Dr Hiley - PNS Flashcards
What is the synaptic vesicle cycle?
1) Uptake of transmitter release
2) Synaptic vesicles move to active zone
3) Vesicles dock to the plasma membrane
4) Vesicles undergo ATP-dependent pre-fusion
5) Ca2+ triggers completion of fusion
6) Vesicles retrieved by endocytosis
7) Coated vesicles shed coat and recycle to the interior of nerve terminal
8) Empty vesicles can either refill of pass through an endosomal intermediary sorting station
What is the synaptic structure of the ANS?
Terminal varicose axon branches that form a network in and around the effector cells
Which vesicular proteins are involved in neurotransmitter release?
v-SNAREs and synaptobrevin
Which terminal proteins are involved in neurotransmitter release?
t-SNAREs, SNAP-25 and syntaxin
Tetanus toxin?
Targets synaptobrevin
Stops release of transmitter from inhibitory interneurons
=Motor neuron more excitable = tetani
What is the target of botulinum B, D, F and G?
Synaptobrevin
Which botulinum toxins target synaptobrevin?
B, D, F and G
What do botulinum toxins A and E target?
SNAP-25
Which botulinum toxins target SNAP-25?
A & E
What does botulinum C1 target?
Syntaxin and SNAP-225
Which botulinum toxin targets syntaxin and SNAP-25
C1
Botulinum toxin?
Attack cholinergic neurons
Heavy chain - C-terminus binds to a glycoside and N-terminus translocates light chain
Light chain - Peptidase which cleaves SNARE
=Muscles weakness and ANS stimulation
What is the role of synaptotagmin?
Ca2+ sensor
Binds syntaxin during membrane fusion
What are synapsins?
Proteins on the surface of the vesicles which links them to the cytoskeleton (regulated by phosphorylation)
PKA/CaM Kinase II phosphorylation causes vesicular dissociation
What are the two kinds of synaptic plasticity?
Depression and Facilitation
What is synaptic depression?
Depletion of neurotransmitter after repeated stimulation
What is synaptic facilitation?
Ca2+ builds up as its entry is too fast for removal
Which aspects of the PNS can drugs act upon?
- Neurotransmitters (NTs)
- Synthesis of NTs
- Storage of NTs
- Release of NTs
- Breakdown of NTs
- Uptake of NTs
- Activation of receptors
Draw Cholinergic transmission
Well done have a gold star
What is the rate limiting step of ACh synthesis?
Choline uptake
Hemicholium?
Blocks choline uptake by binding to transporter
Trimethylcholine?
Acts as a competitive substrate to choline in ACh synthesis
Acetylated to acetyltriethylcholine (released instead of ACh)
Vesamicol?
Block the vesicle transport (VAChT)
Inhibition is non-competitive and reversible
β-Bungarotoxin
Acts via a phospholipase A2 and localises to the membrane by a K+ channel-binding moiety
α-Latrotoxin
Black widow spider venom
Targets neurexins, latrophilin receptors, protein tyrosine phosphatase-σ
Τetramer forms ion pore = Ca2+ entry
Where is acetylcholinesterase found?
Soluble presynaptically and bound to the post synaptic membrane
Where is butyrylcholinesterase found?
Predominantly a plasma enzyme, also found in quantities in skin, brain and gut
With which Ach receptors does α-bungarotoxin interact?
Binds to brain (α7)5
What are the compositions of the striated muscles ACh receptors?
(α1)2β1δε
(α1)2β1δγ
What is the composition of the ACh receptors in the autonomic ganglion?
(α3)2(β4)3
What is the composition of the ACh receptors in the brain?
(α4)2(β2)3
Some (α3)2(β4)3
(α7)5
What are the phase of nicotine action?
Phase I block - Cells are depolarised
- VGCs are refractory
- Cell unresponsive to stimulation
Phase II block - cells repolarise
- Neuron becomes electrically re-exitable
Trimetaphan
Competitive ACh receptor antagonist
Mecamylamine?
Non-competitive ACh receptor antagonist
Hexamethonium
Non-competitive ACh receptor antagonist
What effect does increasing the chain length have on hexamethonium’s action?
Changes its selectivity between ganglion and nmj
What were the side effects of hexamethonium?
