Selective muscarinic receptor agonists and antagonists.

Muscarinic receptors are composed of a family of four subtypes each of which can be distinguished pharmacologically and structurally. The physiological role of each subtype in the central and peripheral nervous systems remains to be clarified, due, in part, to a lack of agonists and antagonists with adequate subtype selectivity. Nonetheless, several agonists with functional selectivity for M1 receptors are now in advanced clinical evaluation for Alzheimer's disease, while selective M1/M3 antagonists may prove useful in the treatment of disorders of smooth muscle function. These novel compounds thus provide an advance over earlier therapeutics with which the clinical efficacy was compromised by the side effect profile. This review attempts to assess novel, selective agonists and antagonists, both in terms of their use in defining muscarinic receptor subtypes and their potential clinical utility.