Perivascular substance P: occurrence and distribution in mammalian pial vessels.

Nerve fibres containing immunoreactive substance P (SP) were demonstrated in the wall of cerebral blood vessels of several mammalian species. Pial arteries of cat and guinea-pig were richly supplied with SP nerve fibres, while those of rat, rabbit, pig, and man had a moderate number. SP fibres were more numerous in pial vessels belonging to the rostral parts of the circle of Willis as compared to more caudally located blood vessels. In cat and guinea-pig, blood vessels in the choroid plexus were surrounded by few SP nerve fibres; also spinal cord blood vessels of cat contained few such fibres.