Dendritic development of newly generated neurons in the adult brain

Dendritic morphological development of traumatic brain injury-induced new neurons in the dentate gyrus is important for post-injury cognitive recovery and is regulated by Notch1

Traumatic brain injury (TBI) is a prevalent problem with survivors suffering from chronic cognitive impairments. Following TBI there is a series of neuropathological changes including neurogenesis. It is well established that neurogenesis in the dentate gyrus (DG) of the hippocampus is important for hippocampal dependent learning and memory functions. Following TBI, injury-enhanced hippocampal neurogenesis is believed to contribute to post-injury cognitive recovery. Behavioral function is connected to synaptic plasticity and neuronal dendritic branching is critical for successful synapse formation. To ascertain the functional contribution of injury-induced DG new neurons in post-TBI cognitive recovery, it is necessary to study their dendritic morphological development and the molecular mechanisms controlling this process. Utilizing transgenic mice with tamoxifen-induced GFP expression and Notch1 knock-out in nestin+ neural stem cells, this study examined dendritic morphology, the role of Notch1 in regulating dendritic complexity of injury-induced DG new neurons, and their association to post-TBI cognitive recovery. We found that at 8 weeks after a moderate TBI, injury-induced DG new neurons in the injured control mice displayed a similar dendritic morphology as the cells in non-injured mice accompanied with cognitive recovery. In comparison, in Notch1 conditional knock-out mice, DG new neurons in the injured mice had a significant reduction in dendritic morphological development including dendritic arbors, volume span, and number of branches in comparison to the cells in non-injured mice concomitant with persistent cognitive dysfunction. The results of this study confirm the importance of post-injury generated new neurons in cognitive recovery following TBI and the role of Notch1 in regulating their maturation process.

Traumatic brain injury alters the effects of class II invariant peptide (CLIP) antagonism on chronic meningeal CLIP + B cells, neuropathology, and neurobehavioral impairment in 5xFAD mice

BACKGROUND

Traumatic brain injury (TBI) is a significant risk factor for Alzheimer's disease (AD), and accumulating evidence supports a role for adaptive immune B and T cells in both TBI and AD pathogenesis. We previously identified B cell and major histocompatibility complex class II (MHCII)-associated invariant chain peptide (CLIP)-positive B cell expansion after TBI. We also showed that antagonizing CLIP binding to the antigen presenting groove of MHCII after TBI acutely reduced CLIP + splenic B cells and was neuroprotective. The current study investigated the chronic effects of antagonizing CLIP in the 5xFAD Alzheimer's mouse model, with and without TBI.

METHODS

12-week-old male wild type (WT) and 5xFAD mice were administered either CLIP antagonist peptide (CAP) or vehicle, once at 30 min after either sham or a lateral fluid percussion injury (FPI). Analyses included flow cytometric analysis of immune cells in dural meninges and spleen, histopathological analysis of the brain, magnetic resonance diffusion tensor imaging, cerebrovascular analysis, and assessment of motor and neurobehavioral function over the ensuing 6 months.

RESULTS

9-month-old 5xFAD mice had significantly more CLIP + B cells in the meninges compared to age-matched WT mice. A one-time treatment with CAP significantly reduced this population in 5xFAD mice. Importantly, CAP also improved some of the immune, histopathological, and neurobehavioral impairments in 5xFAD mice over the ensuing six months. Although FPI did not further elevate meningeal CLIP + B cells, it did negate the ability of CAP to reduce meningeal CLIP + B cells in the 5xFAD mice. FPI at 3 months of age exacerbated some aspects of AD pathology in 5xFAD mice, including further reducing hippocampal neurogenesis, increasing plaque deposition in CA3, altering microgliosis, and disrupting the cerebrovascular structure. CAP treatment after injury ameliorated some but not all of these FPI effects.

Adult Hippocampal Neurogenesis in the Human Brain: Updates, Challenges, and Perspectives

The existence of neurogenesis in the adult human hippocampus has been under considerable debate within the past three decades due to the diverging conclusions originating mostly from immunohistochemistry studies. While some of these reports conclude that hippocampal neurogenesis in humans occurs throughout physiologic aging, others indicate that this phenomenon ends by early childhood. More recently, some groups have adopted next-generation sequencing technologies to characterize with more acuity the extent of this phenomenon in humans. Here, we review the current state of research on adult hippocampal neurogenesis in the human brain with an emphasis on the challenges and limitations of using immunohistochemistry and next-generation sequencing technologies for its study.

