NEURO: Neurotransmitter Systems II: GABA and Glycine Flashcards
What is GABA?
Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the CNS.
Approximately one third of synapses utilise GABA as their neurotransmitter. GABA is most commonly found as an inhibitory neurotransmitter in local circuit interneurons.
First identified in 1950, shown to inhibit action potential firing in crayfish neurones in 1957.
Describe GABA synthesis?
Glucose is metabolised to glutamate. Glutamate is converted to GABA - catalysed by enzyme glutamate decarboxylase (GAD) along with cofactor pyridoxal phosphate (derived from Vit B6).
GABA synthesis occurs in nerve terminals and is transported into vesicles by vesicular inhibitory amino acid transporters (VIAAT).
Absence of Vit B6, can lead to diminished GABA synthesis leading to seizures due to lack of neuroinhibitory drive.
Compare the vesicular storage of Glutamate and GABA prior to its release.
Glutamate and GABA differ in terms of their synaptic vesicles. Glutamate is stored in round vesicles whereas GABA is stored in oval shaped vesicles.
Describe GABA re-uptake and degradation
After GABA synthesis and storage, it is released by exocytosis to bind to post synaptic receptors. Eventually, GABA’s activity needs to be terminated - via reuptake and degradation.
Neurons and glial cells contain high-affinity Na+ dependent GABA re-uptake transporters (GATs).
Neurons = GAT-1
Glial cells = GAT-3
Both function to transport GABA from the synaptic cleft back into the neuron/glial cell for subsequent degradation.
Degradation: GABA is converted to succinic semialdehyde catalysed by enzyme GABA transaminase (GABA-T)
Succinic semialdehyde is then converted to succinic acid catalysed by enzyme succinic semialdehyde dehydrogenase (SSADH).
RECAP: what are the 2 broad families of neurotransmitter receptors?
There are 2 broad families of neurotransmitter receptors: the ligand-gated ion channels (ionotropic) and G-protein coupled receptors (metabotropic).
The LGICRs contain a membrane spanning domain which forms an ion channel. Neurotransmitter binding to the LGICR, allows for ions to pass through the membrane where it can increase (excitatory) or decrease (inhibitory) the chance of an action potential firing.
The GPCRs comprise a characteristic 7 transmembrane domain structure with an extracellular domain for neurotransmitter binding. Neurotransmitter binding to the GPCR activates G proteins (alpha, beta, gamma subunits) which can dissociate from the receptor and interact with ion channels or bind to other effector proteins that active secondary messenger pathways that can also open or close ion channels.
LGICRs take milliseconds whereas GPCRs produce a slower response. LGICRs and GPCRs are found mainly on the dendritic membrane. There are also voltage-gated ion channels, these are mainly found on the axonal membrane.
Which receptors does GABA bind to?
GABA binds to both ionotropic and metabotropic receptors.
Ionotropic receptors:
- GABAA receptor
- GABAC receptor (this is similar to the GABAA receptor in structure, function and mechanism, however is insensitive to many several drugs that act on the GABAA receptor thus we’ll concentrate on that).
Metabotropic receptors:
- GABAB receptor
What is the basis of the inhibitory mechanism?
The main aim of the inhibitory mechanism in cells is to hyperpolarise it (ie. to bring it further away from the threshold potential that will induce an action potential).
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
Thus, it could be due to an influx of negative charge, or an eflux of positive charge.
Describe the GABAA receptor.
The GABAA receptor is a LGICR. Upon GABA binding to the receptor, the ion channel allows the influx of negatively charged Cl- ions leading to hyperpolarisation (membrane potential lower than the resting potential). This inhibits action potential firing by increasing the stimulus required to fire an action potential.
The GABAA receptor is pentameric, comprising 5 subunits which form the ion channel pore. There are 6 α subtypes, 3 β subtypes, 3 γ subtypes and more. The most common configuration is 2 α, 2β and 1 γ subunits.
The GABAA receptor is located predominantly on the post synaptic. It is a key drug target due to the multiple binding sites of the receptor. GABA binds between the alpha and beta subunits of the receptor.
Binding sites:
- Agonists/antagonists e.g. GABA (orange in pic)
- Benzodiazepine binding site (between alpha and gamma subunits) (yellow)
- Channel blockers e.g. picrotoxin (purple)
- Channel modulators e.g. GA (green)
- Allosteric modulators e.g. barbiturates (blue)
Describe the GABAB receptor.
