Lecture 5 - Chapter 6: Neurotransmitters Flashcards
True or false:
Excitotoxicity can be caused by stroke.
True
Glutamate, an amino acid that is also a neurotransmitters is released during stroke and causes excitotoxcitiy.
True or false:
Benzodiazepines stimulate glutamate receptors in the amygdala to relieve anxiety.
False
Benzodiazepines stimulate GABA receptors, limiting glutamate neurotransmission
True or false:
Acetylcholine is the main neurotransmitter to drive contraction in skeletal muscles.
True
But note that the same neurotransmitter can cause muscle relaxation in the heart
What is the chemical synapse?
Chemical synapses are junctions between neuronal cells (and between neuronal and non-neuronal cells) where communication occurs. Presynaptically, there are dense corse and synaptic vesicles and multivesicular bodies that carry neurotransmitters and other signals to the synaptic cleft.
This picture will be discussed throughout the lecture. But for now:
What vesicles are used for the transport of these neurotransmitter types (synaptic vesicles or dense core vesicles)?
- The small molecule neurotransmitters, amino acids and purines are transported by small clear core vesicles or so called synaptic vesicles.
- The catecholamines (dopamine, norepinephrine and epinephrine) are transported by both synaptic as dense core vesicles.
- Indoleamine, imidazoleamine and peptide neurotransmitters are transported by dense core vesicles.
What two types of neurotransmitter receptors exist?
- Ionotropic neurotransmitter receptors (ligand-gated ion channels)
- Metabotropic neurotransmitter receptors (G-protein-coupled receptors)
What are second messengers? Also name examples of second messengers and examples of cellular response to second messengers.
- Second messengers are molecules that amplify the signal and trigger an intracellular response.
- Examples of second messengers are cAMP, diacylglycerol and calcium.
- Examples of a cellular response are: protein phosphorylation, gene transcription and opening of ion channels.
What type of neurotransmitter is acetylcholine (ACh)?
A small-molecule neurotransmitter
What type of synapses release acetylcholine (ACh)?
Neuromuscular junctions, where an axon is connected to and communicates with a muscle.
Acetylcholine is a neurotransmitter with many functions. What are functions of ACh?
- It can interact with receptors on heart muscle cells, which causes a decreases rate and force of contraction.
- It can interact with skeletal muscle cell receptors and cause contraction.
- It can interact with salivary gland cell receptors and stimulate the secretion of saliva.
Explain how acetylcholine is synthesized and broken down.
ACh is a product of the citric acid cycle (also the reason why presynaptically many mitochondria are located). ACh is produced by the enzyme choline acetyltransferase. A product of the citric acid cycle acetyl CoA is combined with choline, where subsequently ACh is produced. ACh is brought into vesicles by VAChT. When ACh is released in the synaptic cleft, it’s broken down by acetylcholinesterase (choline can be reused).
What is needed to pump newly made ACh (and other small NTs) into synaptic vesicles?
Small NTs like ACh are pumped inside of a vesicle with the help of a transporter (VAChT or VMATs). These transporters use protons inside the vesicle, where protons are pumped out when NTs are pumped in. So the vesicle needs to have a H+ gradient, which is facilitated by V-ATPase that can pump protons to the inside of the vesicle.
Acetylcholine breakdown is essential. Why?
Because otherwise ACh can stimulate its postsynaptic ACh receptors continuously. This can lead to muscle paralysis. This is the case for a nerve gas (Sarin), which is an AChE blocker and is very dangerous and toxic.
What kind of receptors does acetylcholine have?
- Nicotinic AChR → pore that opens when ACh binds.
- Muscarinic AChR (muscarine 1 and 2 AChR) → when ACh binds, an intracellular cascade is initiated that opens another channel.
What disease has a known shortage of ACh? What can be used to treat this disease (symptomatically)?
Alzheimer’s Disease has a lack of ACh. Therefore an acetylcholine esterase blocker (e.g. tacrine, doneprezil, rivastigmine) can be used, so that more ACh stays in the synaptic cleft for postsynaptic stimulation.
