The encoding of cocaine vs. natural rewards in the striatum of nonhuman primates: categories with different activations.

The behavioral and motivational changes that result from use of abused substances depend upon activation of neuronal populations in the reward centers of the brain, located primarily in the corpus striatum in primates. To gain insight into the cellular mechanisms through which abused drugs reinforce behavior in the primate brain, changes in firing of neurons in the ventral (VStr, nucleus accumbens) and dorsal (DStr, caudate-putamen) striatum to "natural" (juice) vs. drug (i.v. cocaine) rewards were examined in four rhesus monkeys performing a visual Go-Nogo decision task. Task-related striatal neurons increased firing to one or more of the specific events that occurred within a trial represented by (1) Target stimuli (Go trials) or (2) Nogotarget stimuli (Nogo trials), and (3) Reward delivery for correct performance. These three cell populations were further subdivided into categories that reflected firing exclusively on one or the other type of signaled reward (juice or cocaine) trial (20%-30% of all cells), or, a second subpopulation that fired on both (cocaine and juice) types of rewarded trial (50%). Results show that neurons in the primate striatum encoded cocaine-rewarded trials similar to juice-rewarded trials, except for (1) increased firing on cocaine-rewarded trials, (2) prolonged activation during delivery of i.v. cocaine infusion, and (3) differential firing in ventral (VStr cells) vs. dorsal (DStr cells) striatum cocaine-rewarded trials. Reciprocal activations of antithetic subpopulations of cells during different temporal intervals within the same trial suggest a functional interaction between processes that encode drug and natural rewards in the primate brain.