STUDY OF THE ROLE OF THE SEROTONINERGIC SYSTEM IN THE BRAIN FUNCTIONALITY AND IN THE VULNERABILITY TO PSYCHOPATHOLOGIES:A GENETIC APPROACH

Psychiatric pathologies are the second leading cause of disability worldwide with about 10% of the general population that manifest at least one symptom during their lifetime. This, together with the only partial effectiveness of the treatments adopted to date, make these illnesses an important burden for the society as well as for families and patients. The first theory elaborated to explain the onset of psychiatric pathologies is the monoaminergic hypothesis that postulated how low levels of monoamines such as serotonin in the brain could be at the basis of depressive episodes and of other mood disorders. In line, human studies highlighted how alterations in different genes coding for different players involved in the serotoninergic system can be related to an increased susceptibility for psychopathologies. Despite this linkage seems striking, different aspects of these pathologies cannot be explained with a direct causal relationship, making this hypothesis part of a bigger picture that also involves the environmental factors as well as other mechanisms. On these bases, understanding the complexity of the systems that are modulated by serotoninergic alterations could give novel insights in understanding these such heterogeneous diseases. To better clarify this aspect, during my PhD I took advantage of different genetically modified animal models characterized by alterations in the serotoninergic system. In particular, we evaluated the effect of low functionality of the serotonin transporter (SERT) on brain functions and we confirmed the pathological-like behavior of SERT-/- rats, characterized by a massive presence of serotonin in the extracellular compartments of the whole body. Moreover, we perform molecular analyses that confirmed alterations in different systems such as the neuroplastic mechanisms, the GABAergic system, and the dendritic spine functionality. Furthermore, we evaluated if a positive environment could ameliorate the behavior and the molecular abnormalities of SERT-/- rats finding a restorative effect of one month of enriched environment both on the anhedonic and anxiety-like behavior and on the molecular abnormalities. Moreover, by altering the expression of the isoforms 1 or 2 of the tryptophan hydroxylase (TPH) enzyme, responsible for the production of serotonin in the periphery or in the brain respectively, we took into consideration TPH1-/- and TPH2-/- rats. More specifically, we worked on TPH2-/- rats, which do not have serotonin in the central nervous system and are characterized by an aggressive behavior and low anxiety levels at adulthood. Interestingly, we found that the lack of brain-serotonin was counteracted by an upregulation in the neurotrophin Brain-derived neurotrophic factor (Bdnf) expression. Moreover, we found that this compensatory mechanism was present also at early adolescence, while at post-natal day 10, when the blood-brain-barrier is not completely formed and we cannot distinguish between central and peripheral serotonin, Bdnf expression was lowered. Interestingly, this increased neuroplasticity at adulthood was not enough to cope with an acute stress such as one hour of restraint stress. Indeed, while wild-type rats showed an activation of the Bdnf machinery and of the transcription of the immediate early genes, this effect was reduced in TPH2-/- rats. Accordingly, we found that the hypothalamic pituitary adrenal (HPA) axis and specifically its genomic pathway activation was much more consistent in TPH2+/+ than in TPH2-/- rats. Interestingly, we observed similar results also in TPH1-/- rats that showed a dramatic reduction in serotonin levels in different peripheral organs. Indeed, the lack of peripheral serotonin was able to reduce the anxiety-like behavior in these rats suggesting that also the peripheral pool of this molecule can alter brain functionality. Moreover, this positive effect on their behavior was sustained, at central level, by an increased expression of Bdnf. Nevertheless, also in this case, we found deficits in the reaction to the acute stress. Indeed, we found that the increased transcription of Bdnf and of the immediate early gene Arc happened specifically in wild-type while not in TPH1-/- rats. Similarly, the upregulation in the glucocorticorticoid responsive gene transcription was lower in a condition of absence of peripheral serotonin. All the molecular analyses were conducted in the prefrontal cortex, a brain region highly innervated by serotoninergic fibers and involved in environmental changes response and in behavioral adaptations. These results highlighted how these genetically modified animal models can be useful to study the role of serotonin in the modulation of brain functionality and the mechanisms involved in this process. Moreover, these findings supported the serotonin involvement in brain development and functionality. Ultimately, the data collected in this thesis give novel insights on the brain pathways affected by serotonin and on the role of this system in shaping the reaction to environmental stimuli in the field of psychiatric disorders.