Key words L-Tryptophan · Receptor · Serotonin ·

to enhance serotonergic neurotransmission through post-synaptic 5-HT1A receptors. This effect is delayed in ani-mals and may be paralleled by a delay in the onset of a clinical response in humans. In humans, the growth hormone (GH) response to intravenous L-tryptophan (IV L-TRP) is blocked by the 5-HT1A antagonist pindolol and the prolactin response is blunted. Both are therefore thought to be a useful measure of 5-HT1A receptor func-tion. Clomipramine has previously been found to en-hance the GH and prolactin responses to IV L-TRP after only 2 h. Objective: The purpose of this study was to use this method to investigate the effects of newer antide-pressants on 5-HT1A receptor-mediated function. Met-hods: Twelve healthy male volunteers took part in a ran-dom order, double blind study, in which 18.75 mg venla-faxine, 5 mg paroxetine or placebo was administered 3 h before infusion of L-TRP. Results: Pretreatment with venlafaxine significantly enhanced the growth hormone (GH) response to the infusion compared with pretreat-ment with placebo. There was no significant difference between the GH response following paroxetine com-pared with placebo or with venlafaxine. Conclusions: The data suggest enhancement of transmission through postsynaptic 5-HT1A receptors by venlafaxine but not paroxetine, after only 3 h