ACh in the ANS

Question 1

The ANS is a two nerve relay

Answer 1

All nerves leaving the spinal cord are cholinergic.
Parasympathetic secondary nerves in the two nerve relay are also cholinergic (ACh Mus).
Sympathetic secondary nerves in the two nerve relay are adrenergic except in sweat glands and the adrenal medulla.

Question 2

Acetylcholine biosynthesis

Answer 2

CoA + choline in converted to acetylcholine by ChAT (choline acetyletransferase) in synapses.
All cholinergic cells have ChAT so it is a good neuromarker.
ACh is then pumped into vesicles by a membrane pump.
Acetylcholinesterase breaks down ACh that is not in vesicles into inactive substrates. Choline is recycled.
This makes choline availability the rate limiting step.

Question 3

Inhibitors of ACh synthesis and release

Answer 3

Hemicholium-3 - inhibits choline uptake
Vesamicol - causes downregulation of neurotransmitters.
Triethylcholine - false transmitter
Toxins (botulinum) -prevents ACh release
Calcium/sodium channel blockers (streptomycin, neomycin, magnesium ions) - inhibits initiation of ACh release (sodium channel blockers used as LAs.

Question 4

Botulinum toxin

Answer 4

Progressive parasympathetic and motor paralysis
Prevents ACh release
Cleaves snare proteins, preventing membrane fusion of synpatic vesicles and therefore preventing release.
Used to treat hyperhydrosis and cosmetically
Recovery by making New snare proteins which takes a long time.

Question 5

Nicotinic ACh receptors

Answer 5

Ligand gated ion channels
5 subunit structure
Different cell types have different combinations of subunits, this is useful for making cell type specific drugs (muscle,ganglia,CNS all different).
Epibatidine is the most potent agonist at nicotinic receptors.
Curare is a potent nicotinic receptor antagonist - it weakens muscles, a potent neuromuscular blocking agent.

Question 6

Non-depolarising competative antagonists at ACh receptors.

Answer 6

Tubocurarine from curare.
Synthetic drugs - gallamine, pancuronium, atracurium.
Used as muscle relaxants during surgery.
Charged so do not cross BBB
Antagonises ACh competitively, partial block.
Would need to block >90% of receptors to block system.
Respiratory system function lost last.
Causes a decrease in endplate potential, makinv it harder to generate APs.
Low bp is a side affect due to action on ganglia.
Produces decreased ACh release due to inhibitory action on prejunctional nicotinic autoreceptor.
Poorly lipid soluble, poorly protein bound, easily reversible, used for prolonged paralysis.
Over come by increasing ACh, can use depolarising drugs.

Question 7

Drugs blocking neuromuscular transmission (postsynaptic)

Answer 7

Non depolarising blocking drugs (tubocurarine)
Depolarising drugs (suxamethonium)

Question 8

Depolarising drugs

Answer 8

Decamethonium, suxamethonium
Agonists at nicotinic ACh receptors.
Overstimulate receptors to desensitise them.
AChE can’t degrade these drugs quickly, spasm if drug lingers in synaptic cleft in animals with multiple end plates per musvle fibre.
Phase I block - membrane depolarisation, constant but diminished TOF.
Phase II block - aftef minutes, receptor desensitisation, unable to bind any more agonists.
Reversal occurs by drug diffusing away or with tubocurarine (non-depolarising) which reduces depolarisation and vice versa.
May induce muscular pain.
Used for paralysis of rapid onseg and short duration ie tracheal intubation.

Question 9

TOF

Answer 9

Tetanic fade
The failure of muscle to maintain a fused tetany.
Difference between response 1 and 4 gives TOF, an indication of relaxation.
TOF 0.15-0.25 - Adequate surgical relaxation
TOF >0.9 - Adequate for safe inthbation and Recovery after surgery.

