Session-long modulations of accumbal firing during sucrose-reinforced operant behavior

The nucleus accumbens is involved in the selection and expression of motivated behaviors. Attempts to understand how activity of single neurons in the accumbens relates to behavior have largely concentrated on brief modulations in accumbal firing that occur in the seconds around events during operant sessions. However, a small number of studies have reported modulations that last the entire duration of a behavioral session. In all of these reports, the operant session was a drug self-administration session. The present study tested the hypothesis that session-long modulations, like phasic firing patterns, are components of normal accumbal activity during periods of instrumental behavior. Eight rats were chronically implanted (unilaterally) with microwire arrays in the nucleus accumbens, and trained to lever press on a Fixed-Ratio 1 schedule of sucrose reinforcement. Activity of 51 single units was recorded, and both session-long increases (n = 14) and session-long decreases (n = 13) were observed. These findings show that session-long modulations are a normal component of the response of accumbal neurons during periods of operant behavior. Moreover, although session-long modulations during cocaine self-administration sessions might reflect pharmacological actions, aspects of the modulations might additionally or alternatively correspond to afferent-driven responses. Further characterization of the firing patterns may elucidate novel mechanisms that mediate accumbal contributions to behavior.