Ca2+Syntillas, Miniature Ca2+Release Events in Terminals of Hypothalamic Neurons, Are Increased in Frequency by Depolarization in the Absence of Ca2+Influx

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Abstract
Localized, brief Ca2+transients (Ca2+syntillas) caused by release from intracellular stores were found in isolated nerve terminals from magnocellular hypothalamic neurons and examined quantitatively using a signal mass approach to Ca2+imaging. Ca2+syntillas (scintilla,L., spark, from a synaptic structure, a nerve terminal) are caused by release of ∼250,000 Ca ions on average by a Ca2+flux lasting on the order of tens of milliseconds and occur spontaneously at a membrane potential of –80 mV. Syntillas are unaffected by removal of extracellular Ca2+, are mediated by ryanodine receptors (RyRs) and are increased in frequency, in the absence of extracellular Ca2+, by physiological levels of depolarization. This represents the first direct demonstration of mobilization of Ca2+from intracellular stores in neurons by depolarization without Ca2+influx. The regulation of syntillas by depolarization provides a new link between neuronal activity and cytosolic [Ca2+] in nerve terminals.