Effects of high K⁺ on mitochondrial dynamics and morphology in neurons. (A) Neurons (10 DIV) were transfected with pDsRed2-Mito to label mitochondria. Neurons (at 11 DIV) were then treated with 45 mM K⁺ for 15 min, and mitochondrial fluorescence was analyzed by time-lapse imaging. Arrows show ringlike mitochondria formation in dendrites. Bar, 10 μm. (B) Higher magnification in dendrites show details of ringlike mitochondrial formation induced by treatment with 45 mM K⁺. Arrows show mitochondrial fission. Time in minutes after application of high K⁺ is indicated at the bottom of each panel. Bar, 1 μm. (C) Ultrastructure of mitochondria analyzed by electron microscopy. Panels i and ii show examples of mitochondria from control neurons. The remaining panels (iii–vii) show examples of mitochondria in neurons treated with 45 mM K⁺ for 15 min. (ii) Mitochondria in dendrites from control neurons were rod shaped, and their cristae were clear. (iii–vii) Mitochondria from neurons treated with 45 mM K⁺ formed clusters that exhibited less electron-dense matrices and contained less cristae. (v–vii) 45-mM K⁺ treatment caused mitochondrial fission. Arrows in v–vii show connections between dividing mitochondria. The dividing mitochondria shared continuous outer membrane with separate inner membranes (arrowhead in vi). Bars: (i and iii)1 μm; (ii and iv–vii) 200 nm.