Anhedonia has long been recognized as a core symptom of major depressive disorder (MDD). Preclinical animal models of effort-based decision-making provide a strong basis for hypotheses regarding the neural and neuropharmacological substrates of anhedonia. Such models indicate the importance of mesolimbic dopamine projections to the nucleus accumbens in overcoming response costs in order to obtain rewards, and the importance of the anterior cingulate in evaluating response costs. On the surface, the behaviors displayed by dopamine depleted animals appear similar to the motivational deficits that characterize anhedonic MDD. However, it has been difficult to integrate these preclinical models with clinical studies of MDD, because of: 1) a failure to distinguish between deficits in hedonics (pleasure) and motivation in the clinical literature, 2) the lack of objective experimental measures of motivational deficits in humans, and 3) the dearth of studies specifically targeting patients with motivational or hedonic deficits. To address this issue, we have developed and validated a novel behavioral paradigm for exploration of motivational elements of anhedonia in humans. The Effort Expenditure for Rewards Task (EEfRT) measures individual differences in the willingness to expend effort for a given level of monetary reward. The EEfRT was carefully adapted from effort-based decision-making paradigms used in animal studies of the mesolimbic dopamine system and anterior cingulate. In humans, peformance on the EEfRT has been shown to predict individual differences in trait anhedonia and dopamine function in healthy controls. The present proposal seeks to:1) use event-related fMRI to elucidate the neural correlates of effort-based decisions in healthy controls (n =24), and test hypotheses regarding the specific roles of the ventral striatum and anterior cingulate in predicting effort-based decisions;(2) validate the sensitivity of the EEfRT for detecting behavioral deficits in individuals with MDD and self-reported motivational deficits (n=24);and (3) test the hypothesis that patients with MDD and self-reported motivational deficits will show abnormal ventral striatal and anterior cingulate activations during performance of the EEfRT. If successful, the proposed research will significantly enhance our ability to identify specific behavioral deficits of anhedonic symptoms in MDD, as well as their neurobiological substrates, and provide the basis for future studies on the utility of the EEfRT and fMRI to serve as a behavioral or biomarker for clinical and clinical outcome research.
MDD is predicted to become the second leading cause of death and disability in the United States by the year 2020, and anhedonia remains among the most difficult symptoms to treat in MDD. This proposal provides a unique translation of preclinical models to attempt to both understand the neural substrates of anhedonia and to develop an objective biomarker that could be used to develop treatments particularly tailored for addressing motivational deficits in MDD.
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