RESUMO
Inducible nitric oxide synthase (iNOS) is an enzyme upregulated in the brain during neuroimmune stimuli which is associated with an oxidative and pro-inflammatory environment in several brain regions, including the hippocampal formation and the prefrontal cortex. The dentate gyrus of the hippocampal formation is the site of a process known as adult hippocampal neurogenesis (AHN). Although many endogenous and extrinsic factors can modulate AHN, the exact participation of specific proinflammatory mediators such as iNOS in these processes remains to be fully elucidated. Here, we investigated how the total genetic ablation of iNOS impacts the hippocampal neurogenic niche and microglial phenotype and if these changes are correlated to the behavioral alterations observed in iNOS knockout (K.O.) mice submitted or not to the chronic unpredictable stress model (CUS - 21 days protocol). Contrary to our initial hypothesis, at control conditions, iNOS K.O. mice displayed no abnormalities on microglial activation in the dentate gyrus. However, they did exhibit impaired newborn cells and immature neuron survival, which was not affected by CUS. The reduction of AHN in iNOS K.O. mice was accompanied by an increased positive coping response in the tail suspension test and facilitation of anxiety-like behaviors in the novelty suppressed feeding. Next, we investigated whether a pro-neurogenic stimulus would rescue the neurogenic capacity of iNOS K.O. mice by administering in control and CUS groups the antidepressant escitalopram (ESC). The chronic treatment with ESC could not rescue the neurogenic capacity or the behavioral changes observed in iNOS K.O. mice. Besides, in the ventromedial prefrontal (vmPFC) cortex there was no change in the expression or the chronic activation of PV neurons (evaluated by double labeling PV with FOSB) in the prelimbic (PrL) or infralimbic subregions. FOSB expression, however, increased in the PrL of iNOS K.O. mice. Our results suggest that iNOS seems essential for the survival of newborn cells and immature neurons in the hippocampus and seem to partially explain the anxiogenic-like behavior observed in iNOS K.O. mice. On the other hand, the iNOS ablation appears to result in increased activity of the PrL which could explain the antidepressant-like behaviors of iNOS K.O mice.
Assuntos
Giro Denteado/citologia , Neurônios/fisiologia , Óxido Nítrico Sintase Tipo II/fisiologia , Animais , Sobrevivência Celular , Citocinas/fisiologia , Escitalopram/farmacologia , Masculino , Camundongos , Camundongos Knockout , Microglia/fisiologia , Neurogênese/efeitos dos fármacos , Óxido Nítrico Sintase Tipo II/genética , Estresse Psicológico/psicologiaRESUMO
The generation of new neurons in the adult brain is a currently accepted phenomenon. Over the past few decades, the subventricular zone and the hippocampal dentate gyrus have been described as the two main neurogenic niches. Neurogenic niches generate new neurons through an asymmetric division process involving several developmental steps. This process occurs throughout life in several species, including humans. These new neurons possess unique properties that contribute to the local circuitry. Despite several efforts, no other neurogenic zones have been observed in many years; the lack of observation is probably due to technical issues. However, in recent years, more brain niches have been described, once again breaking the current paradigms. Currently, a debate in the scientific community about new neurogenic areas of the brain, namely, human adult neurogenesis, is ongoing. Thus, several open questions regarding new neurogenic niches, as well as this phenomenon in adult humans, their functional relevance, and their mechanisms, remain to be answered. In this review, we discuss the literature and provide a compressive overview of the known neurogenic zones, traditional zones, and newly described zones. Additionally, we will review the regulatory roles of some molecular mechanisms, such as miRNAs, neurotrophic factors, and neurotrophins. We also join the debate on human adult neurogenesis, and we will identify similarities and differences in the literature and summarize the knowledge regarding these interesting topics.
Assuntos
Giro Denteado/citologia , Ventrículos Laterais/citologia , Neurogênese/fisiologia , Neurônios/citologia , Estriado Ventral/citologia , Adulto , Animais , Hipocampo/citologia , Humanos , Camundongos , MicroRNAs/genética , Células-Tronco Neurais/citologia , Neurogênese/genética , RatosRESUMO
Axon initial segments (AIS) of dentate granule cells in the hippocampus exhibit prominent spines (AISS) during early development that are associated with microglial contacts. In the present study, we investigated whether developmental changes in AISS could be modified by early-life stress (ELS), specifically neonatal maternal separation (MS), through stress hormones and microglial activation and examined the potential behavioural consequences. We examined AISS at postnatal day (PND)5, 15 and 50, using Golgi-Cox staining and anatomical analysis. Neurone-microglial interaction was assessed using antibodies against ankyrin-G, PSD-95 and Iba1, for AIS, AISS and microglia visualisation, respectively, in normally reared and neonatal maternally separated male and female rats. We observed a higher density of AISS in ELS rats at both PND15 and PND50 compared to controls. Effects were more pronounced in females than males. AIS-associated microglia in ELS rats showed a hyper-ramified morphology and less co-localisation with PSD-95 compared to controls at PND15. ELS-associated alteration in microglial morphology and synaptic pruning was mimicked by treatment of acute hippocampal slices of normally reared rats with vasopressin. ELS rats exhibited increased freezing behaviour during auditory fear memory testing, which was more pronounced in female subjects and corresponded with increased Fos expression in dorsal and ventral dentate granule cells. Thus, microglial synaptic pruning in dentate AIS of hippocampus is influenced by ELS, with demonstrable sex bias regarding its anatomical characteristics and subsequent fear-induced defensive behaviours.
