Interaction between Neurogenesis and Hippocampal Memory System: New Vistas

High-Caloric Diets in Adolescence Impair Specific GABAergic Subpopulations, Neurogenesis, and Alter Astrocyte Morphology

We compared the effects of two different high-caloric diets administered to 4-week-old rats for 12 weeks: a diet rich in sugar (30% sucrose) and a cafeteria diet rich in sugar and high-fat foods. We focused on the hippocampus, particularly on the gamma-aminobutyric acid (GABA)ergic system, including the Ca2+-binding proteins parvalbumin (PV), calretinin (CR), calbindin (CB), and the neuropeptides somatostatin (SST) and neuropeptide Y (NPY). We also analyzed the density of cholinergic varicosities, brain-derived neurotrophic factor (BDNF), reelin (RELN), and cyclin-dependent kinase-5 (CDK-5) mRNA levels, and glial fibrillary acidic protein (GFAP) expression. The cafeteria diet reduced PV-positive neurons in the granular layer, hilus, and CA1, as well as NPY-positive neurons in the hilus, without altering other GABAergic populations or overall GABA levels. The high-sugar diet induced a decrease in the number of PV-positive cells in CA3 and an increase in CB-positive cells in the hilus and CA1. No alterations were observed in the cholinergic varicosities. The cafeteria diet also reduced the relative mRNA expression of RELN without significant changes in BDNF and CDK5 levels. The cafeteria diet increased the number but reduced the length of the astrocyte processes. These data highlight the significance of determining the mechanisms mediating the observed effects of these diets and imply that the cognitive impairments previously found might be related to both the neuroinflammation process and the reduction in PV, NPY, and RELN expression in the hippocampal formation.

The role of resveratrol in neurogenesis: a systematic review

CONTEXT

Resveratrol (RV) is a natural compound found in grapes, wine, berries, and peanuts and has potential health benefits-namely, neurogenesis improvement. Neurogenesis, which is the process through which new neurons or nerve cells are generated in the brain, occurs in the subventricular zone and hippocampus and is influenced by various factors. RV has been shown to increase neural stem cell proliferation and survival, improving cognitive function in hippocampus-dependent tasks. Thus, to provide a convergent and unbiased conclusion of the available evidence on the correlation between the RV and neurogenesis, a systematic review needs to be undertaken meticulously and with appropriate attention.

OBJECTIVE

This study aimed to systematically review any potential connection between the RV and neurogenesis in animal models.

DATA SOURCES AND EXTRACTION

Based on the particular selection criteria, 8 original animal studies that investigated the relationship between RV and neurogenesis were included. Studies written in English and published in peer-reviewed journals with no restrictions on the starting date of publication on August 17, 2023, were searched in the Google Scholar and PubMed databases. Furthermore, data were extracted and analyzed independently by 2 researchers and then reviewed by a third researcher, and discrepancies were resolved by consensus. This project followed PRISMA reporting standards.

DATA ANALYSIS

In the studies analyzed in this review, there is a definite correlation between RV and neurogenesis, meaning that RV intake, irrespective of the mechanisms thereof, can boost neurogenesis in both the subventricular zone and hippocampus.

CONCLUSION

This finding, albeit with some limitations, provides a plausible indication of RV's beneficial function in neurogenesis. Indeed, RV intake may result in neurogenesis benefits-namely, cognitive function, mood regulation, stress resilience, and neuroprotection, potentially preventing cognitive decline.

Genomic glucocorticoid action in embryonic mouse neural stem cells

Prenatal exposure to synthetic glucocorticoids (sGCs) reprograms brain development and predisposes the developing fetus towards potential adverse neurodevelopmental outcomes. Using a mouse model of sGC administration, previous studies show that these changes are accompanied by sexually dimorphic alterations in the transcriptome of neural stem and progenitor cells (NSPCs) derived from the embryonic telencephalon. Because cell type-specific gene expression profiles tightly regulate cell fate decisions and are controlled by a flexible landscape of chromatin domains upon which transcription factors and enhancer elements act, we multiplexed data from four genome-wide assays: RNA-seq, ATAC-seq (assay for transposase accessible chromatin followed by genome wide sequencing), dual cross-linking ChIP-seq (chromatin immunoprecipitation followed by genome wide sequencing), and microarray gene expression to identify novel relationships between gene regulation, chromatin structure, and genomic glucocorticoid receptor (GR) action in NSPCs. These data reveal that GR binds preferentially to predetermined regions of accessible chromatin to influence gene programming and cell fate decisions. In addition, we identify SOX2 as a transcription factor that impacts the genomic response of select GR target genes to sGCs (i.e., dexamethasone) in NSPCs.

A role for flavonoids in the prevention and/or treatment of cognitive dysfunction, learning, and memory deficits: a review of preclinical and clinical studies

OBJECTIVE

Natural food substances, due to high rates of antioxidants, antiviral and anti-inflammatory properties, have been proposed to have the potential for the prevention or treatment of cognitive deficits, learning and memory deficits and neuro inflammation. In particular, medicinal plants with rich amounts of beneficial components such as flavonoids are one of the most promising therapeutic candidates for the cognitive deficit and memory loss. Herein, we aimed to review the impact of medicinal plants with focus on flavonoids on cognitive dysfunction, learning and memory loss by considering their signaling pathways.

METHODS

We extracted 93 preclinical and clinical studies related to the effects of flavonoids on learning and memory and cognition from published papers between 2000 and 2021 in the MEDLINE/PubMed, Cochrane Library, SCOPUS, and Airiti Library databases.

RESULTS

In the preclinical studies, at least there seem to be two main neurological and biological processes in which flavonoids contribute to the improvement and/or prevention of learning, memory deficit and cognitive dysfunction: (1) Regulation of neurotransmission system and (2) Enhancement of neurogenesis, synaptic plasticity and neuronal survival.

CONCLUSION

Although useful effects of flavonoids on learning and memory in preclinical investigations have been approved, more clinical trials are required to find out whether flavonoids and/or other ingredients of plants have the potent to prevent or treat neurodegenerative disorders.

Dexmedetomidine Attenuates Methotrexate-Induced Neurotoxicity and Memory Deficits in Rats through Improving Hippocampal Neurogenesis: The Role of miR-15a/ROCK-1/ERK1/2/CREB/BDNF Pathway Modulation

Methotrexate (MTX) is a widely used neurotoxic drug with broad antineoplastic and immunosuppressant spectra. However, the exact molecular mechanisms by which MTX inhibits hippocampal neurogenesis are yet unclear. Dexmedetomidine (Dex), an α2-adrenergic receptor agonist, has recently shown neuroprotective effects; however, its full mechanism is unexplored. This study investigated the potential of Dex to mitigate MTX-induced neurotoxicity and memory impairment in rats and the possible role of the miR-15a/ROCK-1/ERK1/2/CREB/BDNF pathway. Notably, no former studies have linked this pathway to MTX-induced neurotoxicity. Male Sprague Dawley rats were placed into four groups. Group 1 received saline i.p. daily and i.v. on days 8 and 15. Group 2 received Dex at 10 μg/kg/day i.p. for 30 days. Group 3 received MTX at 75 mg/kg i.v. on days 8 and 15, followed by four i.p. doses of leucovorin at 6 mg/kg after 18 h and 3 mg/kg after 26, 42, and 50 h. Group 4 received MTX and leucovorin as in group 3 and Dex daily dosages as in group 2. Bioinformatic analysis identified the association of miR-15a with ROCK-1/ERK1/2/CREB/BDNF and neurogenesis. MTX lowered hippocampal doublecortin and Ki-67, two markers of neurogenesis. This was associated with the downregulation of miR-15a, upregulation of its target ROCK-1, and reduction in the downstream ERK1/2/CREB/BDNF pathway, along with disturbed hippocampal redox state. Novel object recognition and Morris water maze tests demonstrated the MTX-induced memory deficiencies. Dex co-treatment reversed the MTX-induced behavioral, biochemical, and histological alterations in the rats. These neuroprotective actions could be partly mediated through modulating the miR-15a/ROCK-1/ERK1/2/CREB/BDNF pathway, which enhances hippocampal neurogenesis.

