Dissociable roles of dopamine within the core and medial shell of the nucleus accumbens in memory for objects and place

Abstract

There is increasing focus on the role of the nucleus accumbens (NAc) in learning and memory, but there is little consensus as to how the core and medial shell subregions of the NAc contribute to these processes. In the current experiments, we used spontaneous object recognition to test rats with 6-hydroxydopamine lesions targeted at the core or medial shell of the NAc on a familiarity discrimination task and a location discrimination task. In the object recognition variant, control animals were able to discriminate the novel object at both 24-hr and 5-min delay. However, in the lesion groups, performance was systematically related to dopamine (DA) levels in the core but not the shell. In the location recognition task, sham-operated animals readily detected the object displacement at test. In the lesion groups, performance impairment was systematically related to DA levels in the shell but not the core. These results suggest that dopamine function within distinct subregions of the NAc plays dissociable roles in the modulation of memory for objects and place.

Figure 1. Forebrain regions dissected for postmortem neurochemical analysis. Regions of interest were dissected by pushing micropunch needles of 0.84 or 1.6 mm diameter into the posterior face of the coronal slices, as is indicated. Adapted from “The Rat Brain in Stereotaxic Coordinates, 5e,” by George Paxinos and Charles Watson, Copyright 2005. Reprinted with permission of Elsevier. Numbers indicate distance from bregma in millimeters.

Figure 2. Effect of 6-hydroxydopamine (6-OHDA) lesions to the core and medial shell nucleus accumbens (NAc) on recognition memory for objects after a 24-hr (Experiment 1A) or 5-min (Experiment 1B) retention interval. Mean discrimination ratios (plus standard error of the mean) by lesion group are shown.

Figure 3. Correlation between dopamine (DA) levels in core and medial shell of nucleus accumbens (NAc) and performance of core (plus shell) lesioned (CL) and shell-lesioned (SL) animals on a test of object recognition memory after delays of 24 hr (top half) and 5 min (bottom half). DA levels are presented as a proportion of sham levels. Test performance is the discrimination ratio. The best fit slopes correspond to the Pearson correlations. The partial correlations (controlling for DA levels in the adjacent accumbal region) are presented in the boxes.

Figure 4. Effect of 6-hydroxydopamine (6-OHDA) lesions to the core and medial shell nucleus accumbens (NAc) on novel location recognition memory (Experiment 2). Mean discrimination ratios (plus standard error of the mean) by lesion group are shown.

Figure 5. Correlation between dopamine (DA) levels in core and medial shell of nucleus accumbens (NAc) and performance of core (plus shell) lesioned (CL) and shell-lesioned (SL) animals on a test of location recognition memory. DA levels are presented as a proportion of sham levels. Test performance is the discrimination ratio. The best fit slopes correspond to the Pearson correlations. The partial correlations (controlling for DA levels in the adjacent accumbal region) are presented in the boxes.