Hypothalamic thermoregulatory pathways in the rat.

The cutaneous blood flow (mbl), rate of oxygen consumption (Vo2), rectal (Tre) and cutaneous (Tsk) temperatures, and shivering activity were measured in unanthetized male rats during a 2-h exposure to 26, 33, or 5 degrees C 2 wk after selective bilateral hypothalamic microknife cuts. Animals with preoptic-anterior hypothalamic (PO/AH) junction cuts 1.5 or 3.0 mm lateral to the midline, as well as parasagittal cuts which separated connections between the PO/AH and medial forebrain bundle exhibited a higher mbl at 26 degrees C than did sham-operated rats. At 5 degrees C the extended (3.0 mm) PO/AH cuts as well as the parasagittal cuts prevented cutaneous vasoconstriction but had no effect on shivering activity; hence Tre was not maintained. None of the cuts demonstrably impaired thermoregulation in the 33 degrees C environment. These results suggest that different sites in the hypothalamus may separately control cold-induced skin vasoconstriction and shivering activity, as well as heat-induced skin vasodilation. It would seem therefore that the integrity of the PO/AH is indispensable in rats for cold-induced cutaneous vasoconstriction but not for cold thermogenesis, and also not for heat-induced cutaneous vasodilation.