Dysregulation of the serotonergic signaling system has been implicated in the pathology of mood disorders including depression, and various rodent models of disrupted serotonergic signaling display depression-related behavioral phenotypes. Depression is a common neuropsychiatric feature of preclinical Huntington's disease (HD) but the underlying changes in the HD brain contributing to the development of depression are unknown. Using the R6/1 transgenic mouse model of HD, we show that pre-motor symptomatic HD mice display sex-specific depressive-related behaviors on the forced-swim (FST), tail-suspension (TST) and novelty-suppressed feeding (NSFT) tests while having muted responses to acute anti-depressant administration. The baseline behaviors of HD mice were similar to the behavioral phenotypes of serotonin (5-HT) receptor and transporter null mutants, and gene expression of specific serotonin receptors were subsequently found to be reduced in the hippocampus and cortex of HD mice. Female HD mice had an additional deficit in cortical expression of serotonin transporter (SerT). Environmental enrichment normalized the FST behavioral response of female HD mice corresponding with increased gene expression of specific 5-HT receptors in the hippocampus and cortex. Our findings implicate altered serotonergic signaling as the basis for the development of depression during the preclinical stages of HD.

Brain-derived neurotrophic factor (BDNF) is an essential neurotrophin and regulation of its expression is complex due to multiple 5' untranslated exons which are separately spliced to a common coding exon to form unique mRNA transcripts. Disruption of BDNF gene expression is a key to the development of symptoms in Huntington's disease (HD), a fatal neurodegenerative condition. Abnormal epigenetic modifications are associated with reduced gene expression in late-stage HD but such regulation of BDNF gene expression has yet to be investigated. We hypothesized that BDNF gene expression is altered in the HD hippocampus of pre-motor symptomatic R6/1 transgenic HD mice, correlating with a change in the DNA methylation profile. The effects of wheel-running and environmental enrichment on wild-type mice, in association with a proposed environment-mediated correction of BDNF gene expression deficits in HD mice, were also investigated. Using real-time PCR, levels of total BDNF mRNA were found to be reduced in the hippocampus of both male and female HD mice. Wheel-running significantly increased total BDNF gene expression in all groups of mice except male HD mice. In contrast, environmental enrichment significantly increased expression only in male wild-type animals. Further quantification of BDNF exon-specific transcripts revealed sex-specific changes in relation to the effect of the HD mutation and differential effects on gene expression by wheel-running and environmental enrichment. The HD-associated reduction of BDNF gene expression was not due to increased methylation of the gene sequence. Furthermore, environment-induced changes in BDNF gene expression in the wild-type hippocampus were independent of the extent of DNA methylation. Overall, the results of this study provide new insight into the role of BDNF in HD pathogenesis in addition to the mechanisms regulating normal BDNF gene expression.

Depression is the most common psychiatric disorder in Huntington's disease (HD) patients. In the general population, women are more prone to develop depression and such susceptibility might be related to serotonergic dysregulation. There is yet to be a study of sexual dimorphism in the development and presentation of depression in HD patients. We investigated whether 8-week-old male and female R6/1 transgenic HD mice display depressive-like endophenotypes associated with serotonergic impairments. We also studied the behavioral effects of acute treatment with sertraline. We found that only female HD mice exhibited a decreased preference for saccharin as well as impaired emotionality-related behaviors when assessed on the novelty-suppressed feeding test (NSFT) and the forced-swimming test (FST). The exaggerated immobility time displayed by female HD in the FST was reduced by acute administration of sertraline. We also report an increased response to the 5-HT1A receptor agonist 8-OH-DPAT in inducing hypothermia and a decreased 5-HT2A receptor function in HD animals. While tissue levels of serotonin were reduced in both male and female HD mice, we found that serotonin concentration and hydroxylase-2 (TPH2) mRNA levels were higher in the hippocampus of males compared to female animals. Finally, the antidepressant-like effects of sertraline in the FST were blunted in male HD animals. This study reveals sex-specific depressive-related behaviors during an early stage of HD prior to any cognitive and motor deficits. Our data suggest a crucial role for disrupted serotonin signaling in mediating the sexually dimorphic depression-like phenotype in HD mice.

