Antinociceptive effect of morphine, but not μ opioid receptor number, is attenuated in the spinal cord of diabetic rats

Background: The mechanisms of decreased analgesic potency of mu opioids in diabetic neuropathic pain are not fully known. The authors recently found that G protein activation stimulated by the mu opioid agonist is significantly reduced in the spinal cord dorsal horn in diabetes. in the current study, they determined potential changes in the number and binding affinity of mu opioid receptors in the spinal cord in diabetic rats. Methods: Rats were rendered diabetic with an intraperitoneal injection of streptozotocin. The nociceptive withdrawal threshold was measured before and after intrathecal injection of morphine by applying a noxious pressure stimulus to the hind paw. The mu opioid receptor was determined with immunocytochemistry labeling and a specific mu opioid receptor radioligand, [H-3]-(D-Ala(2),N-Me-Phe(4), Gly-ol(5))-enkephalin ([H-3]-DAMGO), in the dorsal spinal cord obtained from age-matched normal and diabetic rats 4 weeks after streptozotocin treatment. Results: The antinociceptive effect of intrathecal morphine (2-10 mug) was significantly reduced in diabetic rats, with an ED50 about twofold higher than that in normal rats. However, both the dissociation constant (3.99 +/- 0.22 vs. 4.01 +/- 0.23 nm) and the maximal specific binding (352.78 +/- 37.26 vs. 346.88 +/- 35.23 fmol/mg protein) of [H-3]-DAMGO spinal membrane bindings were not significantly different between normal and diabetic rats. The mu opioid receptor immunoreactivity in the spinal cord dorsal horn also was similar in normal and diabetic rats. Conclusions: The reduced analgesic effect of intrathecal morphine in diabetes is probably due to impairment of mu opioid receptor-G protein coupling rather than reduction in mu opioid receptor number in the spinal cord dorsal horn.