Understanding mechanisms underlying normal complex behaviours and the abnormalities that accompany most neuropathologies is a primary challenge in fundamental and biomedical research. Optimal use of the large body of genetic, molecular, electrophysiological, behavioural and imaging techniques that provide new insights into cellular organisation at the microscopic level, and functional circuits at the macroscopic level, is hampered by the usual dissociation of these techniques. Today, the crucial challenge lies in the integration of these approaches in order to target a unified scientific question at multiple levels. The subject of this presentation is the functional analysis of brain circuits with a multi-level approach, to understand how nicotine acts on the brain, affects cognition, and causes addiction. Our goal is, using a simple animal model, to explore the molecular mechanisms underlying addictive and cognitive behaviours in their relation to reward. Addiction to nicotine is typically an 'integrated' pathology including short-term receptor modification, and long term modification of circuit equilibrium and behaviours. Understanding such phenomena requires the development of new tools. Nicotine addiction presents a serious social and public health problem. Worldwide, 100 million people are expected to die this century from the consequences of nicotine addiction, yet nicotine is also known to enhance cognitive performance. Additionally, nicotinic acetylcholine receptors (nAChRs) are down-regulated in serious human psychiatric conditions like autism. Hence, the identification of the molecular mechanisms and circuits involved in nicotine reinforcement and cognition is urgent and requires the development of novel tools that allow genetic and molecular manipulation in vivo.