Neurotransmitters

Question 1

How is Glutamate synthesised?

Answer 1

• Occurs in the pre-synaptic neuronal terminal
• From glucose via tri-carboxylic acid cycle
• From glutamine (by action of glutaminase)

Question 2

Describe the storage, release and reuptake of Glutamate.

Answer 2

• Glutamate is stored in synaptic vesicles prior to release
• Uptake into vesicles occurs via the vesicular glutamate transporter (VGlut) in exchange for H+
• Glutamate is released by Ca2+-dependent exocytosis
• Action is terminated by reuptake mechanisms
• Neurons and non-neuronal cells use excitatory amino acid transporters (EAATs) - Na+ linked to co-transporters - remove glutamate
• In glial cells, glutamate is converted to glutamine by glutamine synthase
• Glutamine is a pool/store of inactive transmitter

Question 3

What different Glutamate recpetors are there?

Answer 3

• Metabotropic glutmate receptors - GPCRs
• Ionotropic glutamate receptors - ligand gated ion channels (NMDA, AMPA, kainate)

Question 4

Describe the structure of NMDA receptors.

Answer 4

• 7 potential subunits encoded by 7 genes
• TETRAMERIC complex
• Hetero-tetramers - mixture of subunits
• Each subunit has: extracellular N-terminal domain (amino terminal domain), extracellular ligand binding domain (binds the agonist), 3 transmembrane spanning alpha-helical domains (M1, M3, M4), 1 re-entrant loop (M2) and an intracellular C-terminus (carboxyl terminal domain)

Question 5

Describe the different drug binding sites on the NMDA receptor.

Answer 5

• Glutamate (agonist site)
• Glycine site
• Polyamine binding site
• Mg2+ site
• Channel blocking site

Question 6

How is the NDMA receptor agonised?

Answer 6

• Each tetramer binds 2 molecules of Glutamate
• 2 molecules of glycine are requried to co-agonise
• GluN1 is the glycine binding-binding subunit

Question 7

What drug competitively antagonises the glutamate site on NMDA receptors?

Answer 7

D-AP5

Question 8

What antagonises the Glycine site?

Answer 8

Kynurenic acid and CGP 61594

Question 9

Describe the Mg2+ site on NMDA receptors.

Answer 9

• Channel is blocked by Mg2+ at resting neuronal membrane potential
• This feature gives NMDA receptor voltage dependent channel block

Question 10

Describe the polyamine site on NMDA receptors.

Answer 10

• Polyamines act as allosteric modulators
• Examples of polyamines - spermine, spermidine
• Can be extra and intracellular
• Can increase affinity for glycine and increases NMDAR responses

Question 11

What drugs act at NMDA receptors?

Answer 11

• Channel blocking drugs - Ketamine, Phencyclidine, MK-801, Memantine (binds to M2 domain)
• Neurosteroids - can be positive or negative allosteric modulators (Pregnenolone sulfate - positive allosteric modulator)