Medical Pharmacology Chapter 6: Autonomic Pharmacology: Cholinergic Drugs
Introduction: Muscarinic Receptor Antagonists
Antimuscarinic agents were of plant origin.
Belladonna (beautiful woman, a reference to the drug's mydriatic effects,) are found in many plants.
Atropa belladonna (Solanaceae) or the deadly nightshade contains atropine (dl-hyoscyamine) as does Datura stramonium (Jamestown or jimsonweed, thorn-apple, etc.)
Scopolamine, also an alkaloid, is found in the shrub Hyosyamus niger and Scopolia carniolica.
An alkaloid is one of a large group of organic, basic plant substances. They are usually pharmacologically active and bitter in taste
Tertiary and Quaternary Antimuscarinic Agents
Atropine, scopolamine, and the semisynthetic agent homatropine (Isopto Homatropine) are tertiary amines, generally well-absorbed and able to penetrate the CNS.
Each drug can be converted to a quaternary form by addition of a methyl group to the nitrogen, resulting in methylatropine nitrate, methscopolamine bromide and homatropine methybromide.
Quaternary muscarinic receptor antagonists tend to be more potent as muscarinic blockers and have increased ganglionic blocking action.
Quaternary (permanently charged) antagonists do not penetrate the CNS to a significant extent. Therefore, CNS activity is limited.
Brown, J.H. and Taylor, P. Muscarinic Receptor Agonists and Antagonists, In, Goodman and Gillman's The Pharmacological Basis of Therapeutics, (Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) The McGraw-Hill Companies, Inc.,1996, pp.149-150.
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