Down-regulation of serotonergic genes expression in the raphe nuclei of midbrain under chronic social defeat stress in male mice

Background: 
There is ample experimental evidence supporting the hypothesis that the brain serotonergic system is involved in the control of chronic social defeat stress (CSDS), depression and anxiety. The study aimed to analyze mRNA levels of the serotonergic genes in the raphe nuclei of the midbrain that may be associated with chronic social defeats consistently shown by male mice in special experimental settings. 

Methodology/Principal Findings: 
The serotonergic genes were the Tph2, Sert, Maoa and Htr1a. The Bdnf, Creb, Cphn, Gapdh, Hprt, B2M, 18S and Actb genes were also studied. The experimental groups were composed of male mice with experience of defeats in 21 daily encounters and male mice with the same track record of defeats followed by a no-defeat period without agonistic interactions (relative rest for 14 days). It has been shown that mRNA levels of the Tph2, Maoa, Sert, Htr1a, Bdnf and Creb genes in the raphe nuclei of defeated mice are decreased as compared with the controls. Under CSDS the Cphn, Gapdh, Hprt, B2M, 18S, Actb genes are also down-regulated. The expression of the serotonergic genes as well as the Cphn and Creb genes is not restored to the control level after the 2 weeks of relative rest. mRNA levels of other genes are not recovered to the control levels, although some up-regulation was observed in rested losers. Significant positive correlations were found between the total time of avoidance behavior demonstrated by the 21-day defeaters in agonistic interactions and Sert, Maoa, Bdnf, Gapdh and 18S mRNA levels. 

Conclusions: 
CSDS experience inducing the development of mixed anxious/depression-like state in male mice down-regulates the serotonergic genes expression associated with the synthesis, inactivation and reception of serotonin. The Bdnf and Creb genes as well as the cell and metabolic Cphn, Gapdh, Hprt, B2M, Actb and 18S genes in the midbrain raphe nuclei are also down-regulated under CSDS. Period of relative rest is not enough for most genes to recover expression to the control levels

Decrease of vanillin sucrose intake by victorious and defeated mice: development of anhedonia?

Hedonic reactions to various rewards play a key role in various forms of motivated behavior. The influence of repeated experience of social victories or defeats in daily agonistic interactions between male mice on voluntary consumption of vanillin sucrose solution used as hedonic reinforcer was studied. Intake of vanillin sucrose solution was shown to decrease in the winners and losers exposed to social confrontations as compared with the controls. Three days of deprivation failed to restore the intake of vanillin sucrose solution to the control level in the losers and did so in the winners. The results obtained imply that similar reaction of animals to a hedonic non-drug reinforcer may have different motivational origin depending on positive or negative social experience