Microglial proliferation attenuates sickness responses in adult mice during endotoxin-induced inflammation.

We previously reported that the single peripheral administration of lipopolysaccharide (LPS) induced robust and transient microglial proliferation or increased the microglial population in the circumventricular organs (CVOs) and other regions, including the hypothalamus, medulla oblongata, and limbic system. However, the functional significance of an increased microglial population during endotoxin-induced inflammation remains unclear. The present study showed microglial proliferation in the mouse brain during inflammation induced by 50 mg/kg zymosan, 160 nmol/kg prostaglandin E2, and 5 mg/kg LPS. The inhibition of LPS-induced microglial proliferation with a continuous i.c.v. infusion of mitotic inhibitor cytosine arabinoside (AraC) caused persistent decreases in body weight and food and water intakes. The continuous infusion of AraC also prolonged LPS-induced sickness responses, such as lower locomotor activity and core body temperature. Collectively, the present results indicate that a transient increase in the microglial population is beneficial during endotoxin-induced inflammation in the mouse brain because it attenuates sickness responses.

Depletion of microglia and macrophages with clodronate liposomes attenuates zymosan-induced Fos expression and hypothermia in the adult mouse.

Toll-like receptor 2 (TLR2) recognizes a wide range of microbial molecules and plays critical roles in the initiation of innate immune responses. In the present study, we aimed to investigate whether the depletion of microglia and macrophages with clodronate liposomes (Clod-Lips) attenuates the activation of mouse brain circuits for TLR2-mediated inflammation and hypothermia. The peripheral administration of the TLR2 agonist zymosan induced nuclear factor-κB activation in microglia and macrophages and Fos expression in astrocytes/tanycytes and neurons in the circumventricular organs (CVOs). The depletion of microglia and macrophages with Clod-Lips markedly decreased zymosan-induced Fos expression in astrocytes/tanycytes and neurons in the CVOs. The treatment with Clod-Lips significantly attenuated zymosan-induced hypothermia. These results indicate that microglia and macrophages in the CVOs participate in the initiation and transmission of inflammatory responses after the peripheral administration of zymosan.

Microglia are continuously activated in the circumventricular organs of mouse brain.

Microglia are the primary resident immune cells of the brain parenchyma and transform into the amoeboid form in the "activated state" under pathological conditions from the ramified form in the "resting state" under physiologically healthy conditions. In the present study, we found that microglia in the circumventricular organs (CVOs) of adult mice displayed the amoeboid form with fewer branched cellular processes even under normal conditions; however, those in other brain regions showed the ramified form, which is characterized by well-branched and dendritic cellular processes. Moreover, microglia in the CVOs showed the strong protein expression of the M1 markers CD16/32 and CD86 and M2 markers CD206 and Ym1 without any pathological stimulation. Thus, the present results indicate that microglia in the CVOs of adult mice are morphologically and functionally activated under normal conditions, possibly due to the specialized features of the CVOs, namely, the entry of blood-derived molecules into parenchyma through fenestrated capillaries and the presence of neural stem cells.

Brain Region-dependent Heterogeneity and Dose-dependent Difference in Transient Microglia Population Increase during Lipopolysaccharide-induced Inflammation.

Numerous studies have reported the importance of microglial activation in various pathological conditions, whereas little attention has been given to the point for dynamics of microglial population under infection-induced inflammation. In the present study, the single systemic stimulation of 100 µg/kg lipopolysaccharide (LPS) induced robust microglial proliferation only in the circumventricular organs (CVOs) and their neighboring brain regions. More than half of microglia similarly showed proliferative activity in the CVOs and their neighboring brain regions after 1 mg/kg LPS stimulation, while this stimulation expanded microglia-proliferating brain regions including the hypothalamus, medulla oblongata, and limbic system. Microglia proliferation resulted in a transient increase of microglial density, since their density almost returned to basal levels within 3 weeks. Divided microglia survived at the same rate as non-divided ones. Proliferating microglia frequently expressed a resident microglia marker Tmem119, indicating that increase of microglia density is due to the proliferation of resident microglia. Thus, the present study demonstrates that transient increase in microglia density depends on the brain region and dose of LPS during infection-induced inflammation and could provide a new insight on microglia functions in inflammation and pathogenesis of brain diseases.

Lifespan extension in the spontaneous dwarf rat and enhanced resistance to hyperoxia-induced mortality.

Lifespan extension has been demonstrated in dwarfism mouse models relative to their wild-type. The spontaneous dwarf rat (SDR) was isolated from a closed colony of Sprague-Dawley (SD) rats. Growth hormone deficiencies have been indicated to be responsible for dwarfism in SDR. Survival time, the markers of oxidative stress, antioxidant enzymes, and resistance to hyperoxia were compared between SDR and SD rats, to investigate whether SDR, a dwarfism rat model, also extends lifespan and has an enhanced resistance to oxidative stress. SDRs lived 38% longer than SD rats on average. This is the first report to show that dwarf rats exhibit lifespan extensions similar to Ames and Snell mice. Decreased 8-oxo-2'-deoxyguanosine (8-oxodG) content, a marker of oxidative DNA damage, indicated suppressed oxidative stress in the liver, kidney, and lung of SDRs. Increased glutathione peroxidase enzyme activity was consistent with decreased 8-oxodG content in the same tissues. The heart and brain showed a similar tendency, but this was not significant. However, the catalase and superoxide dismutase enzyme activities of SDRs were not different from those of SD rats in any tissue. This was not what the original null hypothesis predicted. SDRs had potent resistance to the toxicity associated with high O2 (85%) exposure. The mean survival time in SDRs was more than 147% that of SD rats with 168h O2 exposure. These results suggest that the enhanced resistance to oxidative stress of SDRs associated with enhanced hydrogen peroxide elimination may support its potential role in lifespan extension.

Spontaneous dwarf rat: a novel model for aging research.

AIM: Dwarf animal models can provide new models for aging research. For the spontaneous dwarf rat (SDR), a dwarf strain derived from the Sprague-Dawley (SD) rat, no data relevant to aging research are available. The present study aimed to examine its growth, hormonal background, lifespan and age-related diseases. METHODS: Male SDR and SD rats were used for growth comparison and for immunohistochemistry and plasma hormonal analysis. SDR of each sex were maintained until natural death and then inspected pathologically. RESULTS: SDR showed an apparent dwarfism in their youth. Immunohistochemistry indicated that the development of growth hormone (GH)-positive cells in the pituitary was insufficient in SDR. In SDR, plasma GH levels were lower than in SD rats. Moreover, both insulin-like growth factor-1 (IGF-1) and insulin levels were decreased compared to levels in SD rats. Male and female SDR showed a mean lifespan of 29.3 +/- 3.3 and 26.8 +/- 5.3 months, respectively. The main neoplastic lesions in SDR were pituitary and mammary tumors. Major non-neoplastic lesions were incisor malocclusion, heart disease, chronic nephropathy (male) and cerebral hemorrhage (female). Most cases of chronic nephropathy were mild. CONCLUSION: Compared with longevity data and pathological data reported for SD rats, the lifespan in SDR was increased by 20-40% in males and 10-20% in females, and SDR had characteristic decreases in pituitary and mammary tumors as well as in severe chronic nephropathy. The SDR, differing in endocrinology, longevity and pathology from the SD rat, is potentially a new animal model for aging research.