Adult neurogenesis and functional plasticity in neuronal circuits

Astrocytes in functional recovery following central nervous system injuries

Astrocytes are increasingly recognised as partaking in complex homeostatic mechanisms critical for regulating neuronal plasticity following central nervous system (CNS) insults. Ischaemic stroke and traumatic brain injury are associated with high rates of disability and mortality. Depending on the context and type of injury, reactive astrocytes respond with diverse morphological, proliferative and functional changes collectively known as astrogliosis, which results in both pathogenic and protective effects. There is a large body of research on the negative consequences of astrogliosis following brain injuries. There is also growing interest in how astrogliosis might in some contexts be protective and help to limit the spread of the injury. However, little is known about how astrocytes contribute to the chronic functional recovery phase following traumatic and ischaemic brain insults. In this review, we explore the protective functions of astrocytes in various aspects of secondary brain injury such as oedema, inflammation and blood-brain barrier dysfunction. We also discuss the current knowledge on astrocyte contribution to tissue regeneration, including angiogenesis, neurogenesis, synaptogenesis, dendrogenesis and axogenesis. Finally, we discuss diverse astrocyte-related factors that, if selectively targeted, could form the basis of astrocyte-targeted therapeutic strategies to better address currently untreatable CNS disorders.

When Maturation is Not Linear: Brain Oscillatory Activity in the Process of Aging as Measured by Electrophysiology

Changes in brain oscillatory activity are commonly used as biomarkers both in cognitive neuroscience and in neuropsychiatric conditions. However, little is known about how its profile changes across maturation. Here we use regression models to characterize magnetoencephalography power changes within classical frequency bands in a sample of 792 healthy participants, covering the range 13 to 80 years old. Our findings unveil complex, non-linear power trajectories that defy the traditional linear paradigm, with notable cortical region variations. Interestingly, slow wave activity increases correlate with improved cognitive performance throughout life and larger gray matter volume in the elderly. Conversely, fast wave activity diminishes in adulthood. Elevated low-frequency activity during aging, traditionally seen as compensatory, may also signify neural deterioration. This dual interpretation, highlighted by our study, reveals the intricate dynamics between brain oscillations, cognitive performance, and aging. It advances our understanding of neurodevelopment and aging by emphasizing the regional specificity and complexity of brain rhythm changes, with implications for cognitive and structural integrity.

Epigenetic Regulation of Neural Stem Cells in Developmental and Adult Stages

The development of the nervous system is regulated by numerous intracellular molecules and cellular signals that interact temporally and spatially with the extracellular microenvironment. The three major cell types in the brain, i.e., neurons and two types of glial cells (astrocytes and oligodendrocytes), are generated from common multipotent neural stem cells (NSCs) throughout life. However, NSCs do not have this multipotentiality from the beginning. During cortical development, NSCs sequentially obtain abilities to differentiate into neurons and glial cells in response to combinations of spatiotemporally modulated cell-intrinsic epigenetic alterations and extrinsic factors. After the completion of brain development, a limited population of NSCs remains in the adult brain and continues to produce neurons (adult neurogenesis), thus contributing to learning and memory. Many biological aspects of brain development and adult neurogenesis are regulated by epigenetic changes via behavioral control of NSCs. Epigenetic dysregulation has also been implicated in the pathogenesis of various brain diseases. Here, we present recent advances in the epigenetic regulation of NSC behavior and its dysregulation in brain disorders.

Emerging Pro-neurogenic Therapeutic Strategies for Neurodegenerative Diseases: A Review of Pre-clinical and Clinical Research

The neuroscience community has largely accepted the notion that functional neurons can be generated from neural stem cells in the adult brain, especially in two brain regions: the subventricular zone of the lateral ventricles and the subgranular zone in the dentate gyrus of the hippocampus. However, impaired neurogenesis has been observed in some neurodegenerative diseases, particularly in Alzheimer's, Parkinson's, and Huntington's diseases, and also in Lewy Body dementia. Therefore, restoration of neurogenic function in neurodegenerative diseases emerges as a potential therapeutic strategy to counteract, or at least delay, disease progression. Considering this, the present study summarizes the different neuronal niches, provides a collection of the therapeutic potential of different pro-neurogenic strategies in pre-clinical and clinical research, providing details about their possible modes of action, to guide future research and clinical practice.

Manganese and Vanadium Co-Exposure Induces Severe Neurotoxicity in the Olfactory System: Relevance to Metal-Induced Parkinsonism

Chronic environmental exposure to toxic heavy metals, which often occurs as a mixture through occupational and industrial sources, has been implicated in various neurological disorders, including Parkinsonism. Vanadium pentoxide (V2O5) typically presents along with manganese (Mn), especially in welding rods and high-capacity batteries, including electric vehicle batteries; however, the neurotoxic effects of vanadium (V) and Mn co-exposure are largely unknown. In this study, we investigated the neurotoxic impact of MnCl2, V2O5, and MnCl2-V2O5 co-exposure in an animal model. C57BL/6 mice were intranasally administered either de-ionized water (vehicle), MnCl2 (252 µg) alone, V2O5 (182 µg) alone, or a mixture of MnCl2 (252 µg) and V2O5 (182 µg) three times a week for up to one month. Following exposure, we performed behavioral, neurochemical, and histological studies. Our results revealed dramatic decreases in olfactory bulb (OB) weight and levels of tyrosine hydroxylase, dopamine, and 3,4-dihydroxyphenylacetic acid in the treatment groups compared to the control group, with the Mn/V co-treatment group producing the most significant changes. Interestingly, increased levels of α-synuclein expression were observed in the substantia nigra (SN) of treated animals. Additionally, treatment groups exhibited locomotor deficits and olfactory dysfunction, with the co-treatment group producing the most severe deficits. The treatment groups exhibited increased levels of the oxidative stress marker 4-hydroxynonenal in the striatum and SN, as well as the upregulation of the pro-apoptotic protein PKCδ and accumulation of glomerular astroglia in the OB. The co-exposure of animals to Mn/V resulted in higher levels of these metals compared to other treatment groups. Taken together, our results suggest that co-exposure to Mn/V can adversely affect the olfactory and nigral systems. These results highlight the possible role of environmental metal mixtures in the etiology of Parkinsonism.

Adult Neurogenesis Reconciles Flexibility and Stability of Olfactory Perceptual Memory

In brain regions featuring ongoing plasticity, the task of quickly encoding new information without overwriting old memories presents a significant challenge. In the rodent olfactory bulb, which is renowned for substantial structural plasticity driven by adult neurogenesis and persistent turnover of dendritic spines, we show that such plasticity is vital to overcoming this flexibility-stability dilemma. To do so, we develop a computational model for structural plasticity in the olfactory bulb and show that the maturation of adult-born neurons facilitates the abilities to learn quickly and forget slowly. Particularly important to achieve this goal are the transient enhancement of the plasticity, excitability, and susceptibility to apoptosis that characterizes young neurons. The model captures many experimental observations and makes a number of testable predictions. Overall, it identifies memory consolidation as an important role of adult neurogenesis in olfaction and exemplifies how the brain can maintain stable memories despite ongoing extensive plasticity.

