Absence of serotonergic innervation from raphe nuclei in rat cerebral blood vessels--II. Lack of tryptophan hydroxylase activity in vitro.

Neurochemical studies performed in vivo have suggested that serotonin-containing and -synthesizing nerves, originating in the raphe nuclei, directly innervate pial blood vessels. Nerve fibres of these vessels have been shown by immunocytochemistry to contain tryptophan hydroxylase (the rate-limiting enzyme of serotonin synthesis) but no serotonin. The present study examines this contradiction by measuring in vitro the tryptophan hydroxylase activity of rat cerebral vessels and femoral arteries (which also contain tryptophan hydroxylase-immunopositive nerves), and comparing them to the tryptophan hydroxylase activity of the rat pineal body, raphe nuclei and brain cortex under identical conditions. Oxygenated incubation solutions contained either [14C]- or "cold" L-tryptophan (2 x 10(-5) to 5 x 10(-4) M) and NSD-1015 (3-hydroxybenzylhydrazine) which inhibits the decarboxylation of 5-hydroxytryptophan, the second step of serotonin synthesis. Tissue fragments were incubated for 35-60 min. High-performance liquid chromatography (on tissue extracts and incubation solutions) as well as determination of 14C activity in the 5-hydroxytryptophan fraction of elution from tissue extracts showed that the pineal body, the raphe nuclei and cortical slices synthesize various amounts of 5-hydroxytryptophan under our experimental conditions. All these tissues contained serotonin. Femoral arteries, but not cerebral vessels, also contained small amounts of serotonin stored before incubation, probably in mast cells. In contrast to brain tissues, no measurable amounts of "cold" or [14C]5-hydroxytryptophan were found in cerebral blood vessel and femoral artery extracts or incubation solutions. Under identical experimental conditions, sympathetic nerves of both types of vessels were able to synthesize large amounts of L-DOPA when incubation solutions contained L-tyrosine instead of L-tryptophan.(ABSTRACT TRUNCATED AT 250 WORDS)