Individual calcium syntillas do not trigger spontaneous exocytosis from nerve terminals of the neurohypophysis

Abstract

Recently, highly localized Ca2+ release events, similar to Ca2+ sparks in muscle, have been observed in neuronal preparations. Specifically, in murine neurohypophysial terminals (NHT), these events, termed Ca2+ syntillas, emanate from a ryanodine-sensitive intracellular Ca2+ pool and increase in frequency with depolarization in the absence of Ca2+ influx. Despite such knowledge of the nature of these Ca2+ release events, their physiological role in this system has yet to be defined. Such localized Ca2+ release events, if they occur in the precise location of the final exocytotic event(s),maydirectly trigger exocytosis. However, directly addressing this hypothesis has not been possible, since no method capable of visualizing individual release events in these CNS terminals has been available. Here, we have adapted an amperometric method for studying vesicle fusion to this system which relies on loading the secretory granules with the false transmitter dopamine, thus allowing, for the first time, the recording of individual exocytotic events from peptidergicNHT.Simultaneous use of this technique along with high-speed Ca2+ imaging has enabled us to establish that spontaneous neuropeptide release and Ca2+ syntillas do not display any observable temporal or spatial correlation, confirming similar findings in chromaffin cells. Although these results indicate that syntillas do not play a direct role in eliciting spontaneous release, they do not rule out indirect modulatory effects of syntillas on secretion. Copyright © 2009 Society for Neuroscience.