Serotonin-synthesizing nerve fibers in rat and cat cerebral arteries and arterioles: immunohistochemistry of tryptophan-5-hydroxylase.

Nerve fibers synthesizing 5-hydroxytryptamine (5-HT or serotonin) were demonstrated in rat and cat cerebral blood vessels by immunohistochemical localization of the rate-limiting enzyme catalyzing the biosynthesis of 5-HT, namely tryptophan-5-hydroxylase (TPOH). TPOH-immunoreactive fibers were present in all parts of the circle of Willis with a somewhat less intense distribution in the vertebro-basilar segment. Single, small nerve fibers intermingled around the vessel wall were found in both species but clear TPOH-immunoreactive varicosities were observed predominantly in the rat. The most striking observation was the dense network of TPOH-positive fibers innervating some but not all small pial arteries, and, to a lesser extent, pial arterioles. These results provide the evidence that, at least in these species, the cerebrovascular bed is innervated by authentic serotonergic fibers which can synthesize their own 5-HT.