MP7: How do neurons convey information? Flashcards
(88 cards)
1
Q
What 2 important concepts were discovered by Edgar Adrian with regards to how neurons use electrical signals to communicate?
A
- Individual nerve impulses are of a consistent size (now known as action potentials)
- Neurons use the frequency of action potentials to convey the intensity of the signal
2
Q
Why are aplysia good model organisms for studying neurons?
A
They don’t have as many neurons as other model organisms so they’re easier to study.
3
Q
Where would you find the following neurons:
- pyramidal neuron
- motor neuron
- basket cell
- sensory neuron
A
- Brain (cerebral cortex, hippocampus…)
- CNS (CNS –> muscles)
- Brain (cortex and cerebellum)
- PNS (form clusters of ganglia)
4
Q
In which direction does information generally flow in vertebrate neurons?
A
Dendrites to cell bodies to axons.
5
Q
What are the functions of glia?
A
- Forming myelin sheaths in the CNS axons
- Maintain homeostasis
- Support and protection for neurons
6
Q
How is the nervous system linked to the vascular system?
A
- The blood-brain barrier (vascular system protects nervous system)
- Autonomic nervous system regulates the functions of the cardiovascular system (sympathetic and parasympathetic)
- Neurovascular coupling - changes in neuronal activity are coupled with changes in blood flow to the brain to ensure active neurons receive adequate oxygen and nutrients.
7
Q
What are the fundamental steps of neural communication?
A
- Dendrites receive signals from other neurons/sensory receptors via their dendritic spines
- Integration of signals in the cell body
- If the integrated signals are strong enough, the signal is transmitted down the axon (action potential).
- Release of neurotransmitters at the synapse.
- Reception at the postsynaptic side, resulting in depolarization or hyperpolarization, depending on the type of neurotransmitter and receptor.
- Integration of postsynaptic potentials.
- Termination by removing neurotransmitters from the synaptic cleft.
8
Q
Describe the process of the knee-jerk reflex.
A
- A tap is delivered to the knee cap
- The tap causes stretching of the quadricep muscles.
- A signal travels back to the spinal cord without interneurons, completely independent of higher centres.
- A motor neuron conducts an outwards impulse back the the femur muscle, triggering contraction.
- This contraction, coordinated with the relaxation of the antagonist hamstring muscles, causes the leg to kick.
9
Q
Give an experiment that highlights the speed at which neurons can convey information. Why is this speed important?
A
Zebrafish larva were exposed to water pulses and resulted in the fish changing its direction of travel within a few milliseconds of the water pulse.
This speed must have been selected for extensively over evolution, allowing for escape from predators.
10
Q
Many neurons have long extensions, so how are proteins synthesized locally or transported from the soma?
What experiments have shown this?
A
While some dendritic and axonal proteins are synthesized from mRNAs locally, most proteins are actively transported from the soma. There are lots of ribosomes in the dendrites and axon, meaning there can be local translation and hence quick changes in the proteome.
Injection of radioactively labeled amino acids into a dendrite allowed for observations of protein/mRNA transport. The proteins were then isolated for gel electrophoresis autoradiography, where it was shown that proteins using fast axonal transport are often membrane proteins and secreted proteins, whereas slow transport is used by cytosolic proteins and cytoskeleton proteins.
11
Q
What is the soma?
A
The cell body
12
Q
What is the membrane-associated periodic skeleton (MPS)? What is it composed of? How has it been studied?
A
A cytoskeletal structures that plays a critical role in maintaining the structural integrity and stability of the cell membrane, as well as in regulating the movement and distribution of membrane-associated proteins and lipids.
Cytoskeletal proteins actin, spectrin, and adducin form periodic structures in the axon to form the MPS.
It’s been studied using super-resolution microscopy to see how these are arranged.
13
Q
How is cargo moved along microtubules/actin filaments?
A
Cargo is moved along microtubules and actin filaments in neurons by specialized motor proteins called kinesins and dyneins for microtubules, and myosins for actin filaments.
- Kinesins = plus end
- Dynein = minus end
Kinesins and dyneins are members of the ATPase family of motor proteins, which use energy from ATP hydrolysis to move along microtubules.
14
Q
How is the cytoskeleton of neurons specialized for transport?
