Abstract
We attempted to elucidate the mechanisms operating in the mediation of the cerebrovasodilator action of the defence area within the periaqueductal gray matter, employing radiolabelled microsphere flowmetry in 50 anaesthetized rats. The caudal two-thirds of this brain region were chemically stimulated with N-methyl- d-aspartate (1 nmol as 100 nl in a 10-mM solution). Besides eliciting a distinctive pattern of haemodynamic defence responses, the stimulation consistently provoked a cerebrovasodilator action, preferentially in the supratentorial brain regions. In particular, 56% of the overall flow increase in the parasagittal cerebral cortex was solely attributable to the unique cerebrocortical vasodilator action of the periaqueductal gray matter, independent of unspecific effects of concomitantly occurring hypertension itself. This flow change was not associated with a specific change in the metabolic rate for O 2 of this cortical region. We conclude that the major part of the unique cerebrocortical vasodilator action provoked from the defence area within the periaqueductal gray matter was mediated by non-metabolic mechanisms. The involvement of the metabolic mechanism could be of only minor importance, if any. Cerebral vasodilatation may be involved in the natural defence reactions for coping with the emergency that threatens the animal.
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