Alterations in the Level of mRNA of <i>Tph1</i>, <i>Tph2</i> Genes, Tryptophan Hydroxylase Activity and Serotonin Metabolism in Mouse Brain 5 Days after Lipopolysaccharide Administration

Abstract

Tryptophan hydroxylases 1 and 2 (TPH1 and TPH2) play the key role in the synthesis of a neurotransmitter and hormone, serotonin (5-HT) in peripheral organs and in the brain, respectively. The main aim of the present study was to clarify the distribution of mRNA of Tph1 and Tph2 genes in brain structures in norm and after inflammation. The experiments were carried out on young (4 weeks old) males of C57BL/6 mice. The animals were divided into three groups: intact, control, injected ip with saline, and injected ip with 2 mg/kg of bacterial lipopolysaccharide (LPS). Markers of inflammation, spleen and thymus mass were assayed 5 days after the saline or LPS administration. In their frontal cortex, hippocampus, striatum, hypothalamus and midbrain the concentrations of 5-HT, its main metabolite, 5-hydroxyindole acetic acid (5-HIAA), and TPH activity were assayed in using HPLC, while Tph1 and Tph2 mRNA were quantified by quantitative real-time RT-PCR. Dramatic increase of spleen mass and decrease of thymus mass 5 days after LPS administration was shown. Significant increase of 5-HT and 5-HIAA levels in midbrain as well as decrease of 5-HIAA concentration and TPH activity in hypothalamus in mice treated with LPS and saline compared with intact animals was revealed. The highest concentration of Tph2 gene mRNA was observed in midbrain in 5-HT neuron bodies, while in this gene mRNA level was lower in 5-HT endings (cortex, hippocampus, striatum and hypothalamus). Trace amounts of Tph1 mRNA was revealed in all studied brain structures in mice of the three groups. Thus, Tph1 gene expression in the mouse brain is too low to significantly affect 5-HT synthesis in normal conditions and during inflammation.