A
Microtubules in the axon have a very particular direction: plus ends are at the synapse and minus ends are at the soma. In the dendrites, this can be mixed.
Actin forms various structures e.g., rings, although it’s not clear what these are for.
15
Q
What causes neurons to be electrically polarized at rest? Name the pumps responsible for maintaining this, and give the inside and outside concentrations of relevant ions.
A
Ion gradients across the plasma membrane and differential ion permeability.
N+-K+ ATPase and K+-Cl- cotransporter
Inside:
- [K+] 120mM
- [Na+] 15mM
- [Cl-] 5mM
Outside:
- [K+] 4mM
- [Na+] 150mM
- [Cl-] 120mM
16
Q
What equation describes the equilibrium potential of the thought experiment when a membrane is fully permeable to only K+ ions?
A
Nernst equation –> -85mV
[intracellular]/[extracellular]
17
Q
What equation describes the membrane potential of the thought experiment when a membrane is permeable to more than one ion?
A
The Goldman-Hodgkin-Katz equation
18
Q
What is the membrane potential of an inactive neuron called? Give its exact value.
A
The resting potential.
-75mV
19
Q
State Ohm’s law equation.
A
Current = Voltage / Resistance
20
Q
How can a neuronal plasma membrane be described in terms of an electrical circuit?
A
When an ion channel is opened in the plasma membrane, ions can flow across the membrane, creating a transient change in the electrical potential of the neuron. This change in potential can be described as a voltage, and it can be measured using an electrode.
The lipid bilayer of the plasma membrane acts as a capacitor, which can store electrical charge.
The ion channels act as resistors, which can control the flow of ions across the membrane. The movement of ions across the membrane can create an electrical current, which can be described using Ohm’s law.
The battery is the equilibrium potential of an ion.
21
Q
Give the equation that relates resistance to conductance.
A
Conductance = 1/Resistance
22
Q
Describe the voltage clamp method.
A
The voltage clamp method is a technique used in electrophysiology to measure the flow of ions across the neuronal membrane in response to changes in membrane potential.
The voltage clamp method involves placing a patch pipette, which is a small glass electrode, onto the neuronal membrane and applying a controlled voltage across the membrane using an amplifier. This voltage is controlled by a feedback mechanism that measures the voltage across the membrane and adjusts the voltage being applied to maintain a constant membrane potential.
By holding the membrane potential at a specific level and recording the current flowing across the membrane, researchers can measure the conductance of ion channels and study their properties. The voltage clamp method can also be used to generate artificial action potentials, which can be used to study the mechanisms of action potential generation and propagation in neurons.
23
Q
What key discoveries were made through the voltage clamp method.
A
- The rising phase of the action potential resulted from an influx of Na+
- Na+ influx was caused by a rapid increase in Na+ conductance as a consequence of membrane depolarization
- Na+ conductance decreases despite continued depolarization, accounting for the falling phase (inactivation)
- Depolarization also caused increase in K+ conductance and this lagged behind Na+ conductance.
- The conductances are independent of each other, but both depend on the membrane potential
24
Q
What are the key three properties of an action potential?
A
- All or nothing
- Regenerative
- Unidirectional
25
How do action potentials propagate in a single direction?
Action potentials propagate in a single direction along the axon of a neuron due to the refractory period of the voltage-gated ion channels involved in generating the action potential.
26
What is saltatory conduction and why is it important for action potentials?
Saltatory conduction is the process by which action potentials propagate rapidly along the axon of a myelinated neuron by "jumping" from one node of Ranvier to the next, rather than propagating continuously along the entire length of the axon.
Saltatory conduction is important for action potentials because it allows for faster and more efficient propagation of electrical signals along the axon, which is essential for proper functioning of the nervous system.
27
What is the patch clamp technique and what does it enable the study of?
Enables the study of current flow through individual ion channels.
The patch clamp technique involves placing a small glass pipette, called a patch pipette, onto the surface of a cell membrane. The pipette is filled with an electrolyte solution that contains ions, and a small electrical current is passed through the pipette to create a seal with the membrane. Once a seal is formed, the electrical activity of the cell can be measured by recording the movement of ions across the membrane.
28
State the equation that describes the current carried by a particular ion species.
