A novel model of sensorineural hearing loss induced by repeated exposure to moderate noise in mice: the preventive effect of resveratrol.

Sensorineural hearing loss (SNHL) induced by noise has increased in recent years due to personal headphone use and noisy urban environments. The study shows a novel model of gradually progressive SNHL induced by repeated exposure to moderate noise (8-kHz octave band noise, 90-dB sound pressure level) for 1 hr exposure per day in BALB/cCr mice. The results showed that the repeated exposure led to gradually progressive SNHL, which was dependent on the number of exposures, and resulted in permanent hearing loss after 5 exposures. Repeated exposure to noise causes a loss of synapses between the inner hair cells and the peripheral terminals of the auditory nerve fibers. Additionally, there is a reduction in the expression levels of c-fos and Arc, both of which are indicators of cochlear nerve responses to noise exposure. Oral administration of resveratrol (RSV, 50 mg/kg/day) during the noise exposure period significantly prevented the noise exposure-induced synapse loss and SNHL. Furthermore, the study found that RSV treatment prevented the noise-induced increase in the gene expression levels of the proinflammatory cytokine interleukin-1ß in the cochlea. These results demonstrated the potential usefulness of RSV in preventing noise-induced SNHL in the animal model established as gradually progressive SNHL.

Treatment with cyclophosphamide in post-weaning mice causes prolonged suppression of neural stem cell proliferation in the hippocampal dentate gyrus.

The effects of anticancer drugs used in childhood on brain function in adulthood are unclear. Here, we report the long-term changes in the proliferation of neuronal stem/progenitor cells (NPCs) in the hippocampal dentate gyrus after treatment with cyclophosphamide (CYP), which is often used as a therapeutic medicine in childhood cancer. A systemic injection of CYP into 3-week-old mice decreased 5-bromo-2'-deoxyuridine (BrdU)-incorporated cells in the hippocampal subgranular zone 2 and 55 days after the injection in a dose-dependent manner. Restraint stress induced increase in corticosterone level, which was enhanced by CYP at day 35 after injection. These findings suggest that CYP injection into post-weaning mice causes prolonged alteration in NPC proliferation in the hippocampus and the stress response.

Interleukin-19 as an Immunoregulatory Cytokine.

IL-19 is a type of anti-inflammatory cytokine. Since the receptor for IL-19 is common to IL-20 and IL-24, it is important to clarify the role of each of the three cytokines. If three different cytokines bind to the same receptor, these three may have been produced to complement the other two. However, perhaps it is unlikely. Recently, the existence of a novel receptor for IL-19 was suggested. The distinction between the roles of the three cytokines still makes sense. On the other hand, because T cells do not produce IL-19, their role in acquired immunity is limited or indirect. It has been reported that IL-19 causes inflammation in some diseases but does not have an anti-inflammatory effect. In this review, we introduce the current role of IL-19 in each disease. In addition, we will describe the molecular mechanism of IL-19 and its development for the prevention of diseases. IL-19 was previously considered an anti-inflammatory cytokine, but we would like to propose it as an immunoregulatory cytokine.

Bacopa monnieri (L.) Wettst. Extract Improves Memory Performance via Promotion of Neurogenesis in the Hippocampal Dentate Gyrus of Adolescent Mice.

Bacopa monnieri L. Wettst. (BM) is a botanical component of Ayurvedic medicines and of dietary supplements used worldwide for cognitive health and function. We previously reported that administration of BM alcoholic extract (BME) prevents trimethyltin (TMT)-induced cognitive deficits and hippocampal cell damage and promotes TMT-induced hippocampal neurogenesis. In this study, we demonstrate that administration of BME improves spatial working memory in adolescent (5-week- old) healthy mice but not adult (8-week-old) mice. Moreover, improved spatial working memory was retained even at 4 weeks after terminating 1-week treatment of adolescent mice. One-week BME treatment of adolescent mice significantly enhanced hippocampal BrdU incorporation and expression of genes involved in neurogenesis determined by RNAseq analysis. Cell death, as detected by histochemistry, appeared not to be significant. A significant increase in neurogenesis was observed in the dentate gyrus region 4 weeks after terminating 1-week treatment of adolescent mice with BME. Bacopaside I, an active component of BME, promoted the proliferation of neural progenitor cells in vitro in a concentration-dependent manner via the facilitation of the Akt and ERK1/2 signaling. These results suggest that BME enhances spatial working memory in healthy adolescent mice by promoting hippocampal neurogenesis and that the effects of BME are due, in significant amounts, to bacopaside I.

