Transient receptor potential vanilloid subtype 1 mediates microglial cell death in vivo and in vitro via Ca²⁺-mediated mitochondrial damage and cytochrome c release

The present study examined the expression of transient receptor potential vanilloid subtype 1 (TRPV1) in microglia, and its association with microglial cell death. In vitro cell cultures, RT-PCR, Western blot analysis, and immunocytochemical staining experiments revealed that rat microglia and a human microglia cell line (HMO6) showed TRPV1 expression. Furthermore, exposure of these cells to TRPV1 agonists, capsaicin (CAP) and resiniferatoxin (RTX), triggered cell death. This effect was ameliorated by the TRPV1 antagonists, capsazepine and iodo-resiniferatoxin (I-RTX), suggesting that TRPV1 is directly involved. Further examinations revealed that TRPV1-induced toxicity was accompanied by increases in intracellular Ca²⁺, and mitochondrial damage; these effects were inhibited by capsazepine, I-RTX, and the intracellular Ca²⁺ chelator BAPTA-AM. Treatment of cells with CAP or RTX led to increased mitochondrial cytochrome c release and enhanced immunoreactivity to cleaved caspase-3. In contrast, the caspase-3 inhibitor z-DEVD-fmk protected microglia from CAP- or RTX-induced toxicity. In vivo, we also found that intranigral injection of CAP or 12-hydroperoxyeicosatetraenoic acid, an endogenous agonist of TRPV1, into the rat brain produced microglial damage via TRPV1 in the substantia nigra, as visualized by immunocytochemistry. To our knowledge, this study is the first to demonstrate that microglia express TRPV1, and that activation of this receptor may contribute to microglial damage via Ca²⁺ signaling and mitochondrial disruption.