Consequences of neurotransmitter exocytosis Flashcards
What dictates what effect a release neurotransmitter will have?
The type of receptor and the signalling pathway activated
What are the main types of receptors?
Fast acting inotropic (IR)
Slow acting metabotropic (MR)
Which inotropic receptors are structurally similar, describe their structure?
ACh, GABA and glycine
Pentamer of 4 different proteins (alpha, beta, gama, omega) each with 4 transmembrane domains, the C and N termini are extracellular
Describe the structure of glutamate inotropic receptors
Tetramers of 2 different proteins with 3 transmembrane domains (N terminus outside and C terminus inside cell)
Describe the structure of the ATP ionotropic receptor
Trimer, of one protein with 2 transmembrane domains
TRUE or FALSE?
Each NT will have a single receptor for itself
FALSE
It will have a family of receptors.
There are different forms of the component proteins of these receptors which can influence their properties
What properties can be affected by the differences in the structure of component proteins of receptors?
Regulation, sensitivity to drugs or toxins, etc.
How many subunits do metabotropic receptors have?
1
What is the structure of a metabotropic receptor?
1 subunit with 7 transmembrane domains
N terminus out C terminus in
Name NTs and their corresponding MRs
Glutamate - mGlut 1-7
Ach - Muscarinic M1-M5
GABA - GABAb
Dopamine - D1, D2
What is the inotropic receptor for GABA called?
GABAa
How do metabotropic receptors generally work?
NT binds receptor • Conformational change exposes Gs protein binding site • Receptor and Gs protein complex bind • GDP is displaced by GTP • αs subunit dissociates from the βγ subunits • αs subunit binds and activates adenylyl cyclase --> cAMP from ATP • αs subunit hydrolyses GTP • Returns to its original conformation • Dissociates from cyclase – inactive • Binds to the βγ subunits • Gs protein complex binds to receptor • Process cycles until the transmitter is inactivated
What are certain effectors (enzymes) which help make second messangers with MRs?
Adenylate cyclase
Phospholipase C
Name some secondary messangers involved in MR signal transduction
cAMP
IP3/DAG
TRUE or FALSE?
All MRs are found all over the body and can do a broad range of things
FALSE
Receptors can be very specific as to where they can be found and what they do
What are the types of receptors for Ach and where are they found/what do they do?
Nicotinic: fast excitatory synaptic transmission especially in NMJ (activates ion channel)
Muscarinic: Both excitatory and inhibitory depending on tissue (slows heart, contraction of visceral smooth muscle) - uses G protein
What are the receptors for glutamate and what are their physiological roles?
AMPA: Fast excitatory synaptic transmission in CNS (inotropic)
Kainate: Inotropic receptor
NMDA: Slow excitatory transmission in the CNS (inotropic)
Metabotropic: Neuromodulation
What ions pass through nicotinic receptors?
Na+ and Ca2+ to enter the post synaptic neurone and some K+ goes out
Nicotinic receptors produce a ___ by ___.
Excitatory post synaptic potential, increasing the chances of depolarisation leading to the firing of an AP
What is the main inhibitory NT in higher brain regions?
GABA-A
Name another (not GABA) inhibitory NT
Glycine
Where is glycine found?
The brain stem and spinal cord
How are GABA-A and glycine receptors similar to nicotinic receptors?
Fast acting
Inotropic
How are GABA-A and glycine different from nicotinic receptors?
Allow Cl- into the cell
Induce inhibitory post synaptic potentials
Make the post synaptic terminal less likely to reach the threshold for AP to fire
How do inotropic receptors create specificity?
If too large they won’t go through
If they are the wrong charge the ions won’t go through
Which receptor is important for status epilepticus and HD?
NMDA
What are the inotropic receptors of glutamate?
Kainate, AMPA, NMDA
How do the AMPA and Kainate IRs work?
When glutamate binds to them, they allow Na+ into the cell and K+ out
Why is NMDA interesting?
- Permeable to Ca2+,K+ and NA+
- They need glycine cofactor to open
- Unique: Opening depends on membrane voltage as well as transmitter
What is the role of Mg2+ in NMDA?
Mg2+ usually is bound tightly to the site in the pore of the channel, blocking the ionic current
How does NMDA work?
Slight depolarisation (due maybe to AMPA opening) –> Mg2+ is expelled by electrostatic repulsion.
At the same time glycine binds to NMDA –> opens and allows Ca+,Na+ out and K+ in.
What effect do the glutamate inotropic receptors have?
They make depolarisation more likely
Fast acting
Cause EPSPs
TRUE or FALSE?
There is little variation in glutamate IRs
FALSE there are many different subunits which are alternatively spliced to give further variations
AMPA subuntis encoded by 4 genes - Glu A1-A4
KA receptor subunits encoded by 5 genes
GluK1-K5
NMDA encoded by 5 genes GluN1 and 4GluN2(A-D), [each NMDA has 2 GluN1 and 2 GluN2 subunits]
What do most NMDA expressing neurones have and why?
AMPA receptors so that upon release of glutamine they will lead to depolarisation
What happens if glutamine degradation is impaired?
Too much calcium enters the cell –> activating proteases, NOS, kinases , and free radicals –> damage mitochondria and cell –> decrease ATP –> cytotoxicity
How do some people propose preventing excitotoxicity and in which diseases
NMDA-R antagonists:
HD
Ischaemia (stroke)
Status epilepticus
Which amino acid residues can be phosphorylated?
