Characterization and use of a Tph2 conditional knockout mouse line to investigate the role of serotonin in the adult CNS: unveiling the impact of serotonin homeostasis perturbation on serotonergic wiring

The neurotransmitter serotonin (5-hydroxytryptamnine, 5-HT) has been implied in the modulation of a plethora of physiological, cognitive and behavioral processes as well as in the regulation of specific morphogenetic events during neurodevelopment. In the last decades, human genetics and animal studies reveled the importance of appropriate serotonin homeostasis maintenance in the developing and in the adult brain, suggesting that perturbation of 5-HT physiological levels during critical temporal windows may contribute to the onset of neuropsychiatric disorders. In line, cumulating evidences have shown that an altered serotonergic neurotransmission during brain development could affect the establishment of neuronal wiring. However the precise role of serotonin in age-dependent activities is only beginning to be elucidated and whether perturbation of the proper brain level of 5-HT during adulthood could affect neuronal circuits remains enigmatic. In this context, the use of suitable genetic tools to allow a time-specific depletion of brain serotonin levels is crucial to further dissect the roles of 5-HT in developing and adult brains. During my PhD I validated a newly generated Tph2flox conditional knockout mouse line and I used it to investigate whether serotonin depletion in the adult brain impacts on serotonergic wiring. Results showed that the Tph2flox mouse line could be efficiently used to conditionally abrogate serotonin synthesis in a time-specific manner, thus representing a valuable tool to dissect the role of serotonin in age-dependent activities. Using this line, I showed that the density of serotonergic fibers was increased in the hippocampus and decreased in the thalamic paraventricular nucleus as a consequence of brain serotonin depletion during adulthood. Nevertheless, these innervation defects were reduced when brain serotonin signaling was restored by chronic treatment with the serotonin precursor 5-hydroxytryptophan. Finally, 3D computer-based analysis of serotonergic fiber morphology revealed that an altered 5-HT homeostasis affected the complexity of axonal branching. Overall, these data unveiled an unexpectedly high plasticity of the adult serotonergic system and demonstrated that a correct serotonin homeostasis is life-long required to preserve the proper serotonergic wiring of the brain.