Enzymes Related to Monoamine Transmitter Metabolism in Brain Microvessels

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Abstract
The activities of tyrosine hydroxylase, aromatic L-aminoacid decarboxylase, monoamine oxidase and catechol-O-methyltransferase were measured in microvessel fractions (capillaries and venules), parenchymal arterioles and pial vessels from rat brains, and the decarboxylase activity was compared in brain microvessels from rabbit, cat, dog, pig, cow, baboon and man. Cranial sympathectomy was performed to estimate the neuronal contribution to the enzyme activities. All vascular regions had substantial activities of the various enzymes. The activity of aromatic L-aminoacid decarboxylase in cerebral microvessels was high in rat, dog, pig, cow and man; intermediate in rabbit and cat; and low in baboon. Cerebral microvessels contained tyrosine hydroxylase and monoamine oxidase. Aromatic aminoacid decarboxylase and monoamine oxidase served an enzymatic barrier function at the microvascular level; the main function of tyrosine hydroxylase was probably to synthesize monoamines within nerve terminals that remain in close association with microvessels under the conditions used for preparation of the microvascular fraction. In larger intracerebral and pial vessels monoamine oxidase was present in the wall and in perivascular sympathetic nerves; the remaining 2 enzymes had a primarily neuronal localization. The latter types of vessels contained catechol-O-methyltransferase in their walls.