Cell Proliferation and Neuroblast Differentiation in the Rat Dentate Gyrus After Intrathecal Treatment with Adipose-Derived Mesenchymal Stem Cells

Mesenchymal stem cells (MSC) have emerged as a new therapeutic tool for a number of clinical applications, because they have multipotency and paracrine effects via various factors. In the present study, we investigated the effects of adipose-derived MSC (Ad-MSC) transplantation via intrathecal injection through the cisterna magna on cell proliferation and differentiation of endogenous stem cells in the hippocampal dentate gyrus (DG) using Ki-67 (a marker for proliferating cells), and doublecortin (DCX, a marker for neuroblasts). The transplanted Ad-MSC were detected in the meninges, not in the hippocampal parenchyma. However, the number of Ki-67-immunoreactive cells was significantly increased by 83% in the DG 2 days after single Ad-MSC injection, and by 67% at 23 days after repeated Ad-MSC treatment compared with that in the vehicle-treated group after Ad-MSC transplantation. On the other hand, the number of DCX-immunoreactive cells in the DG was not changed at 2 days after single Ad-MSC injection; however, it was significantly increased by 62% 9 days after single Ad-MSC injection. At 23 days after repeated Ad-MSC application, the number of DCX-immunoreactive cells was much more increased (223% of the vehicle-treated group). At this time point, DCX protein levels were also significantly increased compared with those in the vehicle-treated group. These results suggest that the intrathecal injection of Ad-MSC could enhance endogenous cell proliferation, and the repeated Ad-MSC injection could be more efficient for an enhancement of endogenous cell proliferation and differentiation in the brain.     

Effects of Adrenalectomy and Replacement Therapy of Corticosterone on Cell Proliferation and Neuroblast Differentiation in the Rat Dentate Gyrus

Newly generated neurons in the dentate gyrus differentiate into mature granule cells. In the present study, we observed the effects of adrenalectomy (ADX) and corticosterone replacement therapy (CRT) on cell death, cell proliferation and neuroblast differentiation in the subgranular zone of the hippocampal dentate gyrus (SZDG). For this, the animals received vehicle or CRT after ADX, and were sacrificed 5 or 42 days later. Plasma corticosterone levels were very low in the adrenalectomized groups, whereas CRT after ADX significant increased serum corticosterone levels at 42 days, not 5 days, after ADX. ADX induced some neuronal damage in the dentate gyrus at 5 days post-ADX. CRT did not significantly reduce the neuronal damage at 5 days post-ADX; however, neuronal damage was not shown at 42 post-ADX with CRT. Ki67 (a marker for cell proliferation) and doublecortin (DCX, a marker for neuronal differentiation) immunoreaction was detected in the SZDG. ADX transiently increased cell proliferation and neuroblast differentiation 5 days after ADX, not 42 days, after ADX, and the CRT 42 days after ADX prominently decreased cell proliferation and neuroblast differentiation in the dentate gyrus. These results suggest that adrenal corticosteroid hormone is not essential for cell proliferation and neuroblast differentiation in long-term period after ADX.     

Pyridoxine Enhances Cell Proliferation and Neuroblast Differentiation by Upregulating the GABAergic System in the Mouse Dentate Gyrus

We investigated the effects of pyridoxine (vitamin B₆) on cell death, cell proliferation, neuroblast differentiation, and the GABAergic system in the mouse dentate gyrus. We administered pyridoxine (350 mg/kg intraperitoneally) to 8 week old mice twice a day for 14 days and sacrificed them at 10 weeks of age. Pyridoxine treatment did not induce neuronal death or activate microglia in the dentate gyrus, while glial fibrillary acidic protein (GFAP)-positive cells were significantly increased in the subgranular zone of the dentate gyrus. The increase in GFAP-positive cells was confirmed to be due to proliferating cells based on double immunofluorescence staining. GFAP-positive cells, which were also labeled with Ki67, a marker for cell proliferation, and doublecortin, a marker for neuroblast differentiation, were significantly increased in the pyridoxine-treated group compared to those in the vehicle-treated group. Pyridoxine treatment also increased the protein levels of glutamic acid decarboxylase (GAD) 67, an enzyme for GABA synthesis, and pyridoxal 5′-phosphate (PNP) oxidase, an enzyme for pyridoxal phosphate synthesis, in the dentate gyrus. These results suggest that pyridoxine treatment distinctly increases cell proliferation, neuroblast differentiation, and upregulated the GABAergic system, as revealed by the increases of GAD67 and PNP oxidase in the mouse dentate gyrus.     

