Selectivity profile of muscarinic toxin 3 in functional assays of cloned and native receptors

By using acetylcholine-induced stimulation of [³⁵S]guanosine-5'-O-(3- thio)triphosphate ([³⁵S]GTPγS) binding to membrane G proteins as a functional assay of the cloned human m1-m4 muscarinic receptor subtypes stably expressed in Chinese hamster ovary cells, muscarinic toxin 3 (MT3) was found to block the m4 receptor with a potency (pA₂ = 8.33) much higher than those displayed at the ml (pA₂ = 6.78), m3 (pA₂ = 6.3), and m2 (pA₂ < 6.3) subtypes. In N1E-115 cells, which have been reported to express m4 receptors coupled to inhibition of cAMP, MT3 potently antagonized the carbachol- induced inhibition of adenylyl cyclase with a pA₂ of 8.81 and displayed monophasic inhibitory curves. Unexpectedly, in NG108-15 cells, known to express only m4 receptors, MT3 counteracted the carbachol inhibition of adenylyl cyclase with a lower potency (pA₂ = 7.60) and showed a biphasic inhibitory curve, suggesting the participation of both m4 and m2 receptors. This possibility was supported by radioligand binding data showing that MT3 failed to completely displace the binding of [³H]Nmethylscopolamine to NG108-15 cell membranes and by reverse transcription-polymerase chain reaction analysis, revealing the presence of mRNAs for both m4 and m2 receptor subtypes. These data demonstrate that MT3 possesses a high functional receptor selectivity for both the cloned and native m4 receptors and that in cell systems containing m4 and m2 receptors coupled to a common response, the toxin constitutes a powerful tool to resolve the relative contribution by each receptor subtype.