Do medial preoptic lesions interfere with the set-point of temperature regulation?

Rats with lesions in the medial preoptic area were placed in a thermocline where they could choose ambient temperatures (T a) ranging from 17 to 31°C. The hypothalamic temperature (T hy) and the selected T a were continuously recorded. Amplitude of T hy circadian oscillations was exaggerated in 23 out of 27 lesioned animals. Only in 6 circadian cycles out of the 158 recorded, the rats chose a higher T a during the night. In 55% of the recordings the preferred T a was significantly lower during the night than during the day and out of phase with T hy. In the other 36% of the recorded cycles no significant difference was found between the night and day preferred T a. Therefore in 91% of the recorded cycles the preferred T a oscillations did not support the idea that the exaggerated circadian oscillations of T hy in lesioned rats would be due to exaggerated cyclical shifts of the set-point temperature. The lesions induced many sporadic anomalies in the behavior of the rats in choosing a preferred T a. Short episodes of almost parallel variations in T hyand T a in phase with each other suggest that some lesions induce an instability of the set-point mechanism.