Distinct forms of structural plasticity of adult-born interneuron spines in the mouse olfactory bulb induced by different odor learning paradigms

The morpho-functional properties of neural networks constantly adapt in response to environmental stimuli. The olfactory bulb is particularly prone to constant reshaping of neural networks because of ongoing neurogenesis. It remains unclear whether the complexity of distinct odor-induced learning paradigms and sensory stimulation induces different forms of structural plasticity. In the present study, we automatically reconstructed spines in 3D from confocal images and performed unsupervised clustering based on morphometric features. We show that while sensory deprivation decreased the spine density of adult-born neurons without affecting the morphometric properties of these spines, simple and complex odor learning paradigms triggered distinct forms of structural plasticity. A simple odor learning task affected the morphometric properties of the spines, whereas a complex odor learning task induced changes in spine density. Our work reveals distinct forms of structural plasticity in the olfactory bulb tailored to the complexity of odor-learning paradigms and sensory inputs.

Restoration of neuronal progenitors by partial reprogramming in the aged neurogenic niche

Partial reprogramming (pulsed expression of reprogramming transcription factors) improves the function of several tissues in old mice. However, it remains largely unknown how partial reprogramming impacts the old brain. Here we use single-cell transcriptomics to systematically examine how partial reprogramming influences the subventricular zone neurogenic niche in aged mouse brains. Whole-body partial reprogramming mainly improves neuroblasts (cells committed to give rise to new neurons) in the old neurogenic niche, restoring neuroblast proportion to more youthful levels. Interestingly, targeting partial reprogramming specifically to the neurogenic niche also boosts the proportion of neuroblasts and their precursors (neural stem cells) in old mice and improves several molecular signatures of aging, suggesting that the beneficial effects of reprogramming are niche intrinsic. In old neural stem cell cultures, partial reprogramming cell autonomously restores the proportion of neuroblasts during differentiation and blunts some age-related transcriptomic changes. Importantly, partial reprogramming improves the production of new neurons in vitro and in old brains. Our work suggests that partial reprogramming could be used to rejuvenate the neurogenic niche and counter brain decline in old individuals.

The neurobiological effects of senescence on dopaminergic system: A comprehensive review

Over time, the body undergoes a natural, multifactorial, and ongoing process named senescence, which induces changes at the molecular, cellular, and micro-anatomical levels in many body systems. The brain, being a highly complex organ, is particularly affected by this process, potentially impairing its numerous functions. The brain relies on chemical messengers known as neurotransmitters to function properly, with dopamine being one of the most crucial. This catecholamine is responsible for a broad range of critical roles in the central nervous system, including movement, learning, cognition, motivation, emotion, reward, hormonal release, memory consolidation, visual performance, sexual drive, modulation of circadian rhythms, and brain development. In the present review, we thoroughly examine the impact of senescence on the dopaminergic system, with a primary focus on the classic delimitations of the dopaminergic nuclei from A8 to A17. We provide in-depth information about their anatomy and function, particularly addressing how senescence affects each of these nuclei.

Signaling mechanisms underlying activity-dependent integration of adult-born neurons in the mouse olfactory bulb

Adult neurogenesis has fascinated the field of neuroscience for decades given the prospects of harnessing mechanisms that facilitate the rewiring and/or replacement of adult brain tissue. The subgranular zone of the hippocampus and the subventricular zone of the lateral ventricle are the two main areas in the brain that exhibit ongoing neurogenesis. Of these, adult-born neurons within the olfactory bulb have proven to be a powerful model for studying circuit plasticity, providing a broad and accessible avenue into neuron development, migration, and continued circuit integration within adult brain tissue. This review focuses on some of the recognized molecular and signaling mechanisms underlying activity-dependent adult-born neuron development. Notably, olfactory activity and behavioral states contribute to adult-born neuron plasticity through sensory and centrifugal inputs, in which calcium-dependent transcriptional programs, local translation, and neuropeptide signaling play important roles. This review also highlights areas of needed continued investigation to better understand the remarkable phenomenon of adult-born neuron integration.

The impact of adult neurogenesis on affective functions: of mice and men

In most mammals, new neurons are not only produced during embryogenesis but also after birth. Soon after adult neurogenesis was discovered, the influence of recruiting new neurons on cognitive functions, especially on memory, was documented. Likewise, the late process of neuronal production also contributes to affective functions, but this outcome was recognized with more difficulty. This review covers hypes and hopes of discovering the influence of newly-generated neurons on brain circuits devoted to affective functions. If the possibility of integrating new neurons into the adult brain is a commonly accepted faculty in the realm of mammals, the reluctance is strong when it comes to translating this concept to humans. Compiling data suggest now that new neurons are derived not only from stem cells, but also from a population of neuroblasts displaying a protracted maturation and ready to be engaged in adult brain circuits, under specific signals. Here, we discuss the significance of recruiting new neurons in the adult brain circuits, specifically in the context of affective outcomes. We also discuss the fact that adult neurogenesis could be the ultimate cellular process that integrates elements from both the internal and external environment to adjust brain functions. While we must be critical and beware of the unreal promises that Science could generate sometimes, it is important to continue exploring the potential of neural recruitment in adult primates. Reporting adult neurogenesis in humankind contributes to a new vision of humans as mammals whose brain continues to develop throughout life. This peculiar faculty could one day become the target of treatment for mental health, cognitive disorders, and elderly-associated diseases. The vision of an adult brain which never stops integrating new neurons is a real game changer for designing new therapeutic interventions to treat mental disorders associated with substantial morbidity, mortality, and social costs.

Synergies in stem cell research: Integrating technologies, strategies, and bionanomaterial innovations

Since the 1960 s, there has been a substantial amount of research directed towards investigating the biology of several types of stem cells, including embryonic stem cells, brain cells, and mesenchymal stem cells. In contemporary times, a wide array of stem cells has been utilized to treat several disorders, including bone marrow transplantation. In recent years, stem cell treatment has developed as a very promising and advanced field of scientific research. The progress of therapeutic methodologies has resulted in significant amounts of anticipation and expectation. Recently, there has been a notable proliferation of experimental methodologies aimed at isolating and developing stem cells, which have emerged concurrently. Stem cells possess significant vitality and exhibit vigorous proliferation, making them suitable candidates for in vitro modification. This article examines the progress made in stem cell isolation and explores several methodologies employed to promote the differentiation of stem cells. This study also explores the method of isolating bio-nanomaterials and discusses their viewpoint in the context of stem cell research. It also covers the potential for investigating stem cell applications in bioprinting and the usage of bionanomaterial in stem cell-related technologies and research. In conclusion, the review article concludes by highlighting the importance of incorporating state-of-the-art methods and technological breakthroughs into the future of stem cell research. Putting such an emphasis on constant innovation highlights the ever-changing character of science and the never-ending drive toward unlocking the maximum therapeutic potential of stem cells. This review would be a useful resource for researchers, clinicians, and policymakers in the stem cell research area, guiding the next steps in this fast-developing scientific concern.

