Repeated maternal separation in the neonatal rat: cellular mechanisms contributing to brain growth sparing

Separation of neonates from their dam has been shown to evoke acutely a variety of biochemical responses, typified by depression of ornithine decarboxylase (ODC) activity. In the current study where rat pups were removed from their nursing dams for 6 h, ODC activities in the liver, heart, kidney and lung were markedly suppressed, but the enzyme in the brain was not altered during the early postnatal ages. These data suggest that the brain was protected from maternal separation insults, a homeostatic response mediated in part, by an increase of circulating corticosterone and glycogen mobilization from peripheral tissues, particularly the liver. In addition, we examined whether these responses were extended to pups who were subject to repeated episodes of maternal deprivation, and whether this stress paradigm might be associated with corresponding changes of cellular growth and maturation. Pups were removed from their dams for 6 h daily beginning at 4 days of age until weaning at 21 days. Plasma corticosterone levels of the deprived pups were elevated significantly at the end of each stress episode but returned to basal (control) levels subsequently. The repeating stress paradigm did not influence the magnitude of this hormonal response at the ensuing ages. Consistent with findings observed in the single episodes of stress, ODC activities in the peripheral tissues were significantly depressed in pups subject to repeated maternal deprivation, but the enzyme appeared to recover to control levels 18 h after each insult. In contrast, brain ODC activity did not exhibit any change throughout the period examined. Moreover, while ontogenetic gains of DNA and protein in the peripheral tissues of the deprived rats lagged slightly but consistently behind those of controls, these macromolecules in the brain were not affected appreciably. These results thus suggest that brain growth was, by and large, spared from insults associated with repeated maternal separation; but this stressful paradigm did produce marked, though reversible biochemical and physiological responses in the peripheral tissues of neonates, which cumulatively led to a lag of cellular development.