The pharmacology of McN-A-343

Abstract

The unusual pharmacology of McN-A-343 was first described by Roszowski in 1961. The agonist appeared to be a selective stimulant of muscarinic receptors in sympathetic ganglia, now known to be the muscarinic M1 receptor subtype. However, subsequent research demonstrated that McN-A-343 is a partial agonist with similar affinity at all five muscarinic acetylcholine receptor subtypes and its relative selectivity depends on a higher efficacy at the M1 (and M4) subtypes. Being a partial agonist its action is also dependent on factors, such as receptor density and coupling efficacy between receptor activation and tissue response. Nevertheless, the relatively high efficacy at M1 receptors led to its widespread use as an aid to distinguish responses mediated through M1 receptors from those utilizing M2 or M3 muscarinic receptor subtypes, especially in the CNS. There is also evidence that it has an allosteric action at some receptor subtypes. Recently, it was demonstrated that McN-A-343 can bind to an allosteric site on the M2 receptor as well as to the orthosteric site and has thus been termed a “bitopic agonist”. This allosteric site differs from that occupied by allosteric modulators, such as gallamine. Comparison of comparable mutagenic changes in M2 and M4 receptors also suggests that McN-A-343 utilizes different regions of the two receptors for ERK1/2 activation. McN-A-343 has a number of non-muscarinic actions. These include activation of some types of nicotinic acetylcholine receptors, antagonism of serotonin 5-HT3 and 5-HT4 receptor subtypes, inhibition of the uptake1 mechanism and a local anesthetic action.