Summary
Peripheral aromatic amino acid decarboxylase inhibitors increase human plasma 5-hydroxytryptophan (5-HTP) levels. However the 5-HTP, which is synthesized in the intestine, is not present in a sufficient quantity to influence brain 5-HT to any significant degree. Thus, peripheral decarboxylase inhibitors are unlikely to affect brain 5-HT function via 5-HTP which is synthesized peripherally.
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Young, S.N., Gauthier, S., Chouinard, G. et al. The effect of carbidopa and benserazide on human plasma 5-hydroxytryptophan levels. J. Neural Transmission 53, 83–87 (1982). https://doi.org/10.1007/BF01243522
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DOI: https://doi.org/10.1007/BF01243522