Longitudinal analyses of operant performance on the serial implicit learning task (SILT) in the YAC128 Huntington's disease mouse line

Home cage-based insights into motor learning and strategy adaptation in a Huntington disease mouse model

Huntington disease (HD) is a genetic neurodegenerative disorder characterized by progressive motor dysfunction, cognitive decline, and neuropsychiatric symptoms. Assessing early motor skill deficits in HD mouse models is challenging with traditional behavioral tasks. This study uses a home cage-based lever-pulling task, PiPaw2.0, to evaluate motor learning in 6-7 months-old zQ175 knock-in HD mice in a more naturalistic environment. In this task, mice learn to pull a lever for a water reward, with the requirement to hold the lever within a specific goal range for a required hold time. As the mice improved, the required hold time increased, thereby gradually increasing the task demands. Both wild type (WT) and zQ175 mice initially showed similar task engagement, but zQ175 mice had significant deficits in adapting to increasing hold time. The WT mice refined their strategies over time, shifting from random to more precise lever pulls, while zQ175 mice failed to make this adjustment, maintaining erratic performance. Additionally, in group-housing WT mouse lever performance benefited from peer interactions, an effect absent in zQ175 mice. Post-task neural assessments revealed that WT mice developed experience-mediated synaptic plasticity in the left striatum (contralateral to lever-pulling paw), while zQ175 mice showed no significant changes, consistent with known corticostriatal plasticity impairments in HD mouse models. In conclusion, our findings demonstrate the effectiveness of group-housed, home cage-based assessments for evaluating motor learning and adaptation in HD mouse models. This study provides insights into the motor control and adaptive learning deficits in HD, emphasizing the value of automated home cage systems in advancing neurodegenerative disease research and highlighting the importance of peer influences on performance.

The contribution of preclinical magnetic resonance imaging and spectroscopy to Huntington's disease

Huntington's disease is an inherited disorder characterized by psychiatric, cognitive, and motor symptoms due to degeneration of medium spiny neurons in the striatum. A prodromal phase precedes the onset, lasting decades. Current biomarkers include clinical score and striatal atrophy using Magnetic Resonance Imaging (MRI). These markers lack sensitivity for subtle cellular changes during the prodromal phase. MRI and MR spectroscopy offer different contrasts for assessing metabolic, microstructural, functional, or vascular alterations in the disease. They have been used in patients and mouse models. Mouse models can be of great interest to study a specific mechanism of the degenerative process, allow better understanding of the pathogenesis from the prodromal to the symptomatic phase, and to evaluate therapeutic efficacy. Mouse models can be divided into three different constructions: transgenic mice expressing exon-1 of human huntingtin (HTT), mice with an artificial chromosome expressing full-length human HTT, and knock-in mouse models with CAG expansion inserted in the murine htt gene. Several studies have used MRI/S to characterized these models. However, the multiplicity of modalities and mouse models available complicates the understanding of this rich corpus. The present review aims at giving an overview of results obtained using MRI/S for each mouse model of HD, to provide a useful resource for the conception of neuroimaging studies using mouse models of HD. Finally, despite difficulties in translating preclinical protocols to clinical applications, many biomarkers identified in preclinical models have already been evaluated in patients. This review also aims to cover this aspect to demonstrate the importance of MRI/S for studying HD.

Alternative processing of human HTT mRNA with implications for Huntington's disease therapeutics

