Agonist- and antagonist-induced up-regulation of surface 5-HT₃A receptors

BACKGROUND AND PURPOSE: The 5-hydroxytryptamine type 3 (5-HT3 ) receptor is a member of the pentameric ligand-gated ion channel (pLGIC) family and are pharmacologically targeted to treat irritable bowel syndrome and nausea / emesis. Furthermore, many antidepressants elevate extracellular concentrations of serotonin. This study investigates the functional consequences of exposure of recombinant 5-HT3 A receptors to agonists and antagonists.EXPERIMENTAL APPROACH: We used human embryonic kidney (HEK) cells stably expressing recombinant 5-HT3 A receptors and the ND7/23 neuroblastoma / dorsal root ganglion hybrid cell line, which expresses endogenous 5-HT3 receptors. Surface expression of recombinant 5-HT3 A receptors, modified to contain the bungarotoxin (BTX) binding sequence, was quantified using fluorescence microscopy to image BTX-conjugated fluorophores. Whole cell voltage-clamp electrophysiology was used to measure the density of current mediated by 5-HT3 A receptors.KEY RESULTS: Our results indicate that 5-HT3 A receptors are up-regulated by the prolonged presence of agonists (5-HT and mCPBG) and antagonist (MDL-72222 and morphine). The up-regulation of 5-HT3 A receptors by 5-HT and MDL-72222 is time and concentration-dependent, but independent of newly translated receptors. The phenomenon was observed for recombinant rodent and human 5-HT3 A receptors and for endogenous 5-HT3 receptors in neuronal ND7/23 cells.CONCLUSIONS AND IMPLICATIONS: Our observation that up-regulation of 5-HT3 A receptors occurs after exposure to either agonists or antagonists suggests that this phenomenon may occur in response to different therapeutic agents. Medications that elevate serotonin levels, such as the antidepressant inhibitors of serotonin reuptake and antiemetic inhibitors of 5-HT3 receptor function may both cause elevated receptor expression. However, this will require further investigation in vivo.