Boosting Neurogenesis in the Adult Hippocampus Using Antidepressants and Mesenchymal Stem Cells

The hippocampus is one of the few privileged regions (neural stem cell niche) of the brain, where neural stem cells differentiate into new neurons throughout adulthood. However, dysregulation of hippocampal neurogenesis with aging, injury, depression and neurodegenerative disease leads to debilitating cognitive impacts. These debilitating symptoms deteriorate the quality of life in the afflicted individuals. Impaired hippocampal neurogenesis is especially difficult to rescue with increasing age and neurodegeneration. However, the potential to boost endogenous Wnt signaling by influencing pathway modulators such as receptors, agonists, and antagonists through drug and cell therapy-based interventions offers hope. Restoration and augmentation of hampered Wnt signaling to facilitate increased hippocampal neurogenesis would serve as an endogenous repair mechanism and contribute to hippocampal structural and functional plasticity. This review focuses on the possible interaction between neurogenesis and Wnt signaling under the control of antidepressants and mesenchymal stem cells (MSCs) to overcome debilitating symptoms caused by age, diseases, or environmental factors such as stress. It will also address some current limitations hindering the direct extrapolation of research from animal models to human application, and the technical challenges associated with the MSCs and their cellular products as potential therapeutic solutions.

A neuro-inspired computational model of life-long learning and catastrophic interference, mimicking hippocampus novelty-based dopamine modulation and lateral inhibitory plasticity

The human brain has a remarkable lifelong learning capability to acquire new experiences while retaining previously acquired information. Several hypotheses have been proposed to explain this capability, but the underlying mechanisms are still unclear. Here, we propose a neuro-inspired firing-rate computational model involving the hippocampus and surrounding areas, that encompasses two key mechanisms possibly underlying this capability. The first is based on signals encoded by the neuromodulator dopamine, which is released by novel stimuli and enhances plasticity only when needed. The second is based on a homeostatic plasticity mechanism that involves the lateral inhibitory connections of the pyramidal neurons of the hippocampus. These mechanisms tend to protect neurons that have already been heavily employed in encoding previous experiences. The model was tested with images from the MNIST machine learning dataset, and with more naturalistic images, for its ability to mitigate catastrophic interference in lifelong learning. The results show that the proposed biologically grounded mechanisms can effectively enhance the learning of new stimuli while protecting previously acquired knowledge. The proposed mechanisms could be investigated in future empirical animal experiments and inspire machine learning models.

Postnatal and Adult Neurogenesis in Mammals, Including Marsupials

In mammals, neurogenesis occurs during both embryonic and postnatal development. In eutherians, most brain structures develop embryonically; conversely, in marsupials, a number of brain structures develop after birth. The exception is the generation of granule cells in the dentate gyrus, olfactory bulb, and cerebellum of eutherian species. The formation of these structures starts during embryogenesis and continues postnatally. In both eutherians and marsupials, neurogenesis continues in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampal formation throughout life. The majority of proliferated cells from the SVZ migrate to the olfactory bulb, whereas, in the dentate gyrus, cells reside within this structure after division and differentiation into neurons. A key aim of this review is to evaluate advances in understanding developmental neurogenesis that occurs postnatally in both marsupials and eutherians, with a particular emphasis on the generation of granule cells during the formation of the olfactory bulb, dentate gyrus, and cerebellum. We debate the significance of immature neurons in the piriform cortex of young mammals. We also synthesize the knowledge of adult neurogenesis in the olfactory bulb and the dentate gyrus of marsupials by considering whether adult-born neurons are essential for the functioning of a given area.

Central FGF21 production regulates memory but not peripheral metabolism

Fibroblast growth factor 21 (FGF21) is a liver-derived endocrine hormone that functions to regulate energy homeostasis and macronutrient intake. Recently, FGF21 was reported to be produced and secreted from hypothalamic tanycytes, to regulate peripheral lipid metabolism; however, rigorous investigation of FGF21 expression in the brain has yet to be accomplished. Using a mouse model that drives CRE recombinase in FGF21-expressing cells, we demonstrate that FGF21 is not expressed in the hypothalamus, but instead is produced from the retrosplenial cortex (RSC), an essential brain region for spatial learning and memory. Furthermore, we find that central FGF21 produced in the RSC enhances spatial memory but does not regulate energy homeostasis or sugar intake. Finally, our data demonstrate that administration of FGF21 prolongs the duration of long-term potentiation in the hippocampus and enhances activation of hippocampal neurons. Thus, endogenous and pharmacological FGF21 appear to function in the hippocampus to enhance spatial memory.

Zhou et al. reveal that the endocrine hormone FGF21 is expressed in the brain. Central FGF21 expression occurs in distinct areas, including the retrosplenial cortex, but not the hypothalamus. Interestingly, brain-derived FGF21 regulates spatial memory formation, but not metabolism, and the converse is true for liver-derived FGF21.

Deep brain stimulation of fornix for memory improvement in Alzheimer's disease: A critical review

Memory reflects the brain function in encoding, storage and retrieval of the data or information, which is a fundamental ability for any live organism. The development of approaches to improve memory attracts much attention due to the underlying mechanistic insight and therapeutic potential to treat neurodegenerative diseases with memory loss, such as Alzheimer's disease (AD). Deep brain stimulation (DBS), a reversible, adjustable, and non-ablative therapy, has been shown to be safe and effective in many clinical trials for neurodegenerative and neuropsychiatric disorders. Among all potential regions with access to invasive electrodes, fornix is considered as it is the major afferent and efferent connection of the hippocampus known to be closely associated with learning and memory. Indeed, clinical trials have demonstrated that fornix DBS globally improved cognitive function in a subset of patients with AD, indicating fornix can serve as a potential target for neurosurgical intervention in treating memory impairment in AD. The present review aims to provide a better understanding of recent progresses in the application of fornix DBS for ameliorating memory impairments in AD patients.

Dominant role of adult neurogenesis-induced structural heterogeneities in driving plasticity heterogeneity in dentate gyrus granule cells

Neurons and synapses manifest pronounced variability in the amount of plasticity induced by identical activity patterns. The mechanisms underlying such plasticity heterogeneity, which have been implicated in context‐specific resource allocation during encoding, have remained unexplored. Here, we employed a systematic physiologically constrained parametric search to identify the cellular mechanisms behind plasticity heterogeneity in dentate gyrus granule cells. We used heterogeneous model populations to ensure that our conclusions were not biased by parametric choices in a single hand‐tuned model. We found that each of intrinsic, synaptic, and structural heterogeneities independently yielded heterogeneities in synaptic plasticity profiles obtained with two different induction protocols. However, among the disparate forms of neural‐circuit heterogeneities, our analyses demonstrated the dominance of neurogenesis‐induced structural heterogeneities in driving plasticity heterogeneity in granule cells. We found that strong relationships between neuronal intrinsic excitability and plasticity emerged only when adult neurogenesis‐induced heterogeneities in neural structure were accounted for. Importantly, our analyses showed that it was not imperative that the manifestation of neural‐circuit heterogeneities must translate to heterogeneities in plasticity profiles. Specifically, despite the expression of heterogeneities in structural, synaptic, and intrinsic neuronal properties, similar plasticity profiles were attainable across all models through synergistic interactions among these heterogeneities. We assessed the parametric combinations required for the manifestation of such degeneracy in the expression of plasticity profiles. We found that immature cells showed physiological plasticity profiles despite receiving afferent inputs with weak synaptic strengths. Thus, the high intrinsic excitability of immature granule cells was sufficient to counterbalance their low excitatory drive in the expression of plasticity profile degeneracy. Together, our analyses demonstrate that disparate forms of neural‐circuit heterogeneities could mechanistically drive plasticity heterogeneity, but also caution against treating neural‐circuit heterogeneities as proxies for plasticity heterogeneity. Our study emphasizes the need for quantitatively characterizing the relationship between neural‐circuit and plasticity heterogeneities across brain regions.

