Toll-Like Receptor 9 Is Required for Opioid-Induced Microglia Apoptosis

Abstract

Opioids have been widely applied in clinics as one of the most potent pain relievers for centuries, but their abuse has deleterious physiological effects beyond addiction. However, the underlying mechanism by which microglia in response to opioids remains largely unknown. Here we show that morphine induces the expression of Toll-like receptor 9 (TLR9), a key mediator of innate immunity and inflammation. Interestingly, TLR9 deficiency significantly inhibited morphine-induced apoptosis in microglia. Similar results were obtained when endogenous TLR9 expression was suppressed by the TLR9 inhibitor CpGODN. Inhibition of p38 MAPK by its specific inhibitor SB203580 attenuated morphine-induced microglia apoptosis in wild type microglia. Morphine caused a dramatic decrease in Bcl-2 level but increase in Bax level in wild type microglia, but not in TLR9 deficient microglia. In addition, morphine treatment failed to induce an increased levels of phosphorylated p38 MAPK and MAP kinase kinase 3/6 (MKK3/6), the upstream MAPK kinase of p38 MAPK, in either TLR9 deficient or µ-opioid receptor (µOR) deficient primary microglia, suggesting an involvement of MAPK and µOR in morphine-mediated TLR9 signaling. Moreover, morphine-induced TLR9 expression and microglia apoptosis appears to require μOR. Collectively, these results reveal that opioids prime microglia to undergo apoptosis through TLR9 and µOR as well. Taken together, our data suggest that inhibition of TLR9 and/or blockage of µOR is capable of preventing opioid-induced brain damage.

Glial and Neuroimmune Mechanisms as Critical Modulators of Drug Use and Abuse

“…In contrast, opioids can easily penetrate CNS tissue and signal directly at neurons and glia, suggesting independent patterns of cell recruitment . Drug-specific recruitment of multiple innate immune signaling pathways may also explain their unique neuroimmune and behavioral outcomes; opioids, for example, signal at TLR4 but there is also evidence that they may participate in signaling through TLR2 and TLR9 (He et al, 2011;Li et al, 2010b), although there is currently less evidence for these alternative pathways. Moreover, the affinity of the opioids morphine and fentanyl at the TLR4/MD-2 complex is considerably less than LPS in reporter HEK 293 cells .…”

Neurobiological Effects of Morphine after Spinal Cord Injury

“…Moreover, the apoptotic effects of morphine are not limited to neurons. Morphine-induced cell death has also been reported for neuroprogenitor cells, 51 T cells, 52 microglia, 53 macrophages, 54 and astrocytes. 49,55 As noted above, some studies suggest that morphine induces cell death by increasing FasL expression and activating downstream targets of the Fas receptor, including Caspases-3 and 8.…”

Glial abnormalities in substance use disorders and depression: Does shared glutamatergic dysfunction contribute to comorbidity?

“…Furthermore, microglial p38 (a mitogen activated protein kinase/extracellular signal-related kinase pathway intermediate) in the NAcc also mediates the acquisition and maintenance of morphine-induced CPP (Zhang et al 2012). Microglial TLRs, e.g., TLR2 (Zhang et al 2011), TLR4 (Watkins et al 2009), and TLR9 (He et al 2011), also modulate the neurochemical and behavioural effects of opioids in rodents.…”