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This study examined a mechanism responsible for the enhanced antihyperalgesic and antinociceptive effects of the μ opioid receptor agonist (ORA) [d-Ala2, NMePhe4, Gly5-ol]enkephalin (DAMGO) microinjected in the rostroventromedial medulla (RVM) of rats with inflammatory injury induced by injection of complete Freund's adjuvant (CFA) in one hindpaw. In rats injected with CFA 4 hr earlier, microinjection of the μ opioid receptor antagonistd-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2(CTAP) in the RVM antagonized both the marginal enhancement of the potency of DAMGO and its antinociceptive effect. The δ opioid receptor antagonist naltriben (NTB) was without effect. In rats injected with CFA 2 weeks earlier, CTAP antagonized the effects of DAMGO to a lesser extent. However, NTB completely prevented the enhancement of the potency of DAMGO, whereas it did not antagonize DAMGO's antinociceptive effects. Microinjection of NTB alone, but not CTAP in the RVM of CFA-treated rats, enhanced the hyperalgesia present in the ipsilateral hindpaw and induced hyperalgesia in the contralateral, uninjured hindpaw. These results suggest that persistent inflammatory injury increased the release in the RVM of opioid peptides with preferential affinity for the δ opioid receptor, which can interact in a synergistic or additive manner with an exogenously administered μ opioid receptor agonist. Indeed, the levels of [Met5]enkephalin and [Leu5]enkephalin were increased in the RVM and in other brainstem nuclei in CFA-treated rats. This increase most likely presents a compensatory neuronal response of the CNS of the injured animal to mitigate the full expression of inflammatory pain and to enhance the antinociceptive and antihyperalgesic effects of exogenously administered μ opioid receptor analgesics.

Contribution of Endogenous Enkephalins to the Enhanced Analgesic Effects of Supraspinal μ Opioid Receptor Agonists after Inflammatory Injury