Spatiotemporal dynamics of multi-vesicular release is determined by heterogeneity of release sites within central synapses

Abstract

Synapses can release multiple vesicles in response to a single action potential. This multi-vesicular release (MVR) occurs at most synapses but its spatiotemporal properties and relation to uni-vesicular release (UVR) are poorly understood. Nanoscale-resolution detection of individual release events in hippocampal boutons revealed a pattern of spatial organization of MVR, which preferentially overlapped with UVR at more central release sites. Pairs of fusion events comprising MVR were also not perfectly synchronized and the earlier event within the pair occurred closer to the active zone (AZ) center. Parallel to this organization, individual release sites had a gradient of release probability extending from the AZ center to periphery. This gradient, and spatial features of MVR, were similarly tightened by buffering intracellular calcium. These observations revealed a heterogeneous landscape of release site properties within individual AZs, which determines the spatiotemporal features of MVR and is controlled by non-uniform calcium elevation across the AZ.