I = NPoɣ(Vm-E)
N= number of relevant channel proteins
Po = probability of being open
E = equilibrium potential of ion
29
How were potassium channel genes discovered?
Analyzing Shaker fly mutants which have mutations in potassium channels, making their legs shake where anesthetized.
30
Describe the structure-driven kinetic cycle for KcsA gating.
1. Closed state - ion conduction pathway is blocked by a gate of valine and leucine.
2. Open state - opened by the movement of the inner helix in response to changes in the electrostatic environment of the channel.
3. Inactivated state - conformational changes results in the collapse of the ion conduction pathway, thought to protect against excessive ion flow.
4. Recovery state - channel returns to its closed state.
31
What is an EPSP?
Excitatory postsynaptic potential
A transient depolarization of the membrane potential of a post-synaptic neuron that makes the neuron more likely to fire an action potential.
32
List 3 excitatory and 3 inhibitory neurotransmitters, and the receptors they bind to.
Excitatory:
- Glutamate (ionotropic receptors e.g., AMPA)
- ACh (nicotinic and muscarinic receptors)
- Histamine (H1 receptors)
Inhibitory:
- GABA (ionotropic e.g., GABA-A)
- Glycine (ionotropic receptors)
- Serotonin/5-HT (5-HT3)
33
What is the neuromuscular junction and how was it discovered?
The neuromuscular junction (NMJ) is a specialized synapse between a motor neuron and a muscle fiber that allows for the transmission of nerve impulses from the motor neuron to the muscle fiber, leading to muscle contraction.
It was shown that applying liquid from heart 1 with a slow heart rate will also reduce the heart rate of heart rate 2. This was because it contained ACh. Later on, the synapses were identified via microscopy.
34
What is end-plate potential? How was this discovered experimentally?
End-plate potential (EPP) is a depolarization of the postsynaptic membrane at the neuromuscular junction (NMJ) that occurs in response to the release of acetylcholine (ACh) from the presynaptic motor neuron.
A motor axon had an intracellular electrode placed in it to trigger an action potential that was then measured by an intracellular microelectrode located in muscle fibre, near the NMJ.
This was repeated, but instead of the electrode triggering an action potential, Ach was injected and triggered an EPP directly.
35
How did Bernard Katz show that neurotransmitters are released in discrete packets?
In the 1950s, Katz used a technique called "noise analysis" to show that neurotransmitters are released from presynaptic terminals in discrete packets or "quanta."
Katz recorded the electrical activity of muscle fibers in response to the release of neurotransmitters from presynaptic motor neurons at the neuromuscular junction. He found that the electrical signals recorded from the muscle fibers were not continuous, but rather consisted of discrete "bursts" or "quanta" of activity. These bursts corresponded to the release of individual packets of neurotransmitter molecules from the presynaptic terminal and the subsequent opening of postsynaptic ion channels.
Katz's discovery of quanta provided strong evidence for the idea that synaptic transmission occurs through the release of discrete packets of neurotransmitter, rather than a continuous stream.
36
What conditions were used in Bernard Katz's 'noise analysis' experiments, and why?
What was so interesting about what happened when there was no stimulation, and what did this mean?
Low calcium concentrations to reduce neurotransmitter release.
Normal [Ca2+] in the absence of stimulation showed spontaneous small EPPs, whereas under low [Ca2+] the stimulation infrequently invoked EPPs that were the same size as the small EPPs.
Using the spontaneous EPPs are a unitary size, the frequency distribution of the evoked EPPs was predicted by a Poisson distribution were the amplitudes could be separated into discrete 'bins' under low [Ca2+].
37
What is the quantal hypothesis of neurotransmitter release?
The quantal hypothesis of neurotransmitter release is a fundamental principle of synaptic physiology that was first proposed by Bernard Katz in the 1950s. The hypothesis states that neurotransmitters are released from presynaptic terminals in discrete packets or "quanta", which are the result of the fusion of individual synaptic vesicles with the presynaptic membrane.
Each quantum contains a fixed number of neurotransmitter molecules, and the release of neurotransmitter occurs in an all-or-nothing fashion, such that a presynaptic terminal either releases one or more complete quanta, or none at all. When a quantum is released, it binds to receptors on the postsynaptic membrane and elicits a response in the postsynaptic neuron, such as the generation of an action potential or the modulation of membrane potential.
