1. We investigated the mechanism of starvation-induced hypothermia in rats. 2. Threshold core temperatures (T(cor)) for tail skin vasodilatation and cold-induced thermogenesis were determined after a 3 day starvation using a chronically implanted intravenous thermode. Food deprivation significantly lowered the threshold T(cor) for heat production, but did not affect the heat loss threshold. 3. Thermogenic response to a fall in T(cor) below its threshold was enhanced by starvation. 4. Preferred ambient temperatures (T(pref)) and T(cor) were measured before and during a 3 day starvation in a thermal gradient. The 3 day starvation significantly lowered T(cor) only in the light phase of the day. The level of hypothermia was the same throughout the fasting period, while T(pref) gradually increased during the 3 days of starvation. 5. When rats were starved at a constant ambient temperature of 25°C (no thermal gradient), their T(cor) levels were comparable with those of the rats kept in the thermal gradient. 6. The results suggest that, in rats, hypothermia caused by starvation was not due to a decrement in thermogenic capability, but was due to a decrease in the threshold for the activation of thermogenesis.
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