Perinatal ethanol exposure affects cell populations in adult dorsal hippocampal neurogenic niche

Neurodevelopment is highly affected by perinatal ethanol exposure (PEE). In the adult brain, neurogenesis takes place in the dentate gyrus (DG) of the hippocampus and in the subventricular zone. This work aimed to analyze the effect of PEE on the cellular types involved in adult dorsal hippocampal neurogenesis phases using a murine model. For this purpose, primiparous female CD1 mice consumed only ethanol 6% v/v from 20 days prior to mating and along pregnancy and lactation to ensure that the pups were exposed to ethanol throughout pre- and early postnatal development. After weaning, pups had no further contact with ethanol. Cell types of the adult male dorsal DG were studied by immunofluorescence. A lower percentage of type 1 cells and immature neurons and a higher percentage of type 2 cells were observed in PEE animals. This decrease in type 1 cells suggests that PEE reduces the population of remnant progenitors of the dorsal DG present in adulthood. The increase in type 2 cells and the decrease in immature neurons indicate that, during neurodevelopment, ethanol alters the capacity of neuroblasts to become neurons in the adult neurogenic niche. These results suggest that pathways implicated in cell determination are affected by PEE and remain affected in adulthood.

Multiparametric MR Evaluation of the Photoperiodic Regulation of Hypothalamic Structures in Sheep

Most organisms on earth, humans included, have developed strategies to cope with environmental day-night and seasonal cycles to survive. For most of them, their physiological and behavioral functions, including the reproductive function, are synchronized with the annual changes of day length, to ensure winter survival and subsequent reproductive success in the following spring. Sheep are sensitive to photoperiod, which also regulates natural adult neurogenesis in their hypothalamus. We postulate that the ovine model represents a good alternative to study the functional and metabolic changes occurring in response to photoperiodic changes in hypothalamic structures of the brain. Here, the impact of the photoperiod on the neurovascular coupling and the metabolism of the hypothalamic structures was investigated at 3T using BOLD fMRI, perfusion-MRI and proton magnetic resonance spectroscopy (1H-MRS). A longitudinal study involving 8 ewes was conducted during long days (LD) and short days (SD) revealing significant BOLD, rCBV and metabolic changes in hypothalamic structures of the ewe brain between LD and SD. More specifically, the transition between LD and SD revealed negative BOLD responses to hypercapnia at the beginning of SD period followed by significant increases in BOLD, rCBV, Glx and tNAA concentrations towards the end of the SD period. These observations suggest longitudinal mechanisms promoting the proliferation and differentiation of neural stem cells within the hypothalamic niche of breeding ewes. We conclude that multiparametric MRI studies including 1H-MRS could be promising non-invasive translational techniques to investigate the existence of natural adult neurogenesis in-vivo in gyrencephalic brains.

Bioactive Compounds and Their Influence on Postnatal Neurogenesis

Since postnatal neurogenesis was revealed to have significant implications for cognition and neurological health, researchers have been increasingly exploring the impact of natural compounds on this process, aiming to uncover strategies for enhancing brain plasticity. This review provides an overview of postnatal neurogenesis, neurogenic zones, and disorders characterized by suppressed neurogenesis and neurogenesis-stimulating bioactive compounds. Examining recent studies, this review underscores the multifaceted effects of natural compounds on postnatal neurogenesis. In essence, understanding the interplay between postnatal neurogenesis and natural compounds could bring novel insights into brain health interventions. Exploiting the therapeutic abilities of these compounds may unlock innovative approaches to enhance cognitive function, mitigate neurodegenerative diseases, and promote overall brain well-being.

The Effects of Stress on Hippocampal Neurogenesis and Behavior in the Absence of Lipocalin-2

Lipocalin-2 (LCN2) is an acute phase protein able to bind iron when complexed with bacterial siderophores. The recent identification of a mammalian siderophore also suggested a physiological role for LCN2 in the regulation of iron levels and redox state. In the central nervous system, the deletion of LCN2 induces deficits in neural stem cells proliferation and commitment, with an impact on the hippocampal-dependent contextual fear discriminative task. Additionally, stress is a well-known regulator of cell genesis and is known to decrease adult hippocampal cell proliferation and neurogenesis. Although voluntary running, another well-known regulator of neurogenesis, is sufficient to rescue the defective hippocampal neurogenesis and behavior in LCN2-null mice by promoting stem cells' cell cycle progression and maturation, the relevance of LCN2-regulated hippocampal neurogenesis in response to stress has never been explored. Here, we show a lack of response by LCN2-null mice to the effects of chronic stress exposure at the cellular and behavioral levels. Together, these findings implicate LCN2 as a relevant mediator of neuronal plasticity and brain function in the adult mammalian brain.

Laminin-coated electronic scaffolds with vascular topography for tracking and promoting the migration of brain cells after injury

In the adult brain, neural stem cells are largely restricted into spatially discrete neurogenic niches, and hence areas of neuron loss during neurodegenerative disease or following a stroke or traumatic brain injury do not typically repopulate spontaneously. Moreover, understanding neural activity accompanying the neural repair process is hindered by a lack of minimally invasive devices for the chronic measurement of the electrophysiological dynamics in damaged brain tissue. Here we show that 32 individually addressable platinum microelectrodes integrated into laminin-coated branched polymer scaffolds stereotaxically injected to span a hydrogel-filled cortical lesion and deeper regions in the brains of mice promote neural regeneration while allowing for the tracking of migrating host brain cells into the lesion. Chronic measurements of single-unit activity and neural-circuit analyses revealed the establishment of spiking activity in new neurons in the lesion and their functional connections with neurons deeper in the brain. Electronic implants mimicking the topographical and surface properties of brain vasculature may aid the stimulation and tracking of neural-circuit restoration following injury.

Olfactory neurogenesis and its role in fear memory modulation

Olfaction is a critical sense that allows animals to navigate and understand their environment. In mammals, the critical brain structure to receive and process olfactory information is the olfactory bulb, a structure characterized by a laminated pattern with different types of neurons, some of which project to distant telencephalic structures, like the piriform cortex, the amygdala, and the hippocampal formation. Therefore, the olfactory bulb is the first structure of a complex cognitive network that relates olfaction to different types of memory, including episodic memories. The olfactory bulb continuously adds inhibitory newborn neurons throughout life; these cells locate both in the granule and glomerular layers and integrate into the olfactory circuits, inhibiting projection neurons. However, the roles of these cells modulating olfactory memories are unclear, particularly their role in fear memories. We consider that olfactory neurogenesis might modulate olfactory fear memories by a plastic process occurring in the olfactory bulb.

Age-related pathophysiological alterations in molecular stress markers and key modulators of hypoxia

Alzheimer's disease (AD) is characterized by an adverse cellular environment and pathological alterations in distinct brain regions. The development is triggered or facilitated by a condition such as hypoxia or ischemia, or inflammation and is associated with disruptions of fundamental cellular functions, including metabolic and ion homeostasis. Increasing evidence suggests that hypoxia may affect many pathological aspects of AD, including oxidative stress, mitochondrial dysfunction, ER stress, amyloidogenic processing of APP, and Aβ accumulation, which may collectively result in neurodegeneration. Further investigation into the relationship between hypoxia and AD may provide an avenue for the effective preservation and pharmacological treatment of this neurodegenerative disease. This review summarizes the effects of normoxia and hypoxia on AD pathogenesis and discusses the underlying mechanisms. Regulation of HIF-1α and the role of its key players, including P53, VEGF, and GLUT1, are also discussed.