Assuntos
Giro Denteado/fisiologia , Medo/psicologia , Microglia/fisiologia , Plasticidade Neuronal/fisiologia , Estresse Psicológico , Envelhecimento/psicologia , Animais , Animais Recém-Nascidos , Segmento Inicial do Axônio/fisiologia , Espinhas Dendríticas/fisiologia , Giro Denteado/citologia , Feminino , Masculino , Privação Materna , Microglia/citologia , Gravidez , Ratos , Ratos Wistar , Caracteres Sexuais , Estresse Psicológico/fisiopatologia , Estresse Psicológico/psicologiaRESUMO
Memory systems ought to store and discriminate representations of similar experiences in order to efficiently guide future decisions. This problem is solved by pattern separation, implemented in the dentate gyrus (DG) by granule cells to support episodic memory formation. Pattern separation is enabled by tonic inhibitory bombardment generated by multiple GABAergic cell populations that strictly maintain low activity levels in granule cells. Somatostatin-expressing cells are one of those interneuron populations, selectively targeting the distal dendrites of granule cells, where cortical multimodal information reaches the DG. Nonetheless, somatostatin cells have very low connection probability and synaptic efficacy with both granule cells and other interneuron types. Hence, the role of somatostatin cells in DG circuitry, particularly in the context of pattern separation, remains uncertain. Here, by using optogenetic stimulation and behavioral tasks in mice, we demonstrate that somatostatin cells are required for the acquisition of both contextual and spatial overlapping memories.
Assuntos
Giro Denteado/citologia , Giro Denteado/metabolismo , Aprendizagem por Discriminação/fisiologia , Memória Episódica , Células Secretoras de Somatostatina/metabolismo , Animais , Giro Denteado/química , Feminino , Ácido Glutâmico/análise , Ácido Glutâmico/metabolismo , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Optogenética/métodos , Somatostatina/análise , Somatostatina/metabolismo , Células Secretoras de Somatostatina/químicaRESUMO
Neural stem cells can generate new neurons in the mouse adult brain in a complex multistep process called neurogenesis. Several factors regulate this process, including neurotransmitters, hormones, neurotrophic factors, pharmacological agents, and environmental factors. Purinergic signaling, mainly the adenosinergic system, takes part in neurogenesis, being involved in cell proliferation, migration, and differentiation. However, the role of the purine nucleoside guanosine in neurogenesis remains unclear. Here, we examined the effect of guanosine by using the neurosphere assay derived from neural stem cells of adult mice. We found that continuous treatment with guanosine increased the number of neurospheres, neural stem cell proliferation, and neuronal differentiation. The effect of guanosine to increase the number of neurospheres was reduced by removing adenosine from the culture medium. We next traced the neurogenic effect of guanosine in vivo. The intraperitoneal treatment of adult C57BL/6 mice with guanosine (8 mg/kg) for 26 days increased the number of dividing bromodeoxyuridine (BrdU)-positive cells and also increased neurogenesis, as identified by measuring doublecortin (DCX)-positive cells in the dentate gyrus (DG) of the hippocampus. Antidepressant-like behavior in adult mice accompanied the guanosine-induced neurogenesis in the DG. These results provide new evidence of a pro-neurogenic effect of guanosine on neural stem/progenitor cells, and it was associated in vivo with antidepressant-like effects.
