(A) M3-R inhibits β-arrestin-2 recruitment to the OR. β-arrestin-2 recruitment of 2 ORs when different proteins were coexpressed. β-arrestin-2 recruitment to activated ORs was inhibited when M3-R was coexpressed as compared to the control transmembrane protein CD28 (Two-way ANOVA, OR-S6: p=0.01, Olfr62: p=0.03, OR-EG: p<10-3, MOR18-2: p<10-4), or to another muscarinic receptor M2-R (OR-S6: p<10-3, Olfr62: p<10-4, OR-EG: p=0.04, MOR18-2: p<10-4). (B) Activated M3-R inhibits β-arrestin-2 recruitment to the OR. In HEK293T cells coexpressing M3-R, coadministration of M3-R agonist Carbachol (0.1 μM) further decreased β-arrestin-2 recruitment of OR-S6 (p<10-4, Two-way ANOVA, right, purple), while coadministration of M3-R antagonist Atropine (1μM) attenuated the block at high odorant concentrations including 100μM and 1mM (p<10-4, Two-way ANOVA, right, green), as compared with the vehicle DMSO (right, red). These alterations were not observed when M3-R was not coexpressed (p=0.1, p=0.1, Two-way ANOVA, left). n=3, error bars indicate standard errors.