Opioid and nonopioid conditional analgesia: The role of spinal opioid, noradrenergic, and serotonergic systems.

The first aim of this study was to test whether opioid mediation of conditional analgesia is inversely related to the relative amount of training that rats received, a prediction derived from the severity hypothesis. Naltrexone attenuated the analgesia among subjects in the low-training group but had no effect in rats given an extended amount of training. The second aim of this study was to investigate whether noradrenergic, opiate, and serotonergic spinal systems play a role in conditional analgesia. Intrathecal administration of yohimbine, an alpha 2 antagonist, completely blocked both opioid and nonopioid forms of conditional analgesia. Spinal administration of either quaternary naltrexone or methysergide failed to have an effect. In addition to providing support for the severity hypothesis, these results indicate that both opioid and nonopioid forms of conditional analgesia are subserved by critical alpha 2-adrenoceptors at the spinal level.