Lec 10- excitatory amino acids (glutamate) Flashcards
1
Q
Overview/Intro
A
- Glutamate is released onto pre-synaptic terminals of excitatory neurones
- Glutamate receptors come in many forms, some opening ion- making them more likely to fire and depolarise channels when glutamate binds, other alterning G-protein activity to activate ‘2nd messengers’- molecules which usually switch proteins on or off
2
Q
Criteria for neurotransmitter status
A
- The chemical must be made by neurones
- It must be released by presynaptic neurones
- It must activate post-synaptic receptors
- It must be inactivated
- By enzymes (e.g. AChE- inhibited by novichok)
- Or by reuptake (e.g. DAT- dopamine transporter)
3
Q
Glutamate fits the criteria
A
- Concentrated in vesicles
- Released by exocytosis- usually calcium-dependent exocytosis
- Binds to receptors- binds to both pre and post-synaptic receptors
- Reuptake into the presynaptic terminal
- Has functional effects
4
Q
Aspartate fits some criteria
A
- Concentrated in vesicles?- Not endogenous to the brain
- Released by exocytosis?-
- Binds to receptors
- Reuptake into presynaptic terminals
- Has functional effects
5
Q
Glutamate
A
- Glutamate is one of the 20 amino acids which are used in the synthesis of proteins
- The main EAA in the mammalian CNS
- Binds to the large family of glutamate receptors
- Is synthesis in neurons from glutamine (by glutaminase)
- Reuptake by the EAA1-4 family of transporters
- A very important molecule in the body- so much so we have a taste bud for it (Umami) which humans find delicious
6
Q
Metabolism of transmitter amino acids in the brain
A
- Glutamate can be broken down by Glutamate decarboxylase (GAD) to form GABA (main inhibitory AA)
- Glutamate can be converted by transaminase into glycine (another inhibitory AA), mainly found in the brain stem
7
Q
Storage and release
A
- Although the evidence was slow to accumulate it is now clear that the excitatory amino acids are stored in the synaptic vesicles
- The release of the excitatory amino acids into the synaptic cleft is by a Ca2+ dependent exocytosis
8
Q
Inactivation of glutamate
A
- Subsequent to its release glutamate is inactivated by reuptake
- Is not generally broken down
- Taken up into astrocytes (also known as GLIAL cells- which are support cells of CNS) as well as taken directly into the neuron
- If Glutamate is taken into glial cells then it is broken down into glutamine (Via Glutamine synthase), then back into the neurone
9
Q
Glutamate receptors- Ionotropic
Classes and some subunits
A
- NMDA
- NR1, NR2A-D, NR3
- AMPA
- GluR1-4
- KA
- GluR5-7; KA1,2
10
Q
Glutamate receptors: Metabotropic
Class and some examples of sub-units
A
- Group I
- mGluR1, mGluR5
- 2nd messenger- Increased Ca2+ AND IP3
- Group II
- mGluR2,3
- 2nd messenger- Decreased cAMP
- Group III
- mGluR4, mGluR6-8
- 2nd messenger- Decreased cAMP
- Second messengers- Because metabotropic= GPCR
11
Q
Ligand-gated ion channels- Ionotropic receptor
A
- Ions are both inside and outside (Na, K)
- Channel forms a pore, opens and closes dependent on transmitter binding
- Cause conformational change allowing ions to flow in or out
12
Q
AMPAR and KAR
A
- AMPA= alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propinate (don’t learn full name)
- KA= Kainic acids
- Pass Na+ and (Ca2+ if no GluR2 subunit- AMPAR)
- Their activation produces a fast voltage-sensitive response- EPSPs (Excitatory Post Synaptic Potentials)
- The channels are permeable to Na+ and K+
- Agonists: glutamate, AMPA, KA
- Antagonists: CNQX, NBQX
13
Q
NMDA receptors
A
- NMDA- N-methyl-D-aspartate
- Mediate slower synaptic responses
- Channel conducts Na+, K+ and Ca2+ ions
- Calcium- a signal transducer
- Activates many enzymes including kinases phosphatases, proteases
- Implicated in LTP and LTD, excitotoxicity, seizures and neurodegeneration
- Experimental NMDA antagonists- AP5, AP-7
- Channel blockers- Mg2+, Phencyclidine, ketamine, MK801
14
Q
NDMA receptors
A
- NMDA receptors have an additional co-agonist site for glycine- required to become active
- Occupancy of the glycine site is obligatory for activation of NMDA receptors
- NMDA receptor activation is increased by endogenous polyamines e.g. spermine, acting on a modulatory site
- Zinc- channel and receptor block
15
Q
NMDAR- Modulation
A
- Mg ion sits to stop activation
- Lots of Ca, some Na influx
- Lots of binding sites: Glycine, Polyamine, H+
- Binding site for drugs of abuse: PCP and Ketamine
16
Q
Fast neurotransmission
A
- Released glutamate binds both kinds of receptors
- The different properties of AMPA and NMDA receptors give a fast and slow time course to the post-synaptic EPSP
17
Q
NMDAR-Dual gating
A
-
NMDAR require glutamate + depolarisation to open
- without depolarising event it will not have an effect
-
Depolarisation relieves Mg2+block of the ion channel pore
- Membrane potential shift from -65=> -30mV will cause Mg to dissociate to allow the pore to open
- Also requires glycine as a co-agonist
- So-NMDAR sense 2 things: Agonist + depolarisation
19
Q
GPCR
A
- Neurotransmitter binds to
- G-protein is activated- splits in half with the Alpha sub-unit moving off
- G-proteins bind with effector protein to increase intracellular messsengers (cAMP, IP3)
- Either excitatory or inhibitory effect is dependent on which G-protein the receptor is coupled to
20
Q
Group 1 mGluR signal transduction (Gq)
A
- Excitatory
- Receptor is coupled to G-protein (Gq)
- Gq increases PIP2, IP3, DAG (increases Protien kinase C) increase Ca
21
Q
Pre and post-synaptic EAA receptors
A
- NMDA receptors presynaptically increase glutamate release
- Group II or III metabotropic receptor reduces glutamate release pre-synaptically
