Muscarinic blocking drugs are compounds that se-lectively antagonize the responses to acetylcholine (ACh) and other parasympathomimetics that are medi-ated by activation of muscarinic receptors. These agents are also referred to as muscarinic antagonists, antimus-carinic drugs, and anticholinergics. The belladonna alka-loids, such as atropine, are the oldest known muscarinic blocking compounds, and their medicinal use preceded the concept of neurochemical transmission.
The best known of the muscarinic blocking drugs are the belladonna alkaloids, atropine (Atropine) and scopol-amine (Scopolamine). They are tertiary amines that con-tain an ester linkage. Atropine is a racemic mixture of DL-hyoscyamine, of which only the levorotatory isomer is pharmacologically active. Atropine and scopolamine are parent compounds for several semisynthetic derivatives, and some synthetic compounds with little structural sim-ilarity to the belladonna alkaloids are also in use. All of the antimuscarinic compounds are amino alcohol esters with a tertiary amine or quaternary ammonium group.
The control of access to muscarinic receptors in the cen-tral nervous system (CNS) by a tertiary amine versus quaternary ammonium group is fundamentally impor-tant in selecting among antimuscarinic agents.