Abstract
The study was undertaken to assess the long term effects of tryptophan (TRP) depletion through diet on the prolactin (PRL) responses to the serotonin (5-hydroxytryptophan, 5-HT) agonists m-chlorophenyl-piperazine (mCPP) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in the male rat. The low TRP diet caused significant reductions in both plasma total TRP and brain cortical 5-HT content together with a significant increase in the PRL responses to mCPP. In contrast the PRL responses to 8-OH-DPAT in animals on the low TRP diet for 1week and 6 weeks were similar to control rats. However, a small but significant increase in PRL was observed at 2 min after dosing in the 1-week group. At the same time the 3H-8-OH-DPAT binding parameters, Kd and Bmax, were similar in both control and TRP depleted animals. The results conform that long-term TRP depletion causes a deficiency of brain TRP and a subsequent reduction of brain 5-HT.This is associated with an enhanced PRL response to mCPP probably resulting from a functional up-regulation of post-synaptic 5-HT2Creceptors. The small or transient changes brought about by long-term TRP depletion on post-synaptic 5-HT1Areceptors, suggests that these receptors may be less susceptible to 5-HT depleting effects than the 5-HT2Csubtype.
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