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The Journal of Neuroscience, December 1, 2000, 20(23):8685-8692

-Latrotoxin Releases Calcium in Frog Motor Nerve Terminals

Christopher W. Tsang, Donald B. Elrick, and Milton P. Charlton

Department of Physiology, University of Toronto, Toronto, Canada M5S 1A8

-Latrotoxin (-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca²⁺. Although -LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that -LTX causes transmitter release by mobilizing intracellular Ca²⁺ in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that -LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca²⁺. Neither the blockade of Ca²⁺ release nor the depletion of Ca²⁺ from endoplasmic reticulum affected Ca²⁺ signals produced by -LTX. The Ca²⁺ source is likely to be mitochondria, because the effects on Ca²⁺ mobilization of CCCP (which depletes mitochondrial Ca²⁺) and of -LTX are mutually occlusive. The release of mitochondrial Ca²⁺ is partially attributable to an increase in intracellular Na⁺, suggesting that the mitochondrial Na⁺/Ca²⁺ exchanger is activated. Effects of -LTX were not blocked when Ca²⁺ increases were reduced greatly in saline lacking both Na⁺ and Ca²⁺ and by application of intracellular Ca²⁺ chelators. Therefore, although increases in intracellular Ca²⁺ may facilitate the effects of -LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca²⁺-independent -LTX mechanisms or uses of -LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca²⁺, which itself can trigger exocytosis.

Key words: -latrotoxin; presynaptic toxin; mitochondria; calcium; sodium; exocytosis; frog neuromuscular junction/motor nerve terminal

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Copyright © 2000 Society for Neuroscience  0270-6474/00/20238685-08$05.00/0

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