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  • anatoxin a Excessive extracellular glutamate may induce exci

    2021-10-14

    Excessive extracellular glutamate may induce excitotoxic neuronal damage in disorders of the CNS (Schwartz et al., 2003). EAATs are considered to contribute to prevention of excitotoxicity by glutamate uptake. Our data revealed that expression levels of membrane EAAT proteins in astrocytes are much higher than those in microglia. As the EAAT expression levels increased, astrocytic glutamate uptake became overwhelming when compared to that by microglia. Glial activation is a neuropathological hallmark in various neurological disorders (Barger and Basile, 2001, Pekny and Nilsson, 2005). It remains undetermined whether glial activation is neuroprotective or neurotoxic. We have previously demonstrated that activated microglia produce glutamate via the upregulation of glutaminase, then release this glutamate from the connexin32 gap junction hemichannel, and thereby induce excitotoxic neuronal death (Takeuchi et al., 2006). In this study, we reconfirmed that activated microglia exert excito-neurotoxicity by glutamate release. In contrast, activated astrocytes counteracted this microglial neurotoxicity by glutamate uptake. Taken together, activated astrocytes seem to be neuroprotective by reducing extracellular glutamate levels whereas activated microglia might be neurotoxic by producing higher extracellular glutamate levels. A recent study has shown that activated microglia act on glutamate transporters in oligodendrocytes, leading to a net increase in extracellular glutamate and subsequent oligodendrocyte death (Domercq et al., 2007). Activated microglia may also downregulate or dysregulate astrocytic EAATs, which may also contribute to neurodegeneration in various neurological diseases. However, further studies are needed to elucidate such an issue. Neurons express a few EAATs compared to glial anatoxin a (Lehre and Danbolt, 1998, Tzingounis and Wadiche, 2007). Our data also suggest that effect of neuronal EAATs is too weak to protect against neurotoxicity by microglial glutamate. In this study, we have not identified which type of EAATs in glial cells plays key neuroprotective roles. We are planning more thorough studies to investigate this issue.
    Experimental procedures
    Acknowledgments
    Introduction Angiotensin II (Ang II) is a vasoactive peptide which elicits a wide range of physiological functions in the body and it is also found to be synthesized locally in the brain and exhibits central actions anatoxin a through angiotensin receptors (AT) [1]. Apart from physiological roles, brain Ang II is associated with commencement and progression of neuroinflammatory events in neurodegenerative conditions like cerebral ischemia [2]. The inflammatory responses of brain Ang II mimic the toll like receptor (TLR) activation mechanism [3]. Ang II also increases the oxidative stress in neurons by enhancing the release of mitochondrial dependent superoxide radicals [4]. The mice which are over expressed with human angiotensinogen and renin have shown increased cerebral damage because of the increase in Ang II formation [5]. It has also been shown that induction of focal ischemia in rats elevated the brain Ang II levels and up-regulated the AT1 receptor expression. Activation of microglia by Ang II/AT1 pathway has resulted in the release of pro-inflammatory cytokines like tumor necrosis factor-α (TNF-α) and interleukin-β (IL-β) [6]. The stimulation of AT1 receptor by Ang II in the nigrostriatal region enhances the activation of nicotinamide adenine dinucleotide phosphate-oxidase (NADPH-oxidase) complex, exacerbates oxidative stress, and the microglial inflammatory processes [7]. Telmisartan, an AT1 receptor blocker, reduced the blood pressure, attenuated Monocyte chemoattractant protein-1 (MCP-1) levels, and TNF-α in the cerebral cortex of spontaneously hypertensive stroke-resistant rats. This indicates the beneficial effect of AT1 blockade in ischemia [8]. Though many studies claim that blockade of AT1 receptors shows beneficial role in neurodegenerative conditions, the neuroprotective role of angiotensin II receptor 2 (AT2) receptors during brain injury cannot be ignored. Administration of AT2 receptor agonist has shown significant neuroprotection in acute ischemic models [9]. Activation of AT2 receptors with selective agonist CGP42112 has shown neuroprotection and neurogenesis after traumatic brain injury in mice [10]. Ang II protected the cortical neurons from neuronal injury through activation of Ang II/AT2 pathway in oxygen-glucose deprivation conditions [11]. However the role of AT2 receptor is not clear, because of published reports of the opposite hypothesis [12]. Therefore, further study in an intact system might help to resolve the ambiguity and elucidate the role of AT2 receptors in cerebral ischemia state.