Effects of constitutive depletion or excess of serotonin during development on the maturation of prefrontal monoaminergic circuits

Serotonin (5-hydroxytryptamine; 5-HT) is a critical neuromodulatory morphogen-like signal that governs prenatal cortical circuit assembly, and altered 5-HT signaling is implicated in a wide range of psychiatric disorders. Yet the structural consequences of lifelong 5-HT imbalance during the initial blueprinting phase remain poorly defined. Here, we used two genetic rat models representing the extremes of 5-HT homeostasis, Tph2 knockout (Tph2–/–; chronic depletion of 5-HT) and 5-HTT knockout (5-HTT–/–; chronic excess of 5-HT), to investigate sub-region-specific alterations in the embryonic medial prefrontal cortex (mPFC) and in prefrontal monoaminergic circuit maturation. Our results demonstrate that the developing mPFC is not uniformly vulnerable to 5-HT dysregulation. Instead, the prelimbic (PL) cortex emerged as a region of heightened sensitivity to low 5-HT levels, exhibiting localized hyperinnervation of catecholaminergic fibers and an increase in deep-layer Tbr1+ neurons. In contrast, within the infralimbic (IL) cortex, 5-HT deficiency induced a selective loss of reelin (Reln)-producing Cajal-Retzius (CR) cells, indicating a sub-region-specific disruption of the cortical scaffold. Transcriptomic analyses identified selective upregulation of the low-affinity monoamine transporter Slc29a4 and the monoamine oxidase A (Maoa) in 5-HTT–/– embryos, suggesting activation of compensatory extracellular 5-HT clearance pathways during embryogenesis. Together, these findings establish prenatal 5-HT levels as an essential determinant of afferent innervation and cortical scaffold integrity, and suggest that late-gestational developmental blueprinting errors may provide a neurodevelopmental substrate for later monoaminergic circuit dysfunction linked to social and cognitive phenotypes.