Huntington's disease (HD) is a heritable neurodegenerative disorder caused by expansion of CAG (glutamine) repeats in the HTT gene. A prodromal stage characterized by psychiatric disturbances normally precedes primary motor symptoms and suppressed motivation represents one of the earliest and most common psychiatric symptoms. Although dopamine in the nucleus accumbens (NAc) critically regulates motivation and altered dopamine signaling is implicated in HD, the nature of dopaminergic deficits and contribution to symptoms in HD is poorly understood. We therefore tested whether altered NAc dopamine release accompanies motivational deficits in the Q175 knock-in HD mouse model. Q175 mice express a CAG expansion of the human mutant huntingtin allele in the native mouse genome and gradually manifest symptoms late in life, closely mimicking the genotypic context and disease progression in human HD. Sub-second extracellular dopamine release dynamics were monitored using fast-scan cyclic voltammetry, whereas motivation was assessed using a progressive ratio reinforcement schedule. As the response ratio (lever presses per reward) escalated, Q175 mice exerted less effort to earn fewer rewards versus wild-type (WT). Moreover, dopamine released at reward delivery dynamically encoded increasing reward cost in WT but not Q175 mice. Deficits were specific to situations of high effortful demand as no difference was observed in locomotion, free feeding, hedonic processing, or reward seeking when the response requirement was low. This compromised dopaminergic encoding of reward delivery coincident with suppressed motivation to work for reward in Q175 mice provides novel, neurobiological insight into an established and clinically relevant endophenotype of prodromal HD.
Key words: accumbens; dopamine; Huntington's; motivation; progressive ratio; voltammetry
IntroductionPsychiatric deficits in Huntington's disease (HD) appear early during a prodromal stage that precedes motor deficits (eg, chorea) by a decade or more (Paulsen et al., 2008;Epping et al., 2016). These nonmotor symptoms severely impact daily functioning and quality of life for HD patients and caregivers (Hamilton et al., 2003;Papoutsi et al., 2014), but are poorly understood and insufReceived Jan. 13, 2016; revised March 10, 2016; accepted March 15, 2016. Author contributions: D.P.C. and J.F.C. designed research; D.P.C., H.M.D., N.E.Z., and I.G. performed research; D.P.C. analyzed data; D.P.C., N.E.Z., and J.F.C. wrote the paper.This work was supported by a research grant from CHDI to J.F.C. The authors declare no competing financial interests.
Significance StatementPsychiatric impairments in Huntington's disease (HD) typically manifest early in disease progression, before motor deficits. However, the neurobiological factors contributing to psychiatric symptoms are poorly understood. We used a mouse HD model and assessed whether impaired dopamine release in the nucleus accumbens (NAc), a brain region critical to goal-directed behaviors, accompanies motivational defici...