Block E Lecture 1 - GABA Systems

Question 1

What is the main inhibitory neurotransmitter in the brain?

Answer 1

GABA

(Slide 4)

Question 2

How is GABA synthesised?

Answer 2

It is synthesised from glutamate by glutamic acid decarboxylase (GAD)

(Slide 4)

Question 3

Where is glutamic acid decarboxylase (GAD)?

Answer 3

In GABA-synthesising neurons in the brain

(Slide 4)

Question 4

What are 4 examples of areas of the brain where GABA is found?

Answer 4

Striatum

Substantia Nigra

Hippocampus

Globus Pallidus

(Slide 5)

Question 5

What are the 8 steps of synapse transmission?

Answer 5

1. Neurotransmitters are synthesised and stored in vesicles
2. Action potential arrives at the presynaptic terminal
3. Voltage-gated Ca2+ channels open allowing influx of Ca2+
4. Ca2+ allows vesicle docking and neurotransmitter release
5. Neurotransmitter binds to receptors, causing channels to open (or close)
6. Excitatory (or inhibitory) postsynaptic potential is generated
7. Neurotransmitter is removed by glial uptake or enzyme degradation
8. Vesicular membrane is retrieved from the plasma membrane

(Slide 6)

Question 6

What terminates GABA action?

Answer 6

GABA transporter (GAT)

(Slide 7)

Question 7

What are the 4 variants of the GABA transporter (GAT)?

Answer 7

GAT1
GAT2
GAT3
BGT1

(Slide 7)

Question 8

What type of transporter are the GABA transporters?

Answer 8

Sodium ion symporters (Sodium goes down the concentration gradient, GABA goes up)

(Slide 7)

Question 9

Where do the GAT1,2 and 3 and BGT1 transporters act?

Answer 9

GAT1 and 3 act in neuronal tissue whereas GAT2 and BGT1 act in non-neuronal tissue

(Slide 7)

Question 10

How is GABA catalysed?

Answer 10

It is converted to succinate by GABA transaminase (GABA-T)

(Slide 7)

Question 11

What are the 5 requirements for a substance to be considered a neurotransmitters?

Answer 11

Must be:

Synthesised in nerve terminals

Stored in nerve terminals (synaptic vesicles)

Released in a Ca2+ dependent manner

Acts on postsynaptic receptors

Is later removed / eliminated

(Slide 8)

Question 12

What 2 descriptors can be applied to GABA binding to the brain?

Answer 12

Saturable ( there is a finite number of GABA receptors in the brain. Once all available receptors are occupied by GABA, no further binding can occur, regardless of how much GABA is present) and specific

(Slide 9)

Question 13

What are the 2 main classes of GABA binding sites and what are the differences between them?

Answer 13

GABA receptors - binding is not sodium dependent

GABA uptake sites - binding is sodium-dependent, and they greatly outnumber the GABA receptor sites

(Slide 9)

Question 14

What is the agonist rank of potency, to displace 3[H]-GABA?

Answer 14

Isoguavine > Muscimol > GABA > Gaboxadol (THIP)

(Slide 10)

Question 15

What are 2 antagonists which have the ability to displace 3[H]-GABA and what are their properties?

Answer 15

Bicuculine - Displaces GABA but not from all receptors

Picrotoxin - Doesn’t displace GABA, it binds to the chloride ion channel pore of the GABA receptor

(Slide 10)

Question 16

What are the 2 subtypes of GABA receptors, what are their properties and how was it confirmed that these subtypes exist?

Answer 16

GABAA - Bicuculine-sensitive and Baclofen-insensitive, ligand-gated (chloride) ion channel receptor

GABAB - Bicuculine-insensitive and Baclofen-sensitive, GPCR receptor

This is confirmed by the agonist baclofen [β-(4-chlorophenyl)-GABA] which is an agonist only at bicuculine-insensitive GABA sites

(Slides 11, 12 and 16)

Question 17

What is the structure of the subunits of the GABAA receptor, and how many subunits does the receptor have?

Answer 17

Subunits have 4 transmembrane domains, and the receptor is comprised of 5 subunits

(Slide 16)

Question 18

How is the chloride pore of the GABAA receptor formed?

Answer 18

By the TM2 domains of each of the subunits

(Slides 17 and 18)

Question 19

What 5 classes of GABAA receptor subunit isoforms exist?

