The role of β2-subunit-containing nicotinic acetylcholine receptors in the brain explored with a mutant mouse

Neuronal nicotinic receptors comprise a family of pentameric oligomers made up of a combination of 10 different subunits. The beta 2 subunit has the widest pattern of expression in the brain and is thus likely to form a significant fraction of neuronal nicotinic receptors. Using mice lacking the beta 2 subunit, we have shown that nAChRs containing this subunit are responsible for most of the high-affinity binding sites for nicotine, cytisine, and epibatidine in the brain. Functional receptors containing the beta 2-subunit are found in the somatodendritic compartment as well as the axonal compartment of neurons. We have examined the contribution of these receptors to the effects of nicotine on the mesolimbic DA system, which mediates the reinforcing properties of many addictive drugs (including nicotine). Submicromolar doses of nicotine, corresponding to the concentrations of nicotine in vivo in self-administration paradigms, increased the firing rate of dopaminergic neurons in vitro in normal mice but not in mice lacking the beta 2 subunit. Consistently, systemic injection of nicotine induced an increase in extracellular dopamine in normal mice but not in mutant mice, and nicotine self-administration was reduced or suppressed in mutant mice. These data support the view that the beta 2-containing receptors are involved in mediating the reinforcing properties of nicotine.