Deconstructing Sox2 Function in Brain Development and Disease

SOX2 is a transcription factor conserved throughout vertebrate evolution, whose expression marks the central nervous system from the earliest developmental stages. In humans, SOX2 mutation leads to a spectrum of CNS defects, including vision and hippocampus impairments, intellectual disability, and motor control problems. Here, we review how conditional Sox2 knockout (cKO) in mouse with different Cre recombinases leads to very diverse phenotypes in different regions of the developing and postnatal brain. Surprisingly, despite the widespread expression of Sox2 in neural stem/progenitor cells of the developing neural tube, some regions (hippocampus, ventral forebrain) appear much more vulnerable than others to Sox2 deletion. Furthermore, the stage of Sox2 deletion is also a critical determinant of the resulting defects, pointing to a stage-specificity of SOX2 function. Finally, cKOs illuminate the importance of SOX2 function in different cell types according to the different affected brain regions (neural precursors, GABAergic interneurons, glutamatergic projection neurons, Bergmann glia). We also review human genetics data regarding the brain defects identified in patients carrying mutations within human SOX2 and examine the parallels with mouse mutants. Functional genomics approaches have started to identify SOX2 molecular targets, and their relevance for SOX2 function in brain development and disease will be discussed.

Adult hippocampal neurogenesis shapes adaptation and improves stress response: a mechanistic and integrative perspective

Adult hippocampal neurogenesis (AHN) represents a remarkable form of neuroplasticity that has increasingly been linked to the stress response in recent years. However, the hippocampus does not itself support the expression of the different dimensions of the stress response. Moreover, the main hippocampal functions are essentially preserved under AHN depletion and adult-born immature neurons (abGNs) have no extrahippocampal projections, which questions the mechanisms by which abGNs influence functions supported by brain areas far from the hippocampus. Within this framework, we propose that through its computational influences AHN is pivotal in shaping adaption to environmental demands, underlying its role in stress response. The hippocampus with its high input convergence and output divergence represents a computational hub, ideally positioned in the brain (1) to detect cues and contexts linked to past, current and predicted stressful experiences, and (2) to supervise the expression of the stress response at the cognitive, affective, behavioral, and physiological levels. AHN appears to bias hippocampal computations toward enhanced conjunctive encoding and pattern separation, promoting contextual discrimination and cognitive flexibility, reducing proactive interference and generalization of stressful experiences to safe contexts. These effects result in gating downstream brain areas with more accurate and contextualized information, enabling the different dimensions of the stress response to be more appropriately set with specific contexts. Here, we first provide an integrative perspective of the functional involvement of AHN in the hippocampus and a phenomenological overview of the stress response. We then examine the mechanistic underpinning of the role of AHN in the stress response and describe its potential implications in the different dimensions accompanying this response.

High Caloric Diet Induces Memory Impairment and Disrupts Synaptic Plasticity in Aged Rats

The increasing consumption of sugar and fat seen over the last decades and the consequent overweight and obesity, were recently linked with a deleterious effect on cognition and synaptic function. A major question, which remains to be clarified, is whether obesity in the elderly is an additional risk factor for cognitive impairment. We aimed at unravelling the impact of a chronic high caloric diet (HCD) on memory performance and synaptic plasticity in aged rats. Male rats were kept on an HCD or a standard diet (control) from 1 to 24 months of age. The results showed that under an HCD, aged rats were obese and displayed significant long-term recognition memory impairment when compared to age-matched controls. Ex vivo synaptic plasticity recorded from hippocampal slices from HCD-fed aged rats revealed a reduction in the magnitude of long-term potentiation, accompanied by a decrease in the levels of the brain-derived neurotrophic factor receptors TrkB full-length (TrkB-FL). No alterations in neurogenesis were observed, as quantified by the density of immature doublecortin-positive neurons in the hippocampal dentate gyrus. This study highlights that obesity induced by a chronic HCD exacerbates age-associated cognitive decline, likely due to impaired synaptic plasticity, which might be associated with deficits in TrkB-FL signaling.

Isoform-Specific Reduction of the Basic Helix-Loop-Helix Transcription Factor TCF4 Levels in Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder with onset of characteristic motor symptoms at midlife, preceded by subtle cognitive and behavioral disturbances. Transcriptional dysregulation emerges early in the disease course and is considered central to HD pathogenesis. Using wild-type (wt) and HD knock-in mouse striatal cell lines we observed a HD genotype-dependent reduction in the protein levels of transcription factor 4 (TCF4), a member of the basic helix-loop-helix (bHLH) family with critical roles in brain development and function. We characterized mouse Tcf4 gene structure and expression of alternative mRNAs and protein isoforms in cell-based models of HD, and in four different brain regions of male transgenic HD mice (R6/1) from young to mature adulthood. The largest decrease in the levels of TCF4 at mRNA and specific protein isoforms were detected in the R6/1 mouse hippocampus. Translating this finding to human disease, we found reduced expression of long TCF4 isoforms in the postmortem hippocampal CA1 area and in the cerebral cortex of HD patients. Additionally, TCF4 protein isoforms showed differential synergism with the proneural transcription factor ASCL1 in activating reporter gene transcription in hippocampal and cortical cultured neurons. Induction of neuronal activity increased these synergistic effects in hippocampal but not in cortical neurons, suggesting brain region-dependent differences in TCF4 functions. Collectively, this study demonstrates isoform-specific changes in TCF4 expression in HD that could contribute to the progressive impairment of transcriptional regulation and neuronal function in this disease.

Sevoflurane Postconditioning Ameliorates Neuronal Migration Disorder Through Reelin/Dab1 and Improves Long-term Cognition in Neonatal Rats After Hypoxic-Ischemic Injury

Sevoflurane postconditioning (SPC) has been widely reported to attenuate brain injury after hypoxia-ischemia encephalopathy (HIE) by inhibiting neural necrosis and autophagy. Moreover, recent reports revealed that sevoflurane facilitated hippocampal reconstruction via regulating migration. Yet, it remains unclear whether the promotion of neural migration by SPC repairs the hippocampal injury after HIE. Here, we hypothesize that SPC exerts a neuroprotective effect by ameliorating neuronal migration disorder after HIE and regulating Reelin expression. Furthermore, the downstream Reelin/Dab1 pathway may be involved. The classical Rice-Vannucci model of hypoxia-ischemia was performed on postnatal day 7 rat pups, which was followed by SPC at 1 minimum alveolar concentration (MAC 2.5%) for 30 min. Piceatannol, causing Reelin aggregation in vivo, was used to detect whether Reelin/Dab1 was involved in the neuroprotection effect of SPC. Hippocampal-dependent learning ability tests were conducted to assess the long-term effects on locomotor activity and spatial learning ability. Our findings suggest that hypoxia-ischemia injury inhibited neurons migrated outward from the basal zone of dentate gyrus, disrupted cytoarchitecture of the dentate gyrus (DG), and led to long-term cognition deficits. However, SPC could relieve the restricted hippocampal neurons and repair the hippocampal-dependent memory function damaged after HIE by attenuating the overactivation of the Reelin/Dab1 pathway. These results demonstrate that SPC plays a pivotal role in ameliorating neuronal migration disorder and maintaining normal cytoarchitecture of the DG via inhibiting overactivated Reelin expression. This process may involve overactivated Reelin/Dab1 signaling pathway and spatial learning ability by regulating the Reelin expression which may associate with its neuroprotection.

