Excitotoxiic overstimulation of glutamate receptors on hippocampal neurons produces an influx of extracellular calcium and sodium. The elevated cytosolic calcium becomes sequestered by intracellular organelles, principally mitochondria, and is subsequently slowly released back to the cytoplasm as plasma membrane Ca2+ pumps slowly remove the calcium from the cell. In response to this tremendous calcium load, mitochondria depolarize and may undergo a permeability transition resulting in cessation of mitochondrial function. Simultaneous with the recovery of cytoplasmic calcium levels, cytoplasmic pH becomes increasingly acidic. This long term continual accumulation of hydrogen ionshas been interpreted as an increase in aerobic metabolism associated with the increased demands on various pumps attempting to restore ionic gradients. To determine if mitochondrial function accelerates or shuts down in response to glutamate, we directly measured neuronal oxygen consumption i n response to glutamate utilizing a self-referencing miniature polarographic oxygen electrode. In response to 500?M glutamate application, cultured hippocampal neurons increased their oxygen consumption approximately two fold. After 10-30 minutes oxygen consumption steadily declined below initial levels approaching background levels in the bath. These measurements of oxygen utilization parallel comparable results obtained by monitoring mitochondrial membrane potential and rhodamine 123. Addition of 1?M FCCP, a mitochondrial uncoupler, doubled O2 consumption in untreated neurons but failed to increase O2 consumption at the peak of the response to glutamate or following the decline after glutamate. In all cases where the differential O2 signal remained above background levels, 1?M antimycin A or 10mM Na azide, mitochondrial inhibitors, decreased O2 consumption to these background levels. These measurements of oxygen utilization parallel comparable results obtained by monitoring mitochondrial membr ane potential with rhodamine 123. Thus in response to glutamate, neuronal oxygen consumption and mitochondrial function accelerate transiently, declining to undetectable levels.
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