Neurokinin-1 receptors in the cerebrovascular vasoactive intestinal polypeptide-containing nerves in the rat

Recently, the functions of several putative neurotransmitters such as catecholmines, acetylcholine (ACh) and neuropeptides have been elucidated in the cerebrovasculature. The interaction of such neurotransmitters and their receptors, however, has not been sufficiently clarified. The purpose of this study is to explore the relation of recently demonstrated neurokinin-1 (NK-1) receptor-containing cerebrovascular nerve fibers with the cerebrovascular vasodilatory nerves by means of the sequential-staining immunohistochemical method. Numerous sites of NK-1 receptor immunoreactivities were noted along the nerve fibers with vasoactive intestinal polypeptide (VIP) immunoreactivity in the pial arteries in all regions of the brain. They consisted of fine, delicate varicose fibers and thick bundles without varicosities. After sequential staining with VIP, NK-1 receptor immunoreactive material was demonstrated in the VIP-positive nerve fibers. The majority of fibers were positive for VIP alone (75%). The number of fibers positive for both NK-1 receptor and VIP was about one-third the number of fibers for VIP alone (22%). Fibers positive for NK-1 receptor alone comprised a small population (3%). This study demonstrated that NK-1 receptors are localized in axonal membrane of VIP-containing parasympathetic nerves. This suggests that the sensory nerves modulate the functions of parasympathetic nerves in peripheral nervous system, such as those on cerebral vessels.