Spinal cord ischemia reduces mu-opioid receptors in rats: correlation with morphine insensitivity.

The present study examined the effects of spinal cord ischemia on the expression of mu-opioid receptors in rat dorsal spinal cord and the effects of intrathecal (i.t.) morphine on the acute mechanical allodynia that developed after spinal ischemia. The mechanical allodynia was not significantly alleviated by up to 10 microg i.t. morphine, a dose which caused strong antinociception in the tail flick test. Immunohistochemical staining demonstrated that 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 that spinal cord ischemia reduces the expression of mu-opioid receptors in rat dorsal horn, which may be one of the underlying mechanisms of morphine insensitivity for treating acute allodynia after spinal ischemia.