Research reportPost-ischemic hypothermia for 24 h in P7 rats rescues hippocampal neuron: Association with decreased astrocyte activation and inflammatory cytokine expression
Introduction
Neonatal hypoxic–ischemic encephalopathy, caused by perinatal asphyxia, remains an important contributor to perinatal mortality and long-term neurological impairments in term and preterm survivors [13], [21]. The mechanisms underlying hypoxic–ischemic brain damage are only partially understood but are commonly suggested to include excitotoxicity, and induction of secondary apoptosis and inflammation [8]. Previous studies have shown that hypoxia–ischemia (HI) results in a cascade of biochemical events and pathological reactions that lead to neuronal death in the hippocampus CA1 region [9], [25].
Astrocytes are the most numerous cell types in the central nervous system (CNS). They provide structural, metabolic and trophic support to neurons and modulate synaptic activity [39]. There is considerable evidence that astrocyte-conditioned media support the survival of neurons in vitro[12], [18]. Long-term recovery after brain injury, through neurite outgrowth, synaptic plasticity, or neuron regeneration, is influenced by astrocyte surface molecule expression and trophic factors such as glial cell line-derived neurotrophic factor (GDNF). Impairment in these astrocyte functions during HI or other insults can critically influence neuron survival. Moreover, the inevitable consequence of ischemic infarct is glial scar formation, which, in the long-term, prevents axonal growth and impedes recovery [26]. Besides, a previous study has shown that astrocytes are the major source of interleukin-6 (IL-6) in CNS injury and inflammation [16]. Reactive astrocytes release inflammatory cytokines such as tumor necrosis factor α (TNF-α) and IL-6 through a carrier-dependent mechanism, which can result in sustained modulatory action on neighboring neurons [6], [42]. These cytokines are thought to be involved in ischemic encephalopathy, as TNF-α and IL-6 are elevated in infants with hypoxic–ischemic encephalopathy [5], [35].
Hypothermia is effective in reducing brain injury and improving behavioral recovery in animal experiments [27] and is also safe and effective in clinical settings [15], [19]. Previous studies have shown that mild hypothermia preserves neurons and reduces astrocyte proliferation in animal models of lethal hemorrhage, transient forebrain ischemia, and asphyxial cardiac arrest [3], [17], [23]. It has been shown recently that post-ischemic hypothermia reduces IL-18 expression and suppresses microglial activation accompanied by decreased loss of MAP-2 immunoreactivity in the cortex [14]. Nevertheless, it is still unclear if mild hypothermia can attenuate neuron loss in the hippocampus of hypoxic–ischemia injured immature rat brain, and the corresponding changes of astrocyte proliferation and its related inflammatory factors response.
In the present study, we investigated whether hypothermia neuroprotection after hypoxia–ischemia in the neonatal rat was associated with changes in astrocyte activation and expression of astrocyte-associated inflammatory factors.
Section snippets
Animal models
This study was conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals, and all protocols were approved by the Committee on Ethics in the Care and Use of Laboratory Animals in Shanghai. Sprague–Dawley rats were purchased from Shanghai Experimental Animal Center of the Chinese Academy of Sciences. Neonatal HI was induced in rats at postnatal day 7, according to the method described by Rice et al. [33]. Seven-day-old pups of either sex were
Hypothermia reduced infarct volume and neuron loss after HI in hippocampus CA1 region
Nissl staining (Fig. 2A and B) revealed a significant reduction in the infarct volume of animals treated with hypothermia in comparison with normothermia (Fig. 2G). Histologically, the injured hippocampus appeared to be well preserved (Fig. 2B and b) in animals treated with hypothermia as compared to normothermia. In the latter, the injured hippocampal tissue appeared to be relatively deformed with great cell loss in the CA1 region (Fig. 2a, yellow arrowhead). To assess the effects of
Discussion
In the present study we found that the reduction in volume of infarct and neuronal loss in the hypothermia treated rats brain after hypoxia–ischemia was accompanied by a marked reduction of astrocytosis and of TNF-α and IL-6 mRNA and protein levels in the ipsilateral hippocampus. Contrary to our initial hypothesis there was no increase in GDNF mRNA expression in the hypothermia group up to 72 h after HI compared with normothermia. Although there was a significant increase in GDNF protein at 24
Conflict of interest
There is no conflict of interest about this manuscript.
Acknowledgement
This study was supported by a grant from National Natural Science Foundation of China (No. 30672256).
References (42)
- et al.
Early microglial reaction following mild forebrain ischemia induced by common carotid artery occlusion in rats
Brain Res.
(2000) - et al.
Profound hypothermia protects neurons and astrocytes, and preserves cognitive functions in a Swine model of lethal hemorrhage
J. Surg. Res.
(2005) - et al.
IL-1beta, IL-6 and TNF-alpha and outcomes of neonatal hypoxic ischemic encephalopathy
Brain Dev.
(2006) - et al.
Free radicals, mitochondria, and hypoxia–ischemia in the developing brain
Free Radic. Biol. Med.
(2006) - et al.
Ionotropic glutamate receptors and glutamate transporters are involved in necrotic neuronal cell death induced by oxygen-glucose deprivation of hippocampal slice cultures
Neuroscience
(2005) - et al.
Postischemic moderate hypothermia inhibits CA1 hippocampal ischemic neuronal injury
Neurosci. Lett.
(1989) - et al.
Astrocyte-mediated trophic support of developing serotonin neurons: effects of ethanol, buspirone, and S100B
Brain Res. Dev. Brain Res.
(2001) - et al.
Post-ischemic hypothermia reduced IL-18 expression and suppressed microglial activation in the immature brain
Brain Res.
(2006) - et al.
Selective head cooling with mild systemic hypothermia after neonatal encephalopathy: multicentre randomised trial
Lancet
(2005) - et al.
Postischemic mild hypothermia reduces neurotransmitter release and astroglial cell proliferation during reperfusion after asphyxial cardiac arrest in rats
Brain Res.
(2004)