Voltage-gated calcium channels contribute to spontaneous glutamate release directly via nanodomain coupling or indirectly via calmodulin

Abstract

Neurotransmitter release occurs either synchronously to action potentials or spontaneously, yet whether molecular machineries underlying evoked and spontaneous release are identical, especially whether voltage-gated Ca2+ channels (VGCCs) can trigger spontaneous events has been in debate. To elucidate this issue, we characterized Ca2+ dependency of miniature excitatory postsynaptic currents (mEPSCs) in autaptic cultured hippocampal neurons. We found that 58 % mEPSC frequency was dependent on extracellular Ca2+([Ca2+]o), and Ca2+cooperativity of spontaneous release was comparable to that of evoked release. Moreover, most (> 90 %) of [Ca2+]o-dependent mEPSCs was attributable to VGCCs. Coupling distance between VGCCs and Ca2+ sensors was estimated as tight for both spontaneous and evoked release (~22 nm). In hippocampal slices, VGCC-dependence on spontaneous release was also observed, but to a different extent, at different areas and ages. At the calyx of Held synapses, mEPSCs showed VGCC-dependence in type 1 mature synapses where VGCCs and Ca2+ sensors are tightly coupled, but not in immature synapses. These data strongly suggest that the distance between VGCCs and Ca2+ sensors is the key factor to determine VGCC dependence of spontaneous release.