Describe the nicotinic AChR (nAChR).
The nAChR consists of 5 different subunits (β, δ, γ and 2x α) that together form a pore in the plasma membrane. The receptor has 2 ACh binding sites and when ACh binds to these binding sites, the domains will revolve around their axis and the pore will open.
Note: the subunits can differ per receptor.
The nicotinic acetylcholine receptor can consist of 5 different subunits, but can also consists of fewer combinations.
- What combination of the subunits is mostly found in neuromuscular junctions?
- What combination of the subunits is mostly found in neuron-neuron synapses?
- Neuromuscular → 2x α, 1x β, 1x δ, 1x γ/ε
- Neuron-neuron synapse → 3x α, 2x β
Why is it important that there are different variations in nicotinic acetylcholine receptors?
Variations in subunits also cause variations in characteristics of the receptor, like affinity or how long or how frequent a pore can stay opened. This is important for e.g. synaptic plasticity.
Describe characteristics of a muscarinic ACh receptor (mAChR).
It is composed of 7 transmembrane subunits. The G-protein complex consists of three subunits (α, β and γ). Activation of the receptor results in exchange from GDP to GTP on the α-subunit. Subsequently, both subunit complexes (α-complex and β/γ-complex) are activated and can take part in cell signaling.
So we’ve already concluded that ACh can interact with receptors on heart muscle, skeletal muscle and salivary gland cells. These receptors that are located here, differ in the type of receptor. Explain what acetylcholine receptors are located on these cells.
- Heart muscle cells → muscarinic 2 receptor
- Skeletal muscle cells → nicotinic receptor
- Salivary gland cells → muscarinic 1 receptor
Some agents can have a toxic effect on the ACh-neurotransmitter system. Describe for the following agents whether they’re agonist or antagonist of the muscarinic or nicotinic ACh receptor:
- Nicotinia tabacum
- Amanita muscaria
- α-bungarotoxin
- Curare
- Atropine
- Nicotinia tabacum → agonist for the nAChR
- Amanita muscaria → agonist for the mAChR
- α-bungarotoxin → antagonist for the nAChR
- Curare → antagonist for the nAChR
- Atropine → antagonist for the mAChR
Explain how glutamate is released into the synaptic cleft and what happens with glutamate that is “left over.
Glutamate is produced from glutamine by the enzyme glutaminase. Via VGLUT transporters glutamate is then pumped into vesicles. Upon stimulation, glutamate is released into the synaptic cleft where it can bind to postsnaptic glutamate receptors. Left over glutamate is taken up by glial cells (astrocytes) by EAAT (excitatory amino acid transporter). Inside glial cells, glutamate is converted back to glutamine and then transported back to the presynaptic terminal.
What kind of receptors does glutamate have?
- Ionotropic receptors → AMPA, NMDA and kainate
- Metabotropic receptor → mGluR
Explain what is seen in the picture.
In the picture the excitatory postsynaptic current is plotted against time. What is seen is that:
- for the AMPA receptor, the response is sharp and the amplitude is large. This means that there’s a lot of current in a very short time window. You can also see that the AMPA receptor closes very quickly. (The neuron is turned on very quickly and also stopped very quickly).
- for the NMDA receptor, the response and closing time are much slower. Import for long-term excitation, i.e. synaptic plasticity.
- for the Kainate receptor, the response is very quick and closes relatively slowly.
How can glutamate cause excitotoxity?
When someone has a stroke, there’s a haemmorage in the brain. Since glutamate is an amino acid, it’s abundant in the blood. So during a stroke, a lot of glutamate is released into the brain, which is able to stimulate loads of glutamate receptors (overstimulation). This results in a prolonged Ca2+ influx, which activates the breakdown of lipids and proteins and induces apoptosis.
Describe the structure of AMPA and NMDA receptors.
Both receptors have a similar structure to nAChR. They contain multiple subunits and when glutamate binds to the receptor, also these subunits will revolve around their axis and the pore will open.
Explain how the NMDA receptor works.
