Adult hippocampal neurogenesis for systems consolidation of memory

Cellular stress and epigenetic regulation in adult stem cells

Stem cells are a unique class of cells that possess the ability to differentiate and self-renew, enabling them to repair and replenish tissues. To protect and maintain the potential of stem cells, the cells and the environment surrounding these cells (stem cell niche) are highly responsive and tightly regulated. However, various stresses can affect the stem cells and their niches. These stresses are both systemic and cellular and can arise from intrinsic or extrinsic factors which would have strong implications on overall aging and certain disease states. Therefore, understanding the breadth of drivers, namely epigenetic alterations, involved in cellular stress is important for the development of interventions aimed at maintaining healthy stem cells and tissue homeostasis. In this review, we summarize published findings of epigenetic responses to replicative, oxidative, mechanical, and inflammatory stress on various types of adult stem cells.

Sleep deprivation in adolescent mice impairs long-term memory till early adulthood via suppression of hippocampal astrocytes

Sleep deficiency is a rampant issue in modern society, serving as a pathogenic element contributing to learning and memory impairment, with heightened sensitivity observed in children. Clinical observations suggest that learning disabilities associated with insufficient sleep during adolescence can persist through adulthood, but experimental evidence for this is lacking. In this study, we examined the impact of early-life sleep deprivation (SD) on both short-term and long-term memory, tracking the effects sequentially into adulthood. We employed a modified multiple-platform method mouse model to investigate these outcomes. SD induced over a 14-day period, beginning on postnatal day 28 (PND28) in mice, led to significant impairment in long-term memory (while short-term memory remained unaffected) at PND42. Notably, this dysfunction persisted into adulthood at PND85. The specific impairment observed in long-term memory was elucidated through histopathological alterations in hippocampal neurogenesis, as evidenced by bromodeoxyuridine (BrdU) signals, observed both at PND42 and PND85. Furthermore, the hippocampal region exhibited significantly diminished protein expressions of astrocytes, characterized by lowered levels of aquaporin 4 (AQP4), a representative molecule involved in brain clearance processes, and reduced protein expressions of brain-derived neurotrophic factors. In conclusion, we have presented experimental evidence indicating that sleep deficiency-related impairment of long-term memory in adolescence can endure into adulthood. The corresponding mechanisms may indicate that the modification of astrocyte-related molecules has led to changes in hippocampal neurogenesis.

The impact of flavonoids and BDNF on neurogenic process in various physiological/pathological conditions including ischemic insults: a narrative review

OBJECTIVE

Ischemic stroke is the leading cause of mortality and disability worldwide with more than half of survivors living with serious neurological sequelae thus, it has recently attracted considerable attention in the field of medical research. Neurogenesis is the process of formation of new neurons in the brain, including the human brain, from neural stem/progenitor cells [NS/PCs] which reside in neurogenic niches that contain the necessary substances for NS/PC proliferation, differentiation, migration, and maturation into functioning neurons which can integrate into a pre-existing neural network.Neurogenesis can be modulated by many exogenous and endogenous factors, pathological conditions. Both brain-derived neurotrophic factor, and flavonoids can modulate the neurogenic process in physiological conditions and after various pathological conditions including ischemic insults.

AIM

This review aims to discuss neurogenesis after ischemic insults and to determine the role of flavonoids and BDNF on neurogenesis under physiological and pathological conditions with a concentration on ischemic insults to the brain in particular.

METHOD

Relevant articles assessing the impact of flavonoids and BDNF on neurogenic processes in various physiological/pathological conditions including ischemic insults within the timeline of 1965 until 2023 were searched using the PubMed database.

CONCLUSIONS

The selected studies have shown that ischemic insults to the brain induce NS/PC proliferation, differentiation, migration, and maturation into functioning neurons integrating into a pre-existing neural network. Flavonoids and BDNF can modulate neurogenesis in the brain in various physiological/pathological conditions including ischemic insults. In conclusion, flavonoids and BDNF may be involved in post-ischemic brain repair processes through enhancing endogenous neurogenesis.

