Effects of reward anticipation, reward presentation, and spatial parameters on the firing of single neurons recorded in the subiculum and nucleus accumbens of freely moving rats

The subiculum is the major output of the hippocampal formation (involved in spatial processing). Subicular afferents innervate the nucleus accumbens, which is thought to integrate limbic reward information with motor output. Rats were chronically implanted with extra-cellular recording electrodes ai med at both structures to investigate the functional relationship between t hem. Animals were then trained on a spatial task in which they searched for random locations where they would receive rewarding medial forebrain bundl e stimulation. At random times a cue tone was sounded, indicating that the reward location was in the center of the environment. Rats quickly learned to run to the center upon hearing the tone in order to receive a reward. Si multaneously recorded groups of up to eight subicular and accumbens neurons were found to display alterations in firing rate after rewarding medial fo rebrain bundle stimulation. Moreover, neurons in both subiculum and accumbe ns displayed alterations in firing rate prior to arrival at the center duri ng cued runs, i.e. they anticipated predictable rewards. Subicular and accu mbens firing was also correlated with spatial location. However, neurons in accumbens were more likely to respond to task events, and these responses were more varied, than those seen in subiculum. Thus, while convergence of spatial and reward information occurs at the level of single cells in both subiculum and nucleus accumbens, these structures also display functional l ocalization. (C) 2000 Elsevier Science B.V. All rights reserved.