Week 3- The neurotransmitter GABA and its receptors

Question 1

what are some neurotransmitters in the CNS?

Answer 1

Acetylcholine
Noradrenaline
Adrenaline
Dopamine
Serotonin (5-HT)
Histamine
Glutamate
GABA
Glycine

Question 2

what is GABA? which neurones use GABA, role, distribution in the brain?

Answer 2

• Gamma-aminobutyric acid
• Main inhibitory neurotransmitter in the brain
• 20% of CNS neurons are GABA-ergic
• Mostly GABA neurons are short interneurons
• Widespread distribution of GABA in brain – ubiquitous function
• Key role in regulating neuronal transmission

Question 3

how is GABA synthesised hows it metabolised?

Answer 3

-its linked to the Krebs cycle
• Precursor is glucose
• GABA is formed from
glutamate by the action of
GAD (glutamic acid
decarboxylase)
• Metabolised by GABA-T
(GABA α-oxoglutarate
transamidase)
• GABA shunt – preserves
GABA

Question 4

where is GABA stored, release and up-taken into?

Answer 4

-the production of GABA relies on the expression of GAD.
• Synthesis in GABAergic nerve terminals
• Packaged into vesicles by
vesicular transporter
• Released GABA can be recycled via GABA transporters (GAT) on presynaptic terminal
• GABA can also be taken up by astrocytes (GAT3)

Question 5

what is the pathway of glutamate in the synapse?

Answer 5

theres glutamate and its converted into GABA

• GABA is loaded into the synaptic vesicles using the vesicular transporter
• the synaptic vesicles are then release when the signal comes into the synaptic terminal of the neurone
• the synaptic vesicle is released due to the calcium dependant exocytosis mechanism and GABA is released into the synaptic cleft
• GABA can act on receptor in the synapse
• GABA can also be taken up back into pre synaptic neurone on glial cells like astrocytes where there will be a GABA transporter

Question 6

what happens when GABA is taken up by astrocytes?

Answer 6

can be converted back to glutamate then glutamine then can be provided back to the neurone

Question 7

what drugs affect GABA transport and metabolsim?

Answer 7

-Guvacine- GABA
transporter-Reuptake inhibitor, prevent activity and increase conc on GABA in synapse
-Tiagabine-GABA
transporter-Reuptake inhibitor, prevent activity and increase conc on GABA in synapse
- Vigabatrine-GABA-transaminase-Inhibitor, prevent break down of GABA, PRESERVE CONC of GABA in neurone and synapse

Question 8

where does GABA act on ? what are the different types

Answer 8

• GABA acts on 2 types
of receptors
• GABA(A-little sub)
– ionotropic
–member of Cys
-loop family
• GABA(B)
- metabotropic
– member of class C
GPCRs

Question 9

what are the main aspects of GABA(A)? LOCATION AND SIGNALLING?

Answer 9

• GABA(A)
receptors are located post-synaptically
• Mediate fast and tonic postsynaptic inhibition
• Ion channel is selective for anions, chloride ions (Cl-)
-at resting potential the channel is open by GABA so chloride can move in
• Typically the equilibrium potential for Cl- is more negative than Vm
(resting membrane potential)
• Cl- influx causes hyperpolarisation thus reducing excitability
• (This can be altered in disease states)
• can be found in both Synaptic and extrasynaptic sites

Question 10

what are the key facts about GABA(A) receptors? structure

Answer 10

• Pentameric, 5 subunits that come together to make up the ion channel complex
• Heteromeric, made up of different sub units
• Made of multiple subunits, 19 have been
identified (6 α, 3 β, 3 γ plus δ, ε, θ, π, ρ)
• Each pentamer contains 2α 2β and one γ
• Most common combinations are:
• α1β2γ2 (60% of all)
• α2β3γ2
• α3β3γ2

Question 11

what is the structure of the GABA receptors?

Answer 11

Individual subunit structure
- long extracellular N-terminus,
- 4 transmembrane domains,
- short extracellular C-terminus,
- long intracellular loop between TM3
and TM4
- TM2 lines the ion channel

Question 12

what are the different types of drug binding sites for drugs on GABA(A) receptors?

