Expression of the NR2A and 2B Subunits of N-Methyl-D-aspartate Receptors in Hypoxic Neonatal Rat Hippocampus

PURPOSE: The developing brain has been reported to be extremely susceptible to toxicity of ischemia and/or hypoxia during a restricted developmental period. Hippocampal neuronal cell death is a typical type of perinatal hypoxic brain lesion and often coexists with other forms of cerebral hypoxic injuries. In the present study, we examined whether transcriptional changes of NR2A and NR2B subunits of the N-Methyl-D-aspartate(NMDA) receptors related to the neuronal cell death to hypoxic toxicity are involved in developing neurons in the hippocampus. METHODS: We examined the lesion produced by in vivo direct exposure of 92% N2 and 8% O2 for 2 hours at postnatal day 7. Hippocampal sections from the 7th and 14th days after hypoxia were obtained, and the amount of the NR2A and NR2B mRNA subunits were measured by in situ hybridization using the antisense probe to the NR2A and NR2B subunits. To determine the effects of molecular changes of NMDA receptor subunits, morphological changes of neurons and/or accompanying astrocytosis were evaluated by H&E and immunohistochemical stain. RESULTS: Fourteen days after hypoxia, the expression of NR2B significantly increased whereas NR2A showed distinct reduction compared with that of control rat pups. At this time, unexpectedly, neurons in CA3 region showed prominant reduction of the actual numbers and accompanied reactive astrocytosis. CONCLUSION: Alteration of NR2A and NR2B expression to hypoxic insults, suggest the possibility that changes of the NR2 subunits which can alter the function of the NMDA receptor play a crucial role in the occurrence of developmentally specific hippocampal neuronal injury.

Reactive Astrocytosis in Post-ischemic Rat Hippocampal Formation / 대한해부학회지

In the present studies, changes of the glial fibrillary acidic protein (GFAP) expression in the astrocytes of the rat hippocampal formation were examined in response to the bilateral carotid artery occlusion for 10 minutes along with a decrease of mean arterial blood pressure (MABP) to 50 mmHg. Their relations to neuronal viability were also studied by H&E staining. In early postischemic period, mild increase of the GFAP expression was observed and this was not only confined to the mild-necrotic (CA3 and dentate gyrus) regions but also in the non-necrotic regions (CA1 and subiculum) at postischemic 8 h. This suggest that astrocytosis during early postischemic period may be resulted from nonspecific reaction associated with changes in brain environment. In contrast, in late phase of the postischemia, a marked increase of the GFAP expression was observed at day 4. Moreover, cell bodies were significantly larger and many prominent and numerous processes were observed, suggesting that this may also contribute to the significant increase in the GFAP expression. Importantly, these cellular changes were only confined to the regions of massive necrosis such as subiculum and inner granular cell layer of dentate gyrus and were not observed in the non-necrotic regions (except CA1). In contrast, the GFAP expression in astrocytes were returned to control levels in mildly damaged CA3 region by 4 days. Thus reactive astrocytosis with upregulation of the GFAP in the late postischemic period with structural transformation in the regions of massive necrosis may contribute to the damages in the neighboring neurons.