Functional relevance of mobile and clustered Ca _V 2.1 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 Ca _V 2.1 channels at glutamatergic synapses of hippocampal neurons revealed that apart from Ca _V 2.1 channels aggregated in stable nanodomain clusters, a substantial fraction of Ca _v 2.1 channels remained mobile, raising the question of whether these dispersed channels contribute to synaptic function. Mathematical modelling predicted that dispersed Ca _v 2.1 channels cooperatively enhance release reliability. Upon repetitive stimulation, mobile Ca _V 2.1 channels enable alternative use of release sites and thereby reduce the probability of failed presynaptic release. Both optogenetic immobilisation of Ca _V 2.1 channels per se or activation of GABA _B receptors (GABA _B Rs) alone increase the failure rate and can lead to synaptic silencing. However, optogenetic clustering Ca _V 2.1 channels prior to GABA _B R activation increases the fraction of synapses that remain active even in presence of GABA _B R 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 Ca _V 2.1 channels increase the frequency range of synaptic transmission but are particularly sensitive to metabotropic GABA _B R-mediated inhibition in glutamatergic hippocampal synapses.