3. GLUTAMATE Flashcards
What are neurotransmitters?
- Neurotransmitters are chemical messengers hat transmit signals from a neurone to a target cell
What is the process of synaptic transmission?
- Neurotransmitter synthesised at nerve terminal or cell body
- Neurotransmitter packaged into vesicles
- Arrival of action potential causes depolarisation
- Voltage gated Ca2+ channels open
- Neurotransmitter is released via exocytosis & binds to receptors on the post-synaptic membrane
What is glutamate?
- Glutamate is the major excitatory neurotransmitter in the CNS
- Approx half of brain synapses = glutamate mediated
How is glutamate synthesised?
- Glutamate can be synthesied but it’s mostly converted from the precursor Glutamine
- Glutamine –> Glutamate by the enzyme GLUTAMINASE
- GLUTAMINASE changes the NH3 group into a carboxyllic acid, changing glutamine into glutamate
What enzyme catalyses the conversion of glutamine into glutamate?
- GLUTAMINASE
- Glutamine -> Glutamate
Describe the transport & storage of glutamate in vesicles
- Glutamate is packaged into vesicles by VGLUT (vesicular glutamate uptake transporters)
- The transport of glutamate into vesicles is driven by the movement of H+ out of the vesicle
- The inside of the vesicle is acidic, which is maintained by the H+ pump. H+ moves out of the vesicle, down the conc gradient
What transporters are involved in the re-uptake of glutamate & what do they do?
- Glutamate is released into the synaptic cleft by exocytosis
- EAAT (Excitatory amino acid transporters) are located on neurones & glial cells
- The EAAT are involved in re-uptake of glutamate. They take up glutamate from the synaptic cleft to be converted into glutamine
What enzyme degrades glutamate into glutamien?
- Once glutamate is taken up into glial cells by EAAT it’s converted to glutamine by GLUTAMINE SYNTHETASE
- Glutamate –> Glutamine
Describe the transport of glutamine out of cells after re-uptake?
- Once glutamate, has been converted to glutamine in the glial cell it will be transported out & back into the neurone
- Glutamine is transported out of the glial cell by SN1
(SYSTEM N TRANSPORTER) - The glutamine will then be transported into the neurone by SAT2 (SYSTEM A TRANPORTER 2)
- The whole process is known as the glutamate-glutamine cycle
What are the two main families of glutamate receptor?
- IONOTROPIC (LIGR)
2. METABOTROPHIC (G-PROTEIN COUPLED)
What 3 ionotropic receptors does glutamate bind to?
- AMPA
- NMDA
- KAINATE
- The ionotropic receptors are all located post-synaptically & allow the influx of Na+ & the efflux of K+. NMDA also allows the influx of Ca2+
What are the 4 sub-units of the AMPA receptor & the most common configuration?
- GluA1
- GluA2
- GluA3
- GluA4
- Most common = 2GluA2 with either 2GluA3 or A4 or A1
- GluA2 is constant but the others are variable
What’s the importance of the GluA2 sub-unit in AMPA receptors?
- The GluA2 sub-unit is constant
- The presence of the GluA2 prevents Ca2+ & therefore prevents the mediated excitotoxicity resulting from it
What are the properties of the AMPA receptor?
- The AMPA receptor has 4 glutamate binding sites
- 2 binding sites need to be occupied in order for the ion channel to open
- Allows: Na+ influx & K+ efflux
- Found post-synaptically
- Forms a hetero-tetramer
- Current produced by AMPA will be greater if more binding sites are occcupied
What are the 3 sub-units for the NMDA receptor & he most common configuration?
- GluN1
- GluN2
- GluN3
- Most common: 2GluN1 & 2GluN2
- The 2GluN2 can be replaced with 2GluN3
- All binding sites need to be occupied for an ion channel to open
- GluN3 sub-units are non-functional & are inhibitory to receptor function
What are the properties of the NMDA receptor?
- The NMDA receptor allows for a Ca2+ influx aswell as the influx of Na+ & efflux of K+
- All binding sites need to be occupied for the ion channel to open
- NMDA receptors are both ligand & voltage gated channels
- Ligand: Both Glutamate and Glycine or D-Serine need to bind
- Voltage: NMDA receptors have a Mg2+ block which blocks the ion channel. Once there’s sufficient depolarisation of the membrane, the Mg2+ block dissociates allow the influx of cations
What ligands bind to the NMDA receptors & what sub-unit do they bind to?
- Glutamate = binds to the GluN2 sub-unit
- Glycine or D-serine = bind to GluN1 sub-unit
What are the 5 sub-unit types for Kainate receptors?
- GluK1
- GluK2
- GluK3
- GluK4
- GluK5
- Glutamate binding is required for ion channel opening, but it’s not well understood
What are the properties of Kainate receptors?
