Background and Objectives: In addition to their blockade of voltage-dependent sodium channels, the action of local anesthetics at 5-hydroxytryptamine-3 (5-HT₃) receptors may be clinically relevant. Because local anesthetics have different clinical properties, we have tested the hypothesis that differences in interactions at the 5-HT₃ receptor may be clinically relevant by investigating the effects of 4 local anesthetics on recombinant wild-type and 4 mutant 5-HT_3A receptors.

Methods: The cRNAs from human wild-type and 4 mutant 5-HT_3A subunit clones were synthesized in vitro and expressed in Xenopus oocytes. Four mutant receptors were obtained by site-directed mutagenesis in the N-terminal extracellular region, which contains the agonist binding domain. Tryptophan (W) at positions 62 and 155 were replaced by tyrosine (Y) and glutamate (E) at position 101 by aspartate (D) or asparagine (N). The 2-electrode voltage clamp technique was used to measure peak currents induced by 5-HT in these receptors in the presence and absence of local anesthetics.

Results: All local anesthetics inhibited 5-HT-induced currents in a dose-dependent manner in the wild-type receptor. Inhibition by procaine and tetracaine were competitive whereas those of bupivacaine and lidocaine were both noncompetitive and competitive. The 4 mutants (W62Y, W155Y, E101D, E101N) could all form functional receptors. All mutant receptors exhibited a major increase (> 10-fold) in the half-maximum inhibitory concentration for procaine. The half-maximum inhibitory concentrations of tetracaine, bupivacaine, and lidocaine in mutant receptors were increased 2- to 3-fold except that of tetracaine in W62Y receptor (6-fold).

Conclusions: The ester type local anesthetics, procaine and tetracaine, may act at a different site on the 5-HT_3A receptor and with a different mechanism than the amide-type local anesthetics. Clinical differences between local anesthetics may be at least partially due to differences in interactions at the 5-HT_3A receptor.