Astrocyte-derived nitric oxide causes both reversible and irreversible damage to the neuronal mitochondrial respiratory chain.

Cytokine-stimulated astrocytes produce nitric oxide (NO), which, along with its metabolite peroxynitrite (ONOO(-)), can inhibit components of the mitochondrial respiratory chain. We used astrocytes as a source of NO/ONOO(-) and monitored the effects on neurons in coculture. We previously demonstrated that astrocytic NO/ONOO(-) causes significant damage to the activities of complexes II/III and IV of neighbouring neurons after a 24-h coculture. Under these conditions, no neuronal death was observed. Using polytetrafluoroethane filters, which are permeable to gases such as NO but impermeable to NO derivatives, we have now demonstrated that astrocyte-derived NO is responsible for the damage observed in our coculture system. Expanding on these observations, we have now shown that 24 h after removal of NO-producing astrocytes, neurons exhibit complete recovery of complex II/III and IV activities. Furthermore, extending the period of exposure of neurons to NO-producing astrocytes does not cause further damage to the neuronal mitochondrial respiratory chain. However, whereas the activity of complex II/III recovers with time, the damage to complex IV caused by a 48-h coculture with NO-producing astrocytes is irreversible. Therefore, it appears that neurons can recover from short-term damage to mitochondrial complex II/III and IV, whereas exposure to astrocytic-derived NO for longer periods causes permanent damage to neuronal complex IV.