Evidence for a voltage-dependent enhancement of neurotransmitter release mediated via the synaptic protein interaction site of N-type Ca21 channels

Secretion of neurotransmitters is initiated by voltage-gated calcium influx through presynaptic, voltagegated N-type calcium channels. These channels interact with the SNARE proteins, which are core components of the exocytosis process, via the synaptic protein interaction (synprint) site in the intracellular loop connecting domains II and III of their a1B subunit. Interruption of this interaction by competing synprint peptides inhibits fast, synchronous transmitter release. Here we identify a voltage-dependent, but calcium-independent, enhancement of transmitter release that is elicited by trains of action potentials in the presence of a hyperosmotic extracellular concentration of sucrose. This enhancement of transmitter release requires interaction of SNARE proteins with the synprint site. Our results provide evidence for a voltage-dependent signal that is transmitted by protein–protein interactions from the N-type calcium channel to the SNARE proteins and enhances neurotransmitter release by altering SNARE protein function. Release of neurotransmitters from presynaptic nerve terminals is initiated by Ca21 influx through presynaptic Ca21 channels (1). N-type Ca21 channels (2) are located in the nerve terminals of many neurons (3, 4) and mediate the Ca21 influx, which triggers transmitter release (5–9). They are composed of pore-forming a1B subunits in association with b and a2d subunits (10–13). Recent experiments have shown that N-type calcium channels also bind directly to the SNARE proteins involved in neurotransmitter release (14–16) through a synaptic protein-interaction (synprint) site in the large intracellular loop connecting domains II and III of their a1B subunits (LII-III) (17–19). Disruption of this interaction by synprint peptides reduces the efficacy of Ca21 entry in stimulating exocytosis (20, 21). These experiments support the hypothesis that interaction of SNARE proteins with the synprint site of N-type calcium channels is required to dock synaptic vesicles near the source of calcium to ensure fast, efficient transmitter