Huntington disease is caused by a CAG repeat expansion in exon 1 of the huntingtin gene (HTT) that is translated into a polyglutamine stretch in the huntingtin protein (HTT). We previously showed that HTT mRNA carrying an expanded CAG repeat was incompletely spliced to generate HTT1a, an exon 1 only transcript, which was translated to produce the highly aggregation-prone and pathogenic exon 1 HTT protein. This occurred in all knock-in mouse models of Huntington's disease and could be detected in patient cell lines and post-mortem brains. To extend these findings to a model system expressing human HTT, we took advantage of YAC128 mice that are transgenic for a yeast artificial chromosome carrying human HTT with an expanded CAG repeat. We discovered that the HTT1a transcript could be detected throughout the brains of YAC128 mice. We implemented RNAscope to visualize HTT transcripts at the single molecule level and found that full-length HTT and HTT1a were retained together in large nuclear RNA clusters, as well as being present as single transcripts in the cytoplasm. Homogeneous time-resolved fluorescence analysis demonstrated that the HTT1a transcript had been translated to produce the exon 1 HTT protein. The levels of exon 1 HTT in YAC128 mice, correlated with HTT aggregation, supportive of the hypothesis that exon 1 HTT initiates the aggregation process. Huntingtin-lowering strategies are a major focus of therapeutic development for Huntington's disease. These approaches often target full-length HTT alone and would not be expected to reduce pathogenic exon 1 HTT levels. We have established YAC128 mouse embryonic fibroblast lines and shown that, together with our QuantiGene multiplex assay, these provide an effective screening tool for agents that target HTT transcripts. The effects of current targeting strategies on nuclear RNA clusters are unknown, structures that may have a pathogenic role or alternatively could be protective by retaining HTT1a in the nucleus and preventing it from being translated. In light of recently halted antisense oligonucleotide trials, it is vital that agents targeting HTT1a are developed, and that the effects of HTT-lowering strategies on the subcellular levels of all HTT transcripts and their various HTT protein isoforms are understood.

Behavioral evidence for two distinct memory systems in rats

Serial reaction time tasks, in which subjects have to match a target to a cue, are used to explore whether non-human animals have multiple memory systems. Predictable sub-sequences embedded in the sequence of cues are responded to faster, demonstrating incidental learning, often considered implicit. Here, we used the serial implicit learning task (SILT) to determine whether rats' memory shows similar effects. In SILT, subjects must nose-poke into a sequence of two lit apertures, S1 and S2. Some S1 are always followed by the same S2, creating predictable sequences (PS). Across groups, we varied the proportion of PS trials, from 10 to 80%, and show that rats with more PS experience do better on them than on unpredictable sequences, and better than rats with less experience. We then introduced test trials in which no S2 was cued. Rats with more PS experience did better on test trials. Finally, we reversed some sequences (from predictable to unpredictable and vice versa) and changed others. We find that rats with more PS experience perseverate on old (now incorrect) responses more than those with less PS experience. Overall, we find a discontinuity in performance as the proportion of PS increases, suggesting a switch in behavioral strategies or memory systems, which we confirm using a Process Dissociation Procedure analysis. Our data suggest that rats have at least two distinct memory systems, one of which appears to be analogous to human implicit memory and is differentially activated by varying the proportion of PS in our task.

C57BL/6 Background Attenuates mHTT Toxicity in the Striatum of YAC128 Mice

Mouse models are frequently used to study Huntington’s disease (HD). The onset and severity of neuronal and behavioral pathologies vary greatly between HD mouse models, which results from different huntingtin expression levels and different CAG repeat length. HD pathology appears to depend also on the strain background of mouse models. Thus, behavioral deficits of HD mice are more severe in the FVB than in the C57BL/6 background. Alterations in medium spiny neuron (MSN) morphology and function have been well documented in young YAC128 mice in the FVB background. Here, we tested the relevance of strain background for mutant huntingtin (mHTT) toxicity on the cellular level by investigating HD pathologies in YAC128 mice in the C57BL/6 background (YAC128/BL6). Morphology, spine density, synapse function and membrane properties were not or only subtly altered in MSNs of 12-month-old YAC128/BL6 mice. Despite the mild cellular phenotype, YAC128/BL6 mice showed deficits in motor performance. More pronounced alterations in MSN function were found in the HdhQ150 mouse model in the C57BL/6 background (HdhQ150/BL6). Consistent with the differences in HD pathology, the number of inclusion bodies was considerably lower in YAC128/BL6 mice than HdhQ150/BL6 mice. This study highlights the relevance of strain background for mHTT toxicity in HD mouse models.

Mouse Models of Huntington's Disease

The identification of the mutation causing Huntington's disease (HD) has led to the generation of a large number of mouse models. These models are used to further enhance our understanding of the mechanisms underlying the disease, as well as investigating and identifying therapeutic targets for this disorder. Here we review the transgenic, knock-in mice commonly used to model HD, as well those that have been generated to study specific disease mechanisms. We then provide a brief overview of the importance of standardizing the use of HD mice and describe brief protocols used for genotyping the mouse models used within the Bates Laboratory.

