Abstract

There is a consensus that high-affinity [3H]-L-nicotine binding sites in the mammalian brain, which are thought to represent a predominant form of central nervous system nicotinic acetylcholine receptor (nAChR) composed of alpha 4 and beta 2 subunits, are increased in number after chronic nicotine exposure. However, mechanisms responsible for this effect have not yet been elucidated. To evaluate this issue, we have used, as models, primary cell cultures of fetal rat brain cortex, in which high-affinity [3H]-L-nicotine binding sites are naturally expressed, and clonal cell cultures of fibroblasts stably transfected to express nAChR composed of transgenic chick alpha 4 and beta 2 subunits under control of dexamethasone-inducible promoters. Chronic nicotine exposure induced an approximately 2.5-fold increase in high-affinity [3H]-L-nicotine binding sites in M10 cells maintained in the presence of dexamethasone or in primary fetal rat brain cortical cultures. Up-regulation of [3H]-L-nicotine binding sites was evident for M10 cells treated at nicotine concentrations as low as 10 nM (EC50 and EC100 values were 100 nM and 10 microM, respectively). Scatchard analyses of [3H]-L-nicotine binding data in M10 cells indicated a change in Bmax with no significant change in affinity for radioligand (KD = 2.5 +/- 0.5 nM in control cells vs. 2.0 +/- 0.4 nM in nicotine-treated cells). Northern blot analyses indicated that nicotine treatment alone had no direct effect on the promoter driving transgenic nAChR subunit gene transcription in M10 cells and that steady state levels of fetal rat brain cortical cell or M10 cell nAChR alpha 4 or beta 2 mRNAs were unaffected under conditions of chronic nicotine treatment that produced up-regulation of high-affinity [3H]-L-nicotine binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)