Loss of sympathetic tone to blood vessels = hypotension
Loss of CV reflexes = Fainting
Inhibition of secretion = Dry skin and mouth
GI paralysis
D-Tubocurarine
ACh competitive antagonist
= flaccid paralysis
Atracurium
ACh competitive antagonist
Ester bond = spontaneous breakdown and hydrolysis by plasma esterases
Pancuronium
Competitive antagonist of the ACh receptor
Decamethonium
ACh agonist
What are the two phases of depolarising blocker action?
Phase I - Depolarising block = spastic paralysis
(short lived on focally-innervated muscles)
Phase 2 -Desensitisation block in focally-innervated muscles = flaccid paralysis
What effect do antiCHE and non-depolarising blockers have on Phase 1 of depolarising blockers action?
Phase 1 is deepened by antiCHE and reversed with non-depolarising blockers
What effect do antiCHE and curare-like drugs have on Phase 2 of depolarising blockers action?
Reversed by antiCHE and deepened by curare-like drugs
What are the 3 main subtypes of the muscarinic ACh receptor and which G protein type are they coupled to?
M1 - Gq
M2 - Gi
M3 - Gq
McN-A-343
Cell-specific stimulator of muscarinic receptors in ganglionic neurons
How do M1 and M3 signal?
Through the generation of IP3 and DAG
M1 also inhibits K+ conductance
M3 receptor stimulation increases [Ca2+]i
How does the M2 receptor function?
Gi inhibits adenylyl cyclase = decrease in cAMP
=Decreased phosphorylation of VGCC
=Decreased excitability of heart muscle/transmitter release
βγ subunit opens GIRK in pacemaker tissue
= decreased heart rate
Bethanecol?
Muscarinic receptor agonist used for urinary retention
Pilocarpine?
Muscarinic receptor agonist used in the treatment of glaucoma (contracts ciliary muscles)
Pirenzipine
Selective M1 antagonist
Decrease gastric acid secretion
effect on local ganglion rather than M3 receptors of oxyntic
Tripitramine
Selective M2 antagonist
Darifenacin?
Selective M3 antagonist
Decreases bladder activity
Atropine
Non-selective muscarinic receptor antagonist
Used to dilate eyes (mydriasis)
Bronchodilation
Decreases GI motility
Increased HR
Hyoscine (Scopolamine)
Non-selective muscarinic receptor antagonist
Used to dilate eyes (mydriasis)
Bronchodilation
Decreases GI motility
Increased HR
Ipatropium?
Muscarinic receptor antagonist used in asthma inhalers
What is a difference between the structures of AChE and BuChE?
AChE has an anionic and esteratic site
BuChE only has an esteratic site
How are the anticholinesterase agents classified?
According to length of action:-
Short acting
Medium acting
Long acting
Edrophonium
Short acting AChE inhibitor
Used to diagnose myesthenia gravis
Neostigmine
Medium acting AChE inhibitor
Binds to esteratic site and carbamoylates the enzyme
Used to reverse NMJ blockade after surgery and as an oral treatment to myasthenia gravis (prevent diminishing response)
Physostigmine
Medium acting AChE inhibitor
Glaucoma treatment
Malathion?
Long acting (irreversible) AChE antagonist
^ Parasympathetic activity
Dyflos?
Long acting (irreversible) AChE antagonist
^ Parasympathetic activity
Pralidoxime?
Causes the dissociation of dyflos/malathion from AChE to prevent poisoning
Draw the mechanism bitch
What is the pathway for the synthesis of Adr?
Tyrosine
Tyrosine hydroxylase (TOH)
DOPA
DOPA decarboxylase (DDC)
Dopamine
Dopamine β-hydroxylase (DBH)
NA
PNMT
Adr
At what point does the synthesis of Adr move from the cytoplasm to the vacuole?
Dopamine is taken in before being converted to NA
α-methyltyrosine
Competitive inhibitor of TOH
rate limiting step therefore reduces the amount of transmitter
L-DOPA
By-passes the rate limiting step of DA production
Increased DA production in the brain
(DA can’t be supplemented as it doesnt cross the BBB)
Used in the treatment of parkinsons
Carbidopa
Peripheral decarboxylase inhibitor
Inhibits DDC but cannot enter the brain = reduced side effects of L-DOPA
Disulfiram?