The effects of Hippophae rhamnoides in neuroprotection and behavioral alterations against iron-induced epilepsy

Epilepsy is a neurological disorder in which malfunctioning of the electrical activity of the brain causes recurrent, unprovoked seizures. Epilepsy causes wide symptoms that include cognitive dysfunction, anxiety, behavioral alterations, and histological impairments. In this study, the effect of Hippophae rhamnoides (Sea buckthorn/Sbt) on electrophysiology, behavior, and histology in iron-induced epilepsy was analyzed. Rats were randomly divided into four groups (n = 8); Control group, Epileptic group, Sbt treated epileptic group, and Sbt treated group. To induce epilepsy, the intracortical iron injection was administered at a dose of 5 μl of 100 mM FeCl₃. A significant increase in epileptiform activity, behavioral abnormalities, and histological impairments was observed in the iron-induced epileptic rats. Hippophae rhamnoides berry extract was administered orally at a dose of 1 ml/kg body wt. for one month. Sbt administration significantly reduced the epileptiform activity, improved behavioral abnormalities, and improved histological impairments in epileptic rats. In conclusion, this study demonstrates the antiepileptic effect of Sbt that probably has exerted by its neuroprotective and behavioral alteration potential against iron-induced epilepsy.

Dysregulation of Myt1 expression acts as a potential peripheral biomarker for major depressive disorder and bipolar disorder

Major depressive disorder (MDD) and bipolar disorder (BPD) are among the most debilitating mental conditions. Diagnostic criteria for MDD include psychological and physical symptoms, such as low mood and changes in appetite or sleep, respectively. BPD in addition to periods of depression represents episodes of mania or hypomania, and elevation in mood and energy levels are associated with this condition. Dysregulation in adult neurogenesis and myelination have been reported in psychiatric disorders. As a key factor in neurogenesis, it was hypothesized that Myt1 gene expression may be altered in these conditions. Using Real-time PCR, Myt1 expression level in 100 MDD patients and 100 BPD patients, compared with healthy control (HC) individuals was evaluated. Results demonstrate significant downregulation of Myt1 in MDD and BPD. Logistic regression analysis and binary classification evaluation reveal potential risk factor and biomarker characteristics of Myt1, respectively. Moreover, forward and backward digit span results denote a significant reduction in the function of working memory (WM) of MDD and BPD subjects. Correlation analysis revealed a significant association between Myt1 downregulation and WM disruption in the affected individuals. In conclusion, due to its altered role in neurogenesis, downregulation of Myt1 can be associated with the pathology of MDD and BPD.

Therapeutic potential of ketamine for alcohol use disorder

Excessive alcohol consumption is involved in 1/10 of deaths of U.S. working-age adults and costs the country around $250,000,000 yearly. While Alcohol Use Disorder (AUD) pathology is complex and involves multiple neurotransmitter systems, changes in synaptic plasticity, hippocampal neurogenesis, and neural connectivity have been implicated in the behavioral characteristics of AUD. Depressed mood and stress are major determinants of relapse in AUD, and there is significant comorbidity between AUD, depression, and stress disorders, suggesting potential for overlap in their treatments. Disulfiram, naltrexone, and acamprosate are current pharmacotherapies for AUD, but these treatments have limitations, highlighting the need for novel therapeutics. Ketamine is a N-methyl-D-Aspartate receptor antagonist, historically used in anesthesia, but also affects other neurotransmitters systems, synaptic plasticity, neurogenesis, and neural connectivity. Currently under investigation for treating AUDs and other Substance Use Disorders (SUDs), ketamine has strong support for efficacy in treating clinical depression, recently receiving FDA approval. Ketamine's effect in treating depression and stress disorders, such as PTSD, and preliminary evidence for treating SUDs further suggests a role for treating AUDs. This review explores the behavioral and neural evidence for treating AUDs with ketamine and clinical data on ketamine therapy for AUDs and SUDs.

Effects of pyrethroids on brain development and behavior: Deltamethrin

Deltamethrin (DLM) is a Type II pyrethroid pesticide widely used in agriculture, homes, public spaces, and medicine. Epidemiological studies report that increased pyrethroid exposure during development is associated with neurobehavioral disorders. This raises concern about the safety of these chemicals for children. Few animal studies have explored the long-term effects of developmental exposure to DLM on the brain. Here we review the CNS effects of pyrethroids, with emphasis on DLM. Current data on behavioral and cognitive effects after developmental exposure are emphasized. Although, the acute mechanisms of action of DLM are known, how these translate to long-term effects is only beginning to be understood. But existing data clearly show there are lasting effects on locomotor activity, acoustic startle, learning and memory, apoptosis, and dopamine in mice and rats after early exposure. The most consistent neurochemical findings are reductions in the dopamine transporter and the dopamine D1 receptor. The data show that DLM is developmentally neurotoxic but more research on its mechanisms of long-term effects is needed.

Histopathological study on neuroapoptotic alterations induced by etomidate in rat hippocampus

In human, there is substantial neurogenesis in the hippocampus that is implicated in memory formation and learning. These new-born neurons can be affected by neuropathological conditions. Anesthesia and surgical procedures are associated with postoperative cognitive changes particularly, impaired memory and learning. Therefore, the aim of this study was to evaluate the possible neurodegenerative effects of etomidate in rat hippocampus. Thirty male Wistar rats weighing 250 ± 30 g were randomly divided into 3 groups: 1) Etomidate group; four times 20 mg intraperitoneal injection with 1-h intervals, 2) Control group; the equal volume of normal saline, and 3) Normal group; without any intervention. 6 h after the last injection, the brains were removed and processed according to routine histological methods. TUNEL assay and toluidine blue staining were performed to evaluate neuro-histopathological changes in different regions of hippocampus. Our results showed that the number of TUNEL positive cells and dark neurons (DNs) in etomidate group were significantly higher in the CA1, CA2, CA3, and dentate gyrus (DG) of hippocampus compared with the control and normal groups (p < 0.05). While, there was no significant difference between the various regions of hippocampus in control and normal groups. Our findings showed that etomidate can increase apoptotic cells and dark neurons induction in different regions of hippocampus mainly in DG.

Adult-born granule cell mossy fibers preferentially target parvalbumin-positive interneurons surrounded by perineuronal nets

Adult-born granule cells (abGCs) integrate into the hippocampus and form connections with dentate gyrus parvalbumin-positive (PV+) interneurons, a circuit important for modulating plasticity. Many of these interneurons are surrounded by perineuronal nets (PNNs), extracellular matrix structures known to participate in plasticity. We compared abGC projections to PV+ interneurons with negative-to-low intensity PNNs to those with high intensity PNNs using retroviral and 3R-Tau labeling in adult mice, and found that abGC mossy fibers and boutons are more frequently located near PV+ interneurons with high intensity PNNs. These results suggest that axons of new neurons preferentially stabilize near target cells with intense PNNs. Next, we asked whether the number of abGCs influences PNN formation around PV+ interneurons, and found that near complete ablation of abGCs produced a decrease in the intensity and number of PV+ neurons with PNNs, suggesting that new neuron innervation may enhance PNN formation. Experience-driven changes in adult neurogenesis did not produce consistent effects, perhaps due to widespread effects on plasticity. Our study identifies abGC projections to PV+ interneurons with PNNs, with more presumed abGC mossy fiber boutons found near the cell body of PV+ interneurons with strong PNNs.

Inhibition of mTOR signaling by genetic removal of p70 S6 kinase 1 increases anxiety-like behavior in mice

The mechanistic target of rapamycin (mTOR) is a ubiquitously expressed kinase that acts through two complexes, mTORC1 and mTORC2, to regulate protein homeostasis, as well as long lasting forms of synaptic and behavioral plasticity. Alteration of the mTOR pathway is classically involved in neurodegenerative disorders, and it has been linked to dysregulation of cognitive functions and affective states. However, information concerning the specific involvement of the p70 S6 kinase 1 (S6K1), a downstream target of the mTORC1 pathway, in learning and memory processes and in the regulation of affective states remains scant. To fill this gap, we exposed adult male mice lacking S6K1 to a battery of behavioral tests aimed at measuring their learning and memory capabilities by evaluating reference memory and flexibility with the Morris water maze, and associative memory using the contextual fear conditioning task. We also studied their anxiety-like and depression-like behaviors by, respectively, performing elevated plus maze, open field, light-dark emergence tests, and sucrose preference and forced swim tests. We found that deleting S6K1 leads to a robust anxious phenotype concomitant with associative learning deficits; these symptoms are associated with a reduction of adult neurogenesis and neuronal atrophy in the hippocampus. Collectively, these results provide grounds for the understanding of anxiety reports after treatments with mTOR inhibitors and will be critical for developing novel compounds targeting anxiety.