Bacopa monnieri (L.) Ameliorates Cognitive Deficits Caused in a Trimethyltin-Induced Neurotoxicity Model Mice.

We previously demonstrated that Bacopa monnier (L.) WETTST. extract (BME) ameliorated cognitive dysfunction in animal models of dementia by enhancing synaptic plasticity-related signaling in the hippocampus and protecting cholinergic neurons in the medial septum. To further clarify the pharmacological features and availability of BME as a novel anti-dementia agent, we investigated whether BME affects neuronal repair using a mouse model of trimethyltin (TMT)-induced neuronal loss/self-repair in the hippocampus. Mice pretreated with TMT (2.8 mg/kg, intraperitoneally (i.p.)) on day 0 were given BME (50 mg/kg, per os (p.o.)) once daily for 15-30 d. Cognitive performance of the animals was elucidated twice by the object location test and modified Y maze test on days 17-20 (Phase I) and days 32-35 (Phase II) or by the passive avoidance test on Phase II. TMT impaired hippocampus-dependent spatial working memory and amygdala-dependent fear-motivated memory. The administration of BME significantly prevented TMT-induced cognitive deficits. The protective effects of BME on the spatial memory deficits were confirmed by Nissl staining of hippocampal tissues and propidium iodide staining of organotypic hippocampal slice cultures. Immunohistochemical studies conducted on days 17 and 32 revealed that thirty days of treatment with BME increased the number of 5-bromo-2'-deoxyuridine (BrdU)-immunopositive cells in the dentate gyrus region of TMT-treated mice, whereas fifteen days of treatment with BME had no effect. These results suggest that BME ameliorates TMT-induced cognition dysfunction mainly via protecting the hippocampal neurons from TMT-induced hippocampal lesions and partly via promoting neuroregeneration in the dentate gyrus regions.

[Adult Neurogenesis-activating Signals as Therapeutic Targets for Neurodegenerative Disorders].

In most mammalian species, adult neurogenesis appears to occur only in the olfactory bulb and hippocampal dentate gyrus, where neural stem/progenitor cells exist to create new neurons. The discovery of multi-potential neural stem/progenitor cells (NPCs) in the adult brain has precipitated a novel therapeutic strategy for harnessing these endogenous cells to aid in recovery from neurodegenerative disorders. During neurodegeneration, a plethora of endogenous factors, including cytokines, chemokines, neurotransmitters, blood-derived factors, and reactive oxygen species, are released by the activation of resident microglia, astrocytes, and infiltrating peripheral macrophages. It is interesting that these endogenous factors affect the proliferation, migration, differentiation, and survival of newly generated cells involved in the incorporation of newly generated neurons into the brain's circuitry. The unique profile of these endogenous factors can vary the degree of neuroregeneration after neurodegeneration. We show that adult neurogenesis-activating signals are regulated by endogenous factors produced during neurodegeneration.

Disruption of Gap Junction-Mediated Intercellular Communication in the Spiral Ligament Causes Hearing and Outer Hair Cell Loss in the Cochlea of Mice.

It is well-known that outer hair cell (OHC) loss occurs in the cochlea of animal models of permanent hearing loss induced by intense noise exposure. Our earlier studies demonstrated the production of hydroxynonenal and peroxynitrite, as well as the disruption of gap junction-mediated intercellular communication (GJIC), in the cochlear spiral ligament prior to noise-induced sudden hearing loss. The goal of the present study was to evaluate the mechanism underlying cochlear OHC loss after sudden hearing loss induced by intense noise exposure. In organ of Corti explant cultures from mice, no significant OHC loss was observed after in vitro exposure to 4-hydroxynonenal (a product of lipid peroxidation), H2O2, SIN-1 (peroxynitrite generator), and carbenoxolone (a gap junction inhibitor). Interestingly, in vivo intracochlear carbenoxolone injection through the posterior semicircular canal caused marked OHC and hearing loss, as well as the disruption of gap junction-mediated intercellular communication in the cochlear spiral ligament. However, no significant OHC loss was observed in vivo in animals treated with 4-hydroxynonenal and SIN-1. Taken together, our data suggest that disruption of GJIC in the cochlear lateral wall structures is an important cause of cochlear OHC loss in models of hearing loss, including those induced by noise.

Preventive effect of curcumin and its highly bioavailable preparation on hearing loss induced by single or repeated exposure to noise: A comparative and mechanistic study.