Inhibits DBH
Clinical used as an inhibitor of aldehyde dehydrogenase
MethylDOPA
Released as a false transmitter (MethylNA) for Adr
Greater effect on α2 receptors than NA = increased negative feedback
How are NA and DA transported into the vesicles?
Transported by VMAT-2, driven by the proton gradient (established by ATP-dependent H+ pump)
Reserpine?
Binds to amine binding site of VMAT
=blocks uptake
Antihypertensive
(triggers depression due to CNS -HT depletion)
Tyramine
Indirectly acting sympatheticomimetic amine
Displaces NA in storage vesicles, NA then transported into synaptic cleft by NET (in exchange for tyramine)
Broken down by MAO
What are the side effects of tyramine?
Large release of NA = large increase in BP
Use of MAO inhibitors increases risk
Octopamine
Adr false transmitter
From tyramine
Ephedrine
Indirectly acting sympatheticomimetic agent
Used in nasal sprays
Dexamfetamine
Indirectly acting sympatheticomimetic agent
Methylphenidate
(Ritalin)
Indirectly acting sympatheticomimetic agent
Used to treat ADHD
MDMA
Indirectly acting sympatheticomimetic agent
Tachyphylaxis
Reduction in effectiveness upon repeated administration
Guanethidine
Blocks the release of NA
Carried into nerve by Uptake 1
Large dose: Indirectly acting sympatheticomimetic amine
Low dose: Blocks NA release (hyperpolarisation of nerve terminal)
Bretylium
Blocks the release of NA
Carried into nerve by Uptake 1
Large dose: Indirectly acting sympatheticomimetic amine
Low dose: Blocks NA release (hyperpolarisation of nerve terminal)
How do presynaptic Adr receptors work?
Opening of G protein regulated K+ channel = hyperpolarisation
What effect do M2 and δ-opioid receptors have on NA release?
They decrease it
What are the features of Uptake 1?
High affinity, low capacity
NA > Adr
Mediated by NET
Cocaine?
Blocks Adr transportation by NET
Imipramine?
Tricyclic antidepressant
Blocks Adr uptake by NET
Amitryptyline?
Tricyclic antidepressant
Blocks Adr uptake by NET
What is NET?
Catecholamine transporter
12 TM domains
Na+ dependent
What are the features of Uptake 2?
Low affinity, high capacity
Adr > NA
Carried by ENT (not Na+ dependent)
Also transports DA, 5-HT and histamine
Normetanephrine
Block Uptake 2 by blocking ENT
metabolite of NA therefore NA uses uptake 1
Corticosterone and hydrocortisone
Block ENT
What is the role of MAO?
It deamines NA or NM
What is the role of COMT?
It methylates catechol OH
either NA, NA aldehyde, DOMA or DOPEG
Where is MAO-A found? and what are its substrates?
In the outer mitochondiral membrane
In the liver, GI, brain and peripheral nerves
NA, Adr, DA and 5-HT
Where is MAO-B found? and what are its substrates?
In the outer mitochondrial membrane in the brain
DA
Tranylcypromine
Nonselective MAO inhibitor
Cause depression
Clorgiline
MAO-A inhibitor
Causes depression
Selegine
MAO-B inhibitor
Used as a treatment for Parkinson’s disease
Entacapone
COMT inhibitor used in Parkinson’s disease
What are the relative affinities of adrenoceptors for their substrates?
α- Na > Adr > isoprenaline
β- isoprenaline > Adr > NA
How does the α1 adrenoceptor signal?
Gq
PLCs action cause Ca2+ release (via IP3) and activation of PKC (via DAG)
What are the principal effects of α1 activation?
Vasoconstriction (Ca2+) Uterine smooth muscle constriction GI smooth muscle relaxation (^I[K(Ca)]) Salivary secretion Glycogenolysis (PKC)
What is the mechanism of α2 signalling?
Gi
Gα decreases [cAMP] which in turn deactivated PKA
This inhibits VGCC activity
Gβγ also inhibits VGCC activity
What are the principal effects of α2 activation?
Inhibition of transmission
Platelet aggregation
Vasoconstriction
What is the mechanism of β1 signalling?