38
How does an action potential bring about the release of neurotransmitters? What techniques were used to show this?
What is the role of SNARES in this?
When an action potential reaches the presynaptic terminal, it depolarizes the membrane and triggers the opening of voltage-gated Ca2+ channels. This influx of Ca2+ ions into the presynaptic terminal triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitter molecules into the synaptic cleft.
The structures of these synapses were revealed by EM, showing them fusing with the presynaptic membrane.
The fusion of synaptic vesicles with the presynaptic membrane is mediated by a series of proteins, including synaptotagmin, which senses the increase in Ca2+ concentration and triggers the assembly of a complex of proteins known as the SNARE complex.
39
How do SNARE proteins mediate synaptic vesicle fusion?
The SNARE complex is formed by the interaction of three types of SNARE proteins: syntaxin, synaptobrevin (also known as VAMP), and SNAP-25. Syntaxin and SNAP-25 are located on the presynaptic membrane, while synaptobrevin is anchored to the vesicle membrane.
The process of vesicle fusion begins with the docking of the synaptic vesicle to the presynaptic membrane. The SNARE complex then assembles by the zippering of the helical domains of the three SNARE proteins. This brings the vesicle membrane into close proximity with the presynaptic membrane, which creates the force needed for the fusion of the two membranes.
The SNARE complex catalyzes the fusion of the two membranes by bringing them into close apposition and destabilizing the lipid bilayers. This allows the vesicle membrane to fuse with the presynaptic membrane, releasing the neurotransmitter into the synaptic cleft.
*ALSO REQUIRES SYNAPTAGMINS AND CALCIUM IONS*
40
What else is required for efficient membrane fusion other than SNARE proteins, and why? What experiment showed this?
Whole-cell patch clamping showed the stimulation of one neuron results in a signal in the other neuron, but when the calcium binding of synaptotagmin is mutated, the signal is reduced.
Synaptotagmin is a calcium-binding protein that plays a critical role in regulating the release of neurotransmitter from synaptic vesicles during synaptic transmission. It acts as a calcium sensor, detecting the presence of calcium ions in the presynaptic terminal and triggering the release of neurotransmitter from the vesicles.
The calcium-dependent action of synaptotagmin is critical for precise and efficient neurotransmitter release. By acting as a calcium sensor, synaptotagmin ensures that the release of neurotransmitter is triggered only when there is sufficient calcium present in the presynaptic terminal, which helps to maintain the fidelity and specificity of synaptic transmission.
41
List the proteins that can be found at presynaptic active zones and what their roles are.
1. SNAREs - membrane fusion
2. Synaptotagmins - calcium sensors
3. Cadherins - maintain cell-cell contacts for synapse formation
4. Rab3/RIM - ensure Ca2+ channels are in active zones
5. Unc13 - keeps SNAREs in a fusion-ready state
6. Complexin - blocks SNARE fusion until released by synaptotagmins.
42
How is the presynaptic active zone a highly organized structure? Describe the differences between the core of the active zone and the surrounding site.
The presynaptic active zone is a highly organized structure that is essential for efficient and precise neurotransmitter release. It is composed of a complex of proteins that are localized to a specialized region of the presynaptic membrane that is in close apposition to the postsynaptic membrane. The active zone is organized into several distinct domains that are important for regulating the assembly and function of the neurotransmitter release machinery.
Core = release site...
- High density of VGCCs
- Synaptotagmin
- Synaptic vesicle docking
Surrounding...
- Cytomatrix - a network of proteins that anchor release machinery to the membrane
- Synaptic scaffold - proteins that organize the active zone
43
How is an action potential terminated at the level of the synapse? What can happen if this doesn't occur?
Neurotransmitters are cleared from the synaptic cleft, either by degradation or transport back into the pre-synaptic cleft.
For example, acetylcholine is broken down by the enzyme acetylcholinesterase, which is located on the postsynaptic membrane and in surrounding cells.
Dysregulation of these processes can lead to neurological disorders such as depression, anxiety, and addiction.
44
What are the three proposed mechanisms for synaptic vesicle recycling?
