The high variance of AMPA receptor- and NMDA receptor-mediated responses at single hippocampal synapses: Evidence for multiquantal release

Abstract

Most of our knowledge about transmission at central synapses has been obtained by studying populations of synapses, but some important properties of synapses can be determined only by studying them individually. An important issue is whether a presynaptic action potential causes, at most, a single vesicle to be released, or whether multiquantal transmission is possible. Previous work in the CA1 region has shown that the response to stimulation of a single axon can be highly variable, apparently because it is composed of a variable number of quantal elements (Ϸ5 pA in amplitude). These quantal events have a low coefficient of variation (CV). Because the number of synaptic contacts involved is not known, the response could be because of uniquantal transmission at a varying number of synapses, or to multliquantal transmission at a single synapse. The former predicts that the CV at individual synapses should be small. We have used optical methods to measure the N-methyl-D-aspartate receptormediated Ca 2؉ elevation at single active synapses. Our main finding is that the amplitude of nonfailure responses could be highly variable, having a CV as large as 0.63. In one fortuitous experiment, the optically studied synapse was the only active synapse, and we could therefore measure both its N-methyl-D-aspartate (NMDA) receptorand ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated signals. At this synapse, both signals varied over a 10-fold range and were highly correlated. These results strongly suggest that transmission at single CA1 synapses can be multiquantal. Furthermore, the individual quantal response is very far from saturation, allowing the effective summation of many quanta. The existence of multiquantal release has important implications for defining synaptic strength and understanding the mechanisms of synaptic plasticity.A lthough there has been rapid progress in understanding the properties of central synapses, most experiments have been done on populations of synapses. Such studies can answer many types of questions, but provide little direct evidence regarding the stochastic properties of individual synapses. Recently, individual central synapses have begun to be studied (reviewed in ref. 1). An important question relates to the number of vesicles that are released and the interaction between them. There are indications that transmission can be uniquantal at some synapses, but multiquantal at others (2-4).The most extensively studied central synapse is the Schaffer collateral synapse onto CA1 hippocampal pyramidal cells. However, even for this synapse, there remain substantial questions about the properties of quantal transmission. A minimal stimulation method has been extensively used to study the ''unitary'' responses generated by single axonal inputs. There are now reports from seven independent laboratories indicating that the amplitude distribution of the excitatory postsynaptic potential (EPSP) can have evenly spaced peaks (5-12), the signature of quantal transmission. Stati...

Spine Neck Plasticity Controls Postsynaptic Calcium Signals through Electrical Compartmentalization

“…Individual Schaffer collateral synapses have been documented to produce up to 40 pA of current (Conti and Lisman, 2003), but the number of active AMPARs in our model spine (supplemental Table 1, available at www.jneurosci.org as supplemental material) is still higher than most previous estimates (Matsuzaki et al, 2001;Nimchinsky et al, 2004). When we tuned our model, we used only data from spines that did produce sizable calcium transients in current clamp.…”

Multivesicular Release at Schaffer Collateral–CA1 Hippocampal Synapses

“…However, the criteria by which single synapses are selected in minimal stimulation experiments will reject those that release more than a single vesicle. In contrast, optical measurements of NMDA receptor-mediated calcium transients in single spines suggested that potency increases with P r , arguing in favor of MVR (Oertner et al, 2002;Conti and Lisman, 2003). Theoretically, these results could be also explained by enhanced spillover from neighboring release sites (Barbour and Häusser, 1997;Rusakov and Kullmann, 1998) or perhaps fusion pore regulation that controls the rate of transmitter release (Choi et al, 2000;Renger et al, 2001).…”

Quantal Size Is Independent of the Release Probability at Hippocampal Excitatory Synapses

“…It is generally agreed that an increase in the P r occurs during facilitation; however, it is still debated whether the increase in P r is solely responsible for the facilitation or whether postsynaptic modifications also take place. Several studies reported an exclusive presynaptic alteration (Gulyas et al, 1993;Stevens and Wang, 1995;Dobrunz and Stevens, 1997;Hanse and Gustafsson, 2001;Silver et al, 2003;Chen et al, 2004;Lawrence et al, 2004), but evidence has also been published supporting significant postsynaptic modifications, including an increased quantal size after multivesicular release (Oertner et al, 2002;Conti and Lisman, 2003) or removing the polyamine block from certain postsynaptic glutamate receptors (Rozov and Burnashev, 1999). To some extent, the reason for the discrepancy is likely attributable to differences in experimental approaches, such as the preparations, the age of the animals, and the identity of the synapses examined.…”

“…This led to the formulation of the one-vesicle hypothesis (Korn et al, 1994): either no vesicle or only one vesicle is released at one release site after the arrival of a presynaptic action potential. Several recent studies tested the one-release site, one-vesicle hypothesis at mammalian central synapses and came to various conclusions (Auger et al, 1998;Wadiche and Jahr, 2001;Oertner et al, 2002;Schneggenburger et al, 2002;Conti and Lisman, 2003) (but see Gulyas et al, 1993;Stevens and Wang, 1995;Dobrunz and Stevens, 1997;Hanse and Gustafsson, 2001;Silver et al, 2003;Lawrence et al, 2004). As mentioned above, the discrepancy may be the consequence of technical differences, most notably distinctions in the way presynaptic axons were stimulated.…”