Abstract
Synapses are important functional units that determine how information flows through the brain. Understanding their biophysical properties and the molecules that underpin them is an important goal of cellular neuroscience. Thus, it is of interest to develop protocols that allow easy measurement of synaptic parameters in model systems that permit molecular manipulations. Here, we used a sensitive and high-time resolution optical approach that allowed us to characterize two functional parameters critical to presynaptic efficacy: vesicle fusion probability (Pv) and readily-releasable pool size (RRP). We implemented two different approaches to determine the RRP size that were in broad agreement: depletion of the RRP by high-frequency stimulation and saturation of the calcium sensor during single action potential stimuli. Our methods are based on reporters that provide a robust, quantitative, purely presynaptic readout and present a new avenue to study molecules that affect synaptic vesicle exocytosis.
Highlights
Information flow in the brain is controlled in part by modulating the efficacy of synaptic transmission
It has generally been assumed that the readily-releasable pool of synaptic vesicles (RRP) consists of vesicles that are docked at the plasma membrane and “primed”
Our measurements showed that ∼6–7% of all the releasable vesicles in a synapse are in a primed state, ready to fuse in response to an action potential (AP) with 0.10 average probability
Summary
Information flow in the brain is controlled in part by modulating the efficacy of synaptic transmission. We developed a high time resolution presynaptic optical approach to measure key biophysical parameters of the neurotransmitter release machinery at central nervous system synapses. Within the conceptual framework of the quantal hypothesis of neurotransmitter release, an important presynaptic variable is the number of vesicles (N) that fuse with the plasma membrane in response to one action potential (AP). Assuming vesicles are uniform and exocytose independently of each other, the average N will be determined by the size of the readily-releasable pool of synaptic vesicles (RRP) and the probability that a vesicle in that pool will undergo exocytosis in response to a single action potential (Pv) (Schneggenburger et al, 2002): N = Pv.RRP (1). Under a binomial model consistent with the aforementioned assumptions, Pr will be set by Pv and RRP: Pr =1 − (1 − Pv)RRP (2)
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