Spontaneous Synaptic Vesicle Release is Mediated by Presynaptic Vesicle Pool Size and Distribution

Synapses release neurotransmitters via synaptic vesicles (SV) in two different ways: through an action potential that leads to an evoked release, or spontaneously. While the mechanisms and probabilistic nature of evoked release is now well understood, the regulation within synapses of spontaneous release is less understood. Spontaneous release plays a significant role in maintenance and plasticity, making its regulation important to understand. Here, we introduce an entropic force model where the volume of the recycling pool, containing the mobile SVs, is constrained by the immobile SVs of the reserve pool, providing an entropic force on the readily releasable pool leading to spontaneous release. We support this model using a combination of SEM and high-resolution fluorescence microscopy, we measure SV spontaneous release, recycling pool size, total pool size, and SV mobility. From our results we observed that the spontaneous release rate increases linearly with increasing total pool size. We further show that acutely changing the SV pool distribution can either increase or decrease spontaneous release rate, which our model accurately predicts. These combined results suggests that spontaneous release is regulated by synapses in order to balance multiple competing processes.