Fig. 2: Release of inhibition model describes the kinetics of vesicular release in the calyx of Held.

a Time-course of vesicular release rate simulated in response to 4, 8, and 16 µM \([{{{{\rm{Ca}}}}}^{2+}]\) steps for the single and dual synaptotagmin/SNARE clamp architectures considered in the model (solid coloured traces) and for the benchmark allosteric model (dashed grey trace) that describes vesicular release kinetics recorded in the calyx of Held⁵². b Dependency of the peak release rate (achieved within 10 ms) on the amplitude of a \([{{{{\rm{Ca}}}}}^{2+}]\) step. The numbers indicate the slopes corresponding to the exponent of the power relationship between peak vesicular release rate and \([{{{{\rm{Ca}}}}}^{2+}]\) in the range of 4–16 µM. c Time evolution of the mean number of unclamped SNAREpins (‘Free SNAREs’) on all docked SVs (solid lines) and on SVs at the instance of fusion (dotted lines) in response to 4, 8, and 16 µM \([{{{{\rm{Ca}}}}}^{2+}]\) steps. Shaded areas indicate 1 standard deviation on each side of the mean. Each time point includes data from a 0.25 ms bin. The colour code is the same as in (a). d The relationship between peak release rate and the fraction of SVs that have three unclamped SNARE complexes at the instance of fusion. The dotted line represents an asymptote for the case when fusion may only occur for vesicles that have exactly 3 unclamped SNARE complexes (i.e. the product of the fraction of vesicles with 3 unclamped SNAREs and the Arrhenius rate \({R}_{{{{{\rm{rate}}}}}}(3)\) = 8.1 ms⁻¹, see Fig. 1c and Methods). Data points represent mean values taken over a 0.25 ms bin centred on the time of peak release rate. For each \([{{{{\rm{Ca}}}}}^{2+}]\) step and fusion clamp architecture, at least N = 500,000 stochastic simulations were performed with at least 2000 vesicular fusion events recorded during the first 10 ms time window. This restricted the normalised root mean squared error in stochastic estimates of the kinetics of SV fusion to less than 1% (see Supplementary Fig. 5).