Question 10

Muscarinic receptors M1 and M3

Answer 10

7 membrane spanning domains.
1 ligand binding domain encoded by 5 genes.
Excitatory receptors
Gq protein coupled
Agonist binding leads to disociation of G protein. This leads to PI hydrolysis to give IP3 and DAG. IP3 leads to calcium release and DAG leads to PKC activation.
M1 - responsible for gastric secretion
M3 - smooth muscle contraction/glandular secretion/vasodilation via NO pathway.

Question 11

Muscarinic receptors M2 and M4

Answer 11

Agonist binding leads to disociation of G protein, decreasing adenylate cyclase which in turn decreases cAMP and PKA activation
M2 - decreased rate and force of contraction of heart
M4 - postsynaptic membrane / plasma membrane /cell junction.

Question 12

Muscarinic agonists

Answer 12

ACh -binds well to both Mus and nic ACH receptors, quickky degraded by AChE.
Carbachol - binds wel to Mus and nic, not broken down by AChE theregore a long acting agonist.
Methacholine - binds well to Mus but not nic, broken down by AChE but not as easily as ACh.
Bethanechol - Mus selective, not broken down by cholinesterases therefore long lasting Mus selective agonist. Muscarine is the same but natural (shrooms).
Pilocarpine - Mus selective but not as strkngly as muscarine, not charged molecule so can cross BBB.

Question 13

Muscarinic antagonists

Answer 13

Much more varied in structure as only have to block the receptor.
Atropine - selective Mus antagonist
Hyoscine - selective Mus antagonist “truth drug”.
Tropicamide - pupil dilator, short acting Mus antagonist.
Pirenzepine - M1 selective, peptic ulcer treatment.
Mamba toxins - MT7 (M1 selective), MT3 (M4 selective).
Bungarotoxin - alpha and beta nic antagonist from vipers.

Question 14

Parasympathetic actions of ACh on the heart

Answer 14

ACh release from vagal input to the heart, actions include -
Slowing of HR, decreased automaticity.
Decreased force of attial contraction.
Decreased AV node conduction.
Decreased cardiac output.
Lowering of BP (partly due to vasodilation).
Cardiac actions due to activation of M2 receptors.
M2 receptors are abundant in attial and nodal tissue butnot in ventricles.
Decreases adenylate cyclase, which decreases cAMP, in turn decreasing slow depolarising current.
M2 also couples to Kach, increasing potassium current, leading to hyperpolarisation, decreased calcium current. This all leads to shortening of AP and increased refractory period.
A pathology of this is Sinus bradycardia, treated with atropine as it blocks parasympathetic drive.

Question 15

Parasympathetic actions of ACh on blood vessels

Answer 15

Most not contr directly by parasympathetic innervation.
Exceptions are dilation of erectile tissue and salivary glands (possible M3 control).
Mus receptors on blood vessel endothelium, produce generalised vasodilation when stimulated.
This is mediated by the NO pathway.
ACh acts on M receptor on the vascular endothelium, this increases calcium concentration leading to activation of endothelial NO synthase, NO is produced, increasing guanylate cyclase and cGMP, therefore decreasing calcium concentration and hence vascular tone.

Question 16

Parasympathetic actions of ACh on bronchial smooth muscle

Answer 16

M3 mediated Bronchial constriction
Mus antagonists (ipratropium) are useful in asthma as they -
Relax bronchial constriction
May inhibit bronchial secretions
Do no affect inflammation response (histamines)
Can be given by aerosol theregore have restricted local action.
Are often used in conjuction with B2 agonists (salbutamol)

Question 17

Parasympathetic actions of ACh on glands

Answer 17

M3 mediated increase of secretion in salivary glands, sewat glands(sebum), eyes (lacrimal), and pancreas (insulin)
Dry skin/mouth is a common side affect when using ACh antagonists.
Anti-cholinergics used pre-op to reduce secretions.

