Background: Neuropathic pain which is a chronic pain state can be produced by injury of peripheral nerves or tissues. Traditionally, opioids have been used to alleviate pain but have not been effective in treating neuropathic pain. The present study was conducted to elucidate whether activation of descending pain inhibition system originating in the nucleus raphe magnus and/or microiontophoretic application of opiates can alleviate neuropathic symptoms in an animal model of neuropathic pain. Methods: Under halothane anesthesia, male Sprague-Dawley rats were subjected to peripheral nerve injury. After behavioral tests for neuropathic pain, electrophysiological study including electrical stimulation! of the nucleus raphe magnus and microiontophoretic application of opiates was performed. Results^ Mechanical allodynia, cold allodynia, and spontaneous pain were developed gradually after injury to the tibial and sural nerves, peaking at 14 days postoperatively. Electrical stimulation of the nucleus raphe magnus inhibited the responses of spinal dorsal horn neurons to von Frey filament and acetone. Microiontophoretically ejected DAMGO, DALDA, and U-62066 which are 사-, 多-Gand at-opioid receptor agonists, respectively, also inhibited the responses of spinal neurons to external stimuli. Conclusions; The results suggest that opioid receptors may be involved in modulation of neuropathic pain at the level of the spinal cord and the modulation may be related to the activation of the descending pain inhibition system from the brain stem.

KeywordsNeuropathic pain, Nucleus raphe magnus, Opioid receptor, Microiontophoresis