The muscarinic M₄ receptor is the functionally predominant subtype in rat and mouse striatum as demonstrated using [³⁵S] GTPγS binding

We have used selective muscarinic receptor antagonists and M₂ and M₄ receptor knockout (KO) mouse tissue to define the functional muscarinic acetylcholine receptor populations in rodent striatum. [ ³H] NMS binding studies in rat and mouse striatum demonstrated that approximately 30% of muscarinic acetylcholine receptors expressed are M ₁ receptors. Radioligand binding studies suggest that the remaining muscarinic acetylcholine receptor population is largely M₄ with small levels of M₂. In agreement, carbachol-induced GTPγS binding studies in M₂ and M₄ receptor KO mouse striatum implicated the M₄ receptor as the predominant functional receptor subtype. Based on these data we have developed a novel, native tissue M₄ receptor [³⁵S] GTPγS binding assay. Pharmacological assessment of M₄ receptor agonist and positive 3modulators revealed clear differences in the potencies observed in a human recombinant CHO-M₄ receptor [³⁵S] GTPγS binding assay as compared to the native tissue [³⁵S] GTPγS binding assay. These differences are believed to reflect differences in receptor reserve between the assay systems as well as differences in compound pharmacology (relative contribution of compound affinity and efficacy to observed potency). These studies have demonstrated the importance of understanding the pharmacology of test compounds in a native environment when predicting in vivo response.