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Glutamate receptors Flashcards

1
Q

Which Ca2+-type channels allow calcium influx in the CNS?

A

N-type channels (CaV2.2)

P/Q-type channels (CaV2.1)

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2
Q

Describe the key steps of neurotransmitter exocytosis

A

Action potential reaches terminal bouton of presynaptic neuron
–> Triggers voltage-gated Ca2+ channels to open
–> influx of Ca2+ –> cytoskeletal rearrangement –> vesicles fuse with plasma membrane –> neurotransmitter release to synapse –> vesicle recycling
• Only pre-docked vesicles can be exocytosed

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3
Q

By which two types of receptors can neurotransmitters function?

A

o Ligands
On ligand-gated ion channels
E.g. Glutamate

o GPCRs
Indirect effect on an ion channel
E.g. GABAB¬

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4
Q

Name 5 key ‘fast’ neurotransmitters, the receptors they bind to, and how fast their responses are

A 
o	Glutamate
iGluRs (kainate, AMPA, NMDA-Rs)
mGluRs (group 1,2,3)
Excitatory
o	ACh
Via nicotinic receptors
o	5HT
Via 5HT3 receptors
o	ATP
Via P2XRs
o	GABA
iGABARs (GABAA,C)
GABABRs
Inhibitory
o	Glycine
iGlyRs
Inhibitory
o	These all trigger responses within a millisecond of binding
	Response is short-lived
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5
Q

Name 5 of the ‘slower neurotransmitters’ and their receptor type

A 
o	Dopamine
o	Noradrenaline
o	5HT
o	ACh 
	Muscarinic receptors
o	Neuropeptides
o	Histamine
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6
Q

Outline the key differences between wired and volume transmission

A 
Wired:
	Direct synaptic transmission
	Reuptake via astrocyte
	Little/no diffusion
	∴ limited to activating 1 synapse
	Equivalent of spraying yourself in the face with a hose 
Volume:
	Not as confined as wired transmission
	Astrocyte reuptake occurs but at a slower rate
	∴ diffusion or ‘spillover’ can occur
	∴ other neurons can be affected
	Slower acting/longer effect duration
	Equivalent to turning on a sprinkler and dancing in the spray
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7
Q

Where are glutamatergic synapses usually found? What else are they known as? What does this allow?

A

• Usually occur on dendritic spines
o Areas known as the Pre-Synaptic Density (PSD) contain a lot of Glu vesicles
 Allows concentrated and direct release

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8
Q

With reference to the key components of a neurotransmitter system, explain what makes glutamate a neurotransmitter

A
  • Requires enzymes to synthesise the neurotransmitter – Glutamate-glutamine shuttle; metabolic processes within presynaptic terminal; glutamate synthase!
  • Transporters to get the molecule into cells – Glutamate transporters – Extracellular Amino Acid Transporter (EAAT) 1-5; EAAT2 does 90% of Glu uptake
  • Transporters to get the molecule into synaptic vesicles – VGLUT transfers cytosolic Glu to a vesicle; VGLUT1-3; don’t transport Asp
  • Receptors that are activated by the molecule – iGluRs, mGluRs; KainateRs, AMPARs, NMDARs
  • Requires a way to terminate neurotransmitter action – diffusion, EAAT-mediated uptake
  • Glutamate is a non-essential neurotransmitter – you can make all you need.
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9
Q

With the use of a diagram to aid you, show the typical structure of an ionotropic receptor and list the subtypes of glutamate receptor and their unique subunits.

