Independent Sources of Quantal Variability at Single Glutamatergic Synapses

“The latter is consistent with electron microscopy studies (Nusser et al., 1998; Takumi et al., 1999) and the observation that bigger synapses produce bigger responses (Andrasfalvy and Magee, 2001; Matsuzaki et al., 2001).”

“Further, the distribution of charge injected during evoked miniature excitatory postsynaptic currents in cultured hippocampal neurons is not a Gaussian distribution, but is strongly skewed towards larger events, which also indicates nonuniform vesicle sizes (Bekkers and Stevens, 1995; Franks et al., 2003).”

“As a consequence, organization of AMPARs inside the PSD, together with their proximity with presynaptic release sites, tune the strength and the variability of synaptic responses (Franks et al., 2002; Franks et al., 2003).”

“Modeling studies indicate that AMPA receptor density within the PSD is more important than the overall size of the PSD (Franks et al., 2003; Raghavachari and Lisman, 2004; Lisman and Raghavachari, 2006).”

“…1997; Liu et al., 1999; Mainen et al., 1999; McAllister and Stevens, 2000), and variability in excitatory quantal size is increasingly considered to be significantly attributable to presynaptic fluctuations in transmitter release (Liu et al., 1999; Hanse and Gustafsson, 2001; Franks et al., 2003).”

“…NMDA receptors, with only a few of the 2–20 conducting at any time (Nimchinsky, 2004); 4) Location of release site relative to AMPA receptors (Franks et al., 2003; Ashby et al., 2006; Kusters et al., 2013) 5) Few voltage-dependent calcium channels (VDCCs) in spines that affect synaptic…”

“In the case of AMPA receptors, intrasynaptic positioning relative to presynaptic release sites is predicted to markedly influence synaptic transmission (Franks et al., 2003; Raghavachari and Lisman, 2004).”

“Based on published computational studies, we made the conservative assumption that n is distributed normally with a large coefficient of variation (CV) (0.58) (Franks et al., 2003), P͑n͒ dn ϭ exp͑ Ϫ ͑n Ϫ ͗n͒͘ 2 /͑0.”

“The mechanisms of receptor insertion and removal at the postsynaptic membrane and of incorporation and maintenance in functional clusters within the postsynaptic density have been the topics of intensive research (Nusser, 2000; Savtchenko et al., 2000; Sheng, 2001; Franks et al., 2003).”

“…the complexity of which is perhaps best reflected by a number of excellent models of glutamate signaling (Rusakov and Kullmann, 1998; Rusakov, 2001, 2011; Franks et al., 2002; Lehre and Rusakov, 2002; Rusakov and Lehre, 2002; Franks et al., 2003; Raghavachari and Lisman, 2004; Zheng et al., 2008).”

“However, the proportion of large and small terminals in different pathways varied and can provide useful information about the robustness, efficacy, and level of activity of a pathway (Pierce and Lewin, 1994; Murthy et al., 2001; Franks et al., 2003; Germuska et al., 2006; Zikopoulos and Barbas, 2006, 2007b; Medalla a…”

“Indeed, simulations have suggested that the variability in the location of vesicle opening contributes to the variability of the PSP — up to 36”

“4E,F) suggests a simple “averaging” principle to reduce the stochastic noise in surface AMPAR insertion inherent to the regulation of individual exocytic events (Franks et al., 2003), potentially enhancing the reliability and precision of surface AMPAR delivery to individual synapses.”

“1A. Glutamate molecules reaching the edge of the synapse at 350 nm were absorbed as a simplified model for diffusion and uptake in the neuropil surrounding the synapse (Franks et al. 2002).”

“The postsynaptic response to the fusion of a single synaptic vesicle is a fundamental determinant of synaptic strength that is controlled in part by the number of neurotransmitter molecules within the vesicle (Bruns et al., 2000; Colliver et al., 2000; Franks et al., 2003; Gong et al., 2003; Pothos et al., 2000).”

“Each component has influence on the efficacy of synaptic transmission via several mechanisms at the level of single synapses (Franks et al., 2003; Liu, 2003).”

“…that the probability of AMPAR opening is highest near vesicle release sites, producing local hotspots (< 0.03 μ m 2 ) of maximally activated AMPARs that cover only a fraction of the total PSD area (~25% of an average PSD in a CA1 synapse) (Franks et al., 2003; Raghavachari and Lisman, 2004).”

“The interplay of these factors with synapse morphology has been investigated in great detail with Monte Carlo simulations (Stiles et al., 1996; Franks et al., 2003; Coggan et al., 2005; Postlethwaite et al., 2007), which are in particular useful to understand the sources of variability at synapses.”