- Kainate receptor are found on the post-synaptic membrane
- GluK1 -GluK3 = from HOMOMERS
- GluK4 & GluK5 form HETEREOMERS with GluK1 - GluK3
- Limited distribution in the brain compared to NMDA & AMPA receptors
What are the three groups of metabotropic glutamate receptors?
- GROUP 1 (POST-SYNAPTIC)
- GROUP 2 (PRE-SYNAPTIC)
- GROUP 3 (PRE-SYNAPTIC)
- Metabotrophic receptors can have both inhibitory & excitatory effects
What sub-types make up the three groups of metabotropic glutamate receptors?
- There are 8 sub-types in total: mGlu1 - mGlu8
- GROUP 1 = mGlu1 & mGlu5
- GROUP 2 = mGlu2 & mGlu3
- GROUP 3 = mGlu4, mGlu6, mGlu7, mGlu8
Describe the properties of Group 1 glutamate receptors?
- Group 1 receptors are located on the post-synaptic membrane
- Group 1 receptors are coupled to Gq, so they’re positively linked to phospholipase C
- Phospholipase C converts PIP2 -> IP3 & DAG leading to an increase in Ca2+
- Group 1 receptors are important for long-term potentiation & synaptic plasticity
Describe the properties of Group 2 & 3 glutamate receptors?
- Group 2 & 3 are located on the pre-synaptic membrane
- Group 2 & 3 receptors are Gi/o coupled, so they’re negatively linked to adenylate cyclase leading to decerased cAMP formation & therefore decreased Ca2+
- Inhibition of neurotransmitter release
Describe the dimerisation of metabotropic glutamate receptors?
- Can form HOMOMERS with each other e.g mGlu5& mGlu1
- Can from HETEROMERS with each other or other classes of receptor such as 5HT
Define depolarisation
- The membrane potential is displaced towards a more positive value
What are EPSCs & how do they differ between the ionotropic receptors?
- Deplarisation generates EPSCs - excitatory post-synaptic current. EPSCs represent the flow of ions & the change in current across the post-synaptic membrane
- EPSCs generate EPSPs which are excitatory post-synaptic potentials
- AMPA = generate a large EPSC & are the primary mediators of neurotransmission
- NMDA & Kainate = slow & longer EPSC. Low conductance (NMDA = due to Mg2+ block)
What is excitotoxicity?
- Excitotoxicity is a pathological process by which excessive excitatory stimulation results in neuronal cell death & damage
How can deficits in the VGLUT cause excitotoxicity|?
- A missing or malfunctioning VGLUT means that glutamate cannot be transported to vesicles, instead it accumulates in the cytosol of the pre-synaptic membrane
- Normally EAAT transport glutamate back into the cell for re-uptake, but the accumulation of glutamate in the pre-synaptic neurone changes the direction of glutamate transport
- EAAT then transport glutamate out of the neurone into the cleft. The glutamate then activates the post-synaptic membrane without a stimulus or depolarisation
- Glutamate binding leads to an excessive influx of Ca2+, causing glutamate mediated excitotoxicity (neuronal cell death & damage)
What are the consequences of glutamate-mediated excitotoxicity?
. Oxidative stress
- Mitochondrial damage
- Apoptosis
- Further complications can lead to Alzheimer’s or stroke
Describe the link between Alzheimer’s & glutamate-mediated excitotoxicity?
- Alzheimer’s is a neurodegenerative disease that is characterised by neuronal cell death in the hippocampus which is associated with memory
- There’s an over-activation of NMDA receptors, leading to excessive Ca2+ influx resulting in excitotoxicity such as neuronal cell death
How can Alzheimer’s caused by glutamate mediated excitotoxicity be treated?
- Memantine is an NMDA receptor antagonist which can block the NMDA receptor preventing Ca2+ influx & therefore prevent the excitotoxicity resulting form it
Define long-term potentiation
- Long-term potentiation refers to the consistent strengthening of synapses due to repeated patterns of activity. The strength of of the synaptic transmission increases
- LTP underlies learning & memory
- LTP is a type of synaptic plasticity
What are the five steps of long term potentiation?
- Glutamate binds to AMPA receptors causing an Na+ influx, resulting in depolarization
- Sufficient depolarisation of the membrane results in the NMDA receptors opening & the Mg2+ block dissociates
- NMDA receptors channel opening leads to Ca2+ & Na+ influx
- Ca2+ acts as a secondary messenger, activating Calmodulin Kinase (CamKII) & Protein Kinase C (PKC)
- PKC & CAMKII trigger a series of reactions which lead to new AMPA receptors being expressed/inserted onto the post-synaptic membrane
- More AMPA receptors = increased sensitivity of the post-synaptic membrane to glutamate & increases ion channel conductance
