Barrier mechanisms for neurotransmitter monoamines in the choroid plexus

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Abstract
Studies were performed on choroid plexuses from mouse and rat. The plexus tissue, sympathectomized to avoid interference from adrenergic nerves, is able to accumulate L‐DOPA as well as NA and 5‐HT against a concentration gradient. Within the plexus tissue these compounds are metabolized by aromaticl‐amino acid decarboxylase, MAO and COMT, whose activities were determined radiometrically. On the basis of this and the fate of systemically administeredl‐DOPA and DA studied by fluorescence histochemistry it is suggested that the epithelium of the choroid plexus provides not only a structural but also an enzymatic barrier for transmitter amines. The enzymatic barrier for amine precursors is considerably less sufficient at this blood‐CSF interphase. The trapping of monoamines resembles the enzymatic barrier present in the endothelial cells and pericytes at the microvascular level of the blood‐brain interphase.