Dissociation in effects of lesions of the nucleus accumbens core and shellon appetitive Pavlovian approach behavior and the potentiation of conditioned reinforcement and locomotor activity by D-Amphetamine

Dopamine release within the nucleus accumbens (NAcc) has been associated wi th both the rewarding and locomotor-stimulant effects of abused drugs. The functions of the NAcc core and shell were investigated in mediating ampheta mine-potentiated conditioned reinforcement and locomotion. Rats were initia lly trained to associate a neutral stimulus (Pavlovian CS) with food reinfo rcement (US). After excitotoxic lesions that selectively destroyed either t he NAcc core or shell, animals underwent additional CS-US training sessions and then were tested for the acquisition of a new instrumental response th at produced the CS acting as a conditioned reinforcer (CR). Animals were in fused intra-NAcc with D-amphetamine (0, 1, 3, 10, or 20 mu g) before each s ession. Shell lesions affected neither Pavlovian nor instrumental condition ing but completely abolished the potentiative effect of intra-NAcc amphetam ine on responding with CR. Core-lesioned animals were impaired during the P avlovian retraining sessions but showed no deficit in the acquisition of re sponding with CR. However, the selectivity in stimulant-induced potentiatio n of the CR lever was reduced, as intra-NAcc amphetamine infusions dose-dep endently increased responding on both the CR lever and a nonreinforced (con trol) lever. Shell lesions produced hypoactivity and attenuated amphetamine -induced activity. In contrast, core lesions resulted in hyperactivity and enhanced the locomotor-stimulating effect of amphetamine. These results ind icate a functional dissociation of subregions of the NAcc; the shell is a c ritical site for stimulant effects underlying the enhancement of responding with CR and locomotion after intra-NAcc injections of amphetamine, whereas the core is implicated in mechanisms underlying the expression of CS-US as sociations.