Clinical studies of 5-HT function using I.V. l-Tryptophan

Abstract

1. Preclinical studies reveal that long-term treatment with antidepressant drugs induces significant changes in serotonergic (5-HT) receptor sensitivity. Similarly, clinical studies suggest that brain 5-HT function is abnormal in depression. Of the available methodologies for conducting such clinical studies, the pharmacological challenge strategy has proven particularly useful. 2. I.V. L-TRP has emerged as the most frequently used challenge agent in diagnostic and neuropsychopharmacological studies of 5-HT function. I.v. L-TRP increases serum prolactin (PRL) in humans, probably via 5-HT mechanisms. Under carefully standardized conditions, this PRL response to L-TRP appears to be a reasonably sensitive and valid measure of net 5-HT function. 3. The PRL response to L-TRP is blunted in depressed patients compared with healthy controls. Blunting has not been observed in panic disorder, obsessive compulsive disorder, or schizophrenia, although preliminary findings suggest it may occur in bulimia. 4. The PRL response to L-TRP is enhanced by certain classes of thymoleptic drugs (TCAs, MAOIs, 5-HT reuptake inhibitors, lithium) in a differentially time-dependent fashion. So-called “atypical” antidepressants (trazodone, mianserin) and benzodiazepines have no effect. Such findings are generally consistent with preclinical electrophysiological findings. 5. These clinical studies of the PRL response to L-TRP, in conjunction with emerging evidence that experimentally reduced plasma TRP can reverse the therapeutic effects of some antidepressants, suggest that antidepressant drug action nay be more accurately conceptualized as 5-HT dependent rather than 5-HT enhancing . The availability of more selective 5-HT-active drugs promises to further clarify 5-HT mechanisms of neuropsychiatric disease and drug action at the clinical level.