Spinal cord ischemia reduces mu-opioid receptors in rats: correlation withmorphine insensitivity

THE present study examined the effects of spinal cord ischemia on the expre ssion of mu-opioid receptors in rat dorsal spinal cord and the effects of i ntrathecal (i.t.) morphine on the acute mechanical allodynia that developed after spinal ischemia. The mechanical allodynia was not significantly alle viated by up to 10 mu g i.t. morphine, a dose which caused strong antinocic eption in the tail flick test. Immunohistochemical staining demonstrated th at the level of mu-opioid receptor expression in the dorsal horn of spinal cord segments receiving input from the allodynic skin was markedly reduced 2 days after ischemia, when the rats were allodynic. The results indicate t hat spinal cord ischemia reduces the expression of mu-opioid receptors in r at dorsal horn, which may be one of the underlying mechanisms of morphine i nsensitivity for treating acute allodynia after spinal ischemia. (C) 1999 L ippincott Williams & Wilkins.