Automated Operant Assessments of Huntington's Disease Mouse Models

Huntington's disease (HD) presents clinically with a triad of motor, cognitive, and psychiatric symptoms. Cognitive symptoms often occur early within the disease progression, prior to the onset of motor symptoms, and they are significantly burdensome to people who are affected by HD. In order to determine the suitability of mouse models of HD in recapitulating the human condition, these models must be behaviorally tested and characterized. Operant behavioral testing offers an automated and objective method of behaviorally profiling motor, cognitive, and psychiatric dysfunction in HD mice. Furthermore, operant testing can also be employed to determine any behavioral changes observed after any associated interventions or experimental therapeutics. We here present an overview of the most commonly used operant behavioral tests to dissociate motor, cognitive, and psychiatric aspects of mouse models of HD.

An Automated Home-Cage System to Assess Learning and Performance of a Skilled Motor Task in a Mouse Model of Huntington's Disease

Behavioral testing is a critical step in assessing the validity of rodent models of neurodegenerative disease, as well as evaluating the efficacy of pharmacological interventions. In models of Huntington's disease (HD), a gradual progression of impairments is observed across ages, increasing the need for sensitive, high-throughput and longitudinal assessments. Recently, a number of automated systems have been developed to perform behavioral profiling of animals within their own home-cage, allowing for 24-h monitoring and minimizing experimenter interaction. However, as of yet, few of these have had functionality for the assessment of skilled motor learning, a relevant behavior for movement disorders such as HD. To address this, we assess a lever positioning task within the mouse home-cage. Animals first acquire a simple operant response, before moving to a second phase where they must learn to hold the lever for progressively longer in a rewarded position range. Testing with this paradigm has revealed the presence of distinct phenotypes in the YAC128 mouse model of HD at three early symptomatic time points. YAC128 mice at two months old, but not older, had a motor learning deficit when required to adapt their response to changes in task requirements. In contrast, six-month-old YAC128 mice had disruptions of normal circadian activity and displayed kinematic abnormalities during performance of the task, suggesting an impairment in motor control. This system holds promise for facilitating high throughput behavioral assessment of HD mouse models for preclinical therapeutic screening.

A Longitudinal Operant Assessment of Cognitive and Behavioural Changes in the HdhQ111 Mouse Model of Huntington's Disease

Huntington's disease (HD) is characterised by motor symptoms which are often preceded by cognitive and behavioural changes, that can significantly contribute to disease burden for people living with HD. Numerous knock-in mouse models of HD are currently available for scientific research. However, before their use, they must be behaviourally characterised to determine their suitability in recapitulating the symptoms of the human condition. Thus, we sought to longitudinally characterise the nature, severity and time course of cognitive and behavioural changes observed in HdhQ111 heterozygous knock-in mice.To determine changes in cognition and behaviour an extensive battery of operant tests including: fixed ratio, progressive ratio, the five choice serial reaction time task and the serial implicit learning task, were applied longitudinally to HdhQ111 and wild type mice. The operant test battery was conducted at 6, 12 and 18 months of age. Significant deficits were observed in HdhQ111 animals in comparison to wild type animals in all operant tests indicating altered cognition (attentional and executive function) and motivation. However, the cognitive and behavioural deficits observed were not shown to be progressive over time in the longitudinal testing paradigm that was utilised. The results therefore demonstrate that the HdhQ111 mouse model of HD reflects some features of the cognitive and behavioural changes shown in the human condition of HD. Although, the cognitive and behavioural deficits demonstrated were not shown to be progressive over time.

The utilisation of operant delayed matching and non-matching to position for probing cognitive flexibility and working memory in mouse models of Huntington's disease

BACKGROUND

Operant behavioural testing provides a highly sensitive and automated method of exploring the behavioural deficits seen in rodent models of neurodegenerative diseases, including Huntington's disease (HD). The delayed matching to position (DMTP) and delayed non-matching to position (DNMTP) tasks probe spatial learning and working memory and when applied serially they can be used to measure reversal learning, which has been shown to be an early symptom of executive dysfunction in HD.

NEW METHOD

The DMTP and DNMTP tasks were conducted in two configurations of operant apparatus; the conventional 9-hole operant apparatus, and a Skinner-like operant apparatus, to compare, contrast and optimise the DMTP and DNMTP operant protocols for use in mice. The optimised tasks were then tested in the Hdh(Q111) mouse model of HD.

