MT-7 (1 - 30 nM), a peptide toxin isolated from the venom of the green mamba Dendroaspis angusticeps and previously found to bind selectively to the muscarinic M(1) receptor, inhibited the acetylcholine (ACh)-stimulated [(35)S]-guanosine-5'-O-(3-thio)triphosphate ([(35)S]-GTPgammaS) binding to membranes of Chinese hamster ovary (CHO) cells stably expressing the cloned human muscarinic M(1) receptor subtype. MT-7 failed to affect the ACh-stimulated [(35)S]-GTPgammaS binding in membranes of CHO cells expressing either the M(2), M(3) or M(4) receptor subtype. In N1E-115 neuroblastoma cells endogenously expressing the M(1) and M(4) receptor subtypes, MT-7 (0.3 - 3.0 nM) inhibited the carbachol (CCh)-stimulated inositol phosphates accumulation, but failed to affect the CCh-induced inhibition of pituitary adenylate cyclase activating polypeptide (PACAP) 38-stimulated cyclic AMP accumulation. In both CHO/M(1) and N1E-115 cells the MT-7 inhibition consisted in a decrease of the maximal agonist effect with minimal changes in the agonist EC(50) value. In CHO/M(1) cell membranes, MT-7 (0.05 - 25 nM) reduced the specific binding of 0.05, 1.0 and 15 nM [(3)H]-N-methylscopolamine ([(3)H]-NMS) in a concentration-dependent manner, but failed to cause a complete displacement of the radioligand. Moreover, MT-7 (3 nM) decreased the dissociation rate of [(3)H]-NMS by about 5 fold. CHO/M(1) cell membranes preincubated with MT-7 (10 nM) and washed by centrifugation and resuspension did not recover control [(3)H]-NMS binding for at least 8 h at 30 degrees C. It is concluded that MT-7 acts as a selective noncompetitive antagonist of the muscarinic M(1) receptors by binding stably to an allosteric site.