Contralateral but not systemic administration of the kappa-opioid agonist U-50,488H induces anti-nociception in acute hindpaw inflammation in rats.

1. The anti-nociceptive effects of contralateral administration of kappa-opioid agonist U-50,488H were investigated in rats. 2. Inflammation was induced by unilateral injection of 1% carrageenan into the right hindpaw. Prior to carrageenan injection, U-50,488H or saline was administered into the left hindpaw. Withdrawal responses to mechanical and heat stimulation and oedema levels were evaluated at 3, 6 and 24 h post-carrageenan injection. 3. The results showed that the inflammatory effect of 1% carrageenan peaked after 6 h with bilateral decreases in withdrawal latencies and ipsilateral oedema formation. 4. Contralateral treatment with 0.01, 0.05, 0.3 and 2 mg of U-50,488H attenuated nociceptive reflexes to mechanical stimulation on the inflamed side at 6 h. The anti-nociceptive effect of contralateral treatment was dose-dependent at 3 and 24 h. The hindpaw withdrawal latencies to heat stimulation were prolonged at 3 and 24 h after contralateral treatment with 0.3 mg U-50,488H. No effect on inflammatory oedema formation was observed, except for a decrease at 3 h after treatment with 2 mg of U-50,488H. 5. Sciatic nerve denervation on the contralateral side abolished the anti-nociceptive effects of U-50,488H (0.3 and 2 mg). In contrast, contralateral injection of 1 mg morphine prolonged paw latencies in denervated rats. 6. Both co-administration of the peripherally selective opioid antagonist naloxone methiodide with 0.3 mg U-50,488H, and alternatively, systemic administration of 0.3 mg U-50,488H reversed the anti-nociceptive effects induced by contralateral injection of U-50,488H. 7. Taken together, our findings indicate that the contralateral administration of U-50,488H attenuates nociceptive behaviour resulting from acute inflammation. The effect is mediated via peripheral neuronal kappa-opioid receptors and, possibly, spinal cord mechanisms, suggesting a new treatment approach for acute inflammatory conditions.