Vasodilatation and enhanced oxidative metabolism of the cerebral cortex provoked by the periaqueductal gray matter in anaesthetized rats.

The aim of the present study was to evaluate the possible contribution of the cerebral cortical oxidative metabolism to the cortical vasodilator action of the periaqueductal gray. In 70 rats with cervical cordotomy, we found that unilateral stimulation of the caudal third of the lateral longitudinal column of the periaqueductal gray with N-methyl-D-aspartate bilaterally provoked the greatest increase in cortical blood flow (laser-Doppler flowmetry and/or microsphere flowmetry). The response was widespread over the entire neocortical regions, elicited in a dose-dependent manner, with little change in arterial blood pressure. The flow was increased effectively by a submaximal dose of the amino acid (1 mM, 100 nl), attaining a peak increase by 99 +/- 41% of the baseline level (mean +/- S.D., n = 30), and was associated with an enhancement of the cortical metabolic rate for oxygen by 51 +/- 26%. We then compared the flow increase with that induced by cold exposure (by 52 +/- 29%, n = 27), the latter response being tightly coupled to an enhanced metabolic rate for oxygen (by 41 +/- 23%). It was thus found that the increase in cortical blood flow provoked by the subdivision was dependent on the cerebrovasodilator mechanism that may be coupled to the cortical oxidative metabolism to the extent of one-half, and on certain other mechanisms for the remaining half. In view of the fact that this particular region serves to generate vigorous defence reactions that involve flight behaviour, the region should also help to meet the urgent demand for an increased cortical blood flow, so as to prepare for the possible generation of cortical hyperactivity in coping vigorously with a threatening emergency.