Gs
Increase in [cAMP] causing increased VGCC activity (via PKA)
Also inactivation of myosin light chain kinase (via PKA)
What is the mechanism for β2 signalling?
Gs
Increase in [cAMP] causing increased VGCC activity (via PKA)
Also inactivation of myosin light chain kinase (via PKA)
What is the mechanism for β3 signalling?
Gi
Decrease in [cAMP] causing a decrease in PKA activity
Increase in NO
What are the principle effects of β1 activation?
Positive chronotropic and ionotropic effects
Relaxation of smooth muscle
What are the principal effects of β2 activation?
- Bronchodilation
- Vasodilation
- Glycogenolysis
- Relaxation of smooth muscle in the uterus
- Muscle tremor
What are the principal effects of β3 stimulation?
Negative chronotropic and ionotropic effects
Vasodilation
Relaxation of detrusor muscle in the bladder
What are the actions of Adr on BP, HR and TPR, and how are they mediated?
Decrease in TPR via α1 and β2
Increase in HR via β1
Increase in systolic and decrease in diastolic pressures
What are the actionf of NA on BP, HR and TPR, and how are they mediated?
Increase in TPR
Increase in BP
Reflex decrease in HR
All via α1
What are the actions of isoprenaline on BP, HR and TRP, and how are the mediated?
Increase in HR (β1)
Decrease in TPR (β2)
Increase in systolic and decrease in diastolic
Adrenaline?
Used in severe acute conditions
Relieves bronchospam in anaphylactic shock (and vasoconstriction)
Stimulates heart in acute cardiac failure
Xylometazoline
α-adrenoceptor agonist
Relieves nasal congestion
Pheylephrine
Selective α1-receptor agonist
Increase BP in acute hypotension
Clonidine
α agonist with some selectivity for α2
Antihypertensive (vasodilation)
Vasoconstriction if applied to the periphery (shaving cream)
Acts on hind brain (likely to be via the imidazoline I1 receptor)
Xylazine
α agonist selective for α2
Sedative effects
Action on CNS
Isoprenaline
Non selective β agonist
Bronchodilation but increased risk of dysrhythmia
Salbutamol/terbutaline
β2-selective agonist
Relieves bronchospasm in asthma
Salmeterol
Long acting β2-selective agonist
Dobutamine
β1-selective agonist
Cardiogenic shock or to improve outflow after cardiac surgery
Mirabegron
β3-selective agonist
Increases bladder capacity by relaxing detrusor muscle
Phentolamine
Non selective α antagonist
Usually associated with increased HR
Prazosin
α1 selective antagonist
Antihypertensive agent
Phenoxybenzamine
Irreversible antagonist
Used to treat phaeochromocytoma
Labetalol
α and β antagonist
Propranolol
β antagonist nonselective
Class IB antidysrhythmic agent
Atenolol
β antagonist (β1 selective)
Reduced risk of broncospasm
Class IB antidysrhythmic agent
Ergotamine
Partial α-adrenoceptor agonist
Mainly act at 5-HT receptors
What is ergotism?
AKA St Anthony’s fire
Intense vasoconstriction cause by eating fungus from mouldy grain
What are the four main ways that drugs can affect noradrenaline release?
- Directly blocking release
- By evoking release in the absence of depolarisation (Indirectly acting sympatheticomimetic agents)
- By acting on presynaptic receptors that indirectly inhibit or enhance release
- By increasing or decreasing the available stores of NA
What is NANC transmission?
Non-adrenergic non-cholinergic transmission
Often involves co transmission
Is ATP stored in the same or different vesicles to NA?
Same vesicles
Are peptides stored in different or the same vesicles to ACh?
Different
Who discovered ATPs action as a neurotransmitter?
Drury & Szent-Gyorgy discovered that ATP and other adenine derivatives acted on cells of the heart
Burnstock found that ATP satisfied Dale’s criteria for a neurotransmitter
What are the two main types of purinoceptor? and how are they distinguished?
P1 - Higher affinity for adenosine
P2 - Higher affinity for ATP
What are the subtypes of the purinoceptors? and what kind of receptor are they?
P2X (1-7) - Ionotropic
P2Y - GPCR
P1A1 - GPCR
P1A2A - GPCR
P1AB - GPCR
P1A3 - GPCR
What happens as ATP release by a nerve is degraded?
