Large Ca2+-Dependent Facilitation of CaV2.1 Channels Induced by Ca2+ Photo-Uncaging

Abstract

CaV2.1 voltage-gated Ca2+ channels are the predominant trigger of neurotransmitter release in the CNS, so their Ca2+/calmodulin-dependent facilitation (CDF) could impact short-term synaptic plasticity. CDF is believed to arise from a Ca2+-dependent increase in channel open probability, but the precise magnitude of this increase has been uncertain, owing to complex superposition of CDF with channel (de)activation under conventional electrophysiological protocols. Here, we utilize photo-uncaging of Ca2+ with CaV2.1 channels fluxing Li+ currents, so that any resulting CDF is driven solely by light-induced increases in Ca2+. The figure's black trace shows that a 1 μM Ca2+ step rapidly triggers an increase in current. No current enhancement is seen in the absence of Ca2+ photo-uncaging (gray trace). Because channel (de)activation is steady the instant before Ca2+ uncaging, the increased current relates exclusively to CDF. Notably, the ∼two-fold boost in current suggests that CDF is far stronger than previously suspected. Moreover, half-maximal CDF was reached by Ca2+ concentrations of ∼0.5 μM, well within the physiological range. Given the fourth-power relation between Ca2+ entry and transmitter release, our results suggest that CaV2.1 channel CDF could play a dominant role in producing short-term synaptic facilitation.View Large Image | View Hi-Res Image | Download PowerPoint Slide