“Most diffusion models predict that the extrasynaptic concentrations of transmitter attained by diffusion out of the cleft are relatively small (Eccles and Jaeger, 1958; Barbour, 2001; Franks et al., 2003).”

“…on choice may be a vestige of an implicit dualism between the mentalism of choice and the physicalism of action, the processes underlying decision as a means for forming intentions seem to be a central aspect of volition even if one rejects dualism, as most contemporary philosophers and…”

“Given the low affinity of AMPA receptors for glutamate, models of quantal synaptic transmission restrict postsynaptic AMPA receptor activation to subsynaptic hotspots, 250 nm in diameter, apposed to presynaptic vesicle release sites (Franks et al., 2003; Raghavachari and Lisman, 2004).”

“Coupling would optimally localize clustered receptors within the post-synaptic density so that they could be efficiently activated by SV fusion (Franks et al., 2003).”

“As morphological characteristics including receptor distributions have been suggested to influence synaptic property (Franks et al., 2002, 2003; Xu-Friedman and Regehr, 2004), it appeared conceivable that vesicular release of glutamate in close proximity to AMPAR microclusters would result in…”

“…size variation Higher glutamate concentrations experienced by postsynaptic receptors could be caused by either a larger amount of released transmitter or a shorter distance (distancefusion_receptor) between the fusion site and the center of the postsynaptic receptor cluster (Franks et al., 2003).”

“The activation of postsynaptic NMDA receptors by glutamate released in an idealized, 3D neuropil has been described elsewhere [31,32,33].”

“For these reasons, the distribution of AMPARs within a synapse is broadly thought to influence synaptic efficacy (Xie et al., 1997; Franks et al., 2003; Raghavachari and Lisman, 2004; Lisman et al., 2007).”

“To reproduce the observed variability of mEPSCs (see Results; Franks et al. 2003; Raghavachari & Lisman, 2004), the precise location of the vesicle was assumed to be uniformly distributed over an area restricted to 200 nm relative to the centre of this PSD.”

“2μm(2)/ms [7] was used in all simulations, we here explore the effect on glutamate spreading and uptake for a doubled diffusion constant Ddouble = 0.”

“The importance of precise postsynaptic localization of AMPARs Because the affinity of AMPARs for glutamate is relatively low (high micromolar range), it had been predicted that only AMPARs that are precisely juxtaposed to presynaptic release sites are effectively activated (84, 85).”

“Amongst them reduction in vesicular glutamate concentration (quantal size) and receptor desensitization play a significant role in lowering the amplitude of the postsynaptic response at central glutamatergic synapses [7–10].”

“Second, synaptic receptor binding and activation is a stochastic process controlled by conformational changes of receptor proteins [22,23].”

“…an EPSC, the areal spread of glutamate from a single vesicle covers 0.04 – 0.05 m 2 of the PSD area (Clements et al., 1992; Diamond and Jahr, 1997; Franks et al., 2003; Raghavachari and Lisman, 2004), which can saturate AMPA receptors on small simple synapses but not on large perforated…”

“These measurements suggest that an AMPAR in a nascent zone would have a much lower probability of response than AMPARs anchored beneath docking sites in the active zone (Franks et al., 2003).”

“A drawback of this approach is that this also requires an appropriate model of the time course of neurotransmitter seen by the receptors, which has to be obtained by more detailed diffusion models (see e.g., Franks et al., 2003; Postlethwaite et al., 2007).”

“Localization of AMPARs in those nanoclusters is thought to be functionally important because affinity of AMPARs for glutamate is fairly low and only AMPARs that are exactly juxtaposed to presynaptic release sites might be effectively activated (Franks et al, 2003; Lisman & Raghavachari, 2006).”

“…factors, numerical modeling suggests that while AMPA receptors directly across from the site of release open with fairly high probability (~0.6), receptors laterally displaced by even 50 nm are much less likely to open (Franks et al., 2003; Raghavachari and Lisman, 2004; Xie et al., 1997).”

“Synaptic strength is modulated by both the density and positioning of AMPARs within the synapse (Xie et al., 1997; Franks et al., 2003; Raghavachari and Lisman, 2004; Lisman et al., 2007; Kessels and Malinow, 2009), so determining the mechanisms that establish their synaptic entry, exit, and…”

“Although theoretical studies have realized that a steep gradient of glutamate concentration is created within the synapse (Trommershäuser et al. 1999; Franks et al. 2003), to understand the effect of such a gradient on postsynaptic response, knowledge of the distribution of the receptors within the cleft was needed.”

“Consequently, major sources of trial-to-trial variability [21] in single postsynaptic response are discussed.”

