Are unreliable release mechanisms conserved from NMJ to CNS?

Abstract

The frog neuromuscular junction (NMJ) is a strong and reliable synapse because, during activation, sufficient neurotransmitter is released to trigger a postsynaptic action potential (AP). Recent evidence supports the hypothesis that this reliability emerges from the assembly of thousands of unreliable single vesicle release sites. The mechanisms that govern this unreliability include a paucity of voltage-gated calcium channels, a low probability of calcium channel opening during an AP, and the rare triggering of synaptic vesicle fusion even when a calcium channel does open and allows calcium flux. Here, we discuss the evidence that these unreliable single vesicle release sites may be the fundamental building blocks of many types of synapses in both the peripheral and central nervous system (PNS and CNS, respectively).

Figure 1

Hypothetical model based on the unreliable single vesicle release site as a building block of mature synapses. Synaptic elements (Ca²⁺ channels and synaptic vesicles) are depicted at the time of AP invasion of the nerve terminal. (a) Recent data has led to a model for synaptic assembly based on unreliable single vesicle release sites. These consist of a synaptic vesicle, sparse numbers of closely associated Ca²⁺ channels that each open with a low probability during an AP stimulus, and vesicle fusion that is triggered by the nanodomain of Ca²⁺ flux from as few as one tightly associated Ca²⁺ channel (dark blue regions near the mouth of an open channel). Low release probability synapses can be constructed using a small number of these basic building blocks within few active zones. On the other hand, large and reliable synapses can be constructed using large numbers of the same unreliable single vesicle release site within hundreds of active zones. Variation in the number and size of active zones in these larger synapses, and the number and spatial organization of the unreliable single vesicle release sites within the active zones, likely contributes to differences in synaptic properties. The top half of this figure shows representative examples of synapses that appear to be constructed in this manner (ie. dentate gyrus basket cell boutons, mature calyx of Held synapses, frog NMJ, and mouse NMJ). (b) In contrast, some CNS synapses appear to be assembled with a large number of presynaptic Ca²⁺ channels that are loosely coupled to synaptic vesicles and open with a relatively high probability in response to an AP stimulus. Vesicle fusion at these synapses is triggered by the microdomain of Ca²⁺ that is created by the flux through many loosely coupled Ca²⁺ channels, essentially increasing background Ca²⁺ in the active zone region (summed light blue regions). The bottom half of this figure shows representative examples of synapses that appear to be constructed in this manner (ie. boutons of cerebellar parallel fibers and immature calyx of Held synapses). Evidence also suggests that during development some synapses initially assembled using microdomain-coupled vesicle release sites may mature into synapses that use unreliable single vesicle release sites [33, 36, 39].

Box 1 Figure I

Schematic diagram that compares microdomain and nanodomain coupling of a single vesicle release site to Ca²⁺ channels. (a) Nanodomain coupling occurs when vesicle fusion at single vesicle release sites is triggered by the local Ca²⁺ flux through a single or very small number of open Ca²⁺ channel(s) (dark blue regions). (b) Microdomain coupling occurs when vesicle fusion at single vesicle release sites is triggered by the summed Ca²⁺ flux through many loosely coupled Ca²⁺ channels that essentially increases background Ca²⁺ in the active zone region (summed light blue regions).