Give a brief explanation on drugs acting on ans
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autonomic nervous system drugs classification sympathetic parasympathetic
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Drugs Acting on the Autonomic Nervous System (ANS)
Overview
The ANS, along with the endocrine system, regulates vital bodily functions without conscious control. It innervates smooth muscle, cardiac muscle, vasculature, and exocrine glands. Drugs acting on the ANS either mimic (agonists) or block (antagonists) the actions of its two major divisions: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS).
- Lippincott Illustrated Reviews: Pharmacology, Ch. 3
Neurotransmitters and Receptors
| Division | Neurotransmitter | Receptor Type |
|---|---|---|
| Parasympathetic (PSNS) | Acetylcholine (ACh) | Muscarinic (M1-M5), Nicotinic (Nm, Nn) |
| Sympathetic (SNS) | Norepinephrine (NE) / Epinephrine | Adrenergic (α1, α2, β1, β2, β3) |
| All ganglia | ACh | Nicotinic (Nn) |
- Adrenergic and muscarinic receptors are G protein-coupled (metabotropic).
- Nicotinic receptors are ion channel-linked (ionotropic).
Classification of ANS Drugs
1. Cholinergic Drugs (Parasympathomimetics)
These mimic or enhance ACh activity.
a) Direct-Acting Cholinergic Agonists
Act directly on muscarinic or nicotinic receptors.
| Drug | Receptors | Key Uses |
|---|---|---|
| Acetylcholine | M + N | Short-acting; limited clinical use |
| Bethanechol | Muscarinic (M) | Urinary retention, GI atony |
| Carbachol | M + N | Glaucoma (topical) |
| Pilocarpine | Muscarinic | Glaucoma, Sjogren's syndrome |
| Muscarine | Muscarinic | Toxicological (mushroom poisoning) |
| Nicotine | Nicotinic | Smoking cessation aids |
SLUDGE effects of muscarinic activation: Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis.
b) Indirect-Acting Cholinergic Agonists (Cholinesterase Inhibitors / Anti-ChE)
Inhibit acetylcholinesterase (AChE), increasing ACh at all cholinergic synapses.
| Drug | Duration | Notes |
|---|---|---|
| Edrophonium | Short (reversible) | Diagnosis of myasthenia gravis |
| Neostigmine | Medium (reversible) | Myasthenia gravis, reverse NMB |
| Physostigmine | Medium (reversible) | Glaucoma, atropine OD antidote |
| Pyridostigmine | Medium (reversible) | Myasthenia gravis |
| Donepezil, Rivastigmine, Galantamine | Long (reversible) | Alzheimer's disease |
| Organophosphates (e.g., Parathion, Sarin) | Irreversible | Insecticides/nerve agents; toxicity |
| Echothiophate | Irreversible | Glaucoma |
2. Anticholinergic Drugs (Parasympatholytics / Muscarinic Blockers)
These block muscarinic receptors, reducing PSNS activity.
| Drug | Key Uses |
|---|---|
| Atropine | Bradycardia, organophosphate poisoning, preanesthetic |
| Scopolamine | Motion sickness, preoperative sedation |
| Ipratropium | COPD, asthma (bronchodilation) |
| Tiotropium | COPD maintenance |
| Benztropine, Trihexyphenidyl | Parkinson's disease |
| Oxybutynin, Tolterodine | Overactive bladder |
| Glycopyrrolate | Preanesthetic, peptic ulcer |
Side effects (mnemonic - "Dry as a bone, blind as a bat, red as a beet, hot as a hare, mad as a hatter"): dry mouth, mydriasis, flushing, hyperthermia, confusion.
3. Ganglionic Blockers
Block nicotinic receptors in autonomic ganglia, blocking both SNS and PSNS.
| Drug | Notes |
|---|---|
| Hexamethonium | Historic; experimental use |
| Mecamylamine | Hypertension (largely obsolete); smoking cessation |
| Trimethaphan | Hypertensive emergencies (rarely used) |
The predominant functional effect depends on which division normally predominates in a given organ. These are rarely used therapeutically today but serve as pharmacological tools.
