Lecture 3- Ionotropic receptors I: Excitation Flashcards
What happens when a neurotransmitter is released?
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What happens when a neurotransmitter binds to a ligand-gated channel?
- Neurotransmitter binds
- Channel opens
- Ions flow across membrane
What happens when a neurotransmitter binds to a G-protein coupled receptor?
- The G protein-coupled receptor is activated by an external signal in the form of a ligand or other signal mediator. This creates a conformational change in the receptor, causing activation of a G protein. Further effect depends on the type of G protein. G proteins are subsequently inactivated by GTPase activating proteins, known as RGS proteins.
- Neurotransmitter binds
- G-protein is activated
- G-protein subunits or intracellular messengers modulate ion channels
- Ion channel opens
- Ions flow across membrane
What are the classes of neurotransmitters?
-Small molecule neurotransmitters -Peptide neurotransmitters
What are the small molecule neurotransmitters like? (ACh and AAs, Purines)
- Acetylcholine-movement of skeletal muscle
- Amino acids: Glutamate (excitatory), aspartate, GABA (inhibitory)
- Purines: ATP, ADP
What are some more small molecule neurotransmitters like? (Biogenic amines)
-Biogenic amines:
1.Catecholamines:
- Dopamine
- Noradrenaline
- Adrenaline
2. Indoleamine:
-Serotonin (5-HT) (mood and sleep)
3. Imidazoleamine:
-Histamine (sleep cycle)
What are the peptide neurotransmitters like?
- released from neurons, responsible for neuromodulation, rarely for quick transmission
- more than 100 peptides, usually 3-30 amino acids long
- eg. Methionine ekephalin
What is the main difference between the neurotransmitter types?
- the Amino Acids (GABA, glycine glutamate) point to point fast transmission, on and off switch -Catecholamines and peptides= changing volume or tone, modulatory
What are the types of ionotropic receptors for the neurotransmitters?
- AMPA 2. NMDA 3. Kainate 4. GABA 5. Glycine 6. nACh (nicotinic) 7. Serotonin 8. Purines
Which of the receptors bind glutamate?
-AMPA (excitatory), NMDA (excitatory) and Kainate (mostly excitatory but also a role in inhibitory) -glutamate is the ligand that activates these channels -generally need 4 or 5 subunits for the formation of these receptors
How many subunits does AMPA have?
-4 types -Glu R1, Glu R2, Glu R3, Glu R4 -the receptor can be made up of any combination of these, need 4 or 5 subunits though so can be 4xR1 or one of each etc. -the variation in the composition accounts for the differential responses to molecules, different affinities, different kinetics -must have at least 4 subunits to be able to bind glutamate -huge range of complexity within the system -in general AMPA receptors the fast, rapid excitation
How many subunits does NMDA have?
-has 5 possible subunits -NR1, NR2A, NR2B, NR2C, NR2D -more modulatory function
How many subunits does Kainate have?
-has 5 possible subunits -Glu R5, Glu R6, Glu R7, KA1, KA2 -usually fast transmission and excitation, but more modulatory, presynaptic inhibition as well
What is the structure of an AMPA receptor like?
- 3 domains that have an extracellular and intracellular part(M1, M3, M4) and 2 extracellular domains (LBD and NTD) and an intracellular domain M2
- the pink (LBD)= ligand binding domain, interaction with M2, M3 and M4 that sets off an effect when ligand binds
- NTD (blue) amino terminal domain, involved in desensitization, if the NTD is removed the cell will not be desensitized to constant exposure to glutamate for example, normal cell would become less sensitive after a prolonged exposure (provided it has this domain still)
- the binding properties are all within the pink bit
What is the chemical difference between Glutamate and GABA?
- almost the same chemically
- the difference is a carboxyl group
- the only difference between global inhibition and excitation!
- recognition of this difference is crucial
How are Glutamate and GABA recognised from one another?
- all depends on the arrangement of the amino acids and their groups within the binding domain of the receptor
- the glutamate has to fit within the binding domain of the receptor in a way that differentiates the carboxyl group presence
- has the D and F ribbon, lysine K730 is incredibly important for keeping the ribbons in exactly the right alignment
- have to have the spacing between the F and D exactly right and the lysine K730 is responsible for that
- once that is done the Argenine 485 has its structure in exactly the right position to interact with the carboxyl group from the Glutamate
- then glutamate can interact and fit perfectly in the pocket of the binding domain
How does the glutamate work in the brain?
- glutamate comes along, binds to the receptor and elicits excitation, for fidelity of information must shut the signaling off swiftly
- glutamate comes from your food, so it will spike in your blood, blood brain barrier excludes it from brain circulation (otherwise you’d have an epileptic fit)
- the glutamate that we use in the nervous system comes from Glutamine (differs by having an oxygen group)
- Glutamine does not activate Glutamate receptors as it cannot bind
- Glutamine can be part of the metabolic cycle or it can be taken up from the extracellular space by excitatory amino acid transporters (EATT)
- Glutamine inside the neuron is made into Glutamate by Glutaminase (removes the oxygen group)
- Glutamate is then packaged using vesicular glutamate transporters (VGLUT) into vesicles -then released in response to ATP Ca2+ releases the vesicles
- the glutamate can diffuse and bind to ionotropic receptors
- quickly taken up from the extracellular space by the EATT into glial cells or back within the terminal of the neuron
- glutamine is converted into glutamine via Glutamine synthesase
- if Glutamate builds up in the extracellular space it can spill out and affect metabotropic receptors that are further away in the membrane
What is this?
- AMPA receptor -binds glutamate, the gate opens and ions can go through
- glutamate and AMPA kainate receptors are non-selective cation channels, they do not differentiate in their conductance of Potassium or Sodium, both can move through, so why is it an excitatory channel? = because of the gradients! there is a much larger gradient for Na to come in so depolarisation! (not so for K+ as too close to equilibrium)
What happens in terms of current when AMPA is activated?
- large and fast spike
- due to movement of Na in (depolarisation)
What is this?
- NMDA receptor
- still a glutamate binding channel
- has a magnesium ion= blocks the channel= the block is removed by depolarisation
- need glutamate and depolarisation to open
- glutamate binds and the gate doesn’t, need depolarisation
- usually to response of AMPA there is depolarisation and that can unblock NMDA
- for NMDA to be fully functional it needs glycine (allosteric modulator of the NMDA),binding site for glycine (principal inhibitory neurotransmitter)
- NMDA is also permeable to Na+, K+ and Ca2+, so drive for Na+ and Ca2+ to enter and some K+ to leave
- so when NMDA opens= get some Ca2+ in the cell, that is a secondary messenger (and that is how it is involved in things like plasticity)
What are the currents in AMPA and NMDA?
-NMDA is a slower conducting than AMPA and over a longer time
Where is Acetylcholine nicotinic receptor utilised in the animal kingdom?
- Acetylcholine nicotinic receptors are responsible for skeletal movement
- alpha bungarra toxin, krait makes it and paralyses the prey with it
- also a snail, coneshell uses it! kill prey by firing a harpoon into them, the substance stops activity of the nACh
What is the Acetylcholine production and system like?
- binding of Acetyl CoA and Choline to make Acetylcholine (via choline acetyl-transferase)
- Acetylcholine is packaged into vesicles by Vesicular ACh transporters
- can be released and binds
- inactivated by breakage of the acetylcholine by acetylcholinestarase, Choline is then recycled and into the neuron via sodium dependent transport
