Asynchronous Ca 2+ current conducted by voltage-gated Ca 2+ (Ca V)-2.1 and Ca V2.2 channels and its implications for asynchronous neurotransmitter release

Abstract

We have identified an asynchronously activated Ca 2+ current through voltage-gated Ca 2+ (Ca V)-2.1 and Ca V2.2 channels, which conduct P/Q and N-type Ca 2+ currents that initiate neurotransmitter release. In nonneuronal cells expressing Ca V2.1 or Ca V2.2 channels and in hippocampal neurons, prolonged Ca 2+ entry activates a Ca 2+ current, I Async, which is observed on repolarization and decays slowly with a halftime of 150-300 ms. I Async is not observed after L-type Ca 2+ currents of similar size conducted by Ca V1.2 channels. I Async is Ca 2+-selective, and it is unaffected by changes in Na +, K +, Cl -, or H + or by inhibitors of a broad range of ion channels. During trains of repetitive depolarizations, I Async increases in a pulse-wise manner, providing Ca 2+ entry that persists between depolarizations. In single-cultured hippocampal neurons, trains of depolarizations evoke excitatory postsynaptic currents that show facilitation followed by depression accompanied by asynchronous postsynaptic currents that increase steadily during the train in parallel with I Async. I Async is much larger for slowly inactivating Ca V2.1 channels containing β 2a-subunits than for rapidly inactivating channels containing β 1b-subunits. I Async requires global rises in intracellular Ca 2+, because it is blockedwhen Ca 2+ is chelated by 10 mM EGTA in the patch pipette. Neither mutations that prevent Ca 2+binding to calmodulin nor mutations that prevent calmodulin regulation of Ca V2.1 block I Async. The rise of IAsync during trains of stimuli, its decay after repolarization, its dependence on global increases of Ca 2+, and its enhancement by β 2a-subunits all resemble asynchronous release, suggesting that I Async is a Ca 2+ source for asynchronous neurotransmission.