GABA

Question 1

What is GABA

Answer 1

1. THE principal inhibitory transmitter in the CNS (>30% of brain synapses)
2. Activates both ionotropic and metabotropic receptors

Question 2

What is GABA synthesised from

Answer 2

1. Enzymatic synthesis from glucose via Kreb’s cycle

Question 3

How is GABA inactivated

Answer 3

1. Inactivated by reuptake by the GABA Transporters (GAT)

Question 4

What can deficits in GABA transmission lead to

Answer 4

1. Deficits in GABA transmission important in epilepsy and anxiety

Question 5

What does glycine do

Answer 5

1. Glycine also has a role as an inhibitory amino acid transmitter

Question 6

Describe the synthesis of GABA

Answer 6

1. Starts with glucose and via krebs cycle the product is alpha-ketoglutamate
2. Then glutamate which is converted to GABA using glutamic acid decarboxylase GAD
3. GABA is transported into vesicles by VGAT- vesicular transporter
4. When membrane terminal depolarises calcium enters, vesicles fuse with membrane and GABA is released

Question 7

Describe how GABA is terminated

Answer 7

1. Action is terminated when taken out of synapse by GABA transporters into astrocytes where it is metabolised
2. Also taken up to nerve terminals where it is either taken up by synaptic vesicles or converted by GABA-transaminase into succinic semialdehyde

Question 8

Describe the distribution of GABA

Answer 8

1. 35% of neurones stain for Glutamic Acid Decarboxylase (GAD) – i.e. GABA neurones
2. Not localized discretely
3. GABA neurones play prominent role in cerebellum, basal ganglia, hippocampus, hypothalamus, cortex
4. Principally in local interneurones- Connections limited to other neurones in local area
5. Can be found in relay or projection neurones

Question 9

What is relationship with GABA and glutamate

Answer 9

1. Balance each other out

2. GABA inhibits, glutamate excites

Question 10

Describe GABA(A) receptor

Answer 10

1. ligand gated ion channels
2. permeable to Cl- ions
3. primarily postsynaptic
4. mediate fast (milliseconds) inhibition

Question 11

Describe GABA(B) receptor

Answer 11

1. G-protein-coupled receptors (GPCR)
2. coupled to Ca2+ and K+ ion channels
3. pre and postsynaptic
4. mediate slow inhibition (seconds-minutes)
5. also inhibit transmitter release

Question 12

Describe GABAC receptors (GABAA-rho (ρ) receptor)

Answer 12

1. ligand gated Cl- channels
2. postsynaptic mainly located in retina
3. mediate fast (ms) inhibition

Question 13

Describe the fast synaptic inhibition caused by GABA(A)

Answer 13

1. Low intracellular Cl-
2. GABA opens channel
3. inward negative Cl- current
4. Membrane hyperpolarizes
5. Inhibitory postsynaptic potential (IPSP)

Question 14

Describe the structure of GABA(A) receptor

Answer 14

1. GABAA receptor subunits have 4 trans-membrane segments
2. TM2 (one of 4 trans-membrane segments) is pore forming segment
3. Functional receptors are pentameric combinations of different subunits arranged to form the integral ion channel
4. Most prevalent receptor in mammalian brain consists of two α, two β and one γ-subunit
5. GABA binds between α and β-subunits – so 2 molecules to activate receptor

Question 15

Describe different GABAA structures

Answer 15

1. Multiple isoforms of α, β and γ subunits
2. Rarer subunits exist – δ, ε, π, θ and ρ
3. Most prevalent receptor is α1/β2/γ2
4. Different subunits confer distinct physiological properties
5. Receptor pharmacology is subunit-dependent
6. Inhibitory effect depends on composition.
7. Differential spatial distribution of receptors

Question 16

What are the binding sites of a GABA A receptor

Answer 16

1. Benzodiazepine
2. GABA
3. barbiturate/anaesthetic binding site
4. neurosteroid binding site
5. channel blocking site

Question 17

Describe benzodiazepine binding site

Answer 17

1. e.g. diazepam, lorazepam
2. potentiate GABA
3. increase frequency of channel opening
4. anxiolytic (reduce anxiety), antidepressant, muscle relaxant

Question 18

Describe GABA binding site

Answer 18

1. agonists: GABA, muscimol
2. antagonists: bicuculline, gabazine
3. At interface of alpha-beta subunits- 2 binding sites

Question 19

Describe barbiturate/anaesthetic binding site

Answer 19

1. pentobarbital, thiopental, propofol
2. potentiate GABA
3. prolong open time of channel
4. sedative, antiepileptic, anaesthetic

Question 20

Describe neurosteroid binding site

Answer 20

1. derivatives of sex hormones
2. increase or decrease
3. endogenous GABA inhibition
4. adjacent to barbiturate binding site

Question 21

Describe channel blocking site

Answer 21

1. picrotoxin, pentylenetetrazole
2. block Cl- permeability
3. convulsant

Question 22

Describe action at GABAB receptors

Answer 22

1. GABAB-receptors are Gαi/o coupled, 7-TM GPCRs
2. Linked to K+-channels in postsynaptic neurones
3. Linked to Ca2+-channels in presynaptic terminals
4. Presynaptically- closes calcium channels to reduce transmitter release
5. Postsynaptically- opens potassium channels eliciting a slow hyperpolarization

Question 23

What are the agonists and antagonists of GABAB recetpors

Answer 23

1. Agonists: GABA, baclofen

2. Antagonists: 2-hydroxy- saclofen, CGP55845A

Question 24

Describe why functional GABAB receptors

are obligatory heterodimers

Answer 24

1. Two GABAB-receptors GABABR1 + GABABR2
2. R1 in endoplasmic reticulum
3. R2 in cell membrane
4. Neither is functional alone
5. Dimerize via C-terminal to form functional GABABR
6. R1 binds GABA not R2

Question 25

Describe the interactions of GABAA and GABAB receptors

Answer 25

1. GABA can mediate fast and slow postsynaptic inhibition 2. GABA can control its own release and strength of inhibition 3. e.g. postsynaptic GABAA + presynaptic GABAB will lead to signal decreasing

Question 26

What are the therapeutic targets for GABAB receptors

Answer 26

1. GABABR agonists (baclofen) for spasticity (muscle spasms, rigidity) in multiple sclerosis and related motor disorders - Activates postsynaptic GABABR on motor neurones - May also have sedative and anxiolytic effects 2. GABABR antagonists for possible use in epilepsy - Blocks presynaptic GABABR on GABA neurones to prevent reduction of GABA release (can't block Ca2+ channels now) and increase synaptic inhibition - May also be sedative