Abstract

Tramadol is a widely used, centrally acting analgesic, but its mechanisms of action are not completely understood. Muscarinic receptors are known to be involved in neuronal function in the brain and autonomic nervous system, and much attention has been paid to these receptors as targets of analgesic drugs in the central nervous system. This study investigated the effects of tramadol on muscarinic receptors by using two different systems, i.e., a Xenopuslaevis oocyte expression system and cultured bovine adrenal medullary cells. Tramadol (10 nM–100 μM) inhibited acetylcholine-induced currents in oocytes expressing the M₁receptor. Although GF109203X, a protein kinase C inhibitor, increased the basal current, it had little effect on the inhibition of acetylcholine-induced currents by tramadol. On the other hand, tramadol did not inhibit the current induced by AlF₄⁻, a direct activator of GTP-binding protein. In cultured bovine adrenal medullary cells, tramadol (100 nM–100 μM) suppressed muscarine-induced cyclic GMP accumulation. Moreover, tramadol inhibited the specific binding of [³H]quinuclidinyl benzilate (QNB). Scatchard analysis showed that tramadol increases the apparent dissociation constant (K_d) value without changing the maximal binding (B_max), indicating competitive inhibition. These findings suggest that tramadol at clinically relevant concentrations inhibits muscarinic receptor function via QNB-binding sites. This may explain the neuronal function and anticholinergic effect of tramadol.