Microglia-Mediated Neurotoxicity Is Inhibited by Morphine through an Opioid Receptor-Independent Reduction of NADPH Oxidase Activity

Abstract

Recent studies have shown that morphine modulates the function of glia cells through both opioid receptor dependent and independent mechanisms. However, the mechanism by which morphine regulates neuronal disorders through the alteration of microglia activity remains unclear. In this study, using rat primary mesencephalic neuron-glia cultures, we report that both l-morphine and its synthetic stereoenantiomer, d-morphine, an ineffective opioid receptor agonist, significantly reduced LPS- or 1-methyl-4-phenylpyridinium-induced dopaminergic neurotoxicity with similar efficacy, indicating a nonopioid receptor-mediated effect. In addition, using reconstituted neuron and glia cultures, subpicomolar concentrations of morphine were found to be neuroprotective only in the presence of microglia, and significantly inhibited the production of inflammatory mediators from LPS-stimulated microglia cells. Mechanistic studies showed that both l- and d- morphine failed to protect dopaminergic neurons in cultures from NADPH oxidase (PHOX) knockout mice and significantly reduced LPS-induced PHOX cytosolic subunit p47phox translocation to the cell membrane by inhibiting ERK phosphorylation. Taken together, our results demonstrate that morphine, even at subpicomolar concentrations, exerts potent anti-inflammatory and neuroprotective effects either through the inhibition of direct microglial activation by LPS or through the inhibition of reactive microgliosis elicited by 1-methyl-4-phenylpyridinium. Furthermore, our study reveals that inhibition of PHOX is a novel site of action for the mu-opioid receptor-independent effect of morphine.

Exploring the Neuroimmunopharmacology of Opioids: An Integrative Review of Mechanisms of Central Immune Signaling and Their Implications for Opioid Analgesia

“…Our work has demonstrated that the (ϩ)-isomers of morphine and methadone are capable of inducing allodynia and hyperalgesia accompanied by expression of spinal proinflammatory cytokines (Hutchinson et al, 2010a). (ϩ)-Morphine has also been demonstrated to induce hyper-responsivity when injected in the periaqueductal gray area (Jacquet et al, 1977), to bind nonstereoselectively to mouse thymocytes and a macrophage cell line (Roy et al, 1991(Roy et al, , 1992(Roy et al, , 1996, and, surprisingly, to protect dopamine neurons from toxicity (Qian et al, 2007). These later neuroprotective actions of (ϩ)-morphine at first seem contrary to the hypothesized detrimental effects of inducing a proinflammatory central immune NEUROIMMUNOPHARMACOLOGIC IMPLICATIONS FOR OPIOID ANALGESIA signal.…”

Physiology of Microglia

“…The mu-opioid receptors increase migration and upregulate expression of P2X 4 purinoceptors in cultured rat microglia (388). On the contrary, in neuronal-glial cultures prepared from rat mesencephalon, morphine significantly reduced microglial neurotoxicity and LPS-induced microglial activation, albeit through an unknown pathway that, most likely, did not involve opioid receptors (737). Reportedly morphine can also cooperate with HIV-1 transactivating protein Tat in stimulating microglial activation and secretion of proinflammatory factors (76).…”

Morphine Protects against Intracellular Amyloid Toxicity by Inducing Estradiol Release and Upregulation of Hsp70

“…In zebrafish embryos, morphine, at certain concentrations, enhances neuron proliferation, increases the number of certain neuronal populations, and protects against glutamate damage in motor neurons and Pax-6-positive neurons in vivo (Sanchez-Simon et al, 2010). Morphine is found to be protective against microglia-mediated lipopolysccharide-or 1-methyl-4-phenylpyridinium-induced dopaminergic neurotoxicity in rat primary mesencephalic neuron/glia cultures (Qian et al, 2007). In rat neuronal/glial cultures, morphine is reported to prevent cell death induced by HIV envelope glycoprotein gp120IIIB or BaL (Avdoshina et al, 2010).…”