temperature persists after suprachiasmatic lesions in the squirrel monkey

Squirrel monkeys (Saimiri sciureus) demonstrate prominent circadian (approx 24 h) rhythms in many behavioral and physiological variables including drinking and body temperature. Both of these rhythms can be entrained by a 24-h light-dark cycle (LD 12:12) but will free-run with an endogenous period in a constantly illuminated (LL:600 lx) environment free of time cues. After radio-frequency lesions were placed stereotaxically in the suprachiasmatic nuclei (SCN) of five monkeys, the circadian rhythm of drinking behavior was disrupted when the monkeys were maintained in LL. However, the circadian rhythm in core body temperature in these animals persisted in LL with a significant circadian spectral component following destruction of the SCN. The SCN thus appear to be of fundamental importance for regulating the circadian organization of drinking; however, an oscillator located elsewhere in the squirrel monkey is capable of generating the core body temperature rhythm.