Paul (Cholinergic pharmacology)

Question 1

Loewi’s experiment

Answer 1

Stimulate vagus in donor heart. Heart rate slows. Remove fluid sample.
Add fluid to recipient heart. Heart rate slows

Question 2

Cholinergic history

Answer 2

Muscarine
- resembled vagus stimulation
- muscarine like substance released from frogs heart after vagal stimulation

Atropine- blocked effects of muscarine and acetylcholine
- abolished muscarinic actions
- larger doses of ACh increased heart rate and BP

Nicotine
- also produced increased heart rate and BP

Question 3

Two different ‘receptors’ for endogenous ACh

Answer 3

(Type 1) Nicotinic receptors
(Type 2) Muscarinic receptors

Functional differences based on
- agonist selectivity
- antagonist selectivity

Question 4

(Type 1) Nicotinic receptors

Answer 4

Agonist-gated channel receptor.
Ionotropic receptors.
Act very quick, ion channel closed until bound

Question 5

(Type 1) Muscarinic receptors

Answer 5

G protein-coupled receptor/metabotropic/7 Trans-membrane
Slower than nicotinic
3 types functionally
- M1 (neural and gastric)
- M2 (cardiac muscle)
- M3 (exocrine glands, smooth muscle)

Question 6

Autonomic nerve terminals

Answer 6

General process we can influence with drugs.
Neurotransmitter synthesised in nerve terminal.

Question 7

Parasympathetic cholinergic nerve terminal

Answer 7

Hemicholinium blocks the re-uptake of choline.
Botulinum blocks the release of ACh.
Vesamicol blocks the uptake of ACh. Build up of ACh in junction causes contraction and paralysis.
Acetylcholinesterase breaks down ACh into acetyl and choline

Question 8

Cholinesterases- breakdown acetylcholine

Answer 8

There are 2 forms of cholinesterases. These are acetylcholinesterase (AChE) and plasma cholinesterases (aka pseudo or butyrykchokinesterase (BuChE)).
They are serine hydrolases that catalyse the hydrolysis of acetylcholine to choline and acetic acid.
The active centre of cholinesterases have 2 areas that interact with ACh. These are the anionic and esteratic site.
Choline is liberated during this reaction and an acetylated enzyme is produced. The enzyme is rapidly regenerated and acetic acid is formed.

Question 9

Anticholinesterases

Answer 9

Inhibition of AChE slows or prevents the degradation of ACh released at synapses.
There are three classes of anticholinesterases. The difference between the groups is their duration of action and this depends on how they interact with AChE.
The three groups are short acting, medium acting and irreversible (long acting) anticholinesterases

Question 10

Short acting anticholinesterases

Answer 10

e.g. Edrophonium
Monoquaternary ammonium alcohols.
they bind to the anionic site of cholinesterases and competitively displace acetylcholine from the active site. Edrophonium has a 5 minute duration (used diagnostically)

Question 11

Medium acting anticholinesterases

Answer 11

These interact with the serine hydroxyl of the esteractic site to give a carbamylated product. The carbamylated enzyme is hydroxylated more slowly.
E.g. physostigmine, neostigmine, and pyridostigmine - eserine.
Neostigmine, and pyridostigmine have direct agonist activity.

Question 12

Long acting anticholinesterases

Answer 12

These irreversibly phosphorylate he serine hydroxyl group of the esteractic site. The regeneration of the phosphorylated enzyme takes days. Most are organophosphorus compounds. The organophosphates are much less selective in that they phosphorylate other enzymes that contain a serine molecule at an active site. They are used as insecticides and chemical weapons.

Question 13

Uses of anticholinesterases

Answer 13

Used for treating myasthenia gravis (muscle weakness) and for the reversal of non-depolarising neuromuscular junction block.
They are also used for treating Alzheimer’s disease (done-evil- centrally acting).
Reversible for dementia. Irreversible for Nerva agent (Novichok)

Question 14

Myasthenia gravis (muscle weakness)

Answer 14

Circulating antibodies block ACh (nicotinic receptors) at the neuromuscular junction

Question 15

Nicotinic receptors

Answer 15

Ionotropic
Embedded in the cell membrane. It is a protein with 5 subunits- 2a, b, g, e.
Found at neuromuscular junctions in skeletal muscle and in autonomic ganglia. At both sites these receptors are post- synaptic.