The NMDA receptor works a little different compared to the AMPA receptor (which is just a normal ion pore). In the pore of the NMDA receptor, there are two blockages. The first is regulated by glutamate and the second block is a Mg+-ion. The first blockage is relieved when glutamata binds to the NMDA receptor, the second blockage is only relieved when the membrane is depolarized. Ions like calcium or sodium can then enter the cell.
The metabotropic glutamate receptors are located pre- and postsynaptically. Why?
The receptors located presynaptically have a much lower affinity for glutamate then the postsynaptic glutamate receptors. So only when glutamate is in high concentrations, it will bind to the presynaptic glutamate receptors. This will initiate a negative feedback loop, where presynaptic SNARE proteins get phosphorylated which makes it harder for glutamate-carrying vesicles to fuse with the membrane.
How is GABA produced and what happens to left over GABA in the synaptic cleft?
GABA is produced out of glutamate (two seperate steps are needed for this). GABA is then pumped inside vesicles by VIAAT (vesicular inhibitory amino acid transporter).
Left over GABA is also taken up by glial cells via GAT (GABA transporter).
Describe the characteristics of GABA-A.
GABA-A is the ionotropic receptor for GABA. It consists of 5 subunits, similar to the ionotropic receptors discussed before. Unlike the other ionotropic receptors, GABA-A mainly lets CI- pass through its pore.
GABA-B receptors also suppress neuronal activity. How?
This receptor activates K+ channels and blocks Ca2+ channels, so that K+ flows out of the cell and Ca2+ cannot go into the cell. This causes the membrane potential to become even more negative.
Note: these actions are due to the activation of the G-proteins and their subunits.
Why can you consider monoamines (dopamine, norepinephrine, epinephrine, serotonin and histamine) as the ‘same’?
They’re all part of one biosynthesis pathway and all originate from the same precursor (namely tyrosine).
Look closely to the picture to see why this is.
Unlike neurotransmitters discussed before, monoamines are not involved in the on/off mechanism of neuronal activation. Monoamines are mostly important in modulating neurotransmission in brain networks and therefore display a wide variety of behaviour. Explain behaviour that is controlled by:
- Norepinephrine
- Serotonin
- Dopamine
(also think of what behaviour is controlled by combinations of these NTs)
- Norepinephrine → alertness, concentration, energy
- Serotonin → obsessions, compulsions, memory
- Dopamine → pleasure, reward, motivation/drive
What kind of drug is fluoxetine (Prozac)?
It’s an SSRI (selective serotonine reuptake inhibitor), it is used in clinic for patients suffering from depression where it’s believed that these patients have less serotonin in their brain. So an SSRI is used to block the reuptake of serotonin, so that serotonin is longer left in the synaptic cleft to stimulate its postsynaptic receptors.
How are neuropeptides produced?
- Pre-propeptide, contain a signal sequence which tells the E.R. to transport it to the E.R. lumen.
- Propeptide, the signal sequence is removed while in the lumen of the E.R.
- Active peptide, several domains are cleaved while being packaged into vesicles and being transported to the synaptic cleft. This way there can be three active peptides inside a vesicle that all originate from the same pre-propeptide.
Explain how many active neuropeptides you’d expect to have from this pre-propeptide.
Since there are 5 domains for enkephalin A and one other domain, there should be 5 active Met-enkephalin peptides and 1 active Leu-enkephalin.
Do neuropeptides signal via synaptic signaling?
No, they rather signal via endocrine and paracrine signalling (compare them to hormonal secretion/signalling).
Endocannabinoids are lipid-based neurotransmitters. From what lipid do these NTs originate from and what are other examples of active endocannabinoids?
They’re derived from the lipid phosphatidylinositol. An example of an active endocannabinoid is 2-AG.
How do endocannabinoids function?
They’re synthesised when a periferal mGluR (so outside the postsynaptic density) is stimulated by glutamate. This activates the production of endocannabinoids like 2-AG, where it then diffuses from the postsynaptic membrane to the presynaptic membrane. Here it can activate the CB1R (endocannabinoid receptor). This can result in e.g. the inhibition of synaptic vesicle release, which overall leads to a diminshed postsynaptic response.