Assuntos
Envelhecimento/fisiologia , Guanosina/farmacologia , Hipocampo/citologia , Células-Tronco Neurais/citologia , Neurogênese , Animais , Antidepressivos/farmacologia , Comportamento Animal/efeitos dos fármacos , Diferenciação Celular/efeitos dos fármacos , Proliferação de Células/efeitos dos fármacos , Giro Denteado/citologia , Proteína Duplacortina , Feminino , Masculino , Camundongos Endogâmicos C57BL , Células-Tronco Neurais/efeitos dos fármacos , Células-Tronco Neurais/metabolismo , Neurogênese/efeitos dos fármacos , Esferoides Celulares/citologia , Esferoides Celulares/efeitos dos fármacosRESUMO
Chronic treatment with agmatine, similarly to fluoxetine, may cause antidepressant-like effects mediated, at least in part, by the modulation of hippocampal plasticity. However, the ability of chronic treatment with agmatine to cause antidepressant-like effects associated with the modulation of mammalian target of rapamycin (mTOR) signaling pathway and protection against neuronal death remains to be established. In this study, we investigated the effects of agmatine (0.1 mg/kg, p.o.) and the conventional antidepressant fluoxetine (10 mg/kg, p.o.) treatment on the levels of phosphorylated mTOR (p-mTOR), neuronal death, and overall volume in the hippocampal dentate gyrus (DG) of mice exposed to chronic corticosterone (20 mg/kg, p.o.) treatment for 21 days, a model of stress and depressive-like behavior. Chronic corticosterone treatment increased cell death in the sub-granular zone (SGZ) of the DG, as assessed by Fluoro-Jade B labeling. Agmatine, similarly to fluoxetine, was capable of reversing this alteration in the entire DG, an effect more evident in the ventral portion of the hippocampus. Additionally, reduced phosphorylation of mTOR (Ser2448), a pro-survival protein that is active when phosphorylated at Ser2448, was observed in the whole hippocampal DG in corticosterone-treated mice, an effect not observed in agmatine or fluoxetine-treated mice. Chronic exposure to corticosterone caused a significant reduction in overall hippocampal volume, although no alterations were observed between the groups with regards to DG volume. Altogether, the results indicate that agmatine, similar to fluoxetine, was able to counteract corticosterone-induced impairment on mTOR signaling and cell death in hippocampal DG.
Assuntos
Agmatina/farmacologia , Anti-Inflamatórios/toxicidade , Corticosterona/toxicidade , Hipocampo/efeitos dos fármacos , Neurônios/efeitos dos fármacos , Fármacos Neuroprotetores/farmacologia , Serina-Treonina Quinases TOR/efeitos dos fármacos , Animais , Morte Celular/efeitos dos fármacos , Giro Denteado/citologia , Giro Denteado/efeitos dos fármacos , Fluoxetina/farmacologia , Hipocampo/metabolismo , Camundongos , Inibidores Seletivos de Recaptação de Serotonina/farmacologia , Serina-Treonina Quinases TOR/metabolismoRESUMO
BACKGROUND: Few studies have explored the glial response to a standard environment and how the response may be associated with age-related cognitive decline in learning and memory. Here we investigated aging and environmental influences on hippocampal-dependent tasks and on the morphology of an unbiased selected population of astrocytes from the molecular layer of dentate gyrus, which is the main target of perforant pathway. RESULTS: Six and twenty-month-old female, albino Swiss mice were housed, from weaning, in a standard or enriched environment, including running wheels for exercise and tested for object recognition and contextual memories. Young adult and aged subjects, independent of environment, were able to distinguish familiar from novel objects. All experimental groups, except aged mice from standard environment, distinguish stationary from displaced objects. Young adult but not aged mice, independent of environment, were able to distinguish older from recent objects. Only young mice from an enriched environment were able to distinguish novel from familiar contexts. Unbiased selected astrocytes from the molecular layer of the dentate gyrus were reconstructed in three-dimensions and classified using hierarchical cluster analysis of bimodal or multimodal morphological features. We found two morphological phenotypes of astrocytes and we designated type I the astrocytes that exhibited significantly higher values of morphological complexity as compared with type II. Complexity = [Sum of the terminal orders + Number of terminals] × [Total branch length/Number of primary branches]. On average, type I morphological complexity seems to be much more sensitive to age and environmental influences than that of type II. Indeed, aging and environmental impoverishment interact and reduce the morphological complexity of type I astrocytes at a point that they could not be distinguished anymore from type II. CONCLUSIONS: We suggest these two types of astrocytes may have different physiological roles and that the detrimental effects of aging on memory in mice from a standard environment may be associated with a reduction of astrocytes morphological diversity.
Assuntos
Astrócitos/metabolismo , Astrócitos/patologia , Fatores Etários , Animais , Cognição/fisiologia , Giro Denteado/citologia , Giro Denteado/metabolismo , Meio Ambiente , Feminino , Proteína Glial Fibrilar Ácida/metabolismo , Hipocampo/citologia , Hipocampo/fisiologia , Memória/fisiologia , CamundongosRESUMO
Experience shapes the development and connectivity of adult-born granule cells (GCs) through mechanisms that are poorly understood. We examined the remodeling of dentate gyrus microcircuits in mice in an enriched environment (EE). Short exposure to EE during early development of new GCs accelerated their functional integration. This effect was mimicked by in vivo chemogenetic activation of a limited population of mature GCs. Slice recordings showed that mature GCs recruit parvalbumin γ-aminobutyric acid-releasing interneurons (PV-INs) that feed back onto developing GCs. Accordingly, chemogenetic stimulation of PV-INs or direct depolarization of developing GCs accelerated GC integration, whereas inactivation of PV-INs prevented the effects of EE. Our results reveal a mechanism for dynamic remodeling in which experience activates dentate networks that "prime" young GCs through a disynaptic feedback loop mediated by PV-INs.
