Nicotinic receptor inactivation after acute and repeated in vivo nicotine exposures in rats

Nicotine tolerance is often accompanied by an upregulation of brain area nicotinic acetylcholine receptors (nAChRs) in both animal and human subjects. This upregulation has been hypothesized to result from repeated or prolonged exposures of these receptors to nicotine. To explore this further, this study examined the level of nAChR desensitization following acute and repeated nicotine administration in the male Lewis rat. Nicotine-stimulated 86Rb+ efflux was measured in synaptosomes prepared from the frontal cortex, hippocampus, striatum, and thalamus. Analysis of receptor functionality was achieved by calculating area-under-the-curve (AUC) for nicotine-induced fractional 86Rb+ efflux. Nicotine-stimulated 86Rb+ efflux from all brain regions was significantly less in rats that received an acute injection of 0.4 mg/kg nicotine (s.c.) 15 min prior to dissection compared to control rats. This decrease in nAChR functional status was also observed in rats treated with 1 day or 14 days of twice-daily nicotine administration. These results are consistent with the concept that acute exposure to nicotine induces rapid desensitization of nAChRs. In addition, following repeated exposure to nicotine, nAChRs did not become tolerant to the loss in receptor function that occurs after an initial nicotine administration. Overall, these data suggest that neuronal adaptations underlying nicotine tolerance may begin upon initial exposure then persist following repeated exposures.