Figure 9.

A simple model for the frequency dependence of synaptic vesicle reuse. A, The figure depicts the kinetic model we used to account for our observations. In this model, synapses initially occupy the “rest” state and transition to the “reuse” state in response to stimulation with a rate α. Synapses transition from the reuse state to a state of “exhaustion” with a rate β. The transition rates between the states (α and β) depend on stimulation frequency. B–I, The difference between folimycin-treated and control depression profiles at 1, 10, 20, and 30 Hz stimulations (data from Figs. 1, 2) plotted with respect to the number of action potentials for excitatory (B, D, F, H) and inhibitory (C, E, G, I) synapses (open circles). Solid lines depict the progression of the reuse state during stimulation estimated by fitting the model to the data by minimizing the mean square of the error. The occupancy of reuse state was calculated by solving the following first-order equations: d(rest)/dn = −α(rest), d(reuse)/dn = α(rest) − β(reuse), and d(exhaustion)/dn = β(reuse), where n denotes the number of action potentials. J, In the case of excitatory synapses, the rates α and β increase monotonically in response to the increase in the frequency of stimulation. K, In inhibitory synapses, the same rates plateau after 10 Hz.