Towards an understanding of psychedelic-induced neuroplasticity

Classic psychedelics, such as LSD, psilocybin, and the DMT-containing beverage ayahuasca, show some potential to treat depression, anxiety, and addiction. Importantly, clinical improvements can last for months or years after treatment. It has been theorized that these long-term improvements arise because psychedelics rapidly and lastingly stimulate neuroplasticity. The focus of this review is on answering specific questions about the effects of psychedelics on neuroplasticity. Firstly, we review the evidence that psychedelics promote neuroplasticity and examine the cellular and molecular mechanisms behind the effects of different psychedelics on different aspects of neuroplasticity, including dendritogenesis, synaptogenesis, neurogenesis, and expression of plasticity-related genes (e.g., brain-derived neurotrophic factor and immediate early genes). We then examine where in the brain psychedelics promote neuroplasticity, particularly discussing the prefrontal cortex and hippocampus. We also examine what doses are required to produce this effect (e.g., hallucinogenic doses vs. "microdoses"), and how long purported changes in neuroplasticity last. Finally, we discuss the likely consequences of psychedelics' effects on neuroplasticity for both patients and healthy people, and we identify important research questions that would further scientific understanding of psychedelics' effects on neuroplasticity and its potential clinical applications.

Psychedelics and Neuroplasticity: A Systematic Review Unraveling the Biological Underpinnings of Psychedelics

Clinical studies suggest the therapeutic potential of psychedelics, including ayahuasca, DMT, psilocybin, and LSD, in stress-related disorders. These substances induce cognitive, antidepressant, anxiolytic, and antiaddictive effects suggested to arise from biological changes similar to conventional antidepressants or the rapid-acting substance ketamine. The proposed route is by inducing brain neuroplasticity. This review attempts to summarize the evidence that psychedelics induce neuroplasticity by focusing on psychedelics' cellular and molecular neuroplasticity effects after single and repeated administration. When behavioral parameters are encountered in the selected studies, the biological pathways will be linked to the behavioral effects. Additionally, knowledge gaps in the underlying biology of clinical outcomes of psychedelics are highlighted. The literature searched yielded 344 results. Title and abstract screening reduced the sample to 35; eight were included from other sources, and full-text screening resulted in the final selection of 16 preclinical and four clinical studies. Studies (n = 20) show that a single administration of a psychedelic produces rapid changes in plasticity mechanisms on a molecular, neuronal, synaptic, and dendritic level. The expression of plasticity-related genes and proteins, including Brain-Derived Neurotrophic Factor (BDNF), is changed after a single administration of psychedelics, resulting in changed neuroplasticity. The latter included more dendritic complexity, which outlasted the acute effects of the psychedelic. Repeated administration of a psychedelic directly stimulated neurogenesis and increased BDNF mRNA levels up to a month after treatment. Findings from the current review demonstrate that psychedelics induce molecular and cellular adaptations related to neuroplasticity and suggest those run parallel to the clinical effects of psychedelics, potentially underlying them. Future (pre)clinical research might focus on deciphering the specific cellular mechanism activated by different psychedelics and related to long-term clinical and biological effects to increase our understanding of the therapeutic potential of these compounds.

Neuroinflammatory mechanisms of post-traumatic epilepsy

BACKGROUND

Traumatic brain injury (TBI) occurs in as many as 64-74 million people worldwide each year and often results in one or more post-traumatic syndromes, including depression, cognitive, emotional, and behavioral deficits. TBI can also increase seizure susceptibility, as well as increase the incidence of epilepsy, a phenomenon known as post-traumatic epilepsy (PTE). Injury type and severity appear to partially predict PTE susceptibility. However, a complete mechanistic understanding of risk factors for PTE is incomplete.

MAIN BODY

From the earliest days of modern neuroscience, to the present day, accumulating evidence supports a significant role for neuroinflammation in the post-traumatic epileptogenic progression. Notably, substantial evidence indicates a role for astrocytes, microglia, chemokines, and cytokines in PTE progression. Although each of these mechanistic components is discussed in separate sections, it is highly likely that it is the totality of cellular and neuroinflammatory interactions that ultimately contribute to the epileptogenic progression following TBI.

CONCLUSION

This comprehensive review focuses on the neuroinflammatory milieu and explores putative mechanisms involved in the epileptogenic progression from TBI to increased seizure-susceptibility and the development of PTE.

