Antagonism of the five cloned human muscarinic cholinergic receptors expressed in CHO-K1 cells by antidepressants and antihistaminics

Based on molecular cloning studies, five different muscarinic receptor subtypes exist: m1, m2, m3, m4, and m5. We determined the affinity and selectivity of binding for sixteen antidepressants, two of their metabolites, and three antihistaminics (h₁) at these subtypes. Using Chinese hamster ovary cells (CHO-K1) transfected with genes for the human muscarinic receptor subtypes, we obtained equilibrium dissociation constants (K_ds) from competitive radioligand binding studies with [³H]-quinuclidinyl benzilate ([³H]QNB) and membranal preparations of these cells. QNB was the most potent compound studied (K_d 30-80 pM). Mequitazine (K_d 6-14 nM) and amitriptyline (K_d 7-16 nM) exhibited the highest affinity among the antihistaminics and antidepressants, respectively. Among the antidepressants examined were the serotonin-selective drugs sertraline and fluoxetine, both of which displayed K_d values > 1 μM. The remaining antidepressants were moderate to weak antagonists with some eliciting no radioligand competition at high concentrations. The compounds studied showed no significant selectivity among the five cloned subtypes.