Nitrergic cerebral vasodilatation provoked by the periaqueductal grey.

The present study evaluated the contribution of nitric oxide to the increase in cerebral cortical blood flow that is provoked by chemical stimulation of the caudal third of the lateral periaqueductal grey (PAG), employing 48 anaesthetized rats with cervical cordotomy. Following topical application over the cortical surface, the inhibitor of nitric oxide synthase NG-nitro-L-arginine inhibited the response stereospecifically and achieved its maximal effect at 1 mM, when the response was reduced to 47 +/- 16% (n = 16). Since the PAG represents a principal site for the central organization of defence reactions, we suggest that nitric oxide may play a definitive role in the central regulation of the cerebral circulation, thereby participating in such important biological processes as the defensive reflex activity.