The ubiquitous nature of multivesicular release

Abstract

'Simplicity is prerequisite for reliability' (E.W. Dijkstra [1]) Presynaptic action potentials trigger the fusion of vesicles to release neurotransmitter onto postsynaptic neurons. Each release site was originally thought to liberate at most one vesicle per action potential in a probabilistic fashion, rendering synaptic transmission unreliable. However, the simultaneous release of several vesicles, or multivesicular release (MVR), represents a simple mechanism to overcome the intrinsic unreliability of synaptic transmission. MVR was initially identified at specialized synapses but is now known to be common throughout the brain. MVR determines the temporal and spatial dispersion of transmitter, controls the extent of receptor activation, and contributes to adapting synaptic strength during plasticity and neuromodulation. MVR consequently represents a widespread mechanism that extends the dynamic range of synaptic processing.

Figure 1. Multiquantal release at single synapses

(Ai and Aii) Electron micrograph and corresponding cartoon of a hippocampal synapse imaged following rapid freezing. Arrows indicate the presence of three docked vesicles at the active zone. (Bi) Electron micrograph of two synaptic vesicles fusing with the within an active zone simultaneously. (Bii) Cartoon representation on (Bi) illustrating neurotransmitter released from MVR interacts with a common pool of postsynaptic receptors (green and orange). Adapted from [22]. (C) Overview of different release modalities and their consequences for postsynaptic currents. UVR causes a low-concentration, short-lived transmitter transient in the synaptic cleft that results in a small postsynaptic EPSC (left). Synchronous fusion of several vesicles, MVR, results in elevated synaptic transmitter concentration and a large postsynaptic EPSC (middle). Desynchronized MVR prolongs but reduces the transmitter concentration transient resulting in a smaller and slowed postsynaptic EPSC (right). Adapted from [39].

Figure 2. Detecting multivesicular release (MVR)

(Ai) Reconstruction from paired recordings of a presynaptic basket (soma and dendrites in black, axon red) and postsynaptic pyramidal cell (soma and dendrites in blue, axon gray). Inset, EM-identified boutons shown at higher magnification. (Aii) Electron micrographs show synaptic contacts between the presynaptic basket terminals and the pyramidal cell soma. Arrowheads indicate synaptic junctions. (Aiii) Presynaptic action potentials (red) and resulting currents (blue) recorded under low P_r (left) and high P_r (right) conditions. Variance versus mean plot from current responses recorded in (Aiii) was used to derive quantal parameters (bottom). Adapted from [30]. (Bi) Top: Slow-off, high-affinity antagonist (labeled as ‘slow’) occupies postsynaptic receptors (in orange). Bottom: As a result of slow antagonist unbinding, synaptically released neurotransmitter (NT) binds only to the fraction of receptors that is not bound by the antagonist. (Bii) Top: Due to its fast off-rate a low-affinity antagonist (labeled as ‘fast’) rapidly unbinds from receptors. Bottom: Synaptically released transmitter competes with the antagonist for receptor binding sites. Therefore, inhibition by the antagonist depends on the transmitter concentration and time course. (Biii) The slow high-affinity antagonist inhibits PSCs recorded under high and low P_r conditions to the same extent (left). The fast low-affinity antagonist inhibits the EPSC recorded under high P_r conditions to a lesser extent than at low P_r conditions (right). Adapted from [39]. (Ci) Raster image of a dendrite with spines (red fluorescence) and the calcium transient after synaptic stimulation (green fluorescence). White lines and arrows indicate position of the line scan and time of synaptic stimulation, respectively. Synaptic stimulation results in a rapid increase of the green fluorescence (calcium sensitive) that does not occur with the red fluorescence (calcium insensitive). (Cii) The ratio of green/red fluorescence in response to a single (left) or a pair of synaptic stimuli (right) shown by arrows. Failures of synaptic transmission can be clearly distinguished from successes. (Ciii) Average fluorescence responses to a single successful stimulus (yellow) and to the second stimulus in a pair when the first stimulation produced a failure (green). The failure to both stimuli is also shown (black). Adapted from [15].