Conjugation of bone grafts with NO-delivery dinitrosyl iron complexes promotes synergistic osteogenesis and angiogenesis in rat calvaria bone defects

Craniofacial/jawbone deformities remain a significant clinical challenge in restoring facial/dental functions and esthetics. Despite the reported therapeutics for clinical bone tissue regeneration, the bioavailability issue of autografts and limited regeneration efficacy of xenografts/synthetic bone substitutes, however, inspire continued efforts towards functional conjugation and improvement of bioactive bone graft materials. Regarding the potential of nitric oxide (NO) in tissue engineering, herein, functional conjugation of NO-delivery dinitrosyl iron complex (DNIC) and osteoconductive bone graft materials was performed to optimize the spatiotemporal control over the delivery of NO and to activate synergistic osteogenesis and angiogenesis in rat calvaria bone defects. Among three types of biomimetic DNICs, [Fe2(μ-SCH2CH2COOH)2(NO)4] (DNIC-COOH) features a steady kinetics for cellular uptake by MC3T3-E1 osteoblast cells followed by intracellular assembly of protein-bound DNICs and release of NO. This steady kinetics for intracellular delivery of NO by DNIC-COOH rationalizes its biocompatibility and wide-spectrum cell proliferation effects on MC3T3-E1 osteoblast cells and human umbilical vein endothelial cells (HUVECs). Moreover, the bridging [SCH2CH2COOH]- thiolate ligands in DNIC-COOH facilitate its chemisorption to deproteinized bovine bone mineral (DBBM) and physisorption onto TCP (β-tricalcium phosphate), respectively, which provides a mechanism to control the kinetics for the local release of loaded DNIC-COOH. Using rats with calvaria bone defects as an in vivo model, DNIC-DBBM/DNIC-TCP promotes the osteogenic and angiogenic activity ascribed to functional conjugation of osteoconductive bone graft materials and NO-delivery DNIC-COOH. Of importance, the therapeutic efficacy of DNIC-DBBM/DNIC-TCP on enhanced compact bone formation after treatment for 4 and 12 weeks supports the potential for clinical application to regenerative medicine.

Chronic unpredictable mild stress alters odor hedonics and adult olfactory neurogenesis in mice

Experiencing chronic stress significantly increases the risk for depression. Depression is a complex disorder with varied symptoms across patients. However, feeling of sadness and decreased motivation, and diminished feeling of pleasure (anhedonia) appear to be core to most depressive pathology. Odorants are potent signals that serve a critical role in social interactions, avoiding danger, and consummatory behaviors. Diminished quality of olfactory function is associated with negative effects on quality of life leading to and aggravating the symptoms of depression. Odor hedonic value (I like or I dislike this smell) is a dominant feature of olfaction and guides approach or avoidance behavior of the odor source. The neural representation of the hedonic value of odorants is carried by the granule cells in the olfactory bulb, which functions to modulate the cortical relay of olfactory information. The granule cells of the olfactory bulb and those of the dentate gyrus are the two major populations of cells in the adult brain with continued neurogenesis into adulthood. In hippocampus, decreased neurogenesis has been linked to development or maintenance of depression symptoms. Here, we hypothesize that chronic mild stress can alter olfactory hedonics through effects on the olfactory bulb neurogenesis, contributing to the broader anhedonia phenotype in stress-associated depression. To test this, mice were subjected to chronic unpredictable mild stress and then tested on measures of depressive-like behaviors, odor hedonics, and measures of olfactory neurogenesis. Chronic unpredictable mild stress led to a selective effect on odor hedonics, diminishing attraction to pleasant but not unpleasant odorants, an effect that was accompanied by a specific decrease in adult neurogenesis and of the percentage of adult-born cells responding to pleasant odorants in the olfactory bulb.

Associations between social health factors, cognitive activity and neurostructural markers for brain health - A systematic literature review and meta-analysis

Social health factors (e.g., social activities or social support) and cognitive activity engagement have been associated with dementia risk, but their neural substrates have not been well established. This systematic review and meta-analysis summarizes the available evidence regarding the association between these factors and cerebral macro- and micro-structure. A comprehensive literature search was conducted in various databases, following predefined criteria. Heterogeneity, risk of publication bias and overall certainty of evidence were assessed using standardized scales and, whenever appropriate, random effects meta-analysis was conducted. Of 6715 identified articles, 43 were included. Overall, consistency of findings was low and methodological heterogeneity high for all outcomes. However, in some studies cognitive and social activities were positively associated with total brain, global and cortical grey matter and hippocampal volume as well as white matter microstructural integrity. Furthermore, structural social network characteristics (e.g., social network size) were associated with regional grey matter volumes, while functional social network characteristics (e.g., social support) were additionally associated with total brain volume. Meta-analyses revealed small but significant partial correlations between cognitive and social activities and hippocampal (three studies; n = 892; rz =0.07) and white matter hyperintensity volume (three studies; n = 2934; rz =-0.04). More prospective studies are needed to assess temporal associations.

Single-cell transcriptomic analysis reveals the adverse effects of cadmium on the trajectory of neuronal maturation

Cadmium (Cd) is an extensively existing environmental pollutant that has neurotoxic effects. However, the molecular mechanism of Cd on neuronal maturation is unveiled. Single-cell RNA sequencing (scRNA-seq) has been widely used to uncover cellular heterogeneity and is a powerful tool to reconstruct the developmental trajectory of neurons. In this study, neural stem cells (NSCs) from subventricular zone (SVZ) of newborn mice were treated with CdCl2 for 24 h and differentiated for 7 days to obtain neuronal lineage cells. Then scRNA-seq analysis identified five cell stages with different maturity in neuronal lineage cells. Our findings revealed that Cd altered the trajectory of maturation of neuronal lineage cells by decreasing the number of cells in different stages and hindering their maturation. Cd induced differential transcriptome expression in different cell subpopulations in a stage-specific manner. Specifically, Cd induced oxidative damage and changed the proportion of cell cycle phases in the early stage of neuronal development. Furthermore, the autocrine and paracrine signals of Wnt5a were downregulated in the low mature neurons in response to Cd. Importantly, activation of Wnt5a effectively rescued the number of neurons and promoted their maturation. Taken together, the findings of this study provide new and comprehensive insights into the adverse effect of Cd on neuronal maturation.

Scaling the Andean Shilajit: A Novel Neuroprotective Agent for Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder without a cure, despite the enormous number of investigations and therapeutic approaches. AD is a consequence of microglial responses to "damage signals", such as aggregated tau oligomers, which trigger a neuro-inflammatory reaction, promoting the misfolding of cytoskeleton structure. Since AD is the most prevalent cause of dementia in the elderly (>60 years old), new treatments are essential to improve the well-being of affected subjects. The pharmaceutical industry has not developed new drugs with efficacy for controlling AD. In this context, major attention has been given to nutraceuticals and novel bioactive compounds, such as molecules from the Andean Shilajit (AnSh), obtained from the Andes of Chile. Primary cultures of rat hippocampal neurons and mouse neuroblastoma cells were evaluated to examine the functional and neuroprotective role of different AnSh fractions. Our findings show that AnSh fractions increase the number and length of neuronal processes at a differential dose. All fractions were viable in neurons. The AnSh fractions inhibit tau self-aggregation after 10 days of treatment. Finally, we identified two candidate molecules in M3 fractions assayed by UPLC/MS. Our research points to a novel AnSh-derived fraction that is helpful in AD. Intensive work toward elucidation of the molecular mechanisms is being carried out. AnSh is an alternative for AD treatment or as a coadjuvant for an effective treatment.

