mu-, delta- and kappa-opiate receptors mediate antinociception in the rat tail flick test following noxious thermal stimulation of one hindpaw.

Experiments were performed to investigate the possible involvement of spinal mu-, delta- and kappa-opiate receptors in mediating the antinociceptive effects of noxious thermal stimulation of one hindpaw on the tail flick reflex in the rat. Male Sprague-Dawley rats were implanted with chronic intrathecal catheters to the lumbar level of the spinal cord. After 5 to 7 days, they were lightly anesthetized with an i.p. injection of a mixture of Na-pentobarbital (20 mg/kg) and chloral hydrate (120 mg/kg). After baseline readings were taken in the tail flick test a conditioning noxious thermal stimulus, which consisted of immersion of one hindpaw in water at 55 degrees C for 90 sec, was applied and the effects on the latency of the tail withdrawal reflex were studied over the next 30 min. In animals pretreated with CSF intrathecally 10 min before the stimulus, an increase in tail flick reaction time was observed peaking at 30 sec after the stimulus. This response was attenuated in a dose-related manner by preadministration of the specific mu-opiate receptor antagonist, beta-funaltrexamine, the specific delta-opiate receptor antagonist, H-Tyr-Tic psi[CH2NH]-Phé-Phe-OH or the specific kappa-opiate receptor antagonist, nor-binaltorphimine. The data show that the antinociceptive effect on the tail withdrawal reflex from a brief noxious thermal stimulus is provoked heterosegmentally by the noxious conditioning stimulus to the hindpaw and is mediated by the endogenous release of ligands that bind to mu-, delta- and kappa-opiate receptors in the spinal cord.