Effects of sciatic nerve injuries on δ-opioid receptor and substance P immunoreactivities in the superficial dorsal horn of the rat

The aim of the present study was to examine the effects of transection combined with tight ligation, and crush of the sciatic nerve on delta-opioid receptor and substance P immunoreactivities in the superficial spinal dorsal horn at different time points after injury. Both the delta-opioid receptor and substance P are primarily localised to primary afferent fibres and terminals. Seven days following transection and ligation, a slight decrease in both delta-opioid receptor and substance P levels was seen in laminae I and II. The maximal reduction appeared to take place around 4 weeks. Restoration of immunoreactivity was observed by 32 weeks, and by 1 year the levels were almost back to normal. Regarding crush injury, the reduction in both delta-opioid receptor and substance P immunoreactivities were less pronounced and recovery was faster than after transection injury. Already by 16 weeks, the levels were almost back to normal. These results show that peripheral nerve injuries dramatically reduce the levels of delta-opioid receptor and substance P immunoreactivities in the superficial dorsal horn after short survivals and demonstrate recovery after long survivals. Whether the marked reduction of delta-opioid receptors in the dorsal horn is involved in the decreased ability of opioid analgesics to alleviate neuropathic pain remains to be studied.