Additionally, cholinergic transmission in MHb has also been implicated in nicotine addiction as MHb neurons not only release ACh, but express an array of nicotinic acetylcholine receptor subunits (nAChRs, such as Chrna3 and Chrnb3;Figure 2—figure supplements 2 and 6) (Fowler et al., 2011; Shih et al., 2014). This evidence concerns the gene CHRNA3 and nicotine dependence.