Relation Among Neuronal Death, Cell Proliferation and Neuronal Differentiation in the Gerbil Main Olfactory Bulb after Transient Cerebral Ischemia

Severe hypoxia exposure inhibits larval brain development but does not affect the capacity to mount a cortisol stress response in zebrafish

Fish nursery habitats are increasingly hypoxic and the brain is recognized as highly hypoxia-sensitive, yet there is a lack of information on the effects of hypoxia on the development and function of the larval fish brain. Here, we tested the hypothesis that by inhibiting brain development, larval exposure to severe hypoxia has persistent functional effects on the cortisol stress response in zebrafish (Danio rerio). Exposing 5 days post-fertilization (dpf) larvae to 10% dissolved O2 (DO) for 16 h only marginally reduced survival, but it decreased forebrain neural proliferation by 55%, and reduced the expression of neurod1, gfap, and mbpa, markers of determined neurons, glia, and oligodendrocytes, respectively. The 5 dpf hypoxic exposure also elicited transient increases in whole body cortisol and in crf, uts1, and hsd20b2 expression, key regulators of the endocrine stress response. Hypoxia exposure at 5 dpf also inhibited the cortisol stress response to hypoxia in 10 dpf larvae and increased hypoxia tolerance. However, 10% DO exposure at 5 dpf for 16h did not affect the cortisol stress response to a novel stressor in 10 dpf larvae or the cortisol stress response to hypoxia in adult fish. Therefore, while larval exposure to severe hypoxia can inhibit brain development, it also increases hypoxia tolerance. These effects may transiently reduce the impact of hypoxia on the cortisol stress response but not its functional capacity to respond to novel stressors. We conclude that the larval cortisol stress response in zebrafish has a high capacity to cope with severe hypoxia-induced neurogenic impairment.

Effects of total saponins of Panax notoginseng on immature neuroblasts in the adult olfactory bulb following global cerebral ischemia/reperfusion

The main active components extracted from Panax notoginseng are total saponins. They have been shown to inhibit platelet aggregation, increase cerebral blood flow, improve neurological behavior, decrease infarct volume and promote proliferation and differentiation of neural stem cells in the hippocampus and lateral ventricles. However, there is a lack of studies on whether total saponins of Panax notoginseng have potential benefits on immature neuroblasts in the olfactory bulb following ischemia and reperfusion. This study established a rat model of global cerebral ischemia and reperfusion using four-vessel occlusion. Rats were administered total saponins of Panax notoginseng at 75 mg/kg intraperitoneally 30 minutes after ischemia then once a day, for either 7 or 14 days. Total saponins of Panax notoginseng enhanced the number of doublecortin (DCX)⁺ neural progenitor cells and increased co-localization of DCX with neuronal nuclei and phosphorylated cAMP response element-binding/DCX⁺ neural progenitor cells in the olfactory bulb at 7 and 14 days post ischemia. These findings indicate that following global brain ischemia/reperfusion, total saponins of Panax notoginseng promote differentiation of DCX⁺ cells expressing immature neuroblasts in the olfactory bulb and the underlying mechanism is related to the activation of the signaling pathway of cyclic adenosine monophosphate response element binding protein.

Calretinin is expressed in the intermediate cells during olfactory receptor neuron development

The field of neurogenesis has greatly benefited from stage-specific marker discoveries. However, such markers are not well defined in the olfactory epithelium (OE), where olfactory receptor neurons (ORNs) are constantly generated throughout lifetime. During OE neurogenesis, there is a lack of markers that label cells that are at the intermediate stage before they are fully mature. In this study, we show that during embryonic development calretinin is expressed transiently in the intermediate cells right before ORNs become mature. Calretinin is expressed between the end of beta-III tubulin (an immature neuronal marker) expression and the beginning of olfactory marker protein (OMP, a mature neuronal marker) expression in ORNs. Therefore, calretinin can serve as a marker of the intermediate ORNs. With this discovery, future studies can use calretinin as a tool to define these intermediate ORNs during olfactory neurogenesis.