Cholinergic Agent – Effects, Uses and Side Effects
A cholinergic agent is a substance that activates the parasympathetic nervous system by mimicking or enhancing the effects of the neurotransmitter acetylcholine.
Things worth knowing about "Cholinergic Agent"
A cholinergic agent is a substance that activates the parasympathetic nervous system by mimicking or enhancing the effects of the neurotransmitter acetylcholine.
What Is a Cholinergic Agent?
A cholinergic agent is a pharmacological substance that promotes the activity of the cholinergic nervous system. This system is a branch of the autonomic (vegetative) nervous system and uses the neurotransmitter acetylcholine to transmit signals between nerve cells and target organs. Cholinergic agents are used to produce effects associated with the parasympathetic nervous system – often referred to as the rest and digest system – which governs relaxation, digestion, and regeneration.
Mechanism of Action
Cholinergic agents work through two main mechanisms:
- Direct cholinergic agents bind directly to acetylcholine receptors (muscarinic or nicotinic receptors) and activate them, mimicking the effect of natural acetylcholine.
- Indirect cholinergic agents (cholinesterase inhibitors) block the enzyme acetylcholinesterase, which normally breaks down acetylcholine. This increases the concentration and duration of acetylcholine in the synaptic cleft.
The two main receptor types involved are:
- Muscarinic receptors: found in smooth muscle, cardiac muscle, and glands.
- Nicotinic receptors: found in skeletal muscle and the central nervous system.
Medical Uses
Cholinergic agents are used in a wide range of medical fields:
- Ophthalmology: treatment of glaucoma by constricting the pupil and improving aqueous humor drainage.
- Neurology: cholinesterase inhibitors such as donepezil and rivastigmine are used in Alzheimer dementia to compensate for the loss of acetylcholine in the brain.
- Anaesthesia and surgery: reversal of neuromuscular blockade after surgical procedures.
- Gastroenterology: stimulation of gastrointestinal motility in postoperative ileus or gastroparesis.
- Urology: management of urinary retention or neurogenic bladder dysfunction.
Examples of Cholinergic Agents
- Pilocarpine – direct cholinergic agent used in ophthalmology
- Neostigmine – indirect cholinergic agent used in myasthenia gravis and reversal of muscle relaxants
- Donepezil – cholinesterase inhibitor for Alzheimer dementia
- Rivastigmine – cholinesterase inhibitor for Alzheimer and Parkinson dementia
- Bethanechol – direct cholinergic agent for bladder dysfunction
Side Effects
Because cholinergic agents affect the entire parasympathetic nervous system, unwanted effects can occur. Common side effects include:
- Increased salivation, lacrimation, and sweating
- Nausea, vomiting, and diarrhoea
- Slowed heart rate (bradycardia)
- Narrowing of the airways (bronchoconstriction)
- Urinary urgency or incontinence
- Visual disturbances due to pupil constriction (miosis)
These effects are commonly summarised by the acronym SLUDGE: Salivation, Lacrimation, Urination, Defaecation, Gastrointestinal distress, Emesis.
Contraindications
Cholinergic agents should be used with caution or avoided in patients with:
- Bronchial asthma or chronic obstructive pulmonary disease (COPD)
- Bradycardia or atrioventricular (AV) block
- Peptic ulcer disease or gastrointestinal obstruction
- Hyperthyroidism
References
- Katzung B.G., Trevor A.J. – Basic and Clinical Pharmacology, 15th edition (2021). McGraw-Hill Education.
- Rang H.P. et al. – Rang and Dale's Pharmacology, 9th edition (2019). Elsevier.
- World Health Organization (WHO) – Model Formulary for Adults (2023). Available at: https://www.who.int
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