Neurotransmitters System l: Glutamate (neuro)

Question 1

Neurotransmission

Answer 1

• fundamental process that drives information transfer between neurons and their targets

Question 2

Criteria for neurotransmitter

Answer 2

• neurotransmitters are chemical messengers that transmit signals from a neuron to a target cell across a synapse (ie neurotransmission)
• molecule must be synthesised and stored in pre-synaptic neuron
• molecule must be released by the pre-synaptic axon terminal upon stimulation
• molecule must produce a response in post-synaptic cell
  process:
• neurotransmitter is synthesised in cell body or in terminal
• neurotransmitter is packaged into vesicles
• neurotransmitter is released when vesicles fuse
• neurotransmitter binds to and activates post-synaptic receptors

Question 3

Classification of neurons

Answer 3

• neurons can be classified by the neurotransmitter they use
• these differences arise due to the differential expression of proteins involved in neurotransmitter synthesis, storage and release

Question 4

Glutamate intro

Answer 4

• the major excitatory neurotransmitter in CNS
• took a while to realise glutamate was a neurotransmitter
• at the crossroad of multiple metabolic pathways
• excitatory role of glutamate in brain and spinal cord, discovered in 1950s
• not until late 1970s that. glutamate became recognised as principle excitatory neurotransmitter in CNS
• nearly all excitatory neurons in CNS are glutamatergic and it has been estimated that over half of all brain synapses release glutamate

Question 5

Glutamate synthesis and storage

Answer 5

• glutamine turns to glutamate using phosphate-activated glutaminase
• synthesised in nerve terminals
• transported into vesicles by vesicular glutamate transporters (VGLUT)
• counter transport with H+ to drive glutamate entry into vesicles

Question 6

Glutamate re-uptake and degradation

Answer 6

reuptake:
- pr-synaptic terminal to post-synaptic neuron
- neurons and glial contain Na+ dependant excitatory amino acid transporters (EAATs)
degradation:
- enzymatic reaction - glutamate to glutamine via glutamine synthetase
- transport - glial cells to neurons via SN1 (system N transporter expressed on glial cells) and SAT2 (system A transporter 2 expressed on neurons)

Question 7

Neurotransmitter receptors

Answer 7

2 broad families of neurotransmitter receptor:

• ligand-gated ion channels (ionotrophic)
• G-protein coupled receptors (metabotrophic)

Question 8

Glutamate receptors

Answer 8

ionotropic:
- AMPA receptors 
- NMDA receptors
- kainate receptors 
metabotropic:
- group l
- group ll
- group lll

Question 9

Ionotropic glutamte receptors

Answer 9

• ionotropic glutamate receptors are named after antagonists that activate them
• AMPA and NMDA (majority of excitatory neurotransmission in the brain) and kainic acid
  influx:
• AMPA - Na+
• NMDA - Na+, Ca2+
• Kainate - Na+
  efflux:
• AMPA, NMDA, Kainate - K+

Question 10

AMPA receptors

Answer 10

• hetero-tetrameric ‘dimer of dimers’
• four orthosteric binding sites
• 2 sites must be occupied for channel opening
• current increases as more binding sites are occupied
  four subunit types (plus alternate splice variants):
• GluA1
• GluA2
• GluA3
• GluA4
• most commonly 2 GluA2 subunits, 2 GluA1, 3 or 4
• presence of GluA2 subunits prevents Ca2+ flow

Question 11

NMDA receptors

Answer 11

• hetero-tetrameric ‘dimer of dimers’
• three subunit types (plus alternate splice variants):
• GluN1 (or NR1)
• GluN2 (or NR2)
• GluN3 (or NR3)

Question 12

Kainate receptors

Answer 12

• tetrametric - GluK1-3 can form hormones or heteromers, GluK4 and 5 only heteromers with GluK1-3 subunits
  ligand-gated ion channel:
• glutamate binding required for channel opening, not well understood
• limited distribution in brain compared to AMPA/NMDA receptors
  five subunit types (plus alternate splice variants):
• GluK1 (GluR5)
• GluK2 (GluR6)
• GluK3 (GluR7)
• GluK4 (KA1)
• GluK5 (KA2)

Question 13

Metabotropic glutamate receptors

Answer 13

• G-protein coupled receptor (GCPR)
• 8 sub types: Group 1-3
• synaptic plasticity and inhibit NT release
• dimers: homomers, heteromers eg mGlu1 and mGlu5, and mGlu2 and 5-HT2A

Question 14

Excitatory neurotransmitters and depolarisation

Answer 14

• excitatory neurotransmitters (eg glutamate) can lead to neuronal membrane depolarisation- displacement of a membrane potential towards a more positive value

Question 15

AMPA, NMDA and kainate receptors and EPSCs

Answer 15

• the excitatory post synaptic current (EPSC) represents the flow of ions, and change in current, across a post-synaptic membrane
• EPSCs lead to the generation of excitatory post synaptic potentials (EPSPs), which increase the likelihood of firing an action potential
• EPSCs produced by the NMDA receptor and kainite receptor are slower and last longer than those produced by AMPA receptors

Question 16

Glutamate-mediated excitotoxicity

Answer 16

• excitoxicity is the pathological process by which excessive excitatory stimulation can lead to neuronal damage and death
• excess Ca2+ can cause mitochondrial damage, oxidative stress and apoptosis
• excitotoxicity linked to stroke, alzhemiers disease

Question 17

Glutamate-mediated excitotoxicity in Alzheimer’s

Answer 17

- memantine is a low affinity NMDA receptor antagonist that blocks the NMDA receptor ion channel to reduce glutamate mediated neurotoxicity 
effects on brain:
- extreme shrinkage of cerebral cortex
- extreme shrinkage of hippocampus 
- severely enlarged vesicles

Question 18

Long-term potentiation (LTP)

Answer 18

• LTP refers to the persistent strengthening of a synapse based upon repeated patterns of activity
• LTP underlies important processes, including learning and memory - initial phase involves glutamatergic neurotransmission
  mechanism of action:
• glutamate activates AMPA receptors, with Na+ flowing into the post-synaptic neuron and causing depolarisation
• NMDA receptors open, due to depolarisation removing the voltage-gated Mg2+ ion block
• Ca2+ ions enter the cell activate post-synaptic protein kinases such as calmodulin kinase ll (CAMKll) and protein kinase C (PKC)
• CaMKll and PKC trigger a series of reactions that lead to the insertion of new AMPA receptors into post-synaptic membrane
• AMPA receptors increase the post-synaptic membranes sensitivity to glutamate and increases ion channel conductance
• this underlies the initial phase of long-term potentiation (LTP)