Endogenous kappa-opioid systems in opiate withdrawal: role in aversion and accompanying changes in mesolimbic dopamine release.

The kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) was recently shown to potentiate certain overt withdrawal signs in morphine-dependent rats. The present study sought to further assess this phenomenon by examining the influence of nor-BNI treatment upon the conditioned place aversion associated with the naloxone-precipitated withdrawal syndrome. In addition, in vivo microdialysis studies were conducted in morphine-dependent rats to determine whether nor-BNI treatment can modify withdrawal-induced changes in basal dopamine (DA) release within the mesolimbic system. Rats were pretreated with either saline or a single dose of nor-BNI and then received ascending doses of morphine for 10 days. A withdrawal syndrome was then precipitated by the administration of naloxone (1 mg/kg SC). In rats which received chronic morphine injections, administration of naloxone produced a characteristic withdrawal syndrome and a marked aversion for an environment previously associated with naloxone-precipitated withdrawal. Nor-BNI treatment potentiated most overt signs of physical dependence. This treatment also resulted in a greater withdrawal-induced place aversion. Morphine-dependent rats exhibited a marked reduction in basal mesolimbic DA release. An even greater decrease in basal DA release was observed in nor-BNI treated rats. These results suggest that endogenous kappa-systems are important in the modulation of mesolimbic DA release and the accompanying place aversion which occurs during opiate withdrawal.