Novel allosteric effects of amiodarone at the muscarinic M₅ receptor

Allosteric sites on muscarinic receptors may present superior therapeutic targets for several central nervous system disorders, due to the potential of allosteric ligands to provide more selective modulation and to preserve the spatiotemporal patterning that is characteristic of synaptic transmission. We have found that the antiarrhythmic drug amiodarone interacts allosterically with M₁ and M₅ muscarinic receptors. At both M₁ and M₅, amiodarone was only able to partially inhibit the binding of the orthosteric antagonist [³H]N-methylscopolamine (NMS). In addition, amiodarone was able to alter the rate of dissociation of [³H]NMS from M₁ and M₅ receptors. These findings suggest that NMS and amiodarone are able to bind to the receptor simultaneously. The pharmacology of the effect on NMS dissociation demonstrated that amiodarone was not interacting at the "common" site at which gallamine, obidoxime, and many other muscarinic allosteric ligands are known to bind. In functional studies, amiodarone enhanced the ability of acetylcholine (at EC₂₀) to activate the M₅ receptor; however, under the same conditions, amiodarone did not enhance M₁ activation. More detailed studies at M₅ found that the effect of amiodarone was to enhance the efficacy of acetylcholine, without increasing its potency. This report describes the first demonstration of allosteric enhancement of efficacy at the M₅ receptor, and the first demonstration of enhancement of efficacy but not potency at any muscarinic receptor. In summary, amiodarone has been shown to be a novel positive allosteric modulator of muscarinic receptors that is selective for the M₅ subtype, relative to M₁.