Amino Acid Neurotransmitters

Question 1

What are the main AA NTs ?

Answer 1

• Glutamate
• GABA (γ-aminobutyric acid)
• Glycine

Question 2

What are the steps of synaptic transmission ?

Answer 2

1. Uptake of precursors
2. Synthesis of transmitter
3. Uptake into vesicles
4. Metabolism of transmitter
5. Invasion of action potential
6. Ca2+ influx
7. Exocytosis of vesicles
8. Diffusion to receptors
9. Action at postsynaptic receptors
10. Diffusion away from synapse
11. Re-uptake by neuron
12. Re-uptake by glia
13. Action on presynaptic receptors

Question 3

What are the main characteristics of Glu ?
How is it taken up from the extracellular space ?
How is it concentrated in vesicles ?

Answer 3

• Most widespread excitatory neurotransmitter in the CNS
• Taken up from the extracellular space by specific transporters - Excitatory Amino Acid Transporters (EAAT1-5)
• Concentrated in vesicles in glutamatergic neurons, but not in others - Vesicular Glutamate Transporter (VGLUT)

Question 4

How is Glu synthesized and metabolized ?

Answer 4

alpha-ketoglutarate (from TCA cycle) –> Glu (via Glu dehydrogenase) + part of the Glu converted to gluthione, polyamines, urea, GABA
Glu –> Gln (via Glu synthetase (glutaminase for Gln –> Glu) + part of the Gln conerted to amino sugars, glucoproteins, nucleotides

Question 5

What are the main characteristics of GABA ?
How is it taken up from the extracellular space ?
How is it concentrated in vesicles ?

Answer 5

• Most widespread inhibitory neurotransmitter in the CNS
• Synthesized from glutamate via cytosolic enzyme - Glutamic Acid Decarboxylase (GAD)
• Taken up from the extracellular space by specific transporters - GABA Transporters (GAT1-4)
• Concentrated in vesicles in GABAergic neurons, but not in others - Vesicular Inhibitory Amino Acid Transporter (VIAAT)

Question 6

What are the main characteristics of Gly ?
How is it taken up from the extracellular space ?
How is it concentrated in vesicles ?

Answer 6

• Inhibitory neurotransmitter in mammalian spinal cord
• Simplest amino acid; derived from metabolism and synthesized in all cells
• Taken up from the extracellular space by specific transporters - Glycine Transporters (GlyT-1 and GlyT-2)
• Concentrated in vesicles in glycinergic neurons - VIAAT (note, some neurons can release both GABA and Glycine)

Question 7

What different types of GluRs exist ?

Answer 7

Glutamate receptors : ionotropic Vs metabotropic
Ionotropic : NMDA Vs non-NMDA
Metabotropic : mGluRs
non-NMDA : AMPA Vs kainate

Question 8

How do ionotropic Vs matebotropic GluRs differ ?

Answer 8

iGluR subunit : 4 membrane domains (M1 to M4)

mGluR subunit : 7 transmembrane domain (M1 to M7)

Question 9

What are the three classes of iGluR ?

Answer 9

• N-methyl-D-aspartate (NMDA)
• α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)
• Kainate

Question 10

Name 4 iGluR agonists.

Answer 10

L-Glutamate, NMDA, Kainate, AMPA

Question 11

What genes are part of the AMPA, kainate and NMDA family ?

What are agonists and co-agonists for these receptors ?

Answer 11

AMPA : GluA1, GluA2, GluA3, GluA4 (Glu, AMPA, Kainate)
Kainate : GluK1, GluK2, GluR3, GluK4, GluK5 (Glu, Kainate, Domoic Acid)
NMDA : GluN1, GluN2A, GluN2B, Glu2C, GluN2D, GluN3A, GluN3B (Glu, Gly, NMDA)

Question 12

Do Glu gated ion channels assemble as :

• trimers ?
• tetramers ?
• pentamers ?

Answer 12

Tetramers.

Question 13

How are mGluR sub-types grouped ?

Answer 13

Group I : mGlu1 + mGlu5 => +PLC (IP3)
Group II : mGlu2-3 => -AC (cAMP)
Group III : mGlu4 + mGlu6-8 -AC (cAMP)

Question 14

How do AMPA/KainateRs VS NMDARs Vs mGluR work ?

