Neuronal calcium elevations are shaped by several key parameters, including the properties, density, and the spatial location of voltage-gated calcium channels (VGCCs). These features allow presynaptic terminals to translate complex firing frequencies and tune the amount of neurotransmitter released. While synchronous neurotransmitter release relies on both P/Q- and N-type VGCCs at hippocampal MF-CA3 synapses, the specific contribution of VGCCs to calcium dynamics, neurotransmitter release and short-term facilitation remains unknown. Here, we used random-access two-photon calcium imaging together with electrophysiology in acute mouse hippocampal slices to dissect the roles of P/Q- and N-type VGCCs. Our results show that N-type VGCCs control glutamate release at a limited number of release sites through highly localized Ca2+ elevations and support short-term facilitation by enhancing multivesicular release. In contrast, Ca2+ entry via P/Q-type VGCCs promotes the recruitment of additional release sites through spatially homogeneous Ca2+ elevations. Altogether, our results highlight the specialized contribution of P/Q- and N-types VGCCs to neurotransmitter release.
In presynaptic terminals, neurotransmitter release is dynamically regulated by the transient opening of different types of voltage-gated calcium channels. Hippocampal giant mossy fiber terminals display extensive short-term facilitation during repetitive activity, with a large several fold postsynaptic response increase. Though, how giant mossy fiber terminals leverage distinct types of voltage-gated calcium channels to mediate short-term facilitation remains unexplored. Here, we find that P/Q- and N-type VGCCs generate different spatial patterns of calcium elevations in giant mossy fiber terminals and support short-term facilitation through specific participation in two mechanisms. While N-type VGCCs contribute only to the synchronization of multivesicular release, P/Q-type VGCCs act through microdomain signaling to recruit additional release sites.
The authors declare no competing financial interests.
This work was supported by the Canadian Institutes of Health Research (KT) and the National Science and Engineering Research Council (KT). SC received PhD fellowships from NSERC and CTRN. AE received a PhD fellowship from CTRN. The authors wish to thank Dr. Stéphane Dieudonné for advice with RAMP microscopy and two-photon calcium imaging, Mr. Benjamin Mathieu for technical support and software development, and Dr. Simon Labrecque for assistance with automation of calcium imaging data analysis.