The effect of tryptophan on brain 5‐HT function: A review

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Abstract
Administration of l‐tryptophan (TRP), the amino acid precursor of 5‐hydroxytryptamine (5‐HT), to animals increases brain 5‐HT synthesis and in some circumstances, the release of 5‐HT in terminal fields. TRP also appears to increase brain 5‐HT neurotransmission in humans because neuroendocrine studies have shown that intravenous TRP produces a dose‐related increase in plasma prolactin concentration, probably via indirect stimulation of postsynaptic 5‐HT1A receptors. The effect of TRP on prolactin release in humans is enhanced by several effective antidepressant treatments including tricyclic antidepressants, selective 5‐HT re‐uptake inhibitors, monoamine oxidase inhibitors and lithium. The ability of these drugs to augment the effect of TRP on brain 5‐HT function may represent the pharmacological mechanism that underpins the utility of TRP in combination drug treatments of refractory depression.