We sought to determine the preventive effects of curcumin and its highly bioavailable preparation on noise-induced hearing loss in a novel murine model of permanent hearing loss developed by repeated exposure to noise. Upon exposure to noise (8-kHz octave band noise, 90 dB sound pressure level, 1 h), hearing ability was impaired in a temporary and reversible manner. During repeated noise exposure (1-h exposure per day, 5 days), there was a progressive increase in the auditory threshold shift at 12 and 20 kHz. The threshold shift persisted for at least 6 days after noise exposure. Oral administration of curcumin for 3 days before and each day during noise exposure significantly alleviated the hearing loss induced by repeated noise exposure. Curcumin abolished intranuclear translocation of nuclear factor-κB-p65 and generation of 4-hydroxynonenal-adducted proteins found in the cochlea after noise exposure. Theracurmin®, a highly absorbable and bioavailable preparation of curcumin, had strong preventive effects on hearing loss induced by repeated noise exposure. Together, these data suggest that curcumin exerts a preventive effect on noise-induced hearing loss and is therefore a good therapeutic candidate for preventing sensorineural hearing loss.

Calpain inhibitor alleviates permanent hearing loss induced by intense noise by preventing disruption of gap junction-mediated intercellular communication in the cochlear spiral ligament.

Our previous studies demonstrated that intense noise-induced hearing loss might be at least in part due to an oxidative stress-induced decrease in the level of gap junction-composing protein connexins in the spiral ligament (SL) of the cochlear lateral wall structures in mice. Further, an in vivo exposure of mice to intense noise activates calpain in the cochlear SL. Based on these studies, we sought to determine whether a calpain inhibitor would prevent an intense noise exposure from causing hearing loss, disruption of gap junction-mediated intercellular communication (GJIC) in the SL. An exposure of mice to intense noise (8-Hz octave band noise, 110-dB sound pressure level, 1h) produced permanent hearing loss and cochlear hair cell death. The results of an ex vivo assay using gap-fluorescence recovery after photobleaching of dissected lateral wall structures revealed that the intense noise disrupted GJIC in the cochlear SL at day-7 post exposure. A prior intracochlear injection of the calpain inhibitor PD150606 significantly abolished this noise-induced hearing loss on days 5 and 7 post exposure. Similarly, PD150606 prevented noise-induced hair cell death and the GJIC disruption on day-7 post exposure. The intense noise temporarily enhanced the gene expression of calpain subtypes Capn1 and Capn2 immediately after exposure. Taken together, our data suggest that calpain inhibitor alleviated the noise-induced hearing loss, at least in part, by preventing disruption of GJIC in the cochlear SL. It possible that calpain inhibitors would be useful as a candidate of therapeutic drugs for sudden sensorineural hearing loss.

Protease-activated receptor-1 negatively regulates proliferation of neural stem/progenitor cells derived from the hippocampal dentate gyrus of the adult mouse.

Thrombin-activated protease-activated receptor (PAR)-1 regulates the proliferation of neural cells following brain injury. To elucidate the involvement of PAR-1 in the neurogenesis that occurs in the adult hippocampus, we examined whether PAR-1 regulated the proliferation of neural stem/progenitor cells (NPCs) derived from the murine hippocampal dentate gyrus. NPC cultures expressed PAR-1 protein and mRNA encoding all subtypes of PAR. Direct exposure of the cells to thrombin dramatically attenuated the cell proliferation without causing cell damage. This thrombin-induced attenuation was almost completely abolished by the PAR antagonist RWJ 56110, as well as by dabigatran and 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), which are selective and non-selective thrombin inhibitors, respectively. Expectedly, the PAR-1 agonist peptide (AP) SFLLR-NH2 also attenuated the cell proliferation. The cell proliferation was not affected by the PAR-1 negative control peptide RLLFT-NH2, which is an inactive peptide for PAR-1. Independently, we determined the effect of in vivo treatment with AEBSF or AP on hippocampal neurogenesis in the adult mouse. The administration of AEBSF, but not that of AP, significantly increased the number of newly-generated cells in the hippocampal subgranular zone. These data suggest that PAR-1 negatively regulated adult neurogenesis in the hippocampus by inhibiting the proliferative activity of the NPCs.

Involvement of calpain in 4-hydroxynonenal-induced disruption of gap junction-mediated intercellular communication among fibrocytes in primary cultures derived from the cochlear spiral ligament.

