Effect of kappa-opioid receptor agonists on morphine analgesia in morphine-naive and morphine-tolerant rats

The effect of i.p. administration of kappa-opioid receptor agonists, bremazocine, tifluadom and U-50,488H on morphine (8 mg/kg i.p.)-induced analgesia in morphine-naive and morphine tolerant male Sprague-Dawley rats was determined using the tail-flick test. The tolerance to morphine in the rats was induced by s.c., implantation of six morphine pellets during a 7-day period. Implantation of morphine pellets resulted in the development of tolerance as evidenced by the decrease in the analgesic response to morphine when compared to placebo pellets implanted rats. Bremazocine (0.3, 1.0 and 3.0 mg/kg) and U-50,488H (16 mg/kg) antagonized morphine-induced analgesia in morphine-naive rats while tifluadom (8 and 16 mg/kg) potentiated the effect. In morphine-tolerant rats, bremazocine (3 mg/kg) and U-50,488H (16 mg/kg) potentiated morphine-induced analgesia. Tifluadom at any of the doses had no effect on morphine-induced analgesia in morphine-tolerant rats. These results provide evidence that different kappa-opioid agonists modify morphine-induced analgesia differentially in morphine-naive and morphine-tolerant rats.