High Concentration of Ketone Body β-Hydroxybutyrate Modifies Synaptic Vesicle Cycle and Depolarizes Plasma Membrane of Rat Brain Synaptosomes

Ketoacidosis is a dangerous complication of diabetes mellitus in which plasma levels of ketone bodies can reach 20–25 mM. This condition is life-threatening. In contrast, a ketogenic diet, achieving plasma levels of ketone bodies of about 4–5 mM, can be used for treating different brain diseases. However, the factors leading to the conversion of the neuroprotective ketone bodies’ action to the neurotoxic action during ketoacidosis are still unknown. We investigated the influence of high concentration (25 mM) of the main ketone body, β-hydroxybutyrate (BHB), on intrasynaptosomal pH (pHi), synaptic vesicle cycle, plasma membrane, and mitochondrial potentials. Using the fluorescent dye BCECF-AM, it was shown that BHB at concentrations of 8 and 25 mM did not influence pHi in synaptosomes. By means of the fluorescent dye acridine orange, it was demonstrated that 25 mM of BHB had no effect on exocytosis but inhibited compensatory endocytosis by 5-fold. Increasing buffer capacity with 25 mM HEPES did not affect endocytosis. Glucose abolished BHB-induced endocytosis inhibition. Using the fluorescent dye DiSC3(5), it was shown that 25 mM of BHB induced a significant plasma membrane depolarization. This effect was not impacted by glucose. Using the fluorescent dye rhodamine-123, it was shown that BHB alone (25 mМ) did not alter the potential of intrasynaptosomal mitochondria. Importantly, the high concentration of BHB (25 mМ) causes the depolarization of the plasma membrane and stronger inhibition of endocytosis compared with the intermediate concentration (8 mM).