Drugs Acting on the Autonomic Nervous System
The autonomic nervous system controls involuntary actions such as heart rate, breathing, digestion, glandular secretion, and smooth muscle activity. Drugs acting on this system modify these functions by either stimulating or blocking nerve responses. These medicines are widely used to treat asthma, hypertension, glaucoma, allergic reactions, heart disorders, and gastrointestinal conditions.
Sympathomimetic (Adrenergic Agonists)
Sympathomimetic drugs stimulate the sympathetic nervous system by activating adrenergic receptors. They mimic the actions of the natural neurotransmitters epinephrine and norepinephrine. These drugs may act directly on receptors or indirectly by releasing stored norepinephrine. They are useful in emergencies such as bronchial asthma, cardiac arrest, shock, and severe allergies. They are also used as nasal decongestants and appetite suppressants.
Classification Based on Therapeutic Use
- Vasoconstrictors (raise blood pressure): Norepinephrine, Metaraminol
- Cardiac stimulants: Dopamine, Dobutamine, Xamoterol
- CNS stimulants: Amphetamine
- Smooth muscle relaxants: Epinephrine, Isoprenaline, Isoxsuprine
- Selective β2 stimulants: Salbutamol
- Anti-allergic agents: Epinephrine, Ephedrine
- Local vasoconstrictors: Phenylephrine, Naphazoline
- Appetite suppressants: Fenfluramine, Phentermine
Direct Acting Sympathomimetic Agents
These agents act directly on α or β adrenergic receptors. Some are selective, while others stimulate multiple receptor subtypes.
Examples
- Norepinephrine
- Epinephrine
- Phenylephrine
- Dopamine
- Terbutaline
- Salbutamol
- Naphazoline
- Tetrahydrozoline
Example: Norepinephrine
Norepinephrine acts mainly on α-receptors causing vasoconstriction and helps maintain blood pressure in acute hypotension due to trauma, surgery, or hemorrhage.
Indirect Acting Sympathomimetic Agents
These drugs do not bind receptors directly. They release stored norepinephrine from nerve endings, which then stimulates adrenergic receptors.
Examples
- Hydroxyamphetamine
- Pseudoephedrine
Example: Pseudoephedrine
Pseudoephedrine relieves nasal and sinus congestion by vasoconstriction and also has mild bronchodilator action through β-receptor stimulation.
Mixed Acting Sympathomimetic Agents
These drugs act as both direct agonists and cause release of norepinephrine from nerve terminals. They stimulate α and β receptors and mimic sympathetic responses.
Examples
- Ephedrine
- Metaraminol
Example: Ephedrine
Ephedrine is used in asthma, nasal congestion, allergic reactions, and to manage anesthesia-related low blood pressure.
Adrenergic Antagonists (Sympatholytic Agents)
These drugs block α or β receptors and inhibit the effects of catecholamines. They are used in hypertension, heart diseases, and urinary conditions. They may act selectively or non-selectively and may be reversible or irreversible blockers.
Classification
- α-Blockers: Prazosin, Phenoxybenzamine, Phentolamine
- β-Blockers: Propranolol, Atenolol, Carvedilol
Example: Propranolol
A non-selective β-blocker used for hypertension, anxiety, migraine prevention, and heart rhythm disorders.
Cholinergic Drugs (Parasympathomimetic Agents)
These drugs mimic acetylcholine and stimulate muscarinic or nicotinic receptors. They increase secretions, slow heart rate, constrict pupils, and stimulate gastrointestinal movement.
Classification
- Direct-acting agents: Acetylcholine, Carbachol, Pilocarpine
- Indirect-acting agents (cholinesterase inhibitors): Neostigmine, Edrophonium, Tacrine, Echothiophate
Example: Pilocarpine
Pilocarpine increases secretions and causes pupil constriction, used in glaucoma and dry mouth associated with Sjögren’s syndrome.
Cholinergic Antagonists (Parasympatholytic Agents)
These agents block muscarinic receptors and prevent acetylcholine action. They reduce secretions, relax smooth muscles, raise heart rate, and dilate pupils. They are used in COPD, eye examinations, peptic ulcers, and motion sickness.
Examples
- Atropine Sulphate
- Ipratropium Bromide
- Tropicamide
- Cyclopentolate
- Clidinium Bromide
- Dicyclomine
Example: Ipratropium Bromide
A bronchodilator used for chronic obstructive pulmonary disorders, reducing bronchospasm by blocking muscarinic receptors in the lungs.
