Studies indicate that nucleus accumbens (NAcc) dopamine neurotransmission is involved in the reinforcing and direct effects of cocaine. The present study was initiated to explore further the relationship of NAcc extracellular dopamine concentrations ([DA]e) and cocaine self-administration using a yoked littermate design. In the first experiment, one rat from each litter was trained to self-administer cocaine i.v. (SA: 0.33 mg/inf) under a fixed ratio 2 schedule, while a second rat received simultaneous infusions of cocaine yoked to the infusions of the SA (YC). NAcc [DA]e and cocaine concentrations ([COC]) were assessed during the test sessions using in vivo microdialysis combined with microbore HPLC procedures. [DA]e and [COC] were significantly elevated in the SA and YC groups during the self-administration session; however, [DA]e were greater in the SA group compared to the YC group in the first hour of the session, even though [COC] were not significantly different. On the following day, the rats previously allowed to self-administer cocaine were administered response-independent cocaine infusions yoked to the infusion pattern from the previous day. [DA]e were significantly elevated above baseline levels during the session but were significantly less than concentrations obtained when cocaine was self-administered by these subjects. [COC] during the sessions were not significantly different between the two days. Baseline [DA]e were not significantly different between the SA and YC groups or between Day 1 and Day 2. Furthermore, there was no significant difference in the in vitro probe recovery between one and two days following probe implantation. These results suggest that the context in which cocaine was administered significantly altered the neurochemical response to equivalent brain concentrations of cocaine. NAcc [DA]e was significantly increased when the delivery of cocaine infusions was contingent on the behavior of the rat, indicative of a role in the neural processes underlying cocaine reinforcement.
The concurrent use of cocaine and opiate combinations (speedball) has increased since the 1970s and now represents a growing subset of intravenous drug abusers. An isobolographic analysis was applied to the ascending limb of the dose-effect curves for rat selfadministration of cocaine, heroin, and their combination to determine the nature of the interaction. The addition of heroin to cocaine shifted the dose-effect curve for self-administration to the left, and the modulation in reinforcing efficacy of the combination of cocaine and heroin was found to be additive. A second experiment used microdialysis to determine the effects of this drug combination on nucleus accumbens (NAc) extracellular levels of dopamine ([DA] e ) in rats self-administering low doses of cocaine, heroin, or cocaine/ heroin combinations. These doses of cocaine and cocaine/heroin combinations significantly increased NAc [DA] e , while heroin alone did not. The ratio of the % baseline of [DA] e (or the dialysate concentrations of DA) to cocaine in the dialysate was higher during selfadministration of cocaine/heroin combinations than with cocaine alone. These data indicate that although the interaction between cocaine and heroin in maintaining self-administration is additive, a potentiation of NAc dopaminergic neurotransmission is present, suggesting that NAc [DA] e may not be a direct measure of reinforcing efficacy and/or it is not central to the mediation of the selfadministration of this drug combination.
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