Intranucleus Accumbens Amphetamine Infusions Enhance Responding Maintained by a Stimulus Complex Paired With Oral Ethanol Self-Administration

Abstract

Six male Long–Evans rats were trained to self-administer 10% ethanol (v/v) during 30 min operant sessions. A licking response on an empty drinking tube resulted in the presentation of reinforcement from an automatic dipper. During the initiation of ethanol self-administration, a tone–light stimulus complex was paired with all ethanol presentations. When 10% ethanol maintained responding, guide cannulae aimed at the nucleus accumbens (NAcc) were implanted into the brain. The ability of the paired stimulus complex to reinforce a new operant response (i.e., a lever press) was then examined. To test for the development of the new response, responding on one lever resulted in presentation of only the paired tone–light stimulus complex (contingency-associated lever) while responding on an alternate lever had no programmed consequences (no contingency-associated lever). Prior to some new response sessions, amphetamine (5–20 μg/μl) was infused into the NAcc to examine the influence of dopamine on responding maintained by the stimulus complex. Ethanol intake during the sessions prior to new response testing averaged 0.49 ± 0.07 g/g. During new response sessions no significant differences in lever pressure during no-drug conditions (control, sham, injection or vehicle injection) were observed between the contingency-associated and no contingency-associated levers. Intra-NAcc infusion of amphetamine (5–20 μg/μl) resulted in significant increases in lever pressing only on the contingency-associated lever. These data suggest that increasing NAcc dopamine levels with amphetamine enhanced the ability of the stimulus complex to function as a reinforcer. Further studies examining the ability of potentially more salient stimuli (i.e., taste of ethanol) to function as conditioned reinforcers associated with ethanol self-administration are warranted due to the apparent inability of the paired tone–light stimulus complex to function as a reinforcer without amphetamine-induced activation of the NAcc.