Smoking decreases appetite and smokers often report that they smoke to control their weight. Understanding the neurobiological mechanisms underlying the anorexic effects of smoking would facilitate the development of novel treatments to help with smoking cessation and to prevent or treat obesity. Using a combination of pharmacological, molecular genetic, electrophysiological and feeding studies, we found that activation of hypothalamic α3β4 nicotinic acetylcholine receptors (nAChRs) leads to activation of pro-opiomelanocortin (POMC) neurons. POMC neurons and subsequent activation of melanocortin 4 receptors were critical for nicotinic-induced decreases in food intake in mice. This study demonstrates that nicotine decreases food intake and bodyweight by influencing the hypothalamic melanocortin system and identifies critical molecular and synaptic mechanisms involved in nicotine-induced decreases in appetite.

In humans, tobacco withdrawal produces symptoms that contribute to the difficulty associated with smoking cessation. Nicotine withdrawal symptoms can also be observed in rodents. A major standing question is which nicotinic receptor subtypes and which areas of the brain are necessary for nicotine withdrawal to occur. Using knock-out mice, we previously showed that the ␤4, but not the ␤2 subunit of nicotinic acetylcholine receptors, is necessary for the somatic manifestations of nicotine withdrawal. Since the ␤4 subunit is highly expressed in the medial habenula, we focused our studies on the medial habenula and its primary target, the interpeduncular nucleus. In particular, we studied nicotine withdrawal in mice lacking the ␣2 or the ␣5 nicotinic receptor subunits, which are highly expressed in the interpeduncular nucleus. We precipitated withdrawal by systemically injecting the nicotinic antagonist mecamylamine in mice chronically treated with nicotine. Both the ␣2 and the ␣5 null mutations abolished the somatic manifestations of nicotine withdrawal. In addition, in wild-type mice chronically treated with nicotine, mecamylamine precipitated withdrawal when microinjected into the habenula or the interpeduncular nucleus, but not into the cortex, ventral tegmental area or hippocampus. Our results demonstrate a major role for the habenulo-interpeduncular system and the nicotinic receptor subunits expressed therein, in nicotine withdrawal symptoms. Our data suggest that the efforts to develop new smoking cessation therapies should concentrate on these areas and receptor types.

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Withdrawal from chronic exposure to nicotine, the main addictive component of tobacco, produces distinctive symptoms in humans. The appearance of these symptoms is a major deterrent when people try to quit smoking. To study which type of nicotine receptor is relevant for the onset of the withdrawal syndrome, we used a mouse model of nicotine withdrawal. Wild-type mice and mice null for the ␤4 (␤4Ϫ/Ϫ) or the ␤2 (␤2Ϫ/Ϫ) nicotinic acetylcholine receptor subunits were implanted with osmotic minipumps delivering 24 mg ⅐ kg Ϫ1 ⅐ d Ϫ1 nicotine for 13 d. Subsequently, a single intraperitoneal injection of the nicotinic receptor antagonist mecamylamine induced behavioral symptoms of withdrawal measured as increased grooming, chewing, scratching, and shaking, plus the appearance of some unique behaviors such as jumping, leg tremors, and cage scratching. Mecamylamine injection triggered comparable withdrawal signs in wild-type and in ␤2Ϫ/Ϫ mice, whereas the ␤4Ϫ/Ϫ mice displayed significantly milder somatic symptoms. In addition, nicotine withdrawal produced hyperalgesia in wild-type but not ␤4 Ϫ/Ϫ mice. Finally, chronic nicotine produced an increase in epibatidine binding in several areas of the brain in both wild-type and in ␤4Ϫ/Ϫ mice, but such receptor upregulation did not correlate with the severity of withdrawal signs.Our results demonstrate a major role for ␤4-containing nicotinic acetylcholine receptors in the appearance of nicotine withdrawal symptoms. In contrast, the ␤2 subunit does not seem to greatly influence this phenomenon. We also show that the upregulation of epibatidine binding sites attributable to chronic nicotine, an effect associated with ␤2-containing receptors, is probably not related to the mechanisms underlying withdrawal.

Nicotine, acting at pentameric neuronal nicotinic acetylcholine receptors (nAChRs), is the primary addictive component in tobacco. At low doses, it affects attention, learning, memory, anxiety, cardiovascular responses, thermoregulation, and nociception. At high doses, nicotine produces more drastic behaviors and eventually induces tonic-clonic seizures in rodents. In mammals, several subunits of the nAChRs have been cloned, including eight ␣ and three ␤ subunits. To study the physiological role of the ␣5 subunit, we have generated ␣5-deficient mice. These mice have a generally healthy appearance and are normal in a standard battery of behavioral tests. However, the sensitivity of ␣5 mutant mice to nicotine-induced behaviors and seizures is dramatically reduced compared with their wild-type littermates. These animals have a normal brain anatomy and normal levels of mRNA for other nAChR subunits, namely ␣4, ␣6, ␣7, ␤2, and ␤4. In addition, 125 I-epibatidine and [ 125 I]␣-bungarotoxin binding in the brains of ␣5-deficient mice is normal. Together, these results suggest a direct involvement of the ␣5 subunit in the observed phenotypes.