Brain stem tonic inhibition of thermoregulation in the rat.

It was shown, by selective levels of decerebration, that a drop in body temperature occurred in a midpontine rat due to impairment in thermoregulatory responses. Lowering the level of decerebration to the lower pons restored a coordinated thermoregulatory response. The response was inadequate to raise the core temperature to normal level, but it was nevertheless an integrated response of vasoconstriction, piloerection, and shivering and was appropriately turned off by warming the skin or core. In intact unanesthetized rats, microinjection of a local anesthetic in the pontine tegmentum resulted in thermoregulatory responses, such as curling of the body, piloerection, tremor in facial muscles, and shivering. Similar injections in the pontine tegmentum of the midpontine decerebrate rat also restored some heat gain responses, including shivering. These experiments suggest that in rats the heat gain responses are powerfully regulated by a tonic inhibitory mechanism located in the midbrain and upper pons that exerts a complete inhibitory effect on the thermoregulatory areas caudal to the midbrain in decerebrate rats and a partial inhibitory effect in intact animals. These experiments also confirm the previous observation in the cat of the presence of a facilitatory area(s) in lower levels of the central nervous system which is responsible for coordinated heat gain responses observed in low-level decerebrate animals. In addition, these experiments suggest that the brain stem thermoregulatory mechanisms are in fact functioning in intact unanesthetized animals.