Effects of Cu,Zn-Superoxide Dismutase on Cell Proliferation and Neuroblast Differentiation in the Mouse Dentate Gyrus

Oxidative stress is one of the most important factors in reducing adult hippocampal neurogenesis in the adult brain. In this study, we observed the effects of Cu,Zn-superoxide dismutase (SOD1) on lipid peroxidation, cell proliferation, and neuroblast differentiation in the mouse dentate gyrus using malondialdehyde (MDA), Ki67, and doublecortin (DCX), respectively. We constructed an expression vector, PEP-1, fused PEP-1 with SOD1, and generated PEP-1-SOD1 fusion protein. We administered PEP-1 and 100 or 500 μg PEP-1-SOD1 intraperitoneally once a day for 3 weeks and sacrificed at 30 min after the last administrations. PEP-1 administration did not change the MDA levels compared to those in the vehicle-treated group, while PEP-1-SOD1 treatment significantly reduced MDA levels compared to the vehicle-treated group. In the PEP-1-treated group, the number of Ki67-positive nuclei was similar to that in the vehicle-treated group. In the 100 μg PEP-1-SOD1-treated group, the number of Ki67-positive nuclei was slightly decreased; however, in the 500 μg PEP-1-SOD1-treated group, Ki67-positive nuclei were decreased to 78.5% of the vehicle-treated group. The number of DCX-positive neuroblasts in the PEP-1-treated group was similar to that in the vehicle-treated group. However, the arborization of DCX-positive neuroblasts was significantly decreased in both the 100 and 500 μg PEP-1-SOD1-treated groups compared to that in the vehicle-treated group. The number of DCX-positive neuroblasts with tertiary dendrites was markedly decreased in the 500 μg PEP-1-SOD1-treated group. These results suggest that a SOD1 supplement to healthy mice may not be necessary to modulate cell proliferation and neuroblast differentiation in the dentate gyrus.     

Phosphoglycerate Mutase 1 Promotes Cell Proliferation and Neuroblast Differentiation in the Dentate Gyrus by Facilitating the Phosphorylation of cAMP Response Element-Binding Protein

Neurochemical Research - In a previous study, we observed a significant increase in phosphoglycerate mutase 1 (PGAM1) levels after pyridoxine treatment. In the present study, we investigated the...     

Effects of Sensitive to Apoptosis Gene Protein on Cell Proliferation, Neuroblast Differentiation, and Oxidative Stress in the Mouse Dentate Gyrus

Sensitive to apoptosis gene (SAG) protein is a redox-inducible protein that protects cells against apoptosis induced by redox agents. In this study, we observed effects of SAG on cell proliferation and neuroblast differentiation in the mouse hippocampal dentate gyrus (DG) using Ki67 and doublecortin (DCX), respectively. For easy penetration into neurons, Tat-SAG expression vector was constructed by ligation with SAG and expression vector, Tat, in-frame with six histidine open-reading frames to generate the expression vector, and cloned into E. coli DH5α cells. One or 5?mg/kg Tat-SAG fusion protein (Tat-SAG) was intraperitoneally administered to mice once a day for 3?weeks. The administration of Tat-SAG significantly increased the number of 5-bromodeoxyuridine positive cells, Ki67 positive cells and DCX immunoreactive neuroblast in the mouse DG: Especially, in the 5?mg/kg Tat-SAG-treated mice, DCX positive neuroblasts showed a well-developed arborization of tertiary dendrites in the DG. On the other hand, we examined that the administration of Tat-SAG significantly reduced the DNA damage and lipid peroxidation judging from 8-hydroxy-2'-deoxyguanosine and 4-hydroxynonenal immunohistochemistry: The decrease was much more distinct in the 5?mg/kg Tat-SAG-treated mice than 1?mg/kg Tat-SAG-treated mice. This result suggests that SAG significantly increases cell proliferation, neuroblast differentiation and oxidative stress in normal states.     

