Effects of neonatal corticosteroid treatment on hippocampal synaptic function

There is growing concern about long-term neurodevelopmental outcomes after neonatal corticosteroid treatment for chronic lung disease (CLD). Here, we use a protocol with tapering doses of dexamethasone (DEX) or hydrocortisone (HC) proportional to those used in preterm infants to examine the long-term consequences of these treatments on hippocampal synaptic plasticity and associative memory in later life. We found that neonatal DEX, but not HC, treatment impairs long-term potentiation (LTP) but enhances long-term depression (LTD) induction in adolescent rats. The effects of neonatal DEX treatment on LTP and LTD were prevented when the animals were given glucocorticoid receptor antagonist, RU38486, before DEX administration. We also found that neonatal DEX, but not HC, treatment induces a profound increase in the autophosphorylation of α isoform of Ca/calmodulin-dependent protein kinase II at threonine-286 and a decrease in the protein phosphatase 1 expression. In addition, only neonatal DEX treatment disrupts memory retention in rats subjected to passive avoidance learning tasks. These results demonstrate that only neonatal DEX treatment alters the hippocampal synaptic plasticity and associative memory formation in later life and thus suggest that HC may be a safer alternative to DEX for the treatment of CLD in the neonatal period.