A number of studies have focused on the ability of the habenula, particularly the MHb to oppose the behavioral processes mediated through the VTA (for reviews, see (Fowler and Kenny, 2012; Hikosaka, 2010)). The MHb-interpeduncular pathway is cholinergic, and it has been proposed that its effects on VTA neuron firing are mediated indirectly through inhibition of the PPTg (Maskos, 2008). Decreasing the expression of nAChRs containing the α5 subunit in the MHb results in increased nicotine self-administration (Fowler et al., 2011), suggesting that this cholinergic system normally acts as a brake on drug reward.
