Repeated and single maternal separation specifically alter microglial morphology in the prefrontal cortex and neurogenesis in the hippocampus of 15-day-old male mice

Early-life adversity impairs neuronal plasticity of the developing brain. In rodents, brain maturation processes, including neuro- and synaptogenesis, myelination, microglial maturation, and hypothalamic-pituitary-adrenal (HPA) axis development continue in the postnatal period. In our study, two models of early-life stress were used: repeated maternal separation (MS) from postnatal day (PND) 2 to PND14 for 3 h daily and single maternal deprivation (MD) on PND9 for 24 h. Effects of each type of early-life stress on neuron density, neurogenesis, microglial morphology, and HPA axis programming were studied in 15-day-old male mice. Neither early-life stress paradigm affected the expression of stress-related genes (Crh, Avp, Crhr1, Crhr2, Nr3c1, and Nr3c2) and the serum level of corticosterone on PND15. Immunohistochemical analysis was performed on slices of the hippocampus and prefrontal cortex (PFC) with antibodies against a marker of mature neurons (NeuN), of microglia (Iba1), proliferating cells (Ki67), and immature neurons (DCX). We found higher density of ameboid microglia and intermediate microglia in the PFC in groups MS and MD, respectively, than in a control group. In both stressed groups, a higher number of Ki67-positive cells was noted in the dentate gyrus. Thus, in mice, the process of transformation of ameboid microglia into ramified ones as well as a neurogenesis reduction take place during the second postnatal week, whereas early-life stress can disturb these processes in a stress- and region-dependent manner.