Combination Effects of Sodium Butyrate and Pyridoxine Treatment on Cell Proliferation and Neuroblast Differentiation in the Dentate Gyrus of d-Galactose-Induced Aging Model Mice

We previously reported that sodium butyrate (SB), a histone deacetylase inhibitor, robustly increased pyridoxine-induced cell proliferation and neuroblast differentiation in the dentate gyrus of the adult mouse. In this study, we investigated the effects of treatment with SB combined with pyridoxine on cell proliferation and neuroblast differentiation in the dentate gyrus of a mouse model of aging induced by d-galactose (d-gal). d-gal was administered to 20-week-old male mice (d-gal mice) for 10 weeks to induce changes that resemble natural aging in animals. Seven weeks after d-gal (100 mg/kg) treatment, vehicle (physiological saline; d-gal-vehicle mice) and SB (300 mg/kg) combined with pyridoxine (Pyr; 350 mg/kg) were administered to the mice (d-gal-Pyr-SB mice) for 3 weeks. Escape latency under water maze in the d-gal mice was longer than that in the control mice. In the d-gal-Pyr-SB mice, escape latency was similar to that in the control mice. In the d-gal mice, many cells in the granule cell layer of the dentate gyrus showed pyknosis and condensation of the cytoplasm. However, in the d-gal-Pyr-SB mice, such cellular changes were rarely found. Furthermore, the d-gal mice showed a great reduction in cell proliferation (Ki67-positive cells) and neuroblast differentiation (doublecortin-positive neuroblasts) in the dentate gyrus compared to control mice. However, in the d-gal-Pyr-SB mice, cell proliferation and neuroblast differentiation were markedly increased in the dentate gyrus. Furthermore, the administration of pyridoxine with sodium butyrate significantly increased Ser133-phosphorylated cyclic AMP response element binding protein in the dentate gyrus. These results indicate that the combination treatment of Pyr with SB in d-gal mice ameliorated the d-gal-induced reduction in cell proliferation, neuroblast differentiation, and memory deficits.     

Effects of Treadmill Exercise on Cell Proliferation and Differentiation in the Subgranular Zone of the Dentate Gyrus in a Rat Model of Type II Diabetes

In the present study, we investigated the effects of a treadmill exercise on serum glucose levels and Ki67 and doublecortin (DCX) immunoreactivity, which is a marker of cell proliferation expressed during cell cycles except G0 and early G1 and a marker of progenitors differentiating into neurons, respectively, in the subgranular zone of the dentate gyrus (SZDG) using a type II diabetic model. At 6 weeks of age, Zucker lean control (ZLC) and Zucker diabetic fatty (ZDF) rats were put on a treadmill with or without running for 1 h/day/5 consecutive days at 22 m/min for 5 weeks. Body weight was significantly increased in the control (without running)-ZDF rats compared to that in the other groups. In the control groups blood glucose levels were increased by 392.7 mg/dl in the control-ZDF rats and by 143.3 mg/dl in the control-ZLC rats. However, in the exercise groups, blood glucose levels were similar between the exercise-ZLC and ZDF rats: The blood glucose levels were 110.0 and 118.2 mg/dl, respectively. Ki67 positive nuclei were detected in the SZDG in control and exercise groups. The number of Ki67 positive nuclei was significantly high in exercise groups compared to that in the control groups. In addition, Ki67 positive cells were abundant in ZLC groups compared to those in ZDF groups. DCX-immunoreactive structures in the control-ZDF rats were lower than that in the control-ZLC rats. In the exercise groups, DCX-immunoreactive structures (somata and processes with tertiary dendrites) and DCX protein levels were markedly increased in both the exercise-ZLC and ZDF rats compared to that in the control groups. These results suggest that a treadmill exercise reduces blood glucose levels in ZDF rats and increases cell proliferation and differentiation in the SZDG in ZLC and ZDF rats compared to those in control groups.     

Effects of Treadmill Exercise on Neural Stem Cells, Cell Proliferation, and Neuroblast Differentiation in the Subgranular Zone of the Dentate Gyrus in Cyclooxygenase-2 Knockout Mice

