Parasympathomimetic Drugs (Cholinergic Agonists)

Parasympathomimetic Drugs (Cholinergic Agonists) are agents that mimic acetylcholine, stimulating parasympathetic activity to control functions like smooth muscle contraction, gland secretion, and reduced heart rate.

Definition of Parasympathomimetic Drugs:

• Parasympathomimetics are drugs that mimic the action of the parasympathetic nervous system.
• They enhance the effects of acetylcholine (ACh) at muscarinic or nicotinic

Classification with MOA:

• Direct-acting parasympathomimetics: Act directly on cholinergic receptors (usually muscarinic).
• Indirect-acting parasympathomimetics (Anticholinesterases): Inhibit the enzyme acetylcholinesterase, which breaks down ACh, thereby increasing ACh levels at synapses.

Examples:

1. Direct-acting:

   • Pilocarpine – used in glaucoma
   • Bethanechol – used for urinary retention

2. Indirect-acting:

   • Neostigmine – used in myasthenia gravis
   • Physostigmine – used in atropine poisoning

Pharmacological Effects:

1. Eye

• Miosis (pupil constriction): Contraction of circular muscles (sphincter pupillae) via M3 receptors.
• Accommodation: Contraction of ciliary muscle → improved near vision (helps drain aqueous humor in glaucoma).

2. Cardiovascular System

• Bradycardia: Activation of M2 receptors in the SA node decreases heart rate.
• Vasodilation (indirect): Endothelial M3 receptor activation increases nitric oxide (NO) release → smooth muscle relaxation → decreased blood pressure.
• Overall effect: Decreased cardiac output and peripheral vascular resistance.

3. Respiratory System

• Bronchoconstriction: M3 receptor stimulation on bronchial smooth muscle.
• Increased secretion: More mucous production from airway glands.

4. Gastrointestinal Tract

• Increased motility: Enhanced peristalsis via M3 receptors.
• Increased secretions: Salivary, gastric, pancreatic, and intestinal secretions stimulated.
• Relaxation of sphincters: Promotes digestion and defecation.

5. Urinary Tract

• Detrusor muscle contraction (M3 receptor): Promotes urination.
• Internal sphincter relaxation: Facilitates urine outflow.

6. Glands

• Increased secretions: Lacrimal (tears), salivary, sweat, and digestive glands all increase activity.

Clinical Uses:

• Glaucoma
• Urinary retention
• Myasthenia gravis
• Alzheimer’s disease (some central acting anticholinesterases)
• Reversal of neuromuscular blockade

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