Regional blood flow and nociceptive stimuli in rabbits: patterning by medullary raphe, not ventrolateral medulla

1. Regional blood flow was measured with Doppler ultrasonic probes in anaes thetized rabbits. We used focal microinjections of pharmacological agents t o investigate medullary pathways mediating ear pinna vasoconstriction elici ted by electrical stimulation of the spinal tract of the trigeminal nerve o r by pinching the lip, and pathways mediating mesenteric vasoconstriction e licited by electrical stimulation of the afferent abdominal vagus nerve. 2. Bilateral injection of kynurenate into the rostral ventrolateral medulla reduced arterial pressure and prevented the mesenteric vasoconstriction an d the rise in arterial pressure elicited by abdominal vagal stimulation. Ho wever, kynurenate did not prevent ear pinna vasoconstriction or the fall in pressure elicited by trigeminal tract stimulation. Similar injections of m uscimol also failed to prevent the trigeminally elicited cardiovascular cha nges. 3. Injections of kynurenate into the raphe-parapyramidal area did not dimin ish trigeminally elicited ear vasoconstriction or the depressor response. H owever, injections of muscimol substantially reduced or abolished the trige minally elicited ear vasoconstriction, without affecting the depressor resp onse. Raphe-parapyramidal muscimol injections also entirely abolished ear v asoconstriction elicited by pinching the rabbit's lip. 4. The trigeminal depressor response does not depend on either the rostral ventrolateral medulla or the raphe-parapyramidal region. 5. Mesenteric vasoconstriction elicited by stimulation of the afferent abdo minal vagus nerve is mediated via the rostral ventrolateral medulla, but ea r vasoconstriction elicited by lip pinch or by stimulation of the trigemina l tract is mediated by the raphe-parapyramidal region. Our study is the fir st to suggest a brainstem pathway mediating cutaneous vasoconstriction elic ited by nociceptive stimulation.