The temporal origin of dentate granule neurons dictates their role in spatial memory

The dentate gyrus is one of the only brain regions that continues its development after birth in rodents. Adolescence is a very sensitive period during which cognitive competences are programmed. We investigated the role of dentate granule neurons (DGNs) born during adolescence in spatial memory and compared them with those generated earlier in life (in embryos or neonates) or during adulthood by combining functional imaging, retroviral and optogenetic tools to tag and silence DGNs. By imaging DGNs expressing Zif268, a proxy for neuronal activity, we found that neurons generated in adolescent rats (and not embryos or neonates) are transiently involved in spatial memory processing. In contrast, adult-generated DGNs are recruited at a later time point when animals are older. A causal relationship between the temporal origin of DGNs and spatial memory was confirmed by silencing DGNs in behaving animals. Our results demonstrate that the emergence of spatial memory depends on neurons born during adolescence, a function later assumed by neurons generated during adulthood.

The Ketamine Antidepressant Story: New Insights

Ketamine is a versatile agent primarily utilized as a dissociative anesthetic, which acts by blocking the excitatory receptor N-methyl-d-aspartate receptor (NMDA). It functions to inhibit the current of both Na⁺ and K⁺ voltage-gated channels, thus preventing serotonin and dopamine reuptake. Studies have indicated that administering a single subanesthetic dose of ketamine relieves depression rapidly and that the effect is sustained. For decades antidepressant agents were based on the monoamine theory. Although ketamine may not be the golden antidepressant, it has opened new avenues toward mechanisms involved in the pathology of treatment-resistant depression and achieving rapid antidepressant effects. Thus, preclinical studies focusing on deciphering the molecular mechanisms involved in the antidepressant action of ketamine will assist in the development of a new antidepressant. This review was conducted to elucidate the emerging pathways that can explain the complex dose-dependent mechanisms achieved by administering ketamine to treat major depressive disorders. Special attention was paid to reviewing the literature on hydroxynorketamines, which are ketamine metabolites that have recently attracted attention in the context of depression.

Regular Leisure-Time Physical Activity is Effective in Boosting Neurotrophic Factors and Alleviating Menopause Symptoms

BACKGROUND

The study investigated the effects of regular leisure-time physical activity on the parameters of cognitive function (plasma brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and cathepsin B) and menopausal symptoms (the climacterium, depression, and cognitive impairment) in obese middle-aged women.

METHODS

All subjects were middle-aged and obese women (n = 52, % body fat > 30%). The participants were divided into premenopausal (PRM) (n = 18, age = 47.56 ± 6.11 years) and postmenopausal (POM) (n = 34, age = 57.79 ± 5.68 years) groups. The participants completed a survey questionnaire related to depression and the climacterium, as well as cognitive tests. Physical activity was performed for 12 weeks. Blood samples from the forearm vein were analyzed after 12 h of fasting. Blood levels of BDNF, NGF, and cathepsin B were analyzed using an R&D kit.

RESULTS

Regular leisure-time physical activity had a positive effect on reducing the percentage of body fat in premenopausal and postmenopausal obese women. In addition, the results of the questionnaire showed that regular exercise had a positive effect on body composition caused by lifestyle change and enhanced psychological stability. The BDNF concentration was significantly lower in postmenopausal than in premenopausal obese women. In addition, regular physical activity significantly increased the cathepsin B and NGF levels in postmenopausal obese women.

CONCLUSIONS

Continuous leisure-time physical activity improved body composition and neurotrophic factors and alleviated menopausal symptoms in obese Korean women.

Disruption of NREM sleep and sleep-related spatial memory consolidation in mice lacking adult hippocampal neurogenesis

Cellular plasticity at the structural level and sleep at the behavioural level are both essential for memory formation. The link between the two is not well understood. A functional connection between adult neurogenesis and hippocampus-dependent memory consolidation during NREM sleep has been hypothesized but not experimentally shown. Here, we present evidence that during a three-day learning session in the Morris water maze task a genetic knockout model of adult neurogenesis (Cyclin D2^-/-) showed changes in sleep macro- and microstructure. Sleep EEG analyses revealed a lower total sleep time and NREM fraction in Cyclin D2^-/- mice as well as an impairment of sleep specific neuronal oscillations that are associated with memory consolidation. Better performance in the memory task was associated with specific sleep parameters in wild-type, but not in Cyclin D2^-/- mice. In wild-type animals the number of proliferating cells correlated with the amount of NREM sleep. The lack of adult neurogenesis led to changes in sleep architecture and oscillations that represent the dialog between hippocampus and neocortex during sleep. We suggest that adult neurogenesis-as a key event of hippocampal plasticity-might play an important role for sleep-dependent memory consolidation and modulates learning-induced changes of sleep macro- and microstructure.

Ganglioside GD3 regulates dendritic growth in newborn neurons in adult mouse hippocampus via modulation of mitochondrial dynamics

Ganglioside GD3, a major ganglioside species in neural stem cells, plays a crucial role in maintenance of the self-renewal capacity of these cells. However, its bioactivity in postnatally differentiated neurons in the neurogenic regions of adult brains has not been elucidated. Here, we describe for the first time that deletion of GD3 not only impairs neurotrophin-induced stem cell proliferation, but also alters the dendritic structure as well as the number of synapses of nascent neurons in the dentate gyrus of adult brain. When examining the behavioral phenotypes, GD3 synthase-knockout (GD3S-KO) mice displayed impairment in hippocampus-dependent memory function. To further gain insight into its cellular function, we examined GD3-binding partners from mouse brain extract using a GD3-specific monoclonal antibody, R24, followed by LC-MS/MS analysis and identified a mitochondrial fission protein, the dynamin-related protein-1 (Drp1), as a novel GD3-binding protein. Biochemical and imaging analyses revealed mitochondrial fragmentation in GD3-depleted dentate gyrus neurons, suggesting that GD3 is essential for the mitochondrial Drp1 turnover that is required for efficient mitochondrial fission. These results suggest that GD3 is required for proper dendritic and spine maturation of newborn neurons in adult brain through the regulation of mitochondrial dynamics.

Understanding the Real State of Human Adult Hippocampal Neurogenesis From Studies of Rodents and Non-human Primates

The concept of adult hippocampal neurogenesis (AHN) has been widely accepted, and a large number of studies have been performed in rodents using modern experimental techniques, which have clarified the nature and developmental processes of adult neural stem/progenitor cells, the functions of AHN, such as memory and learning, and its association with neural diseases. However, a fundamental problem is that it remains unclear as to what extent AHN actually occurs in humans. The answer to this is indispensable when physiological and pathological functions of human AHN are deduced from studies of rodent AHN, but there are controversial data on the extent of human AHN. In this review, studies on AHN performed in rodents and humans will be briefly reviewed, followed by a discussion of the studies in non-human primates. Then, how data of rodent and non-human primate AHN should be applied for understanding human AHN will be discussed.

Responsiveness of dentate neurons generated throughout adult life is associated with resilience to cognitive aging

During aging, some individuals are resilient to the decline of cognitive functions whereas others are vulnerable. These inter-individual differences in memory abilities have been associated with differences in the rate of hippocampal neurogenesis measured in elderlies. Whether the maintenance of the functionality of neurons generated throughout adult life is linked to resilience to cognitive aging remains completely unexplored. Using the immediate early gene Zif268, we analyzed the activation of dentate granule neurons born in adult (3-month-old), middle-aged (12-month-old), or senescent (18-month-old) rats (n = 96) in response to learning when animals reached 21 months of age. The activation of neurons born during the developmental period was also examined. We show that adult-born neurons can survive up to 19 months and that neurons generated 4, 10, or 19 months before learning, but not developmentally born neurons, are activated in senescent rats with good learning abilities. In contrast, aged rats with bad learning abilities do not exhibit activity-dependent regulation of newborn cells, whatever their birthdate. In conclusion, we propose that resilience to cognitive aging is associated with responsiveness of neurons born during adult life. These data add to our current knowledge by showing that the aging of memory abilities stems not only from the number but also from the responsiveness of adult-born neurons.

