Neurochemical Transmission and Cholinergic Pharmacology

Introduction to Neurochemical Transmission

Neurochemical transmission involves the release of neurotransmitters from nerve terminals, their interaction with specific receptors, and subsequent physiological effects. Drugs can modulate these processes at various steps, especially at cholinergic and noradrenergic synapses.

• Key neurotransmitters: Acetylcholine (ACh) and noradrenaline (NA)

• Receptor types: G-protein coupled receptors (GPCRs) and ligand-gated ion channels

Cholinergic Receptors

Types of Cholinergic Receptors

Cholinergic receptors are classified into muscarinic and nicotinic types, each mediating distinct physiological responses.

• Muscarinic receptors (mAChR): GPCRs, activated by ACh and muscarine, blocked by atropine

• Nicotinic receptors (nAChR): Ligand-gated ion channels, activated by ACh and nicotine

Muscarinic Receptor Subtypes and Functions

Note: M1, M3, M5 are generally excitatory; M2, M4 are inhibitory.

Nicotinic Receptor Subtypes

• NM: Neuromuscular junction – mediates skeletal muscle contraction

• NN: Autonomic ganglia, adrenal medulla – mediates firing of postganglionic neurons and catecholamine secretion

Direct Agonists at Muscarinic Receptors

Comparison of Muscarinic Agonists

Muscarinic Antagonists

Major Muscarinic Antagonists and Their Uses

• Atropine: Non-selective; used in anesthesia, organophosphate poisoning, as antispasmodic

• Hyoscine (Scopolamine): Prevents motion sickness

• Ipratropium: Inhaled for asthma treatment

• Tropicamide: Eye drops for mydriasis (pupil dilation)

• Pirenzepine: Treats peptic ulcer by reducing gastric acid secretion

Drugs Acting on Autonomic Ganglia

Ganglion Stimulants

• Stimulate both sympathetic and parasympathetic ganglia

• More specific for nAChR in ganglia

• No clinical use; example: Nicotine at low dose

Ganglion-Blocking Drugs

• Decrease ACh release and postsynaptic effects

• Main effects: ↓ BP, loss of cardiovascular reflexes, ↓ secretions, GIT paralysis, impaired micturition

• Examples: Hexamethonium, decamethonium, trimethaphan, high dose nicotine

• Rarely used clinically due to systemic side effects

Neuromuscular Junction (NMJ) Blockers: Skeletal Muscle Relaxants

Types of NMJ Blockers

• Non-depolarizing blockers: Competitive antagonists of ACh at nAChR (e.g., tubocurarine, pancuronium, vecuronium, atracurium)

• Depolarizing blockers: Agonists of AChR causing persistent depolarization (e.g., suxamethonium, decamethonium)

Mechanism of Action

• Non-depolarizing: Block ACh binding, prevent depolarization and muscle contraction

• Depolarizing: Cause initial depolarization (Phase I block), followed by desensitization (Phase II block)

Example: Suxamethonium (Succinylcholine)

• Molecular structure: Two ACh molecules joined end to end

• Not easily hydrolyzed by cholinesterase, leading to persistent depolarization

• Therapeutic use: Short-acting muscle relaxant during procedures (e.g., bronchoscopy, laryngoscopy)

Example: d-Tubocurarine

• Plant-derived alkaloid, long-acting

• Blocks nAChR at motor endplate and ganglia

• Side effects: Decreased BP

Acetylcholinesterase and Cholinesterase Inhibitors

Types of Cholinesterases

• Acetylcholinesterase (AChE): Found at synapses, hydrolyzes ACh to choline and acetate

• Pseudocholinesterase (Butyrylcholinesterase): Found in plasma, liver, etc.

Drugs that Interfere with Transmitter Metabolism

Anti-cholinesterases (AChE Inhibitors)

• Reversible inhibitors:

  • Physostigmine (crosses BBB, CNS effects)

  • Neostigmine, pyridostigmine (do not cross BBB, used for myasthenia gravis)

  • Edrophonium (short-acting, diagnostic for myasthenia gravis)

  • Tacrine, rivastigmine, donepezil (Alzheimer's disease)

• Irreversible inhibitors:

  • Organophosphorus compounds (pesticides, nerve agents)

  • Form irreversible covalent bonds with AChE

  • Cause persistent depolarization and paralysis

Mechanism of Action of Reversible Anticholinesterase

• Neostigmine resembles ACh, binds to anionic and esteratic sites of AChE, blocking ACh binding

• Hydrolyzed by AChE much more slowly than ACh

Clinical Uses of Anticholinesterase Agents

• Neostigmine, pyridostigmine: Myasthenia gravis, reversal of non-depolarizing muscle relaxants, urinary bladder atony, ileus

• Physostigmine: Glaucoma

• Edrophonium: Diagnostic agent for myasthenia gravis

• Tacrine, rivastigmine, donepezil: Alzheimer's disease

Disease at Neuromuscular Junction: Myasthenia Gravis

Pathophysiology and Clinical Features

• Autoimmune disorder affecting nAChR at NMJ

• Results in skeletal muscle weakness and fatigue

• Symptoms: Drooping eyelids, difficulty swallowing, respiratory difficulty

Summary Table: Key Drug Classes and Actions

Key Equations

• Hydrolysis of Acetylcholine:

Additional info:

• Muscarinic and nicotinic receptors are central to understanding drug effects on the autonomic and somatic nervous systems.

• NMJ blockers are essential in anesthesia and emergency medicine for muscle relaxation.

• Anticholinesterase agents have both therapeutic and toxicological significance.