Readily releasable pool of synaptic vesicles measured at single synaptic contacts

Abstract

To distinguish between different models of vesicular release in brain synapses, it is necessary to know the number of vesicles of transmitter that can be released immediately at individual synapses by a high-calcium stimulus, the readily releasable pool (RRP). We used direct stimulation by calcium uncaging at identified, singlesite inhibitory synapses to investigate the statistics of vesicular release and the size of the RRP. Vesicular release, detected as quantal responses in the postsynaptic neuron, showed an unexpected stochastic variation in the number of quanta from stimulus to stimulus at high intracellular calcium, with a mean of 1.9 per stimulus and a maximum of three or four. The results provide direct measurement of the RRP at single synaptic sites. They are consistent with models in which release proceeds from a small number of vesicle docking sites with an average occupancy around 0.7.release site | exocytosis | interneurons | cerebellum

Impact of single-site axonal GABAergic synaptic events on cerebellar interneuron activity

“…2 A, inset; see also Fig. 3 C), that correspond to GABA release sites (Llano et al, 1997;Forti et al, 2000;Rowan et al, 2014;Trigo et al, 2012). As in the paired experiment of Fig.…”

“…The procedure was iteratively repeated over a range of synaptic conductance values. and photolyzed DM-nitrophen (see Trigo et al, 2012). Photolysis of DM-nitrophen induces a fast and homogeneous calcium transient in the presynaptic varicosity and, subsequently, a fast postsynaptic current ( Fig.…”

Numbers of presynaptic Ca 2+ channel clusters match those of functionally defined vesicular docking sites in single central synapses

“…Electron microscopy studies indicate the presence of several docked SVs per AZ with average numbers of approximately eight at PF or climbing fiber terminals (25) and two to seven at the calyx of Held (26)(27)(28). Such numbers, however, are merely indicative of docking-site numbers because SVs may bind to the plasma membrane without being fully release-competent (5) whereas, conversely, only 50-70% of docking sites are occupied by SVs at rest (10,11,29 Error bars show ± SEM. *P < 0.05 and ***P < 0.001, Student's t test.…”

“…1G). In simple GABAergic synapses, variations in quantal size are related with variations in N D , and both parameters are thought to scale with synaptic size (10,11). To test whether size variations could underlie the parallel downward trends observed for quantal size and N D , we plotted the quantal size as a function of N D , revealing a significant correlation between these two parameters ( Fig.…”

“…Despite these advances, electrophysiological methods remain the most rapid and sensitive method to assess SV exocytosis at central synapses such as the calyx of Held (7) or cerebellar mossy fiber terminals (8). In recent years, electrophysiological recordings performed at special synapses containing a single AZ and a single postsynaptic density (PSD), called "simple synapses," revealed a fixed maximum in the number of SVs that can be released per AZ by a short calcium stimulus (9)(10)(11), providing a direct evaluation of the number of docking sites per AZ. This number ranges between 1 and 6 at synapses between molecular layer interneurons (MLIs) of the cerebellum and between 3 and 6 at synapses between parallel fibers (PFs) and MLIs (9)(10)(11).…”