Upregulation of surface alpha 4 beta 2 nicotinic receptors is initiated byreceptor desensitization after chronic exposure to nicotine

It is hypothesized that desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) induced by chronic exposure to nicotine initiates upreg ulation of nAChR number. To test this hypothesis directly, oocytes expressi ng alpha 4 beta 2 receptors were chronically incubated (24-48 hr) in nicoti ne, and the resulting changes in specific [H-3]nicotine binding to surface receptors on intact oocytes were compared with functional receptor desensit ization. Four lines of evidence strongly support the hypothesis. (1) The ha lf-maximal nicotine concentration necessary to produce desensitization (9.7 nM) was the same as that needed to induce upregulation (9.9 nM). (2) The c oncentration of [H-3]nicotine for half-maximal binding to surface nAChRs on intact oocytes was also similar (11.1 nM), as predicted from cyclical dese nsitization models. (3) Functional desensitization of alpha 3 beta 4 recept ors required 10-fold higher nicotine concentrations, and this was mirrored by a 10-fold shift in concentrations necessary for upregulation. (4) Mutant alpha 4 beta 2 receptors that do not recover fully from desensitization, b ut not wild-type channels, were upregulated after acute (1 hr) applications of nicotine. Interestingly, the nicotine concentration required for half-m aximal binding of alpha 4 beta 2 receptors in total cell membrane homogenat es was 20-fold lower than that measured for surface nAChRs in intact oocyte s. These data suggest that cell homogenate binding assays may not accuratel y reflect the in vivo desensitization affinity of surface nAChRs and may ac count for some of the previously reported differences in the efficacy of ni cotine for inducing nAChR desensitization and upregulation.