An analytical method to measure the contribution of clear synaptic and dense-core peri-synaptic vesicles to neurotransmitter release from synaptic terminals with two classes of secretory vesicles.

Two types of secretory vesicles co-exist at some presynaptic terminals. Clear synaptic vesicles (CSV) release their contents at the synaptic active zone, upon single impulses, while dense-core vesicles (DCV) usually release their contents in the periphery of the terminal upon repetitive stimulation. Part of the transmitter released by DCV diffuses to produce paracrine effects, and part of it reaches the postsynaptic terminal, adding its effect to that of synaptic release. This article presents an analytical method to separate the contribution of CSV and DCV to the postsynaptic responses, based on the kinetics of postsynaptic currents (PSCs). Since stimulation with single presynaptic impulses usually triggers release only from CSV, the kinetics of the resulting PSC can be used as a template to model the postsynaptic response to release from CSV during stimulation trains, accounting for the variations in the amplitude of PSCs due to short-term synaptic plasticity. Subtraction of this model simulation to the total recorded PSC renders the response to DCV peri‑synaptic release, which has slower kinetics. The method can be further simplified by measuring only the amplitudes of the PSC peaks for synaptic release and the integral of the current for peri‑synaptic release.•The postsynaptic current in response to presynaptic release from clear synaptic vesicles is modeled using the kinetics of the PSC in response to single impulses.•The model synaptic response is subtracted from the total recorded PSC to obtain the response to peri‑synaptic release from dense-core vesicles.