RESULTS

Optimisation of the operant apparatus demonstrated that the mice learned the DMTP and DNMTP tasks more rapidly and effectively in the Skinner-like apparatus configuration in comparison to the conventional 9-hole apparatus configuration. When tested in the Hdh(Q111) mouse model of HD, the DMTP and DNMTP tasks revealed significant deficits in reversal learning.

COMPARISON WITH EXISTING METHOD

We found that mice were capable of performing the DMTP and DNMTP tasks in both apparatus configurations, but in comparison to the 9-hole configuration, the Skinner-like configuration produced more efficient, robust and reliable results.

CONCLUSIONS

The results presented here suggest that DMTP and DNMTP tasks, incorporating a reversal learning manipulation, are valid and robust methods for probing selected cognitive deficits in mouse models of neurodegenerative diseases.

Similar striatal gene expression profiles in the striatum of the YAC128 and HdhQ150 mouse models of Huntington's disease are not reflected in mutant Huntingtin inclusion prevalence

Background

The YAC128 model of Huntington’s disease (HD) shows substantial deficits in motor, learning and memory tasks and alterations in its transcriptional profile. We examined the changes in the transcriptional profile in the YAC128 mouse model of HD at 6, 12 and 18 months and compared these with those seen in other models and human HD caudate.

Results

Differential gene expression by genotype showed that genes related to neuronal function, projection outgrowth and cell adhesion were altered in expression. A Time-course ANOVA revealed that genes downregulated with increased age in wild-type striata were likely to be downregulated in the YAC128 striata. There was a substantial overlap of concordant gene expression changes in the YAC128 striata compared with those in human HD brain. Changes in gene expression over time showed fewer striatal YAC128 RNAs altered in abundance than in the HdhQ150 striata but there was a very marked overlap in transcriptional changes at all time points. Despite the similarities in striatal expression changes at 18 months the HdhQ150 mice showed widespread mHTT and ubiquitin positive inclusion staining in the striatum whereas this was absent in the YAC128 striatum.

Conclusions

The gene expression changes in YAC128 striata show a very closely matched profile to that of HdhQ150 striata and are already significantly different between genotypes by six months of age, implying that the temporal molecular gene expression profiles of these models match very closely, despite differences in the prevalence of brain inclusion formation between the models. The YAC128 gene expression changes appear to correlate well with gene expression differences caused by ageing. A relatively small number of genes showed significant differences in expression between the striata of the two models and these could explain some of the phenotypic differences between the models.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2251-4) contains supplementary material, which is available to authorized users.

Mouse Models of Huntington's Disease

In this review, we explore the similarities and differences in the behavioural neurobiology found in the mouse models of Huntington's disease (HD) and the human disease state. The review is organised with a comparative focus on the functional domains of motor control, cognition and behavioural disturbance (akin to psychiatric disturbance in people) and how our knowledge of the underlying physiological changes that are manifest in the HD mouse lines correspond to those seen in the HD clinical population. The review is framed in terms of functional circuitry and neurotransmitter systems and how abnormalities in these systems impact on the behavioural readouts across the mouse lines and how these may correspond to the deficits observed in people. In addition, interpretational issues associated with the data from animal studies are discussed.

Correlations of behavioral deficits with brain pathology assessed through longitudinal MRI and histopathology in the R6/1 mouse model of Huntington's disease

Huntington's disease (HD) is caused by the expansion of a CAG repeat in the huntingtin (HTT) gene. The R6 mouse models of HD express a mutant version of exon 1 HTT and typically develop motor and cognitive impairments, a widespread huntingtin (HTT) aggregate pathology and brain atrophy. Unlike the more commonly used R6/2 mouse line, R6/1 mice have fewer CAG repeats and, subsequently, a less rapid pathological decline. Compared to the R6/2 line, fewer descriptions of the progressive pathologies exhibited by R6/1 mice exist. The association between the molecular and cellular neuropathology with brain atrophy, and with the development of behavioral phenotypes remains poorly understood in many models of HD. In attempt to link these factors in the R6/1 mouse line, we have performed detailed assessments of behavior and of regional brain abnormalities determined through longitudinal, in vivo magnetic resonance imaging (MRI), as well as an end-stage, ex vivo MRI study and histological assessment. We found progressive decline in both motor and non-motor related behavioral tasks in R6/1 mice, first evident at 11 weeks of age. Regional brain volumes were generally unaffected at 9 weeks, but by 17 weeks there was significant grey matter atrophy. This age-related brain volume loss was validated using a more precise, semi-automated Tensor Based morphometry assessment. As well as these clear progressive phenotypes, mutant HTT (mHTT) protein, the hallmark of HD molecular pathology, was widely distributed throughout the R6/1 brain and was accompanied by neuronal loss. Despite these seemingly concomitant, robust pathological phenotypes, there appeared to be little correlation between the three main outcome measures: behavioral performance, MRI-detected brain atrophy and histopathology. In conclusion, R6/1 mice exhibit many features of HD, but the underlying mechanisms driving these clear behavioral disturbances and the brain volume loss, still remain unclear.

