The reorganization of μ opioid receptors in the rat dorsal horn following peripheral axotomy

The μ opioid receptor is concentrated in laminae I and II (LI and LII, respectively) of the normal rat dorsal horn. Fourteen days after transection of the L4-L6 segmental peripheral nerves, image analysis demonstrates a 49, 34 and 17% decrease in μ opioid staining density in the medial, middle and lateral thirds of the superficial dorsal horn, respectively, when comparing the operated to the unoperated side. Intralaminar analysis demonstrates that the greatest change in density occurs in LI and LII outer, compared to LII inner. By 31 days post-surgery, staining has returned to normal with side to side differences no longer present. These results imply that μ opioid ligands such as morphine might be less effective in ameliorating pain 2 weeks after a peripheral nerve lesion than they are in the normal condition but that this effectiveness should return as the receptors are restored to their normal levels. Thus, the time following a lesion may be an important variable in assessing the effectiveness of μ opioid ligands in alleviating neuropathic pain. Furthermore, this study shows that the organization of opioid receptors in the superficial dorsal horn is malleable and could lead to changes in drug efficacy.