Vasoactive sites in the diencephalon of the rabbit.

Stimulation of the brain stem of the anaesthetized rabbit, in the lateral hypothalamic area and the zona incerta 0.5-3 mm from the midline and a similar distance dorsal to the lateral mammillary nucleus, evoked vasoconstriction in the skin of the ear and of the hindpaw. Only weak and inconstant effects on muscle blood flow were evoked from this region of the brain stem. Muscle vasodilatation was obtained by stimulation of more medial regions of the brain stem, extending almost all the way from the supramammillary nucleus to the dorsal surface. This vasodilatation was not diminished by atropine. Alpha-adrenergic blocking agents diminished the cutaneous vasoconstrictor responses and also reduced or even reversed the bradycardia that could be evoked by hypothalamic stimulation.