Role of the Cytoplasmatic M3-M4 Loop for the Homopentameric Assembly of a Chimeric Nicotinic Alpha 7 Receptor

Abstract

The FDA-approved drug bupropion has been prescribed as an antidepressant (Wellbutrin) for over three decades, and more recently as a smoking cessation aid (Zyban). The presumed mechanism of action of bupropion was inhibition of norephinephrine and dopamine reuptake by their respective transporters. Recently, bupropion's non-competitive antagonistic effect was demonstrated in nicotinic acetylcholine receptors (a1bεd, Torpedo, a1bgd, a3b4a55b2, a3b2, a4b2, a7) of the Cys-loop superfamily providing an alternate pharmacological pathway. Our laboratory has shown that another cationselective member of the Cys-loop superfamily, the serotonin type 3 receptor (5-HT 3 -R), is modulated by bupropion at clinically relevant concentrations. Specifically, we determined that bupropion acts as a non-competitive antagonist at 5-HT 3A subunits. 5-HT 3 -Rs are found pre-and postsynaptically, and are currently targeted by anti-emetics and irritable bowel syndrome treatments. They also hold promise as potential future targets for multiple neurological disorders, such as Alzheimer's disease, schizophrenia, and bipolar disorder. The 5-HT 3 -R family consists of five different subunits (A-E) but the assembly of this receptor requires the 3A subunit, yielding either a homomer or heteromer with another subunit. To date, only the interaction of bupropion with the 3A subunit has been studied. Here, we extend our investigations to heteromeric 5-HT 3AB -Rs, which are found in the central and peripheral nervous system, predominantly in the amygdala, caudate nucleus, and hippocampus. The functional interaction of bupropion with 5-HT 3AB was characterized in Xenopus oocytes using two-electrode voltage clamp and patch clamp techniques. Docking studies and site-directed mutagenesis were used to identify the binding site/s in 5-HT 3 -R. Our studies confirmed that bupropion, similar to other non-competitive antagonists, evokes different responses in 5-HT 3AB -Rs as compared to the homomeric 5-HT 3A -Rs.