The GABAB receptor is metabotropic. The GPCRs comprise a large extracellular domain for GABA binding (can be termed a venusfly trap domain), a characteristic 7 transmembrane structure and an intracellular C terminal. It is modulated by the αGi/o G protein cascade.
They assemble as heterodimers GABAB1 and GABAB2.
GABA binding activates the G protein subunit αGi/o, which dissociates from the GABAB receptor. This inhibits enyme adenlyl cyclase and reduces the secondary messenger cAMP levels. This has two major effects:
- it opens potassium channels, causing the efflux of positive charge
- it blocks VGCCs, blocking the influx of positive charge
Thus, overall, it causes the hyperpolarisation of the cell.
Give a brief overview of the cerebellum.
The cerebellum is a prominent hindbrain structure - it accounts for approximately 10% of the human brain volume.
Cerebellum function:
• The cerebellum does not initiate movement but detects differences in “motor error” between an intended movement and the actual movement
• Aids the motor cortex to produces precise and co-ordinated movement
• It is also responsible for a number of functions including motor skills such as balance, coordination, and posture.
The cerebellum function is conserved in context of synchonisation of movement with musical rhythm. This may be widespread across the animal kingdom.
What are the GABAergic projections of the cerebellum?
Purkinje cells are a class of GABAergic neurons that comprise the principle projection neurons of the cerebellar cortex.
Purkinje cells have elaborate dendritic trees that receive convergent input from cells in the molecular layer. Purkinje cells send GABAergic projections to deep cerebellar neurons. Purkinje cell output to the deep cerebellar neurons, generates an error connection signal that can modify movements. This provides the basis for real-time control of precise and synchronous movement.
What controls the brain’s overall level of exciation?
GABA and glutamate are the major neurotransmitters in the brain – both work together to control the brain’s overall level of excitation.
Remarkably, in one step, the major excitatory neurotransmitter (glutamate) in the brain is converted into the major inhibitory neurotransmitter (GABA) in the brain - via the action of glutamate decarboxylase (and pyridoxal phosphate).
What is epilepsy?
Epilepsy is a brain disorder characterised by periodic and unpredictable seizures mediated by the rhythmic firing of large groups of neurons.
List some different epilepsy drugs based on their mechanism of action.
If there is too much glutamatergic excitation, the GABA system can be targeted in order to increase GABAergic inhibition and restore the excitatory-inhibition balance. This can be achieved by:
GABAA RECEPTOR ENHANCERS: increase GABAergic neurotransmission by acting as positive allosteric modulators at the GABAA receptors.
- Barbiturates (not used anymore because of the risk of addiction and overdose)
- Benzodiazepines
GAT BLOCKERS: block reuptake of GABA into pre-synaptic neuron and thus increase GABA availabilty in the synaptic cleft
- Tiagabine
GABA-TRANSAMINASE INHIBITOR: Inhibits GABA transaminase to increase GABA availabilty.
- Vigabatrine
GAD MODULATORS: increase activity of enzyme GAD to increase conversion of glutamate to GABA
- Gabapentin
- Valproate
PRODRUG: can be metabolised in the body to GABA to increase GABA availability
- Progabide (exogenous analogue of glutamate)
Other anti-epileptic drugs direcly decrease excitation.
The GABAergic system has also been implicated in a number of anxiety disorders. Give an example of a positive allosteric modulator at the GABAA receptor in order to increase GABAergic neurotransmission.
Anxiety can be defined as a feeling of unease (e.g. worry or fear), which can range from mild to severe.
Anxiety disorders can be generalised anxiety disorder or panic disorders. Benzodiazepines are an a form of anxiolytics which act as a postive allosteric modulator at the GABAA receptor to increase GABAerigic neurotransmission.
Describe glycine synthesis and storage.
Glycine is synthesised from 3-phosphoglycerate (a product in glycolysis).
3-phosphoglycerate is converted to serine, which then gets converted to glycine (catalysed by enzyme serine hydroxymethyl-transferase).
It is synthesised in the nerve terminals, and is transported into vesicles by vesicular inhibitory amino acid transporters (VIAAT).
Glutamate is stored in round vescles whereas GABA and glycine are stored in more oval shaped vesicles (electron micrograph images.)
