Aberrations of circadian body temperature rhythms in rats with medial preoptic lesions.

The normal amplitude of the circadian rhythm of telemetered body temperature (Tb) in male rats was 2 degrees C (between 36 and 38 degrees C). For several weeks after large medial preoptic lesions Tb cycled from as low as 29 degrees C to as high as 41 degrees C in a single day. With recovery, or in rats with smaller lesions, peak-to-trough Tb amplitudes decreased, ranging between 3 and 5 degrees C for many months, with normal or slightly raised troughs in the light part and grossly higher peaks in the dark part of a 12:12 light-dark cycle. Rats in whom some periventricular tissue was spared had normal amplitudes of Tb, but the whole curve was displaced 0.5-1.0 degrees C above control values. These effects persisted in constant darkness. We conclude that in the weeks after medial preoptic lesions thermoregulatory responses are not activated until Tb is abnormally high or low. As recovery progresses, or if the lesions are smaller to begin with, there may be an imbalance between heat loss and heat production systems so that heat production mechanisms are either always dominant or overshoot when activated.