The prefrontal cortex (PFC) is responsible for higher cognitive tasks, and its ‘malfunction’ is shared across serotonin (5-HT)-related neuropsychiatric disorders and often find its origins during development. Proper cortical development is directed by a sequence of intrinsic ( transcription factors, local placental-derived 5-HT) and extrinsic (instructing 5-HT and catecholaminergic afferents) events. Recently, our group has shown a functional interplay between the 5-HT and catecholaminergic prefrontal projection systems during development leading to a distortion in PFC cytoarchitecture. Lack of the 5-HT transporter (5-HTT), resulting in high 5-HT levels, affects not only the 5-HT but also the TH-positive projections towards the PFC. Moreover, in 5-HTT-/- rats, the number and distribution of reelin-releasing Cajal-Retzius cells is affected in early postnatal stages, as is cortical layer identity. Further, we showed that 5-HTT is also expressed in the cortex during embryonic development even before any neurotransmitter system reaches the PFC. Therefore, we hypothesize that there is an intrinsic as well as extrinsic effect of 5-HT levels during PFC development and that any change in this can affect the functional interplay between neurotransmitter systems. We are currently assessing how lifelong high (5-HTT-/-) and low 5-HT levels (TPH2-/-) affect 5-HT and dopaminergic prefrontal innervation and PFC microstructure during prenatal development. Preliminary data shows that the expression of several monoaminergic neurotransmitter receptors and transporters is altered in the absence of 5-HTT. Ultimately, we aim to longitudinally assess 5-HT-driven developmental consequences in early postnatal and adult transdiagnostic-like behavior to broad the scope on risk factors for neurodevelopmental psychiatric disorders.