5-Hydroxytryptophan, a tryptophan metabolite, is a direct 5-hydroxytryptamine (5-HT) precursor and an L-aromatic amino acid decarboxylase substrate. [1][2][3].
References:
[1]. Gartside SE, et al. Mediation of ACTH and prolactin responses to 5-HTP by 5-HT2 receptors. Eur J Pharmacol. 1990 Apr 10;179(1-2):103-9.
[2]. Gijsman HJ, et al. Placebo-controlled comparison of three dose-regimens of 5-hydroxytryptophan challenge test in healthy volunteers. J Clin Psychopharmacol. 2002 Apr;22(2):183-9.
[3]. Schruers K, et al. Acute L-5-hydroxytryptophan administration inhibits carbon dioxide-induced panic in panic disorder patients. Psychiatry Res. 2002 Dec 30;113(3):237-43.