Streptozotocin-induced diabetes selectively alters the potency of analgesia produced by μ-opioid agonists, but not by δ- and κ-opioid agonists

To investigate the possible mechanisms of the alterations in morphine-induced analgesia observed in diabetic mice, we examined the influence of streptozotocin-induced (STZ-induced) diabetes on analgesia mediated by the different opioid receptors. The antinociceptive potency of morphine (10 mg/kg), administered s.c., as determined by both the tail-pinch and the tail-flick test, was significantly reduced in diabetic mice as compared to that in controls. Mice with STZ-induced diabetes had significantly decreased sensitivity to intracerebroventricularly (i.c.v.) administered μ-opioid agonists, such as morphine (10 μg) and [d-Ala², N-Me Phe⁴,Gly-ol⁵]enkephalin (DAMGO, 0.5 μg). However, i.c.v. administration of [d-Pen^2,5]enkephalin (DPDPE, 5 μg), a δ-opioid agonist, and U-50,488H (50 μg), a κ-opioid agonist, produced pronounced antinociception in both control and diabetic mice. Furthermore, there were no significant differences in antinociceptive potency between diabetic and control mice when morphine (1 μg), DAMGO (10 μg), DPDPE (0.5 μg) or U-50,488H (50 μg) was administered intrathecally. In conclusion, mice with STZ-induced diabetes are selectively hyporesponsive to supraspinal μ-opioid receptor-mediated antinociception, but they are normally responsive to activation of δ- and κ-opioid receptors.