Raphe region mediates changes in cutaneous vascular tone elicited by stimulation of amygdala and hypothalamus in rabbits

Raphe pallidus/parapyramidal neurons control cutaneous vasoconstriction ind uced by noxious stimuli. To determine whether they mediate forebrain-induce d cutaneous vasoconstriction, we assessed changes in ear pinna blood flow e licited by electrical stimulation of amygdala and hypothalamus before and a fter injection of muscimol into the raphe/parapyramidal region. We compared ear flow with simultaneously recorded mesenteric flow. Experiments were pe rformed in rabbits anesthetized with urethane (1.25-1.5 g/kg), paralysed an d mechanically ventilated. Amygdala stimulation reduced skin conductance fr om 0.32+/-0.06 to 0.10+/-0.02 cm/s per mm Hg (P<0.05, n=9), without effect on mesenteric conductance. Hypothalamic stimulation caused vasoconstriction in both cutaneous and mesenteric beds (conductances fell from 0.27+/-0.05 to 0.05+/-0.02 cm/s per mm Hg and from 0.27+/-0.06 to 0.14+/-0.04 cm/s per mm Hg (P<0.05, n=9:), respectively). Muscimol microinjection (5 nmol in 100 nl) to raphe/parapyramidal region eliminated amygdala- and hypothalamus-in duced skin vasoconstriction (pre-stimulus conductance 0.42+/-0.13 and 0.41/-0.11 cm/s per mm Hg, post-stimulus 0.41+/-0.12 and 0.39+/-0.10 cm/s per m m Hg, respectively), but not hypothalamically-induced mesenteric vasoconstr iction (pre-stimulus 0.29+/-0.06, post-stimulus 0.16+/-0.03 cm/s per mm Kg: P<0.05, n=8). The latter was strongly attenuated by bilateral injection of muscimol to the rostral ventrolateral medulla. Data suggest that descendin g hypothalamo-spinal and amygdala-spinal pathways constricting the cutaneou s vascular bed relay in the raphe/parapyramidal area. A relay in the rostra l ventrolateral medulla contributes substantially to mesenteric vasoconstri ction elicited from the hypothalamus. (C) 2001 Elsevier Science B.V. All ri ghts reserved.