Cyclooxygenase-2 (COX-2) function has been implicated in a number of physiological processes, including inflammatory responses, synaptic transmission, and synaptic plasticity in the brain. However, the specific role of COX-2 in exercise-induced neurogenesis is still debatable. Here, we assessed the role of COX-2 in exercise-induced plasticity by comparing COX-2 knockout mice to wild-type control littermates. We investigated the number of neural stem cells, and the degree of cell proliferation and neuronal differentiation in COX-2 knockout and its wild-type mice that either exercised or remained inactive. Wild-type and COX-2 knockout mice were put on a treadmill and were either sedentary or were forced to run 1 h/day for five consecutive days at a pace of 10–12 m/min for 5 weeks. Loss of COX-2 expression in the knockout mice was confirmed with two measures: (1) COX immunolabeling in the hippocampus, and (2) the identification of abnormal kidney development using hematoxylin and eosin staining, including subcapsular glomerular hypoplasia and hypertrophy of the deeper cortical glomeruli. Compared to wild-type mice, COX-2 knockout mice exhibited a significant reduction in the neural stem cells (nestin-positive cells), cell proliferation (Ki67-positive cells), and neuroblast differentiation (doublecortin-positive cells). In contrast, exercise significantly increased the neural stem cells, cell proliferation, and neuroblast differentiation in both the wild-type and COX-2 knockout mice although the NeuN-immunoreactive neurons were similar in all groups. Expression of phosphorylated cAMP-response element binding protein was decreased in knockout mice. Exercise increased its expression in the subgranular zone of the dentate gyrus in both wild-type and knockout mice. These results suggest that the COX-2 pathway is one of important factors on neural stem cells, cell proliferation and neuroblast differentiation in sedentary mice. The ability of exercise to increase these types of neural plasticity, regardless of COX-2 signaling, suggests that the effects of exercise on neural stem cells, cell proliferation, and neuroblast differentiation are induced via a pathway that is independent of COX-2.     

Reduced Hippocampal Cell Differentiation in the Subgranular Zone of the Dentate Gyrus in a Rat Model of Type II Diabetes

It has recently been reported that diabetes mellitus is strongly associated with neurodegenerative and functional disorders of the central nervous system. In the present study, we investigated the changes in proliferating neurons in the dentate gyrus of type II diabetic rats using doublecortin (DCX), a marker of progenitors differentiating into neurons. At 4 weeks after birth, there were no differences in the blood glucose levels of Zucker diabetic fatty (ZDF) rats or Zucker lean control (ZLC) rats. DCX-immunoreactive neurons were detectable in the subgranular zone of the dentate gyrus in both the ZDF and ZLC rats; however, DCX immunoreactivity was higher in the ZLC rats than in the ZDF rats. At 12 weeks after birth, the blood glucose level was significantly increased by 400 mg/dl in the ZDF rats, but the blood glucose level in the ZLC rats was only slightly increased by 152.3 mg/dl. DCX immunoreactivity was significantly decreased in 12-week-old rats in comparison to 4-week-old rats. Some DCX-immunoreactive neurons were detectable in the subgranular zone of the dentate gyrus in the ZLC rats. However, only a few DCX-immunoreactive neurons were observed in the ZDF rats, and the DCX-immunoreactive neurons in the ZDF rats did not show fully developed processes. These results suggest that DCX-immunoreactive neurons were significantly decreased in an age-dependent manner and that DCX-immunoreactive neurons were also reduced in diabetic rats. In addition, the reduction in DCX-immunoreactive neurons in age matched rats may be associated with type II diabetes.     

Additive or Synergistic Effects of Aluminum on the Reduction of Neural Stem Cells, Cell Proliferation, and Neuroblast Differentiation in the Dentate Gyrus of High-Fat Diet-Fed Mice

Aluminum is the most plentiful metal on the Earth’s crust, and its usage in cooking utensils, cosmetics, drinking containers, food additives, pharmaceutical products, and building materials provides many opportunities for potential aluminum consumption. However, its toxicity is low and harmful effects only develop with large-scale deposition of aluminum. In this study, we investigated the effects of subchronic exposure to aluminum (40 mg/kg/day) on neural stem cells, cell proliferation, neuroblast differentiation, and mature neurons in the dentate gyrus of the hippocampus. These experiments were performed in both high-fat diet and low-fat diet-fed C57BL/6J mice via immunohistochemistry using the relevant marker for each cell type, including nestin, Ki67, doublecortin, and NeuN. Subchronic exposure to aluminum in both low-fat and high-fat diet-fed mice reduced neural stem cells, cell proliferation, and neuroblast differentiation without any changes in mature neurons. Furthermore, this reduction effect was exacerbated in high-fat diet-fed mice. These results suggest that aluminum accelerates the reduction of neural stem cells, cell proliferation, and neuroblast differentiation additively or synergistically in high-fat diet-fed mice without any harmful changes in mature neurons.     

