Co-expression of the 5-HT_3B serotonin receptor subunit alters the biophysics of the 5-HT₃ receptor

Homomeric complexes of 5-HT_3A receptor subunits form a ligand-gated ion channel. This assembly does not fully reproduce the biophysical and pharmacological properties of native 5-HT₃ receptors which might contain the recently cloned 5-HT_3B receptor subunit. In the present study, heteromeric assemblies containing human 5-HT_3A and 5-HT_3B subunits were expressed in HEK 293 cells to detail the functional diversity of 5-HT₃ receptors. We designed patch-clamp experiments with homomeric (5-HT_3A) and heteromeric (5-HT_3AB) receptors to emphasize the kinetics of channel activation and desensitization. Co-expression of the 5-HT_3B receptor subunit reduced the sensitivity for 5-HT (5-HT_3A receptor: EC₅₀ 3 μM, Hill coefficient 1.8; 5-HT_3AB receptor: EC₅₀ 25 μM, Hill coefficient 0.9) and markedly altered receptor desensitization. Kinetic modeling suggested that homomeric receptors, but not heteromeric receptors, desensitize via an agonist-induced open-channel block. Furthermore, heteromeric 5-HT_3AB receptor assemblies recovered much faster from desensitization than homomeric 5-HT_3A receptor assemblies. Unexpectedly, the specific 5-HT₃ receptor agonist mCPBG induced an open-channel block at both homomeric and heteromeric receptors. Because receptor desensitization and resensitization massively affect amplitude, duration, and frequency of synaptic signaling, these findings are evidence in favor of a pivotal role of subunit composition of 5-HT₃ receptors in serotonergic transmission.