Part III: Modeling Neurotransmission – A Cholinergic Synapse
Operation of the nervous system is dependent on the flow of information through chains of neurons
functionally connected by synapses. The neuron conducting impulses toward the synapse is the
presynaptic neuron, and the neuron transmitting the signal away from the synapse is the
postsynaptic neuron. Chemical synapses are specialized for release and reception of chemical
neurotransmitters. For the most part, neurotransmitter receptors in the membrane of the
postsynaptic cell are either 1.) channel-linked receptors, which mediate fast synaptic transmission,
or 2.) G protein-linked receptors, which oversee slow synaptic responses. Channel-linked
receptors are ligand-gated ion channels that interact directly with a neurotransmitter and are called
ionotropic receptors. Alternatively, metabotropic receptors do not have a channel that opens
or closes but rather, are linked to a G-protein. Once the neurotransmitter binds to the metabotropic
receptor, the receptor activates the G-protein which, in turn, goes on to activate another molecule.
3a. Model the ionotropic cholinergic synapse shown below. Be sure to label all of the following:
voltage-gated sodium channel, voltage-gated potassium channel, neurotransmitter, synaptic
vesicle, presynaptic cell, postsynaptic cell, potassium leak channel, sodium-potassium pump,
synaptic cleft, acetylcholine receptor, acetylcholinesterase, calcium channel.
When a nerve impulse (action potential) reaches the axon terminal, it sets into motion a chain of
events that triggers the release of neurotransmitter. You will next model the events of
neurotransmission at a cholinergic synapse. Cholinergic synapses utilize acetylcholine as the
chemical of neurotransmission.
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Synapse Kit: Section 3-6 | 1
Step 1 - Action potential arrives at the
terminal end of the presynaptic cell.
Step 2 - Calcium channels open in the
presynaptic axon terminal. Open the
calcium channels (red) and move some
calcium ions to the interior of the neuron.
Calcium ions bind to synaptotagmin.
Step 3 - Acetylcholine is released. Move the
synaptic vesicle to the terminal end of the neuron.
SNAP/SNARE proteins interact to bring the vesicle
in position to fuse with the cell membrane.
Step 4 - Acetylcholine binds to
postsynaptic receptors. Acetylcholine
traverses the synaptic cleft to bind to
the acetylcholine receptor on the
postsynaptic membrane
MSOE Center for BioMolecular Modeling
Synapse Kit: Section 3-6 | 2
Step 5 - Ion channels open in the
postsynaptic membrane. The
acetylcholine receptor opens to allow
sodium ions to follow their concentration
gradient into the postsynaptic cell.
Depolarization of the postsynaptic cell
occurs and an action potential may be
generated in the post synaptic cell.
Step 6 - Termination of acetylcholine
effects. The enzyme acetylcholinesterase
breaks the neurotransmitter down into
acetic acid and choline. An uptake receptor
transports choline back into the presynaptic
cell for use in the synthesis of more
acetylcholine in the presynaptic cell.
3b.
Identify the ion that triggers the SNAP/SNARE proteins to cause the synaptic vesicle to “kiss
and run”.
(Ca2+)
3c.
What molecule is released into the synaptic cleft in this model?
(acetylcholine)
3d.
Why is it important that acetylcholinesterase is present in the synaptic cleft?
(Acetylcholinesterase removes acetylcholine from the synaptic cleft in order to terminate
nervous transmission.)
3e.
What type of channel is the acetylcholine receptor in the post synaptic cell?
(a ligand-gated sodium channel)
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Synapse Kit: Section 3-6 | 3
3f.
What happens in the post synaptic cell when acetylcholine binds to the receptor?
(Sodium ions enter through the acetycholine receptor in the post synaptic cell resulting in a
depolarization of the cell.)
3g.
Identify the cholinergic synapse modeled excitatory or inhibitory. Explain your choice.
(The cholinergic synapse is excitatory because it results in a depolarization of the
postsynaptic cell.)
Part IV: Modeling a Perturbance in a Cholinergic Synapse
Next you will introduce two substances that perturb neural transmission in the cholinergic synapse.
Nicotine binds to the acetylcholine receptor to open the channel. Sarin binds to
acetylcholinesterase. Model where these substances bind in your synapse.
4a.
How would nicotine affect nervous transmission in this synapse.
(The acetylcholine channel would be held open allowing an influx of sodium ions,
depolarizing the cell.)
4b.
How does sarin perturb the cholinergic synapse?
(Acetylcholine will not be able to be removed from the synaptic cleft.)
MSOE Center for BioMolecular Modeling
Synapse Kit: Section 3-6 | 4
4c.
