Brain and blood indole metabolites after peripheral administration of14C-5-HT in rat

Radioactive techniques were used to reexamine the reports that pharmacological quantities of peripheral serotonin (5-hydroxytryptamine or 5-HT) gain access to brain parenchyma. Intravenous injection of 0.108–4.19 mg/kg of¹⁴C-5-HT (3.55 Ci/100 g weight) produced significant metabolic differences in brain but not blood as a function of dose at up to 10 min after injection. Neither of the metabolites, 5-hydroxyindoleacetic acid nor 5-hydroxytryptophol, were detectable in brain following their intravenous injection, suggesting that when such metabolites are found in brain they represent central metabolism. It has also been shown that peripheral compartments in general, and specifically blood in the cerebral vasculature and the adrenergic nerve endings in the cerebral blood vessels, contribute to the uptake and metabolism of 5-HT. We conclude that doses up to 0.435 mg/kg 5-HT do not cross the blood-brain barrier in the rat but are being totally metabolized in nonneuronal tissues that are invariably removed and assayed along with brain parenchyma. The level at which 5-HT actually passes the blood-brain barrier was found to be at least 0.863 mg/kg. This value is one-third lower than that previously reported.