Cholinergic Pharmacology – Acetylcholine & Receptors (Lecture Notes)

• Acetylcholine (ACh) is a neurotransmitter in both the central and peripheral nervous systems, functioning primarily at the neuromuscular junction and within various brain regions.It plays a crucial role in muscle contraction, attention, arousal, and memory processes, making it essential for numerous physiological functions.

Page 4 – Cholinergic Pharmacology (macro-theme)

• The pharmacology of all drugs/agents that either mimic (agonists) or block (antagonists) acetylcholine at muscarinic or nicotinic receptors.

Page 5 – Overview of ACh “life cycle”

• Key headings: Synthesis – Storage – Release – Degradation – Action.
• Central organizing question: “How does each step influence clinical pharmacology?”

Page 6 – Diagram: Neurotransmission at a Cholinergic Synapse

1. Action potential arrives at presynaptic terminal → $Na^+$ influx depolarises membrane.

2. Choline uptake from ECF via high-affinity, Na⁺-dependent transporter (rate-limiting step).

3. Synthesis: Choline + Acetyl-CoA (from mitochondria) → Acetylcholine via choline acetyl-transferase (ChAT/CAT).
   $\text{Choline} + \text{Acetyl-CoA} \xrightarrow{ChAT} \text{ACh} + \text{CoA}$

4. Vesicular packaging in synaptic vesicles (antiporter exchanges $H^+$ for ACh).

5. Release: Action potential opens voltage-gated $Ca^{2+}$ channels → vesicle fusion & exocytosis (requires SNARE proteins).

6. Postsynaptic binding: ACh diffuses across cleft → binds muscarinic or nicotinic receptors (details p.13–17).

7. Termination: Rapid hydrolysis by acetylcholinesterase (AChE) → choline + acetate.

8. Recycling: Choline re-uptake completes cycle.

Page 7 – Synthesis of Acetylcholine

• Transport step: Choline uptake is energy- and Na⁺-dependent; inhibited by hemicholinium-3 (pharmacological point).
• Enzyme: Choline acetyl-transferase (ChAT).
• Substrates: Cytoplasmic acetyl-CoA (from pyruvate dehydrogenase in mitochondria) + choline.
• Site: Cytoplasm of presynaptic cholinergic neurons.

Page 8 – Storage & Release of ACh

• Storage: Small, clear, membrane-bound vesicles crowd presynaptic terminal (≈1000–50,000 ACh molecules/vesicle).
• Trigger for release: Arrival of action potential → depolarisation → voltage-gated $Ca^{2+}$ influx.
• Mechanism: $Ca^{2+}$ interacts with synaptotagmin → SNARE complex (syntaxin, SNAP-25, synaptobrevin) mediates partial exocytosis (quantum release).
• Termination: Extracellular ACh persists only milliseconds because of abundant AChE.

Page 9 – Degradation of Acetylcholine (Structural View)

• ACh is an ester: quaternary ammonium “cationic head” + acetate ester link.
• AChE active site:
– Anionic site: attracts quaternary N⁺.
– Esteratic site: catalytic triad (Ser-His-Glu) forms covalent bond with acetyl group.

Page 10 – Three-Step Hydrolysis Mechanism

1. Binding: ACh aligns; cationic N⁺ to anionic site, ester to serine hydroxyl.

2. Cleavage: Choline leaves; acetylated enzyme intermediate forms.

3. Hydrolysis: Water hydrolyses acetyl–enzyme → releases acetate + regenerates free AChE.
   • Time scale: $\approx 1\,\text{msec}$; explains need for continuous ACh release during sustained signalling.

Page 11 – Types of Cholinesterases

1. True/Specific AChE
   – Location: synaptic clefts of cholinergic neurons & RBC membranes.
   – High specificity; essential for life.
   – Slow regeneration in RBCs (≈120 days, lifespan of erythrocyte).

2. Pseudo/Butyryl-Cholinesterase (BChE)
   – Location: plasma, liver.
   – Broad substrate profile (succinylcholine, ester local anaesthetics).
   – Not essential; rapid hepatic synthesis.

Page 12 – General Statement on Receptors

• ACh exerts effects by binding to Muscarinic (M1–M5) and Nicotinic (Nm, Nn) receptors in both CNS & periphery.

Page 13 – Receptor Map

• Muscarinic (G-protein-coupled) – “M”: M1, M2, M3 clinically dominant slides.

• Nicotinic (ligand-gated ion channels) – “N”: Nn (neuronal, autonomic ganglia & CNS) & Nm (neuromuscular junction).

• Downstream signalling distinguishes therapeutic/side-effect profiles of drugs.

Page 14 – Muscarinic Receptors Blocked by Atropine

• Practical pharmacology: Atropine = competitive antagonist at all M sub-types (prototype anti-muscarinic).