Human Cytomegalovirus IE2 Disrupts Neural Progenitor Development and Induces Microcephaly in Transgenic Mouse

Human cytomegalovirus (HCMV) is a significant contributor to congenital birth defects. Limited by the lack of animal models, the pathogenesis of neurological damage in vivo caused by HCMV infection and the role of individual viral genes remain to be elucidated. Immediate early (IE2) protein may play a function in neurodevelopmental problems caused by HCMV infection. Here, this study intended to investigate IE2's long-term effects on development of the brain in IE2-expressing transgenic mice (Rosa26-LSL-IE2^+/-, Camk2α-Cre) aimed to observe the phenotype of postnatal mice. The expression of IE2 in transgenic mice was confirmed by PCR and Western blot technology. We collected mouse brain tissue at 2, 4, 6, 8, and 10 days postpartum to analyze the developmental process of neural stem cells by immunofluorescence. We discovered that transgenic mice (Rosa26-LSL-IE2^+/-, Camk2α-Cre) can reliably produce IE2 in the brain at various postpartum phases. Furthermore, we also observed the symptoms of microcephaly in postnatal transgenic mice, and IE2 can damage the amount of neural stem cells, prevent them from proliferating and differentiating, and activate microglia and astrocytes, creating an unbalanced environment in the brain's neurons. In conclusion, we demonstrate that long-term expression of HCMV-IE2 can cause microcephaly through molecular mechanisms affecting the differentiation and development of neural stem cells in vivo. This work establishes a theoretical and experimental foundation for elucidating the molecular mechanism of fetal microcephaly brought by HCMV infection in throughout the period of neural development of pregnancy.

Endogenous but not sensory-driven activity controls migration, morphogenesis and survival of adult-born juxtaglomerular neurons in the mouse olfactory bulb

The development and survival of adult-born neurons are believed to be driven by sensory signaling. Here, in vivo analyses of motility, morphology and Ca2+ signaling, as well as transcriptome analyses of adult-born juxtaglomerular cells with reduced endogenous excitability (via cell-specific overexpression of either Kv1.2 or Kir2.1 K+ channels), revealed a pronounced impairment of migration, morphogenesis, survival, and functional integration of these cells into the mouse olfactory bulb, accompanied by a reduction in cytosolic Ca2+ fluctuations, phosphorylation of CREB and pCREB-mediated gene expression. Moreover, K+ channel overexpression strongly downregulated genes involved in neuronal migration, differentiation, and morphogenesis and upregulated apoptosis-related genes, thus locking adult-born cells in an immature and vulnerable state. Surprisingly, cells deprived of sensory-driven activity developed normally. Together, the data reveal signaling pathways connecting the endogenous intermittent neuronal activity/Ca2+ fluctuations as well as enhanced Kv1.2/Kir2.1 K+ channel function to migration, maturation, and survival of adult-born neurons.

The gut-brain connection: Exploring the influence of the gut microbiota on neuroplasticity and neurodevelopmental disorders

Neuroplasticity refers to the ability of brain circuits to reorganize and change the properties of the network, resulting in alterations in brain function and behavior. It is traditionally believed that neuroplasticity is influenced by external stimuli, learning, and experience. Intriguingly, there is new evidence suggesting that endogenous signals from the body's periphery may play a role. The gut microbiota, a diverse community of microorganisms living in harmony with their host, may be able to influence plasticity through its modulation of the gut-brain axis. Interestingly, the maturation of the gut microbiota coincides with critical periods of neurodevelopment, during which neural circuits are highly plastic and potentially vulnerable. As such, dysbiosis (an imbalance in the gut microbiota composition) during early life may contribute to the disruption of normal developmental trajectories, leading to neurodevelopmental disorders. This review aims to examine the ways in which the gut microbiota can affect neuroplasticity. It will also discuss recent research linking gastrointestinal issues and bacterial dysbiosis to various neurodevelopmental disorders and their potential impact on neurological outcomes.

The Pathological Effects of Circulating Hydrophobic Bile Acids in Alzheimer's Disease

Recent clinical studies have revealed that the serum levels of toxic hydrophobic bile acids (deoxy cholic acid, lithocholic acid [LCA], and glycoursodeoxycholic acid) are significantly higher in patients with Alzheimer's disease (AD) and amnestic mild cognitive impairment (aMCI) when compared to control subjects. The elevated serum bile acids may be the result of hepatic peroxisomal dysfunction. Circulating hydrophobic bile acids are able to disrupt the blood-brain barrier and promote the formation of amyloid-β plaques through enhancing the oxidation of docosahexaenoic acid. Hydrophobic bile acid may find their ways into the neurons via the apical sodium-dependent bile acid transporter. It has been shown that hydrophobic bile acids impose their pathological effects by activating farnesoid X receptor and suppressing bile acid synthesis in the brain, blocking NMDA receptors, lowering brain oxysterol levels, and interfering with 17β-estradiol actions such as LCA by binding to E2 receptors (molecular modelling data exclusive to this paper). Hydrophobic bile acids may interfere with the sonic hedgehog signaling through alteration of cell membrane rafts and reducing brain 24(S)-hydroxycholesterol. This article will 1) analyze the pathological roles of circulating hydrophobic bile acids in the brain, 2) propose therapeutic approaches, and 3) conclude that consideration be given to reducing/monitoring toxic bile acid levels in patients with AD or aMCI, prior/in combination with other treatments.

Thymol in Trachyspermum ammi seed extract exhibits neuroprotection, learning, and memory enhancement in scopolamine-induced Alzheimer's disease mouse model

Several reports have stated the neuroprotective and learning/memory effects of Tachyspermum ammi seed extract (TASE) and its principal component thymol; however, little is known about its underlying molecular mechanisms and neurogenesis potential. This study aimed to provide insights into TASE and a thymol-mediated multifactorial therapeutic approach in a scopolamine-induced Alzheimer's disease (AD) mouse model. TASE and thymol supplementation significantly reduced oxidative stress markers such as brain glutathione, hydrogen peroxide, and malondialdehyde in mouse whole brain homogenates. Tumor necrosis factor-alpha was significantly downregulated, whereas the elevation of brain-derived neurotrophic factor and phospho-glycogen synthase kinase-3 beta (serine 9) enhanced learning and memory in the TASE- and thymol-treated groups. A significant reduction in the accumulation of Aβ 1-42 peptides was observed in the brains of TASE- and thymol-treated mice. Furthermore, TASE and thymol significantly promoted adult neurogenesis, with increased doublecortin positive neurons in the subgranular and polymorphic zones of the dentate gyrus in treated-mice. Collectively, TASE and thymol could  potentially act as natural therapeutic agents for the treatment of  neurodegenerative disorders, such as  AD.