1. Kiss and run:
vesicle fuses briefly with the presynaptic membrane, before rapidly resealing and being recycled
2. Clathrin-mediated endocytosis:
membrane invaginates to form a clathrin-coated pit of neurotransmitters
3. Bulk endocytosis:
large portions of the presynaptic membrane are taken up into the presynaptic terminal to form large endosomes which then separates into smaller vesicles
45
What are AMPA and NMDA receptors an example of? What happens when these are activated?
Ligand-gated cation channels in many excitatory synapses.
Opening of these postsynaptic channels triggers a net inward current of cations, producing an EPSP.
46
What are the three sub-types of glutamate receptors?
1. AMPA
2. NMDA
3. Kainate (very similar to AMPA)
47
Describe the structure of an AMPA receptor.
Heterotetramers, with each subunit consisting of 3 domains:
1. N-terminal domain (glutamate-binding site)
2. TMD
3. Ligand binding domain
The NTD and LBD are clamshell-like domains, but the properties of individual receptors are specified by the subunit composition.
48
Compare AMPA and NMDA receptors.
AMPA:
- Fast-opening
- Conduct mainly K+ and Na+
NMDA:
- must bind glutamate and glycine to open
- only opens when the membrane has already been depolarized to relieve the magnesium ion block
- conduct mainly calcium to trigger biochemical changes in the postsynaptic cell
49
Why are NMDA receptors called coincidence receptors?
They contain a magnesium block that means they only open when glutamate is bound AND the postsynaptic membrane is depolarized.
NB: also requires glycine to bind, but this site is so high affinity that glycine is almost always bound
50
What are TARPs, and how were they discovered?
TARPs (Transmembrane AMPA receptor Regulatory Proteins) are a family of transmembrane proteins that interact with and modulate the function of AMPA receptors by regulating AMPA stability and gating.
They were discovered using the Stargazer mutant mouse which had a mutation in the Stargazer gene. This encodes the Stargazin protein, a TARP. Mutations in this protein leads to a lack of the Stargazin protein, which leads to a lack of AMPA receptor activity.
51
What is the role of PSD-95 in the postsynaptic density?
It organizes the postsynaptic proteins, such as TARPs, via its PDZ domains.
52
What is a gap junction and how does it relate to electrical synapses?
A gap junction is a specialized intercellular junction that directly connects the cytoplasm of two adjacent cells, allowing for the direct exchange of small molecules and ions between them. Gap junctions are composed of connexin proteins that form hexameric channels called connexons, which are docked on the plasma membranes of the two adjacent cells.
In an electrical synapse, the presynaptic and postsynaptic neurons are connected by gap junctions, which allow the flow of electrical current between the two cells. The electrical signals can be bidirectional, meaning that they can flow in either direction across the synapse, unlike chemical synapses, which typically transmit signals in only one direction.
53
Why are electrical synapses important?
Electrical synapses are particularly important in networks of neurons that need to synchronize their activity quickly and efficiently, such as in the pacemaker cells of the heart, smooth muscle cells, and some regions of the brain.
54
How does botox and black widow spider venom work?
Botox stops Ach release.
Venom releases too much Ach release, leading to overstimulation and desensitization.
55
What are the two major families of ligand-gated ion channels?
1. Cys-loop receptors: This family of ion channels includes nicotinic acetylcholine receptors, GABA-A receptors, glycine receptors, and 5-HT3 receptors. They are named after a conserved cysteine loop in their primary structure and are activated by ligands such as neurotransmitters and drugs.
2. Glutamate receptors: This family of ion channels includes AMPA receptors, kainate receptors, and NMDA receptors. They are activated by the neurotransmitter glutamate and are important for mediating fast excitatory neurotransmission in the central nervous system.
56
What are the two major families of ligand-gated ion channels?
1. Cys-loop receptors: This family of ion channels includes nicotinic acetylcholine receptors, GABA-A receptors, glycine receptors, and 5-HT3 receptors. They are named after a conserved cysteine loop in their primary structure and are activated by ligands such as neurotransmitters and drugs.
2. Glutamate receptors: This family of ion channels includes AMPA receptors, kainate receptors, and NMDA receptors. They are activated by the neurotransmitter glutamate and are important for mediating fast excitatory neurotransmission in the central nervous system.
57
Describe the subunits and stoichiometries of nicotinic acetylcholine receptors in muscle, neuronal, and neuronal a7 cells.
