Dissociable Cost and Benefit Encoding of Future Rewards by Mesolimbic Dopamine
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Dopamine is a neural substrate implicated in the complex computation of weighing the costs and benefits of future actions. Midbrain dopamine neurons encode fundamental economic parameters pertaining to predicted rewards in their subsecond firing pattern and innervate areas that have been implicated in economic decision-making such as the nucleus accumbens. Disruptions of dopamine in the nucleus accumbens core (NAcc) diminish animals’ ability to respond to cues and overcome effortful response costs. However, electrolytic and chemical depletions of dopamine disrupt more than just the subsecond dopamine signal shown to encode economic parameters. Thus, to deconstruct the nature of the signal transmitted by phasic dopamine to the NAcc, new technology to detect dopamine at a subsecond time scale viable over months needs be developed. With this new technology we may characterized how subsecond dopamine accounts for behaviors by determining how it reacts to changes in anticipated costs and benefits. Part I of this thesis will discuss dopamine in general. I will discuss dopamine as a neurochemical, anatomy of the dopaminergic systems involved with decision making, and dopamine neuron firing patterns. I will briefly review some theories of dopamine’s role in behavior, with attention to dopamine’s role in decision making. In Part II, I discuss the development and characterization of chronically implantable fast-scan cyclic voltammetry (FSCV) microsensors. Though in vitro validation of these electrodes was performed, I will primarily describe the in vivo validation of these electrodes in this thesis. With the advent of these electrodes, we extend the ability to detect subsecond dopamine transmission in awake, behaving animals from a handful of recordings to a multitude of recordings across months. In Part III, we examine the valuation signal transmitted by phasic dopamine in the NAcc. Animals implanted with chronic FSCV microsensors were asked to distinguish cues that predicted either differing amounts of food reward or differing efforts (amounts of lever presses). We found that dopamine release in rat nucleus accumbens encodes anticipated benefits, but not effort-based response costs unless they are atypically low. This neural separation of costs and benefits indicates that mesolimbic dopamine scales with the value of pending rewards, but does not encode the net utility of the action to obtain them.