A 
•	Ligand-gated ion channel assembly
o	Channel is inherent to the receptor
•	Ligand-binding  channel opening
•	4 subunits, vary between receptors:
o	4 transmembrane domains 
o	NMDA:
	GluN1
	GluN2A-D
	GluN3A,B
o	AMPA:
	GluA1-4
o	Kainate:
	GluK1-5

Typical ionotropic structure: https://www.sciencedirect.com/topics/neuroscience/ionotropic-receptors (scroll down)

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10
Q

With the use of a diagram to aid you, show the typical structure of a metabotropic receptor

A

• G-protein coupled receptors (GPCRs)
o Signal via intracellular G-proteins to second messenger cascades
• Often function as dimers but can work fine as monomers
o 7 transmembrane domains

Typical metabotropic structure: https://www.sciencedirect.com/topics/neuroscience/ionotropic-receptors (scroll down)

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11
Q

List the 3 key features of an AMPA receptor

A

• Housekeeping receptors of synaptic function
o =/= boring
• Fast transmission, short-lived (normally)
• Na+ influx to cell

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12
Q

With the aid of a diagram, show the subunit topology of an AMPA receptor (including the glutamate binding site) AND explain which subunits in which combinations are required for function

A

• Tetramer – requires GluA1-4 in any combo
o Homotetramer or heterotetramer
• Q/R site determines the Ca2+ permeability of GluA2
o If have GluA2 as any one of the subunits per receptor, the receptor is then Ca2+ impermeable!
o Normally at least 1 GluA2 subunit per receptor

Subunit topology: https://ars.els-cdn.com/content/image/3-s2.0-B9780123694379500141-f09-01-9780123694379.jpg

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13
Q

List the 4 key properties of an AMPA receptor

A

• Mediate majority of fast excitatory synaptic transmission (mainly postsynaptic localization) –AMPARs non-selective cation channels permeable to Na+ (goes in) and K+ (goes out) and in some cases Ca2+
• Like all ionotropic glutamate receptors comprised of four subunits to form a tetrameric receptor.
• Four different AMPA receptor subunits in mammals GluA1, GluA2, GluA3 and GluA4 these “mix and match” to produce subtly different receptors.
• AMPA receptors containing GluA2 subunit have very low Ca2+ permeability – due to mRNA editing – positively charged arginine (R) residue expressed instead of neutral glutamine (Q) in pore forming M2 region of GluA2.
o Thus activation of all AMPA receptors leads to an influx of Na+ but these receptors are only permeable to Ca2+ only in the absence of any GluA2(R) subunits (and most AMPA receptors have GluA2(R)).

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14
Q

Explain how RNA editing alters AMPA receptors

A

RNA editing alters Ca2+ permeability by deaminating adenosine bases in a site-specific manner in double-stranded RNA substrates.

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15
Q

Pharmacology of AMPA receptors: list 2 agonists and antagonists, respectively, and the effects of PAMs

A

• Agonists: Glu, AMPA (fast desensitisation)
• Antagonists: NBQX (competitive), Telampanel (non-competitive)
• Positive allosteric modulators (PAMs): increase Glu affinity, inhibits Glu-induced fast-desensitisation (cyclothiazide, LY404187)
o AKA Ampakines:
 These enhance currents via AMPA receptor channels
 Have no effect on their own but enhance the effect of Glu
• Have been in clinical trials for disorders such as Scz, but none have registered for use yet

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16
Q

List 2 facts about Kainate receptors relating to the function and topology, their subunits required for function, and the pores formed during receptor formation

A

• Not housekeeping
• Similar topology to AMPA
• Subunits:
o GluK1-5
 Homotetramers of GluK4/5 do not form functional receptors
o GluK1 and 2 undergo RNA editing at a pore Q/R site; this is regulated in development
• 3 Transmembrane domains (M1, 3 & 4) and 1 re-entrant loop (M2)
• M2 involved in pore formation
• S1 and S2 form ligand binding domain
• Q/R site in M2 domain controls Ca2+ permeability

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17
Q

Pharmacology of kainate receptors: list 2 agonists (not including kainate), and 2 competitive antagonists

A
  • Agonists: kainate, glutamate, domoate
  • Antagonists: CNQX, ACET (both competitive antagonists; CNQX also antagonises AMPARs; ACET is selective for GluK1)
  • Plant lectin concanavalin A blocks kainate-induced desensitisation
18
Q

What effects do domoic acid and kainic acid have on animals?