A 20-Year Journey from Axonal Injury to Neurodegenerative Diseases and the Prospect of Immunotherapy for Combating Alzheimer's Disease

The understanding of the dialogue between the brain and the immune system has undergone dramatic changes over the last two decades, with immense impact on the perception of neurodegenerative diseases, mental dysfunction, and many other brain pathologic conditions. Accumulated results have suggested that optimal function of the brain is dependent on support from the immune system, provided that this immune response is tightly controlled. Moreover, in contrast to the previous prevailing dogma, it is now widely accepted that circulating immune cells are needed for coping with brain pathologies and that their optimal effect is dependent on their type, location, and activity. In this perspective, we describe our own scientific journey, reviewing the milestones in attaining this understanding of the brain-immune axis integrated with numerous related studies by others. We then explain their significance in demonstrating the possibility of harnessing the immune system in a well-controlled manner for the treatment of neurodegenerative diseases.

Environmental Enrichment Improved Learning and Memory, Increased Telencephalic Cell Proliferation, and Induced Differential Gene Expression in Colossoma macropomum

Fish use spatial cognition based on allocentric cues to navigate, but little is known about how environmental enrichment (EE) affects learning and memory in correlation with hematological changes or gene expression in the fish brain. Here we investigated these questions in Colossoma macropomum (Teleostei). Fish were housed for 192 days in either EE or in an impoverished environment (IE) aquarium. EE contained toys, natural plants, and a 12-h/day water stream for voluntary exercise, whereas IE had no toys, plants, or water stream. A third plus maze aquarium was used for spatial and object recognition tests. Compared with IE, the EE fish showed greater learning rates, body length, and body weight. After behavioral tests, whole brain tissue was taken, stored in RNA-later, and then homogenized for DNA sequencing after conversion of isolated RNA. To compare read mapping and gene expression profiles across libraries for neurotranscriptome differential expression, we mapped back RNA-seq reads to the C. macropomum de novo assembled transcriptome. The results showed significant differential behavior, cell counts and gene expression in EE and IE individuals. As compared with IE, we found a greater number of cells in the telencephalon of individuals maintained in EE but no significant difference in the tectum opticum, suggesting differential plasticity in these areas. A total of 107,669 transcripts were found that ultimately yielded 64 differentially expressed transcripts between IE and EE brains. Another group of adult fish growing in aquaculture conditions were either subjected to exercise using running water flow or maintained sedentary. Flow cytometry analysis of peripheral blood showed a significantly higher density of lymphocytes, and platelets but no significant differences in erythrocytes and granulocytes. Thus, under the influence of contrasting environments, our findings showed differential changes at the behavioral, cellular, and molecular levels. We propose that the differential expression of selected transcripts, number of telencephalic cell counts, learning and memory performance, and selective hematological cell changes may be part of Teleostei adaptive physiological responses triggered by EE visuospatial and somatomotor stimulation. Our findings suggest abundant differential gene expression changes depending on environment and provide a basis for exploring gene regulation mechanisms under EE in C. macropomum.

Chronic Bumetanide Infusion Alters Young Neuron Morphology in the Dentate Gyrus Without Affecting Contextual Fear Memory

Young neurons in the adult brain are key to some types of learning and memory. They integrate in the dentate gyrus (DG) of the hippocampus contributing to such cognitive processes following timely developmental events. While experimentally impairing GABAergic transmission through the blockade or elimination of the ionic cotransporter NKCC1 leads to alterations in the proper maturation of young neurons, it is still unknown if the in vivo administration of common use diuretic drugs that block the cotransporter, alters the development of young hippocampal neurons and affects DG-related functions. In this study, we delivered chronically and intracerebroventricularly the NKCC1 blocker bumetanide to young-adult rats. We analyzed doublecortin density and development parameters (apical dendrite length and angle and dendritic arbor length) in doublecortin positive neurons from different subregions in the DG and evaluated the performance of animals in contextual fear learning and memory. Our results show that in bumetanide-treated subjects, doublecortin density is diminished in the infra and suprapyramidal blades of the DG; the length of primary dendrites is shortened in the infrapyramidal blade and; the growth angle of primary dendrites in the infrapyramidal blade is different from control animals. Behaviorally, treated animals showed the typical learning curve in a contextual fear task, and freezing-time displayed during contextual fear memory was not different from controls. Thus, in vivo icv delivery of bumetanide negatively alters DCX density associated to young neurons and its proper development but not to the extent of affecting a DG dependent task as aversive context learning and memory.

Endonuclease VIII-like 1 deficiency impairs survival of newly generated hippocampal neurons and memory performance in young-adult male mice

AIMS

Efficient memory formation in rodents depends on adult neurogenesis in the subgranular zone of the hippocampus, and mounting evidence suggests that deficiencies in initiating repair of oxidatively induced DNA damage may impair neurogenesis. Hence, we aimed to determine whether loss of the DNA glycosylase, endonuclease VIII-like 1 (Neil1), affects hippocampal neurogenesis and memory performance in young-adult mice.

MAIN METHODS

Eight-week-old male wild-type and Neil1-deficient (Neil1^-/-) mice were treated with bromodeoxyuridine to track neuronal proliferation and differentiation. A neurosphere formation assay was further used to measure neuroprogenitor proliferative capacity. Hippocampus-related memory functions were assessed with Y-maze spontaneous alternation and novel object recognition tests.

KEY FINDINGS

Young-adult male Neil1^-/- mice exhibited diminished adult hippocampal neurogenesis in the dentate gyrus, probably as a result of poor survival of newly proliferated neurons. Furthermore, the Y-maze spontaneous alternation and novel object recognition tests respectively revealed that Neil1 deficiency impairs spatial and non-spatial hippocampus-related memory functions. We also found that expression of p53, a central regulator of apoptosis, was upregulated in the dentate gyrus of Neil1^-/- mice, while the level of β-catenin, a key cell survival molecule, was downregulated.

SIGNIFICANCE

The DNA glycosylase, Neil1, promotes successful hippocampal neurogenesis and learning and memory in young-adult mice.

Training on an Appetitive Trace-Conditioning Task Increases Adult Hippocampal Neurogenesis and the Expression of Arc, Erk and CREB Proteins in the Dorsal Hippocampus

Adult hippocampal neurogenesis (AHN) plays an essential role in hippocampal-dependent memory consolidation. Increased neurogenesis enhances learning, whereas its ablation causes memory impairment. In contrast, few reports suggest that neurogenesis reduces after learning. Although the interest in exploring the role of adult neurogenesis in learning has been growing, the evidence is still limited. The role of the trace- and delay-appetitive-conditioning on AHN and its underlying mechanism are not known. The consolidation of trace-conditioned memory requires the hippocampus, but delay-conditioning does not. Moreover, the dorsal hippocampus (DH) and ventral hippocampus (VH) may have a differential role in these two conditioning paradigms. Here, we have investigated the changes in: (A) hippocampal cell proliferation and their progression towards neuronal lineage; and (B) expression of Arc, Erk1, Erk2, and CREB proteins in the DH and VH after trace- and delay-conditioning in the rat. The number of newly generated cells significantly increased in the trace-conditioned but did not change in the delay-conditioned animals compared to the control group. Similarly, the expression of Arc protein significantly increased in the DH but not in the VH after trace-conditioning. Nonetheless, it remains unaltered in the delay-conditioned group. The expression of pErk1, pErk2, and pCREB also increased in the DH after trace-conditioning. Whereas, the expression of only pErk1 pErk2 and pCREB proteins increased in the VH after delay-conditioning. Our results suggest that appetitive trace-conditioning enhances AHN. The increased DH neuronal activation and pErk1, pErk2, and pCREB in the DH may be playing an essential role in learning-induced cell-proliferation after appetitive trace-conditioning.