Adult-born dentate granule cells show a critical period of dendritic reorganization and are distinct from developmentally born cells

Adult-born dentate granule cells (abGCs) exhibit a critical developmental phase during function integration. The time window of this phase is debated and whether abGCs become indistinguishable from developmentally born mature granule cells (mGCs) is uncertain. We analyzed complete dendritic reconstructions from abGCs and mGCs using viral labeling. AbGCs from 21-77 days post intrahippocampal injection (dpi) exhibited comparable dendritic arbors, suggesting that structural maturation precedes functional integration. In contrast, significant structural differences were found compared to mGCs: AbGCs had more curved dendrites, more short terminal segments, a different branching pattern, and more proximal terminal branches. Morphological modeling attributed these differences to developmental dendritic pruning and postnatal growth of the dentate gyrus. We further correlated GC morphologies with the responsiveness to unilateral medial perforant path stimulation using the immediate-early gene Arc as a marker of synaptic activation. Only abGCs at 28 and 35 dpi but neither old abGCs nor mGCs responded to stimulation with a remodeling of their dendritic arbor. Summarized, abGCs stay distinct from mGCs and their dendritic arbor can be shaped by afferent activity during a narrow critical time window.

Astrocyte Hypertrophy Contributes to Aberrant Neurogenesis after Traumatic Brain Injury

Traumatic brain injury (TBI) is a widespread epidemic with severe cognitive, affective, and behavioral consequences. TBIs typically result in a relatively rapid inflammatory and neuroinflammatory response. A major component of the neuroinflammatory response is astrocytes, a type of glial cell in the brain. Astrocytes are important in maintaining the integrity of neuronal functioning, and it is possible that astrocyte hypertrophy after TBIs might contribute to pathogenesis. The hippocampus is a unique brain region, because neurogenesis persists in adults. Accumulating evidence supports the functional importance of these newborn neurons and their associated astrocytes. Alterations to either of these cell types can influence neuronal functioning. To determine if hypertrophied astrocytes might negatively influence immature neurons in the dentate gyrus, astrocyte and newborn neurons were analyzed at 30 days following a TBI in mice. The results demonstrate a loss of radial glial-like processes extending through the granule cell layer after TBI, as well as ectopic growth and migration of immature dentate neurons. The results further show newborn neurons in close association with hypertrophied astrocytes, suggesting a role for the astrocytes in aberrant neurogenesis. Future studies are needed to determine the functional significance of these alterations to the astrocyte/immature neurons after TBI.

The effects of hormones and physical exercise on hippocampal structural plasticity

The hippocampus plays an integral role in certain aspects of cognition. Hippocampal structural plasticity and in particular adult hippocampal neurogenesis can be influenced by several intrinsic and extrinsic factors. Here we review how hormones (i.e., intrinsic modulators) and physical exercise (i.e., an extrinsic modulator) can differentially modulate hippocampal plasticity in general and adult hippocampal neurogenesis in particular. Specifically, we provide an overview of the effects of sex hormones, stress hormones, and metabolic hormones on hippocampal structural plasticity and adult hippocampal neurogenesis. In addition, we also discuss how physical exercise modulates these forms of hippocampal plasticity, giving particular emphasis on how this modulation can be affected by variables such as exercise regime, duration, and intensity. Understanding the neurobiological mechanisms underlying the modulation of hippocampal structural plasticity by intrinsic and extrinsic factors will impact the design of new therapeutic approaches aimed at restoring hippocampal plasticity following brain injury or neurodegeneration.

Hypocellularity in the Murine Model for Down Syndrome Ts65Dn Is Not Affected by Adult Neurogenesis

