ENHANCED ACQUISITION OF DISCRIMINATIVE APPROACH FOLLOWING INTRAAMYGDALA D-AMPHETAMINE

This study examined the role of the mesoamygdaloid dopamine projection in stimulus-reward learning. Bilateral post-session intra-amygdala mi croinjections of d-amphetamine were carried out in rats during trainin g in a discriminative approach task known to be sensitive to experimen tal manipulations of the amygdala. The experiment consisted of two pha ses: discriminative approach training, and a subsequent assessment of instrumental conditioned reward efficacy. During discriminative approa ch training, subjects were trained to associate a neutral stimulus wit h 10% w/v sucrose reward. Each trial consisted of a 1-s light stimulus followed by a 5-s presentation of the sucrose reward. Approach behavi our into the recess housing sucrose reward was measured during each tr ial. Inappropriate approach behaviour (approach outside of the trial p eriods) was punished by delaying the next trial. Intra-amygdala cl-amp hetamine (10 mu g/side) enhanced the rate of acquisition of discrimina tive approach behaviour. This effect was most evident early during tra ining (sessions 2-4) and by the tenth session both groups had reached similar asymptotic performance. Horizontal and vertical activity incre ased slightly across sessions, but there was no indication of a differ ential effect of d-amphetamine. Thus, intraamygdala microinjections of d-amphetamine enhanced selectively the acquisition of the stimulus-re ward association. During a subsequent test of instrumental conditioned reward, presentation of the conditioned light stimulus was made conti ngent upon performance of a novel lever-pressing response (probability 0.5). Responding on a second, control lever was without programmed co nsequences. Sucrose reward was not available at any point, and subject s were tested drug-free. In both groups the conditioned stimulus was f ound to possess significant conditioned rewarding efficacy. Extraneous behaviour was increased in the cl-amphetamine group but the rewarding properties of the conditioned stimulus were unaltered. These findings demonstrate that the mesoamygdaloid dopamine projection modulates the acquisition of a stimulus-reward association, but is apparently witho ut subsequent effect on the rewarding efficacy of a conditioned stimul us.