Compare the chemical structures of nicotine, sarin and acetylcholine. How might Aricept
perturb the cholinergic synapse?
(Aricept binds to acetylcholinesterase preventing the breakdown of acetylcholine.)
4d.
Imagine where other perturbances might occur in the synapse. Record your thoughts.
(various)
Part V: Modeling Neurotransmission – A Dopaminergic Synapse
5a. Model the metabotropic dopaminergic synapse shown below. Be sure to label all of the
following: voltage-gated sodium channel, voltage-gated potassium channel, dopamine,
synaptic vesicle, presynaptic cell, postsynaptic cell, potassium leak channel, sodiumpotassium pump, synaptic cleft, G-protein coupled receptor, calcium channel, dopamine
transporter, and vesicular transporter
When a nerve impulse (action potential) reaches the axon terminal, it sets into motion a chain of
events that triggers the release of neurotransmitter. You will next model the events of
neurotransmission at a dopaminergic synapse.
MSOE Center for BioMolecular Modeling
Synapse Kit: Section 3-6 | 5
Step 1 - Action potential arrives at the
terminal end of the presynaptic cell.
Step 3 - Dopamime is released. Move the
synaptic vesicle to the terminal end of the
neuron. SNAP/SNARE proteins interact to
bring the vesicle in position to fuse with the
cell membrane.
MSOE Center for BioMolecular Modeling
Step 2 - Calcium channels open in the
presynaptic axon terminal. Open the
calcium channels (red) and move some
calcium ions to the interior of the neuron.
Calcium ions bind to synaptotagmin.
Step 4 - Dopamine traverses the synaptic
cleft to bind to the extracellular domain of
the metabotropic receptor in the
postsynaptic membrane. The intracellular
domain of the metabotropic receptor binds
to G-proteins. The G-protein has three
subunits: alpha(α), beta(β), and
gamma(γ).
Synapse Kit: Section 3-6 | 6
Step 5 - Bound dopamine activates the
metabotropic receptor. The α subunit
dissociates from the βγ complex. The α
subunit triggers a signal cascade that
ends in the opening of ion channels,
depolarizing the postsynaptic cell.
Step 6 - The effects of dopamine are
terminated when dopamine is removed from
the synaptic cleft by the dopamine uptake
transporter and returned to the vesicle.
5b.
What neurotransmitter is released into the synaptic cleft in this model?
(dopamine)
5c.
Identify the dopaminergic synapse modeled as excitatory or inhibitory. Explain your choice.
(The dopaminergic synapse modeled is an excitatory metabotropic synapse. The
postsynaptic cell is depolarized setting up an action potential.)
5d.
Describe how an ionotropic receptor differs from a metabotropic receptor.
(The acetylcholine receptor is an ionotropic receptor in that when the neurotransmitter binds
to the receptor, a channel opens allowing ions to traverse the membrane. The dopamine
receptor is a metabotropic receptor in that the eventual movement of ions through a channel
depends on one or more metabolic steps.)
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Synapse Kit: Section 3-6 | 7
5e.
Do ions move through this metabotropic receptor? (no)
5f.
Identify the subunits of the G-protein.
(alpha, beta and gamma subunits)
5g.
Which of these subunits activates adenylyl cyclase? (alpha)
5h.
What is the role of adenylyl cyclase in this signal cascade?
(Adenylyl cyclase converts ATP to cyclic AMP which in turns binds to the regulatory domain
of protein kinase.)
5i.
What is the role of protein kinase in this model of neurotransmission?
(Protein kinase eventually phorphorylates an ion channel allowing ions to flow through the
membrane depolarizing the neuron.)
MSOE Center for BioMolecular Modeling
Synapse Kit: Section 3-6 | 8
Part VI: Modeling a Perturbance in a Dopaminergic Synapse
Next you will introduce a substance that perturbs neural transmission in the dopaminergic synapse.
Cocaine binds to the dopamine reuptake protein in the presynaptic cell.
6a.
How would cocaine affect nervous transmission in this synapse?
(Cocaine prevents the reuptake of dopamine by the dopamine reuptake receptor resulting in
a buildup of dopamine in the synapse.)
6b.
What long term effect might repeated exposure to cocaine have on the dopaminergic
synapse?
(The brain starts to adapt and becomes less sensitive to natural reinforcers and to the drug
inteself. Tolerance may develop meaning that higher doses and/or more frequent use of
cocaine is needed to register the same level of pleasure experienced during the initial use.)
MSOE Center for BioMolecular Modeling
Synapse Kit: Section 3-6 | 9