The Dialogue Between Neuroinflammation and Adult Neurogenesis: Mechanisms Involved and Alterations in Neurological Diseases

Adult neurogenesis occurs mainly in the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricles. Evidence supports the critical role of adult neurogenesis in various conditions, including cognitive dysfunction, Alzheimer's disease (AD), and Parkinson's disease (PD). Several factors can alter adult neurogenesis, including genetic, epigenetic, age, physical activity, diet, sleep status, sex hormones, and central nervous system (CNS) disorders, exerting either pro-neurogenic or anti-neurogenic effects. Compelling evidence suggests that any insult or injury to the CNS, such as traumatic brain injury (TBI), infectious diseases, or neurodegenerative disorders, can provoke an inflammatory response in the CNS. This inflammation could either promote or inhibit neurogenesis, depending on various factors, such as chronicity and severity of the inflammation and underlying neurological disorders. Notably, neuroinflammation, driven by different immune components such as activated glia, cytokines, chemokines, and reactive oxygen species, can regulate every step of adult neurogenesis, including cell proliferation, differentiation, migration, survival of newborn neurons, maturation, synaptogenesis, and neuritogenesis. Therefore, this review aims to present recent findings regarding the effects of various components of the immune system on adult neurogenesis and to provide a better understanding of the role of neuroinflammation and neurogenesis in the context of neurological disorders, including AD, PD, ischemic stroke (IS), seizure/epilepsy, TBI, sleep deprivation, cognitive impairment, and anxiety- and depressive-like behaviors. For each disorder, some of the most recent therapeutic candidates, such as curcumin, ginseng, astragaloside, boswellic acids, andrographolide, caffeine, royal jelly, estrogen, metformin, and minocycline, have been discussed based on the available preclinical and clinical evidence.

α2-Adrenergic modulation of Ih in adult-born granule cells in the olfactory bulb

In the olfactory bulb (OB), a large population of axon-less inhibitory interneurons, the granule cells (GCs), coordinate network activity and tune the output of principal neurons, the mitral and tufted cells (MCs), through dendrodendritic interactions. Furthermore, GCs undergo neurogenesis throughout life, providing a source of plasticity to the neural network of the OB. The function and integration of GCs in the OB are regulated by several afferent neuromodulatory signals, including noradrenaline (NA), a state-dependent neuromodulator that plays a crucial role in the regulation of cortical function and task-specific decision processes. However, the mechanisms by which NA regulates GC function are not fully understood. Here, we show that NA modulates hyperpolarization-activated currents (I_h) via the activation of α₂-adrenergic receptors (ARs) in adult-born GCs (abGCs), thus directly acting on channels that play essential roles in regulating neuronal excitability and network oscillations in the brain. This modulation affects the dendrodendritic output of GCs leading to an enhancement of lateral inhibition onto the MCs. Furthermore, we show that NA modulates subthreshold resonance in GCs, which could affect the temporal integration of abGCs. Together, these results provide a novel mechanism by which a state-dependent neuromodulator acting on I_h can regulate GC function in the OB.

Compensatory cognition in neurological diseases and aging: A review of animal and human studies

Specialized individual circuits in the brain are recruited for specific functions. Interestingly, multiple neural circuitries continuously compete with each other to acquire the specialized function. However, the dominant among them compete and become the central neural network for that particular function. For example, the hippocampal principal neural circuitries are the dominant networks among many which are involved in learning processes. But, in the event of damage to the principal circuitry, many times, less dominant networks compensate for the primary network. This review highlights the psychopathologies of functional loss and the aspects of functional recuperation in the absence of the hippocampus.

Differential vulnerability of adult neurogenic niches to dosage of the neurodevelopmental-disorder linked gene Foxg1

The transcription factor FOXG1 serves pleiotropic functions in brain development ranging from the regulation of precursor proliferation to the control of cortical circuit formation. Loss-of-function mutations and duplications of FOXG1 are associated with neurodevelopmental disorders in humans illustrating the importance of FOXG1 dosage for brain development. Aberrant FOXG1 dosage has been found to disrupt the balanced activity of glutamatergic and GABAergic neurons, but the underlying mechanisms are not fully understood. We report that FOXG1 is expressed in the main adult neurogenic niches in mice, i.e. the hippocampal dentate gyrus and the subependymal zone/olfactory bulb system, where neurogenesis of glutamatergic and GABAergic neurons persists into adulthood. These niches displayed differential vulnerability to increased FOXG1 dosage: high FOXG1 levels severely compromised survival and glutamatergic dentate granule neuron fate acquisition in the hippocampal neurogenic niche, but left neurogenesis of GABAergic neurons in the subependymal zone/olfactory bulb system unaffected. Comparative transcriptomic analyses revealed a significantly higher expression of the apoptosis-linked nuclear receptor Nr4a1 in FOXG1-overexpressing hippocampal neural precursors. Strikingly, pharmacological interference with NR4A1 function rescued FOXG1-dependent death of hippocampal progenitors. Our results reveal differential vulnerability of neuronal subtypes to increased FOXG1 dosage and suggest that activity of a FOXG1/NR4A1 axis contributes to such subtype-specific response.

Olfaction in (Social) Context: The Role of Social Complexity in Trajectories of Older Adults' Olfactory Abilities

Objectives: Olfaction is an important correlate of later-life health, including cognition and mortality risk. Environmental enrichment protects against olfactory decline, yet little research considers the social context as a source of sensory enrichment or stimulation. This study examines how exposure to social complexity (i.e., diversity or novelty in social networks and activities) shapes later-life olfaction. Methods: Cross-sectional and longitudinal ordered logit models analyze data from 1,447 older adults interviewed at Rounds 1 and 2 of the National Social Life, Health, and Aging Project. Results: Exposure to greater social complexity (larger social networks, greater network diversity) is associated with significantly better olfaction at baseline. Increases in network diversity and fewer network losses significantly protect against olfactory decline over time. Discussion: Findings highlight the social context as an important, yet relatively overlooked source of sensory enrichment, and underscore the need for biological applications to integrate social life dynamics into studies of health trajectories.

What can traditional Chinese medicine do for adult neurogenesis?

Adult neurogenesis plays a crucial role in cognitive function and mood regulation, while aberrant adult neurogenesis contributes to various neurological and psychiatric diseases. With a better understanding of the significance of adult neurogenesis, the demand for improving adult neurogenesis is increasing. More and more research has shown that traditional Chinese medicine (TCM), including TCM prescriptions (TCMPs), Chinese herbal medicine, and bioactive components, has unique advantages in treating neurological and psychiatric diseases by regulating adult neurogenesis at various stages, including proliferation, differentiation, and maturation. In this review, we summarize the progress of TCM in improving adult neurogenesis and the key possible mechanisms by which TCM may benefit it. Finally, we suggest the possible strategies of TCM to improve adult neurogenesis in the treatment of neuropsychiatric disorders.

