Rationale: The neurobiological mechanisms underlying the persistence of drug craving in detoxified addicts are still poorly understood.
Objective: The present study was designed to evaluate dopaminergic mechanisms in drug-seeking behaviour following long-term (>3 weeks) extinction of IV drug self-administration in rats.
Methods: To that end, we studied the effects of direct and indirect dopamine (DA) agonists on reinstatement of previously extinguished responding for heroin (50 microg/kg per injection; 14-15 daily 3-h sessions) and cocaine (500 microg/kg per injection; 10-11 daily 2-h sessions).
Results: In animals with a cocaine history, priming with cocaine, the selective DA reuptake inhibitor GBR-12909 and the DA D2 receptor agonist quinpirole resulted in robust and selective reinstatement of non-reinforced nose poking behaviour in the previously drug-paired hole. In contrast, the D1 agonist SKF-82958 failed to reinstate responding and the non-selective DA agonist apomorphine even suppressed responding in these animals. In heroin-trained rats, heroin and GBR-12909 strongly reinstated responding, whereas all direct DA agonists were ineffective. Again, the two highest doses of apomorphine decreased responding in these animals. In a parallel study, the ability of DA ligands to express behavioural sensitization in animals pretreated with amphetamine or morphine was evaluated. Interestingly, all agonists that reinstated responding in the present study caused expression of locomotor sensitization and vice versa.
Conclusions: The differences between direct and indirect agonists indicate a clear, but complex, involvement of DA in drug-seeking behaviour long after detoxification. Moreover, the results show an important role of D2 receptor activation in the persistence of cocaine- but not heroin-seeking behaviour. Finally, the results from both studies suggest a relationship between drug-induced reinstatement and drug hyperresponsiveness in long-term abstinent rats.