Potential role of oxytocin in the regulation of memories and treatment of memory disorders

Oxytocin (OXT) is an "affiliative" hormone or neurohormone or neuropeptide consists of nine amino acids, synthesized in magnocellular neurons of paraventricular (PVN) and supraoptic nuclei (SON) of hypothalamus. OXT receptors are widely distributed in various region of brain and OXT has been shown to regulate various social and nonsocial behavior. Hippocampus is the main region which regulates the learning and memory. Hippocampus particularly regulates the acquisition of new memories and retention of acquired memories. OXT has been shown to regulate the synaptic plasticity, neurogenesis, and consolidation of memories. Further, findings from both preclinical and clinical studies have suggested that the OXT treatment improves performance in memory related task. Various trials have suggested the positive impact of intranasal OXT in the dementia patients. However, these studies are limited in number. In the present study authors have highlighted the role of OXT in the formation and retrieval of memories. Further, the study demonstrated the outcome of OXT treatment in various memory and related disorders.

Hippocampal neurodegeneration induces transient endogenous regeneration and long-term exhaustion of the neurogenic niche

The hippocampal dentate gyrus, responds to diverse pathological stimuli through neurogenesis. This phenomenon, observed following brain injury or neurodegeneration, is postulated to contribute to neuronal repair and functional recovery, thereby presenting an avenue for endogenous neuronal restoration. This study investigated the extent of regenerative response in hippocampal neurogenesis by leveraging the well-established kainic acid-induced status epilepticus model in vivo. In our study, we observed the activation and proliferation of neuronal progenitors or neural stem cell (NSC) and their subsequent migration to the injury sites following the seizure. At the injury sites, new neurons (Tuj1+BrdU+ and NeuN+BrdU+) have been generated indicating regenerative and reparative roles of the progenitor cells. We further detected whether this transient neurogenic burst, which might be a response towards an attempt to repair the brain, is associated with persistent long-term exhaustion of the dentate progenitor cells and impairment of adult neurogenesis marked by downregulation of Ki67, HoPX, and Sox2 with BrdU+ cell in the later part of life. Our studies suggest that the adult brain has the constitutive endogenous regenerative potential for brain repair to restore the damaged neurons, meanwhile, in the long term, it accelerates the depletion of the finite NSC pool in the hippocampal neurogenic niche by changing its proliferative and neurogenic capacity. A thorough understanding of the impact of modulating adult neurogenesis will eventually be required to design novel therapeutics to stimulate or assist brain repair while simultaneously preventing the adverse effects of early robust neurogenesis on the proliferative potential of endogenous neuronal progenitors.

Effect of 2-Week Naringin Supplementation on Neurogenesis and BDNF Levels in Ischemia-Reperfusion Model of Rats

BACKGROUND

Ischemic stroke is the leading cause of mortality and disability worldwide with more than half of survivors living with serious neurological sequelae; thus, it has recently attracted a lot of attention in the field of medical study.

PURPOSE

The aim of this study was to determine the effect of naringin supplementation on neurogenesis and brain-derived neurotrophic factor (BDNF) levels in the brain in experimental brain ischemia-reperfusion.

STUDY DESIGN

The research was carried out on 40 male Wistar-type rats (10-12 weeks old) obtained from the Experimental Animals Research and Application Center of Selçuk University. Experimental groups were as follows: (1) Control group, (2) Sham group, (3) Brain ischemia-reperfusion group, (4) Brain ischemia-reperfusion + vehicle group (administered for 14 days), and (5) Brain ischemia-reperfusion + Naringin group (100 mg/kg/day administered for 14 days).

METHODS

In the ischemia-reperfusion groups, global ischemia was performed in the brain by ligation of the right and left carotid arteries for 30 min. Naringin was administered to experimental animals by intragastric route for 14 days following reperfusion. The training phase of the rotarod test was started 4 days before ischemia-reperfusion, and the test phase together with neurological scoring was performed the day before and 1, 7, and 14 days after the operation. At the end of the experiment, animals were sacrificed, and then hippocampus and frontal cortex tissues were taken from the brain. Double cortin marker (DCX), neuronal nuclear antigen marker (NeuN), and BDNF were evaluated in hippocampus and frontal cortex tissues by Real-Time qPCR analysis and immunohistochemistry methods.

RESULTS

While ischemia-reperfusion increased the neurological score values, DCX, NeuN, and BDNF levels decreased significantly after ischemia in the hippocampus and frontal cortex tissues. However, naringin supplementation restored the deterioration to a certain extent.

