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.electrophysiology | lentivirus | nicotinic receptor | voltammetry | ventral striatum

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 important to understand reinforcement, and finally to develop efficient smoking cessation treatments. We used a novel lentiviral system to re-express exclusively high-affinity nAChRs on either dopaminergic (DAergic) or γ-aminobutyric acid-releasing (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 dopamine (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.

Midbrain dopamine (DA) neurons are key players in motivation and reward processing. Increased DA release is thought to be central in the initiation of drug addiction. Whereas dopamine neurons are generally considered to be activated by drugs such as nicotine, we report here that nicotine not only induces excitation of ventral tegmental area (VTA) DA cells but also induces inhibition of a subset of VTA DA neurons that are anatomically segregated in the medial part of the VTA. These opposite responses do not correlate with the inhibition and excitation induced by noxious stimuli. We show that this inhibition requires D2 receptor (D2-R) activation, suggesting that a dopaminergic release is involved in the mechanism. Our findings suggest a principle of concurrent excitation and inhibition of VTA DA cells in response to nicotine. It promotes unexplored roles for DA release in addiction contrasting with the classical views of reinforcement and motivation, and give rise to a new interpretation of the mode of operation of the reward system.

Addiction is a maladaptive pattern of behavior following repeated use of reinforcing drugs in predisposed individuals, leading to lifelong changes. Common among these changes are alterations of neurons releasing dopamine in the ventral and dorsal territories of the striatum. The serotonin 5-HT receptor has been involved in various behaviors, including impulsivity, response to antidepressants, and response to psychostimulants, pointing toward putative interactions with the dopamine system. Despite these findings, it remains unknown whether 5-HT receptors directly modulate dopaminergic activity and the possible mechanisms involved. To answer these questions, we investigated the contribution of 5-HT receptors to cocaine-dependent behavioral responses. Male mice permanently lacking 5-HT receptors, even restricted to dopamine neurons, developed heightened cocaine-induced locomotor responses. Retrograde tracing combined with single-cell mRNA amplification indicated that 5-HT receptors are expressed by mesolimbic dopamine neurons. and electrophysiological recordings showed that 5-HT-receptor inactivation in dopamine neurons affects their neuronal activity and increases AMPA-mediated over NMDA-mediated excitatory synaptic currents. These changes are associated with lower ventral striatum dopamine activity and blunted cocaine self-administration. These data identify the 5-HT receptor as a pharmacological intermediate and provide mechanistic insight into attenuated dopamine tone following exposure to drugs of abuse. Here we report that mice lacking 5-HT receptors totally or exclusively in dopamine neurons exhibit heightened cocaine-induced locomotor responses. Despite the sensitized state of these mice, we found that associated changes include lower ventral striatum dopamine activity and lower cocaine operant self-administration. We described the selective expression of 5-HT receptors in a subpopulation of dopamine neurons sending axons to the ventral striatum. Increased bursting properties of these dopamine neurons and a concomitant increase in AMPA synaptic transmission to dopamine neurons were found in mice lacking 5-HT receptors. These data support the idea that the chronic 5-HT-receptor inhibition makes mice behave like animals already exposed to cocaine with higher cocaine-induced locomotion associated with changes in dopamine neuron reactivity.