Nicotine addiction represents a serious social and public health problem. In this project, we will address a series of fundamental questions regarding the roles of nicotinic receptors (nAChRs) in the mechanisms underlying choice modification under drug exposure. Dopaminergic signaling is critical to the neurobiology of reward and motivation and the dopamine system constitutes the primary target of addictive drugs. Numerous theories have been developed to model addiction, but an important framework states that addictive drugs change the characteristics of dopamine activity in reward signaling and in decision making. Using behavioral analysis, in vivo electrophysiological recording and genetic tools to modify nAChRs expression, we will analyze the modifications of DA network dynamics in the different steps leading to nicotine addiction, including genetic and environmental factors, and the consequences of these modifications on decision making. Three main issues can be distinguished:
- The first one refers to the existence of functional subnetworks within the ventral tegmental area (VTA) DA cell population and the effect of chronic nicotine exposure on these subnetworks. Indeed, DA neurons do not constitute a uniform population but rather encompass multiple cell types with distinct roles in motivational control. Understanding the functional balance between these sub-network, the role of nAChRs in these defined populations and how this balance is modified after nicotine exposure should provide important insight into the complex effects of nicotine.
- The second one is related to the genetic and environmental factors that increase the risk of nicotine consumption. Stress and alcohol are two factors that will be particularly investigated. Genetic factor analysis will focus on specific nAChR subunits. Indeed, human genetic studies have recently revealed that human coding and non-coding polymorphisms at locus 15q25 for α3-containing (*), α5*- and β4*-nAChRs are correlated with smoking and/or lung cancer. These subunits will be particularly examined using animal models of nicotine neuronal, reinforcing or cognitive effects.
- The third issue regards the role of these subnetworks in reinforcement learning and exploration, the drug–induced alterations of their dynamics and the consequences on decision making. Using in vivo electrophysiology in behaving mice, we will decipher the role of nicotinic modulation in the transition by which an arbitrary stimulus becomes relevant from a behavioral point of view and leads to DA release that predicts drug availability. Finally, since drug users have impaired evaluation and decision processes, we will undertake a detailed analysis of natural behaviors in closed economy paradigm as well as in socially and environmentally relevant situations to assess the impact of drug on decision making.
Neuronal nicotinic acetylcholine receptors (nAChRs) play a major regulatory role on DA transmission, controlling bursting activity or DA release. nAChRs have been strongly implicated in the pathophysiology of several psychiatric disorders as well as nicotine addiction and Alzheimer’s disease. However, progress in this regard has been held back by the lack of subtype-selective nAChR pharmacology and the difficulties associated with selectively targeting nAChRs in defined cell types and in different parts of the brain.
Optogenetic pharmacology combines the advantages of photochemical control with the specificity of genetics to enable precise manipulation of particular receptors and ion channels in particular cells (1, 2). This strategy involves the genetic engineering of receptors and their conjugation to a chemical photoswitch, allowing light to precisely activate or inhibit that receptor subtype in a given neuronal population.
The photoswitch is made of a ligand (agonist or pore blocker/antagonist) on one end of the molecule, a central photoisomerizable azobenzene core, and a cysteine-reactive group on the other end of the molecule. The photoswitch can react with engineered cysteine point mutations in the receptor of interest, covalently tethering the ligand to the designed protein. Different wavelengths of light are used to isomerize the photoswitch between its trans and cis configurations, allowing light to toggle the ligand in and out of its binding pocket.
We have engineered light-activated and –inhibited neuronal nAChRs, where the photoswitch is covalently attached to an engineered cysteine (red) on the beta subunit (grey) of the receptor. Under green light condition, the photoswitch (green) has an extended configuration and cannot bind to the acetylcholine binding pocket. Under violet light, the photoswitch (violet) bends, reaches the agonist binding pocket located under the alpha subunit (orange) and activates (or inhibits) the receptor (3).
Such strategy will allow us to control an individual receptor subtype, i.e. the one containing the cysteine mutation, in a targeted neuron with high spatiotemporal precision, while leaving native receptors unaffected. The combination of rational design, specificity and precision makes this optogenetic pharmacological approach a very powerful tool for dissecting the roles of particular receptor subtypes in complex neural networks, both in vitro and in vivo.
Souris City is an innovative project that will allow to explore mouse behaviours in an environment more complex than the standard housing conditions. Indeed, Souris City is composed of several compartments connected through “tubes and scales”, resembling the natural living conditions of mice. Other modules allowing standardised testing of the animals will be additionally connected. The environment is also socially enriched, the group housed together being larger than in standard conditions. Mice are observed throughout night and day in the different compartments. Such a setting will allow to gather information on the hierarchical structure within a group and to combine this information with the performances of the animals in the standardised tests conducted in parallel.