CONCLUSION

The results of the study show that 2 weeks of naringin supplementation may have protective effects on impaired neurogenesis and BDNF levels after brain ischemia and reperfusion in rats.

Visuotactile integration facilitates mirror-induced self-directed behavior through activation of hippocampal neuronal ensembles in mice

Remembering the visual features of oneself is critical for self-recognition. However, the neural mechanisms of how the visual self-image is developed remain unknown because of the limited availability of behavioral paradigms in experimental animals. Here, we demonstrate a mirror-induced self-directed behavior (MSB) in mice, resembling visual self-recognition. Mice displayed increased mark-directed grooming to remove ink placed on their heads when an ink-induced visual-tactile stimulus contingency occurred. MSB required mirror habituation and social experience. The chemogenetic inhibition of dorsal or ventral hippocampal CA1 (vCA1) neurons attenuated MSB. Especially, a subset of vCA1 neurons activated during the mirror exposure was significantly reactivated during re-exposure to the mirror and was necessary for MSB. The self-responding vCA1 neurons were also reactivated when mice were exposed to a conspecific of the same strain. These results suggest that visual self-image may be developed through social experience and mirror habituation and stored in a subset of vCA1 neurons.

Understanding Others' Distress Through Past Experiences: The Role of Memory Engram Cells in Observational Fear

For individuals to survive and function in society, it is essential that they recognize, interact with, and learn from other conspecifics. Observational fear (OF) is the well-conserved empathic ability of individuals to understand the other's aversive situation. While it is widely known that factors such as prior similar aversive experience and social familiarity with the demonstrator facilitate OF, the neural circuit mechanisms that explicitly regulate experience-dependent OF (Exp OF) were unclear. In this review, we examine the neural circuit mechanisms that regulate OF, with an emphasis on rodent models, and then discuss emerging evidence for the role of fear memory engram cells in the regulation of Exp OF. First, we examine the neural circuit mechanisms that underlie Naive OF, which is when an observer lacks prior experiences relevant to OF. In particular, the anterior cingulate cortex to basolateral amygdala (BLA) neural circuit is essential for Naive OF. Next, we discuss a recent study that developed a behavioral paradigm in mice to examine the neural circuit mechanisms that underlie Exp OF. This study found that fear memory engram cells in the BLA of observers, which form during a prior similar aversive experience with shock, are reactivated by ventral hippocampal neurons in response to shock delivery to the familiar demonstrator to elicit Exp OF. Finally, we discuss the implications of fear memory engram cells in Exp OF and directions of future research that are of both translational and basic interest.

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and clear dead cells generated during development or disease. The phagocytic process shapes the microglia phenotype, which affects the local environment. A unique population of microglia resides in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence the neurogenic niche is not well understood. Here, we demonstrate that phagocytosis contributes to a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering a neuroinflammatory microglia phenotype that resembles dysfunctional microglia in neurodegeneration and aging and that reduces neural precursor proliferation via elevated interleukin-1β signaling; interleukin-1 receptor inhibition rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to the maintenance of a pro-neurogenic phenotype in the developing V-SVZ.

Neuroprotective, neurogenic, and anticholinergic evidence of Ganoderma lucidum cognitive effects: Crucial knowledge is still lacking

Ganoderma lucidum is a mushroom that has been widely used for centuries in Asian countries for its antiaging properties. It is popularly known as "Ling Zhi," "Reishi," and "Youngzhi," and because of its benefits, it is known as the "immortality mushroom." Pharmacological assays have revealed that G. lucidum ameliorates cognitive impairments through inhibition of β-amyloid and neurofibrillary tangle formation, antioxidant effect, reduction of inflammatory cytokine release and apoptosis, genic expression modulation, among other activities. Chemical investigations on G. lucidum have revealed the presence of metabolites such as triterpenes, which are the most explored in this field, as well as flavonoids, steroids, benzofurans, and alkaloids; in the literature, these have also been reported to have mnemonic activity. These properties of the mushroom make it a potential source of new drugs to prevent or reverse memory disorders, as actual medications are able to only alleviate some symptoms but are unable to stop the progress of cognitive impairments, with no impact on social, familiar, and personal relevance. In this review, we discuss the cognitive findings of G. lucidum reported in the literature, converging the proposed mechanisms through the several pathways that underlie memory and cognition processes. In addition, we highlight the gaps that deserve particular attention to support future studies.

