Different distributions of substance P and vasoactive intestinal polypeptide in the cerebral arterial innervation in rat and guinea pig.

In immunofluorescence studies, nerve fibers containing substance P were sparse and limited to the anterior and middle cerebral arteries in rats but in moderate supply and associated with all of the major cerebral arteries in guinea pigs. Fibers containing vasoactive intestinal polypeptide (VIP) were abundant in the rostral part of the circle of Willis in both species; moderate numbers were seen in the basilar and vertebral arteries in rats, but few in the rostral half of the basilar arteries and none in the caudal half of the basilar and vertebral arteries in guinea pigs. These differences in distribution in the 2 rodents indicate wide variation in peptide-containing nerves around cerebral vasculature and preclude interspecies extrapolation of findings.