An agonist that is selective for adenylate cyclase-coupled muscarinic receptors

Compound BM5 [N-methyl-N(1-methyl-4-pyrrolidino-2-butynyl) acetamide] has previously been described as an agonist at postsynaptic muscarinic receptors and as an antagonist at presynaptic receptors. In the current work, we studied the ability of this compound to selectively stimulate phosphoinositide (PI) turnover in Chinese hamster ovary cells transfected with m1 muscarinic receptors and in SK-N-SH neuroblastoma cells that express only m3 receptors. We also studied the ability of this compound to stimulate adenylate cyclase inhibition in m2 muscarinic receptors from heart tissue and in m4 receptors expressed in NG108-15 cells. BM5 stimulated the two muscarinic receptor subtypes coupled to adenylate cyclase inhibition. In NG108-15 cells, 100 microM BM5 inhibited prostaglandin E1-stimulated cAMP formation by 36 +/- 1.5%, whereas 100 microM of the full agonist oxotremorine-M inhibited cAMP formation by 64.1 +/- 1.9%. The half-maximal concentration for BM5 inhibition of cAMP formation was 0.4 +/- 0.1 microM. In heart membranes, BM5 inhibited isoproterenol-stimulated adenylate cyclase by 24 +/- 2%, whereas oxotremorine inhibited this activity by 34 +/- 3%. In contrast to its activity at these receptor subtypes, BM5 did not stimulate the m1 or m3 receptor subtypes, which couple to PI turnover. In these latter two subtypes, BM5 inhibited oxotremorine-M-stimulated PI turnover with IC50 values of 10-20 microM. Therefore, BM5 is a partial agonist at adenylate cyclase-coupled muscarinic receptor subtypes and is a pure antagonist at PI turnover-coupled muscarinic receptor subtypes. These studies also suggest that, at least in some parts of the brain, postsynaptic muscarinic receptors are coupled to adenylate cyclase, whereas presynaptic muscarinic receptors are coupled to PI turnover.