Role of mitochondrial dysfunction in calcium signalling alterations in dorsal root ganglion neurons of mice with experimentally-induced diabetes

The role of mitochondrial dysfunction in alterations of calcium signalling in primary sensory neurons has been studied in mice with streptozotocin-induced and genetically predisposed diabetes mellitus before and after additional treatment with insulin infusions. Cytosolic calcium transients triggered by membrane depolarization were measured using a membrane-permeable form of fluorescent indicator indo-1, and their changes after application of mitochondrial uncoupler carbonyl cyanide m-chlorphenylhydrazone were compared in cells of control and diabetic animals. Considerable prolongation of residual elevation of cytosolic calcium after termination of membrane depolarization was observed in diabetic mice, which was expressed mainly in small-sized (nociceptive) neurons. This correlated with the level of hyperglycemia, which was maximal in cells from streptozotocin-treated mice. Insulin partly reversed these changes. Carbonyl cyanide m-chlorophenylhydrazone application to neurons of control mice enlarged the peak of calcium transients and decreased residual calcium elevations, indicating that mitochondria in physiological conditions participate in shaping of these transients by diminishing their peak due to rapid uptake of calcium ions and by prolonging them due to subsequent slow calcium release back into the cytosol. Depression of the calcium accumulating function of mitochondria by carbonyl cyanide m-chlorophenylhydrazone eliminated these changes. The prolonged residual elevation of cytosolic calcium characteristic for neurons of diabetic animals was also eliminated by carbonyl cyanide m-chlorophenylhydrazone, confirming the suggestion that such elevation is determined mainly by mitochondrial dysfunction, the latter being dependent on the level of hyperglycemia. Predominant expression of such changes in small-sized neurons can be explained by the absence in them of effective calcium-buffering by the endoplasmic reticulum. Possible role of the described calcium signalling changes in the origin of neuropathic syndromes is discussed.