The endocochlear potential in the inner ear is essential for hearing ability, and maintained by various K(+) transport apparatuses including Na(+), K(+)-ATPase and gap junction-mediated intercellular communication (GJ-IC) in the lateral wall structures of the cochlea. Noise-induced hearing loss is known at least in part due to disruption of GJ-IC resulting from an oxidative stress-induced decrease in connexins (Cxs) level in the lateral wall structures. The purpose of this study was to investigate, using primary cultures of fibrocytes from the cochlear spiral ligament of mice, the mechanism underlying GJ-IC disruption induced by 4-hydroxynonenal (4-HNE), which is formed as a mediator of oxidative stress. An exposure to 4-HNE produced the following events: i.e., an increase in 4-HNE-adducted proteins; a decrease in the protein levels of Cx43, ß-catenin, and Cx43/ß-catenin complex along with intracellular translocation of this complex from the cell membrane to the cytoplasm; enhanced calpain-dependent degradation of endogenous α-fodrin; and disruption of GJ-IC. The 4-HNE-induced decrease in these protein levels and disruption of GJ-IC were most completely abolished by the calpain inhibitor PD150606. Taken together, our data suggest that 4-HNE disrupted GJ-IC through calpain-mediated degradation of Cx43 and ß-catenin in primary cultures of fibrocytes derived from the cochlear spiral ligament.

Beneficial effect of cilostazol-mediated neuronal repair following trimethyltin-induced neuronal loss in the dentate gyrus.

Cilostazol acts as an antiplatelet agent and has other pleiotropic effects based on phosphodiesterase-3-dependent mechanisms. We evaluated whether cilostazol would have a beneficial effect on neuronal repair following hippocampal neuronal damage by using a mouse model of trimethyltin (TMT)-induced neuronal loss/self-repair in the hippocampal dentate gyrus [Ogita et al. (2005) J Neurosci Res 82:609-621]; these mice will hereafter be referred to as impaired animals. A single treatment with cilostazol (10 mg/kg, i.p.) produced no significant change in the number of 5-bromo-2'-deoxyuridine (BrdU)-incorporating cells in the dentate granule cell layer (GCL) or subgranular zone on day 3 after TMT treatment. However, chronic treatment with cilostazol on days 3-15 posttreatment resulted in an increase in the number of BrdU-incorporating cells in the dentate GCL of the impaired animals, and these cells were positive for neuronal nuclear antigen or doublecortin. Cilostazol was effective in elevating the level of phosphorylated cyclic adrenosine monophosphate response element-binding protein (pCREB) in the dentate gyrus of impaired animals. The results of a forced swimming test revealed that the chronic treatment with cilostazol improved the depression-like behavior seen in the impaired animals. In the cultures of hippocampal neural stem/progenitor cells, exposure to cilostazol produced not only enhancement of proliferation activity but also elevation of pCREB levels. Taken together, our data suggest that cilostazol has a beneficial effect on neuronal repair following neuronal loss in the dentate gyrus through promotion of proliferation and/or neuronal differentiation of neural progenitor cells in the subgranular zone.

Possible involvement of caspases in proliferation of neocortical neural stem/progenitor cells in the developing mouse brain.

Caspases are well-known enzymes that work as initiators and effectors of apoptosis. To elucidate the role of caspases in neurodevelopment, we sought to determine if caspases are involved in the proliferation of neural stem/progenitor cells (NPCs) in the developing mouse brain. Labeling with 5-bromo-2'-deoxyuridine (BrdU) from days 14 to 18 of pregnant mice revealed that the 18-d old fetus had many BudU-positive cells in its brain. Double-labeling revealed that active caspase-3 was co-localized with these BrdU-positive cells in the neocortex, hippocampus, and subventricular zone of the fetal brain. Active caspase-3 was detected in cultures of NPCs derived from the neocortex of 15-d old fetuses during culture periods. Importantly, the pan-caspase inhibitor z-VAD-FMK was effective at completely inhibiting neurosphere formation by the NPCs. These results suggest the possibility that the caspase cascade is essential for the proliferation of neocortical NPCs in the developing mouse brain.

Disruption of ion-trafficking system in the cochlear spiral ligament prior to permanent hearing loss induced by exposure to intense noise: possible involvement of 4-hydroxy-2-nonenal as a mediator of oxidative stress.