Tyrosine and Lysine
What is the effect of Ca2+ inside the cell?
It activates calmodulin dependent kinase II (CaMKII) which phosphorylates other proteins to activate them
Why do synapses further from the soma have more receptors?
To give greater depolarisation to account for decrease in depolarisation as moves through the soma
What are the differences between type 1 and 2 synapses?
Type 1 v. 2:
Excitatory v. inhibitory Glutaminergic v. GABAnergic SSV-round v.SSV-flattened Cleft wider v cleft narrow Active zone larger v. Smaller Densities more prominent v less prominent
Name 3 NTs that make a fast EPSP
Glutamate
Aspartate
Acethylcholine
What does an increased number of excitatory synapses do to the post synaptic potential?
Even though presynaptic outputs are standard (although they can increase frequency), this can have an additive effect on the post synaptic potential and make it more likely for that neurone to fire an action potential
The reverse happens with IPSPs
TRUE or FALSE
Summation can occur in any part of the NS not just the CNS
TRUE
Is a single input sufficient to depolarise a neuron below its threshold?
No
How is the axon hillock specialised to its role of deciding whether the AP should fire or not?
Has more VG Na+ channels
So more sensitive to milder depolarisation
What 2 properties of a neuron influence summation?
Temporal summation - over what time inputs have an influence
Length constant - how much depolatisation lost in transfer to axon hillock due to e.g. leakage of ions across the plasma membrane
What makes a neuron more likely to fire?
Longer temporal summation have longer length constants
Dendrites also have VG Na+, K+ , and Ca2+ VG channels so can influence depolarisation
Describe feed forward inhibition
Where you want to stimulate the extensor muscle and relax the flexor:
Afferent neurons innervate both extensor and flexor motor neurons but the one that innervates the extensor synapses with an inhibitory interneuron which synapses with the flexor motor neuron, cancelling out the other afferent neuron
Describe feedback inhibition
Stimulation of the extensor muscle will be self limiting:
Afferent neuron synapses with motor extensor neuron, but this motor neuron also synapses with an inhibitory interneuron which inhibits itself
Why are metabotropic receptors described as modulatory?
They modulate the influence of any de/repolarisation in the neuron
There is no direct effect on ion movement, just changes in shape in response to a ligand which can cause changes (e.g. opening channels) in other parts of the cell
What does activated PLC do?
IT makes IP3 and DAG from PIP2 which cause CA2+ release and protein kinase activation respectively –> Ca2+ regulates ion channels and enzymes (NOS, proteases), while PKC phosphorylated proteins do the same plus affect transcription
What do cAMP do after being made by adenylyl cyclase?
Bind to PKA binding site–> release the catalytic subunit (no more inhibitory binding) –>Affect protein phosphorylation –> affects K+, Ca2+, inotropic receptor channels –> influences when AP fires
What does the G protein gated K+ channel activated by the muscarinic acid receptor for Ach do?
Affects membrane potential
Do different NTs have different G proteins?
Yes
What is the role of Gs?
Activation of adenylyl cyclase
Activation of Ca2+ channels
What do G0 and G1 do?
Inhibit Adenylyl cyclase
Inhibit Ca2+ channels
Activate K+ channels
hat do G11 and Gq do?
Activate PLC
What provides the huge amount of heterogeneity in MRs?
Different receptors
Different typesof G protein
Different G protein subunits
Different down stream differences
Explain the convergent and divergent effects of NTs
Convergent: Each NT has many diff MRs with diff G protein specificities. So, one NT can activate many diff receptors
Divergent: An NT may activate a different receptor but lead to the same result down stream if (e.g. they share the same G protein)
What are 3 ways (examples) of modulating PSPs with MR activation?
- Post synaptic - prolonged opening of inotropic receptor –> increase depolarisation
- Cell body-prolonged depolarisation (K channel is closed to stop repolarisation)
- Pre-synaptically - increase in the size and duration –> more NT release –> more depolarisation post synaptically
What is the effect if a metabotropic receptor causes a K+ channel to stay open longer?
Hyperpolarisation –> Ca2+ channel decreases Ca2+ entry (decreased depolarisation) –> decreased NT release –> less likely to get an AP post synaptically
Reverse is true if it causes the K channel to close
Explain the gill withdrawl mechanism of aplysia
Touching the siphon leads to gill withdrawl. This can be sensitised by the role of interneurons, increasing the effect on the motor neurons.
5HT –> activates MR1 –> Gs–> Ad cyclase activation, cAMP activaties PKA –> phosphorylates K+ channel which closes–> enhancing
depolarisation and NT release
5HT also activates MR2 linked to PLC activation–>DAG –> PKC activation
enhances the exocytosis machinery increasing NT release. (Process lasts several mins)
Now sensitised to touch of the siphon which generates AP in main terminal –> Increased depolarisation in post synaptic neuron –> firing –> stimulate movement away
What happens if you keep poking at aplysia’s siphon?
Repetitive touching leads to the adaptive response of habituation
Influence on transcription factors–>
sensitisation: increases synapses/receptors
Habituation:
- decreases SSV, NT repease
- decreases nr. of synapses
All this leads to a slower response
What are 3 drugs which directly activate the post-synaptic Dopamine receptors (for PD)?
Bromocriptine
Pramiexole
Ropinirole
What treamtent mimicks PD?
Haloperidol (DA antagonist-antipsychotic)
