SELECTIVE OPIOID RECEPTOR AGONISTS MODULATE MECHANICAL ALLODYNIA IN AN ANIMAL-MODEL OF NEUROPATHIC PAIN

This study evaluated the antinociceptive effects of systemically admin istered selective opioid agonists of mu (DAMGO), delta (BUBU) and kapp a (U 69593) receptors on the vocalization threshold to paw pressure in a rat model of peripheral unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve. DAMGO (0.5-2 mg/kg), BUBU (1.5-6 mg/kg) and U 69593 (0.75-3 mg/kg) injected intravenously (i.v.) produced a potent long-lasting antinociceptive effect on both hind pa ws. The effects on the lesioned paw were clearly and statistically mor e potent than for the non-lesioned paw. The selective antinociceptive effect of 2 mg/kg DAMGO, 3 mg/kg BUBU and 1.5 mg/kg U 69593 were compl etely prevented by prior administration of the appropriate antagonists : 0.1 mg/kg naloxone, 1 mg/kg naltrindole and 0.4 mg/kg MR 2266. The p resent data clearly show that an acute i.v. injection of these selecti ve opioid agonists induces potent antinociceptive effects in a rat mod el of peripheral neuropathy. These data are discussed with regard to t he classical view that there is opioid resistance in neuropathic pain.