Assuntos
Giro Denteado/fisiologia , Retroalimentação Fisiológica , Rede Nervosa/fisiologia , Neurogênese , Neurônios/fisiologia , Animais , Giro Denteado/citologia , Feminino , Interneurônios/citologia , Interneurônios/metabolismo , Interneurônios/fisiologia , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Modelos Neurológicos , Neurônios/citologia , Parvalbuminas/metabolismo , Meio Social , Sinapses/fisiologia , Ácido gama-Aminobutírico/metabolismoRESUMO
Adult hippocampal neurogenesis can be modulated by various physiological and pathological conditions, including stress, affective disorders, and several neurological conditions. Given the proposed role of this form of structural plasticity in the functioning of the hippocampus (namely learning and memory and affective behaviors), it is believed that alterations in hippocampal neurogenesis might underlie some of the behavioral deficits associated with these psychiatric and neurological conditions. Thus, the search for compounds that can reverse these deficits with minimal side effects has become a recognized priority. In the present study we tested the pro-neurogenic effects of isoxazole 9 (Isx-9), a small synthetic molecule that has been recently identified through the screening of chemical libraries in stem cell-based assays. We found that administration of Isx-9 for 14days was able to potentiate cell proliferation and increase the number of immature neurons in the hippocampal DG of adult rats. In addition, Isx-9 treatment was able to completely reverse the marked reduction in these initial stages of the neurogenic process observed in vehicle-treated animals (which were submitted to repeated handling and exposure to daily intraperitoneal injections). Based on these results, we recommend that future neurogenesis studies that require repeated handling and manipulation of animals should include a naïve (non-manipulated) control to determine the baseline levels of hippocampal cell proliferation and neuronal differentiation. Overall, these findings demonstrate that Isx-9 is a promising synthetic compound for the mitigation of stress-induced deficits in adult hippocampal neurogenesis. Future studies are thus warranted to evaluate the pro-neurogenic properties of Isx-9 in animal models of affective and neurological disorders associated with impaired hippocampal structural plasticity.
Assuntos
Proliferação de Células/efeitos dos fármacos , Fármacos do Sistema Nervoso Central/farmacologia , Giro Denteado/efeitos dos fármacos , Isoxazóis/farmacologia , Neurogênese/efeitos dos fármacos , Neurônios/efeitos dos fármacos , Tiofenos/farmacologia , 2-Hidroxipropil-beta-Ciclodextrina , Animais , Fármacos do Sistema Nervoso Central/síntese química , Corticosterona/sangue , Giro Denteado/citologia , Giro Denteado/fisiologia , Avaliação Pré-Clínica de Medicamentos , Imuno-Histoquímica , Isoxazóis/síntese química , Masculino , Estrutura Molecular , Neurônios/citologia , Neurônios/fisiologia , Ratos Sprague-Dawley , Estresse Psicológico/tratamento farmacológico , Estresse Psicológico/patologia , Estresse Psicológico/fisiopatologia , Tiofenos/síntese química , beta-Ciclodextrinas/farmacologiaRESUMO
BACKGROUND: In the adult hippocampus new neurons are continuously generated from neural stem cells (NSCs) present at the subgranular zone of the dentate gyrus. This process is controlled by Wnt signaling, which plays a complex role in regulating multiple steps of neurogenesis including maintenance, proliferation and differentiation of progenitor cells and the development of newborn neurons. Differential effects of Wnt signaling during progression of neurogenesis could be mediated by cell-type specific expression of Wnt receptors. Here we studied the potential role of Frizzled-1 (FZD1) receptor in adult hippocampal neurogenesis. RESULTS: In the adult dentate gyrus, we determined that FZD1 is highly expressed in NSCs, neural progenitors and immature neurons. Accordingly, FZD1 is expressed in cultured adult hippocampal progenitors isolated from mouse brain. To evaluate the role of this receptor in vivo we targeted FZD1 in newborn cells using retroviral-mediated RNA interference. FZD1 knockdown resulted in a marked decrease in the differentiation of newborn cells into neurons and increased the generation of astrocytes, suggesting a regulatory role for the receptor in cell fate commitment. In addition, FZD1 knockdown induced an extended migration of adult-born neurons within the granule cell layer. However, no differences were observed in total dendritic length and dendritic arbor complexity between control and FZD1-deficient newborn neurons. CONCLUSIONS: Our results show that FZD1 regulates specific stages of adult hippocampal neurogenesis, being required for neuronal differentiation and positioning of newborn neurons into the granule cell layer, but not for morphological development of adult-born granule neurons.