NICOTINE REGULATES NICOTINIC CHOLINERGIC RECEPTORS AND SUBUNIT MESSENGER-RNAS IN PC-12 CELLS THROUGH PROTEIN-KINASE-A

To understand the up-regulation of neuronal nicotinic cholinergic rece ptors (nAcChRs) that results from chronic in vivo treatment with nicot ine, we studied the effect of nicotine on [H-3]nicotine binding sites on PC 12 cells. PC 12 cells were grown in nicotine hemisulfate (10(-6) to 10(-3) M) or vehicle for 7 days, and specific [H-3]nicotine bindin g was measured. Nicotine (10(-6) to 10(-4) M) dose-dependently increas ed specific binding by up to 2.6-fold over basal levels in 5-7 days, w hereas a 10(-3) M concentration failed to do so. In contrast, [3H]nico tine binding to PC 12 cell mutants (A126.1B2 and A123.7), deficient in cAMP-responsive protein kinase A Types I and/or II, was unaffected by nicotine. Northern gel analysis of nAcChR subunit mRNAs from wild typ e PC 12 cells showed that the mRNA encoding the dominant agonist-bindi ng subunit, alpha 3, was significantly reduced by nicotine, as early a s 4 h after treatment, whereas mRNA for the structural beta 2 subunit was slightly increased. In contrast, the alpha 3 subunit mRNA from the PC 12 cell mutant A123.7 was not significantly decreased after 4 h an d 7 days of nicotine treatment. These studies indicate that nicotine u p-regulates expression of nAcChRs on wild type PC 12 cells and reduces the content of alpha 3 subunit mRNA; these effects require an intact protein kinase A system. The divergent effects of nicotine on the nAcC hR compared to its alpha 3 subunit mRNA suggests that enhanced express ion of nicotinic receptors may not involve synthesis of new receptor s ubunit proteins.