formation, storage and release resembles NA. Its precursor is tryptophan, an amino acid
derived from dietary protein, the plasma content of which varies considerably according to
food intake and time of day. Tryptophan is actively taken up into neurons, converted by
tryptophan hydroxylase to 5-hydroxytryptophan and then decarboxylated to 5-HT. The
decarboxylase is very similar, if not identical, to DOPA decarboxylase, and does not play
any role in regulating 5-HT synthesis. Following release, 5-HT is largely recovered by
neuronal uptake, this mechanism being inhibited by many of the same drugs (tricyclic
antidepressants) that inhibit catecholamine uptake.