Role of spinal opioid receptor on the antiallodynic effect of intrathecal nociceptin in neuropathic rat

The purpose of this study was to examine the effects of intrathecal nociceptin for neuropathic pain and determine the role of spinal opioid receptor types. Neuropathic pain was induced by ligation of L5 and L6 spinal nerves in male Sprague-Dawley rats. Several antagonists were intrathecally administered to evaluate the action mechanisms of nociceptin: nonselective opioid receptor antagonist (naloxone), μ opioid receptor antagonist (CTOP), δ opioid receptor antagonist (naltrindole) and κ opioid receptor antagonist (GNTI). The levels of opioid receptor proteins were examined by Western blotting. Intrathecal nociceptin produced dose-dependent antiallodynia. Intrathecal naloxone reversed the antinociception of nociceptin. Intrathecal CTOP, naltrindole and GNTI reversed the antinociceptive effect of nociceptin. Western blots showed that the levels of spinal opioid receptor proteins did not differ between rats with neuropathic pain and naïve rats. Intrathecal nociceptin increased the level of δ opioid receptor protein compared with that of nerve ligated rats, while the levels of μ, and κ opioid receptor proteins were unchanged. These results suggest that intrathecal nociceptin produced antiallodynic effect in spinal nerve ligation-induced neuropathic pain. All three types of spinal μ, δ, and κ opioid receptors were involved in the antiallodynic mechanism of nociceptin.