NEURO: Neurotransmitter Systems II: GABA and Glycine Flashcards
Give examples of Inhibitory Neurotransmitters and what they do?
GABA (gamma-aminobutyric acid) and glycine
hyperpolarise the postsynaptic cell, meaning an action potential is less likely to take place ( (ie. to bring it further away from the threshold potential that will induce an action potential).
What are the main mechanisms which cause hyperpolarisation?
There are two ways in which ion channels can do this:
- when the ligand binds, it will allow negative ions (eg. Cl-) to flow in, decreasing the membrane potential
- when the ligand binds, it will allow K+ ions to flow out, thus decreasing the membrane potential
Describe GABA synthesis and storage.
Glutamate (excitatory) converted to GABA (inhibitory) in via the action of glutamate decarboxylase (GAD) using a cofactor called pyridoxal phosphate (derived from vitamin B6)
It is synthesised in the nerve terminals. Then, it is transported into vesicles by vesicular inhibitory amino acid transporters (VIAATs).
If we were to microscopically look at a nerve terminal/synapse, would we be able to distinguish vesicles holding glutamate or GABA/glycine?
glutamate vesicles are more rounded, whereas GABA vesicles are more oval-shaped
The different vesicle shapes are due to the electrostatic interactions of the different molecules in the vesicles
Describe the GABA(A) receptor.
Most common GABAa configuration
ligand-gated Cl- channel
pentameric, consisting of 5 subunits, and each of the subunits has different subtypes:
- Six ⍺ subtypes (⍺1-⍺6)
- Three β subtypes (β1-β3)
- Three 𝛾 subtypes ( 𝛾1-𝛾3)
- Also ẟ,ε, pi, theta subunits is a ligand-gated Cl- channel. It sits in a pentameric structure.
The most common configuration is with 2 α, 2 β and a γ subunit.
The receptor is most commonly found post-synaptically.
GABAa receptor binding sites
There are multiple binding sites (on the receptor) for:
- agonists/antagonists (eg. GABA) [between α & β]
- benzodiazepines [between α & γ]
- channel modulators (eg. GA, alcohol)
- allosteric modulators (eg. barbiturates)
- channel blockers (eg. picrotoxin)
*because GABAa has multiple binding sites, it makes an attractive drug target for multiple disorders (e.g. epilepsy)
Stimulation of GABAa receptor
GABA binds receptor
Cl- channel opens
Cl- influx
Hyperpolarisation
In what ways is GABA activity terminated?
- upon reuptake by GABA reuptake transporter (GAT) on the presynaptic membrane
- upon breakdown by GABA transaminase
- upon diffusion away from the synaptic cleft
Describe GABA(B) receptor structure
It is a G protein-coupled receptor modulated by the αGi/o G protein cascade.
They exist as dimers, sometimes from two different subtypes (eg. GABA(B1) and GABA(B2))
· Extracellular Venus Flytrap Domain: for ligand binding
· 7 Transmembrane Domains
· Intracellular C-terminal Domain: coupled to a G-protein (Gi/o)
Stimulation of GABAb receptor
GABA binds GABAb receptor (Gi/o) causing hyperpolarisation by:
- activating K+ channels to facilitate K+ efflux
- blocking Ca2+ influx into the cell
GABA Reuptake
neurones and glial contain high-affinity Na+ dependent GABA reuptake transporters (GATs):
- neurones contain GAT-1
- glial cells contain GAT-3
GABA Degradation
1) GABA is degraded via an enzyme called GABA transaminase (GABA-T) into a compound called Succinic semialdehyde.
2) Succinic semialdehyde then converted into Succinic acid via an enzyme called Succinic semialdehyde dehydrogenase (SSADH)
How is GABA implicated in epilepsy?
By modulating how much GABA we have in the brain, we can produce the occurrence of seizures.
Epilepsy is a lot of excitation in the brain, so we can amp up the inhibition in different ways that could help us to level out the imbalance in excitation and inhibition.
