Kappa-opioid modulation of human microglial cell superoxide anion generation

Opioids have been postulated to play an immunomodulatory role in the CNS. Recently, we found that priming microglia with interferon (IFN)-gamma or tumor necrosis factor (TNF)-alpha resulted in an enhanced production of superoxide anion, a reactive oxygen intermediate that may be pathogenic during brain inflammation. In the present study, we investigated the effects of trans-3,4-dichloro-N-methyl-N[2-(1-pyrolidinyl)cyclohexyl]benze neaceamide methanesulfonate (U50,488), a selective kappa-opioid ligand, on microglial cell superoxide production when cells were primed with cytokines or stimulated with phorbol myristate acetate. While treatment of microglial cells with U50,488 had little effect on nonstimulated or stimulated superoxide production, this opioid inhibited (by >70%) the priming effects of cytokines. Maximal inhibition of microglial cell superoxide generation by U50,488 was observed at 10 nM for the priming effect of interferon-gamma and at 1 microM for tumor necrosis factor-alpha. Pretreatment of microglial cell cultures for 30 min with an equal concentration of the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) completely blocked the inhibitory effect of U50,488. The results of this study suggest that kappa-opioids may have therapeutic potential in inflammatory diseases of the CNS involving reactive oxygen intermediates produced by activated microglia.