Sleep and hypometabolism

Metabolic rate (MR) of endotherms is lower during sleep than quiet wakefulness (W), largely as a result of a regulated lowering of body temperature (Tb) during slow-wave sleep (SWS). In mammals this decrease in regulated Tb is evidenced by a lowered hypothalamic thermosensitivity. In avian thermoregulatory systems spinal thermosensitivity is lower in SWS than in W. In rapid eye movement sleep (REMS) there is a severe inhibition of the thermoregulatory systems. Hypothalamic cells that are thermosensitive during W become less so during SWS and become totally insensitive to temperature during REMS. The adjustments in thermoregulatory systems of endotherms that result in lower Tb and MR during SWS have been shaped by natural selection in different species to produce a variety of adaptations for energy conservation. Bouts of torpor, hibernation, or nocturnal hypothermia consist mostly of SWS, and these states involve a downward resetting of hypothalamic thermosensitivity in mammals and spinal thermosensitivity in birds. A physiological mechanism underlying this change in regulation of Tb may be related to the regulation of breathing and acid–base balance. Retention of CO2 occurs at the onset of sleep, shallow torpor, and hibernation and release of excess CO2 occurs when these states are reversed by arousal. Increased plasma CO2 may have a direct effect on hypothalamic neurons involved in thermoregulation, resulting in a decline in regulated Tb.

Thermoregulatory responses of febrile sheep to spinal and hypothalamic heating

Fever was induced by the intravenous injection of 0.25 microgram/kg of lipopolysaccharide (LPS) from Escherichia coli in eight conscious sheep exposed to ambient temperatures adjusted to the lower range of thermoneutrality. Chronic spinal or hypothalamic thermodes were perfused with water of 44 degrees C for 20 min or for most of the rising phase of fever (100 min of the mean 166 min total rise time). The effects of spinal and hypothalamic heating were identical. Thus, before LPS, spinal or hypothalamic heating did not affect the rate of O2 consumption (VO2) but increased skin blood flow (as indicated by skin temperatures) and elicited panting; therefore rectal temperature (Tre) fell. During fever rise, the already reduced skin blood flow and respiratory rate were not affected by spinal or hypothalamic heating, but the increased VO2 was reduced; consequently, the rise in Tre was attenuated. During the plateau phase of fever, all responses were similar to those seen before LPS. In febrilysis, heating strongly enhanced the operating heat loss mechanisms and, hence, augmented the fall in Tre. Thus, although the thermoeffectors activated by spinal or hypothalamic heating were modified during the different stages of fever, the effect on body temperature was nearly the same. Therefore there seems to be no change in spinal or hypothalamic thermosensitivity during fever in sheep.

Central control of expired air temperature and other thermoregulatory effectors in reindeer

Norwegian reindeer (Rangifer tarandus tarandus) were provided with chronically implanted hypothalamic perfusion thermodes to determine to what extent expired air temperature (Tex) is centrally controlled. The effects that alterations of hypothalamic temperature (Thy) had on metabolic heat production, respiratory frequency, respiratory evaporative heat loss, and rectal and leg-skin temperature were simultaneously recorded. Experiments were conducted in conscious animals exposed to ambient temperatures (Ta) of -20, 0, and 20 degrees C in summer, when fur insulation was minimal, and in winter, when it was prime. Altering Thy induced changes in Tex, the degree and direction of which depended on the prevailing Ta, season, and intensity of hypothalamic thermal stimulation. These results indicate that Tex, and hence nasal heat exchange, is under central control. The Thy-induced changes in the other measured parameters confirm that hypothalamic thermosensitivity in these animals is similar to that previously found in other mammalian species.