This dissertation, "The Role of Peroxynitrite in Neurotoxicity and Neurogenesis, Friend or Foe?" by Xingmiao, Chen, 陈幸苗, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Peroxynitrite (ONOO-), produced from the reaction of superoxide anion and nitric oxide (NO), plays important roles in many physiological and pathological processes as it is far more active than its precursors. However, the lack of specific and direct detection methods slow down the pace of its related research. To this end, five new peroxynitrite probes have been tested with different properties, including HKGreen-4A, 9-32, HKYellow-AM, 9-40 and MitoPN-1. Overwhelming evidence shows peroxynitrite is responsible for neurotoxicity in cerebral ischemia-reperfusion injury. Development of drugs for scavenging ONOO- becomes an important therapeutic strategy for ischemic stroke. Danshen, the dried root of salvia miltiorrhiza Bunge, is a traditional Chinese Medicine commonly used for cerebrovascular diseases. With the screening platform based on the new probe HKYellow-AM, several hydrophilic compounds from Danshen were identified to be peroxynitrite scavengers. Among them, Salvianolic acid B (SAB) possessed the highest peroxynitrite scavenging activity. Another compound, Sodium Danshensu (SDSS), the sodium salt of Danshensu, is not only a representative active compound of Danshen but also the degradation product of these hydrophilic compounds in vivo. Therefore, the potential neuroprotective effects of SAB and SDSS were studied. Both SAB and SDSS possessed strong ONOO- scavenging activity and consequently protected neuronal cell line SH-SY5Y and cortical neurons from both ONOO- and oxygen-glucose deprivation-reoxygenation induced cell death. SDSS also inhibited ONOO- formation partly by scavenging NO and superoxide. In vivo focal cerebral ischemia-reperfusion experiments revealed that both SAB and SDSS remarkably reduced protein nitration level, cell death and infarct volume in ischemia-reperfused rat brains. However, as "coin has its two sides," whether peroxynitrite could also act as a signaling molecule has not been well known yet. The discovery of adult neurogenesis brings hope for brain repair after ischemic stroke. Recent studies indicate low concentration of peroxynitrite promotes endothelial cell growth for angiogenesis and contributes to hypoxia-induced muscle cell proliferation. To investigate the role of peroxynitrite in neurogenesis, several experiments on neural stem/progenitor cells (NSCs) were performed. The results indicated low concentrations of peroxynitrite promoted NSCs proliferation, self-renewal and neuronal differentiation. The increased peroxynitrite during hypoxia has been visualized by MitoPN-1 staining. Meanwhile, peroxynitrite decomposition catalysts (PDCs, FeTMPyP and FeTPPS) treatment reduced hypoxia-induced peroxynitrite formation, NSCs proliferation, self-renewal and neuronal differentiation. Moreover, effects of peroxynitrite on neurogenesis were partly through activating HIF-1α correlated with enhanced Wnt/β-catenin signaling pathway. In addition, the different effects of peroxynitrite on neurogenesis and neurotoxicity depended on its concentration. In summary, five new probes with different properties have been tested to be sensitive and specific peroxynitrite probes. With these probes, I found low concentration of peroxynitrite promoted neurogenesis, and endogenous peroxynitrite generation contributed to hypoxia-induced neurogenesis. On the other hand, high concentrations of peroxynitrite-induced neurotoxicity could be attenuated by SAB and SDSS,