Down syndrome (DS) is caused by the presence of an extra copy of the chromosome 21 and it is the most common aneuploidy producing intellectual disability. Neural mechanisms underlying this alteration may include defects in the formation of neuronal networks, information processing and brain plasticity. The murine model for DS, Ts65Dn, presents reduced adult neurogenesis. This reduction has been suggested to underlie the hypocellularity of the hippocampus as well as the deficit in olfactory learning in the Ts65Dn mice. Similar alterations have also been observed in individuals with DS. To determine whether the impairment in adult neurogenesis is, in fact, responsible for the hypocellularity in the hippocampus and physiology of the olfactory bulb, we have analyzed cell proliferation and neuronal maturation in the two major adult neurogenic niches in the Ts656Dn mice: the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ). Additionally, we carried out a study to determine the survival rate and phenotypic fate of newly generated cells in both regions, injecting 5'BrdU and sacrificing the mice 21 days later, and analyzing the number and phenotype of the remaining 5'BrdU-positive cells. We observed a reduction in the number of proliferating (Ki67 positive) cells and immature (doublecortin positive) neurons in the subgranular and SVZ of Ts65Dn mice, but we did not observe changes in the number of surviving cells or in their phenotype. These data correlated with a lower number of apoptotic cells (cleaved caspase 3 positive) in Ts65Dn. We conclude that although adult Ts65Dn mice have a lower number of proliferating cells, it is compensated by a lower level of cell death. This higher survival rate in Ts65Dn produces a final number of mature cells similar to controls. Therefore, the reduction of adult neurogenesis cannot be held responsible for the neuronal hypocellularity in the hippocampus or for the olfactory learning deficit of Ts65Dn mice.

How morphological constraints affect axonal polarity in mouse neurons

Neuronal differentiation is under the tight control of both biochemical and physical information arising from neighboring cells and micro-environment. Here we wished to assay how external geometrical constraints applied to the cell body and/or the neurites of hippocampal neurons may modulate axonal polarization in vitro. Through the use of a panel of non-specific poly-L-lysine micropatterns, we manipulated the neuronal shape. By applying geometrical constraints on the cell body we provided evidence that centrosome location was not predictive of axonal polarization but rather follows axonal fate. When the geometrical constraints were applied to the neurites trajectories we demonstrated that axonal specification was inhibited by curved lines. Altogether these results indicated that intrinsic mechanical tensions occur during neuritic growth and that maximal tension was developed by the axon and expressed on straight trajectories. The strong inhibitory effect of curved lines on axon specification was further demonstrated by their ability to prevent formation of multiple axons normally induced by cytochalasin or taxol treatments. Finally we provided evidence that microtubules were involved in the tension-mediated axonal polarization, acting as curvature sensors during neuronal differentiation. Thus, biomechanics coupled to physical constraints might be the first level of regulation during neuronal development, primary to biochemical and guidance regulations.

Comparing adult hippocampal neurogenesis in mammalian species and orders: influence of chronological age and life history stage

Adult hippocampal neurogenesis is a prominent event in rodents. In species with longer life expectancies, newly born cells in the adult dentate gyrus of the hippocampal formation are less abundant or can be completely absent. Several lines of evidence indicate that the regulatory mechanisms of adult neurogenesis differ between short- and long-lived mammals. After a critical appraisal of the factors and problems associated with comparing different species, we provide a quantitative comparison derived from seven laboratory strains of mice (BALB, C57BL/6, CD1, outbred) and rats (F344, Sprague-Dawley, Wistar), six other rodent species of which four are wild-derived (wood mouse, vole, spiny mouse and guinea pig), three non-human primate species (marmoset and two macaque species) and one carnivore (red fox). Normalizing the number of proliferating cells to total granule cell number, we observe an overall exponential decline in proliferation that is chronologically equal between species and orders and independent of early developmental processes and life span. Long- and short-lived mammals differ with regard to major life history stages; at the time points of weaning, age at first reproduction and average life expectancy, long-lived primates and foxes have significantly fewer proliferating cells than rodents. Although the database for neuronal differentiation is limited, we find indications that the extent of neuronal differentiation is subject to species-specific selective adaptations. We conclude that absolute age is the critical factor regulating cell genesis in the adult hippocampus of mammals. Ontogenetic and ecological factors primarily influence the regulation of neuronal differentiation rather than the rate of cell proliferation.

Suspension of mitotic activity in dentate gyrus of the hibernating ground squirrel

Neurogenesis occurs in the adult mammalian hippocampus, a region of the brain important for learning and memory. Hibernation in Siberian ground squirrels provides a natural model to study mitosis as the rapid fall in body temperature in 24 h (from 35-36°C to +4–6°C) permits accumulation of mitotic cells at different stages of the cell cycle. Histological methods used to study adult neurogenesis are limited largely to fixed tissue, and the mitotic state elucidated depends on the specific phase of mitosis at the time of day. However, using an immunohistochemical study of doublecortin (DCX) and BrdU-labelled neurons, we demonstrate that the dentate gyrus of the ground squirrel hippocampus contains a population of immature cells which appear to possess mitotic activity. Our data suggest that doublecortin-labelled immature cells exist in a mitotic state and may represent a renewable pool for generation of new neurons within the dentate gyrus.

