Thermoregulatory responses to hypothalamic cooling in unanesthetized dogs

Abstract

A method is described for local heating or cooling of regions of the hypothalamus of the intact unanesthetized dog. Needle thermodes carrying warm or cold water are inserted through metal guides fixed permanently in the skull and placed symetrically across the mid-line thus permitting the controlled raising or lowering of the hypothalamic temperature. Insertion of the thermodes is painless and causes no hyperpyrexia or other reaction. The thermodes are withdrawn at the end of an experimental period and the animal can be used repeatedly without ill effects. The following observations were made: a) mild cooling causes vasoconstriction and shivering. There is a marked rise in internal temperature and in temperature of skin over trunk as the result of increased heat production. There is also a large drop in skin temperature of extremities as the result of vasoconstriction. b) With continued cooling, shivering weakens and finally ceases; vasoconstriction continues but rise in body temperature slows. c) Rapidly alternating heating and cooling permits the simultaneous stimulation of panting and shivering responses. These observations suggest that in the hypothalamus there are areas sensitive to cooling, which have thermoregulatory responses. As a result of continued cooling of these regions, inhibitory responses are also evoked which are probably associated with warmth stimulation due to elevated skin and internal body temperature outside of the hypothalamus.