TPH2 knockout male rats are aggressive, show less anxiety, and exhibit an altered oxytocin system

The central serotoninergic system is critical for stress responsivity and social behavior, and its dysregulations has been centrally implicated in virtually all neuropsychiatric disorders. Genetic serotonin depletion animal models could provide a tool to elucidate the causes and mechanisms of diseases and to develop new treatment approaches. Previously mice lacking tryptophan hydroxylase 2 (Tph2) have been developed, showing altered behaviors and neurotransmission. However, the effect of congenital serotonin deficiency on emotional and social behavior in rats is still largely unknown, as are the underlying mechanisms. In this study, we used Tph2 knockout (Tph2-/-) male rat model to study how the lack of serotonin in the rat brain affects anxiety-like and social behaviors. Since oxytocin is centrally implicated in these behaviors, we furthermore explored whether effects of Tph2 knockout on behavior would relate to changes in the oxytocin system. We show that Tph2-/- rats display reduced anxiety-like behavior and a high level of aggression in social interactions. In addition, oxytocin receptor expression was increased in the infralimbic and prelimbic cortex, paraventricular nucleus, dorsal raphe nucleus and some subregions of hippocampus, which was paralleled by increased levels of oxytocin in the medial frontal cortex, paraventricular nucleus, but not the dorsal raphe nucleus, central amygdala and hippocampus. In conclusion, our study demonstrated reduced anxiety but exaggerated aggression in Tph2-/- male rats and reveals for the first time a potential involvement of altered oxytocin system function. SIGNIFICANCE STATEMENTWe explored the changes in behavior and oxytocin system functioning in the tryptophan hydroxylase 2 (Tph2) knockout rat model, lacking serotonin in the brain. This rat model contributes to our understanding of the role of serotonin in psychiatric transdiagnostic features and underlying mechanisms. We found that Tph2 knockout male rats are aggressive, less anxious, and exhibit an altered oxytocin system. The observed changes in oxytocin signaling may lead to a new target for the treatment of diseases caused by genetic serotonin deficiency.