Choline Promotes Nicotinic Receptor α4+β2 Up-regulation

Neuronal nicotinic acetylcholine receptors ( nAChR) composed of alpha 4 + beta 2 subunits, the high affinity nicotine-binding site in the mammalian brain, up-regulate in response to chronic nicotine exposure. The identities of endogenous mediators of this process are unknown. We find that choline also up-regulates alpha 4 + beta 2 nAChRs stably expressed by HEK293 cells as measured by increased [H-3] epibatidine density. Choline-mediated up-regulation is dose-dependent and corresponds with an increase in beta 2 subunit protein expression. The choline kinase inhibitor hemicholinium-3 inhibits similar to 60% of choline-mediated up-regulation revealing both an HC3-dependent and -independent pathway. Furthermore, choline-mediated up-regulation is not additive with up-regulation agents such as nicotine, but it is additive with weaker promoters of the up-regulation process. When co-applied with the pro-inflammatory cytokine tumor necrosis factor alpha, choline- mediated up- regulation is increased further through a mechanism that includes an increase in both alpha 4 and beta 2 protein expression, and this is inhibited by the p38 MAPK inhibitor SB202190. These findings extend the view that up- regulation of alpha 4 + beta 2 nAChRs is a normal physiological response to altered metabolic and inflammatory conditions.