Hussmann_2011_J.Biol.Chem_286_39726

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 alpha4beta2 nAChR binding site density and beta2 subunit protein. Carbachol, a known nAChR and muscarinic receptor agonist, up-regulated both alpha4beta2 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 alpha4 and beta2 mRNA. Muscarinic receptor activation in these cells appears to affect CMV promoter activity only minimally ( approximately 1.2 fold), suggesting that the increase in alpha4 and beta2 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 augmenting nAChR expression in a cell model through mechanisms and targets other than the nAChR receptor itself.