The effects of hypothalamic temperature variation and intracarotid cooling on behavioural thermoregulation in sheep.

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Abstract
Shorn sheep, placed in cold environments, were trained to turn on IR heaters. The effect, on this thermoregulatory behavior, of warming and cooling the hypothalamus by means of a thermode was examined. At ambient temperatures of 5, 15, 25 and 35.degree. C, cooling the anterior hypothalamus by means of a thermode, for periods of 20 min, resulted in a marked increase in the rate of using the heaters. At ambient temperatures of 5 and 15.degree. C, warming the anterior hypothalamus for periods of 20 min caused a considerable reduction in the rate of using the radiant heaters. At an ambient temperature of 10.degree. C, a 2 h period of hypothalamic cooling resulted in an increase in the rate at which the heaters were used for the first 70 min, but after this the effect was reduced and the reduction coincided with a rise of deep body temperature of about 0.75.degree. C. At an ambient temperature of 10.degree. C, a 2 h period of hypothalamic warming resulted in a reduction in the rate of operating the heaters during the first 85 min, but after this period the use of the heaters increased and this increase coincided with a fall of about 0.75.degree. C in deep body temperature. At ambient temperatures of 15, 25 and 35.degree. C, the cephalic region was cooled by intracarotid injections of cold saline for periods of 15 min. This procedure lowered hypothalamic temperature by about 1.degree. C and produced increases in the rate at which the heaters were used similar to those seen when the thermode was cooled. It was not necessary to lower hypothalamic temperature outside the normal range to elicit marked increases in the rate at which the heaters were used.