There has been remarkable recent progress in our understanding of the sequence of steps that mediate chemical transmission at mammalian central glutamatergic synapses. This work provides insight into how neurotransmitter is released and how the quantal response is generated.
Work on large central synapses has made it possible to voltage-clamp the presynaptic terminal, to control the Ca2+ concentration and to monitor release. This work confirms earlier studies in the squid giant axon showing that the current through voltage-dependent Ca2+ channels that triggers release occurs on the falling phase of the action potential.
The Ca2+ that triggers release occurs in a microdomain near the Ca2+ channels. This Ca2+ elevation is sensed by the molecule synaptotagmin 1. It appears that the synaptic vesicle membrane is already partially fused with the plasma membrane and awaits the activation of synaptotagmin. When this occurs, a fusion pore opens. Whether this pore is like a protein channel or is lipid-lined remains to be resolved.
The size of the fusion pore can vary and this determines the rate at which the neurotransmitter diffuses into the synaptic cleft. Modulation of fusion pore size is likely to be important for regulation.
The resulting activation of AMPA channels in the postsynaptic membrane depends on the channel's properties and much has been learnt about this through structural, molecular and electrophysiological experiments. Glutamate can cause either activation or inactivation of AMPA channels. To be effectively activated, the glutamate concentration in the synaptic cleft must be high, a condition that only holds near the site of vesicle release.
Each vesicle that is released generates a quantal response in the postsynaptic cell. Many of the factors that shape the quantal response and make it reproducible are now understood. At some synapses multiple vesicles are released, allowing fluctuations in transmission efficacy.
The properties of synaptic transmission were first elucidated at the neuromuscular junction. More recent work has examined transmission at synapses within the brain. Here we review the remarkable progress in understanding the biophysical and molecular basis of the sequential steps in this process. These steps include the elevation of Ca2+ in microdomains of the presynaptic terminal, the diffusion of transmitter through the fusion pore into the synaptic cleft and the activation of postsynaptic receptors. The results give insight into the factors that control the precision of quantal transmission and provide a framework for understanding synaptic plasticity.
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The authors would like to thank M. Jackson, J. Zimmerberg, A. Silver, S. Marty, L.-G. Wu, Z. Nusser, R. Schneggenburger, G. Fain and E. Marder for useful discussions. J.E.L. has been supported by the US National Institutes of Health (NIH) grants R01 NS27337 and R01 NS50944 as part of the Collaborative Research in Computational Neuroscience Program. R.W.T. has been supported by NIH grants NS24067 and MH64070 and generous support from the Mathers Foundation. S.R. has been supported by the National Science Foundation grant 0642000.
The authors declare no competing financial interests.
The term synapse can be used either in a structural sense or to describe an entire connection. According to the structural definition, a synapse consists of a single presynaptic active zone and postsynaptic density, together with the specialized membranes and cleft in-between. Synapse diameter is between 0.2 and 1 micron. At most dendritic spines, there is a single such synapse. Giant synaptic connections have many structural synapses; the mossy fibre boutons in CA3 have over 10, whereas the Calyx of Held has approximately 50.
The elementary building block of the EPSC, which contains an integral number of these events. Quantal size is derived from the distance between the peaks in the amplitude histogram of the EPSC, and equals the amplitude of the mEPSC at synapses where these events are uniquantal.
- Fusion pore
Provides the passage from the interior of the vesicle into the synaptic cleft, through which neurotransmitter diffuses.
- Excitatory postsynaptic current
(EPSC). The inward postsynaptic membrane current evoked by a single presynaptic action potential. It is measured using the voltage-clamp technique.
- Electrochemical driving force
The difference between the Nernst equilibrium potential for an ion and the membrane voltage. The current through a channel is the product of the channel conductance and the electrochemical driving force.
- Uncaging methods
The standard form of a compound is modified by chemical adducts, which can be cleaved by bright light to produce the standard form. One application of this technology allows rapid light-induced Ca2+ elevation.
- Release site
A site in the active zone where the vesicle is released. It remains unclear whether there is a structural basis for such sites, or whether release can occur anywhere in the active zone provided that the appropriate vesicle priming has occurred.
- Impedance method
A rapidly oscillating voltage is applied by voltage-clamp. The in-phase component of the resulting current is termed the 'real' component and can be related to patch conductance. The out-of-phase component is called the imaginary component and can be related to patch capacitance. The advantage of this method is that both conductance and capacitance can be measured simultaneously.
- Monte-Carlo simulation
A simulation that involves keeping track of the position and state of each molecule. At each short time step the computer calculates the new position or state of each molecule according to the probability of each change.
- Kinetic model
The kinetic model of an ion channel specifies the rate constants for the binding and unbinding of neurotransmitter to sites on the channel, and the rate of conformational transitions between various states that control channel opening and closing, as well as transitions to a desensitized state.
(miniature EPSC). An mEPSC is a spontaneously occurring synaptic event caused by spontaneous vesicle release. It is generally measured after blocking action potentials with tetrodotoxin to insure that there is no release due to spontaneous action potentials. The amplitude of mEPSCs is determined by AMPA channel density and is taken as a measure of postsynaptic processes in traditional quantal analysis. However, mEPSC amplitude can also be affected by vesicle glutamate concentration, multi-vesicular release and glutamate release mode.
- Outside-out patch
A variant of the patch-clamp technique, in which a patch of plasma membrane covers the tip of the electrode. The outside of the membrane is exposed to bathing solution.
- Coefficient of variation
(CV). The standard deviation divided by the mean. The CV is thus a convenient measure of the relative variability of a quantity. For instance, a CV of 0.2 would mean that most measurements were within plus or minus 20% of the mean.
- Amplitude histogram
An amplitude histogram of the EPSC is made by defining amplitude bins and then plotting the number of occurrences that fall within each bin as a function of amplitude. For responses comprising a variable number of elementary quantal units, the histogram should show multiple peaks at integral multiples of quantal size. In practice, the peaks become smeared because of the CV of quantal size and because of measurement noise.
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Lisman, J., Raghavachari, S. & Tsien, R. The sequence of events that underlie quantal transmission at central glutamatergic synapses. Nat Rev Neurosci 8, 597–609 (2007). https://doi.org/10.1038/nrn2191
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