Triptolide up-regulates metabotropic glutamate receptor 5 to inhibit microglia activation in the lipopolysaccharide-induced model of Parkinson's disease.

Metabotropic glutamate receptor (mGlu) regulates microglia activation, which contributes to inflammation. However, the role of mGlu in neuroinflammation associated with Parkinson's disease (PD) remains unclear. Triptolide (T10) exerts potent immunosuppressive and anti-inflammatory effects and protects neurons by inhibiting microglia activation. In this study, we investigated the role of mGlu in the anti-inflammatory effect of T10 in a lipopolysaccharide (LPS)-induced PD model. In cultured BV2 cells and primary microglia, blocking mGlu activity or knocking down its expression abolished T10-inhibited release of proinflammatory cytokines induced by LPS. Moreover, T10 up-regulated mGlu expression decreased by LPS through enhancing mRNA expression and protein stability. T10 also reversed the reduction in mGlu membrane localization and modulated receptor-mediated mitogen-activated protein kinase activity induced by LPS. Pharmacological inhibition of signaling molecules increased nitric oxide level and inducible nitric oxide synthase (iNOS), tumor necrosis factor-α, and interleukin (IL)-1β and -6 transcript levels that were downregulated by treatment with T10. Consistent with these in vitro findings, blocking mGlu attenuated the anti-inflammatory effects of T10 in an LPS-induced PD model and blocked the decreases in the number and morphology of ionized calcium binding adaptor molecule 1-positive microglia and LPS-induced iNOS protein expression caused by T10 treatment. Besides, mGlu mediated the effect of T10 on microglia-induced astrocyte activation in vitro and in vivo. The findings provide evidence for a novel mechanism by which mGlu regulates T10-inhibited microglia activation via modulating protein expression of the receptor and its intracellular signaling. The study might contribute to the biological effects of Chinese herbs as an approach for protecting against neurotoxicity in PD.