Brain hyperthermia during physiological and pathological conditions: causes, mechanisms, and functional implications.

Although brain metabolism consumes high amounts of energy and is accompanied by intense heat production, brain temperature is usually considered a stable, tightly regulated homeostatic parameter. Current animal research, however, has shown that different forms of functional neural activation are accompanied by relatively large brain hyperthermia (2-3 degrees C), which has an intra-brain origin; cerebral circulation plays a crucial role in dissipating this potentially dangerous metabolic heat from brain tissue. Brain hyperthermia, therefore, reflects enhanced brain metabolism and is a normal physiological phenomenon that can be enhanced by interaction with common elements of an organism's environment. There are, however, instances when brain hyperthermia becomes pathological. Both exposure to extreme environmental heat and intense physical activity in a hot, humid environment restrict heat dissipation from the brain and may push brain temperatures to the limits of physiological functions, resulting in acute life-threatening complications and destructive effects on neural cells and functions of the brain as a whole. Brain hyperthermia may also result from metabolic activation induced by various addictive drugs, such as heroin, cocaine, and meth-amphetamine (METH). In contrast to heroin and cocaine, whose stimulatory effects on brain metabolism invert with increases in dose, METH increases brain metabolism dose-dependently and diminishes heat dissipation because of peripheral vasoconstriction. The thermogenic effects of this drug, moreover, are enhanced during physiological activation, resulting in pathological brain hyperthermia. Since brain hyperthermia exacerbates drug-induced toxicity and is destructive to neural cells, uncontrollable use of amphetamine-like drugs under conditions restricting heat dissipation from the brain may result both in acute life-threatening complications and clinically latent but dangerous morphological and functional brain destruction.