Influence of Intra- and Extracellular Acidification on Free Radical Formation and Mitochondria Membrane Potential in Rat Brain Synaptosomes

Brain ischemia is accompanied by lowering of intra- and extracellular pH. Stroke often leads to irreversible damage of synaptic transmission by unknown mechanism. We investigated an influence of lowering of pH_i and pH_o on free radical formation in synaptosomes. Three models of acidosis were used: (1) pH_o 6.0 corresponding to pH_i decrease down to 6.04; (2) pH_o 7.0 corresponding to the lowering of pH_i down to 6.92: (3) 1 mM amiloride corresponding to pH_i decrease down to 6.65. We have shown that both types of extracellular acidification, but not intracellular acidification, increase 2′,7′-dichlorodihydrofluorescein diacetate fluorescence that reflects free radical formation. These three treatments induce the rise of the dihydroethidium fluorescence that reports synthesis of superoxide anion. However, the impact of amiloride on superoxide anion synthesis was less than that induced by moderate extracellular acidification. Superoxide anion synthesis at pH_o 7.0 was almost completely eliminated by mitochondrial uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone. Furthermore, using fluorescent dyes JC-1 and rhodamine-123, we confirmed that pH_o lowering, but not intracellular acidification, led to depolarization of intrasynaptosomal mitochondria. We have shown that pH_o but not pH_i lowering led to oxidative stress in neuronal presynaptic endings that might underlie the long-term irreversible changing in synaptic transmission.