Neurobiological reduction: From cellular explanations of behavior to interventions

Scientific reductionism, the view that higher level functions can be explained by properties at some lower-level or levels, has been an assumption of nervous system analyses since the acceptance of the neuron doctrine in the late 19th century, and became a dominant experimental approach with the development of intracellular recording techniques in the mid-20th century. Subsequent refinements of electrophysiological approaches and the continual development of molecular and genetic techniques have promoted a focus on molecular and cellular mechanisms in experimental analyses and explanations of sensory, motor, and cognitive functions. Reductionist assumptions have also influenced our views of the etiology and treatment of psychopathologies, and have more recently led to claims that we can, or even should, pharmacologically enhance the normal brain. Reductionism remains an area of active debate in the philosophy of science. In neuroscience and psychology, the debate typically focuses on the mind-brain question and the mechanisms of cognition, and how or if they can be explained in neurobiological terms. However, these debates are affected by the complexity of the phenomena being considered and the difficulty of obtaining the necessary neurobiological detail. We can instead ask whether features identified in neurobiological analyses of simpler aspects in simpler nervous systems support current molecular and cellular approaches to explaining systems or behaviors. While my view is that they do not, this does not invite the opposing view prevalent in dichotomous thinking that molecular and cellular detail is irrelevant and we should focus on computations or representations. We instead need to consider how to address the long-standing dilemma of how a nervous system that ostensibly functions through discrete cell to cell communication can generate population effects across multiple spatial and temporal scales to generate behavior.

Frontotemporal phase lag index correlates with seizure severity in patients with temporal lobe epilepsy

Objectives

To find the brain network indicators correlated with the seizure severity in temporal lobe epilepsy (TLE) by graph theory analysis.

Methods

We enrolled 151 patients with TLE and 36 age- and sex-matched controls with video-EEG monitoring. The 90-s interictal EEG data were acquired. We adopted a network analyzing pipeline based on graph theory to quantify and localize their functional networks, including weighted classical network, minimum spanning tree, community structure, and LORETA. The seizure severities were evaluated using the seizure frequency, drug-resistant epilepsy (DRE), and VA-2 scores.

Results

Our network analysis pipeline showed ipsilateral frontotemporal activation in patients with TLE. The frontotemporal phase lag index (PLI) values increased in the theta band (4–7 Hz), which were elevated in patients with higher seizure severities (P < 0.05). Multivariate linear regression analysis showed that the VA-2 scores were independently correlated with frontotemporal PLI values in the theta band (β = 0.259, P = 0.001) and age of onset (β = −0.215, P = 0.007).

Significance

This study illustrated that the frontotemporal PLI in the theta band independently correlated with seizure severity in patients with TLE. Our network analysis provided an accessible approach to guide the treatment strategy in routine clinical practice.

Cell proliferation and neurogenesis in the adult telencephalon of the newt Cynops pyrrhogaster

Urodele amphibians have the ability to regenerate several organs, including the brain. For this reason, the research on neurogenesis in these species after ablation of some parts of the brain has markedly progressed. However, detailed information on the characteristics and fate of proliferated cells as well as the function of newly generated neurons under normal conditions is still limited. In this study, we focused on investigating the proliferative and neurogenic zones as well as the fate of proliferated cells in the adult brain of the Japanese red-bellied newt to clarify the significance of neurogenesis in adulthood. We found that the proximal region of the lateral ventricles in the telencephalon and the preoptic area in the diencephalon were the main sites for continuous cell proliferation in the adult brain. Furthermore, we characterized proliferative cells and analyzed neurogenesis through a combination of 5-ethynyl-2'-deoxyuridine (EdU) labeling and immunohistochemistry using antibodies against the stem cell marker Sox2 and neuronal marker NeuN. Twenty-four hours after EdU injection, most of the EdU-positive cells were Sox2-immunopositive, whereas, EdU-positive signals and NeuN-immunoreactivities were not colocalized. Two months after EdU injection, the colocalization ratio of EdU-positive signals with Sox2-immunoreactivities decreased to approximately 10%, whereas the ratio of colocalization of EdU-positive signals with NeuN-immunoreactivities increased to approximately 60%. Furthermore, a portion of the EdU-incorporated cells developed into γ-aminobutyric acid-producing cells, which are assumed to function as interneurons. On the basis of these results, the significance of newly generated neurons was discussed with special reference to their reproductive behavior.

What microglia depletion approaches tell us about the role of microglia on synaptic function and behavior

Microglia are dynamic cells, constantly surveying their surroundings and interacting with neurons and synapses. Indeed, a wealth of knowledge has revealed a critical role of microglia in modulating synaptic transmission and plasticity in the developing brain. In the past decade, novel pharmacological and genetic strategies have allowed the acute removal of microglia, opening the possibility to explore and understand the role of microglia also in the adult brain. In this review, we summarized and discussed the contribution of microglia depletion strategies to the current understanding of the role of microglia on synaptic function, learning and memory, and behavior both in physiological and pathological conditions. We first described the available microglia depletion methods highlighting their main strengths and weaknesses. We then reviewed the impact of microglia depletion on structural and functional synaptic plasticity. Next, we focused our analysis on the effects of microglia depletion on behavior, including general locomotor activity, sensory perception, motor function, sociability, learning and memory both in healthy animals and animal models of disease. Finally, we integrated the findings from the reviewed studies and discussed the emerging roles of microglia on the maintenance of synaptic function, learning, memory strength and forgetfulness, and the implications of microglia depletion in models of brain disease.

The rearing environment persistently modulates mouse phenotypes from the molecular to the behavioural level

The phenotype of an organism results from its genotype and the influence of the environment throughout development. Even when using animals of the same genotype, independent studies may test animals of different phenotypes, resulting in poor replicability due to genotype-by-environment interactions. Thus, genetically defined strains of mice may respond differently to experimental treatments depending on their rearing environment. However, the extent of such phenotypic plasticity and its implications for the replicability of research findings have remained unknown. Here, we examined the extent to which common environmental differences between animal facilities modulate the phenotype of genetically homogeneous (inbred) mice. We conducted a comprehensive multicentre study, whereby inbred C57BL/6J mice from a single breeding cohort were allocated to and reared in 5 different animal facilities throughout early life and adolescence, before being transported to a single test laboratory. We found persistent effects of the rearing facility on the composition and heterogeneity of the gut microbial community. These effects were paralleled by persistent differences in body weight and in the behavioural phenotype of the mice. Furthermore, we show that environmental variation among animal facilities is strong enough to influence epigenetic patterns in neurons at the level of chromatin organisation. We detected changes in chromatin organisation in the regulatory regions of genes involved in nucleosome assembly, neuronal differentiation, synaptic plasticity, and regulation of behaviour. Our findings demonstrate that common environmental differences between animal facilities may produce facility-specific phenotypes, from the molecular to the behavioural level. Furthermore, they highlight an important limitation of inferences from single-laboratory studies and thus argue that study designs should take environmental background into account to increase the robustness and replicability of findings.

The phenotype of an organism results not only from its genotype but also the influence of its environment throughout development. This study shows that common environmental differences between animal facilities can induce substantial variation in the phenotype of mice, thereby highlighting an important limitation of inferences from single-laboratory studies in animal research.

Immature olfactory sensory neurons provide behaviourally relevant sensory input to the olfactory bulb

Postnatal neurogenesis provides an opportunity to understand how newborn neurons integrate into circuits to restore function. Newborn olfactory sensory neurons (OSNs) wire into highly organized olfactory bulb (OB) circuits throughout life, enabling lifelong plasticity and regeneration. Immature OSNs form functional synapses capable of evoking firing in OB projection neurons but what contribution, if any, they make to odor processing is unknown. Here, we show that immature OSNs provide odor input to the mouse OB, where they form monosynaptic connections with excitatory neurons. Importantly, immature OSNs respond as selectively to odorants as mature OSNs and exhibit graded responses across a wider range of odorant concentrations than mature OSNs, suggesting that immature and mature OSNs provide distinct odor input streams. Furthermore, mice can successfully perform odor detection and discrimination tasks using sensory input from immature OSNs alone. Together, our findings suggest that immature OSNs play a previously unappreciated role in olfactory-guided behavior.

