Microglia-mediated neurotoxicity is inhibited by morphine through an opioid receptor-independent reduction of NADPH oxidase activity.
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 p47(phox) 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.
Qian
Li
L
Comprehensive Center for Inflammatory Disorders, University of North Carolina, Chapel Hill, NC 27599, USA.
Tan
Kai Soo
KS
Wei
Sung-Jen
SJ
Wu
Hung-Ming
HM
Xu
Zongli
Z
Wilson
Belinda
B
Lu
Ru-Bin
RB
Hong
Jau-Shyong
JS
Flood
Patrick M
PM
D015655:1-Methyl-4-phenylpyridinium / Q000633:toxicity; D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D018920:Coculture Techniques; D048049:Extracellular Signal-Regulated MAP Kinases / Q000187:drug effects / Q000378:metabolism; D005434:Flow Cytometry; D006540:Herbicides / Q000633:toxicity; D007150:Immunohistochemistry; D008070:Lipopolysaccharides / Q000633:toxicity; D051379:Mice; D018345:Mice, Knockout; D017628:Microglia / Q000187:drug effects* / Q000201:enzymology / Q000473:pathology; D018613:Microscopy, Confocal; D009020:Morphine / Q000494:pharmacology*; D019255:NADPH Oxidases / Q000187:drug effects / Q000378:metabolism*; D009294:Narcotics / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects; D018696:Neuroprotective Agents / Q000494:pharmacology*; D051381:Rats; D017382:Reactive Oxygen Species / Q000378:metabolism; D011957:Receptors, Opioid / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013237:Stereoisomerism
D006540:Herbicides; D008070:Lipopolysaccharides; D009294:Narcotics; D018696:Neuroprotective Agents; D017382:Reactive Oxygen Species; D011957:Receptors, Opioid; D009020:Morphine; D019255:NADPH Oxidases; D048049:Extracellular Signal-Regulated MAP Kinases; D015655:1-Methyl-4-phenylpyridinium