What is the most important subunit and why?
Muscle: 1y, 2a, 1B, 1 delta
Neuronal: 2a, 3B
a7: all alpha
Alpha subunits are essential for binding acetylcholine, with 2 of them being essential for function.
58
What's the predicted TM topology of nAchRs? What techniques were used to study these receptors, and what did it show?
4 predicted TMs per subunit with a large N-terminal loop that binds Ach.
Electric organ in Torpedo rays contains lots of nicotinic receptors that, when extracted, form arrays which makes high resolution XRC much easier.
These XRC structures allowed for the determination of subunits, showing that the Ach binding site is between the alpha and gamma/delta subunits.
59
What's the difference between loop C and the cys-loop on nicotinic receptors?
Loop C is part of a binding site, containing 2 cysteines that aid in Ach binding.
The cys-loop is a conserved sequence amongst cys-loop receptors that undergoes a conformational change when the neurotransmitter binds, leading to opening of the ion channel pore.
60
What are the roles of the rings of sidechains from the M2 helices of nicotinic receptors? How were these roles discovered?
1' ring: selectivity
4' and 8' ring: control permeation
9/11' rings: control gating via leucines
Discovered using mutagenesis experiments.
61
How was the acetylcholine binding protein (corresponding to the EC domain of nicotinic receptors) studied?
What was it linked to for studying gating? What did this show?
Initially, it was discovered by accident from snail synapses and was easy enough to crystallize for structural studies. This sequence was found to be homologous to that of the nicotinic receptor EC domain.
This could then be linked to the TM region of a 5HT receptor to make a channel gated by Ach.
It showed Loop C clamps down and around Ach.
62
How did cryo-EM aid in uncovering the mechanism of ion conduction through nicotinic receptors?
CryoEM was able to show the structures of a zebrafish glycine receptor in 3 states.
These structures were used to assign states based on different properties:
- Narrow 9' ring = resting state
- Very open pore = open state
- Open at 9' but closed at 2' = dessensitized
63
How did a cryo-EM structure of nicotinic receptors and MD lead to the discovery of a hydrophobic gate? How does this hydrophobic gate work?
The closed state of the cryo-EM structures showed water could fit through.
MD was used to model a hydrophobic cylinder with varying radii and length. Increasing the radius increased the chance of water moving through, and adding dipoles into the channel opened it when it would normally be closed.
Hydrophobic gating refers to the process by which the ion channel pore opens and closes in response to changes in the hydrophobicity of the M2 lining. When an agonist molecule such as acetylcholine binds to the receptor, it induces a conformational change in the receptor protein that causes the M2 domains to tilt and expand, resulting in the opening of the ion channel pore. This conformational change also leads to a shift in the position of the bulky hydrophobic residues at the tip of the M2 domains, which allows ions to pass through the pore.
64
How are anaesthetics and lipids thought to modulate nicotinic receptors?
Studies have shown that some general anesthetics, such as propofol and etomidate, can potentiate the effects of GABA on nAChRs. This occurs because both GABA and anesthetics bind to different sites on the receptor protein, with the anesthetic binding to a site located in the transmembrane domain of the receptor subunit. Binding of the anesthetic to this site enhances the effect of GABA on the receptor, resulting in increased chloride ion influx and hyperpolarization of the cell membrane, leading to anesthesia.
One way that lipids can modulate nAChR activity is by directly interacting with the receptor protein. Specifically, cholesterol has been shown to modulate the fluidity and thickness of the lipid bilayer, which in turn can affect the conformation and function of the receptor.
65
How can nicotinic receptor responses be 'tuned'?
Each of the subunits may react to Ach differently, so the responses need to be 'tuned'.
This is achieved via alternative splicing or RNA editing which mainly occurs at regions around the binding pocket or at the top of the TM section.
66
How are nicotinic receptors associated with disease and smoking?
Muscular diseases, such as slow channel syndrome, are caused by prolonged channel activation due to mutations stabilizing the open state. This causes Na-channel inactivation and hence muscle cells are non-excitable for prolonged periods of time.
Nicotine binds to these receptors to stimulate dopamine release, but over time desensitization occurs and so the smoker builds up a tolerance.
67
What is the function of ionotropic glutamate receptors, and what diseases are these associated with?
