Figure 6. Gene expression of 5-HT transporter (5-HTT), 5-HT1A receptor (5-HT1AR) and tryptophan hydroxylase-2 (TPH2) in the raphe. Measuring mRNA levels of several genes encoding proteins that regulate 5-HT homeostasis, we found significant genotype-sex interactions for both mRNA levels of (A) 5-HTT and (B) TPH2 but not for (C) 5-HT1AR gene expression. Both 5-HTT and TPH2 mRNA levels were decreased in female WT when compared to male WT. In addition, HD mutation decreased both 5-HTT and TPH2 gene expressions in male only. Finally 5-HT1AR mRNA levels were not affected by either the sex or the genotype. Values represent means (6 SEM) of n = 5–6 mice per group. WT vs. HD: (**) p,0.01. Male vs. female: (##) p,0.01. doi:10.1371/journal.pone.0022133.g006 Collectively, our results suggest that the female-specific depression-like behavior was associated with an increased response to 8-OH-DPAT and a global reduction of serotonergic transmis- sion as suggested by the decreased tissue levels of serotonin (5-HT) and its metabolite (5-HIAA) in various brain areas of HD animals. The 5-HT and 5-HIAA deficits observed in our 8-week-old female R6/1 HD mice are consistent with similar findings in the R6/2 model [53]. Interestingly in male HD mice, 5-HT concentration was reduced only in the cortex although levels of 5-HIAA were massively decreased in the three regions studied. These observa- tions suggested an impaired serotonin turnover in male HD. Detailed exploration of the enzymes involved in 5-HT catabolism (e.g. monoamine oxidase, aldehyde dehydrogenease) would be informative to address this matter. In addition, both hippocampal serotonin content and TPH2 mRNA levels were higher in male when compared to female mice. Whether such sex differences in neurochemistry are the cause or the consequence of the reported sex-dependent 5-HT1A autoreceptor dysregulation, is an impor- tant point to address in the future to facilitate understanding of the pathophysiology of depression. Supersensitivity of 5-HT1A autoreceptors has already been reported in other animal models of depression [22,24,54] but also see ([55,56]). In addition, enhanced 5-HT1A autoreceptor function has been associated with reduced serotonergic neurotransmission (as a lower baseline firing rate of DRN 5-HT neurons) in several models of depression [39,57] and a significant negative correlation between serotonin synthesis and 5-HT1A binding potential has been reported in healthy volunteers [58]. Interestingly, an increased 5-HT1A receptor density has also been observed in the DRN of depressed patients [18,59]. However, there exists disagreement within the literature regarding the presence and direction of 5-HT1A receptor abnormalities [15,60]. Interestingly, a sensitization of 5-HT7 receptors in HD mice cannot be excluded since the involvement of these receptors in 8-OH-DPAT-induced hypothermia has been reported in mice [37]. In addition, selective 5-HT7 receptor antagonists have been recently proposed as a new class of antidepressants [61]. Whether such an adaptive change occurs in HD mice is an interesting question to be addressed in future studies. The brain serotonin deficits reported in our 8-week-old female HD mice are more likely to be driven by the resultant increased 5- HT1A receptor-mediated inhibitory feedback on serotonergic neuronal firing than impairment in the synthesis process. Indeed in females, we did not find any genotype effect on the gene expression of TPH2, the rate-limiting enzyme in the biosynthesis of 5-HT. However, this interpretation is limited by a possible floor effect in female mice (which showed lower levels of mRNA relative to males). In contrast, TPH2 mRNA levels were reduced in HD male mice when compared with control animals, although 5-HT concentrations were not affected by the mutation, at least in male hippocampus and striatum. Male HD mice could possibly have developed adaptive changes to counteract the TPH2 deficit. Whether such adaptation(s) occur in a region-specific manner would be an interesting question to address. Interestingly, recent data suggest that differences in brain 5-HT synthesis may be larger between healthy and depressed men that between healthy and depressed women [62]. Consistent with our observations in WT animals, it has been found that females had lower levels of mRNA relative to males quantifying mRNA in post-mortem human brain [63]. In addition, the mean rate of serotonin synthesis in healthy males was found to be 50% higher than in normal females [64]. Interestingly, a sex-specific increased in Hypothalamic-pitui- tary-adrenal (HPA) axis reactivity has been recently reported in women with chronic major depressive disorder [65]. As well as an increased levels of the stress hormone (corticosterone/cortisol) in both R6/2 HD mice and late-stage HD patients [66]. We therefore assessed the effect of 5-HT1A post-synaptic receptor activation on corticosterone levels [52] in both male and female HD mice. The similar 8-OH-DPAT-induced increase in cortico- sterone in both WT and HD animals was unexpected given that HD mice have been demonstrated to have decreased 5-HT1A receptor density [28] and gene expression [27] in the hippocampus and the cortex. However, it is possible that the hypothalamic 5- HT1A receptors (involved in mediating corticosterone release) were PLoS ONE | www.plosone.org 6 July 2011 | Volume 6 | Issue 7 | e22133