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Unit 10 - Neuronal Synapse
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Neurons in the brain are highly interconnected with each other.
The complexity of the interconnections is a basis of complex, fine-tuned behavior.
The all-or-none action potential carries the neuron's signal down the axon to synapses.
Chemical transmission at the synapse is the basis of graded levels of communication between neurons.
The process of release of a synaptic vesicle involves a large number of interacting proteins.
The large number of proteins provide numerous points for control of modulation of synaptic function from the presynaptic side.
Synaptic vesicle release is a particular case of the more general process of vesicular budding from cellular organelles and of exocytosis. Similar mechanisms apply to all.
Neuronal input and output regions are segregated
Dendrites integrate many inputs
Axon terminals provide many outputs
It is important to realize that dendrites let multiple inputs converge onto an individual neuron and that the axon terminals allow the resultant signal to diverge to many other neurons.
A typical mammalian neuron in the cortex may be in synaptic contact with 100-1000 other neurons.
This animation illustrates information flow through neuronal circuits
The action potential is a stereotyped voltage change that travels down the axon.
This shows the voltage change at a single point on the axon.
This summarizes the ionic changes that are the basis of the action potential and its propagation.
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This lecture does not deal with the action potential and assumes you have learned about it in previous courses. Some references are available in my NeuraLinksPlus bookmarks if you want to review the action potential.
The signal on the postsynaptic side is either an excotattory post-synaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).
Often, a single EPSP is not strong enough to trigger an action potential; temporal summation of many PSPs is necessary. This is shown in the next figure.
The above two figures are shown to emphasize the fact that it is the addition of many synaptic events at many synapses that are "added up" to ultimately fire the ttarget neuron.
thus, having ways to modulate this release is an important method of "tuning" the nervous system.
The Synapse and the Mechanism of Vesicle Release
This electron micrograph illustrates the important componants of a CNS synapse
Here, a cartoon highlights these components
Here are some neurotransmitters
This movie illustrates that the current from the action potential in the axon invades a synaptic bouton and causes one or more synaptic vesicles to fuse with the presynaptic membrane and release transmitter into the synaptic cleft. Association of transmitter molecules with receptors on the postsynaptic membrane opens channels that allow ions to flow, generating the PSP.
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click picture to stop
A question that has been studied for the past 50 years is, how does the current flow into the synaptic bouton trigger fusion of a synaptic vesicle with the presynaptic membrane and release of its contents into the cleft?
Historical Note
When I first learned neurophysiology in the late 1960s the "vesicle hypothesis" was gaining broad acceptance. Further, there was evidence that vesicle release required in influx of Ca++ into the bouton. But, the mechanism by which Ca++ triggered fusion of a vesicled with the membrane was untirely unknown. There was speculation that somehow speculation that calcium ions allowed a rearrangement of the bilayer-lipid membrane of the vesicle and that of the bouton to somehow intermix. It's important to note that there was no hint of a complex of proteins being involved in the process.
By the mid-1980s, little progress had been made, as can be seen from this quote from a major textbook, Principles of Neural Science, 2nd ed., by Kandel and Schwartz:
"What are the molecular mechanisms by which Ca++ leads to transmitter release? . . . One possibility is that Ca++ simply acts to facilitate directly the physical fusion of two lipid bilayer membranes, that of the vesicle membrane and that of the external membrane. A second possibility is that Ca++ acts through one or more proteins such as calmodulin, a calmodulin-sensitive protein kinase, or a phospholipid kinase to accomplish vesicle fusion."
Interestingly, as you will see in the rest of this lecture prepared in 2002, we know know a great deal of molecular detail--except for the last step: it is still not known exactly what the Ca++ does!
As summarized in the next two figures, the vesicle cycle involves:
- targeting, tethering, docking
- release
- membrane recovery and transmitter breakdown/recovery
- vesicle replenishment/recycling
The remainder of this lecture concentrates on docking and release, although much is known about the other parts of the cycle. Even with this narrowing details of some accessory proteins are omitted.
Positionion of a vesicle for release occurs at specialized sites that show up as part of a regularlly arrayed set of densities on the presynaptic membrane. Some of the proteins at the densities are involved in docking and tethering and docking, others in vesicle membrane recycling.