Neurite outgrowth: this process, first discovered by Santiago Ramon y Cajal, is sustained by the exocytosis of two distinct types of vesicles

Neurite outgrowth is a fundamental process in the differentiation of neurons. The first, seminal study documenting the generation of "appendages" (now known as filopodia and lamellipodia) on the "cones d'accroissement," the specialized growth cones at the tips of neurites, was reported by Cajal still in the XIXth century, investigating chicken neurons embryos stained by the Golgi's reazione nera. Since then, studies have continued using, in addition to brain tissues, powerful in vitro models, i.e. primary cultures of pyramidal neurons from the hippocampus and neurosecretory cell lines, in particular PC12 cells. These studies have documented that neuronal neurites, upon sprouting from the cell body, give rise to both axons and dendrites. The specificity of these differentiated neurites depends on the diffusion barrier established at the initial segment of the axon and on the specialized domains, spines and presynaptic boutons, assembled around complexes of scaffold proteins. The two main, coordinate mechanisms that support neurite outgrowth are (a) the rearrangement of the cytoskeleton and (b) the expansion of the plasma membrane due to the exo/endocytosis of specific vesicles, distinct from those filled with neurotransmitters (clear and dense-core vesicles). The latter process is the main task of this review. In axons the surface-expanding exocytoses are concentrated at the growth cones; in dendrites they may be more distributed along the shaft. At least two types of exocytic vesicles appear to be involved, the enlargeosomes, positive for VAMP4, during early phases of development, and Ti-VAMP-positive vesicles later on. Outgrowth studies, that are now intensely pursued, have already yielded results of great importance in brain cell biology and function, and are playing an increasing role in pathology and medicine.

Structural plasticity and hippocampal function

The hippocampus is a region of the mammalian brain that shows an impressive capacity for structural reorganization. Preexisting neural circuits undergo modifications in dendritic complexity and synapse number, and entirely novel neural connections are formed through the process of neurogenesis. These types of structural change were once thought to be restricted to development. However, it is now generally accepted that the hippocampus remains structurally plastic throughout life. This article reviews structural plasticity in the hippocampus over the lifespan, including how it is investigated experimentally. The modulation of structural plasticity by various experiential factors as well as the possible role it may have in hippocampal functions such as learning and memory, anxiety, and stress regulation are also considered. Although significant progress has been made in many of these areas, we highlight some of the outstanding issues that remain.

Self-organized dendritic sidebranching in directional solidification: sidebranch coherence within uncorrelated bursts

We experimentally study the level of organization of dendritic sidebranching in directional solidification. For this, we extract successive interface positions at a fixed distance from the dendrite tips and we perform various correlation analyses. The sidebranching signals appear composed of randomly distributed bursts in which sidebranching coherence is surprisingly large and robust. This is attested by the large autocorrelation found in single bursts and the large cross-correlation found in any couple of bursts, even belonging to different sides of a dendrite or to different dendrites. However, the phase coherence of sidebranching breaks down at the transition between bursts. This restricts the coherence of extended sidebranching signals to a mean burst length and prevents the occurrence of large scale cross-correlation between them. This balanced view on sidebranching coherence stresses the capability of self-organization of dendrites in material science and sheds light on the nature of sidebranching on curved growing forms.

Partial kindling induces neurogenesis, activates astrocytes and alters synaptic morphology in the dentate gyrus of freely moving adult rats

A partial kindling procedure was used to investigate the correlation between focal seizure development and changes in dendritic spine morphology, ongoing neurogenesis and reactive astrogliosis in the adult rat dentate gyrus (DG). The processes of neurogenesis and astrogliosis were investigated using markers for doublecortin (DCX), 5-bromo-2-deoxyuridine (BrdU) and glial fibrillary acidic protein (GFAP). Our data demonstrate that mild focal seizures induce a complex series of cellular events in the DG one day after cessation of partial rapid kindling stimulation consisting (in comparison to control animals that were electrode implanted but unkindled), firstly, of an increase in the number of postmitotic BrdU labeled cells, and secondly, an increase in the number of DCX labeled cells, mainly in subgranular zone. Ultrastructural changes were examined using qualitative electron microscope analysis and 3-D reconstructions of both dendritic spines and postsynaptic densities. Typical features of kindling in comparison to control tissue included translocation of mitochondria to the base of the dendritic spine stalks; a migration of multivesicular bodies into mushroom dendritic spines, and most notably formation of "giant" spinules originating from the head of the spines of DG neurons. These morphological alterations arise at seizure stages 2-3 (focal seizures) in the absence of signs of the severe generalized seizures that are generally recognized as potentially harmful for neuronal cells. We suggest that an increase in ongoing neurogenesis, reactive astrogliosis and dendritic spine reorganization in the DG is the crucial step in the chain of events leading to the progressive development of seizure susceptibility in hippocampal circuits.

