Metformin reduces morphine tolerance by inhibiting microglial-mediated neuroinflammation

Abstract

BackgroundTolerance seriously impedes the application of morphine in clinical medicine. Thus, it is necessary to investigate the exact mechanisms and efficient treatment. Microglial activation and neuroinflammation in the spinal cord are thought to play pivotal roles on the genesis and maintaining of morphine tolerance. Activation of adenosine monophosphate-activated kinase (AMPK) has been associated with the inhibition of inflammatory nociception. Metformin, a biguanide class of antidiabetic drugs and activator of AMPK, has a potential anti-inflammatory effect. The present study evaluated the effects and potential mechanisms of metformin in inhibiting microglial activation and alleviating the antinociceptive tolerance of morphine.MethodsThe microglial cell line BV-2 cells and mouse brain-derived endothelial cell line bEnd3 cells were used. Cytokine expression was measured using quantitative polymerase chain reaction. Cell signaling was assayed by western blot and immunohistochemistry. The antinociception and morphine tolerance were assessed in CD-1 mice using tail-flick tests.ResultsWe found that morphine-activated BV-2 cells, including the upregulation of p38 mitogen-activated protein kinase (p38 MAPK) phosphorylation, pro-inflammatory cytokines, and Toll-like receptor-4 (TLR-4) mRNA expression, which was inhibited by metformin. Metformin suppressed morphine-induced BV-2 cells activation through increasing AMPK phosphorylation, which was reversed by the AMPK inhibitor compound C. Additionally, in BV-2 cells, morphine did not affect the cell viability and the mRNA expression of anti-inflammatory cytokines. In bEnd3 cells, morphine did not affect the mRNA expression of interleukin-1β (IL-1β), but increased IL-6 and tumor necrosis factor-α (TNF-α) mRNA expression; the effect was inhibited by metformin. Morphine also did not affect the mRNA expression of TLR-4 and chemokine ligand 2 (CCL2). Furthermore, systemic administration of metformin significantly blocked morphine-induced microglial activation in the spinal cord and then attenuated the development of chronic morphine tolerance in mice.ConclusionsMetformin significantly attenuated morphine antinociceptive tolerance by suppressing morphine-induced microglial activation through increasing AMPK phosphorylation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0754-9) contains supplementary material, which is available to authorized users.

Morphine-Mediated Brain Region-Specific Astrocytosis Involves the ER Stress-Autophagy Axis

“…Morphine has been shown to induce autophagy in the neuroblastoma cell line [52], in bone marrow-derived macrophages [64], and in the mice hippocampus [2,65,52,43]. Our previous studies have also shown that morphine exposure resulted in synaptic injury in the hippocampal neurons and that this was mediated via the ER stress-autophagy axis [43].…”

Spinal glucocorticoid receptor‑regulated chronic morphine tolerance may be through extracellular signal‑regulated kinase 1/2

“…During the past two decades, a number of associated intracellular signaling cascades, including the mitogen-activated protein kinase (MAPK) signaling cascades have been described. MAPKs may be activated by morphine via opioid receptors, and their activation has been observed in synaptic plasticity and addiction (8)(9)(10)(11)(12)(13)(14). In vivo, a role of MAPKs in opioid analgesia and sedation has additionally been proposed (15).…”

Exosome derived from mesenchymal stem cells enhance autophagy and inhibit iNOS/TXNIP/NLRP3 inflammasome axis in injured spinal cord

“…All these factors may affect the functional recovery by limiting axonal outgrowth and preventing reformation of functional synapses. 27 Exosome administration has been shown to attenuatemicroglial-mediated neuroinflammation and reduce neuronal damage in a SCI model 28 .Furthermore, it was reported that exosomeadministration ameliorated insulin resistance by inhibiting oxidative stress in adipocytes 29 . Pandeyetal have demonstrated that exosome treatment inhibited iNOS signaling in experimental model of colitis 30 .…”