How we weigh up the costs and benefits of a potential outcome to guide choice is paramount to our everyday functioning. In making these economic decisions one must consider costs such as physical effort, the delay before the outcome is presented, the certainty of the outcome and possible aversive consequences, as well as social and financial costs. Dopamine plays an important role in dealing with these costs: antagonism of dopamine is sufficient to lower the physical effort rats will make, or the amount of time they will wait to obtain greater rewards. Dopamine is also implicated in risk-taking. Subsecond activity in dopamine neurons encodes the outcome value within a range of available rewards, and these rapid signals can promote decisions to engage in behaviors to obtain rewards. This fast transmission may provide a threshold signal for the costs an animal should endure to obtain a reward. Since they encode the expected reward amplitude, this should drive animals to tolerate a higher cost for better (expected) outcomes. However, during normal aging, there are several changes in dopaminergic systems, including a reduction in the expression of striatal dopamine receptors and in the biological capacity for striatal dopamine release. This raises a number of questions: 1) Is the diminished capacity for dopamine release reflected in the amount of dopamine released during economic decision making? 2) Do these age-related effects contribute to a change in economic decision making? To address these questions, adult (6-month-old) and aged (24-month-old) rats will be studied while they perform decision making to """"""""purchase"""""""" food options by lever pressing. First potential behavioral differences in economic decision making between age groups will be probed. Quantitative differences in the cost the rats will forgo to obtain a larger reward offered concurrently with a smaller but cheaper reward will be tested. In one set of experiments, the currency of the imposed cost will be physical effort, where animals will be required to make a higher number of lever presses to receive the larger reward. In another set of experiments, risk-taking behavior will be tested by allowing rats to choose between smaller rewards and larger, but uncertain, rewards. Next, dopamine release will be measured in the ventral striatum (nucleus accumbens) of adult or aged rats during economic decision making. Dopamine will be monitored with subsecond resolution using fast-scan cyclic voltammetry. Using a single-choice paradigm, the way in which dopamine encodes reward value when reward is available for effort or probabilistic costs will be tested. Finally, the capacity of dopamine to modulate economic decision making in adult and aged rats will be tested by electrically stimulating dopamine neurons at the time within the task that an endogenous dopamine signal is normally generated. This evoked dopamine release will boost the endogenous signal and thus test the capacity of dopamine to overcome effortful or probabilistic costs. Collectively these experiments will provide unique insight into age-related changes in economic choice, dopamine transmission during decision making and their interaction. Cognitive, emotional and physical capabilities change during aging, in part by age-related changes in brain motivational systems. How these changes impact economic choices is unknown. The proposed work will use behavioral and neurochemical methods to study age-related changes in economic decision making in rats.
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| Wanat, Matthew J; Kuhnen, Camelia M; Phillips, Paul E M (2010) Delays conferred by escalating costs modulate dopamine release to rewards but not their predictors. J Neurosci 30:12020-7 |
