5-HT 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.