Abstract

A place preference conditioning procedure was used to examine the influence of kappa-opioid receptor ligands upon the development of sensitization to the conditioned rewarding effects of cocaine. Previous exposure to cocaine (10-20 mg/kg; i.p.; days 1-5) resulted in an enhancement of the conditioned rewarding effects of this agent, e.g., sensitization. Thus, doses of cocaine (5.0-10.0 mg/kg; i.p.) that failed to produce place preferences in control rats produced significant place preferences in cocaine-experienced animals. In animals that had received the kappa-agonist U50,488H (5.0 mg/kg; s.c.) in combination with the repeated cocaine treatment regimen, no enhancement of cocaine-induced place conditioning was seen. Similarly, the kappa-agonist U69593 administered on days 1 to 5 or only on days 3 to 5 of the cocaine treatment regimen prevented the enhanced response to cocaine. This effect occurred after either systemic (0.04-0.16 mg/kg; s.c.) or intracerebroventricular (1.0 mg) treatment and was abolished by the kappa-opioid receptor antagonist, nor-binaltorphimine. In contrast to its effects when administered in combination with cocaine, prior administration of U69593, alone, failed to modify the conditioned response to cocaine. Microdialysis studies revealed a marked elevation of extracellular dopamine levels within the ventral striatum after repeated cocaine administration. In animals that had received U69593 in combination with cocaine, no elevation of dopamine was seen. These data demonstrate that sensitization develops to the conditioned rewarding effects of cocaine and that the activation of central nervous system kappa-opioid receptors prevents the development of this phenomenon. An involvement of the mesolimbic dopamine system in mediating the interaction of kappa-agonists with cocaine is suggested.