Structural changes of hippocampi and expression of HSP70, C-FOS and dream proteins in the spinal cord after acute thermal stimulus or early stress-experienced rats
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Date
2014
Authors
Mat Taib, Che Norma
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Publisher
Universiti Sains Malaysia
Abstract
This current study was to explore the effect of acute thermal stress on the expression of HSP70, c-Fos and DREAM proteins in an attempt to understand the mechanisms of neonatal stress and the effect on stress later in life. Phase one of this study evaluated forced swimming test (FST) as a model for induction of moderate stress in neonatal rats. Forced swimming test was applied to neonatal rats on days 7, 8 and 9 of life. All the pups survived and remained healthy after FST. From day 14 to day 42, the FST group had significantly lower body weights when compared to the control group. Their body weights were back to control levels on day 49 onwards. Forced swimming test significantly increased neutrophil/lymphocyte ratios in the stress group compared to controls. There was downregulation of neurogenesis as evidenced by the decreased BrdU scores in the FST group when compared to controls. 5’- bromo-2’-deoxyuridine scores in both upper and lower blades of dentate gyrus were similar but higher than other subfields of hippocampus (CA1-3), thalamus and retrosplenial granular (RSG) areas. BrdU scores in the thalamus and RSG were significantly lower than that of the CA1-3 subfields of the hippocampus. Forced swimming test was found to be a good model for studying acute stress in neonatal pups. It was able to show differentiation of the stress response in the hippocampus.
Phase two involved the same group of rats subjected to FST in the neonatal period (stress-experienced rats) and included a comparison with non stress-experienced rats. Both groups were subdivided into control (C), pretreatment with either nor-BNI (N) or corticosterone (CO) groups, before exposure to thermal stress at 42±1°C for 15 minutes. The animals were sacrificed two hours later, followed by determination of HSP70, c-Fos and DREAM proteins expression. Neuronal cell death was evaluated with immunohistochemistry (IHC) and TEM respectively. In the control group, HSP70 was upregulated in stress-experienced and thermal rats (CTH F). Regardless of experience, the corticosterone group had the highest expression of HSP70. The highest expression of c-Fos was in the stress-experienced control group (CTH C) while stress-experienced groups (CTH F and COTH F) showed lower levels of c-Fos expression. Similarly, the highest expression of DREAM protein was in the control groups (CWTH C and CTH C) and COTH F, followed by CWTH F and CTH F. Thus, stress-experienced rats were better able to withstand subsequent stress compared to rats with no previous stress experience. In the nor-BNI group almost all proteins were less expressed, showing it worked as a kappa opioid antagonist, where the animals experienced less pain and minimum stress Thus, neonatal stress experience caused changes in the peripheral nervous system indicating that stress-experienced rats are better able to withstand subsequent acute stress during the adult stage.
However, neuronal cell death remained present in the hippocampus in CA3, followed by CA2 and CA1 subfields, suggesting that nor-BNI was tissue specific. Transmission electron microscope showed damaged cytoplasm, nucleus and mitochondria, whereas the Golgi apparatus was unaffected. COTH F group has severe damage of organelles and death occurred immediately after heat stress. Rectal temperature showed an incremental pattern even though it was not significant except
for COTH C where there was hypothermia and probably shock. Corticosterone level was not significant after two hours thermal stress suggesting it had returned to basal levels. The structure and function of the hippocampus was altered by neonatal stress experience resulting in depression of neurogenesis and increased neuronal cell death. Thus, acute thermal stress in stress-experienced rats result in a dual manifestation: (1) in the spinal cord, the acute thermal stress appears to be neuroprotective and this effect is tissue specific to the kappa-opioid receptors in the spinal cord; (2) in the brain (especially), acute thermal stress appears to cause persistent neuronal cell death and depression of neurogenesis.
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Keywords
Acute thermal