International Neuropsychiatric Disease Journal

Aims: The existence of a dopamine (DA)-serotonin (5-HT) interaction in the brain has been validated by numerous studies. Nevertheless, interaction between DA- 5-HT synthesis in aging brain has not been highly considered. The central aim of this study was to investigate the interaction between DA and 5-HT synthesis in rat brain striatal synaptosomes.

Methodology: Male Wistar rats (3 and 30 months old) were killed by decapitation and the brain sriatal synaptosomes were prepared by discontinuous Ficoll/sucrose gradient technique. DA or 5-HT synthesized during an appropriate incubation period was measured by the synaptosomes in the presence of added substrates (tyrosine or tryptophan) and a monoamine oxidase inhibitor (pargiline).

Results: Dopamine synthesis in the synaptosomes prepared from young animals was markedly inhibited (about 30%) by the addition of 5 μM 5-HT and increasing 5-HT up to 50 μM caused only a relatively small additional inhibition. However, different concentrations of 5-HT (0-50μM) had little effect on dopamine synthesis of the synaptosomes preparations from old animals. In case of 5-HT synthesis, exogenously added 5 μM DA inhibited 5-HT synthesis in the synaptosomes of both ages by about 40%, whereas with higher concentration of DA (10-50 μM) the rate of inhibition was highly pronounced in the old rats as compared to that of young animals.

Conclusion: It is concluded that DA- 5-HT cross reaction might be considered, where long-term treatment with L-DOPA of patients suffering from Parkinson's disease renders patients experience variations in response and even psychiatric problems.