Stimulus-activated changes in brain tissue temperature in the anesthetized rat.

A new thin-film, multisensor probe was used to determine tissue oxygen tension, tissue temperature, and electrical activity at two depths below the brain surface in chloral hydrate- or nitrous oxide/halothane-anesthetized rats. Brain tissue temperature at both depths was found to be lower than core temperature by 1-2 degrees C. Electrical activation, spreading depression, and pentylenetetrazol seizures all resulted in transient increases of brain tissue temperature of a few tenths degree centigrade. Vasodilation, induced by hypercapnia or hypoxia, caused a warming of brain tissue. Near-maximum oxygen metabolism, reached upon reoxygenation after severe hypoxia, was accompanied by tissue temperature rises of greater than 1 degree C. It was concluded that brain tissue temperature in the anesthetized rat is lower than core temperature due to extensive radiative and conductive heat loss to the environment through the head. Transient increases in tissue temperature during activation are caused by vasodilation and increased metabolism.