ABSTRACT
Muscarinic receptors are pharmacologically identified on the basis of selective antagonism by the antagonist atropine (1-3). The classification of muscarinic receptor subtypes, of which there are five, is more problematic. This difficulty arises from a lack of agonists and antagonists selective for one muscarinic receptor subtype over the remaining four. Nonetheless, the therapeutic potential for subtype selective antagonists has ensured that research in the area has continued for at least 25 years, with several compounds in advanced clinical study.