A 
•	Domoic acid:
o	Natural product isolated from marine diatoms from west coast USA & Canada. Responsible for amnesic shellfish  poisoning due to consumption of contaminated mussels.   
o	Domoate is a potent AMPA/kainate receptor agonist that can cross the blood brain barrier.
o	Outcomes include: 
	Loss of short-term memory 
	Motor weakness 
	Seizures 
	Death
•	Kainic acid  seizures in mammals
19
Q

Explain the roles of kainate receptors and the cells these occur in

A

• Kainate receptors often found alongside AMPA receptors – need to block AMPA to study Kainate
o Often done with GYKI53655
• KainateRs make excitatory post-synaptic responses
o Confirmed by single-cell patch clamp measurements
o GluK2 is essential for kainate-R mediated EPSPs.
 Shown by knockout mice
 GluKRs aren’t housekeeping
• CNQX blocks AMPA-Rs and kainate-Rs
• Presynaptic GluKRs can act on the presynaptic neuron
o Can enhance Pr of Glu from mossy fibres
o Also has effects on GABA release
o and has metabotropic effects on neuronal excitability

Cells this occurs in:
Dentate gyrus cells
CA3 pyramidal cells
CA1 pyramidal cells

20
Q

NMDA receptor structure and function:

A

• Same structure as AMPA and kainate
• 4 subunits
o Can only arrange as heterotetramers
 Obligate heterotetramers due to NR1+2, or NR1+3
o Responsible for excitotoxicity during stroke

21
Q

Explain the key differences between the 7 subunit genes of glutamate receptors

A

 GluN1 – required
• If you KO the GluN1 gene, no NMDARs can form
• Binds glycine – required for NMDAR function!
• Only 1 gene but 8 spliced forms
• Expressed everywhere in the brain
 GluN2A-D – requires at least 1
• Binds Glu
• These genes are differentially expressed in different brain areas and at different ages
• Little A at birth, increases with age
• Always B
• C doesn’t show up until cerebellum develops – largely localised to the cerebellum
• D is lost over time
 GluN3A-B
• Comparably little known about these.

22
Q

List 3 important properties of NMDA receptors and explain how these relate to function:

A
  1. High Ca2+ permeability – always, unlike with GluA2
  2. Mg2+ blocks the channel at negative (resting) Vmem
    a. This is voltage-dependent:
    i. In order for Mg2+ block of NMDAR to occur the channel must be open - ∴ glycine and glutamate must be bound to their binding sites on the NMDA receptor.
    ii. As membrane potential is depolarised, the Mg2+ block of the NMDA receptor channel is progressively removed.
    iii. AMPARs often present at same synapse as NMDARs. Activation of AMPARs depolarises the membrane sufficiently to remove Mg2+ block of NMDARs.
    iv. Thus Ca2+ entry through NMDARs is dependent on pre- and postsynaptic elements being active at the same time i.e. the NMDAR is a coincidence detector.
    v. By ~-35 mV, the Mg2+ block of the NMDA receptor channel is mostly removed ∴ inward current through the NMDAR is voltage dependent and transmitter gated
    vi. Mg2+ presence doesn’t lead to a slower response in NMDARs; slow responses are inherent to NMDARs. Slow responses continue in the absence of Mg2+
  3. Glycine is a necessary co-agonist –> obligate heterotetramer
23
Q

Explain the synaptic physiology of NMDARs

A

• NMDARs mediate slowly rising, long-lasting EPSPs via Na+ and Ca2+ influx through the channel – NOT BECAUSE OF Mg2+
o Studies in absence of Mg2+ have shown this
• Subunit composition affects the time of the EPSP decay
o Glu isn’t in the synapse for any more than 5ms
 Just diffuses away very quickly
• Ca2+ influx –> activation of enzymes; regulation of ion channel opening; affects gene expression
o Can also –>
 Synaptic plasticity – ability to change strength of synaptic connections, and consolidate new pathways. NMDARs are specifically involved in:
• Long-term potentiation (LTP) – long-lasting potentiation of synaptic transmission
• Long-term depression (LTD) – long-lasting depression of synaptic transmission
• NMDAR antagonists block these processes (e.g. AP5)

24
Q

Molecular pharmacology and structure of NMDARs - overview:

