The serotonin 6 (5-HT6) receptor is almost exclusively localized in the CNS, predominantly in regions involved in the regulation of cognitive processes such as the cerebral cortex, the hippocampus and the striatum. Correspondingly, it has become an increasingly promising target for the treatment of cognitive deficits associated with various CNS disorders including Alzheimer’s disease and schizophrenia. 5-HT6 receptors also participate in neuro-developmental processes, controlling the migration and normal positioning of cortical interneurons. Although 5-HT6 receptors are known to signal through Gs-adenylyl cyclase, coupling pathways and mechanisms controlling their activity remain poorly explored. To address these issues, we have recently used proteomic approaches associating co-immunoprecipitation or affinity chromatography experiments and high-resolution mass spectrometry, to identify novel signalling proteins associated with the receptor. These studies revealed interaction of the receptor with several members of the mammalian target of rapamycin (mTOR) cascade, including mTOR itself and Neurofibromin (Nf1), which are known to control synaptic plasticity, learning and memory formation. They also demonstrated association of the receptor with a network of proteins, including Cdk5 (Cyclin-dependent kinase 5) and one of its substrates WAVE-1 (Wiskott-Aldrich syndrome protein (WASP)-family verprolin homologous protein 1), involved in the actin cytoskeleton dynamics, which is crucial for neurite growth, dendritic spine formation and neuronal migration. In preliminary studies, we demonstrated that stimulation of 5-HT6 receptor activated the mTOR pathway in various brain areas including the prefrontal cortex, the site of modulation of social recognition by 5-HT6 ligands. Correspondingly, blocking the mTOR pathway by peripheral administration of rapamycin, in rats, prevented the reduction of short-term memory evoked by injection of a 5-HT6 agonist in a social recognition test but did not blunt pro-cognitive effects of 5-HT6 receptor antagonists. Our preliminary data also showed that expression of 5-HT6 receptor in neuroblastoma cell lines (SH-SY5Y, NG108-15) induced drastic morphological changes that are usually associated with neuronal differentiation (neurite outgrowth, formation of spine-like structures). This project, which combines cell biology, behavioural and quantitative phosphoproteomics approaches, will pursue four objectives: 1) to characterize the mechanisms underlying activation of mTOR pathway by 5-HT6 receptor and its behavioural impact in various cognition paradigms and in a developmental model of schizophrenia; 2) to determine the role of Nf1 in 5-HT6 receptor-operated signalling (notably activation of mTOR pathway) and in regulation of cognitive functions by 5-HT6 ligands; 3) to demonstrate a role of 5-HT6 receptors and of the associated Cdk5/WAVE-1 signalling network in neuronal differentiation, focusing on neurite growth and dendritic spine formation; and 4) to identify mTOR- and Cdk5-dependent signalling mechanisms engaged by 5-HT6 receptor. This program should define whether activation of Nf1/mTOR signalling pathway by 5-HT6 receptors compromise cognition and whether manipulating this pathway would be a valuable strategy for the treatment of cognitive deficits in psychoses such as schizophrenia. It should also identify novel molecular substrates of neuro-developmental effects of serotonin.