On the interaction between BDNF and serotonin systems: The effects of long-term ethanol consumption in mice

We investigated the effect of chronic (6 weeks) consumption of 10% alcohol on the principal elements of BDNF (BDNF, proBDNF, p75, and TrkB receptors) and 5-HT (5-HT, 5-HIAA, tryptophan hydroxylase-2 [Tph-2], 5-HT transporter [5-HTT], 5-HT_1A, 5-HT_2A, and 5-HT₇ receptors) systems in the brain of C57Bl/6 mice. BDNF mRNA level in the raphe nuclei area and BDNF protein level in the hippocampus were lowered in ethanol-treated mice. The increase in proBDNF protein level in the raphe nuclei area, cortex, and amygdala and the increase of p75 receptor protein levels in the raphe nuclei area were revealed after ethanol exposure. Alcohol intake reduced the protein level and increased the activity of Tph-2, the key enzyme for serotonin biosynthesis in the brain, and increased the main 5-HT metabolite 5-HIAA level and 5-HIAA/5-НТ ratio as well as the 5-HT₇ receptor mRNA level in the raphe nuclei area. In the cortex, 5-HT_2A receptor protein level was reduced, and 5-HIAA/5-HT ratio was increased. These data showed considerable impact of alcoholization on the BDNF system, resulting in proBDNF and p75 receptor expression enhancement. Alcohol-induced changes in BDNF and 5-HT systems were revealed in the raphe nuclei area where the majority of the cell bodies of the 5-HT neurons are localized, as well as in the cortex, hippocampus, and amygdala. Our data suggest that the BDNF/5-HT interaction contributes to the mechanism underlying chronic alcohol-induced neurodegenerative disorders.