Functional relevance of mobile and clustered CaV2.1 channels in central synapses

Reliable neurotransmitter release critically depends on the spatial relationship between voltage- gated calcium channels (VGCCs) and presynaptic release sites. Single particle tracking of endogenous CaV2.1 channels at glutamatergic synapses of hippocampal neurons revealed that apart from CaV2.1 channels aggregated in stable nanodomain clusters, a substantial fraction of Cav2.1 channels remained mobile, raising the question of whether these dispersed channels contribute to synaptic function. Mathematical modelling predicted that dispersed Cav2.1 channels cooperatively enhance release reliability. Upon repetitive stimulation, mobile CaV2.1 channels enable alternative use of release sites and thereby reduce the probability of failed presynaptic release. Both optogenetic immobilisation of CaV2.1 channels per se or activation of GABAB receptors (GABABRs) alone increase the failure rate and can lead to synaptic silencing. However, optogenetic clustering CaV2.1 channels prior to GABABR activation increases the fraction of synapses that remain active even in presence of GABABR agonist. The contribution of mobile channels to reliable neurotransmitter release is frequency-dependent and is minor at stimulation frequencies 1 Hz but becomes strong at frequencies over 10 Hz. These results demonstrate that mobile presynaptic CaV2.1 channels increase the frequency range of synaptic transmission but are particularly sensitive to metabotropic GABABR-mediated inhibition in glutamatergic hippocampal synapses.