Huntington’s disease is characterised by a triad of motor, cognitive, and psychiatric impairments, as well as unintended weight loss. Although much of the research has focused on cognitive, motor and psychiatric symptoms, the extent of peripheral pathology and the relationship between these factors, and the core symptoms of Huntington’s disease, are relatively unknown. Gut microbiota are key modulators of communication between the brain and gut, and alterations in microbiota composition (dysbiosis) can negatively affect cognition, behaviour and affective function, and may be implicated in disease progression. Furthermore, gut dysbiosis was recently reported in Huntington’s disease transgenic mice. Our main objective was to characterise the gut microbiome in people with Huntington’s disease and determine whether the composition of gut microbiota are significantly related to clinical indicators of disease progression. We compared 42 Huntington’s disease gene expansion carriers (HDGECs), including 19 people who were diagnosed with Huntington’s disease (Total Functional Capacity > 6) and 23 in the premanifest stage, with 36 age and gender-matched healthy controls. Participants were characterised clinically using a battery of cognitive tests and using results from 16S V3-V4 rRNA sequencing of faecal samples to characterise the gut microbiome. For gut microbiome measures, we found significant differences in the microbial communities (beta diversity) based on unweighted UniFrac distance (p = 0.001), as well as significantly lower alpha diversity (species richness and evenness) between our combined HDGEC group and healthy controls (p = 0.001). We also found major shifts in the microbial community structure at Phylum and Family levels, and identified functional pathways and enzymes affected in our HDGEC group. Within the HDGEC group, we also discovered associations between gut bacteria, cognitive performance and clinical outcomes. Overall, our findings suggest an altered gut microbiome in HDGECs. These results highlight the importance of gut biomarkers and raise interesting questions regarding the role of the gut in Huntington’s disease, and whether it may be a potential target for future therapeutic intervention.

Spatial cognition is fundamental for survival in the topographically complex environments inhabited by humans and other animals. The hippocampus, which has a central role in spatial cognition, is characterized by high concentration of serotonin (5-hydroxytryptamine; 5-HT) receptor binding sites, particularly of the 1A receptor (5-HT1A) subtype. This review highlights converging evidence for the role of hippocampal 5-HT1A receptors in spatial learning and memory. We consider studies showing that activation or blockade of the 5-HT1A receptors using agonists or antagonists, respectively, lead to changes in spatial learning and memory. For example, pharmacological manipulation to induce 5-HT release, or to block 5-HT uptake, have indicated that increased extracellular 5-HT concentrations maintain or improve memory performance. In contrast, reduced levels of 5-HT have been shown to impair spatial memory. Furthermore, the lack of 5-HT1A receptor subtype in single gene knockout mice is specifically associated with spatial memory impairments. These findings, along with evidence from recent cognitive imaging studies using positron emission tomography (PET) with 5-HT1A receptor ligands, and studies of individual genetic variance in 5-HT1A receptor availability, strongly suggests that 5-HT, mediated by the 5-HT1A receptor subtype, plays a key role in spatial learning and memory.

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