PNAS Plus: Asynchronous Ca2+ current conducted by voltage-gated Ca2+ (CaV)-2.1 and CaV2.2 channels and its implications for asynchronous neurotransmitter release

摘要

We have identified an asynchronously activated Ca²⁺ current through voltage-gated Ca²⁺ (CaV)-2.1 and CaV2.2 channels, which conduct P/Q- and N-type Ca²⁺ currents that initiate neurotransmitter release. In nonneuronal cells expressing CaV2.1 or CaV2.2 channels and in hippocampal neurons, prolonged Ca²⁺ entry activates a Ca²⁺ current, IAsync, which is observed on repolarization and decays slowly with a half-time of 150–300 ms. IAsync is not observed after L-type Ca²⁺ currents of similar size conducted by CaV1.2 channels. IAsync is Ca²⁺-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, IAsync increases in a pulse-wise manner, providing Ca²⁺ 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 IAsync. IAsync is much larger for slowly inactivating CaV2.1 channels containing β2a-subunits than for rapidly inactivating channels containing β1b-subunits. IAsync requires global rises in intracellular Ca²⁺, because it is blocked when Ca²⁺ is chelated by 10 mM EGTA in the patch pipette. Neither mutations that prevent Ca²⁺ binding to calmodulin nor mutations that prevent calmodulin regulation of CaV2.1 block I_Async. The rise of I_Async during trains of stimuli, its decay after repolarization, its dependence on global increases of Ca²⁺, and its enhancement by β_2a-subunits all resemble asynchronous release, suggesting that I_Async is a Ca²⁺ source for asynchronous neurotransmission.