Noise-induced hearing loss is at least in part due to disruption of endocochlear potential, which is maintained by various K(+) transport apparatuses including Na(+), K(+)-ATPase and gap junction-mediated intercellular communication in the lateral wall structures. In this study, we examined the changes in the ion-trafficking-related proteins in the spiral ligament fibrocytes (SLFs) following in vivo acoustic overstimulation or in vitro exposure of cultured SLFs to 4-hydroxy-2-nonenal, which is a mediator of oxidative stress. Connexin (Cx)26 and Cx30 were ubiquitously expressed throughout the spiral ligament, whereas Na(+), K(+)-ATPase α1 was predominantly detected in the stria vascularis and spiral prominence (type 2 SLFs). One-hour exposure of mice to 8 kHz octave band noise at a 110 dB sound pressure level produced an immediate and prolonged decrease in the Cx26 expression level and in Na+, K(+)-ATPase activity, as well as a delayed decrease in Cx30 expression in the SLFs. The noise-induced hearing loss and decrease in the Cx26 protein level and Na(+), K(+)-ATPase activity were abolished by a systemic treatment with a free radical-scavenging agent, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, or with a nitric oxide synthase inhibitor, N(ω)-nitro-L-arginine methyl ester hydrochloride. In vitro exposure of SLFs in primary culture to 4-hydroxy-2-nonenal produced a decrease in the protein levels of Cx26 and Na(+), K(+)-ATPase α1, as well as Na(+), K(+)-ATPase activity, and also resulted in dysfunction of the intercellular communication between the SLFs. Taken together, our data suggest that disruption of the ion-trafficking system in the cochlear SLFs is caused by the decrease in Cxs level and Na(+), K(+)-ATPase activity, and at least in part involved in permanent hearing loss induced by intense noise. Oxidative stress-mediated products might contribute to the decrease in Cxs content and Na(+), K(+)-ATPase activity in the cochlear lateral wall structures.

Lithium promotes neuronal repair and ameliorates depression-like behavior following trimethyltin-induced neuronal loss in the dentate gyrus.

Lithium, a mood stabilizer, is known to ameliorate the stress-induced decrease in hippocampal neurogenesis seen in animal models of stress-related disorders. However, it is unclear whether lithium has beneficial effect on neuronal repair following neuronal damage in neuronal degenerative diseases. Here, we evaluated the effect of in vivo treatment with lithium on the hippocampal neuronal repair in a mouse model of trimethyltin (TMT)-induced neuronal loss/self-repair in the hippocampal dentate gyrus (such mice referred to as "impaired animals") [Ogita et al. (2005) J Neurosci Res 82: 609-621]. The impaired animals had a dramatically increased number of 5-bromo-2'-deoxyuridine (BrdU)-incorporating cells in their dentate gyrus at the initial time window (days 3 to 5 post-TMT treatment) of the self-repair stage. A single treatment with lithium produced no significant change in the number of BrdU-incorporating cells in the dentate granule cell layer and subgranular zone on day 3 post-TMT treatment. On day 5 post-TMT treatment, however, BrdU-incorporating cells were significantly increased in number by lithium treatment for 3 days. Most interestingly, chronic treatment (15 days) with lithium increased the number of BrdU-incorporating cells positive for NeuN or doublecortin in the dentate granule cell layer of the impaired animals, but not in that of naïve animals. The results of a forced swimming test revealed that the chronic treatment with lithium improved the depression-like behavior seen in the impaired animals. Taken together, our data suggest that lithium had a beneficial effect on neuronal repair following neuronal loss in the dentate gyrus through promoted proliferation and survival/neuronal differentiation of neural stem/progenitor cells in the subgranular zone.

Beneficial in vivo effect of aripiprazole on neuronal regeneration following neuronal loss in the dentate gyrus: evaluation using a mouse model of trimethyltin-induced neuronal loss/self-repair in the dentate gyrus.

Aripiprazole is used clinically as an atypical antipsychotic. We evaluated the effect of in vivo treatment with aripiprazole on the proliferation and differentiation of neural stem/progenitor cells in a mouse model, trimethyltin-induced neuronal loss/self-repair in the hippocampal dentate gyrus (referred as "impaired animals") [Ogita et al., J Neurosci Res. 82, 609 - 621 (2005)]. In the impaired animals, an increased number of 5-bromo-2'-deoxyuridine (BrdU)-positive cells was seen in the dentate gyrus at the initial time window of the self-repair stage. At the same time window, a single treatment with aripiprazole significantly increased the number of cells positive for both BrdU and nestin in the dentate gyrus of the impaired animals. Chronic treatment with aripiprazole promoted the proliferation/survival and neuronal differentiation of the cells newly-generated following the neuronal loss in the dentate gyrus of the impaired animals. The chronic treatment with aripiprazole improved depression-like behavior seen in the impaired animals. Taken together, our data suggest that aripiprazole had a beneficial effect on neuronal regeneration following neuronal loss in the dentate gyrus through indirectly promoted proliferation/survival and neuronal differentiation of neural stem/progenitor cells in the subgranular zone of the dentate gyrus.