Expression patterns of AEG-1 in the normal brain

Astrocyte elevated gene-1 (AEG-1) is a well-known oncogene implicated in various types of human cancers, including brain tumors. Recently, AEG-1 has also been reported to play pivotal roles in glioma-associated neurodegeneration and neurodegenerative diseases like Parkinson's disease and amyotrophic lateral sclerosis. However, the normal physiological functions and expression patterns of AEG-1 in the brain are not well understood. In this study, we investigated the expression patterns of AEG-1 in the normal mouse brain and found that AEG-1 is widely expressed in neurons and neuronal precursor cells, but little in glial cells. We observed differential expression levels of AEG-1 in various brain regions, and its expression was mainly localized in the cell body of neurons rather than the nucleus. Additionally, AEG-1 was expressed in the cytoplasm of Purkinje cells in both the mouse and human cerebellum, suggesting its potential role in this brain region. These findings suggest that AEG-1 may have important functions in normal brain physiology and warrant further investigation. Our results may also shed light on the differential expression patterns of AEG-1 in normal and pathological brains, providing insights into its roles in various neurological disorders.

Novel Psychopharmacological Herbs Relieve Behavioral Abnormalities and Hippocampal Dysfunctions in an Animal Model of Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is an anxiety disorder caused by traumatic or frightening events, with intensified anxiety, fear memories, and cognitive impairment caused by a dysfunctional hippocampus. Owing to its complex phenotype, currently prescribed treatments for PTSD are limited. This study investigated the psychopharmacological effects of novel COMBINATION herbal medicines on the hippocampus of a PTSD murine model induced by combining single prolonged stress (SPS) and foot shock (FS). We designed a novel herbal formula extract (HFE) from Chaenomeles sinensis, Glycyrrhiza uralensis, and Atractylodes macrocephala. SPS+FS mice were administered HFE (500 and 1000 mg/kg) once daily for 14 days. The effects of HFE of HFE on the hippocampus were analyzed using behavioral tests, immunostaining, Golgi staining, and Western blotting. HFE alleviated anxiety-like behavior and fear response, improved short-term memory, and restored hippocampal dysfunction, including hippocampal neurogenesis alteration and aberrant migration and hyperactivation of dentate granule cells in SPS+FS mice. HFE increased phosphorylation of the Kv4.2 potassium channel, extracellular signal-regulated kinase, and cAMP response element-binding protein, which were reduced in the hippocampus of SPS+FS mice. Therefore, our study suggests HFE as a potential therapeutic drug for PTSD by improving behavioral impairment and hippocampal dysfunction and regulating Kv4.2 potassium channel-related pathways in the hippocampus.

Systems consolidation induces multiple memory engrams for a flexible recall strategy in observational fear memory in male mice

Observers learn to fear the context in which they witnessed a demonstrator's aversive experience, called observational contextual fear conditioning (CFC). The neural mechanisms governing whether recall of the observational CFC memory occurs from the observer's own or from the demonstrator's point of view remain unclear. Here, we show in male mice that recent observational CFC memory is recalled in the observer's context only, but remote memory is recalled in both observer and demonstrator contexts. Recall of recent memory in the observer's context requires dorsal hippocampus activity, while recall of remote memory in both contexts requires the medial prefrontal cortex (mPFC)-basolateral amygdala pathway. Although mPFC neurons activated by observational CFC are involved in remote recall in both contexts, distinct mPFC subpopulations regulate remote recall in each context. Our data provide insights into a flexible recall strategy and the functional reorganization of circuits and memory engram cells underlying observational CFC memory.

Dissecting cell-type-specific pathways in medial entorhinal cortical-hippocampal network for episodic memory

Episodic memory, which refers to our ability to encode and recall past events, is essential to our daily lives. Previous research has established that both the entorhinal cortex (EC) and hippocampus (HPC) play a crucial role in the formation and retrieval of episodic memories. However, to understand neural circuit mechanisms behind these processes, it has become necessary to monitor and manipulate the neural activity in a cell-type-specific manner with high temporal precision during memory formation, consolidation, and retrieval in the EC-HPC networks. Recent studies using cell-type-specific labeling, monitoring, and manipulation have demonstrated that medial EC (MEC) contains multiple excitatory neurons that have differential molecular markers, physiological properties, and anatomical features. In this review, we will comprehensively examine the complementary roles of superficial layers of neurons (II and III) and the roles of deeper layers (V and VI) in episodic memory formation and recall based on these recent findings.

