Dr. Kruse's Pharmocology of NM junction Flashcards
ACh synthesis: Choline transporter
channel that brings choline into the cell
ACh synthesis: ChAT
Choline acetyltransferase enzyme that combines acetyl CoA and choline to form ACh
Alzheimer’s patients have reduced cerebral production of
Choline acetyltransferase enzyme (ChAT)
ACh storage: ACh vesicular transporter
ATP dependent transporter that immediately shuffles ACH into storage vesicles after ACh synthesis
1K-50K molecules of ACh per synaptic vesicle
a motor nerve terminal may contain over 300K vesicles
ACh release: CA channels
open upon depolarization and allow Ca to enter the cell, Ca promotoes vesicle membrane fusion
Ach release: VAMP and SNAPs
vesicular and plasma membrane proteins that initiate vesicle-plasma membrane fusion and release of ACh
roughly 125 vesicles rupture per action potential
hemicholinium will block what
the choline transporter
the Botulinium toxin will block what
Snaps from binding snares (vesicle fusion and release of NT)
Acetylcholinesterase (AChE)
enzyme that cleaves ACh into choline and acetate
choline is recycled back into the motoneuron via the choline transporter
endycytosis occurs at the nerve terminal to replenish the number of available vesicles
What does calcium do really?
it allows for SNAPS, SNAREs, and VAMPs to take conformations that allow them to interact merge ve
vesicles to the membrane for release of the NT
botulin interferes with this process, thereby blocking acetylcholine
what does botulin toxin ultimately block at the synaptic terminal?
Acetylcholine release
Where is AchE located?
on the post-synaptic membrane
inhibition of the AChE will do what?
it will increase acetylcholine
ACh signaling: two ACh activates two subsets of receptors
nicotinic (n) and muscarinic (m) receptors
Nicotinic receptors
nAChRs
activated by ACh and nicotine
ligand gated ion channel (Na)
Pre-and-Postjunctional
NMJ: Na increase causes muscle action potential
Muscarinic receptors
mAChRs
Activated by ACh and muscarine
G-protein coupled receptor
Pre-and-Post junctional
NOT LOCATED at skeletal NMJ
would find them on smooth muscle (in the GI for example)
who destroys the acetylcholine signal?
the acetycholinesterase
which of the two ACh receptors are located on skeletal muscle?
the nicotinic
nicotine does what
acts on acetycholine receptors
What does the pre-junction nicotinic receptor do?
facilitates more vesicle release
what do post-junctional nicotinic receptors do?
initiate an excitatory signal
nAChR: location, function, agonist
skeletal muscle, contraction, acetylcholine/nicotine = agonist
causes muscle contraction
mAChR: location, function, agonists
smooth muscle, contraction, acetylcholine and muscarine
mAChR and cardiac muscle
located at the SA and AV nodes, atrium and ventricle
reduce heart rate, reduce conduction velocity
reduce slight contraction, reduce contraction
agonists are acetylcholine and muscarine
Which of the two receptors is a ligand gated channel and which is a 7 transmembrane domain channel?
Nicotinic is a voltage gated channel
Muscarinic uses G coupled proteins
mAChRs are
GPCRs
How many subtypes of GPCRs are there in mammals?
5 M1-M5
activation leads to a series of intracellular events triffered by second messengers (metabotropic)
cellular effects measured in seconds
use ACh and muscarine
nAChRs are what? what do they do? what is their speed?
ligand gated ion channels
allow ions to pass through the channel pore when activated (ionotropic)
fastest synaptic event in the nervous system (miliseconds)
how many subunits does a the nAChR have?
four distinct subunit assembled as a pentamer surrounded by a central pore where ions pass when activated
ACh binding site lies between a and adjacent subunit
Ionotropic versus metabotropic channels
Ionotropic: nAChRs
metabotropic: mAChRs
What kind of amino acids do we expect to line the LGIC?