Answer 12

• Contain multiple (5) drug
binding sites:
1. Receptor site (GABA site)
2. Benzodiazepine site
3. Modulatory site (barbiturate)
4. Steroid site
5. Picrotoxin site (channel
blocking) 
• Ethanol is also thought to bind
and increase activity

Question 13

what are the main facts about GABA(A) receptors binding sites, GABA site?

Answer 13

The GABA site – agonist (GABA) binding opens the Clion channel
• Located between an α and β subunit interface
• There are 2 sites per pentamer
• Muscimol (GABA analog from psychoactive mushrooms) binds to this
site as a potent agonist
• Bicuculline (natural product) competes with GABA and blocks the site
(antagonist)
• Bicuculline blocks IPSPs in CNS synapses and causes convulsions

Question 14

what does GABA act as ?

Answer 14

• GABA acting as a agonist is the authosteric ligand
• other binding sites on the receptors and the ion channels known as allosteric binding site which can affect the agonist
• affinity of agonist
• efficacy of agonist

Question 15

how is the affinity of a agonist negatively modulated what does it mean?

Answer 15

makes it harder for agonist to open another chancel so need higher concentration for response

Question 16

how is the efficacy of a agonist negatively modulated what does it mean?

Answer 16

less of an effect in response of the agonist

Question 17

what are the main facts about GABA(A) receptors binding sites, Benzodiazepine site?

Answer 17

• Benzodiazepine site – drugs that selectively
enhance the effects of GABA
• Bind with high affinity to the BDZ site located
between α and γ subunit interface
• Alters the receptor affinity for GABA – the agonist
effect is increased
• Certain α subunits are not modulated; α4, α6
• Examples are diazepam, Valium
• Used clinically – anxiety, epilepsy, anaesthesia,
sleep
• Benzodiazepine antagonists also exist – e.g.
Flumazenil

Question 18

what are the main facts about GABA(A) receptors binding Barbiturates site , site?

Answer 18

• Barbiturates site – drugs binding here increase ligand binding to BDZ and
GABA sites
• Single channel recordings suggest mean channel open time is increased
by barbiturates
• Barbiturates are CNS depressants used in anaesthesia and epilepsy
• Previously used in sleep disorders and anxiety but now replaced by
other drugs
• e.g. Pentobarbitol

Question 19

what are the main facts about GABA(A) receptors binding Neurosteroid site , site?

Answer 19

• Neurosteroid site – endogenous modulators that
enhance the effects of GABA
• Thought to act on transmembrane regions of α
subunit (M1 and M2)
• Examples are metabolites of progesterone
(allopregnanolone), cortisone (THDOC) and
androgens
• Synthetic steroid alphaxalone also binds here
• Ganaxolone – clinical trials for epilepsy

Question 20

what are the main facts about GABA(A) receptors binding site number 5 site?

Answer 20

• Channel blocking site – inside the channel pore
• Picrotoxin (natural product) binds here
• Requires channel opening to gain access to TM
domains
• Inhibitors such as picrotoxin cause convulsions

Question 21

what are the main factors about. GABA (B)

Answer 21

• GPCRs
• Couple through Gαi/o
• Signalling inhibits CaV channels
and reduces neurotransmitter
release from presynaptic terminals
• Open K+ channels to increase
hyperpolarisation
• Dimeric structure – 2 GPCRs B1 and B2

Baclofen is an agonist at GABAB
Competitive antagonist e.g. 2-hydroxyl-saclofen

Question 22

what is glycine? main facts?

Answer 22

• Another inhibitory neurotransmitter in the CNS
• Also a co-agonist at NMDA receptors
• The glycine receptor is similar to the GABAA
receptors – cys-loop family
• Ligand gated ion channel, chloride selective
• 5 glycine receptor subunits cloned
• Strychnine (plant alkaloid) is an antagonist. Acts as a convulsant
• Tetanus toxin prevents glycine release from inhibitory interneurons –
causes lockjaw due to reflex hyperexcitability
• Glycine transporters remove glycine from extracellular fluid
• GlyT1 and GlyT2
• Expressed on astrocytes and neurons