The neurobiological study of reward was launched by the discovery of intracranial self-stimulation (ICSS). Subsequent investigation of this phenomenon provided the initial link between reward-seeking behavior and dopaminergic neurotransmission. We re-evaluated this relationship by psychophysical, pharmacological, optogenetic, and computational means. In rats working for direct, optical activation of midbrain dopamine neurons, we varied the strength and opportunity cost of the stimulation and measured time allocation, the proportion of trial time devoted to reward pursuit. We found that the dependence of time allocation on the strength and cost of stimulation was similar formally to that observed when electrical stimulation of the medial forebrain bundle served as the reward. When the stimulation is strong and cheap, the rats devote almost all their time to reward pursuit; time allocation falls off as stimulation strength is decreased and/or its opportunity cost is increased. A 3D plot of time allocation versus stimulation strength and cost produces a surface resembling the corner of a plateau (the “reward mountain”). We show that dopamine-transporter blockade shifts the mountain along both the strength and cost axes in rats working for optical activation of midbrain dopamine neurons. In contrast, the same drug shifted the mountain uniquely along the opportunity-cost axis when rats worked for electrical MFB stimulation in a prior study. Dopamine neurons are an obligatory stage in the dominant model of ICSS, which positions them at a key nexus in the final common path for reward seeking. This model fails to provide a cogent account for the differential effect of dopamine transporter blockade on the reward mountain. Instead, we propose that midbrain dopamine neurons and neurons with non-dopaminergic, MFB axons constitute parallel limbs of brain-reward circuitry that ultimately converge on the final-common path for the evaluation and pursuit of rewards.
Author summary
To succeed in the struggle for survival and reproductive success, animals must make wise choices about which goals to pursue and how much to pay to attain them. How does the brain make such decisions and adjust behaviour accordingly? An animal model that has long served to address this question entails delivery of rewarding brain stimulation. When the probe is positioned appropriately in the brain, rats will work indefatigably to trigger such stimulation. Dopamine neurons play a crucial role in this phenomenon. The dominant model of the brain circuitry responsible for the reward-seeking behavior treats these cells as a gateway through which the reward-generating brain signals must pass. Here, we challenge this idea on the basis of an experiment in which the dopamine neurons were activated selectively and directly. Mathematical modeling of the results argues for a new view of the structure of brain reward circuitry. On this view, the pathway(s) in which the dopamine neurons are embedded is one of a set of parallel channels that process reward s...