A
  • Agonists and antagonists for every binding site on the molecule
  • Pore blockers exist, bind the pore and block it
  • can study the receptor through mutagenesis – KOs etc
25
Molecular pharmacology and structure of NMDARs - Glu site agonists:
``` o Glu o NMDA (N-methyl-D-aspartate) ```
26
Molecular pharmacology and structure of NMDARs - Glu site competitive antagonists:
```  AP5 • Glu analogue • Widely used as a blocker for NMDA • Been around for years • Can’t cross BBB  CPPene • Crosses BBB, can be dosed and enter brain ```
27
Molecular pharmacology and structure of NMDARs - Gly site agonists:
o Gly  Affinity of glycine for GluN1 depends on type of GluN2 subunit in tetramer o D-serine  D-serine is released by astrocytes and may act as a transmitter in some areas of the CNS
28
Molecular pharmacology and structure of NMDARs - Gly site antagonists:
o Kynurenate – an endogenous compound produced by glial cells. Antagonist at the glycine binding site  Many potent and selective antagonists such as 5,7-dichlorokynurenate (5,7-DCKN) have been developed by medicinal chemists in the search for therapeutic agents.
29
Molecular pharmacology and structure of NMDARs - N-terminal domain:
o Eliprodil – selectively binds to the dimer interface between N-terminal domains of the GluN1 and GluN2B subunits leading to inhibition of the NMDA receptor  is a negative allosteric modulator (binds to a site other than the glutamate/glycine binding sites).
30
Channel blockers (uncompetitive antagonists) - low affinity:
 Ketamine (Ketalar): dissociative general anaesthetic; induces a state of sedation, immobility and analgesia. Used in veterinary practice and also licensed for use in humans in short-term treatment of pain resulting from cancer, peripheral nerve disease or spinal cord injury. “Special K” – drug of abuse  Memantine: well tolerated low affinity channel blocker used in the clinic for the treatment of memory impairment in moderate-severe Alzheimer’s disease, vascular dementia and the dementia of Parkinson’s disease.
31
Channel blockers (uncompetitive antagonists) - high affinity:
 Phencyclidine (PCP): once used as general anaesthetic withdrawn due to hallucinogenic effects. Now a commonly abused drug (street name - angel dust).  MK-801 (dizocilpine): high affinity channel blocker, withdrawn from clinical trials for stroke due to adverse side effects such as hallucinations and memory impairment.
32
List 4 pathophysiological roles of NMDA receptors:
* Excitotoxicity (through excessive entry of Ca2+) --> neuronal cell death (e.g. in cerebral ischaemia, Alzheimer’s and Parkinson’s disease). * Epilepsy – NMDARs involved in development and expression of seizures. Anticonvulsant activity of NMDAR antagonists correlates with their affinity for the NMDAR. * Transmission of pain responses - NMDARs expressed on sensory neurones. NMDAR-dependent maladaptive plasticity (e.g. wind-up) in neuronal pain pathways (hyperalgesia) can be blocked with antagonists. * Schizophrenia is associated with NMDAR hypofunction e.g. PCP causes psychotic episodes that resemble schizophrenia.
33
List 4 facts about GluN3 subunits:
• Excitatory ionotropic Glycine Receptor • Doesn’t require Glu binding to activate the receptor and have the response • Not blocked by strychnine o D-serine is a partial agonist • Gly inhibits NR1 but activates NR3, but we still don’t know how these work o If you block NR1, NR3 works better o Redox modulation may have some effect on this
34
Explain what kind of receptors metabotropic glutamate receptors are, and list the subfamilies of mGluRs
``` • GPCRs – Glu-activated • 8 in mammals; 3 subfamilies o mGluR1-8 o subfamilies:  Group 1: • mGluR1, 5 • signal via Gq • agonised by DHPG  Group 2: • mGlu2+3 • signal via Gi/o o inhibitory • agonised by LY404039  Group 3: • mGlu4,6-8 • signal via Gi/o o inhibitory • agonised by L-AP4 ```
35
Which signalling pathway are mGlu group 1 Rs coupled to? What are the intracellular effects of this pathway?
