Naloxone-induced analgesia: involvement of kappa-opiate receptors.

Rats treated with an acute high dose (30 mg/kg) or 4 days with a lower dose (5 mg/kg) of naloxone or naltrexone show an analgesic response at the hot-plate test. This paradoxical analgesic effect of the two mu-opiate receptor antagonists is blocked by the kappa opiate receptor antagonist MR 1452, and is modulated by the kappa opiate receptor agonist U 50-488. Our results suggest that kappa opiate receptors are involved in naloxone-induced analgesia and are consistent with a high degree of plasticity of the opiatergic system.