Selective activation of the M1 muscarinic acetylcholine receptor achieved by allosteric potentiation

The forebrain cholinergic system promotes higher brain function in part by signaling through the M-1 muscarinic acetylcholine receptor (mAChR). During Alzheimer's disease (AD), these cholinergic neurons degenerate, therefore selectively activating M-1 receptors could improve cognitive function in these patients while avoiding unwanted peripheral responses associated with non-selective muscarinic agonists. We describe here benzyl quinolone carboxylic acid (BQCA), a highly selective allosteric potentiator of the M-1 mAChR. BQCA reduces the concentration of ACh required to activate M-1 up to 129-fold with an inflection point value of 845 nM. No potentiation, agonism, or antagonism activity on other mAChRs is observed up to 100 mu M. Furthermore studies in M-1(-/-) mice demonstrates that BQCA requires M-1 to promote inositol phosphate turnover in primary neurons and to increase c-fos and arc RNA expression and ERK phosphorylation in the brain. Radioligand-binding assays, molecular modeling, and site-directed mutagenesis experiments indicate that BQCA acts at an allosteric site involving residues Y179 and W400. BQCA reverses scopolamine-induced memory deficits in contextual fear conditioning, increases blood flow to the cerebral cortex, and increases wakefulness while reducing delta sleep. In contrast to M-1 allosteric agonists, which do not improve memory in scopolamine-challenged mice in contextual fear conditioning, BQCA induces beta-arrestin recruitment to M-1, suggesting a role for this signal transduction mechanism in the cholinergic modulation of memory. In summary, BQCA exploits an allosteric potentiation mechanism to provide selectivity for the M-1 receptor and represents a promising therapeutic strategy for cognitive disorders.