EXOGENOUS L-5-HYDROXYTRYPTOPHAN IS DECARBOXYLATED IN NEURONS OF THE SUBSTANTIA-NIGRA PARS COMPACTA AND LOCUS-COERULEUS OF THE RAT

The aim of the present study is to examine by immunohistochemistry whe ther exogenous L-5-hydroxytryptophan (L-5HTP) is decarboxylated in neu rons of the substantia nigra pars compacta (SNC) and locus coeruleus ( LC) of the rat. In normal rats, neurons of the SNC and LC stained inte nsely for aromatic L-amino acid decarboxylase (AADC). No serotonin (5H T)-positive cells were found in the two regions of the normal rats. In rats that were intraperitoneally injected with L-5HTP alone, the SNC neurons stained deeply for 5HT, but the LC neurons showed only a faint staining for 5HT. In rats that intraperitoneally received both a mono amine oxidase (MAO) inhibitor and L-5HTP, when compared with the L-5HT P-injected rats, the LC neurons became much darker in 5HT staining, bu t the SNC neurons showed only a slight increase in 5HT staining. The p resent findings suggest that (i) AADC in dopaminergic neurons of the S NC and in noradrenergic neurons of the LC can catalyze the in vivo dec arboxylation of exogenous L-5HTP to produce 5HT, and (ii) most of the newly produced 5HT in the LC neurons is rapidly degraded by endogenous MAO.