Role of ventrolateral medulla in vasomotor response to cerebral ischemia

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Abstract
The ventrolateral medulla is postulated to contain chemosensitive neurons. The role of this region in the generation of the pressor response to cerebral ischemia (CIR) was investigated in anesthetized paralyzed artificially ventilated rabbots. A circumscribed and highly sensitive pressor area in the ventrolateral medullary reticular formation 2-4 mm rostral to the obex, separate from the well-known dorsal pressor area, was mapped using a stimulating electrode. Electrolytic destruction of this area resulted in a profound reduction in resting mean arterial pressure (MAP). After restoration of baseline MAP with norepinephrine infusion, the CIR was greatly reduced (by mean 70.2% of controls) but pressor responses from the dorsal medulla were unaffected. Lesions of greater size placed in the ventrolateral medulla more caudally did not significantly alter resting MAP and only slightly reduced the CIR (by mean 17.0% of control). Vasomotor responses to stimulation of the ventrolateral pressor area were unaffected by caudal ventrolateral lesions, but greatly reduced by dorsomedial lesions in the same plane. The ventrolateral area is the site of origin or an essential part of the central vasomotor pathway mediating the CIR; this pathway projects dorsomedially before descending to the spinal cord.