Neuropeptide Mechanisms Affecting Temperature Control

Abstract

The internal body temperature of mammals is remarkably constant, being regulated at approximately 37°–39°C in most mammals and, in individuals, varying by less than a degree under remarkably diverse metabolic and environmental influences. This condition of homeothermia is a feature of mammals and birds. It appears to be brought about through the defense of a specific set-point for body temperature. Animals which are unable to defend their body temperature are said to be poikilothermic, and such animals exhibit changes in body temperature related directly to environmental demands. While poikilotherms also appear to regulate around a set-point for body temperature, they lack the physiological mechanisms enjoyed by homeotherms in regulating body temperature. When the body temperature of a homeotherm falls below its set-point, the animal is said to be hypothermic, and appropriate mechanisms are engaged to return the body temperature to normal; a body temperature above the normal set-point temperature is known as hyperthermia, and such animals engage in mechanisms designed to reduce body temperature. The state of fever resembles that of hyperthermia in that body temperature is above normal; however, in contrast to hyperthermia, a febrile animal appears to have an altered set-point for body temperature regulation such that the temperature is defended at a new, elevated level. The febrile condition can be initiated by a number of pathological and experimental situations but is most commonly brought about through the action of pyrogens or bacterial endotoxins. Certain substances, such as the common drugs acetylsalicylic acid (aspirin) and acetaminophen, reduce febrile body temperature but, at the doses effective in lowering fever, are ineffective in altering normal body temperatures; these substances are known as antipyretics.