In vivo and in vitro treatment with edaravone promotes proliferation of neural progenitor cells generated following neuronal loss in the mouse dentate gyrus.

Edaravone is clinically used in Japan for treatment of patients with acute cerebral infarction. To clarify the effect of edaravone on neurogenesis in the hippocampus following neuronal injury in the hippocampal dentate gyrus, we investigated the effect of in vitro and in vivo treatment with edaravone on the proliferation of neural stem/progenitor cells prepared from the mouse dentate gyrus damaged by trimethyltin (TMT). Histological assessment revealed the presence of large number of nestin(+) cells in the dentate gyrus on days 3 - 5 post-TMT treatment. We prepared cells from the dentate gyrus of naïve, TMT-treated mice or TMT/edaravone-treated mice. The cells obtained from the dentate gyrus of TMT-treated animals were capable of BrdU incorporation and neurosphere formation when cultured in the presence of growth factors. The TMT-treated group had a larger number of nestin(+) cells and nestin(+)GFAP(+) cells than the naïve one. Under the culture condition used, sustained exposure of the cells from the damaged dentate gyrus to edaravone at 10(-11) and 10(-8) M promoted the proliferation of nestin(+) cells. The systemic in vivo treatment with edaravone for 2 days produced a significant increase in the number of nestin(+) cells among the cells prepared from the dentate gyrus on day 4 post-TMT treatment, and as well as one in the number of neurospheres formed from these cells in the culture. Taken together, our data indicated that edaravone had the ability to promote the proliferation of neural stem/progenitor cells generated following neuronal damage in the dentate gyrus.

Opposing roles of glucocorticoid receptor and mineralocorticoid receptor in trimethyltin-induced cytotoxicity in the mouse hippocampus.

The organotin trimethyltin (TMT) is known to cause neuronal degeneration in the murine brain. Earlier studies indicate that TMT-induced neuronal degeneration is enhanced by adrenalectomy and prevented by exogenous glucocorticoid. The aim of this study was to investigate the regulation of TMT neuroxicity by corticosterone receptors including type I (mineralocorticoid receptor, MR) and type II (glucocorticoid receptor, GR) in adult mice. The systemic injection of TMT at the dose of 2.0 or 2.8 mg/kg produced a marked elevation in the level of plasma corticosterone that was both dose and time dependent. The MR agonist aldosterone had the ability to exacerbate TMT cytotoxicity in the dentate granule cell layer, whereas its antagonist spironolactone protected neurons from TMT cytotoxicity there. In contrast, the GR antagonist mifepristone exacerbated the TMT cytotoxicity. Taken together, our data suggest TMT cytotoxicity is oppositely regulated by GR and MR signals, being exacerbated by MR activation in adult mice.

[Activated microglial cells trigger neurogenesis following neuronal loss in the dentate gyrus of adult mice].

Neurological injuries are widely known to promote neurogenesis in the adult hippocampal dentate gyrus. Our previous studies demonstrated that the granule cells in the hippocampal dentate gyrus are injured and eradicated by treatment with trimethyltin (TMT), with being regenerated in the dentate granule cell layer (GCL) after neuronal loss. Recent collective reports indicate that during brain injury and in neurodegenerative disorders, neurogenesis is controlled by cytokines, chemokines, neurotransmitters, and reactive oxygen/nitrogen species, which are released by dying neurons as well as by activated macrophages, micro-glia, and astrocytes. To elucidate the role of activated microglia in the neuroregeneration following the dentate granule cell loss, in this study, we evaluated the involvement of activated microglial cells and a related factor in the generation of newly-generated cells of the hippocampal dentate gyrus following neuronal loss induced by TMT. Our results support the possibility that pro-inflammatory cytokines released from activated microglial cells may be involved in promotion of the neurogenesis mechanism through activation of the NF-kappaB signaling pathway following the dentate neuronal loss induced by TMT treatment.