Central nervous regulation of body temperature in vertebrates: comparative aspects.

Thermal fluctuations affect, and are responded to by, nearly all forms of life. The basic vertebrate template has guided and shaped the ways that animals in this subphylum cope with thermal challenges. This has led to a situation where there are major similarities in the neuronal mechanisms which sense temperature and control the responses to temperature change in all vertebrates, from fish to mammals. The PO/AH is the most important single integration site for temperature regulation and (except for birds) is also important in the sensing of core temperature. Other portions of the brainstem as well as the spinal cord are also involved in thermal control and can sense, integrate, and produce appropriate efferent signals to varying degrees. Peripheral thermal input to the hypothalamic areas is via the brainstem reticular areas. A number of studies has related the thermal response characteristics of CNS single neurons to the thermoregulatory output of intact animals. These studies have been performed on neurons in whole animal, brain slice, and tissue culture preparation. These neurophysiological studies of central neurons are informative, but are sometimes difficult to interpret because of the chronic lack of definite criteria to differentiate generalized thermal sensitivity from thermal sensitivity utilized for regulating body temperature. Recent neuroanatomical work has illustrated that many areas previously implicated in the thermoregulatory network (such as the septum, various hypothalamic nuclei, the midbrain reticular formation, and the midbrain raphé nuclei) receive direct projections from PO/AH neurons. When compared, the neurophysiological and neuroanatomical characteristics of the preoptic nucleus and anterior hypothalamic area are similar, but not identical. The broad differences in the responses that vertebrates utilize to deal with thermal change is largely determined by the respiratory medium (water or air) and whether metabolic energy (endothermy) or ambient temperature (ectothermy) is of primary importance in the determination of internal temperature. A number of physiological systems are perturbed in water breathing ectotherms when the ambient temperature is altered. In these vertebrates long-term acclimation is very important and has a major effect on temperature selection. Air breathing ectotherms are less adversely affected by temperature change; long-term thermal acclimation is less important and has little effect upon temperature selection; large thermal changes are often initiated by these animals. Endotherms rely on insulation and a high, variable metabolic rate to maintain a constant internal temperature.(ABSTRACT TRUNCATED AT 400 WORDS)