The effects of a low tryptophan diet on brain 5-HT metabolism and 5-HT-mediated neuroendocrine responses in the male rat

The study was undertaken to assess the effects of periods of dietary tryptophan (TRP) depletion on (i) plasma total and free TRP together with brain TRP, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) and (ii) the prolactin (PRL) responses to the 5-HT-releasing agent, n-fenfluramine (FEN), and the 5-HT agonist, m-chlorophenylpiperazine (mCPP). The TRP-deficient diet caused significant reductions in plasma total and free TRP for a short-lived period; however, it caused longer lasting and significant reductions in brain TRP, 5-HT and 5-HIAA content. Following periods of dietary TRP depletion, plasma PRL responses to FEN were significantly reduced for 14 days but had normalised by day 21. In contrast, the PRL responses to mCPP were significantly enhanced after 6 and 21 days, the response at 14 days being similar to control. The result suggests that periods of dietary invoked TRP depletion cause a deficiency of brain TRP content which subsequently leads to a fall in brain 5-HT. This is associated with a reduced PRL response to FEN and an enhanced PRL response to mCPP, the latter possibly resulting from functional up-regulation of post-synaptic 5-HT receptors.