Summary Background Chronic exposure to nicotine elicits physical dependence in smokers, yet the mechanism and neuroanatomical bases for withdrawal symptoms are unclear. As in humans, rodents undergo physical withdrawal symptoms after cessation from chronic nicotine characterized by increased scratching, head nods, and body shakes. Results Here we show that induction of physical nicotine withdrawal symptoms activates GABAergic neurons within the interpeduncular nucleus (IPN). Optical activation of IPN GABAergic neurons via light stimulation of channel rhodopsin elicited physical withdrawal symptoms in both nicotine-naïve and chronic nicotine-exposed mice. Dampening excitability of GABAergic neurons during nicotine withdrawal through IPN-selective infusion of an NMDA receptor antagonist or through blocking IPN neurotransmission from the medial habenula reduced IPN neuronal activation and alleviated withdrawal symptoms. During chronic nicotine exposure, nicotinic acetylcholine receptors containing the β4 subunit were upregulated in somatostatin interneurons clustered in the dorsal region of the IPN. Blockade of these receptors induced withdrawal signs more dramatically in nicotine-dependent compared to nicotine-naïve mice and activated non-somatostatin neurons in the IPN. Conclusions Together, our data indicate that therapeutic strategies to reduce IPN GABAergic neuron excitability during nicotine withdrawal, for example, by activating nicotinic receptors on somatostatin interneurons, may be beneficial for alleviating withdrawal symptoms and facilitating smoking cessation.

Increased anxiety is a predominant withdrawal symptom in abstinent smokers, yet the neuroanatomical and molecular bases underlying it are unclear. Here, we show that withdrawal-induced anxiety increases activity of neurons in the interpeduncular intermediate (IPI), a subregion of the interpeduncular nucleus (IPN). IPI activation during nicotine withdrawal was mediated by increased corticotropin releasing factor (CRF) receptor-1 expression and signaling, which modulated glutamatergic input from the medial habenula (MHb). Pharmacological blockade of IPN CRF1 receptors or optogenetic silencing of MHb input reduced IPI activation and alleviated withdrawal-induced anxiety; whereas IPN CRF infusion in mice increased anxiety. We identified a meso-interpeduncular circuit, consisting of ventral tegmental area (VTA) dopaminergic neurons projecting to the IPN, as a potential source of CRF. Knock-down of CRF synthesis in the VTA prevented IPI activation and anxiety during nicotine withdrawal. These data indicate that increased CRF receptor signaling within a VTA-IPN-MHb circuit triggers anxiety during nicotine withdrawal.

Recently, the smoking cessation therapeutic varenicline, a nicotinic acetylcholine receptor (nAChR) partial agonist, has been shown to reduce alcohol consumption. However, the mechanism and nAChR subtype(s) involved are unknown. Here we demonstrate that varenicline and alcohol exposure, either alone or in combination, selectively activates dopaminergic (DAergic) neurons within the posterior, but not the anterior, ventral tegmental area (VTA). To gain insight into which nAChR subtypes may be involved in the response to alcohol, we analyzed nAChR subunit gene expression in posterior VTA DAergic neurons. Ethanol-activated DAergic neurons expressed higher levels of ␣4, ␣6, and ␤3 subunit genes compared with nonactivated neurons. To examine the role of nicotinic receptors containing the ␣4 subunit (␣4* nAChRs) in varenicline-induced reduction of alcohol consumption, we examined the effect of the drug in two complementary mouse models, a knock-out line that does not express the ␣4 subunit (␣4 KO) and another line that expresses ␣4* nAChRs hypersensitive to agonist (Leu9ЈAla). While varenicline (0.1-0.3 mg/kg, i.p.) reduced 2% and 20% alcohol consumption in wild-type (WT) mice, the drug did not significantly reduce consumption in ␣4 KO animals. Conversely, low doses of varenicline (0.0125-0.05 mg/kg, i.p.) that had little effect in WT mice dramatically reduced ethanol intake in Leu9ЈAla mice. Infusion of varenicline into the posterior, but not the anterior VTA was sufficient to reduce alcohol consumption. Together, our data indicate that activation of ␣4* nAChRs is necessary and sufficient for varenicline reduction of alcohol consumption.