Group 1s are coupled via Gq to PLC: Glu/DHPG binding --> αq is given GTP--> PLCβ (GTP-->GDP, αq dephosphorylated) --> PIP2 --> IP3 + DAG IP3 --> IP3R (on ER)  Ca2+ released from ER into cell, increases i[Ca2+] --> Ca2+-dependent processes DAG + Ca2+ activate PKC --> protein phosphorylation
36
Which signalling pathway are mGlu group 2+3 Rs coupled to? What are the intracellular effects of this pathway?
Group 2 + 3 are negatively coupled to adenylyl cyclase: LY404039/L-AP4 --> αi/o phosphorylated; inhibited adenylyl cyclase --> decreased ATP-->cAMP decreases βγ complex activated G-protein coupled Inwardly Rectifying K+ channels (GIRKs) + inhibits VDCCs
37
With the aid of a diagram, describe typical mGluR structure, showing drug binding sites:
• Bi-lobed N-terminal extracellular domain; contains Glu binding site (orthosteric) – can bind 2 Glu molecules if needed • Cysteine-rich domain – involved in maintaining 3° structure • 7 transmembrane domains (TMDs) – shared with other GPCR families o Allosteric modulators bind here • 2nd intracellular loop o Involved in G-protein coupling o Determines transduction mechanism Typical structure: https://www.abcam.com/content/metabotropic-glutamate-receptors (scroll down)
38
Is mGluR1 hetero- or homodimeric? Explain its structure and relate this to function
bridge between Cys residues present on Ligand-Binding site 1 of the Ligand Binding Regions o Bilobed structures (LB1+2) of each protomer are flexible – can form open/closed conformations o X-ray structure showed Glu bound to both protomers:  1 with lobes closed  1 with lobes open  Corresponds to activated state o Research suggests lobes should be closed in both protomers to achieve the fully activated state o Spontaneous lobe closure in agonist absence  mGluR activation (constitutive activity) & basal activity of receptor – e.g. basal levels of phosphoinositide hydrolysis  Blocked by inverse agonists  Glu binding stabilises activate state; orthosteric antagonists bind to the resting state – prevents lobe closure which would lead to receptor activating
39
What is MPEP and what does it do?
MPEP is a negative allosteric modulator of mGlu5: Increasing [MPEP]  decreased max response of a [Gu] response curve on rat mGlu5 MPEP binds to a site in the TM region of mGlu5 to inhibit the Glu-stimulated rise in i[Ca2+] MPEP but not MCPG acts as an inverse agonist to block constitutive activity of rat mGlu5a: MPEP blocks basal production of inositol phosphates (IP) (occurs through constitutive activation of mGlu5a) – control shows IP production in cells not expressing mGlu5a MPEP but not orthosteric antagonist (MCPG) blocks basal IP production ∴ MPEP is a NAM and inverse agonist of mGlu5.
40
What is Ro01-6128?
A positive allosteric modulator of mGlu1: o Conc.-response curves for effect of (S)-DHPG on mGlu1 are shifted to the left in presence of Ro01-6128 o 5-fold decrease in EC50 value, and 1.2-fold increase in maximum response as observed in the presence of Ro01-6128 = a change in the efficacy of orthosteric agonist DHPG.
41
List the general properties of mGluRs:
* Depending on subtype can be pre- and/or post-synaptically localised. * Generally play a modulatory role in synaptic transmission. * Postsynaptic group I mGlu receptors mediate slow depolarization (EPSP) * Presynaptic group II and III mGlu receptors decrease neurotransmitter release. * Involved in the modulation of signalling through K+ and Ca2+– control excitability of neurones. * Metabotropic glutamate receptors were desirable targets for drug discovery. * mGluR activation is also involved in various forms of synaptic plasticity * This include short term plasticity at a variety of synapses * Also reported involvement in forms of LTP and in particular LTD * A few minutes' application of the group-1 agonist DHPG will induce hippocampal LTD

Neuropharmacology (16 decks)

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