Effects of carbidopa on the cerebral accumulation of exogenous L-5-hydroxytryptophan in mice.

The concentration of L-5-hydroxytryptophan (5-HTP) was measured in plasma and whole brain tissue from mice by use of high pressure liquid chromatography with fluorimetric detection after reaction with phthaldialdehyde. 5-HTP was injected intraperitoneally and the rise in plasma and brain 5-HTP concentrations was found directly related to the size of amino acid dose. Carbidopa, an extracerebral inhibitor of L-aromatic amino acid decarboxylation, increased dose dependently the brain bioavailability of systemically administered 5-HTP not only by peripheral enzyme inhibition, but also by inhibition of the decarboxylase activity within the blood-brain barrier as indicated by a rise in the calculated brain/plasma 5-HTP concentration ratio.