Regional vulnerability in Huntington's disease: fMRI-guided molecular analysis in patients and a mouse model of disease

Although the huntingtin gene is expressed in brain throughout life, phenotypically Huntington's disease (HD) begins only in midlife and affects specific brain regions. Here, to investigate regional vulnerability in the disease, we used functional magnetic resonance imaging (fMRI) to translationally link studies in patients with a mouse model of disease. Using fMRI, we mapped cerebral blood volume (CBV) in three groups: HD patients, symptom-free carriers of the huntingtin genetic mutation, and age-matched controls. In contrast to a region in the anterior caudate, in which dysfunction was linked to genotype independent of phenotype, a region in the posterior body of the caudate was differentially associated with disease phenotype. Guided by these observations, we harvested regions from the anterior and posterior body of the caudate in postmortem control and HD human brain tissue. Gene-expression profiling identified two molecules whose expression levels were most strongly correlated with regional vulnerability - protein phosphatase 1 regulatory subunit 7 (PPP1R7) and Wnt inhibitory factor-1 (WIF-1). To verify and potentially extend these findings, we turned to the YAC128 (C57BL/6J) HD transgenic mice. By fMRI we longitudinally mapped CBV in transgenic and wildtype (WT) mice, and over time, abnormally low fMRI signal emerged selectively in the dorsal striatum. A relatively unaffected brain region, primary somatosensory cortex (S1), was used as a control. Both dorsal striatum and S1 were harvested from transgenic and WT mice and molecular analysis confirmed that PPP1R7 deficiency was strongly correlated with the phenotype. Together, converging findings in human HD patients and this HD mouse model suggest a functional pattern of caudate vulnerability and that variation in expression levels of herein identified molecules correlate with this pattern of vulnerability.

The search for genetic mouse models of prodromal Parkinson's disease

Parkinson's disease is characterized and diagnosed by bradykinetic motor symptoms caused by the loss of dopamine neurons in the substantia nigra. The pathological and non-motor behavioral changes that occur prior to degeneration are less well characterized, although changes in gait, olfaction and cognition have been recognized in familial Parkinson's disease subjects. Gene mutations associated familial Parkinson's disease give rise to mitochondrial changes, altered energy homeostasis and intracellular trafficking deficits, and these can be modeled in transgenic mice. Here we discuss the recent finding of prodromal behavioral disturbances in a PINK1 deficient mouse that manifest prior to dopaminergic cell death and correlate to 5-HT fiber losses and mitochondrial morphological changes. We discuss the representation of the PINK1 deficient mouse and other genetic models to accurately recapitulate early Parkinson's disease. Prodromal symptoms and underlying pathology modeled in mice and cell lines from human subjects may have wide implications for earlier diagnosis. Current and emerging therapies need to be tailored to target both early cognitive and late stage motor symptoms.

Mouse models of polyglutamine diseases: review and data table. Part I

Polyglutamine (polyQ) disorders share many similarities, such as a common mutation type in unrelated human causative genes, neurological character, and certain aspects of pathogenesis, including morphological and physiological neuronal alterations. The similarities in pathogenesis have been confirmed by findings that some experimental in vivo therapy approaches are effective in multiple models of polyQ disorders. Additionally, mouse models of polyQ diseases are often highly similar between diseases with respect to behavior and the features of the disease. The common features shared by polyQ mouse models may facilitate the investigation of polyQ disorders and may help researchers explore the mechanisms of these diseases in a broader context. To provide this context and to promote the understanding of polyQ disorders, we have collected and analyzed research data about the characterization and treatment of mouse models of polyQ diseases and organized them into two complementary Excel data tables. The data table that is presented in this review (Part I) covers the behavioral, molecular, cellular, and anatomic characteristics of polyQ mice and contains the most current knowledge about polyQ mouse models. The structure of this data table is designed in such a way that it can be filtered to allow for the immediate retrieval of the data corresponding to a single mouse model or to compare the shared and unique aspects of many polyQ models. The second data table, which is presented in another publication (Part II), covers therapeutic research in mouse models by summarizing all of the therapeutic strategies employed in the treatment of polyQ disorders, phenotypes that are used to examine the effects of the therapy, and therapeutic outcomes.