Memory-related process in physiological status and alzheimer's disease

Rejecting central dogma around static status of adult mammalian brain, CNS has the nascent neurons generated in subgranular zone of dentate gyrus in hippocampus which develop to novel glutamatergic granule cells, with the innate feature of transmuting to memory disks. Structural plasticity proceeds with synaptic plasticity to process all the developing stages required to successful maturation and functional integration, whereby the memory context is ready to leave the hippocampus toward cortex network through consolidation process, for being installed and run the memory disk forever. However, in Alzheimer's disease, brain deal with subtle deadly progressive loss of synapsis, neuronal dysfunction and ultimately network failure, resulting in memory decay and cognitive decline-concluding that AD destroys memory formation related-pathways.

The polarity and properties of radial glia-like neural stem cells are altered by seizures with status epilepticus: Study using an improved mouse pilocarpine model of epilepsy

In the adult mouse hippocampus, new neurons are produced by radial glia-like (RGL) neural stem cells in the subgranular zone, which extend their apical processes toward the molecular layer, and express the astrocyte marker glial fibrillary acidic protein, but not the astrocyte marker S100β. In rodent models of epilepsy, adult hippocampal neurogenesis was reported to be increased after acute and mild seizures, but to be decreased by chronic and severe epilepsy. In the present study, we investigated how the severity of seizures affects neurogenesis and RGL neural stem cells in acute stages of epilepsy, using an improved mouse pilocarpine model in which pilocarpine-induced hypothermia was prevented by maintaining body temperature, resulting in a high incidence rate of epileptic seizures and low rate of mortality. In mice that experienced seizures without status epilepticus (SE), the number of proliferating progenitors and immature neurons were significantly increased, whereas no changes were observed in RGL cells. In mice that experienced seizures with SE, the number of proliferating progenitors and immature neurons were unchanged, but the number of RGL cells with an apical process was significantly reduced. Furthermore, the processes of the majority of RGL cells extended inversely toward the hilus, and about half of the aberrant RGL cells expressed S100β. These results suggest that seizures with SE lead to changes in the polarity and properties of RGL neural stem cells, which may direct them toward astrocyte differentiation, resulting in the reduction of neural stem cells producing new granule cells. This also suggests the possibility that cell polarity of RGL stem cells is important for maintaining the stemness of adult neural stem cells.

Schisandrin A and B enhance the dentate gyrus neurogenesis in mouse hippocampus

Schisandrin A and B (Sch A and B) are the main effective components of Schisandra chinensis (S. chinensis), which is traditionally used to enhance mental and intellectual functions in eastern Asia. Previously, we reported Sch A and B remarkably affect adult neurogenesis in the subventricular zone of mouse lateral ventricle. Since the neurogenesis in the hippocampal dentate gyrus (DG) is more important to learning, memory and cognition, here we further examined their effects on the adult DG neurogenesis. Phosphohistone H3 (PHH3) immunostaining showed that Sch B significantly enhanced the cell proliferation in the DG. Glial fibrillary acidic protein (GFAP, mostly labels astrocytes and some stem cells) staining was used to further identify the proliferating cell type. Dramatically, increases of GFAP⁺ cells in both Sch A and B treated groups were observed. What's more, the total numbers of the mature neurons labeled by neuron-specific nuclear protein (NeuN) were also increased in both Sch A and B treated groups compared with the controls. Together, Sch A and B enhance the adult DG neurogenesis by increasing astrocytes/stem cells and improving the survival and maturation of DG neurons. Our study shed a new light on the neuropharmacological functions of the herbal medicine S. chinensis.

Analysis of proliferating neuronal progenitors and immature neurons in the human hippocampus surgically removed from control and epileptic patients

Adult neurogenesis in the mammalian hippocampus is a well-known phenomenon. However, it remains controversial as to what extent adult neurogenesis actually occurs in the adult human hippocampus, and how brain diseases, such as epilepsy, affect human adult neurogenesis. To address these questions, we analyzed immature neuronal marker-expressing (PSA-NCAM+) cells and proliferating neuronal progenitor (Ki67+/HuB+/DCX+) cells in the surgically removed hippocampus of epileptic patients. In control patients, a substantial number of PSA-NCAM+ cells were distributed densely below the granule cell layer. In epileptic patients with granule cell dispersion, the number of PSA-NCAM+ cells was reduced, and aberrant PSA-NCAM+ cells were found. However, the numbers of Ki67+/HuB+/DCX+ cells were very low in both control and epileptic patients. The large number of PSA-NCAM+ cells and few DCX+/HuB+/Ki-67+ cells observed in the controls suggest that immature-type neurons are not recently generated neurons, and that the level of hippocampal neuronal production in adult humans is low. These results also suggest that PSA-NCAM is a useful marker for analyzing the pathology of epilepsy, but different interpretations of the immunohistochemical results between humans and rodents are required.

Non-radial tortuous migration with cell polarity alterations of newly generated granule neurons in the neonatal rat dentate gyrus

To establish functional neuronal circuits, newborn neurons generally migrate from the ventricular germinal zones to their final positions during embryonic periods. However, most excitatory neurons of the hippocampal dentate gyrus are born postnatally in the hilus, far from the lateral ventricle. Newly generated granule neurons must then migrate to the surrounding granule cell layer (GCL), which suggests that newborn granule cells may migrate by unique cellular mechanisms. In the present study, we describe the migratory behaviors of postnatally generated granule neurons using combined retroviral labeling and time-lapse imaging analysis. Our results show that whereas half of the newly generated neurons undergo radial migration, the remainder engages in more complex migratory patterns with veering and turning movements accompanied by process formation and cell polarity alterations. These data reveal a previously unappreciated diversity of mechanisms by which granule neurons distribute throughout the GCL to contribute to hippocampal circuitry.

Threat Memory Reminder Under Matrix Metalloproteinase 9 Inhibitor Doxycycline Globally Reduces Subsequent Memory Plasticity

Associative memory can be rendered malleable by a reminder. Blocking the ensuing reconsolidation process is suggested as a therapeutic target for unwanted aversive memories. Matrix metalloproteinase-9 (MMP-9) is required for structural synapse remodeling involved in memory consolidation. Inhibiting MMP-9 with doxycycline is suggested to attenuate human threat conditioning. Here, we investigated whether MMP-9 inhibition also interferes with threat memory reconsolidation. Male and female human participants (N = 78) learned the association between two visual conditioned stimuli (CS⁺) and a 50% chance of an unconditioned nociceptive stimulus (US), and between CS^- and the absence of US. On day 7, one CS⁺ was reminded without reinforcement 3.5 h after ingesting either 200 mg of doxycycline or placebo. On day 14, retention of CS memory was assessed under extinction by fear-potentiated startle. Contrary to our expectations, we observed a greater CS⁺/CS^- difference in participants who were reminded under doxycycline compared with placebo. Participants who were reminded under placebo showed extinction learning during the retention test, which was not observed in the doxycycline group. There was no difference between the reminded and the nonreminded CS⁺ in either group. In contrast, during relearning after the retention test, the CS⁺/CS^- difference was more pronounced in the placebo group than in the doxycycline group. To summarize, a single dose of doxycycline before threat memory reminder appeared to have no specific impact on reconsolidation, but to globally impair extinction learning, and threat relearning, beyond drug clearance.SIGNIFICANCE STATEMENT Matrix metalloproteinase-9 inhibition appears to attenuate memory consolidation. It could also be a target for blocking reconsolidation. Here, we test this hypothesis in human threat conditioning. We find that doxycycline has no specific impact on a reminded cue, but confers a global reduction in extinction learning and threat learning beyond the clearance of the drug. This may point toward a more long-lasting impact of doxycycline treatment on memory plasticity.

