Action potential-induced changes in O ₂ consumption, NADH regeneration and mitochondrial membrane potential in adult mouse spinal motor neurons

Studies of central neurons suggest that stimulation activates Ca 2+ -dependent processes that increase NADH regeneration and O 2 consumption. This study asked whether similar processes dominate respiratory responses in spinal motor neurons. Lumbar ventral horn segments from adult mice were perfused in a flow-through chamber. Motor neurons were stimulated with trains of action potentials (5-80 Hz for 20-40 s) conducted antidromically from ventral roots. Perfusion solutions contained antagonists of ionotropic cholinergic and glutaminergic receptors to inhibit synaptic transmission. The stimulation-induced increase in O 2 consumption, calculated from changes in the phosphorescence/fluorescence of porphyrin compounds, was inhibited reversibly by procaine and low [K + ] solutions, and blocked by inhibitors of mitochondrial respiration (rotenone, oligomycin). These findings suggest that the increase in O 2 consumption was due mainly to activation of ATPase pumps to counteract the changes in transmembrane ion gradients produced by repetitive action potential discharge. In glucose, pyruvate or lactate (1-10 mM) peak O 2 consumption and NADH regeneration increased with action potential workload. For all 3 tested substrates a fraction of O 2 consumption (median 12-40%) was Ca 2+ -dependent; NADH regeneration showed significant Ca 2+ dependence only in low [lactate] (2.5 mM). Stimulation produced a depolarizing shift in the mitochondrial membrane potential (ΔΨ m ) that increased with workload. The relationship between O 2 consumption and NADH regeneration was flexible, in a manner suggesting that resting NADH is sufficient for the initial respiratory response to stimulation. A scenario consistent with most respiration in glucose is action potential workload →increased pump activity →↑ADP/ATP ratio → ↑ATP synthase activity → ΔΨ m depolarization→↑O 2 consumption.