Abstract

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. A series of studies were conducted in order to elucidate whether adrenergic agents can inhibit neuropathic pain and are related to opioid system, and how adrenergic system and opioid system interact. Mechanical allodynia, cold allodynia, and spontaneous pain were developed gradually after injury to the tibial and sural nerves, peaking at 14 days postoperatively. Intraperitoneally injected clonidine, an α2-adrenergic agonist, inhibited mechanical allodynia and cold allodynia. Pretreated naloxone, an opioid antagonist, reversed the effects of clonidine. Yohimbine, an α2-adrenergic antagonist, rather tended to increase neuropathic pain. Pretreated naloxone tended to reverse the effects of yohimbine. The results suggest that alpha-2 adrenergic receptors interact with opioid receptors in the modulation of neuropathic pain.