Chronic nicotine treatment up-regulates alpha 3 and alpha 7 acetylcholine receptor subtypes expressed by the human neuroblastoma cell line SH-SY5Y

Chronic exposure to nicotine has been reported to increase the number of nicotinic acetylcholine receptors (AChRs) in brain. The mechanism of up-regulation for the alpha 4 beta 2 AChR subtype, which accounts for the majority of high affinity nicotine binding in mammalian brain, has previously been shown to involve a decrease in the rate of alpha 4 beta 2 AChR turnover. Here, we report an investigation of the extent and mechanism of nicotine-induced up-regulation of alpha 3 AChRs and alpha 7 AChR subtypes expressed in the human neuroblastoma cell line SH-SY5Y. Up-regulation of human alpha 3 AChRs and alpha 7 AChRs, unlike alpha 4 beta 2 AChRs, requires much higher nicotine concentrations than are encountered in smokers; the extent of increase of surface AChRs is much less; and the mechanisms of up-regulation are different than with alpha 4 beta 2 AChRs. The mechanisms of up-regulation may be different for alpha 3 AChRs or alpha 7 AChRs. Chronic treatment with nicotine or carbamylcholine, but not d-tubocurarine, mecamylamine, or dihydro-p-erythroidine, induced a 500-600% increase in the number of alpha 3 AChRs but only a 30% increase in alpha 7 AChRs. Chronic nicotine treatment did not increase affinity for nicotine or increase the amount of RNA for alpha 3 or alpha 7 subunits. The effect of nicotine on up-regulation of alpha 7 AChRs was partially blocked by either d-tubocurarine or mecamylamine. The effect of nicotine treatment on the number of alpha 3 AChRs was only slightly blocked by the antagonists d-tubocurarine, mecamylamine, or dihydro-beta-erythroidine at concentrations that efficiently block alpha 3 AChR function. Most of the nicotine-induced increase in alpha 3 AChRs was found to be intracellular. The alpha 3 AChRs, which accumulate intracellularly, were shown to have been previously exposed on the cell surface by their susceptibility to antigenic modulation. The data suggest that chronic exposure to nicotine may induce a conformation of cell surface alpha 3 AChRs that at least in this cell line are consequently internalized but not immediately destroyed.