Answer 19

α, β, γ, ρ and other

(Slide 19)

Question 20

What is the most common subunit arrangement of GABAA receptor?

Answer 20

2 X α, 2 X β and 1 X γ subunits

(Slide 19)

Question 21

How are GABAA receptors activated?

Answer 21

They have 2 interfaces between the α and β subunits, with GABA having to bind to both before the receptor becomes activated

(Slide 20)

Question 22

Where does benzodiazepine bind and what does it act as regarding GABAA receptors?

Answer 22

It binds at the interface between the α and γ subunits, activating as a positive allosteric modulator

(Slide 20)

Question 23

What is the structure of the GABAA-Rho receptor?

Answer 23

Also a 5 subunit ligand-gated ion channel, like normal GABAA receptors, but comprised solely of the three rho(ρ) subunit varieties

(Slide 21)

Question 24

What cells are GABAA-Rho receptors found in?

Answer 24

Retinal bipolar cells

(Slide 21)

Question 25

What is the GABAA-Rho receptor pharmacology?

Answer 25

It's insensitive to: Bicuculine, barbiturates, benzodiazepine and baclofen But it's sensitive to: CACA (agonist), TPMPA (antagonist) and picrotoxin (Slide 21)

Question 26

What is the structure of GABAB receptors?

Answer 26

It is a heterodimer made up of GABAB1 and GABAB2 subunits, which each have 7 transmembrane domains each (Slide 22)

Question 27

What do GABAB1 and GABAB2 subunits bind?

Answer 27

GABAB1 binds GABA whereas GABAB2 bind positive allosteric modulators (Slide 22)

Question 28

What G protein do GABAB receptors couple to?

Answer 28

Gi/Go (Slide 22)

Question 29

How does GABA cause presynaptic inhibition in the spinal cord?

Answer 29

It reduces transmitter release from the terminals of primary afferent fibres. It also involves axo-axonal synaptic connections (a GABA neuron connecting to a primary afferent fibre) (Slide 28)

Question 30

How is GABA pharmacologically distinct from glycine?

Answer 30

Bicuculline and picrotoxin block GABA's effects whereas strychnine blocks glycine's effects (Slide 28)

Question 31

How does GABA cause postsynaptic inhibition in the brain?

Answer 31

It increases chloride ion (Cl-) flux postsynaptically, resulting in the neuron becoming hyperpolarised, resulting in a decreased likelihood of the neuron firing an action potential (Slide 30)

Question 32

How does the brain have a negative feedback system for dopamine?

Answer 32

Overactivity of dopamine results in an increase in GABA activity which results in the inhibition of dopaminergic firing in the substantia nigra (Slide 31)

Question 33

What is an axon collateral?

Answer 33

It is a side branch which extends from an axon, allowing a neuron to communicate with multiple other neurons (Slide 32)

Question 34

How can GABA inhibit a pyramidal neuron recurrently using axon collaterals, and how does this prevent epilepsy?

Answer 34

The axon collateral connects to GABA, with an interneuron and a GABA neuron also connecting to the pyramidal neuron. 1 A pyramidal neuron axon collateral activates a GABA interneuron 2. The interneuron, in turn, inhibits firing of the pyramidal neuron 3. This prevents a bursting discharge and reduces the max firing rate of the pyramidal cell. 4. Bursting discharge is a feature of epilepsy, so preventing this stops epilepsy occuring (Slide 32)

Question 35

How is recurrent inhibition measured?

Answer 35

1. A pyramidal neuron is stimulated to fire an action potential. 2. The resulting inhibitory postsynaptic potential (IPSP), caused by activation of inhibitory interneurons are recorded in the same neuron. 3. The IPSPs can serve as evidence for recurring inhibition (Slide 32)

Question 36

How do drugs effect the recurring inhibition of GABA?

Answer 36

GABA agonists (such as muscimol) enhance recurring inhibition, whereas GABA antagonists (such as bicuculine) depress it. (Slide 33)

Question 37

What are the pre and postsynaptic effects of GABAB receptors?

Answer 37

Presynaptic: Decreased Ca2+ fluxes Postsynaptic: Increased K+ fluxes (Slide 36)

Question 38

What are GABAB receptors implicated as a target for management of?

Answer 38

Pain Absence epilepsy (brief, sudden lapse of consciousness) Cocaine addiction Asthma (Slide 36)