We found that development of obesity was coupled with the emergence of a progressively worsening brain reward deficit. Similar changes in reward homeostasis induced by cocaine or heroin is considered a critical trigger in the transition from casual to compulsive drug-taking. Accordingly, we detected compulsive-like feeding behavior in obese but not lean rats, measured as palatable food consumption that was resistant to disruption by an aversive conditioned stimulus. Striatal dopamine D2 receptors (D2R) were downregulated in obese rats, similar to previous reports in human drug addicts. Moreover, lentivirus-mediated knockdown of striatal D2R rapidly accelerated the development of addiction-like reward deficits and the onset of compulsive-like food seeking in rats with extended access to palatable high-fat food. These data demonstrate that overconsumption of palatable food triggers addiction-like neuroadaptive responses in brain reward circuitries and drives the development of compulsive eating. Common hedonic mechanisms may therefore underlie obesity and drug addiction.

Genetic variation in CHRNA5, the gene encoding the α5 nicotinic acetylcholine receptor (nAChR) subunit, increases vulnerability to tobacco addiction and lung cancer, but underlying mechanisms are unknown. Here, we report dramatically increased nicotine consumption in mice with null mutation in Chrna5. This effect was `rescued' in knockout mice by re-expressing α5 subunits in medial habenula (MHb), and recapitulated in rats through α5 subunit knockdown in MHb. Remarkably, α5 subunit knockdown in MHb did not alter the rewarding effects of nicotine but abolished the inhibitory effects of higher nicotine doses on brain reward systems. The MHb extends projections almost exclusively to the interpeduncular nucleus (IPN). We found diminished IPN activation in response to nicotine in α5 knockout mice and disruption of IPN signaling increased nicotine intake in rats. Our findings suggest that nicotine activates the habenulo-interpeduncular pathway through α5-containing nAChRs, triggering an inhibitory motivational signal that acts to limit nicotine intake.

The number of negative nodes is an important independent prognostic factor for patients with stage IIIB and IIIC colon cancer.