Null mutant analysis of responses to nicotine: deletion of beta2 nicotinic acetylcholine receptor subunit but not alpha7 subunit reduces sensitivity to nicotine-induced locomotor depression and hypothermia.

The nicotinic acetylcholine receptor (nAChR) subtypes alpha4beta2 and alpha7 comprise the majority of brain nicotine-binding sites. Classical genetic strategies using inbred mice and their hybrids suggest that nicotine's effects on locomotor activity and body temperature are influenced by alpha4beta2 but not alpha7 receptors. To evaluate directly the role of these nicotinic subtypes on responses to nicotine, beta2 and alpha7 null mutant (-/-) mice, as well as wild-type (+/+) and heterozygous (+/-) mice, were tested for baseline body temperature and locomotion and nicotine (0-1.5 mg/kg)-induced changes in these responses. Basal responses for these measures were similar for all beta2 genotypes, but baseline Y-maze activity was higher in alpha7-/- mice compared with alpha7+/+ mice. Following nicotine injection, dose-dependent decreases in body temperature and locomotor activity were observed for all three genotypes of both beta2 and alpha7 mice. Although responses in alpha7 mice did not differ among genotypes, beta2 gene deletion was found to have a gene-dependent effect on nicotine's effects. beta2-/- mice were less sensitive to nicotine-induced locomotor depression and hypothermia at low nicotine doses (.25-.5 mg/kg) but were no different from beta2+/+ mice at the highest doses tested (1.0-1.5 mg/kg). Residual responses at high nicotine doses in beta2-/- mice as well as responses in all alpha7 and beta2 mouse genotypes were mediated by nicotinic receptors, since mecamylamine (1.0 mg/kg) blocked all responses following 1.0 mg/kg nicotine. This finding suggests receptors that include the beta2 nAChR subunit partially mediate nicotine's effects on locomotor activity and body temperature.