Enhanced acquisition of discriminative approach following intra-amygdala d-amphetamine

This study examined the role of the mesoamygdaloid dopamine projection in stimulus-reward learning. Bilateral post-session intra-amygdala microinjections of d-amphetamine were carried out in rats during training in a discriminative approach task known to be sensitive to experimental manipulations of the amygdala. The experiment consisted of two phases: discriminative approach training, and a subsequent assessment of instrumental conditioned reward efficacy. During discriminative approach training, subjects were trained to associate a neutral stimulus with 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 behaviour into the recess housing sucrose reward was measured during each trial. Inappropriate approach behaviour (approach outside of the trial periods) was punished by delaying the next trial. Intra-amygdala d-amphetamine (10 μg/side) enhanced the rate of acquisition of discriminative approach behaviour. This effect was most evident early during training (sessions 2–4) and by the tenth session both groups had reached similar asymptotic performance. Horizontal and vertical activity increased slightly across sessions, but there was no indication of a differential effect of d-amphetamine. Thus, intra-amygdala microinjections of d-amphetamine enhanced selectively the acquisition of the stimulus-reward association. During a subsequent test of instrumental conditioned reward, presentation of the conditioned light stimulus was made contingent upon performance of a novel lever-pressing response (probability 0.5). Responding on a second, control lever was without programmed consequences. Sucrose reward was not available at any point, and subjects were tested drug-free. In both groups the conditioned stimulus was found to possess significant conditioned rewarding efficacy. Extraneous behaviour was increased in the d-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 without subsequent effect on the rewarding efficacy of a conditioned stimulus. Received: 24 October 1996/Final version: 28 February 1997