What are agonists (endogenous and exogenous) and antagonists at these receptors ?

Answer 14

AMPA/Kainate : work by Na+/K+ influx, activate by Glu and exogenously by AMPA/Kainate, inhibited by CNQX
NMDA : work by Na+/K+/Ca2+ influx, activated Glu/Gly and exogenously by NMDA, inhibited by AP5 and 7-CK
mGluR : work via 2nd messengers (IP3, cAMP), activate by Glu and exogenously by ACPD, inhibited by various compounds

Question 15

What are the two types of integration that can occur in neurons ?

Answer 15

Spatial and temporal.

Question 16

What is the special feature of NMDARs ?

Answer 16

The ion channel is normally blocked by a Mg2+ ion.

Relief of Mg block leads to further depolarization and Ca2+ entry.

Question 17

Glu is taken up into vesicles in exchange fo H+. How is the proton concentration maintained in the vesicle ?

Answer 17

Via a proton transporter (an ATPase : ATP –> ADP + Pi)

Question 18

What are the 2 classes of GABA receptors ?

Answer 18

Ionotropic : GABA(A) and sometimes GABA(C)...
Metabotropic GABA(B)

Question 19

How do the different GABARs work ?

What molecules are agonists/antagonists at these receptors ?

Answer 19

GABA(A) = increases Cl- influx, activated by GABA and exogenously by muscimol, inhibited by bicuculiline
GABA(B) = increase K+ efflux and decrease Ca2+ influx (Gi/o, cAMP), activated by GABA and exogenously by baclofen, inhibited by CGP 35348

Question 20

What is another name for the cys-loop receptor family ?
What are the characteristics of receptors belonging to this family ?
Give examples of receptors.

Answer 20

Cys-loop receptor superfamily = nACh superfamily
All pentameric : 
nACh receptors
5-HT(3) receptors
GABA(A) rceptors
Glycine receptors

Question 21

What are the members of the Glu receptor family ?

What is their common feature ?

Answer 21

All tetramers :
NMDA receptors
AMPA receptors
Kainate receptors

Question 22

How many GABA(A) subunits do we currently know ?

Answer 22

6 alpha
3 beta
3 gamma
1 delta
1 epsilon
1 pi
1 theta
2 rho

Question 23

Name 4 GABA(B) agonists and 3 antagonists.

Answer 23

Agonists : 
- GABA
-  APPA
- (R)-baclofen
- Gabapentin
Antagonists : 
- (R)-phaclofen
- CGP 54626
- CGP 64213

Question 24

What is the effect of 2-OH-saclofen on GABAergic transmission ?

Answer 24

In prevents the late IPSP in the postsynaptic cell from occurring.

Question 25

How is GABA metabolized ?

Answer 25

In the mitochondria (of neurons or glial cells) :
GABA –> Succinic semialdehyde (SSA) via GABA-T
SSA –> Succinate via SSA dehydrogenase (SSADH)

Question 26

Name 6 molecules or classes of molecules that are modulators of GABA.

Answer 26

• benzodiazepines
• barbiturates
• steroids
• intravenous anaesthetics
• inhalation anaesthetics
• alcohol

Question 27

What is diazepam ?

What is it used for ?

Answer 27

Diazepam is an BDZ.
It is an anxiolytic, sedative, muscle relaxant anti-convulsant and attenuates autonomic endocrine response.
It is used to treat :
- acute anxiety
- insomnia
- epilepsy
It is also used as pre-medication during surgery, and as a sedative during minor medical procedures.

Question 28

What did Leo Sternbach (1908-2005) synthesize in the 1950s-1960s ?
How did this have an impact ?

Answer 28

Synthesized :
- chlordiazepoxide in 1955 –> marketed by Roche as Librium (1960)
- diazepam in 1959 –> marketed as Valium (1963)
Between 1969 and 1982 Valium was the most prescribed drug in the world.

Question 29

What do the effects of BDZ depend on ?

Answer 29

Effects of BDZ correlate with binding.

Question 30

How do BDZs affects the dose-response curve ?

Answer 30

BDZs are positive allosteric modulators (cause a parallel shift in the dose-response curve to the left).