Hypothalamic network for thermoregulatory vasomotor control.

Warming one side of a rat's preoptic area and anterior hypothalamus (POAH) causes skin vasodilation on both sides of the body, and the present study evaluated the extent to which signals mediating this vasodilation cross the midline within and below the POAH. Hind paw vasomotion during unilateral POAH thermal or electrical stimulation was measured for rats in which the POAH had been midsagittally transected and for rats in which one side of the hypothalamus had been coronally transected just below the POAH. In midsagittally transected rats, unilateral POAH thermal or electrical stimulation produced bilateral paw vasodilation, but the ipsilateral dilation occurred at a hypothalamic temperature lower than that at which contralateral dilation occurred. In the unilaterally transected rats, unilateral POAH warming produced bilateral vasodilation and, regardless of which side of the POAH was warmed, the threshold stimulus temperature was always lower for vasodilation on the intact side. Unilateral transection of a part of the hypothalamus that included the medial forebrain bundle had the same effect as did unilateral transection of the whole hypothalamus. Information controlling thermoregulatory vasomotion thus crosses the midline both within and below the POAH. Although efferent signals descending through the medial forebrain bundle innervate skin blood vessels on both sides of the body, this innervation is stronger on the ipsilateral side.