The cerebrovascular parasympathetic innervation.

Until recent years, little has been known about the parasympathetic innervation of cerebral vessels, in contrast to the sympathetic innervation. Recent histochemical and biochemical studies on cerebrovascular parasympathetic nerves have revealed their sources and pathways. Histochemical studies have demonstrated nerve fibers containing choline acetyltransferase, a reliable marker for cholinergic nerves, and vasoactive intestinal polypeptide (VIP) in the cerebral vessels. By combining histochemistry with a retrograde tracer technique and selective denervations, the cerebrovascular parasympathetic innervation has been mapped in the rat, cat, and monkey. Acetylcholine (ACh) has been measured biochemically in the pial arteries of several species. A high-affinity uptake, local synthesis, and tetrodotoxin-sensitive release of ACh have been demonstrated in the pial vessels. Pharmacological studies on isolated pial arteries have revealed ACh- and VIP-induced relaxation through the stimulation of muscarinic and VIPergic receptors, respectively. The action of ACh requires an intact endothelial function. An increase in cerebral blood flow upon stimulation of pre- or postganglionic fibers of the sphenopalatine ganglion has been demonstrated in some animals, and can be mimicked by local administration of ACh and VIP in vivo. This indicates a role of the parasympathetic nerves in tone regulation of the cerebral vessels. The pathophysiological conditions during which these nerves become activated are currently being revealed.