Relationship between body core and peripheral temperatures at the onset of thermoregulatory responses in rats

The relationships between body core temperature and environmental temperature (T(a)) at the onset of tail skin vasodilation, thermally-induced salivary secretion and cold-induced thermogenesis were examined in rats. Hypothalamic temperature (Thy) was measured as an index of body core temperature. Internal body heating and cooling were made using an intravenous thermode chronically implanted in the inferior vena cava at various T(a)s. External heating and cooling were made by changing T(a) at a constant rate, while Thy was controlled at constant levels with the thermode. Internal heating induced tail skin vasodilation but not thermally-induced salivary secretion, and internal cooling induced cold-induced thermogenesis. External warming caused tail skin vasodilation and thermally-induced salivary secretion, but external cooling failed to induce cold-induced thermogenesis. There were significant correlations between Thy and T(a) at the onset of tail skin vasodilation, thermally-induced salivary secretion and cold-induced thermogenesis. When T(a) was taken as an indicator of thermal input from the skin, the ratios of the potency of inputs from the body core and skin were estimated to be between 8:1 and 6:1 at the onset of tail skin vasodilation, 8:1 at the onset of thermally-induced salivary secretion, and 5:1 for the control of cold-induced thermogenesis. These results suggest that, in rats, the contribution of thermal input from the skin to the onset of thermoregulatory responses is relatively small.