Abstract:Variance analysis of postsynaptic current amplitudes suggests the presence of distinct docking sites (also called release sites) where vesicles pause before exocytosis. Docked vesicles participate in the readily releasable pool (RRP), but the relation between docking site number and RRP size remains unclear. It is also unclear whether all vesicles of the RRP are equally release competent, and what cellular mechanisms underlie RRP renewal. We address here these questions at single glutamatergic synapses, counting released vesicles using a new developed method based on deconvolution analysis. Vesicular release counts at simple synapses follow binomial statistics with a maximum that varies from 2 to 10 among experiments. This maximum presumably reflects the number of docking sites at a given synapse. We find a remarkably low variance of cumulative vesicle counts during action potential trains. This, combined with Monte Carlo simulations, indicates that vesicles transit through 2 successive states before exocytosis, so that the RRP is up to 2-fold higher than the docking site number. The transition to the second state has a very rapid rate constant, and is specifically inhibited by latrunculin B and blebbistatin, suggesting the involvement of actin and myosin.

Reference:

Miki, T., Malagon, G., Pulido, C., Llano, I., Neher, E., and Marty, A. (2016). Actin- and myosin-dependent vesicle loading of presynaptic docking sites prior to exocytosis. Neuron 91, 808–823.

Malagon, G., Miki, T., Llano, I., Neher, E., and Marty, A. (2016). Counting vesicular release events reveals binomial release statistics at single glutamatergic synapses.
J. Neurosci. 36, 4010–4025.