Chronic opioid antagonist treatment increases mu and delta receptor mediated spinal opioid analgesia

Chronic systemic treatment with the opioid antagonist, naltrexone, increased the analgesic potency of intrathecally administered opioid agonists at spinal mu and delta receptors, but not at kappa receptors. These results indicate that, contrary to a previous report, opioid receptors in the spinal cord display up-regulation and supersensitivity and are no different in this regard from supraspinal receptors.