Ligand-gated ion channels that mediates nearly all rapid excitatory neurotransmissions in the CNS. It's been implied in learning and memory processes.
Diseases:
- Huntington's
- Epilepsy
- Parkinson's
- Alzheimer's
68
What are the key differences between the 3 main families of glutamate receptors?
They all have different response times.
69
What's the proposed mechanism for ionotropic glutamate receptors?
1. Ligand binding to the NTD
2. Conformational change to open - D2 closes the 'clam' and pulls TM3 upwards
3. Ion flux
4. Desensitization - D2 opens again, pushing TM3 back into a closed conformation
70
How does cyclothiazide (CTZ) block desensitization of ionotropic glutamate receptors? How might this be used in therapies?
Cyclothiazide (CTZ) is a pharmacological agent that is known to block the desensitization of ionotropic glutamate receptors (iGluRs), specifically the AMPA subtype of iGluRs. It's thought to do so via binding at the dimer interface.
Inhibition of desensitization of AMPA receptors increases the response of receptors. For example, CTZ has been shown to enhance synaptic plasticity and improve cognitive function in animal models of Alzheimer's disease and schizophrenia.
71
How does RNA-editing and splicing impact AMPA receptors?
What is the link between editing in iGluR and ALS?
Alternative splicing at the 'flip-flop cassette' effects desensitization.
mRNA editing of the Q/R site effects channel properties. Q has a high calcium permeability, but changing it to R gives a low calcium permeability.
Patients with ALS show a random efficiency of RNA editing in motor neurons.
72
How do AMPA and NMDA receptors work together?
AMPA receptor triggers action potential and depolarizes the membrane. This depolarization pushes the magnesium ion out of the NMDA receptor so it can be activated for calcium influx.
73
What happens when NMDA receptors are overstimulated?
Excess calcium influx causes the collapse of the neuronal cell and release of more glutamate. This will ultimately cause a stroke.
74
What's the difference between an ionotropic and a metabotropic receptor? Give 3 examples of each.
Ionotropic receptors are a type of receptor that directly open ion channels upon binding of a ligand, while metabotropic receptors indirectly activate ion channels through a signaling pathway inside the cell.
Ionotropic:
1. Nicotinic ACh receptor
2. iGluR (AMPA, NMDA)
3. GABA receptor
Metabotropic:
1. mGluR
2. GABA receptor
3. B-adrenergic receptor
75
How do metabotropic neurotransmitter receptors trigger G protein cascades?
Metabotropic neurotransmitter receptors trigger G protein cascades through a process known as G protein coupling. When a neurotransmitter binds to a metabotropic receptor, it causes a conformational change in the receptor, which allows it to interact with a G protein that is coupled to the receptor.
The activated alpha subunit can then interact with downstream effector proteins, such as adenylate cyclase or phospholipase C, which generate intracellular second messengers, such as cyclic AMP or inositol triphosphate. These second messengers can then activate or inhibit ion channels or enzymes, leading to a wide range of physiological responses. The beta and gamma subunits can also modulate the activity of downstream effector proteins or ion channels.
76
How were GPCRs discovered? What were the issues in this?
Bacteriorhodopsin channels were studied using cryoEM, but GPCRs for hormones and neurotransmitters aren't expressed in tissues at sufficient levels for studies so there was a lot of focus on achieving this.
We're now able to use Fab fragments to stabilize the crystals, and clone the proteins.
77
How does its asymmetric dimer underly GABA-B's activation mechanism?
Unlike many other metabotropic receptors, the GABA-B receptor is a heterodimeric receptor, consisting of two subunits: GABA-B1 and GABA-B2.
The mechanism of activation of the GABA-B receptor involves a conformational change in the receptor that is initiated by the binding of GABA to the GABA-B1 subunit. This conformational change leads to the activation of a G protein signaling cascade, similar to other metabotropic receptors.
78
What is the common mechanism of activation in class C GPCRs?
A twisting motion of each subunit along the pseudo-symmetry axis in which TM5s move apart while TM6s approach closer, forming the heterodimeric interface.
79
Describe the B-adrenergic G-protein signaling cascade that speeds up heart rate.
