Behavioural responses to hypothalamic cooling and heating in the rat

Rats with hypothalamic thermodes had their hypothalamus cooled or warmed for short sessions. In a first series of experiments, rats could bar-press to obtain fanning with cool air. Cooling the hypothalamus did not suppress or inhibit this behaviour although rectal temperature was markedly increased. In a second series of experiments, bar-pressing would warm the water flowing in the thermode. The rats thus self suppressed the cooling of their hypothalamus. This behaviour was absent at 10 degrees C ambient temperature, and increased with increasing ambient temperature up to 35 degrees C. The result of this behaviour was a small hyperthermia in warm and hot environment compared to control when the rats could not self-suppress the cooling of hypothalamus. The results of both experiments suggest that no conscious direct sensation is aroused by hypothalamic cooling. Hypothalamic heating increased the rat's bar-pressing for cool air and decreased the rat's rectal temperature. When pressure on the lever would suppress a warm hypothalamic stimulus rats self-cooled their hypothalamus, especially in warm environments. Such behaviour resulted in an increased somatic hyperthermia due to the warm environment and hypothalamic cooling. These results are compatible with the hypothesis of a direct conscious sensation from a warm hypothalamus.