The SNARE complex initially consists of:
- synaptobrevin (also called VAMP), anchored in the vesicle membrane
- active syntaxin, anchored in the bouton's presynaptic membrane
- SNAP-25, a protein folded over into two strands, that helps the other two associate.
The association region of these four strands forms a coiled-coil that positions the membranes for fusion.
A crystal structure solution of the complex is pictured here. Click on it to view a movie that views this complex end-on and emphasizes the numerous hydrophobic interactions within the complex all along its length.
The protein complexin contains an alpha-helical region that must be bound to the SNARE complex to fully prime the vesicle for pore opening triggered by calcium influx.. It is shown in position in this picture (the upper pink helix).
The role of conmplexin is described in the Rizo & Sudhof review as follows:
"Binding involves a network of salt bridges, hydrogen bonds and hydrophobic interactions between complexin and the SNARE motifs of synaptobrevin and syntaxin, indicating that complexin might act as a 'tape' that seals and stabilizes the interface between the synaptobrevin and syntaxin SNARE motifs."
This figure shows some of these bonds.
It is suggested that addition of the complexin is needed to abut the vesicle to the membrane.
Experiments to date suggest that the stabilized SNARE complex is not suffficient for pore opening.
Synaptotagmin is a calcium-binding protein that needs to be bound to the SNARE complex for pore opening to occur. How it links calcium influx to pore opening is still not understood.
- There are actually a number of different synaptotagmin subtypes and more than one type is part of the SNARE complex. Subtype interaction may be important for pore opening.
- The calcium channel itself is closely associated with the SNARE complex. Allosteric changes due to its opening are suggested to be part of the rearrangement that allows the pore to open. This figure shows regions of synaptotagmin III that interaact with various release complex proteins
Presynaptic Modulation
In lamprey spinal cord the reticulospinal synapse onto a motoneuron itself receives a serotonergic synapse that causes presynaptic inhibition. The serotonin receptor on the presynaptic bouton is a GPCR. the beta-gamma subunit of the activated g-protein is the messanger than brings about the transmission and it appears to interact with the exocytotic machinery.
In other systems, interaction of g-protein beta-gamma subunits can occur directly with the calcium channel, modulating its effectiveness.
Useful References
The most current and comprehensive review is: Rizo and Sudhof. (2002) SNARES and mucn18 in synaptic fusion. Nature Reviews Neuroscience. 3:641-653.
Chen, et al. (2002) Three-Dimensional Structure of the Complexin/SNARE Complex. Neuron. 33:397-409.
Brunger (2001) Structural insights into the molecular mechanism of calcium-dependent vesicle-membrane fusion. Current Opinion in Structural Biology. 163-173.
Brunger. (2001) Structure of proteins involved in synaptic vesicle fusion in neurons. Annual Reviews Biophys. Biomol. Structure. 30:157-171.
Ybe, et al. (2000)Molecular structures of proteins involved in vesicular fusion. Traffic. 1:474-479
Fon and Edwards. (2001) Molecular mechanisms of neurotransmitter release. Muscle and Nerve. 24:581-601.
Lin and Scheller (2000) Mechanisms of synaptic vesicle exocytosis. Annual Reviews Cell Dev. Biol. 16:19-49.
Tucker and Chapman (2002) Role os synaptogamin in Ca2+ triggered exocytosis. Biochemical Journal. 366:1-13.
Atlas. (2001) Functional and physical coupling of voltage-sensitive calcium channels with exocytotic proteins. J. Neurochemistry. 77:972-985.
Augustine. (2001) How does calcium trigger neurotransmitter release? Current Opinion in neurobiology. 3:320-326.
Blackmer, et al. (2001) G Protein beta-gamma Subunit-Mediated Presynaptic Inhibition: Regulation of Exocytotic Fusion Downstream of Ca2+ Entry. Science. 292:293-297.
Jarvis and Zamponi. (2001) Interactions between presynaptic Ca2+ channels, cytoplasmic messengers and proteins of the synaptic vesicle release complex . Trends in Pharmacological Sciences. 10:519-525.