The facets of olfactory learning

Volatile chemicals in the environment provide ethologically important information to many animals. However, how animals learn to use this information is only beginning to be understood. This review highlights recent experimental advances elucidating olfactory learning in rodents, ranging from adaptations to the environment to task-dependent refinement and multisensory associations. The broad range of phenomena, mechanisms, and brain areas involved demonstrate the complex and multifaceted nature of olfactory learning.

Inhibition of Hippocampal Neurogenesis Starting in Adolescence Increases Anxiodepressive Behaviors Amid Stress

Stressors during the adolescent period can affect development of the brain and have long-lasting impacts on behavior. Specifically, adolescent stress impairs hippocampal neurogenesis and can increase risk for anxiety, depression, and a dysregulated stress response in adulthood. In order to model the functional effects of reduced hippocampal neurogenesis during adolescence, a transgenic neurogenesis ablation rat model was used to suppress neurogenesis during the adolescent period and test anxiodepressive behaviors and stress physiology during adulthood. Wildtype and transgenic (TK) rats were given valganciclovir during the first two weeks of adolescence (4-6 weeks old) to knock down neurogenesis in TK rats. Starting in young adulthood (13 weeks old), blood was sampled for corticosterone at several time points following acute restraint stress to measure negative feedback of the stress response, and rats were tested on a battery of anxiodepressive tests at baseline and following acute restraint stress. Although TK rats had large reductions in both cell proliferation during adolescence, as measured by bromodeoxyuridine (BrdU), and ongoing neurogenesis in adulthood (by doublecortin), resulting in decreased volume of the dentate gyrus, negative feedback of the stress response following acute restraint was similar across all rats. Despite similar stress responses, TK rats showed higher anxiety-like behavior at baseline. In addition, only TK rats had increased depressive-like behavior when tested after acute stress. Together, these results suggest that long-term neurogenesis ablation starting in adolescence produces hippocampal atrophy and increases behavioral caution and despair amid stressful environments.

Neuroprotective properties of chrysin on decreases of cell proliferation, immature neurons and neuronal cell survival in the hippocampal dentate gyrus associated with cognition induced by methotrexate

Methotrexate (MTX) is a drug widely used for chemotherapy and can reduce cancer cell production by inhibiting dihydrofolate reductase and decreasing cancer cell growth. MTX has a neurotoxic effect on neural stem and glial cells, leading to memory deficits. Chrysin is a natural flavonoid that contains essential biological activities, such as neuroprotective and cognitive-improving properties. Therefore, the aim of the present study was to investigate the protective effect of chrysin against MTX-induced memory impairments related to hippocampal neurogenesis. Seventy-two male Sprague Dawley rats were divided into six groups: control, MTX, chrysin (10 and 30 mg/kg), and MTX+ chrysin (10 and 30 mg/kg) groups. Chrysin (10 and 30 mg/kg) was administered by oral gavage for 15 days. MTX (75 mg/kg) was administered by intravenous injection on days 8 and 15. Spatial and recognition memories were evaluated using the novel object location (NOL) and novel object recognition (NOR) tests, respectively. Moreover, cell proliferation, neuronal cell survival, and immature neurons in the subgranular zone of the hippocampal dentate gyrus were quantified by Ki-67, bromodeoxyuridine/neuronal nuclear protein (BrdU/NeuN), and doublecortin (DCX) immunohistochemistry staining. The results of the MTX group demonstrated that spatial and recognition memories were both impaired. Furthermore, cell division reduction, neuronal cell survival reduction, and immature neuron decreases were detected in the MTX group and not observed in the co-administration groups. Therefore, these results revealed that chrysin could alleviate memory and neurogenesis impairments in MTX-treated rats.

Morphological Diversity of Calretinin Interneurons Generated From Adult Mouse Olfactory Bulb Core Neural Stem Cells

Neural stem cells (NSCs) in the olfactory bulb (OB) core can generate mature interneurons in the adult mice brain. The vast majority of these adult generated cells express the calcium-binding protein Calretinin (CalR), and they migrate towards different OB layers. However, these cells have yet to be fully characterized and hence, to achieve this we injected retroviral particles expressing GFP into the OB core of adult animals and found that the CalR⁺ neurons generated from NSCs mainly migrate to the granule cell layer (GCL) and glomerular layer (GL) in similar proportions. In addition, since morphology and function are closely related, we used three-dimensional imaging techniques to analyze the morphology of these adult born cells, describing new subtypes of CalR⁺ interneurons based on their dendritic arborizations and projections, as well as their localization in the GCL or GL. We also show that the migration and morphology of these newly generated neurons can be altered by misexpressing the transcription factor Tbr1 in the OB core. Therefore, the morphology acquired by neurons located in a specific OB layer is the result of a combination of both extrinsic (e.g., layer allocation) and intrinsic mechanisms (e.g., transcription factors). Defining the cellular processes and molecular mechanisms that govern adult neurogenesis might help better understand brain circuit formation and plasticity, as well as eventually opening the way to develop strategies for brain repair.

Spatial Learning Promotes Adult Neurogenesis in Specific Regions of the Zebrafish Pallium

Adult neurogenesis could be considered as a homeostatic mechanism that accompanies the continuous growth of teleost fish. As an alternative but not excluding hypothesis, adult neurogenesis would provide a form of plasticity necessary to adapt the brain to environmental challenges. The zebrafish pallium is a brain structure involved in the processing of various cognitive functions and exhibits extended neurogenic niches throughout the periventricular zone. The involvement of neuronal addition as a learning-related plastic mechanism has not been explored in this model, yet. In this work, we trained adult zebrafish in a spatial behavioral paradigm and evaluated the neurogenic dynamics in different pallial niches. We found that adult zebrafish improved their performance in a cue-guided rhomboid maze throughout five daily sessions, being the fish able to relearn the task after a rule change. This cognitive activity increased cell proliferation exclusively in two pallial regions: the caudal lateral pallium (cLP) and the rostral medial pallium (rMP). To assessed whether learning impinges on pallial adult neurogenesis, mitotic cells were labeled by BrdU administration, and then fish were trained at different periods of adult-born neuron maturation. Our results indicate that adult-born neurons are being produced on demand in rMP and cLP during the learning process, but with distinct critical periods among these regions. Next, we evaluated the time course of adult neurogenesis by pulse and chase experiments. We found that labeled cells decreased between 4 and 32 dpl in both learning-sensitive regions, whereas a fraction of them continues proliferating over time. By modeling the population dynamics of neural stem cells (NSC), we propose that learning increases adult neurogenesis by two mechanisms: driving a chained proliferation of labeled NSC and rescuing newborn neurons from death. Our findings highlight adult neurogenesis as a conserved source of brain plasticity and shed light on a rostro-caudal specialization of pallial neurogenic niches in adult zebrafish.