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 t ryptophan (TRP) depletion on (i) plasma total and free TRP together wi th brain TRP, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic aci d (5-HIAA) and (ii) the prolactin (PRL) responses to the 5-HT-releasin g agent, n-fenfluramine (FEN), and the 5-HT agonist, m-chlorophenylpip erazine (mCPP). The TRP-deficient diet caused significant reductions i n plasma total and free TRP for a short-lived period; however, it caus ed longer lasting and significant reductions in brain TRP, 5-HT and 5- HIAA content. Following periods of dietary TRP depletion, plasma PRL r esponses to FEN were significantly reduced for 14 days but had normali sed by day 21. In contrast, the PRL responses to mCPP were significant ly enhanced after 6 and 21 days, the response at 14 days being similar to control. The result suggests that periods of dietary invoked TRP d epletion cause a deficiency of brain TRP content which subsequently le ads to a fall in brain 5-HT. This is associated with a reduced PRL res ponse to FEN and an enhanced PRL response to mCPP, the latter possibly resulting from functional up-regulation of post-synaptic 5-HT recepto rs.