Dopamine D1- and D2-like receptor mechanisms in relapse to cocaine-seeking behavior: effects of selective antagonists and agonists.

Dopaminergic mechanisms are thought to be critical in mediating relapse to cocaine-seeking behavior. This study examined the different roles of D1- and D2-like receptor mechanisms in the relapse process. Squirrel monkeys were given extended histories of i. v. cocaine self-administration under conditions in which responding was maintained jointly by response-contingent cocaine injections and a cocaine-paired visual stimulus (second-order schedule). Responding was then extinguished by substituting saline for cocaine injections and omitting presentations of the cocaine-paired stimulus. Subsequently, noncontingent priming injections of cocaine combined with restoration of the cocaine-paired stimulus induced dose-dependent reinstatement of drug-seeking behavior, with response rates approaching those maintained by active cocaine self-administration. The priming effects of cocaine were attenuated by several D1- and D2-like receptor antagonists and low efficacy agonists but not by the D3-preferring antagonists UH 232 and AJ-76. The priming effects of cocaine were mimicked by the D2-like receptor agonists R(-)-propylnorapomorphine hydrochloride (NPA) and quinpirole, less consistently by 7-OH-DPAT, and not by the D1-like receptor agonists SKF-81297 and SKF-82958, the D3-preferring agonist PD-128,907, or any low efficacy agonist. Cotreatment with NPA, PD-128,907, and 7-OH-DPAT did not alter reinstatement of drug-seeking behavior induced by a maximally effective priming dose of cocaine, whereas cotreatment with D1-like receptor agonists attenuated the priming effects of cocaine. The results suggest that D1- and D2-like receptors play fundamentally different roles in the relapse process. Although stimulation of D2-like, but probably not D3-like, receptors appears necessary for induction of relapse, either stimulation or blockade of D1-like receptors appears to be inhibitory with respect to relapse.