Question 18

Parasympathetic actions of ACh on the GI tract

Answer 18

M3 mediated increase in motility
Anti-muscarinics are anti motily drugs.
Cisapride stimulates ACh release via 5HT receptors, this raises oesophageal sphincter tone, increanses GI motility and is used to treat reflux/gut emptying disorders.
Gastric acid secretion is M1 mediated, pirenzepine used to inhibit this and treat peptic ulcers.

Question 19

Parasympathetic actions of ACh on the eye

Answer 19

Thin rings of muscle around the pupil are the sphincter muscles (constrictor muscles).
Thick longitudinal muscles are the radial muscles (dilator muscles).
Alpha1 adrenoceptors cause dilation via radial muscle contraction.
M3 receptors cause constriction via sphincter muscle contraction.
Glaucoma - build up of intraocular pressure within the eye, affects the retina. Intraocular pressure builds up when the canal of schiem is blocked. Pupil constriction pulls open the canal draining it of aqueous humor. Miotics (pupil constrictors) can be used to do this, carbachol and pilocarpine.
Anti-muscarinics are dilators (mydriatics) - used by optitions aid eye examinations, can be dangerous in glaucoma as dilation reduces drainage. (cyclopentolate, tropicamide)

Question 20

Other parasympathetic actions of ACh

Answer 20

Kidney function regulation
Liver metabolism regulation
Bladder-antagonists to treat incontinence (oxybutynin, tolterodine)
Bladder agonists to promate bladder emptying or stimulate GI motility. (bethanochol)

Question 21

Ganglion activating drugs

Answer 21

Decamethonium and suxamethonium stimulate NMJ

DMPP is a selective ganglion stimulating hypertensive agent.

Question 22

Effects of peripheral ganglion stimulation

Answer 22

Ie by smoking
Nicotine triggers dopamine effects in the brain but also has big effects on the ANS.
It changes lipid profiles in the blood, blocking stimulation of sympathetic nerves.
This causes increased cardiac output and arterial pressure (tachycardia) and sweating and decreased GI motility. Decreased ADH theregore decreased urine production and increased plasma free fatty acids.

Question 23

Nicotinic receptor subtypes

Answer 23

Nm - skeletal muscle
Nn - brain
Ng - ganglia
All are ligand gated ion channels.

Question 24

Ganglion blocking drugs

Answer 24

Depolarising drugs inhibit post synaptic action of ACh at Nn.
Can develop drugs which are selective for ganglia or NMJ by changing carbon backbone length.
C6= ganglia selective
C10= NMJ selective
Selective antagonists also exist, blocking ACh receptor channel (hexamethonium) or ACh binding site (trimetaphan, mecamylamine).
Trimetaphan also reduces sympathetic tone, original anti-hypertensive but side effect of postural hypotension.
Blocking ganglia leads to decreased BP due to sympathetic block, GI tract motility and secretion is also inhibited as is Bladder emptying.
If both sympathetic and parasympathetic are nlocked there is little change elsewere but HR and postural reflexes are blocked.
Botulinum toxin - non receptor compound, prevents hyoerhydrosis due to ganglionic effects.

Question 25

Cholinesterase

Answer 25

Rapidly breaks down ACh by hydrolysis
Two forms -
1. Acetylcholinesterase - selective, often tethered.
2. Butyrylcholinesterase - non-selective, in plasma.

Anti-cholinesterase drugs -
Edrophonium - short acting
Physostigmine - medium acting
Organophosphates/dyflos - irreversible
Main actikns result in enhancement of ganglia /NMJ.

Question 26

Uses of AChE inhibitors

Answer 26

Anaesthetics to reverse Non-polarising muscle relaxants.
Neostigmine used in myasthemia gravis.
Edrophonium used as diagnostic for myasthemia gravis.
Ecothiopate eye drops in glaucoma.
Also used as chemical weapons, sarin attacks.
Also used as insectacides.

Question 27

Myasthemia gravis

Answer 27

An auto immune disease where antinodies develop for nicotine receptors.
Fatigued, compromised muscle function, saggy eyelids, hypersensitive to non-depolarising blockers.