Stem- and Progenitor Cell Proliferation in the Dentate Gyrus of the Reeler Mouse

Adult hippocampal neurogenesis has been implicated in hippocampus-dependent learning and memory. Furthermore, the decline of neurogenesis accompanying aging could be involved in age-related cognitive deficits. It is believed that the neural stem cell niche comprises a specialized microenvironment regulating stem cell activation and maintenance. However, little is known about the significance of the extracellular matrix in controlling adult stem cells. Reelin is a large glycoprotein of the extracelluar matrix known to be of crucial importance for neuronal migration. Here, we examined the local interrelation between Reelin expressing interneurons and putative hippocampal stem cells and investigated the effects of Reelin deficiency on stem cell and progenitor cell proliferation. Reelin-positive cells are found in close vicinity to putative stem cell processes, which would allow for stem cell regulation by Reelin. We investigated the proliferation of stem cells in the Reelin-deficient reeler hippocampus by Ki67 labeling and found a strong reduction of mitotic cells. A detailed analysis of dividing Type 1, type 2 and type 3 cells indicated that once a stem cell is recruited for proliferation, the progression to the next progenitor stage as well as the number of mitotic cycles is not altered in reeler. Our data point to a role for Reelin in either regulating stem cell quiescence or maintenance.     

Effects of Treadmill Exercise Combined with MK 801 Treatment on Neuroblast Differentiation in the Dentate Gyrus in Rats

N-methyl-d-aspartate receptor (NR) is involved in activity-dependent synaptic plasticity, such as associative long-term potentiation, and in related central functions, such as learning and memory. In this study, we observed effects of treadmill exercise on NR1 and doublecortin (DCX, a marker for neuroblast differentiation) in the subgranular zone of the dentate gyrus (DG). At 6 weeks of age, rats were put on a treadmill with or without running for 1 h/day for 5 consecutive days at 22 m/min for 5 weeks. Exercise increased NR1 immunoreactivity and protein level in the hippocampus. To identify the correlations between NR and neuroblasts, we intraperitoneally administered a NR antagonist, MK-801, to the exercised rats. MK-801 treatment reduced NR1 protein level in the hippocampus of the exercised rats. In addition, in the MK-801-treated group, the number of DCX cells was significantly decreased in the subgranular zone of the DG. These results suggest that NR may be one of the important factors that modulate neuroblast differentiation during exercise in rats.     

Differential Involvement of the Dentate Gyrus in Adaptive Forgetting in the Rat

How does the brain discriminate essential information aimed to be stored permanently from information required only temporarily, and that needs to be cleared away for not saturating our precious memory space? Reference Memory (RM) refers to the long-term storage of invariable information whereas Working Memory (WM) depends on the short-term storage of trial-unique information. Previous work has revealed that WM tasks are very sensitive to proactive interference. In order to prevent such interference, irrelevant old memories must be forgotten to give new ones the opportunity to be stabilized. However, unlike memory, physiological processes underlying this adaptive form of forgetting are still poorly understood. Here, we precisely ask what specific brain structure(s) could be responsible for such process to occur. To answer this question, we trained rats in a radial maze using three paradigms, a RM task and two WM tasks involving or not the processing of interference but strictly identical in terms of locomotion or motivation. We showed that an inhibition of the expression of Zif268 and c-Fos, two indirect markers of neuronal activity and synaptic plasticity, was observed in the dentate gyrus of the dorsal hippocampus when processing such interfering previously stored information. Conversely, we showed that inactivating the dentate gyrus impairs both RM and WM, but improves the processing of interference. Altogether, these results strongly suggest for the first time that the dentate gyrus could be a key structure involved in adaptive forgetting.     

