ABSENCE OF SEROTONERGIC INNERVATION FROM RAPHE NUCLEI IN RAT CEREBRAL BLOOD-VESSELS .1. HISTOLOGICAL EVIDENCE

Anterograde tracing from dorsal raphe neurons by Phaseolus vulgaris leucoagglutinin and serotonin immunocytochemistry revealed no serotonergic projections from raphe nuclei to cerebral pial vessels in the rat. However, cerebrovascular nerve fibres, mainly located in major pial arteries, were immunoreactive to tryptophan-5-hydroxylase antibodies as previously shown by others. It thus seems that the rate-limiting enzyme catalysing the biosynthesis of serotonin, tryptophan-5-hydroxylase, is present in cerebrovascular nerve fibres which do not originate in the dorsal raphe nucleus and which do not contain enough serotonin to be labelled by serotonin immunocytochemistry. We also observed tryptophan hydroxylase-immunoreactive but no serotonin-immunoreactive nerve fibres in the femoral artery and, occasionally, in the dura mater. The femoral artery, like the dura mater, contained numerous mast cells reacting positively to both tryptophan hydroxylase and to serotonin immunocytochemistry. The co-localization of the enzyme and its final product thus appears to be a general feature, since it has already been demonstrated within the central nervous system. The only exception appears to be the tryptophan hydroxylase-immunoreactive nerves present in cerebral and peripheral vessels. These results suggest that there is not a true serotonergic (i.e. serotonin-containing) innervation in cerebral blood vessels. They also strongly suggest that the cerebrovascular nerve fibres which appear to contain tryptophan hydroxylase do not originate in the raphe nuclei.