1. When adrenaline binds the B-adrenergic receptor, it causes a conformational change that activates Gs.
2. The alpha subunit of Gs activates adenylate cyclase, converting ATP to cAMP.
3. cAMP levels cause PKA levels to rise that can phosphorylate voltage-gated calcium channels.
4. The channels open and cause an influx of calcium into the cell that triggers muscle contraction in the pacemaker cells, and hence increases heart rate.
80
How does acetylcholine release due to vagus nerve stimulation slow down heart rate?
When acetylcholine binds to the muscarinic ACh receptor, it activates a G protein known as G_i, which inhibits the activity of an enzyme known as adenylate cyclase.
This leads to a decrease in cyclic AMP (cAMP) levels in the cell, which in turn reduces the activity of protein kinase A (PKA), a key enzyme involved in regulating heart rate and contractility.
K+ channels open and hyperpolarize the cell.
81
What is the role of CaMKII in neurons, and how is it activated?
In neurons, CaMKII is activated by the influx of calcium ions through voltage-gated calcium channels or by the release of calcium from intracellular stores in response to neuronal activity. This is triggered by activation of metabotropic GPCRs.
The activation of CaMKII has been linked to several processes in neurons, including the strengthening and weakening of synaptic connections between neurons, a phenomenon known as synaptic plasticity.
82
What is the role of alpha-adrenergic receptors at the presynapse?
Alpha-adrenergic receptors located at the presynaptic terminal of neurons modulate the release of neurotransmitters by feedback inhibition.
Released G-By binds and inactivates voltage-gated calcium channels, thereby stopping synaptic vesicle fusion and release of more neurotransmitter.
83
How do metabotropic receptors act on the presynaptic terminal to modulate neurotransmitter release?
1. Presynaptic facilitation e.g., serotonin release, facilitates depolarization.
2. Presynaptic inhibition e.g., GABA-B, inhibits depolarization.
84
What is the hippocampal circuit? What hypotheses are there to describe how the brain stores information?
The hippocampal circuit is a neural circuit that plays a key role in learning, memory, and spatial navigation. These cells (CA1 and CA3) provide huge capacity for memory acquisition and storage.
Hypothesis 1: memory is stored as strengths of synaptic connections in neural circuits.
Hypothesis 2: learning modifies the strengths of synaptic connections.
85
What is long-term depression?
While LTP strengthens the synaptic connection between two neurons and enhances their communication, LTD weakens the connection and decreases their communication.
The exact mechanisms by which LTD is induced and maintained are still not fully understood, but it is thought to involve a decrease in the number of AMPA receptors at the synapse due to calcium levels only being high enough to activate phosphatases that remove the phosphate groups added to AMPA receptors by CaMKII.
86
Describe the gill-withdrawal reflex in Aplysia.
The gill-withdrawal reflex is triggered by a noxious stimulus, such as a light touch to the siphon, which is a structure on the side of the animal's body. This touch activates sensory neurons, which send signals to the central nervous system, specifically to a ganglion called the abdominal ganglion. In response to the sensory input, the abdominal ganglion sends signals to motor neurons, which control the contraction of muscles in the gill and siphon.
During the gill-withdrawal reflex, the animal rapidly withdraws its gill and siphon, which are normally extended and exposed to the surrounding water. The reflex is an adaptive response that helps the animal avoid potential predators or other harmful stimuli.
87
Why has the gill-withdrawal reflex of Aplysia been extensively studied?
The gill-withdrawal reflex of Aplysia has been extensively studied as a model for neural plasticity, particularly in the context of habituation and sensitization. Habituation refers to a decrease in the strength of the reflex over time as the animal becomes accustomed to repeated presentations of the same stimulus.
Experiments revealed that protein synthesis is required for long-term memory.
88
What are the molecular mechanisms behind short-term and long-term memory in Aplysia?
Short-term:
Tail shock induces serotonin release at presynapse of sensory neuron, activating a GPCR. This leads to activation of adenylate cyclase, producing cAMP. cAMP activates PKA, which phosphorylates K+ channels and thereby inhibits them. This elevates the resting potential of the presynaptic terminal and increases the action-potential-triggered neurotransmitter release.
Long-term:
Repeated activation of PKA leads to its catalytic subunit being transported to the nucleus where it phosphorylates TFs e.g., CREB, inducing new gene expression. The result is a structural change and the growth of synaptic contacts.
