Animals.

All animal procedures were approved by the University of Toronto Animal Care Committee in accordance with the Canadian Council for Animal Care guidelines. All efforts were made to minimize animal suffering and to reduce the number of animals used. Two HD mouse models were used in this study: transgenic R6/2 mice containing the mutated Htt gene expressing exon 1 of the human Htt gene carrying ∼120 ± 5 CAG repeat expansions (31) and YAC128 mice containing full-length human Htt with 128 CAG repeats (37). Both males and females were used for biochemistry, imaging, electrophysiology, and behavioral experiments.