Endogenously Expressed Muscarinic Receptors in HEK293 Cells Augment Up-regulation of Stably Expressed α4β2 Nicotinic Receptors

Nicotine-induced up-regulation of neuronal nicotinic receptors (nAChRs) has been known and studied for more than 25 years. Other nAChR ligands can also up-regulate nAChRs, but it is not known if these ligands induce up-regulation by mechanisms similar to that of nicotine. In this study, we compared up-regulation by three different nicotinic agonists and a competitive antagonist of several different nAChR subtypes expressed in HEK293 cells. Nicotine markedly increased alpha 4 beta 2 nAChR binding site density and beta 2 subunit protein. Carbachol, a known nAChR and muscarinic receptor agonist, up-regulated both alpha 4 beta 2 nAChR binding sites and subunit protein 2-fold more than did nicotine. This increased up-regulation was shown pharmacologically to involve endogenously expressed muscarinic receptors, and stimulation of these muscarinic receptors also correlated with a 2-fold increase in alpha 4 and beta 2 mRNA. Muscarinic receptor activation in these cells appears to affect CMV promoter activity only minimally (similar to 1.2 fold), suggesting that the increase in alpha 4 and beta 2 nAChR mRNA may not be dependent on enhanced transcription. Instead, other mechanisms may contribute to the increase in mRNA and a consequent increase in receptor subunits and binding site density. These studies demonstrate the possibility of augmentingn ChR expression in a cell model through mechanisms and targets other than the nAChR receptor itself.