Learning Objective
By the end of this section, students should be able to differentiate direct and indirect cholinomimetic agents, recognize their mechanisms of action, therapeutic applications, and clinical pearls—a high-yield topic for USMLE Step 1 pharmacology.
Cholinomimetic Agents
Clinical caution: Watch for exacerbation of COPD, asthma, and peptic ulcers in susceptible patients.
1. Direct-Acting Muscarinic Agonists
| Drug | Mechanism | Clinical Application | Mnemonic/Tip |
|---|---|---|---|
| Bethanechol | Activates muscarinic receptors in bladder smooth muscle; resistant to AChE | Urinary retention | “Bethany, call me to activate your bladder.” |
| Carbachol | Mimics acetylcholine; resistant to AChE; acts on muscarinic receptors | Induces pupil constriction (miosis) during surgery | “Carbon copy of ACh” |
| Methacholine | Stimulates muscarinic receptors in the airways when inhaled | Asthma challenge test | High-yield Step 1 question: triggers bronchoconstriction |
| Pilocarpine | Contracts ciliary muscle (open-angle glaucoma) and pupillary sphincter (closed-angle glaucoma); resistant to AChE; crosses BBB | Open- and closed-angle glaucoma, xerostomia (Sjögren syndrome) | “You cry, drool, and sweat on your pilow” |
2. Indirect-Acting (Anticholinesterases)
| Drug | Mechanism | Clinical Application | Mnemonic/Tip |
|---|---|---|---|
| Donepezil, Rivastigmine, Galantamine | ↑ ACh by inhibiting AChE | Alzheimer’s disease (first-line) | “Dona Riva forgot the gala.” |
| Neostigmine | ↑ ACh; does not cross BBB (positively charged) | Postoperative/neurogenic ileus, urinary retention, myasthenia gravis, reversal of NMJ blockade | “Neo = no CNS penetration.” |
| Pyridostigmine | ↑ ACh at NMJ; does not penetrate CNS | Long-acting myasthenia gravis treatment; often combined with glycopyrrolate or hyoscyamine to reduce side effects | “Pyridostigmine gets rid of myasthenia gravis.” |
| Physostigmine | ↑ ACh; crosses BBB freely | Antidote for anticholinergic toxicity (e.g., atropine overdose) | “Phyxes atropine overdose” |
High-Yield Step 1 Tips
- Direct agonists: Bind muscarinic receptors directly → rapid effect
- Indirect agonists: Inhibit acetylcholinesterase → ↑ endogenous ACh
- CNS penetration: Pilocarpine and physostigmine cross the BBB; neostigmine and pyridostigmine do not
- Mnemonic: “Bethany calls, Carbon copies, Meth blows, Pillow sweats” for direct agonists