A single intranasal dose of human mesenchymal stem cell-derived extracellular vesicles after traumatic brain injury eases neurogenesis decline, synapse loss, and BDNF-ERK-CREB signaling

An optimal intranasal (IN) dose of human mesenchymal stem cell-derived extracellular vesicles (hMSC-EVs), 90 min post-traumatic brain injury (TBI), has been reported to prevent the evolution of acute neuroinflammation into chronic neuroinflammation resulting in the alleviation of long-term cognitive and mood impairments. Since hippocampal neurogenesis decline and synapse loss contribute to TBI-induced long-term cognitive and mood dysfunction, this study investigated whether hMSC-EV treatment after TBI can prevent hippocampal neurogenesis decline and synapse loss in the chronic phase of TBI. C57BL6 mice undergoing unilateral controlled cortical impact injury (CCI) received a single IN administration of different doses of EVs or the vehicle at 90 min post-TBI. Quantifying neurogenesis in the subgranular zone-granule cell layer (SGZ-GCL) through 5'-bromodeoxyuridine and neuron-specific nuclear antigen double labeling at ~2 months post-TBI revealed decreased neurogenesis in TBI mice receiving vehicle. However, in TBI mice receiving EVs (12.8 and 25.6 × 10⁹ EVs), the extent of neurogenesis was matched to naive control levels. A similar trend of decreased neurogenesis was seen when doublecortin-positive newly generated neurons were quantified in the SGZ-GCL at ~3 months post-TBI. The above doses of EVs treatment after TBI also reduced the loss of pre-and post-synaptic marker proteins in the hippocampus and the somatosensory cortex. Moreover, at 48 h post-treatment, brain-derived neurotrophic factor (BDNF), phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2), and phosphorylated cyclic AMP response-element binding protein (p-CREB) levels were downregulated in TBI mice receiving the vehicle but were closer to naïve control levels in TBI mice receiving above doses of hMSC-EVs. Notably, improved BDNF concentration observed in TBI mice receiving hMSC-EVs in the acute phase was sustained in the chronic phase of TBI. Thus, a single IN dose of hMSC-EVs at 90 min post-TBI can ease TBI-induced declines in the BDNF-ERK-CREB signaling, hippocampal neurogenesis, and synapses.

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and are responsible for clearing dead cells generated during development or disease. The phagocytic process shapes the phenotype of the microglia, which affects the local environment. A unique population of microglia reside in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence this neurogenic niche is not well-understood. Here, we demonstrate that phagocytosis creates a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering the development of a neuroinflammatory phenotype, reminiscent of neurodegenerative and-age-associated microglia, that reduces neural precursor proliferation via elevated interleukin (IL)-1β signaling; inhibition of IL-1 receptor rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to a phenotype that promotes neurogenesis in the developing V-SVZ.

Adult Neurogenesis Is Altered by Circadian Phase Shifts and the Duper Mutation in Female Syrian Hamsters

Cell birth and survival in the adult hippocampus are regulated by a circadian clock. Rotating shift work and jet lag disrupt circadian rhythms and aggravate disease. Internal misalignment, a state in which abnormal phase relationships prevail between and within organs, is proposed to account for adverse effects of circadian disruption. This hypothesis has been difficult to test because phase shifts of the entraining cycle inevitably lead to transient desynchrony. Thus, it remains possible that phase shifts, regardless of internal desynchrony, account for adverse effects of circadian disruption and alter neurogenesis and cell fate. To address this question, we examined cell birth and differentiation in the duper Syrian hamster (Mesocricetus auratus), a Cry1-null mutant in which re-entrainment of locomotor rhythms is greatly accelerated. Adult females were subjected to alternating 8 h advances and delays at eight 16 d intervals. BrdU, a cell birth marker, was given midway through the experiment. Repeated phase shifts decreased the number of newborn non-neuronal cells in WT, but not in duper hamsters. The duper mutation increased the number of BrdU-IR cells that stained for NeuN, which marks neuronal differentiation. Immunocytochemical staining for proliferating cell nuclear antigen indicated no overall effect of genotype or repeated shifts on cell division rates after 131 days. Cell differentiation, assessed by doublecortin, was higher in duper hamsters but was not significantly altered by repeated phase shifts. Our results support the internal misalignment hypothesis and indicate that Cry1 regulates cell differentiation. Phase shifts may determine neuronal stem cell survival and time course of differentiation after cell birth. Figure created with BioRender.