• Diagnostic implication: Any effect reversed by atropine is muscarinic in origin.

Page 15 – M1 (Excitatory, G_q → ↑IP₃/Diacylglycerol)

CNS roles:
a. Learning & Short-term memory – loss of cholinergic tone → Alzheimer’s disease.
b. Basal ganglia – DA–ACh balance modulates movement; cholinergic hyperactivity contributes to Parkinsonian rigidity/tremor.
c. Vestibular pathway – M1 activation may trigger vomiting; explains anti-muscarinics as anti-emetics.
Peripheral (GIT):
– On gastric enterochromaffin-like (ECL) cells → histamine release → parietal cell ↑HCl secretion (clinical: atropine decreases acid).

Page 16 – M2 (Inhibitory, G_i → ↓cAMP, opens K⁺ channels)

• Presynaptic auto-receptors: inhibit further ACh release (negative feedback).
• Heart
– SA Node: ↓ firing rate (negative chronotropy).
– AV Node: ↓ conduction velocity (negative dromotropy).
– Atria: ↓ contractility.
– Shorter action potential & refractory period → predisposition to atrial arrhythmias if overstimulated.
• CNS respiratory centres (RC) – not elaborated but implied inhibition.

Page 17 – M3 (Excitatory, G_q)

• Smooth Muscle
– Bronchi, GIT, urinary bladder: contracts wall, relaxes sphincters (promotes airway constriction, peristalsis, urination).
• Eye
– Constrictor pupillae: miosis.
– Ciliary muscle: accommodation for near vision → opens trabecular meshwork, lowers IOP (basis for pilocarpine in glaucoma).
• Exocrine Glands
– ↑ secretions: lacrimal, salivary, bronchial, gastric, intestinal, sweat (eccrine), pancreatic [milk unaffected].
• Vascular Endothelium
– M3 on endothelial cells → NO release → smooth-muscle relaxation → vasodilation (VD) → ↓BP (if endothelium intact).
– In damaged endothelium, direct M3 on vascular smooth muscle can cause vasoconstriction (paradoxical pharm effect).

Page 18 – Review Question: Effects of Acetylcholine

Prompted organs & answers:
• Blood Pressure: ↓ (via M3-mediated endothelial NO).
• Heart Rate: ↓ (M2 at SA node).
• Lacrimal/Salivary/Sweat Glands: ↑ secretions (M3).
• Urination: Facilitated – detrusor contraction + sphincter relaxation (M3).
• Bowel Habit: Increased motility & possible diarrhoea (M3, M1 in ENS).

Page 19 – Lecture Quiz 1

Question: Vesicle fusion & ACh release depend on what?
Correct answer: B. Dependent on calcium influx through voltage-sensitive calcium channels.
Explanations of distractors:
– Cl⁻ not primary determinant.
– Sodium carrier = choline uptake step, not exocytosis.
– Botulinum toxin blocks (not facilitates) release.
– Hexamethonium blocks N_n ganglia, not release.

Page 20 – Lecture Quiz 2

Muscarinic activation causes?
Correct answer: C. Miosis due to stimulation of the constrictor pupillae muscle.
Rationales:
A. Sweat glands ↑, not ↓.
B. Bronchial smooth muscle contracts (so B is false).
D. Blood vessels vasodilate via NO (not vasoconstrict).
E. AV node conduction ↓ (not ↑).

Page 21 – Suggested Textbooks

1. Whalen, Finkel & Panavelil – Lippincott’s Illustrated Reviews: Pharmacology 7th ed.

2. Katzung & Trevor – Basic and Clinical Pharmacology 14th ed.

Page 22 – Closing

• “Thank you” slide; military college crest.

• Signals end of lecture.

Integrative Connections & Clinical Correlates

• Cholinergic crisis vs. Myasthenic crisis: Distinction based on excessive vs. insufficient N_m stimulation; atropine only reverses muscarinic symptoms.
• Alzheimer therapy: Centrally acting AChE inhibitors (donepezil, rivastigmine) compensate for lost M1 signalling.
• Glaucoma: Direct muscarinic agonists (pilocarpine) exploit M3-mediated miosis & ciliary contraction to enhance aqueous outflow.
• Succinylcholine metabolism: Variations in pseudo-cholinesterase prolong paralysis; genetic testing & dibucaine number.
• Organophosphate poisoning: Irreversible AChE inhibition → cholinergic crisis; treated with atropine (muscarinic) + pralidoxime (reactivates AChE).

Key Equations & Numbers

1. ACh synthesis: $\text{Choline} + \text{Acetyl-CoA} \xrightarrow{ChAT} \text{ACh} + \text{CoA}$

2. AChE hydrolysis rate: $t_{1/2}\approx10^{-3}\,s$ (functional half-life of synaptic ACh).

3. Cholinesterase regeneration (RBC): $\approx120\,\text{days}$.