- Lippincott Illustrated Reviews: Pharmacology, p. 138
4. Adrenergic Drugs (Sympathomimetics)
These mimic or enhance SNS activity by acting on adrenergic receptors.
a) Direct-Acting
| Drug | Receptor | Key Uses |
|---|---|---|
| Epinephrine | α1, α2, β1, β2 | Anaphylaxis, cardiac arrest, bronchospasm |
| Norepinephrine | α1, α2, β1 | Shock/hypotension |
| Dopamine | D1, β1 (α at high doses) | Cardiogenic shock, septic shock |
| Dobutamine | β1 (predominant) | Heart failure, cardiogenic shock |
| Isoproterenol | β1, β2 | Bradycardia, bronchospasm |
| Salbutamol (Albuterol), Terbutaline | β2 | Asthma, premature labor |
| Salmeterol, Formoterol | β2 (long-acting) | COPD/asthma maintenance |
| Phenylephrine | α1 | Nasal decongestant, hypotension |
| Clonidine | α2 (central) | Hypertension, ADHD, withdrawal |
b) Indirect-Acting
Release stored NE from terminals.
| Drug | Mechanism | Notes |
|---|---|---|
| Amphetamine | NE/DA release | ADHD, narcolepsy |
| Tyramine | NE release | Dietary (cheese reaction with MAOIs) |
| Ephedrine | Mixed (direct + indirect) | Nasal congestion, hypotension |
5. Adrenergic Blockers (Sympatholytics)
a) Alpha Blockers (α-antagonists)
| Drug | Selectivity | Uses |
|---|---|---|
| Phentolamine | α1 + α2 (non-selective) | Pheochromocytoma diagnosis/crisis |
| Phenoxybenzamine | α1 + α2 (irreversible) | Pheochromocytoma preoperative |
| Prazosin, Doxazosin, Terazosin | α1 selective | Hypertension, BPH |
| Tamsulosin | α1A selective | BPH |
b) Beta Blockers (β-antagonists)
| Drug | Selectivity | Notable Feature |
|---|---|---|
| Propranolol | Non-selective (β1 + β2) | Angina, HTN, arrhythmias, migraine prophylaxis |
| Metoprolol, Atenolol | β1 selective (cardioselective) | HTN, heart failure, post-MI |
| Carvedilol, Labetalol | α + β blockade | Heart failure (carvedilol), HTN in pregnancy (labetalol) |
| Timolol | Non-selective | Glaucoma (topical) |
| Nebivolol | β1 + NO release | HTN |
General beta-blocker effects: reduced heart rate, reduced BP, reduced renin release. Contraindicated in bronchospasm (non-selective agents).
6. Drugs Affecting Synthesis/Release of Neurotransmitters
These act presynaptically:
| Drug | Mechanism | Use |
|---|---|---|
| Methyldopa | False neurotransmitter (α-methylNE) | Hypertension, especially in pregnancy |
| Metyrosine (α-methyltyrosine) | Inhibits tyrosine hydroxylase (blocks NE synthesis) | Pheochromocytoma |
| Reserpine | Depletes NE/DA from vesicles (blocks VMAT2) | Hypertension (historic); Huntington's chorea |
| Guanethidine | Blocks NE release | Hypertension (largely obsolete) |
| Botulinum toxin | Blocks ACh vesicle release | Dystonias, spasticity, cosmetic use |
- Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 2121-2136
Quick Summary Diagram
ANS Drugs
├── Cholinergic (mimic PSNS)
│ ├── Direct agonists (pilocarpine, bethanechol)
│ └── Indirect (AChE inhibitors: neostigmine, donepezil)
├── Anticholinergic (block PSNS) → atropine, ipratropium
├── Ganglionic blockers → mecamylamine (rare)
├── Adrenergic (mimic SNS)
│ ├── Direct agonists (epinephrine, salbutamol)
│ └── Indirect (amphetamine, ephedrine)
└── Adrenergic blockers (block SNS)
├── Alpha blockers → prazosin, phentolamine
└── Beta blockers → propranolol, metoprolol
Sources:
- Lippincott Illustrated Reviews: Pharmacology, Unit II, Chapters 3-6 - ANS overview, neurotransmitters, receptor pharmacology
- Goodman & Gilman's The Pharmacological Basis of Therapeutics - Cholinergic and adrenergic pharmacology, p. 2121-2163
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