aspartic acid and glutamic acid (negatively charged)
channels selective to Na, Ca, and K
nAChRs antagonists
Atracurium
Vecuronium
d-tubocuranine
Pancuronium
What happens when an antagonist interacts with the nAChRs
a signal cant be sent, so its an anesthetic
Nm vs Nn
Nm include a, b, d, e and g subunits
found ONLY at skeletal muscle motor end plates
receptors are pre-post ganglionic
Nn include a and b combinations or all a subunit
found in the CNS, autonomic ganglia, adrenal medulla
receptors are pre-posjunctional
Nn
include a and b combinations or all a subunit
found in the CNS, autonomic ganglia, adrenal medulla
receptors are pre-posjunctional
Nm
nclude a, b, d, e and g subunits
found ONLY at skeletal muscle motor end plates
receptors are pre-post ganglionic
NAChR postjunctional activity
Two ACh molecules bind to both binding sites
N, K, Ca pass through the channel down concentration gradients
the muscle cell is depolarized and an action potential is initiated
End plate depolarization
spreads by local currents to activate voltage gated Na channels in the adjacent membrane
influx of Na initiates an action potential after threshold is reached (although the motor end plate itself cannot fire action potentials, it depolarized sufficiently to initiate the process in the adjacent muscle membrane
does the motor end plate fire an action potential?
no: it depolarizes sufficiently to initiate the process in the adjacent muscle membrane
what does flow between adjacent membrane and end plate is a local current
Na/K influx/efflux
Na diffuses inwards in large concentrations while K diffuses outwards in smaller concentrations
Activation of what receptor will inhibit further ACh release?
mAChR
Tetrodotoxin ‘
Affects the nerve AP
Puffer fish toxin (fugu, blowfish)
Inhibits voltage gated Na channels, blocks axonal conduction
weakness, dizziness, loss of reflexes, hypotension, generalized paralysis. death due to respiratory failure
local anesthetics
MOA inhibition of voltage gated Na channels–> inhibition of action potential
utilized for pain control
lidocain
bupivacaine
procaine
Batrachotoxin
poison dart frog:
causes an increase in permeability of Na channels and induces persistent depolarization
very toxic
Botulinum toxin
clostridium botulinum
a group of heterogenous, gram + rod shaped, spor forming obligate anaerobic bacteria
8 types cause human disease ABE, rarely FGH
Botulinum toxin action
cleaves SNARE complex involved in exocytosis prevents release of ACh
How is botulism classified?
as the acute onset of bilateral cranial neuropathies associated with symmetric descending weakness
no sensory deficits except blurred vision
foodborne symptoms: vomiting, diarrhea, dry mouth, abdominal pain, nausea
has a clinical use to remove wrinkles and prophylaxis of chronic migraine headache
Synaptobrevin is the target of
tetanus and Botulinum BDFG
SNAP-25 is the target of
Botulinum AVE
Syntaxin is the target of
Botulinum C1
Which toxin blocks Sodium channels from conducting AP?
Which toxin blocks the SNARE complex?
Which toxin blocks synaptic vesicle fusion but does so by being internalized and traveling via retroaxonally to the spinal cord?
1) Tetrodoxin
2) Botulinum toxin
3) Tetanus
Tetanus act specifically on the
interneurons of the spinal cord: blocks release of inhibitory NTs
how does tetanus present?
spasms/muscle spasms
Which symptom corresponds to which toxin?
1) Diarrhea, nausea, vomiting, abdominal pain
2) Respiration failure
3) spastics
- botulism
- tetrodoxin
- tetanus
NAChR agonists and antagonists
both can prevent synaptic transmission
agonist can activate receptor to signal as a direct result of binding to it
antagonist binds to receptor but does not activate generation of signal
Curare alkaloids
competes with Ach for the nAChR receptors on the motor end plate
Curare binding to the nAChR does what
inhibition: flaccid paralysis of skeletal muscle
when are curares used?
to inhibit nAChR receptors to induce flaccid paralysis
used during anesthesia to relax skeletal muscle
paralysis by curares can be reversed how?
by increasing ACh in NMJ
Succinylcholine
depolarizing neuromuscular blocker that binds to nAChR on skeletal muscle and causes depolarization
continued depolarization leads to receptor blockage and paralysis
What is succinycholine used for?
it induces anesthesia
How is succinylcholine’s paralytic effects reversed?
time. gotta wait it out.
