Background and purpose: To compare loss in binding to muscarinic receptor (mAChR) subtypes with their known functions, the total density of muscarinic receptors was measured in peripheral tissues from wild type (WT) and mAChR knockout (KO) mice.
Experimental approach: Binding parameters of [N-methyl-3H]scopolamine methyl chloride ([3H]NMS) were determined in 10 peripheral tissues of WT and M1-M5 receptor KO mice. Competition between [3H]NMS and darifenacin (selective M3 receptor antagonist) was also measured.
Key results: There was an extensive loss of [3H]NMS-binding sites (maximal number of binding sites, Bmax) in heart and smooth muscle from M2KO mice, compared with WT mice. Smooth muscle from M3KO mice also showed a moderate loss of Bmax. Bmax fell in pancreas and bladder of M4KO mice and in prostate in M1KO and M3KO mice. There was a large loss of Bmax in exocrine and endocrine glands of M3KO mice with a moderate decrease in M2KO mice. Darifenacin inhibited specific [3H]NMS binding in submandibular gland and bladder of WT, M2KO and M3KO mice. Ki (inhibition constant) values for darifenacin in the submandibular gland were the same in WT and M2KO mice but increased in M3KO mice. However, Ki values in bladder were decreased in M2KO mice and increased in M3KO mice.
Conclusions and implications: Single mAChR KO mice exhibit a loss of mAChR in peripheral tissues that generally paralleled the reported loss of function. Quantitative analysis of data, however, also suggested that, in some instances, normal expression of a receptor subtype depended on expression of other subtypes.