Chronic Nicotine Treatment Up-Regulates a3 and a7 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 a4b2 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 a4b2 AChR turnover. Here, we report an investigation of the extent and mechanism of nicotine-induced up-regulation of a3 AChRs and a7 AChR subtypes expressed in the human neuroblastoma cell line SH-SY5Y. Up-regulation of human a3 AChRs and a7 AChRs, unlike a4b2 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 a4b2 AChRs. The mechanisms of up-regulation may be different for a3 AChRs or a7 AChRs. Chronic treatment with nicotine or carbamylcholine, but not d-tubocurarine, mecamylamine, or dihydro-b-erythroidine, induced a 500–600% increase in the number of a3 AChRs but only a 30% increase in a7 AChRs. Chronic nicotine treatment did not increase affinity for nicotine or increase the amount of RNA for a3 or a7 subunits. The effect of nicotine on up-regulation of a7 AChRs was partially blocked by either d-tubocurarine or mecamylamine. The effect of nicotine treatment on the number of a3 AChRs was only slightly blocked by the antagonists d-tubocurarine, mecamylamine, or dihydro-b-erythroidine at concentrations that efficiently block a3 AChR function. Most of the nicotineinduced increase in a3 AChRs was found to be intracellular. The a3 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 a3 AChRs that at least in this cell line are consequently internalized but not immediately destroyed.