SELECTIVE BRAIN COOLING IN RATS RESTING IN HEAT AND DURING EXERCISE

1. Hypothalamic, sagittal sinus venous blood and rectal temperatures were recorded in 8 rats during resting in heat (40-degrees-C) and during running at 31-degrees-C. 2. In normothermic animals the brain was warmer than rectal temperature. In hyperthermic animals, both resting and running, it became cooler than rectal temperature. This selective brain cooling response to hyperthermia was highly significant (P < 0.001). 3. Sagittal sinus was always cooler than the hypothalamus. Thermal changes in sagittal sinus reflected thermolytic responses and were followed by parallel shifts in hypothalamic temperature which were independent of trunk temperature changes. 4. The basis of selective brain cooling in rats may be collecting cool blood from the head skin by the system of dural venous sinuses.