Cognitive deficits in animal models of basal ganglia disorders

The two most common neurological disorders of the basal ganglia are Parkinson's disease (PD) and Huntington's disease (HD). The most overt symptoms of these diseases are motoric, reflecting the loss of the striatal medium spiny neurons in HD and ascending substantia nigra dopaminergic cells in PD. However, both disease processes induce insidious psychiatric and cognitive syndromes that can manifest well in advance of the onset of motor deficits. These early deficits provide an opportunity for prophylactic therapeutic intervention in order to retard disease progression from the earliest possible point. In order to exploit this opportunity, animal models of HD and PD are being probed for the specific cognitive deficits represented in the disease states. At the neuronal level, these deficits are typically, but not exclusively, mediated by disruption of parallel corticostriatal loops that integrate motor information with sensory and higher order, "executive" cognitive functions. Dysfunction in these systems can be probed with sensitive behavioural tests that selectively probe these cognitive functions in mouse models with focal lesions of striatal or cortical regions, or of specific neurotransmitter systems. Typically these tests were designed and validated in rats. With the advent of genetically modified mouse models of disease, validated tests provide an opportunity to screen mouse models of disease for early onset cognitive deficits. This review seeks to draw together the literature on cognitive deficits in HD and PD, to determine the extent to which these deficits are represented in the current animal models of disease, and to evaluate the viability of selecting cognitive deficits as potential therapeutic targets. This article is part of a Special Issue entitled 'Animal Models'.

Comparative analysis of pathology and behavioural phenotypes in mouse models of Huntington's disease

The longitudinal characterisation of Huntington's disease (HD) mouse lines is essential for the understanding of the differential developmental time course, nature and severity of phenotype progression over time. This overview outlines detailed behavioural, neuropathological and gene expression studies in four HD mouse lines: R6/1, YAC128, HdhQ92 and HdhQ150 and outlines their relevance to human HD. The review describes the similarities and differences between the models at the behavioural, anatomical and genetic levels of pathology and how these phenotypes interact in the development of disease in the lines. The HdhQ150 mouse demonstrates the most similarities to the functional deficits observed in human HD. The neuropathological profile with early cortical development of intense aggregate/inclusion pathology in the YAC128 mouse suggests that this line most resembles the development of inclusion pathology in the human disease. The gene expression analyses of the mouse lines find significant similarities between each of the lines and human HD, which converge as the mice age. In the YAC128 and HdhQ92 mouse lines some severe functional deficits are progressive whilst others are not, despite the concomitant ongoing development of neuropathological and gene expression changes. We suggest that the YAC128 and R6/1 lines may be more representative of the juvenile form of HD. The suitability of the different mouse models studied here for different types of pre-clinical therapeutic trials is discussed.

Profiles of motor and cognitive impairment in the transgenic rat model of Huntington's disease

The transgenic Huntington's disease (tgHD) rat strain provides a well regarded transgenic animal model of Huntington's disease, offering the prospect for a more detailed functional analysis in rats, along with neurological and therapeutic interventions, than is possible in the more widely available mouse models. In the present experiments, we compare the performance of heterozygous and homozygous tgHD rats against wildtype littermates on a range of motor and cognitive assessments in five separate cohorts of rats between 8 and 22 months of age. Male but not female heterozygous tgHD rats exhibit modest motor deficits in rotarod and staircase reaching tests, whereas most cognitive tests (including object recognition, exploration of novelty, delayed alternation, choice reaction time, and serial implicit learning tasks) revealed at best small or inconsistent deficits, in homozygous as well as heterozygous animals, up to 22 months of age. Thus, although we have observed modest but clear-cut deficits in motor phenotype, with a sex difference in line with previous reports, we have not established a robust cognitive impairment in this strain on a range of tasks sensitive to frontostriatal function, as required for testing novel (symptomatic, protective or reparative) therapeutics in a robust, valid, animal model of human Huntington's disease.