Temporal Changes in Prosaposin Expression in the Rat Dentate Gyrus after Birth

Neurogenesis in the hippocampal dentate gyrus occurs constitutively throughout postnatal life. Adult neurogenesis includes a multistep process that ends with the formation of a postmitotic and functionally integrated new neuron. During adult neurogenesis, various markers are expressed, including GFAP, nestin, Pax6, polysialic acid-neural cell adhesion molecule (PSA-NCAM), neuronal nuclei (NeuN), doublecortin, TUC-4, Tuj-1, and calretinin. Prosaposin is the precursor of saposins A–D; it is found in various organs and can be excreted. Strong prosaposin expression has been demonstrated in the developing brain including the hippocampus, and its neurotrophic activity has been proposed. This study investigated changes in prosaposin in the dentate gyrus of young and adult rats using double immunohistochemistry with antibodies to prosaposin, PSA-NCAM, and NeuN. Prosaposin immunoreactivity was intense in the dentate gyrus at postnatal day 3 (P3) and P7, but decreased gradually after P14. In the dentate gyrus at P28, immature PSA-NCAM-positive neurons localized exclusively in the subgranular zone were prosaposin-negative, whereas mature Neu-N-positive neurons were positive for prosaposin. Furthermore, these prosaposin-negative immature neurons were saposin B-positive, suggesting that the neurons take up and degrade prosaposin. In situ hybridization assays showed that prosaposin in the adult dentate gyrus is dominantly the Pro+9 type, a secreted type of prosaposin. These results imply that prosaposin secreted from mature neurons stimulates proliferation and maturation of immature neurons in the dentate gyrus.     

Topiramate Improves Neuroblast Differentiation of Hippocampal Dentate Gyrus in the <Emphasis Type="SmallCaps">d</Emphasis>-Galactose-Induced Aging Mice via Its Antioxidant Effects

Some anticonvulsant drugs are associated with cognitive ability in patients; Topiramate (TPM) is well known as an effective anticonvulsant agent applied in clinical settings. However, the effect of TPM on the cognitive function is rarely studied. In this study, we aimed to observe the effects of TPM on cell proliferation and neuronal differentiation in the dentate gyrus (DG) of the d-galactose-induced aging mice by Ki-67 and doublecortin (DCX) immunohistochemistry. The study is divided into four groups including control, d-galactose-treated group, 25 and 50 mg/kg TPM-treated plus d-galactose-treated groups. We found, 50 mg/kg (not 25 mg/kg) TPM treatment significantly increased the numbers of Ki-67⁺ cells and DCX immunoreactivity, and improved neuroblast injury induced by d-galactose treatment. In addition, we also found that decreased immunoreactivities and protein levels of antioxidants including superoxide dismutase and catalase induced by d-galactose treatment were significantly recovered by 50 mg/kg TPM treatment in the mice hippocampal DG (P < 0.05). In conclusion, our present results indicate that TPM can ameliorate neuroblast damage and promote cell proliferation and neuroblast differentiation in the hippocampal DG via increasing SODs and catalase levels in the d-galactose mice.     

The Selective Antagonism of P2X7 and P2Y1 Receptors Prevents Synaptic Failure and Affects Cell Proliferation Induced by Oxygen and Glucose Deprivation in Rat Dentate Gyrus

Purinergic P2X and P2Y receptors are broadly expressed on both neurons and glial cells in the central nervous system (CNS), including dentate gyrus (DG). The aim of this research was to determine the synaptic and proliferative response of the DG to severe oxygen and glucose deprivation (OGD) in acute rat hippocampal slices and to investigate the contribution of P2X₇ and P2Y₁ receptor antagonism to recovery of synaptic activity after OGD. Extracellular field excitatory post-synaptic potentials (fEPSPs) in granule cells of the DG were recorded from rat hippocampal slices. Nine-min OGD elicited an irreversible loss of fEPSP and was invariably followed by the appearance of anoxic depolarization (AD). Application of MRS2179 (selective antagonist of P2Y₁ receptor) and BBG (selective antagonist of P2X₇ receptor), before and during OGD, prevented AD appearance and allowed a significant recovery of neurotransmission after 9-min OGD. The effects of 9-min OGD on proliferation and maturation of cells localized in the subgranular zone (SGZ) of slices prepared from rats treated with 5-Bromo-2′-deoxyuridine (BrdU) were investigated. Slices were further incubated with an immature neuron marker, doublecortin (DCX). The number of BrdU⁺ cells in the SGZ was significantly decreased 6 hours after OGD. This effect was antagonized by BBG, but not by MRS2179. Twenty-four hours after 9-min OGD, the number of BrdU⁺ cells returned to control values and a significant increase of DCX immunofluorescence was observed. This phenomenon was still evident when BBG, but not MRS2179, was applied during OGD. Furthermore, the P2Y₁ antagonist reduced the number of BrdU⁺ cells at this time. The data demonstrate that P2X₇ and P2Y₁ activation contributes to early damage induced by OGD in the DG. At later stages after the insult, P2Y₁ receptors might play an additional and different role in promoting cell proliferation and maturation in the DG.     

