Inflammatory neurodegeneration mediated by nitric oxide from activated glia-inhibiting neuronal respiration, causing glutamate release and excitotoxicity

Glia undergo inflammatory activation in most CNS pathologies and are capabl e of killing cocultured neurons. We investigated the mechanisms of this inf lammatory neurodegeneration using a mixed culture of neurons, microglia, an d astrocytes, either when the astrocytes were activated directly with lipop olysaccharide (LPS) and interferon-gamma (IFN-gamma) or LPS/IFN-gamma -acti vated microglia were added to mixed neuronal cultures. In either case, acti vated glia caused 75-100% necrotic cell death within 48 hr, which was compl etely prevented by inhibitors of inducible nitric oxide synthase (iNOS) (am inoguanidine or 1400W). Activated astrocytes or microglia produced nitric o xide (NO) (steady-state level similar to0.5 muM), which immediately inhibit ed the cellular respiration of cocultured neurons, as did authentic NO. NO donors also decreased ATP levels and stimulated lactate production by neuro ns, consistent with NO-induced respiratory inhibition. NO donors or a speci fic respiratory inhibitor caused rapid (<1 min) release of glutamate from n euronal and neuronal-astrocytic cultures and subsequent neuronal death that was blocked by an antagonist of NMDA receptor (MK-801). MK-801 also blocke d neuronal death induced by activated glia. High oxygen also prevented NO-i nduced neuronal death, consistent with death being induced by NO inhibition of cytochrome c oxidation in competition with oxygen. Thus activated glia kill neurons via NO from iNOS, which inhibits neuronal respiration resultin g in glutamate release and subsequent excitotoxicity. This may contribute t o neuronal cell death in inflammatory, infectious, ischemic, and neurodegen erative diseases.