Anticholinergic agents Muscarinic receptor

Mechanism of Action A peripheral anticholinergic agent that has limited ability to cross the blood-brain barrier and provides a peripheral blockade of muscarinic receptors. Therapeutic Effect Reduces the volume and the total acid content of gastric secretions, inhibits salivation, and reduces gastrointestinal motility. 

Anticholinergic effects Many of the Hj-antagonists also tend to inhibit responses to acetylcholine that are mediated by muscarinic receptors. The newer agents, terfenadine and astemizole have no effect on muscarinic receptors. 

Hyoscine (scopolamine), a prototypic muscarinic receptor antagonist, is one of the best agents for the prevention of motion sickness. However, it has a very high incidence of anticholinergic effects when given orally or parenterally. It is better tolerated as a transdermal patch. Superiority to dimenhydrinate has not been proved. 

The anticholinergic bronchodilators include ipratropium and tiotropium, which are muscarinic receptor blockers that are similar in structure and function to atropine. Although atropine is the prototypical muscarinic antagonist, its use in respiratory conditions is usually limited because it is readily absorbed into the systemic circulation and tends to produce many side effects even when administered by inhalation. Alternatively, ipratropium (Atrovent) is an anticholinergic agent that is poorly absorbed into the systemic circulation and can be administered by an aerosol inhaler.110 Thus, inhaled ipratropium is associated with substantially fewer systemic side effects. 

Tertiary-amine muscarinic receptor antagonists gain access to the central nervous system and are therefore the anticholinergic drugs used to treat parkinsonism and the extrapyramidal side effects of antipsychotic drugs. Specific agents used primarily for these conditions include benztropine mesylate (Cogentin) and trihexyphenidyl hydrochloride (Artane, others). 

In the past 40 years a wide variety of synthetic and semi-synthetic anticholinergic agents have been developed for their selectivity in blocking muscarinic receptors in the brain (see Figure 13.8 for the structure of some... 

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