Effects of Neonatal Corticosteroid Treatment on Hippocampal Synaptic Function

0031-3998/07/6203-0267 PEDIATRIC RESEARCH Copyright © 2007 International Pediatric Research Foundation, Inc. Vol. 62, No. 3, 2007 Printed in U.S.A. Effects of Neonatal Corticosteroid Treatment on Hippocampal Synaptic Function CHIUNG-CHUN HUANG, HSIUE-RU LIN, YING-CHING LIANG, AND KUEI-SEN HSU Department of Pharmacology, College of Medicine, National Cheng Kung University, Tainan City 701, Taiwan study, we used a clinically relevant rodent model to compare long-term effects of neonatal treatment with DEX and HC on hippocampal synaptic plasticity and associative memory in adolescent rats. Our results indicate that neonatal DEX, but not HC, treatment switched the direction of synaptic plasticity, favoring LTD over LTP, in the hippocampal CA1 region, as well as an impaired memory retention subjected to a one-trial passive avoidance learning task and that these alterations are highly correlated with an increase in the autophosphorylation of ␣ isoform of Ca2⫹/calmodulin-dependent protein kinase II (␣CaMKII) at threonine-286 and a decrease in the protein phosphatase 1 (PP1) expression. ABSTRACT: 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 ␣ isoform of Ca2⫹/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. (Pediatr Res 62: 267–270, 2007) MATERIALS AND METHODS orticosteroids, in particular DEX, have been widely used to prevent or treat CLD in preterm infants (1–3). However, there is growing clinical and experimental evidence that this therapy may adversely affect somatic growth and neurodevelopmental outcomes (4 – 8). Three recent retrospective studies have shown that postnatal HC treatment for CLD had less prominent or even absence of adverse effects on neurodevelopment compared with DEX therapy at school age (e.g. a reduction of cortical gray matter volume) (9 –11). In addition, it has also been shown that HC-treated infants had significantly better school performance in comparison with their DEX-treated counterparts at 5–7 y of age (9). Therefore, it was deemed of interest to compare in detail the long-term effects of DEX and HC treatment on brain function during early development. Because controlled prospective and mechanistic studies in human material are limited, a plausible way to address these questions is using an experimental animal model that closely reproduces a clinical setting for corticosteroid treatment in preterm human neonates. In the present Animals. All procedures were performed in accordance with National Institutes of Health guidelines for animal research and approved by the Institutional Animal Care and Use Committee at National Cheng Kung University. Within 6 – 8 h after delivery (d 0), pups were removed from their nests and six male pups were placed back with each dam. Pups were weaned at 21 d of age and remained group housed with littermates until experimentation at 5 wk of age. Drug treatment. Three treatment groups were assigned for each litter: saline (SAL)-, DEX-, and HC-treated animals. Pups in the DEX and HC groups received a daily i.p. injection of DEX or HC (Sigma Chemical Co., St. Louis, MO) between P1 and P3. DEX was given in tapering doses of 0.5 mg/kg on P1, 0.3 mg/kg on P2, and 0.1 mg/kg on P3. HC was given in tapering doses of 5 mg/kg on P1, 3 mg/kg on P2, and 1 mg/kg on P3. Doses of DEX and HC were selected based on published studies (7,8). Animals in the vehicle group received equivalent volumes of intraperitoneal injection of sterile SAL. Hippocampal slice preparations and electrophysiology. Hippocampal slices (400 ␮m thick) were prepared from 5-wk-old rats using standard procedures (8); the rats were allowed to recover for a minimum of 1 h and then were transferred to a submersion-type recording chamber continually perfused with 30°C–32°C oxygenated artificial cerebrospinal fluid (ACSF) solution (in mM: NaCl, 117; KCl, 4.7; CaCl2, 2.5; MgCl2, 1.2; NaHCO3, 25; NaH2PO4, 1.2; glucose, 11). Extracellular field potential recordings were carried out with Axoclamp-2B amplifier (Molecular Devices, Union City, CA). The responses were low pass–filtered at 2 kHz, digitally sampled at 5–10 kHz, and analyzed using pCLAMP software (Version 8.0; Molecular Devices). Postsynaptic responses were evoked in CA1 stratum radiatum by stimulation of Schaffer collateral/commissural afferents at 0.033 Hz with a bipolar stimulating electrode. Field excitatory postsynaptic potentials (fEPSPs) were recorded with a glass pipette filled with 1 M NaCl (2–3 M⍀ resistance) and the initial slope was measured. The long-term potentiation (LTP) was induced by high-frequency stimulation (HFS) at the test pulse intensity, consisting of two 1-s trains of stimuli separated by an intertrain Received January 11, 2007; accepted March 30, 2007. Correspondence: Kuei-Sen Hsu, Ph.D., Department of Pharmacology, College of Medicine, National Cheng Kung University, No. 1, University Rd., Tainan City 701, Taiwan; e-mail: This work was supported by a research grant (NSC95-2752-B-006-002-PAE) from the National Science Council of Taiwan. Abbreviations: ␣CaMKII, ␣ isoform of Ca2⫹/calmodulin-dependent protein kinase II; CLD, chronic lung disease; DEX, dexamethasone; HC, hydrocortisone; HFS, high-frequency stimulation; LFS, low-frequency stimulation; LTD, long-term depression; LTP, long-term potentiation; PP1, protein phosphatase 1; PSD, postsynaptic density; SAL, saline C 267 268 HUANG ET AL. interval of 20 s at 100 Hz. Long-term depression (LTD) was induced by low-frequency stimulation (LFS) delivered at 1 Hz for 15 min (900 pulses). Passive avoidance training and testing. One-trial passive avoidance learning task was selected as the tool for behavioral assessment to measure associative memory retention performance in rats from each group as described pr (...truncated)