Alteration of Ca²⁺ dependence of neurotransmitter release by disruption of Ca²⁺ channel/syntaxin interaction

Presynaptic N-type calcium channels interact with syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) through a binding site in the intracellular loop connecting domains II and III of the α₁ subunit. This binding region was loaded into embryonic spinal neurons of Xenopus by early blastomere injection. After culturing, synaptic transmission of peptide-loaded and control cells was compared by measuring postsynaptic responses under different external Ca²⁺ concentrations. The relative transmitter release of injected neurons was reduced by ~25% at physiological CA²⁺ concentration, whereas injection of the corresponding region of the L- type Ca²⁺ channel had virtually no effect. When applied to a theoretical model, these results imply that 70% of the formerly linked vesicles have been uncoupled after action of the peptide. Our data suggest that severing the physical interaction between presynaptic calcium channels and synaptic proteins will not prevent synaptic transmission at this synapse but will make it less efficient by shifting its Ca²⁺ dependence to higher values.