5-3 Neurotransmitter Release
Casey Henley
Action Potential
As we have covered, when an action potential propagates down the axon to the presynaptic terminal, the electrical signal will result in a release of chemical neurotransmitters that will communicate with the postsynaptic cell.
Ion flow in Terminal
Depolarization of the terminal causes voltage-gated calcium channels to open
When the action potential reaches the terminal, there is an influx of sodium ions, just like when the action potential moves down the axon. This inward current causes a depolarization of the terminal, and that depolarization activates voltage-gated calcium channels. There is a strong electrochemical gradient that moves calcium into the terminal.
Active Zones
The voltage-gated calcium channels are concentrated in the presynaptic terminal at active zones, the regions of the membrane where small molecule neurotransmitters are released. At active zones, some synaptic vesicles are docked and are ready for immediate release upon arrival of the action potential. Other neurotransmitter-filled vesicles remain in a reserve pool outside of the active zone.
Vesicles filled with neuropeptides do not dock at active zones. They are located outside of the active zone, further away from the membrane and the high density of voltage-gated calcium channels and are therefore slower to release than the small molecule transmitters.
Vesicle Docking
Docking of synaptic vesicles packaged with small molecule neurotransmitters occurs through the interaction of three membrane-bound proteins called SNARE proteins. Synaptobrevin is called a v-SNARE because it is located on the Vesicular membrane. Syntaxin and SNAP-25 are called t-SNARES because they are located on the terminal membrane, which is the Target membrane. The interaction of these three proteins leads to vesicle docking at the active zone.
Exocytosis
Exocytosis of neurotransmitters is dependent on calcium
The influx of calcium through the voltage-gated calcium channels initiates the exocytosis process that leads to neurotransmitter release. Calcium enters the cell and interacts with another vesicle-bound protein called synaptotagmin. This protein is a calcium sensor, and when calcium is present at the active zone, synaptotagmin interacts with the SNARE proteins. This is the first step toward exocytosis of the synaptic vesicle.
Once synaptotagmin interacts with the SNARE proteins, the synaptic vesicle membrane fuses with the presynaptic terminal membrane, exocytosis occurs, and the neurotransmitters released.
Neurotransmitter Action
After exocytosis of the transmitter molecules, they enter the synaptic cleft and bind to receptors on the postsynaptic membrane. Receptors fall into two main categories: ligand-gated channels and G-protein coupled receptors. The next two chapters cover these receptors.
Key Takeaways
- Neurotransmitter release is dependent on the influx of calcium into the terminal
- SNARE proteins are important for vesicle docking at active zones and exocytosis
- Synaptotagmin is a calcium sensor
Test Yourself!
- Describe the events that occur in the presynaptic terminal when an action potential arrives. Include the role of Ca2+.
Video Lecture
Media Attributions
- Active Zones © Casey Henley is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
- SNARE Proteins © Casey Henley is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
- Neurotransmitter In Synapse © Casey Henley is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license