Activation of accumbens cell firing by stimuli associated with cocaine delivery during self-administration

Electrophysiological recording procedures were used in behaving rats (n = 11) to examine the responsiveness of nucleus accumbens (Acb) neurons to stimuli associated with intravenous cocaine delivery during self-administration sessions. Of 208 Acb neurons recorded during cocaine (0.33 mg/inf) self-administration sessions, 78 cells exhibited one of four types of patterned discharges (increases and/or decreases in firing rate) immediately before and/or following the cocaine-reinforced response. All phasically active Acb cells were examined in test sessions consisting of "probe" trials (18-20 per session) during which either the drug only was delivered (0.33 mg/inf cocaine, 6 sec) or the stimulus only (tone-houselight, 20 sec) was randomly presented by the computer, interspersed between reinforced lever press responses. Results show that Acb cells that exhibit postresponse changes in firing rate within seconds of the reinforced response appear to be controlled, at least in part, by the stimulus paired with cocaine delivery during the self-administration session and not via a direct pharmacological action of cocaine. In contrast, neurons displaying exclusively preresponse activity do not appear to be influenced by presentation of the drug-associated stimulus or by a direct pharmacological action of the cocaine, but may be related to initiation of the operant response. These findings are discussed with respect to the role of the Acb in mediating conditioned responses associated with drug reinforcement.