To better understand the roles of α4-containing nAChRs, a KI approach was adopted with the goal of making a hypersensitive nAChR that might generate more noticeable phenotypes. One of the most successful examples of this approach was the generation of two lines of α4 KI mice by introducing a point mutation into the M2 transmembrane region of the α4 subunit.77,78 In Leu9′Ser KI mice, this mutation produces α4-containing nAChRs with a ~30-fold increase in sensitivity to acetylcholine and nicotine. Heterozygous KI mice exhibit greater anxiety, impaired motor learning, excessive ambulation that is eliminated by small amounts of nicotine, and reduction of dopaminergic function on aging.78 In contrast, both homozygous and heterozygous Leu9′Ala KI mice are exceptionally sensitive to nicotine and ND-related behaviors, including reward, tolerance, and sensitization.77 These findings suggest that genetic variability in the α4 subunit can produce dramatic changes in nicotine sensitivity, implying that a variant(s) in the human CHRNA4 gene alters sensitivity to nicotine and vulnerability to ND.