BACKGROUND AND PURPOSEDepression is the most common psychiatric disorder in Huntington's disease (HD) patients. Women are more prone to develop depression and such susceptibility might be related to 5-hydroxytryptaminergic (serotonergic) dysregulation. EXPERIMENTAL APPROACHWe performed tests of depression-related behaviours on female R6/1 HD mice that had been chronically treated with sertraline or provided with running-wheels. Functional assessments of 5-HT1A and 5-HT2A receptors were performed by measuring behavioural and physiological responses following administration of specific agonists, in combination with analysis of hippocampal gene expression. Finally we assessed the effect of exercise on hippocampal cell proliferation. KEY RESULTSFemale HD mice recorded increased immobility time in the forced-swimming test, reduced saccharin preference and a hyperthermic response to stress compared with wild-type animals. These alterations were improved by chronic sertraline treatment. Wheel-running also resulted in similar improvements with the exception of saccharin preference but failed to correct the hippocampal cell proliferation deficits displayed by HD mice. The benefits of sertraline treatment and exercise involved altered 5-HT1A autoreceptor function, as demonstrated by modulation of the exaggerated 8-OH-DPAT-induced hypothermia exhibited by female HD mice. On the other hand, sertraline treatment was unable to restore the reduced 5-HT1A and 5-HT2 heteroceptor function observed in HD animals. CONCLUSIONS AND IMPLICATIONSWe report for the first time a crucial role for 5-HT1A autoreceptor function in mediating the sex-specific depressive-like phenotype of female R6/1 HD mice. Our data further support a differential effect of chronic sertraline treatment and exercise on hippocampal cell proliferation despite common behavioural benefits.Abbreviations 5-CT, 5-carboxamidotryptamine; 5-HTT, 5-HT transporter; AUC, area under curve; BDNF, brain-derived neurotrophic factor; BrdU, 5-bromo-2′-deoxyuridine; FST, forced-swimming test; GR, glucocorticoid receptor; HD, Huntington's disease; HPA, hypothalamic-pituitary-adrenal; RW, running-wheel; SH, standard-housed; SSRIs, selective 5-HT re-uptake inhibitors; TST, tail-suspension test; WT, wild-type BJP British Journal of Pharmacology DOI:10.1111DOI:10. /j.1476DOI:10. -5381.2011 British Journal of Pharmacology (2012) IntroductionHuntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an abnormal expansion of CAG repeats in exon 1 of the huntingtin gene (The Huntington's Disease Collaborative Research Group, 1993). Clinical diagnosis of HD is determined on the basis of motor symptoms; however, the pre-motor stages of the disease are commonly associated with psychiatric manifestations including depression (Paulsen et al., 2005;Duff et al., 2007;Julien et al., 2007;Marshall et al., 2007). Depression is one of the most prevalent causes of disability worldwide and is diagnosed in women more frequently than in men (Fava and Kendler, 2000;Kornstein e...

Huntington's disease is caused by polyglutamine (polyQ)-expansion mutations in the CAG tandem repeat of the Huntingtin gene. The central feature of Huntington's disease pathology is the aggregation of mutant Huntingtin (Htt) protein into micrometer-sized inclusion bodies. Soluble mutant Htt states are most proteotoxic and trigger an enhanced risk of death whereas inclusions confer different changes to cellular health, and may even provide adaptive responses to stress. Yet the molecular mechanisms underpinning these changes remain unclear. Using the flow cytometry method of pulse-shape analysis (PulSA) to sort neuroblastoma (Neuro2a) cells enriched with mutant or wild-type Htt into different aggregation states, we clarified which transcriptional signatures were specifically attributable to cells before versus after inclusion assembly. Dampened CREB signalling was the most striking change overall and invoked specifically by soluble mutant Httex1 states. Toxicity could be rescued by stimulation of CREB signalling. Other biological processes mapped to different changes before and after aggregation included NF-kB signalling, autophagy, SUMOylation, transcription regulation by histone deacetylases and BRD4, NAD+ biosynthesis, ribosome biogenesis and altered HIF-1 signalling. These findings open the path for therapeutic strategies targeting key molecular changes invoked prior to, and subsequently to, Httex1 aggregation.

Huntington’s disease (HD) is a neurodegenerative disorder caused by a tandem repeat mutation encoding an expanded polyglutamine tract in the huntingtin protein, which leads to cognitive, psychiatric and motor dysfunction. Exposure to environmental enrichment (EE), which enhances levels of cognitive stimulation and physical activity, has therapeutic effects on cognitive, affective and motor function of transgenic HD mice. The present study investigated gene expression changes and behavioral pharmacology in male and female R6/1 transgenic HD mice at an early time-point in HD progression associated with onset of cognitive and affective abnormalities, following EE and exercise (wheel running) interventions. We have demonstrated changes in expression levels of the serotonin (5-HT) receptor Htr1a, Htr1b, Htr2a and Htr2c genes (encoding the 5-HT1A, 5-HT1B, 5-HT2A and 5-HT2C receptors, respectively) in HD brains at 8 weeks of age, using quantitative real-time PCR. In contrast, expression of the serotonin transporter (SerT, also known as 5-HTT or Slc6a4) was not altered in these brains. Furthermore, we identified region-specific, sex-specific and environmentally regulated (comparing EE, exercise and standard housing conditions) impacts on gene expression of particular 5-HT receptors, as well as SerT. For example, SerT gene expression was upregulated by exercise (wheel running from 6 to 8 weeks of age) in the hippocampus. Interestingly, when EE was introduced from 6 to 8 weeks of age, Htr2a gene expression was upregulated in the cortex, striatum and hippocampus of male mice. EE also rescued the functional activity of 5-HT2 receptors as observed in the head-twitch test, reflecting sexually dimorphic effects of environmental stimulation. These findings demonstrate that disruption of the serotonergic system occurs early in HD pathogenesis and, together with previous findings, show that the timing and duration of environmental interventions are critical in terms of their ability to modify gene expression. This study is the first to show that EE is able to selectively enhance both gene expression of a neurotransmitter receptor and the functional consequences on behavioral pharmacology, and links this molecular modulation to the therapeutic effects of environmental stimulation in this neurodegenerative disease.