Using the mitochondrial potential (ΔΨm) marker JC-1 (5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazolylcarbocyanine iodide) and high-resolution imaging, we functionally analyzed mitochondria in cultured rat hippocampal astrocytes. Ratiometric detection of JC-1 fluorescence identified mitochondria with high and low ΔΨm. Mitochondrial density was highest in the perinuclear region, whereas ΔΨm tended to be higher in peripheral mitochondria. Spontaneous ΔΨm fluctuations, representing episodes of increased energization, appeared in individual mitochondria or synchronized in mitochondrial clusters. They continued upon withdrawal of extracellular Ca2+, but were antagonized by dantrolene or 2-aminoethoxydiphenylborate (2-APB). Fluo-3 imaging revealed local cytosolic Ca2+ transients with similar kinetics that also were depressed by dantrolene and 2-APB. Massive cellular Ca2+ load or metabolic impairment abolished ΔΨm fluctuations, occasionally evoking heterogeneous mitochondrial depolarizations. The detected diversity and ΔΨm heterogeneity of mitochondria confirms that even in less structurally polarized cells, such as astrocytes, specialized mitochondrial subpopulations coexist. We conclude that ΔΨm fluctuations are an indication of mitochondrial viability and are triggered by local Ca2+ release from the endoplasmic reticulum. This spatially confined organelle crosstalk contributes to the functional heterogeneity of mitochondria and may serve to adapt the metabolism of glial cells to the activity and metabolic demand of complex neuronal networks. The established ratiometric JC-1 imaging—especially combined with two-photon microscopy—enables quantitative functional analyses of individual mitochondria as well as the comparison of mitochondrial heterogeneity in different preparations and/or treatment conditions.

Reactive oxygen species (ROS) released from (dys-)functioning mitochondria contribute to normal and pathophysiological cellular signaling by modulating cytosolic redox state and redox-sensitive proteins. To identify putative redox targets involved in such signaling, we exposed hippocampal neurons to hydrogen peroxide (H 2 O 2 ). Redox-sensitive dyes indicated that externally applied H 2 O 2 may oxidize intracellular targets in cell cultures and acute tissue slices. In cultured neurons, H 2 O 2 (EC 50 118 µM) induced an intracellular Ca 2+ rise which could still be evoked upon Ca 2+ withdrawal and mitochondrial uncoupling. It was, however, antagonized by thapsigargin, dantrolene, 2-aminoethoxydiphenyl borate, and high levels of ryanodine, which identifies the endoplasmic reticulum (ER) as the intracellular Ca 2+ store involved. Intracellular accumulation of endogenously generated H 2 O 2 -provoked by inhibiting glutathione peroxidase-also released Ca 2+ from the ER, as did extracellular generation of superoxide. Phospholipase C (PLC)-mediated metabotropic signaling was depressed in the presence of H 2 O 2 , but cytosolic cyclic adenosine-5′-monophosphate (cAMP) levels were not affected. H 2 O 2 (0.2-5 mM) moderately depolarized mitochondria, halted their intracellular trafficking in a Ca 2+ -and cAMPindependent manner, and directly oxidized cellular nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH 2 ). In part, the mitochondrial depolarization reflects uptake of Ca 2+ previously released from the ER. We conclude that H 2 O 2 releases Ca 2+ from the ER via both ryanodine and inositol trisphosphate receptors. Mitochondrial function is not markedly impaired even by millimolar concentrations of H 2 O 2 . Such modulation of Ca 2+ signaling and organelle interaction by ROS affects the efficacy of PLC-mediated metabotropic signaling and may contribute to the adjustment of neuronal function to redox conditions and metabolic supply.

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