Hyperbaric Oxygenation Prevents Loss of Immature Neurons in the Adult Hippocampal Dentate Gyrus Following Brain Injury

A growing body of evidence suggests that hyperbaric oxygenation (HBO) may affect the activity of adult neural stem cells (NSCs). Since the role of NSCs in recovery from brain injury is still unclear, the purpose of this study was to investigate the effects of sensorimotor cortex ablation (SCA) and HBO treatment (HBOT) on the processes of neurogenesis in the adult dentate gyrus (DG), a region of the hippocampus that is the site of adult neurogenesis. Ten-week-old Wistar rats were divided into groups: Control (C, intact animals), Sham control (S, animals that underwent the surgical procedure without opening the skull), SCA (animals in whom the right sensorimotor cortex was removed via suction ablation), and SCA + HBO (operated animals that passed HBOT). HBOT protocol: pressure applied at 2.5 absolute atmospheres for 60 min, once daily for 10 days. Using immunohistochemistry and double immunofluorescence labeling, we show that SCA causes significant loss of neurons in the DG. Newborn neurons in the subgranular zone (SGZ), inner-third, and partially mid-third of the granule cell layer are predominantly affected by SCA. HBOT decreases the SCA-caused loss of immature neurons, prevents reduction of dendritic arborization, and increases proliferation of progenitor cells. Our results suggest a protective effect of HBO by reducing the vulnerability of immature neurons in the adult DG to SCA injury.

Lung-brain axis: Metabolomics and pathological changes in lungs and brain of respiratory syncytial virus-infected mice

The lung-brain axis is an emerging area of study that got its basis from the gut-brain axis biological pathway. Using Respiratory Synctial Virus (RSV) as the model of respiratory viral pathogen, this study aims to establish some biological pathways. After establishing the mice model, the inflammation in lung and brain were assayed using Hematoxylin-eosin staining, indirect immunofluorescence (IFA), and quantitative reverse-transcription polymerase chain reaction. The biological pathways between lung and brain were detected through metabolomics analysis. In lung, RSV infection promoted epithelial shedding and infiltration of inflammatory cells. Also, RSV immunofluorescence and titerss were significantly increased. Moreover, interleukin (IL)-1, IL-6 and tumor necrosis factor-α (TNF-α) were also significantly increased after RSV infection. In brain, the cell structure of hippocampal CA1 area was loose and disordered. Inflammatory cytokines IL-6 and IL-1β expression in the brain also increased, however, TNF-α expression showed no differences among the control and RSV group. We observed an increased expression of microglia biomarker IBA-1 and decreased neuronal biomarker NeuN. In addition, RSV mRNA expression levels were also increased in the brains. 15 metabolites were found upregulated in the RSV group including nerve-injuring metabolite glutaric acid, hydroxyglutaric acid and Spermine. ɑ-Estradiol increased significantly while normorphine decreased significantly at Day 7 of infection among the RSV group. This study established a mouse model for exploring the pathological changes in lungs and brains. There are many biological pathways between lung and brain, including direct translocation of RSV and metabolite pathway.

Delayed effects of neonatal immune activation on brain neurochemistry and hypothalamic-pituitary-adrenal axis functioning

During the postnatal period, the brain is highly sensitive to stress and inflammation, which are hazardous to normal growth and development. There is increasing evidence that inflammatory processes in the early postnatal period increase the risk of psychopathologies and cognitive impairment later in life. On the other hand, there are few studies on the ability of infectious agents to cause long-term neuroinflammation, leading to changes in the hypothalamic-pituitary-adrenal axis functioning and an imbalance in the neurotransmitter system. In this review, we examine short- and long-term effects of neonatal-induced inflammation in rodents on glutamatergic, GABAergic and monoaminergic systems and on hypothalamic-pituitary-adrenal axis activity.