Interplay between a Mental Disorder Risk Gene and Developmental Polarity Switch of GABA Action Leads to Excitation-Inhibition Imbalance

Excitation-inhibition (E-I) imbalance is considered a hallmark of various neurodevelopmental disorders, including schizophrenia and autism. How genetic risk factors disrupt coordinated glutamatergic and GABAergic synapse formation to cause an E-I imbalance is not well understood. Here, we show that knockdown of Disrupted-in-schizophrenia 1 (DISC1), a risk gene for major mental disorders, leads to E-I imbalance in mature dentate granule neurons. We found that excessive GABAergic inputs from parvalbumin-, but not somatostatin-, expressing interneurons enhance the formation of both glutamatergic and GABAergic synapses in immature mutant neurons. Following the switch in GABAergic signaling polarity from depolarizing to hyperpolarizing during neuronal maturation, heightened inhibition from excessive parvalbumin+ GABAergic inputs causes loss of excitatory glutamatergic synapses in mature mutant neurons, resulting in an E-I imbalance. Our findings provide insights into the developmental role of depolarizing GABA in establishing E-I balance and how it can be influenced by genetic risk factors for mental disorders.

Histone Deacetylase 4 Downregulation Elicits Post-Traumatic Psychiatric Disorders through Impairment of Neurogenesis

An enduring deficit in neurogenesis largely contributes to the development of severe post-traumatic psychiatric disorders such as anxiety, depression, and memory impairment following traumatic brain injury (TBI); however, the mechanism remains obscure. Here we have shown that an imbalance in the generation of γ-aminobutyric acid (GABA)ergic and glutamatergic neurons due to aberrant induction of vesicular glutamate transporter 1 (vGlut1)-positive glutamatergic cells is responsible for impaired neuronal differentiation in the hippocampus following TBI. To elucidate the molecular mechanism, we found that TBI activates a transcription factor, Pax3, by increasing its acetylation status, and subsequently induces Ngn2 transcription. This event, in turn, augments the vGlut1-expressing glutamatergic neurons and accumulation of excess glutamate in the hippocampus that can affect neuronal differentiation. In our study the acetylation of Pax3 was increased due to loss of its interaction with a deacetylase, histone deacetylase 4 (HDAC4), which was downregulated after TBI. TBI-induced activation of GSK3β was responsible for the degradation of HDAC4. We also showed that overexpression of HDAC4 before TBI reduces Pax3 acetylation by restoring an interaction between HDAC4 and Pax3 in the hippocampus. This event prevents the aberrant induction of vGlut1-positive glutamatergic neurons by decreasing the Ngn2 level and subsequently reinforces the balance between GABAergic and glutamatergic neurons following TBI. Further, we found that overexpression of HDAC4 in the hippocampus improves anxiety, depressive-like behavior, and memory functions following TBI.

Knockdown of the CXCL12/CXCR7 chemokine pathway results in learning deficits and neural progenitor maturation impairment in mice

In adult brain, the chemokine CXCL12 and its receptors CXCR4 and CXCR7 are expressed in neural progenitor and glial cells. Conditional Cxcl12 or Cxcr4 gene knockout in mice leads to severe alterations in neural progenitor proliferation, migration and differentiation. As adult hippocampal neurogenesis is involved in learning and memory processes, we investigated the long-term effects of reduced expression of CXCL12 or CXCR7 in heterozygous Cxcl12^+/- and Cxcr7^+/- animals (KD mice) on hippocampal neurogenesis, neuronal differentiation and memory processing. In Cxcl12 KD mice, Cxcr4 mRNA expression was reduced, whereas Cxcr7 was slightly increased. Conversely, in Cxcr7 KD mice, both Cxcr4 and Cxcl12 mRNA levels were decreased. Moreover, Cxcl12 KD animals showed marked behavioral and learning deficits that were associated with impaired neurogenesis in the hippocampus. Conversely, Cxcr7 KD animals showed mild learning deficits with normal neurogenesis, but reduced cell differentiation, measured with doublecortin immunolabeling. These findings suggested that a single Cxcl12 or Cxcr7 allele might not be sufficient to maintain the hippocampal niche functionality throughout life, and that heterozygosity might represent a susceptibility factor for memory dysfunction progression.

In utero infection of Zika virus leads to abnormal central nervous system development in mice

The World Health Organization has declared ZIKA virus (ZIKV) a global public health emergency, prompted by the association of ZIKV infections with severe brain abnormalities in the human fetus. ZIKV preferentially targets human neuronal precursor cells (NPCs) in both monolayer and cortical brain organoid culture systems and stunts their growth. Although ZIKV is well recognized to cause microcephaly, there is no systematic analysis to demonstrate the effect of ZIKV on central nervous system (CNS) development, including brain malformations and spinal cord dysfunction. Here, we conducted a longitudinal analysis to show that a novel mouse model (infected in utero and monitored after birth until adulthood) recapitulates the effects of ZIKV infection affecting neural stem cells fate and leads to a thinner cortex and a smaller brain. Furthermore, we demonstrate the effect of ZIKV on spinal cord function. Specifically, we found significant reductions in neuron numbers in the anterior horn of grey matter of the spinal cord and muscle dystrophy with a significant decrease in forepaw grip strength in the ZIKV group. Thus, the established mouse model of ZIKV infection leading to abnormal CNS development will help to further advance our understanding of the disease pathogenesis.

The ontogeny of memory persistence and specificity

Interest in the ontogeny of memory blossomed in the twentieth century following the initial observations that memories from infancy and early childhood are rapidly forgotten. The intense exploration of infantile amnesia in subsequent years has led to a thorough characterization of its psychological determinants, although the neurobiology of memory persistence has long remained elusive. By contrast, other phenomena in the ontogeny of memory like infantile generalization have received relatively less attention. Despite strong evidence for reduced memory specificity during ontogeny, infantile generalization is poorly understood from psychological and neurobiological perspectives. In this review, we examine the ontogeny of memory persistence and specificity in humans and nonhuman animals at the levels of behavior and the brain. To this end, we first describe the behavioral phenotypes associated with each phenomenon. Looking into the brain, we then discuss neurobiological mechanisms in the hippocampus that contribute to the ontogeny of memory. Hippocampal neurogenesis and critical period mechanisms have recently been discovered to underlie amnesia during early development, and at the same time, we speculate that similar processes may contribute to the early bias towards memory generalization.

Increased Anxiety-Related Behavior, Impaired Cognitive Function and Cellular Alterations in the Brain of Cend1-deficient Mice

Cend1 is a neuronal-lineage specific modulator involved in coordination of cell cycle exit and differentiation of neuronal precursors. We have previously shown that Cend1^-/- mice show altered cerebellar layering caused by increased proliferation of granule cell precursors, delayed radial granule cell migration and compromised Purkinje cell differentiation, leading to ataxic gait and deficits in motor coordination. To further characterize the effects of Cend1 genetic ablation we determined herein a range of behaviors, including anxiety and exploratory behavior in the elevated plus maze (EPM), associative learning in fear conditioning, and spatial learning and memory in the Morris water maze (MWM). We observed significant deficits in all tests, suggesting structural and/or functional alterations in brain regions such as the cortex, amygdala and the hippocampus. In agreement with these findings, immunohistochemistry revealed reduced numbers of γ amino butyric acid (GABA) GABAergic interneurons, but not of glutamatergic projection neurons, in the adult cerebral cortex. Reduced GABAergic interneurons were also observed in the amygdala, most notably in the basolateral nucleus. The paucity in GABAergic interneurons in adult Cend1^-/- mice correlated with increased proliferation and apoptosis as well as reduced migration of neuronal progenitors from the embryonic medial ganglionic eminence (MGE), the origin of these cells. Further we noted reduced GABAergic neurons and aberrant neurogenesis in the adult dentate gyrus (DG) of the hippocampus, which has been previously shown to confer spatial learning and memory deficits. Our data highlight the necessity of Cend1 expression in the formation of a structurally and functionally normal brain phenotype.