Corticostriatal synaptic function in mouse models of Huntington's disease: early effects of huntingtin repeat length and protein load

Huntington's disease (HD) is an autosomal dominant, late onset, neurodegenerative disease characterized by motor deficits and dementia that is caused by expansion of a CAG repeat in the HD gene. Clinical manifestations result from selective neuronal degeneration of predominantly GABAergic striatal medium-sized spiny neurons (MSNs). A growing number of studies demonstrate that personality, mood and cognitive disturbances are some of the earliest signs of HD and may reflect synaptic dysfunction prior to neuronal loss. Previous studies in striatal MSNs demonstrated early alterations in NMDA-type glutamate receptor currents in several HD mouse models, as well as evidence for presynaptic dysfunction prior to disease manifestations in the R6/2 HD fragment mouse model. We have compared corticostriatal synaptic function in full-length, human HD gene-carrying YAC transgenic mice expressing a non-pathogenic CAG repeat (YAC18; control) with three increasingly severe variants of pathogenic HD gene-expressing mice (YAC72 and two different lines of YAC128), at ages that precede any detectable disease phenotype. We report presynaptic dysfunction and a propensity towards synaptic depression in YAC72 and YAC128 compared to YAC18 mice, and, in the most severe model, we also observed altered AMPA receptor function. When normalized to evoked AMPAR currents, postsynaptic NMDAR currents are augmented in all three pathogenic HD YAC variants. These findings demonstrate multiple perturbations to corticostriatal synaptic function in HD mice, furthering our understanding of the early effects of the HD mutation that may contribute to cognitive dysfunction, mood disorders and later development of more serious dysfunction. Furthermore, this study provides a set of neurophysiological sequelae against which to test and compare other mouse models and potential therapies in HD.