Functional Differentiation of Adult-Born Neurons along the Septotemporal Axis of the Dentate Gyrus

Over the past several decades, the proliferation and integration of adult-born neurons into existing hippocampal circuitry has been implicated in a wide range of behaviors, including novelty recognition, pattern separation, spatial learning, anxiety behaviors, and antidepressant response. In this review, we suggest that the diversity in behavioral requirements for new neurons may be partly caused by separate functional roles of individual neurogenic niches. Growing evidence shows that the hippocampal formation can be compartmentalized not only along the classic trisynaptic circuit, but also along a longitudinal septotemporal axis. We suggest that subpopulations of hippocampal adult-born neurons may be specialized for distinct mnemonic- or mood-related behavioral tasks. We will examine the literature supporting a functional and anatomical dissociation of the hippocampus along the longitudinal axis and discuss techniques to functionally dissect the roles of adult-born hippocampal neurons in these distinct subregions.Since the presence of dividing cells in the mostly postmitotic adult brain was first described (Altman and Das 1965), the generation of new neurons in adulthood has been proposed to be involved in a variety of behaviors (Doetsch and Hen 2005; Becker and Wojtowicz 2007; Sahay and Hen 2007; Deng et al. 2010; Ming and Song 2011; Miller and Hen 2014). Adult neurogenesis in the healthy mammalian brain is consistently seen in the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). Recent studies have implicated hippocampal neurogenesis in learning- and memory-related tasks, such as contextual discrimination and spatial navigation and, specifically, in behavioral pattern separation (Clelland et al. 2009; Sahay et al. 2011; Nakashiba et al. 2012; Niibori et al. 2012; see also reviews in Deng et al. 2010; Ming and Song 2011; Marin-Burgin and Schinder 2012), but also in some behavioral effects of antidepressants (Santarelli et al. 2003; see also reviews in Sahay and Hen 2007; Kheirbek et al. 2012; Tanti and Belzung 2013). However, the exact role of adult hippocampal neurogenesis in some of these behaviors has been debated as some studies have shown no effects of altering adult neurogenesis on spatial navigation or antidepressant response. Proposed explanations have included differences in the behavioral tasks used to measure cognition or emotion, motivational state of subjects, species differences, or in how neurogenesis is defined, either as proliferation, survival, or differentiation (see reviews in Zhao et al. 2008; Aimone et al. 2011; Petrik et al. 2012b; Miller and Hen 2014).It must also be noted, however, that these hippocampal neurons are not born into a singular structure. Work in the past several decades has shown that the hippocampus can be divided, not only along the classic trisynaptic loop, but also longitudinally along a septotemporal axis. The septal (dorsal in rodents; posterior in primates) and temporal (ventral in rodents; anterior in primates) poles, as well as potential intermediate zones of the hippocampus, have different anatomic connections and electrophysiological properties, express a gradient of molecular markers, and play different functional roles, such as performance in spatial learning tasks and stress responses (see reviews in Moser and Moser 1998; Fanselow and Dong 2010). Consequently, adult-born neurons in the hippocampal DG may also be segregated along this longitudinal axis, and conflicting functional roles for neurogenesis may be a result of attempting to examine hippocampal neurogenesis as a unitary phenomenon. It is possible that there are intrinsic, cell-autonomous differences in adult-born neurons generated at opposite poles of the DG. An alternative, although not mutually exclusive, hypothesis is that progenitor cells are initially identical, but differentiate in a dissimilar manner as a result of integration into distinct network circuitry. We will, therefore, first discuss heterogeneity of the hippocampus along its longitudinal axis before reviewing differences in neurogenesis between the septal and temporal poles of the DG. As these topics have been reviewed extensively elsewhere (Moser and Moser 1998; Deng et al. 2010; Fanselow and Dong 2010; Koehl and Abrous 2011; Samuels and Hen 2011; Kheirbek et al. 2012; Petrik et al. 2012b), we will not try to exhaustively cover all the current literature. Rather, we attempt to gather key studies examining a septotemporal gradient of the hippocampus and hippocampal neurogenesis. We will then suggest possible approaches to examine neurogenesis in specific subregions of the hippocampal DG. Finally, a short section will examine segregation of the DG along its transverse axis.