The affinity of amitriptyline for muscarine M₁ receptors was studied in the rat cerebral cortex and rabbit vas deferens utilizing binding studies as well as inhibition of carbachol-induced phosphoinositide hydrolysis in the cerebral cortex and blockade of the muscarinic prejunctional inhibition of sympathetic nerve stimulation in the rabbit vas deferens. The inhibition constants (K₁) or dissociation constants (K_b) obtained were approximately 6- to 20-fold lower than those obtained at muscarine M₂ receptors in rat atria (binding and negative inotropic response) indicating that amitriptyline exhibits a degree of muscarine M₁ receptor selectivity.

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