Anticholinergic Drugs

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Chapter: Medicinal Chemistry : Anticholinergic Drugs

The parasympatholytics or cholinergic blocking agents include atropine and related alkaloids obtained from plants, such as Atropa belladona, A. accuminata, Hyosyamus niger, Scoplola carniolica, Datura strammonium, and synthetic or semisynthetic atropine substitutes.


Anticholinergic Drugs

INTRODUCTION

The parasympatholytics or cholinergic blocking agents include atropine and related alkaloids obtained from plants, such as Atropa belladona, A. accuminata, Hyosyamus niger, Scoplola carniolica, Datura strammonium, and synthetic or semisynthetic atropine substitutes. These drugs in the therapeutic doses predominantly block the muscarinic actions of acetylcholine, but the ganglionic and skeletal neuromuscular actions of acetylcholine are not affected.

Cholinergic receptor blocking drugs or antagonists are divided into muscarinic and nicotinic subgroups on the basis of their specific receptor affinities. The antinicotinic drugs consist of ganglion blockers and neuromuscular junction blockers. The ganglion-blocking drugs have little clinical use. Muscarinic antagonists are often called as parasympatholytics. There are naturally occurring compounds with antimuscarinic effects that have been known and used in medicines and cosmetics as well as in poisons. Atropine is a prototype of these drugs. Many similar plant alkaloids are known and hundreds of synthetic antimuscarinic compounds have been prepared.

Muscarinic receptor antagonists prevent the effects of acetylcholine by blocking its binding to muscarinic cholinergic receptors at neuroeffector sites on smooth muscles, cardiac muscles and gland cells; in peripheral ganglia; and in the central nervous system. In general, M receptor antagonists cause little effect on the N receptor sites. However, quaternary ammonium analogues of atropine and related drugs, generally, exhibit a greater degree of nicotinic blocking activity, and consequently, are more likely to interfere with the ganglions on neuromuscular transmission. In the central nervous system (CNS) both muscarinic and nicotinic receptors are distributed at the spinal, subcortical, and cortical levels in the brain. At a high dose or a toxic dose, the actions in the CNS of the atropine and related drugs, generally, produce stimulation followed by depression.

Parasympathetic neuroeffector junctions in the different organs are not equally sensitive to even the nonselective muscarinic receptor antagonists. A small dose of atropine depresses salivary and bronchial secretion and sweating. With large doses, the pupil dilates, accommodation of lenses to near vision is inhibited. The heart rate is increased due to the vagal block. The actions of many of the clinically available muscarinic receptor antagonists differ only quantitatively from those of atropine considered as the prototype. The currently available classes of drugs include the following:

  1. Naturally occurring alkaloid-atropine and scopolamine (atropine from a plant A. belladona).

  2. Semisynthetic derivatives such as Homatropine, Atropine methonitrate, Ipratropium bromide, and Tiotropium bromide.

  3. Synthetic congeners (show some selectivity for nicotinic and muscarinic receptors).

Out of these synthetic compounds, according to their various therapeutic uses, they have been segregated into mydriatics (cyclopentolate), antisecretory, and antispasmodics (propantheline, oxyphenonium, dicyclomine, and pirenzepine) and also antiparkinson’s drugs (trihexyphenidyl, procyclidine, and biperiden). The cholinergic blocking agents can be grouped pharmacologically according to their location and type of cholinergic receptors into the following:

1. Parasympathetic postganglionic blocking agents or antimuscarinic agents. 

Example: Atropine, Scopolamine.

2. Ganglionic blocking agents.

Example: Hexamethonium, Curare alkaloids, Pancuronium bromide.

3. Neuromuscular blocking agents. 

Example: Tubocurarine, Metocurine.


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