Calcium and Vesicle Recruitment at the Calyx of Held

When it comes to synaptic vesicles, synapses seem to know a thing or two about supply and demand. High-frequency stimulation increases exocytosis and results in an accelerated recruitment of additional vesicles within the nerve terminal, a process thought to rely on intracellular calcium. This week, Hosoi et al. took advantage of the accessibility of the presynaptic terminal at the calyx of Held to examine the relationship between intracellular calcium and vesicle recruitment. The authors used paired presynaptic and postsynaptic recordings to measure simultaneously presynaptic calcium levels and postsynaptic responses. Replenishment of the fast-releasing vesicle pool increased with presynaptic calcium levels, thus helping to sustain release during high-frequency firing. This relationship was quasilinear, similar to the calcium dependence of facilitation and posttetanic potentiation. These processes have much slower kinetics than the highly nonlinear calcium dependence of transmitter release and thus appear to be tuned to slowly changing global calcium levels in nerve terminals.