Question 16

Drugs acting on nicotinic receptors

Answer 16

Agonists
- Neuromuscular junction (depolarising neuromuscular blockers)
-Suxamethonium
- Decamethonium
- Autonomic ganglia and CNS
- Nicotine
- Varenicline

Antagonists
- Neuromuscular junction (non-depolarising neuromuscular blockers can be reverse using ACh)
- Tubocurarine
- Atracurium
- Pancuronium
- Vecuronium
- Autonomic ganglia and CNS
- Timethaphan (rarely used)

Both agonist and antagonist are neuromuscular blockers as the agonist cause contraction and then antagonist causes relaxation.

Question 17

Neuromuscular blocking agents

Answer 17

Depolarising blockers:
These drugs activate the nicotinic receptor and therefore produce initial contraction of the muscle fibres. The maintained depolarisation produced by these drugs eventually lead to an inactivation of the sodium channels in the muscle membrane adjacent to the end plate. This prevents the end-plate potential from producing a propagated action potential.

Non-depolarising blockers:
These drugs act as competitive antagonists at the nicotinic receptor sites. They compete with acetylcholine at the neuromuscular junction and thereby reduce the size of end plate potential this blocking transmission. the effects of non-depolarising blockers ca be reversed by an increase in acetylcholine concentration (e.g. by administration of anticholinesterases)

Most of the neuromuscular blocking drugs are quaternary ammonium compounds which penetrate cell membrane poorly. These drugs are therefore given intravenously.

Question 18

Ganglion blocking drugs

Answer 18

No longer used.
These drugs have widespread effects due to the fact that they block sympathetic and parasympathetic ganglia e.h. hypotension due to sympathetic ganglia blockade, dry mouth and tachycardia due to parasympathetic blockade.
Among the ganglion blocking drugs are trimetaphan which competitively blocks ganglionic nicotinic receptors, and ehexamethonium which is a direct channel blockers.
Excessive amount of nicotine may also produce a depolarising block at the ganglion.

Question 19

Muscarinic receptors

Answer 19

They are embedded in the cell membrane.
They are members of the GPCR family (Metabotropic) having 7 transmembrane segments in their amino acid sequence.
5 muscarinic receptor subtypes (m1-m5) have so far been clones but only 3 are well characterised.
M1 (neural) and M3 (glandular) couple to Gq protein and increase the concentration of inositol triphosphate and diacylglycerol concentration.
M2 (cardiac) couple to Gi protein and inhibit adenylate cyclase and open K+ channels. M1 receptors can also inhibit K+ channels opening.

Question 20

Actions of muscarinic receptor agonists

Answer 20

Drugs stimulating muscarinic receptors cause:
1. Smooth muscle contractions (gut, bladder) (m3)
2. Pupillary constriction, ciliary muscle contraction (old medicine used to treat glaucoma)
3. Decreased rate and force of contraction of the heart
4. Increased glandular secretion (salivary, sweat, pancreas) (m3)
5. Increased gastric acid secretion (m3)
6. vasodilation through the release of endothelial nitrous oxide (m3)
7. Inhibition of neurotransmitter release

We don’t use muscarinic agonists therapeutically as there is no need to cause the effects

Question 21

Muscarinic antagonists

Answer 21

These antagonists act as competitive antagonists and they show mostly no selectivity between receptor subtypes.
Blockade of muscarinic receptors causes:
1. Inhibition of secretion (e.g. dry mouth)- can help in travel sickness
2. Increase heart rate
3. Paralysis of accomodation
4. Pupillary dilation- used for eye exams
5. Relaxation of smooth muscles- bronchiole smooth muscle relaxation to treat asthma
6. Inhibition of gastric acid secretion- treat stomach ulcers in olden days
7. CNS excitation (atropine, a competitive antagonist for the muscarinic acetylcholine receptor)
8. CNS depression (scopolamine, a competitive antagonist at the muscarinic acetylcholine receptors)
9. Anti-parkinsonian action
10. Anti-emetic action

Question 22

Examples of drugs acting on muscarinic receptors

Answer 22

Agonists
- Bethanechol
- Pilocarpine

Antagonists
- Atropine
- Scopolamine
- Ipratropium/Tiotropium
- Oxybutynin
- Tropicamide
- Darifenacin