“What could be the functional consequences of such distinct sub-synaptic distributions? Monte Carlo simulations of glutamatergic synapses indicated that the location of vesicle fusion within the active zone (AZ) affects postsynaptic AMPAR open probability (Po), indicating that AMPAR unevenness within the PSD would have…”

“…activation from the release site (half-width at halfmaximum distance of peak Po 125 nm; see previous simulations for comparison) (Barbour, 2001; Franks et al., 2003; Raghavachari and Lisman, 2004; Tarusawa et al., 2009) matches well with the extent of AMPAR distribution (50% cumulative…”

“AMPARs exist in extrasynaptic membrane near the PSD (Tao-Cheng et al., 2011; Nair et al., 2013) and likely were able to support synaptic transmission (Franks et al., 2003; Tardin et al., 2003), although their contribution to synaptic transmission is expected to be smaller (Franks et al., 2003).”

“…findings showed that the amplitude of EPSCs is independent of presynaptic Ca2+ influx (Glowatzki & Fuchs, 2002; Grant et al. 2010), indicating that postsynaptic mechanisms could also contribute to the highly variable EPSC amplitude, as proposed for central glutamate synapses (Franks et al. 2003).”

“Previous studies have shown that quantal release of neurotransmitter is sufficient to saturate AMPA receptors within a postsynaptic area of 0.04 – 0.05 m 2 (Franks et al., 2003; Raghavachari and Lisman, 2004; Medalla and Luebke, 2015).”

“(36) modeled different presynaptic and postsynaptic arrangements for a simple central synapse, and 2,000–3,000 molecules of glutamate released per SV was sufficient to yield realistic outcomes for UVR.”

“, 1992), there has been strong support for a presynaptic mechanism explaining the skew in the control miniature amplitude distribution at central synapses (Frerking et al., 1995; Forti et al., 1997; Liu et al., 1999; McAllister and Stevens, 2000; Hanse and Gustafsson, 2001; Franks et al., 2003).”

“Furthermore, “off-center” release should be again accompanied with a profound decrease in charge transfer per SSV signal (Franks et al., 2003; Wu et al., 2007).”

“…mutations drastically reduce asynchronous release and associated synaptic vesicle trafficking providing the molecular framework for the differential timing of these two types of action potential dependent release (Geppert et al., 1994; Sugita et al., 2002; Bacaj et al., 2013; Li et al., 2017).”

“As mentioned above, studies in the early 2000s questioned the existence of a putative sub-PSD organization of postsynaptic proteins.(54,56,59) Unfortunately, optical microscopy is limited by diffraction to 300 nm, rendering it impossible to decipher AMPAR organization with a precision higher than the PSD size.”

“Problems that were addressed using MCell include modeling of glutamate spill-over leading to activation of extra-synaptic receptors and buffered diffusion effects due to trapping of neurotransmitter by receptors at the glutamatergic synapse [9], or variability in the size of postsynaptic potentials caused by release o…”

“In contrast, postsynaptic factors such as the distribution of postsynaptic receptors relative to the release site (Uteshev and Pennefather, 1996; Franks et al., 2003), the degree of their saturation (Mainen et al., 1999; McAllister and Stevens, 2000; Harrison and Jahr, 2003), and biophysical…”

“Computational studies predict that precise alignment between presynaptic sites of glutamate release and postsynaptic receptors increases synaptic efficiency (Xie et al., 1997; Raghavachari et al., 2004; Franks et al., 2003; Freche et al., 2011; Rusakov et al., 2011; Allam et al., 2012).”

“It is still not conclusive whether the fluctuations of synaptic strength under basal condition and plasticity result from the variations in the probability or the contents of transmitter release [16,18,20,23,25,26,35].”

“Thus, the nanoscale clustering of AMPAR is activity-dependent [90–92] and confirms modeling data on subsynaptic receptor organization and their impact on the synapse performance over time [46,93].”

“A similar conclusion has been reached in studies of many single-site synapses (for review, see Franks et al. 2003).”

“Examples include analyses of glutamate release location relative to post-synaptic receptor location during neurotransmitter release in the glutamate synapse [20], or investigating the effect of signal molecule locations on signalling during bacterial chemotaxis [21].”

“…authors have obtained different values either for the Glu concentration (0.4 M (Karunanithi et al. 2002)) and for the estimation of the internal radius and simulation have been made with 2000 molecules of Glu (Franks et al. 2003, based on data of Karunanithi et al. (2002) obtained on Drosophila).”

“01 when the AMPAR is located 200 nm away from the center of release (34).”

“Because only a subset of AMPARs that is in the direct vicinity of the presynaptic site of release becomes activated, the local density of receptors apposing the release site is a critical determinant of synaptic strength [10 – 13].”