Dysregulation of the Basal Ganglia Indirect Pathway in Early Symptomatic Q175 Huntington’s Disease Mice

The debilitating psychomotor symptoms of Huntington’s disease (HD) are linked partly to degeneration of the basal ganglia indirect pathway. At early symptomatic stages, before major cell loss, indirect pathway neurons exhibit numerous cellular and synaptic changes in HD and its models. However, the impact of these alterations on circuit activity remains poorly understood. To address this gap, optogenetic- and reporter-guided electrophysiological interrogation was used in early symptomatic male and female Q175 HD mice. D2 dopamine receptor-expressing striatal projection neurons (D2-SPNs) were hypoactive during synchronous cortical slow-wave activity, consistent with known reductions in dendritic excitability and cortical input strength. Downstream prototypic parvalbumin-expressing external globus pallidus (PV⁺ GPe) neurons discharged at 2-3 times their normal rate, even during periods of D2-SPN inactivity, arguing that defective striatopallidal inhibition was not the only cause of their hyperactivity. Indeed, PV⁺ GPe neurons also exhibited abnormally elevated autonomous firing ex vivo. Optogenetic inhibition of PV⁺ GPe neurons in vivo partially and fully ameliorated the abnormal hypoactivity of postsynaptic subthalamic nucleus (STN) and putative PV^- GPe neurons, respectively. In contrast to STN neurons whose autonomous firing is impaired in HD mice, putative PV^- GPe neuron activity was unaffected ex vivo, implying that excessive inhibition was responsible for their hypoactivity in vivo. Together with previous studies, these data demonstrate that (1) indirect pathway nuclei are dysregulated in Q175 mice through changes in presynaptic activity and/or intrinsic cellular and synaptic properties; and (2) prototypic PV⁺ GPe neuron hyperactivity and excessive target inhibition are prominent features of early HD pathophysiology.