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Smoking is the most important preventable cause of mortality and morbidity worldwide. This nicotine addiction is mediated through the nicotinic acetylcholine receptor (nAChR), expressed on most neurons, and also many other organs in the body. Even within the ventral tegmental area (VTA), the key brain area responsible for the reinforcing properties of all drugs of abuse, nicotine acts on several different cell types and afferents. Identifying the precise action of nicotine on this microcircuit, in vivo, is key to understand reinforcement, and to finally develop efficient smoking cessation treatments. We used a novel lentiviral system to exclusively re-express high-affinity nAChRs on either dopaminergic (DAergic) or GABAergic neurons, or both, in the VTA. Using in vivo electrophysiology, we show that, contrary to widely accepted models, the activation of GABA neurons in the VTA plays a crucial role in the control of nicotine-elicited DAergic activity. Our results demonstrate that both positive and negative motivational values are transmitted through the DA neuron, but that the concerted activity of DA and GABA systems is necessary for the reinforcing actions of nicotine through burst firing of DA neurons. This work identifies the GABAergic interneuron as a potential target for smoking cessation drug development.
 
Tolu et al., Molecular Psychiatry 2012 

 
 
(a) Design of conditional lentiviral vector. (b) Selective reexpression in dopaminergic (DA) and GABAergic neurons of the VTA. (c) Imaging study. (d) Coexpression of lentiviral vectors in DA and GABA cells leads to retoration of burst firing. (e) Restoration of DA bursting restores nicotine self-administration.
 

 
Nicotine is the primary psychoactive component of tobacco. Its reinforcing and addictive properties depend on nicotinic acetylcholine receptors (nAChRs) located within the mesolimbic axis originating in the ventral tegmental area (VTA). The roles and oligomeric assembly of subunit α4- and subunit α6-containing nAChRs in dopaminergic (DAergic) neurons are much debated. Using subunit-specific knockout mice and targeted lentiviral re-expression, we have determined the subunit dependence of intracranial nicotine self-administration (ICSA) into the VTA and the effects of nicotine on dopamine (DA) neuron excitability in the VTA and on DA transmission in the nucleus accumbens (NAc). We show that the α4 subunit, but not the α6 subunit, is necessary for ICSA and nicotine-induced bursting of VTA DAergic neurons, whereas subunits α4 and α6 together regulate the activity dependence of DA transmission in the NAc. These data suggest that α4-dominated enhancement of burst firing in DA neurons, relayed by DA transmission in NAc that is gated by nAChRs containing α4 and α6 subunits, underlies nicotine self-administration and its long-term maintenance.
 
 Exley et al., PNAS 2012

(Left) Lentiviral reexpression of the alpha4 subunit in VTA restores high affinity binding sites for a nicotinic ligand. (Right) Restoration of the alpha4 subunit restores burst firing of DA neurons after nicotine injection.