Figure 4. Effect of sex and HD mutation on 8-OH-DPAT-induced hypothermia. Acute injection with the 5-HT1A receptor agonist 8-OH-DPAT (0.1 and 0.3 mg/kg, s.c.), decreased rectal temperatures in a dose-dependent manner in both (A) male and (B) female mice when compared to saline- treated animals. However, only female HD mice exhibited an augmented response to the high dose of 8-OH-DPAT (0.3 mg/kg). The maximum hypothermic response to 8-OH-DPAT was observed 20 min post-injection in both WT/HD male (23.6960.31 and 23.8560.35uC respectively) and WT/HD female (22.8160.30 and 23.8860.36uC respectively, p,0.05). Values are expressed as change of temperature (uC) compared to baseline and represent means (6 SEM) of n = 8–14 mice per group. WT vs. HD: (*) p,0.05. doi:10.1371/journal.pone.0022133.g004 The administration of the 5-HT2A/2C receptor agonist (6)DOI, dose-dependently induced head-twitches that were reduced in both male (Fig. 5C) and female (Fig. 5D) HD mice compared to their respective WT control groups. There were significant effects of genotype (F(1,61) = 32.8, p,0.001) and treatment (F(1,61) = 59.8, p,0.001), but no effect of sex (F(1,61) = 0.99, p = 0.75). Effect of the HD mutation on gene expression in the raphe Analyzing mRNA levels of several targets involved in serotonin homeostasis (Figure 6), we found an overall significant effect of sex and genotype for both 5-HTT (F(1,19) = 8.9, p,0.01 and F(1,19) = 10.4, p,0.01 respectively) and TPH2 (F(1,19) = 6.45, p,0.05 and F(1,19) = 9.9, p,0.01 respectively). Interestingly, there were genotype-sex interactions for 5-HTT (F(1,19) = 4.8, p,0.05) and TPH2 (F(1,19) = 5.1, p,0.05). Post-hoc analysis showed that both 5-HTT and TPH2 mRNA levels were decreased in female WT when compared to male WT (p,0.01). In addition, HD mutation decreased both 5-HTT and TPH2 gene expression in males only (p,0.01). In contrast, 5-HT1A receptor mRNA levels (Fig. 6B) were not affected by either sex (F(1,19) = 1.5, p.0.05) or genotype (F(1,19) = 1.0, p.0.05). Tissue levels of serotonin and its metabolite in HD mice In order to assess the effect of the HD mutation on the global serotonergic activity, we measured the levels of serotonin (5-HT) and its metabolite (5-HIAA) in several brain areas (Figure 7) including hippocampus (panels A&B), cortex (panels C&D) and striatum (panels E&F). In each of the three brain regions studied, we found an overall effect of genotype for both 5-HT and 5-HIAA tissue levels. Post-hoc tests revealed significant reductions of 5-HT and/or 5-HIAA levels in HD tissue compared to WT mice in both sexes. We also found a significant effect of sex analyzing both 5- HT and 5-HIAA levels in hippocampus (F(1,23) = 65.6, p,0.001 and F(1,23) = 13.2, p,0.001, respectively) as well as striatal 5-HIAA levels (F(1,23) = 19.3, p,0.001). Indeed, in these two regions female WT mice exhibited decreased 5-HIAA levels when compared to control male animals. Interestingly, the 2-way ANOVA revealed significant interaction between sex and genotype only for hippocampal 5-HIAA levels (F(1,23) = 4.58, p,0.05). Discussion Despite evidence of affective disorders in HD and the known susceptibility of female to developing depression in the general population, there has yet to be a study of sexual dimorphism in the development and presentation of depression in HD patients. We report here a sexually dimorphic depression-like phenotype in the R6/1 HD mice at 8 weeks of age, prior to the development of any motor and/or cognitive deficits which may have confounded previous studies [27,29]. We found that latency to eat food in the NSFT as well as immobility times in FST were both specifically prolonged in female but not in male HD mice, and that the acute administration of the SSRI sertraline corrected this latter behavioral impairment. Indeed in the FST, acute injection with sertraline significantly decreased the immobility time of HD animals (when compared to the HD vehicle-injected group), leading to similar FST performances as those displayed by WT controls (vehicle-injected group). In contrast, the HD mutation did not affect TST performance at 8 weeks of age, consistent with the view that distinct neurochemical pathways mediate responses to FST versus TST [27,38,39]. These TST results are in contrast with a previous study [27] which reported a conflicted and unexpected ‘‘anti-depressive’’ phenotype displayed by female HD mice exposed to TST at 12 weeks of age, and support the relevance of studying depression-like behaviors in HD at an earlier stage of the disease. As an indicator of altered reward system function, the female but not male HD mice displayed a decreased preference for saccharin. This is the first report of such sex-specific behavioral deficits in R6/1 HD mice, since Pang et al. (2009) focused on behavioral tests modeling the experience of helpless- ness. Because the diagnosis of a depressive episode requires that an individual has chronically experienced at least 2 out of the 3 core symptoms (according to the ICD-10 criteria: low mood, fatigue and anhedonia), it is critical to used a broad battery of behavioral tests to satisfactorily characterize a depressive-like phenotype in mice. Collectively, our present findings support the well-estab- lished gender differences in the vulnerability to depression reported in general population [5,6]. Similar results have been found in other animal models of depression [40,41] and female- specific impairments have already been reported using other rodent models of HD [42,43,44]. PLoS ONE | www.plosone.org 4 July 2011 | Volume 6 | Issue 7 | e22133