What is epilepsy?
a brain disorder characterised by periodic and unpredictable seizures mediated by the rhythmic firing of large groups of neurones
-TOO MUCH EXCITATION
Targeting Epilepsy
Increase the amount of GABA mediated inhibition to decrease excitation
List some different epilepsy drugs based on their mechanism of action.
GABA(A) RECEPTOR ENHANCERS:
- Barbiturates (not used anymore because of the risk of addiction and overdose)
- Benzodiazepines
- Progesterone
- Ganaloxone
GAT BLOCKERS:
- Tiagabine
GABA-TRANSAMINASE INHIBITOR:
- Vigabatrine
GAD MODULATORS?:
- Gabapentin
- Valproate
PRODRUG:
- Progabide (exogenous analogue of glutamate)
Anxiety
the feeling of unease (e.g. worry or fear) which can range from mild to severe
-can be normal or pathological
Describe glycine synthesis and storage.
Glycine is synthesised from 3-phosphoglycerate (a product in glycolysis). This gets converted to serine, which then gets converted to glycine in nerve terminals by the enzyme Serine hydroxymethyl-transferase.
It is transported into vesicles by vesicular inhibitory amino acid transporters (VIAAT).
Where is glycine most commonly found?
in the ventral horn
Describe the glycine receptor.
It is an ionotropic ligand-gated Cl- ion channel. It is found both pre- and post-synaptically. It has a pentameric structure.
pentameric structure with:
- Four ⍺ subtypes (⍺1-⍺4)
- One β subtype
- most common configuration is:
- 3α(1)2β
- 4α(1)1β
> binding of glycine will lead to the opening of the channel and an influx of Cl- ions, causing hyperpolarisation
Glycine binding sites
The agonist/antagonist binding sites and their requirements are unclear.
Glycine receptor modulators
modulators of this receptor lag behind the GABA receptor, although there is one known modulator:
· Strychnine- plant alkaloid which potently blocks the glycine receptors
Glycine reuptake
Neurones and glial cells contain high-affinity Na+ dependent glycine re-uptake transporters (GlyTs)
· Glial cells: GlyT-1
· Neurones: GlyT-2
Glycine degradation
Glycine converted back to serine by Serine hydroxymethyl-transferase
What are the mechanisms of glycine activity termination?
- upon the reuptake of glycine by the glycine reuptake transporter Gly1
- upon break down by glycine decarboxylase
- upon glycine’s diffusion away from the synaptic cleft
Hyperekplexia
rare disorder characterised by hypertonia (increased muscle tone) and an exaggerated startle response
Other than glycine receptors, what other receptors can glycine stimulate?
NMDA receptors:
- Glutamate binds to the GluN2 subunits
- Glycine or D-serine binds to the GluN1 subunit
Thus, it can increase inhibition (through its own mechanism), but also enhance excitation (due to NMDA receptor activation), making it a complex pharmacological target.
What causes hyperekplexia?
Gene mutations (e.g. glycine receptors or transporters) disrupt normal inhibitory glycinergic neurotransmission, leading to neuronal hyperexcitability due to impaired glycinergic inhibition
Hyperekplexia in goats
In startle (or myotonic/fainting) goats, there is a decrease in muscle chloride conductance- this can be caused by glycine receptor mutations.
As the goats mature, GABAa receptors (ligand gated Cl- channels) are upregulated to compensate glycine receptor mutations/deficiency.
When the goats/humans have no GABA protein present at the synapses, or the GABA protein is present but isn’t functional, we get the fainting phenomenon.
As a result of the glycine receptor deficiency (or otherwise), they have an inability to produce inhibition.
When their fight or flight response is activated, there is a surge of excitatory and inhibitory signals. However, the inhibitory signals don’t go through, so a transient seizure is induced and they faint.
This phenomenon only really occurs in juvenile goats/humans because the body adapts and upregulates the amount of GABA receptors to compensate.
