Asynchronous Ca ^{2 +} current conducted by voltage-gated Ca ²⁺ (Ca _V )-2.1 and Ca _V 2.2 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 V 2.2 channels, which conduct P/Qand N-type Ca 2+ currents that initiate neurotransmitter release. In nonneuronal cells expressing Ca V 2.1 or Ca V 2.2 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 V 1.2 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 V 2.1 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 blocked when 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 V 2.1 block I Async . The rise of I Async 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.asynchronous synaptic transmission | exocytosis V oltage-gated Ca 2+ (Ca V ) channels are activated by depolarization and conduct inward Ca 2+ currents that initiate many cellular responses to electrical signaling (1). Ca V 2 channels conduct P/Q-, N-, and R-type currents in presynaptic nerve terminals, and Ca 2+ entry through these channels initiates neurotransmitter release at conventional synapses (1-3). P/Q-and N-type Ca 2+ currents conducted by Ca V 2.1 and Ca V 2.2 channels, respectively, are more efficiently coupled to neurotransmitter release than Rtype Ca 2+ currents conducted by Ca V 2.3 channels (4, 5). Neurotransmitter release occurs in two phases: a fast synchronous (phasic) component and a slow asynchronous (tonic) component (6). The slower asynchronous component of release is proposed to result from residual Ca 2+ remaining after an action potential acting on a different Ca 2+ sensor than the sensor for synchronous neurotransmitter release (6-8). Remarkably, when synchronous release is prevented by deletion of its Ca 2+ sensor synaptotagmin, the asynchronous release process can cause exocytosis of the entire readily releasable pool, suggesting a functional competition between synchronous and asynchronous release processes for the ...

Synapsin II desynchronizes neurotransmitter release at inhibitory synapses by interacting with presynaptic calcium channels

“…Another recent paper reports that the SNARE protein VAMP2 drives synchronous release, while its isoform VAMP4 boosts asynchronous release14. Moreover, it was recently shown that both voltage-gated presynaptic Cav-2.1 and Cav-2.2 channels, that conduct P/Q-type and N-type Ca 2+ currents respectively, are characterized by a prolonged Ca 2+ current that promotes asynchronous release49.…”

Molecular Mechanisms for Synchronous, Asynchronous, and Spontaneous Neurotransmitter Release

“…It is well established that voltage-gated Ca 2+ channels provide the brief Ca 2+ signal that evokes synchronous release, but voltage-gated Ca 2+ channels may also provide a longer-lasting phase of Ca 2+ entry that may contribute to asynchronous release (157). In transfected nonneuronal cells, following a large and prolonged depolarization or a train of depolarizations, a fraction of either Ca V 2.1 (P-type) or Ca V 2.2 (N-type) Ca 2+ channels open for hundreds of milliseconds.…”

Calcium regulation of spontaneous and asynchronous neurotransmitter release

“…Intriguingly, P/Q- and N-type VACCs, which trigger evoked release, will also activate sustained calcium entry following prolonged activation. This “asynchronous” Ca 2+ current decays over hundreds of milliseconds, and may directly trigger asynchronous release [91]. TRPV1, the Ca 2+ permeable non-selective cation channel, is another ion channel that may evoke asynchronous release but unlike P/Q- and N-type VACCs TRPV1 will not trigger synchronous release [6].…”