ADP etc. act upon different receptors = different responses
What enzyme degrade ATP and where are they found?
Ecto-nucleotidases are found on the cell surface and nucleotidases can be co released with ATP
What is the structure of the P2X purinoceptor?
Formed as a trimer of subunits (Homo or hetero)
Two TM domains
Large extracellular domain with disulphide bonds
What is the difference in effects between ATP and NA on the postsynaptic membrane?
ATP = Fast depolarisation NA = Slow depolarisation
What is the role of P2X2 receptors?
Nerve induced contraction of the vas deferens
What is the role of P2X4 receptors?
ATP - ^ Force of contraction
Up-regulation in the spinal cord - tactile allodynia
What is the main role of adenosine?
To signal hypoxic conditions (retaliatory metabolite)
Causes vasodilation in smooth muscle and endothelial cells
Causes a decreased rate and force of contraction of the heart (reduces O2 demand)
What are the signalling mechanisms and principle actions of the A1 receptor?
Gi
Decreased HR & force of contraction
Bronchoconstriction
Presynaptic inhibition of transmitter release
Decreased brain glutamate release
What are the signalling mechanisms and principle actions of the A2A receptor?
Gs
Vasodilation
Decreased activation of neutrophils
What are the signalling mechanisms and principle actions of the A
2B receptor?
Gs
Vasodilation
What are the signalling mechanisms and principle actions of the A3 receptor?
Gi/Gq
Decreased eosinophil activation in the lungs
Caffeine
A1 receptor antagonist
Also a PDE inhibitor
How is adenosine transmission inactivated?
By transport into cells
Dipyramidamole
Blocks uptake and potentiates responses to adenosine
What are the features of peptide NANC transmission?
Requires sustained increase in [Ca2+]i for release as peptides-containing vesicles are not found in the active zone
Slowly inactivated
Produce the late slow epsp
What is the synthesis order of a signalling peptide?
Preprohormone
Prohormone
Active molecule
(one precursor can generate several active molecules)
Why do peptides not make very good drugs?
Poorly absorbed orally
Rapidly degraded
Dont cross the BBB
Oxytocin
Peptide drug used to induce labour
How is NO produced?
Made by Ca2+ dependent enzyme nNOS and eNOS
or in a Ca2+ independent manner by iNOS
What is the equation for the production of NO?
L-Arginine + O2 + NADPH -> NO + L-citrulline + NADP+
How does NO function as a signalling molecule?
Freely diffuses in all directions
Activates GC in pre and postjunctional cells
PKG causes effect (eg vasodilation)
Glyceryl trinitrate
NO donor used to treat angina
Taken sublingually
Isosorbide dinitrate
Metabolised to isosorbide mononitrate (active)
Acts as an NO donor used in the treatment of angina
How is NO inactivated?
By binding to haemoglobin or by oxidation to NO2- + NO3-
What is the role of NO in the enteric nervous system? Which type of NOS is used?
ENS neurons express nNOS
Relax the pyloric sphincter
How is penile erection mediated?
Release of NO by NANC nerves
=dilation of vessels supplying the corpus spongiosum and cavernosum
Sildenafil
Selective inhibitor of PDE 5 used to treat impotence
Selectively breaks down cyclic GMP = potentiates NO signallin
What is the role of NO in the CNS?
Acts as a retrograde messenger upon activation of NMDA-Rs
How is NO used by the immune system?
iNOS in macrophages
-NO attacks Fe-S complexes = free Fe = cytotoxicity
(excess production is damaging)
How is NO involved in septic shock?
Excess NO production = vasodilation and decrease in BP
How is NO cytotoxic in the CNS?
Hypoxia = ^ glutamate release
= massive influx of Ca2+ = ^ NO production and cell death
L-NMMA
No inhibitor
Analogue of L-Arginine
L-NAME
No inhibitor
Analogue of L-Arginine
L-NOARG
No inhibitor
Analogue of L-Arginine
7-NI
Selectively inhibits the NOS of neurons
L-NIO
A potent and irreversible inhibitor of iNOS
Shipmanising patients not clinical
ADMA
Asymmetric dimethyl arginine
Endogenous NOSI
Synthesised in post-translational protein methylation
Elevated ADMA = poor prognosis
