Naloxone-induced hypoalgesia: evidence from the formalin test.

Two experiments examined the hypoalgesic effects that accrue from pairing exposure to a heat stressor with the opioid antagonist naloxone. Experiment 1 confirmed that rats given separate exposures to a heated floor and to naloxone are hypoalgesic when then tested on that floor. Further, the results confirmed that pairing the initial exposure to the heat stressor with naloxone resulted in a more profound hypoalgesic response. Experiment 2 provided new evidence for the hypoalgesic effects of separate exposures to the heat stressor and to naloxone, and for naloxone's enhancement of acquisition of the hypoalgesia, in rats tested for responsiveness to formalin. These results, therefore, demonstrate generalisability of the hypoalgesic effects across assays for acute and chronic pain.