Cyclic variations in hypothalamic temperature in unanesthetized rats

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Abstract
Male and female rats, equipped with stainless steel re-entrant tubes implanted in the brain adjacent to one preoptic area, were placed in cages with accessible activity wheels. Brain temperatures were recorded continuously for periods up to 48 hr by means of thermocouples inserted into the tubes. The activities of running, feeding, and drinking were recorded simultaneously. About 90% of feeding and drinking activity occurred irregularly during three to nine periods of running activity at night. Active periods at night were associated with brain temperatures 1.0–2.5 C higher than inactive periods during the night or day. Moderate restraint with absence of food abolished these cyclic temperature variations whereas absence of food alone did not. During the night of estrus (9 pm–3 am), marked increases in activity frequently were associated with elevated mean preoptic temperatures.