Rationale: Chronic nicotine exposure induces both tolerance and upregulation of [3H]nicotine binding sites in rodent and human brain. However, the mechanism for chronic tolerance is unclear because a direct relationship between tolerance and receptor upregulation is not consistently observed.
Objectives: In the present experiments, the role of beta2* nicotinic acetylcholine receptors (nAChRs) on tolerance development and nAChR upregulation was examined following chronic nicotine treatment of beta2 wild-type (+/+), heterozygous (+/-), and null mutant (-/-) mice.
Methods: Saline or nicotine (1, 2, or 4 mg/kg/h) was infused intravenously for 10 days. Locomotor activity and body temperature responses were measured before and after nicotine challenge injection to observe changes in nicotine sensitivity. [3H]Epibatidine binding was then measured in ten brain regions.
Results: Beta2+/+ mice developed dose-dependent tolerance and upregulation of [3H]epibatidine binding sites. In contrast, beta2-/- mice, initially less sensitive to acute nicotine's effects, became more sensitive following treatment with the lowest chronic dose (1 mg/kg/h). Beta2-/- mice treated with 4.0 mg/kg/h nicotine were no longer supersensitive, indicating that tolerance developed at this higher dose. However, these changes in nicotine sensitivity occurred in the absence of any nAChR changes in either low- or high-affinity [3H]epibatidine sites. Responses of beta2+/- mice were intermediate between wild-type and mutant mice.
Conclusions: Upregulation of nAChRs in vivo requires the presence of the beta2 subunit. Changes in nicotine sensitivity occurred both in the presence (beta2+/+) and absence (beta2-/-) of beta2* nAChRs and suggest that mechanisms involving both beta2* and non-beta2* nAChR subtypes modulate adaptation to chronic nicotine exposure.