Status epilepticus during old age is not associated with enhanced hippocampal neurogenesis

Increased production of new neurons in the adult dentate gyrus (DG) by neural stem/progenitor cells (NSCs) following acute seizures or status epilepticus (SE) is a well known phenomenon. However, it is unknown whether NSCs in the aged DG have similar ability to upregulate neurogenesis in response to SE. We examined DG neurogenesis after the induction of continuous stages III-V seizures (SE) for over 4 h in both young adult (5-months old) and aged (24-months old) F344 rats. The seizures were induced through 2-4 graded intraperitoneal injections of the excitotoxin kainic acid (KA). Newly born cells in the DG were labeled via daily intraperitoneal injections of the 5'-bromodeoxyuridine (BrdU) for 12 days, which commenced shortly after the induction of SE in KA-treated rats. New cells and neurons in the subgranular zone (SGZ) and the granule cell layer (GCL) were analyzed at 24 h after the last BrdU injection using BrdU and doublecortin (DCX) immunostaining, BrdU-DCX and BrdU-NeuN dual immunofluorescence and confocal microscopy, and stereological cell counting. Status epilepticus enhanced the numbers of newly born cells (BrdU(+) cells) and neurons (DCX(+) neurons) in young adult rats. In contrast, similar seizures in aged rats, though greatly increased the number of newly born cells in the SGZ/GCL, failed to increase neurogenesis due to a greatly declined neuronal fate-choice decision of newly born cells. Only 9% of newly born cells in the SGZ/GCL differentiated into neurons in aged rats that underwent SE, in comparison to the 76% neuronal differentiation observed in age-matched control rats. Moreover, the number of newly born cells that migrate abnormally into the dentate hilus (i.e., ectopic granule cells) after SE in the aged hippocampus is 92% less than that observed in the young adult hippocampus after similar SE. Thus, SE fails to increase the addition of new granule cells to the GCL in the aged DG, despite a considerable upregulation in the production of new cells, and SE during old age leads to much fewer ectopic granule cells. These results have clinical relevance because earlier studies have implied that both increased and abnormal neurogenesis occurring after SE in young animals contributes to chronic epilepsy development.

Implications of decreased hippocampal neurogenesis in chronic temporal lobe epilepsy

Temporal lobe epilepsy (TLE), characterized by spontaneous recurrent motor seizures (SRMS), learning and memory impairments, and depression, is associated with neurodegeneration, abnormal reorganization of the circuitry, and loss of functional inhibition in the hippocampal and extrahippocampal regions. Over the last decade, abnormal neurogenesis in the dentate gyrus (DG) has emerged as another hallmark of TLE. Increased DG neurogenesis and recruitment of newly born neurons into the epileptogenic hippocampal circuitry is a characteristic phenomenon occurring during the early phase after the initial precipitating injury such as status epilepticus. However, the chronic phase of the disease displays substantially declined DG neurogenesis, which is associated with SRMS, learning and memory impairments, and depression. This review focuses on DG neurogenesis in the chronic phase of TLE and first confers the extent and mechanisms of declined DG neurogenesis in chronic TLE. The available data on production, survival and neuronal fate choice decision of newly born cells, stability of hippocampal stem cell numbers, and changes in the hippocampal microenvironment in chronic TLE are considered. The next section discusses the possible contribution of declined DG neurogenesis to the pathophysiology of chronic TLE, which includes its potential effects on spontaneous recurrent seizures, cognitive dysfunction, and depression. The subsequent section considers strategies that may be useful for augmenting DG neurogenesis in chronic TLE, which encompass stem cell grafting, administration of distinct neurotrophic factors, physical exercise, exposure to enriched environment, and antidepressant therapy. The final section suggests possible ramifications of increasing the DG neurogenesis in chronic epilepsy.