Curate is a ______ blocker; succinylcholine is a ______ blocker
non-depolarizing; depolarizing
depolarizing physically blocks the receptors, nondepolarizing prevents them from opening
Cholinesterase inhibitors
bind to AChE and block enzymatic activity
AChE breaks ACh down in the NM cleft, so inhibition would increase ACh
Clinical use of cholinesterase inhibitor
include dementia associated with alzheimer or parkinson disease, myasthenia gravis, nerve gas,
reversal of neuromuscular blockade during anesthesia
Agents that effect the muscle AP versus muscle contraction (two in mind)
1) tetrodoxin, blocks outer mouth of sodium channels
2) Dantrolene: inhibits ryanodine receptors in the SR, blocking Ca2+ release
clinical uses include malignant hyperthermia
spasticity associated with upper motor neuron disorders
Dantrolene
inhibits ryanodine receptors in the SR: blocks Ca release
used to fight malignant hyperthermia and spasticity associated with upper motor neuron disorders
what clinical use does dantrolene have?
fights against malignant hyperthermia and spasticity
Vesicles in neurons come from where?
motor neuron cell body in the CN
Vesicles for ACh: how do they travel from the motor neuron to the axon terminal?
they travel empty
Vesicles for peptide NTs: how do they travel from the motor neuron to the axon terminal?
they travel carrying their full cargo, already synthesized
Agents that affect nerve action potentials =
tetrodotoxin
local anesthetics
Agens that affect the vesicular acetylcholine release
botulinum toxin
tetanus toxin
Hemicholinium
Agents that affect depolarization
neuromuscular blocking drugs: Curare alkaloids and Succincylcholine
Agents that inhibit acetycholinesterase
AChE inhibitors (duh)
Agents that affect the muscle action potential
Veratridine (stimulates Na passage into cells and leads to increased nerve exctiability)
Tetrodotoxin
Agents that affect muscle contraction
Dantrolene
Choline enters the neuron how?
through a symporter with Na
CHaT
Choline acetyltransferase
combines acetyl Co-A and choline together to make ACh, an it is immediately put into vesicles
How would inhibiting CHaT be helpful?
it wouldn’t be because choline entering the cell is the rate limiting step
Vesicle membrane fusion: Snares and Vamps
SNAREs primary role is mediating vesicle fusion
v-SNARE
synaptobrevin
t-SNARE
syntaxin
SNAP-25 does what?
it interacts with synaptobrevin and syntaxin to form the core SNARE complex that brings the vesicle and presynaptic membranes into close contact
synaptotagmin
the protein that is effected by Ca2+ rushing into the neuron. synaptotagmin triggers the vesicle fusion and exocytosis
alpha-SNAP and ATPase NSF
in synaptic cleft: they dissasemble SNAREs, allowing recycling of vesicle and SNARE proteins
What proteins make up the protein complex causing ACh containing vesicles to come near the membrane?
v-SNARE (synaptobrevin) + t-SNARE (syntaxin) + SNAP-25
What protein triggers the actual vesicle fusion?
synaptotagmin
What proteins breakdown the SNARE complex?
alpha-SNAP and the ATPase NSF.
What are the energy requirements for membrane disassembly?
the alpha-SNAP and NSF complex require one ATP
mAChRs: how many subtypes exist?
5
mAChRs: predominant type found in muscle and cardiac tissue
M2 & M3 = smooth muscle
M2 = cardiac muscle
metabotropic
mAChR, GCPR :activation leads to a series of intracellular events triggered by secondary messengers
mAChR agonist
ACh or muscarine
how do agonist bind?
they bind to the long extracellular amino-terminus and activate intracellular g proteins
mAChRs are located…
at the presynaptic ganglia and the smooth neuromuscular junction where they are involved in ACh-mediated inhibition of further ACh release (opposite of nAChR)
N(m)
skeletal muscle nAChR
contains four distinct subunits: 2alpha, beta, delta, and epsilon in the adult, and epsilon is replaced by gamma in newborns
arranged in a pentameric structure around a central pore
N(N)
peripheral neuronal nAChR
exist at autonomic ganglia and adrenal medulla as pentamers made up of alpha and beta subunits
Central neuronal nAChR
*as opposed to N(N) and N(m)
least understood subtype of nAChRs
Pentameric receptors can be heteromeric or homomeric in organization at both pre and post synaptic locations
Pre-synaptic nAChR
stimulates more ACh vesicles to be mobilized for release