Gut dysbiosis impairs hippocampal plasticity and behaviors by remodeling serum metabolome

Accumulating evidence suggests that gut microbiota as a critical mediator of gut-brain axis plays an important role in human health. Altered gut microbial profiles have been implicated in increasing the vulnerability of psychiatric disorders, such as autism, depression, and schizophrenia. However, the cellular and molecular mechanisms underlying the association remain unknown. Here, we modified the gut microbiome with antibiotics in newborn mice, and found that gut microbial alteration induced behavioral impairment by decreasing adult neurogenesis and long-term potentiation of synaptic transmission, and altering the gene expression profile in hippocampus. Reconstitution with normal gut flora produced therapeutic effects against both adult neurogenesis and behavioral deficits in the dysbiosis mice. Furthermore, our results show that circulating metabolites changes mediate the effect of gut dysbiosis on hippocampal plasticity and behavior outcomes. Elevating the serum 4-methylphenol, a small aromatic metabolite produced by gut bacteria, was found to induce autism spectrum disorder (ASD)-like behavior impairment and hippocampal dysfunction. Together our finding demonstrates that early-life gut dysbiosis and its correlated metabolites change contribute to hippocampal dysfunction and behavior impairment, hence highlight the potential microbiome-mediated therapies for treating psychiatric disorders.

Neuronal Ensembles Organize Activity to Generate Contextual Memory

Contextual learning is a critical component of episodic memory and important for living in any environment. Context can be described as the attributes of a location that are not the location itself. This includes a variety of non-spatial information that can be derived from sensory systems (sounds, smells, lighting, etc.) and internal state. In this review, we first address the behavioral underpinnings of contextual memory and the development of context memory theory, with a particular focus on the contextual fear conditioning paradigm as a means of assessing contextual learning and the underlying processes contributing to it. We then present the various neural centers that play roles in contextual learning. We continue with a discussion of the current knowledge of the neural circuitry and physiological processes that underlie contextual representations in the Entorhinal cortex-Hippocampal (EC-HPC) circuit, as the most well studied contributor to contextual memory, focusing on the role of ensemble activity as a representation of context with a description of remapping, and pattern separation and completion in the processing of contextual information. We then discuss other critical regions involved in contextual memory formation and retrieval. We finally consider the engram assembly as an indicator of stored contextual memories and discuss its potential contribution to contextual memory.

Adult Neurogenesis under Control of the Circadian System

The mammalian circadian system is a hierarchically organized system, which controls a 24-h periodicity in a wide variety of body and brain functions and physiological processes. There is increasing evidence that the circadian system modulates the complex multistep process of adult neurogenesis, which is crucial for brain plasticity. This modulatory effect may be exercised via rhythmic systemic factors including neurotransmitters, hormones and neurotrophic factors as well as rhythmic behavior and physiology or via intrinsic factors within the neural progenitor cells such as the redox state and clock genes/molecular clockwork. In this review, we discuss the role of the circadian system for adult neurogenesis at both the systemic and the cellular levels. Better understanding of the role of the circadian system in modulation of adult neurogenesis can help develop new treatment strategies to improve the cognitive deterioration associated with chronodisruption due to detrimental light regimes or neurodegenerative diseases.

Morris Water Maze and Contextual Fear Conditioning Tasks to Evaluate Cognitive Functions Associated With Adult Hippocampal Neurogenesis

New neurons are continuously generated and functionally integrated into the dentate gyrus (DG) network during the adult lifespan of most mammals. The hippocampus is a crucial structure for spatial learning and memory, and the addition of new neurons into the DG circuitry of rodents seems to be a key element for these processes to occur. The Morris water maze (MWM) and contextual fear conditioning (CFC) are among the most commonly used hippocampus-dependent behavioral tasks to study episodic-like learning and memory in rodents. While the functional contribution of adult hippocampal neurogenesis (AHN) through these paradigms has been widely addressed, results have generated controversial findings. In this review, we analyze and discuss possible factors in the experimental methods that could explain the inconsistent results among AHN studies; moreover, we provide specific suggestions for the design of more sensitive protocols to assess AHN-mediated learning and memory functions.

