Nucleus accumbens neurons in the rat exhibit differential activity to conditioned reinforcers and primary reinforcers within a second‐order schedule of saccharin reinforcement

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Abstract
The nucleus accumbens has been associated with processing information related to primary reinforcement and reward. Most neurophysiological studies report that nucleus accumbens neurons are phasically excited in response to the onsets of salient events during the seeking of reinforcement and to the delivery of primary reinforcers. However, a minority of studies report inhibition during primary reinforcement. We recorded from 65 neurons in the nucleus accumbens whilst thirsty rats performed under a second-order schedule of saccharin reinforcement. This allowed us to analyse neural activity and behaviour during reinforcer-seeking in the presence of conditioned reinforcers (second-order stimuli, also called 'conditioned stimuli'), and during primary reinforcer consumption. Specifically, we sought to examine the valence of potential neural responses to primary reinforcement, to compare these responses to second-order stimulus-evoked responses, and to determine whether responses were differential to second-order stimuli presented at different time points within the schedule. Fifty out of 65 neurons we sampled responded to the second-order stimulus and/or consumption of the primary reinforcer. Most neurons in our sample exhibited excitation following the second-order stimulus and inhibition to the primary reinforcer, a pattern also present over the average response of the neural population. However, there was no systematic variation in neural responses evoked by second-order stimuli presented at different temporal proximities to primary reinforcement. Our results provide evidence that partially overlapping mechanisms within the nucleus accumbens differentially process conditioned reinforcers and primary reinforcers.