Differential Gating and Recruitment of P/Q- N- and R-Type Ca2+ Channels in Hippocampal Mossy Fiber Boutons

摘要

Voltage-gated Ca²⁺ channels in presynaptic terminals initiate the Ca²⁺ inflow necessary for transmitter release. At a variety of synapses, multiple Ca²⁺ channel subtypes are involved in synaptic transmission and plasticity. However, it is unknown whether presynaptic Ca²⁺ channels differ in gating properties and whether they are differentially activated by action potentials or subthreshold voltage signals. We examined Ca²⁺ channels in hippocampal mossy fiber boutons (MFBs) by presynaptic recording, using the selective blockers ω-agatoxin IVa, ω-conotoxin GVIa, and SNX-482 to separate P/Q-, N-, and R-type components. Nonstationary fluctuation analysis combined with blocker application revealed a single MFB contained on average ∼2000 channels, with 66% P/Q-, 26% N-, and 8% R-type channels. Whereas both P/Q-type and N-type Ca²⁺ channels showed high activation threshold and rapid activation and deactivation, R-type Ca²⁺ channels had a lower activation threshold and slower gating kinetics. To determine the efficacy of activation of different Ca²⁺ channel subtypes by physiologically relevant voltage waveforms, a six-state gating model reproducing the experimental observations was developed. Action potentials activated P/Q-type Ca²⁺ channels with high efficacy, whereas N- and R-type channels were activated less efficiently. Action potential broadening selectively recruited N- and R-type channels, leading to an equalization of the efficacy of channel activation. In contrast, subthreshold presynaptic events activated R-type channels more efficiently than P/Q- or N-type channels. In conclusion, single MFBs coexpress multiple types of Ca²⁺ channels, which are activated differentially by subthreshold and suprathreshold presynaptic voltage signals.