Fig. 3.

Superpriming model. (A) An SV can be either in the normally primed or in the superprimed state. Each state has its own replenishment rate and release probability. (B) Two examples of WT synapses. One has a small initial EPSC amplitude, and the other a large one. Both converge toward the same EPSC levels after a few high-frequency stimulations. (C) A similar example for two KO synapses. Even after 20 stimulations, they still retain some degree of their initial differences. (D) Isolation of the superprimed component of the EPSC high-frequency trains (circles). The time constant of an exponential fit to the data points is larger for the KO synapses, indicating a lower superprimed SV release probability for that group. Similarly, the y axis intersection of a straight line fit to the late data points of the cumulative plot indicates a lower release probability for the superprimed SVs of the KO cells (crosses). The small bump in the cumulative plot between stimulus numbers 5 and 15 is an indication of postsynaptic receptor desensitization, mostly at the high EPSC1 synapses. (Inset) The first 10 traces of the cumulative graph, expanded. (E) Four representative examples of EPSC trains predepleted by a low-frequency train. All have a similar time constant of decay, indicating a similar range of release probabilities for the normally primed SVs. Although the curves converge to very similar levels, for the given phenotypes, this level is slightly different for WT and KO synapses.