Adult Hippocampal Neurogenesis in Alzheimer’s Disease: An Overview of Human and Animal Studies with Implications for Therapeutic Perspectives Aimed at Memory Recovery

The mammalian hippocampal dentate gyrus is a niche for adult neurogenesis from neural stem cells. Newborn neurons integrate into existing neuronal networks, where they play a key role in hippocampal functions, including learning and memory. In the ageing brain, neurogenic capability progressively declines while in parallel increases the risk for developing Alzheimer's disease (AD), the main neurodegenerative disorder associated with memory loss. Numerous studies have investigated whether impaired adult neurogenesis contributes to memory decline in AD. Here, we review the literature on adult hippocampal neurogenesis (AHN) and AD by focusing on both human and mouse model studies. First, we describe key steps of AHN, report recent evidence of this phenomenon in humans, and describe the specific contribution of newborn neurons to memory, as evinced by animal studies. Next, we review articles investigating AHN in AD patients and critically examine the discrepancies among different studies over the last two decades. Also, we summarize researches investigating AHN in AD mouse models, and from these studies, we extrapolate the contribution of molecular factors linking AD-related changes to impaired neurogenesis. Lastly, we examine animal studies that link impaired neurogenesis to specific memory dysfunctions in AD and review treatments that have the potential to rescue memory capacities in AD by stimulating AHN.

Carbamylated erythropoietin improves recognition memory by modulating microglia in a rat model of pain

Patients with chronic pain often complain about memory impairments. Experimental studies have shown neuroprotective effects of Carbamylated erythropoietin (Cepo-Fc) in the treatment of cognitive dysfunctions. However, little is currently known about its precise molecular mechanisms in a model of inflammatory pain. Therefore, this study aimed to investigate neuroprotective effects of Cepo-Fc against cognitive impairment induced by the inflammatory model of Complete Freund's Adjuvant (CFA). Carbamylated erythropoietin was administrated Intraperitoneally (i.p) on the day CFA injection, continued for a 21-days period. After conducting the behavioral tests (thermal hyperalgesia and novel object recognition test), western blot and ELISA were further preformed on days 0, 7, and 21. The results of this study indicate that Cepo-Fc can effectively reverse the CFA induced thermal hyperalgesia and recognition memory impairment. Additionally, Cepo-Fc noticeably decreased the hippocampal microglial expression, production of hippocampal IL-1β, and hippocampal apoptosis and necroptosis induced by the inflammatory pain. Therefore, our findings suggest that neuroprotective effects of Cepo-Fc in the treatment of pain related recognition memory impairment may be mediated through reducing hippocampal microglial expression as well as IL-1β production.

Prior maternal separation stress alters the dendritic complexity of new hippocampal neurons and neuroinflammation in response to an inflammatory stressor in juvenile female rats

Stress during critical periods of neurodevelopment is associated with an increased risk of developing stress-related psychiatric disorders, which are more common in women than men. Hippocampal neurogenesis (the birth of new neurons) is vulnerable to maternal separation (MS) and inflammatory stressors, and emerging evidence suggests that hippocampal neurogenesis is more sensitive to stress in the ventral hippocampus (vHi) than in the dorsal hippocampus (dHi). Although research into the effects of MS stress on hippocampal neurogenesis is well documented in male rodents, the effect in females remains underexplored. Similarly, reports on the impact of inflammatory stressors on hippocampal neurogenesis in females are limited, especially when female bias in the prevalence of stress-related psychiatric disorders begins to emerge. Thus, in this study we investigated the effects of MS followed by an inflammatory stressor (lipopolysaccharide, LPS) in early adolescence on peripheral and hippocampal inflammatory responses and hippocampal neurogenesis in juvenile female rats. We show that MS enhanced an LPS-induced increase in the pro-inflammatory cytokine IL-1β in the vHi but not in the dHi. However, microglial activation was similar following LPS alone or MS alone in both hippocampal regions, while MS prior to LPS reduced microglial activation in both dHi and vHi. The production of new neurons was unaffected by MS and LPS. MS and LPS independently reduced the dendritic complexity of new neurons, and MS exacerbated LPS-induced reductions in the complexity of distal dendrites of new neurons in the vHi but not dHi. These data highlight that MS differentially primes the physiological response to LPS in the juvenile female rat hippocampus.

Synergistic gene regulation by thyroid hormone and glucocorticoid in the hippocampus

The hippocampus is considered the center for learning and memory in the brain, and its development and function is greatly affected by the thyroid and stress axes. Thyroid hormone (TH) and glucocorticoids (GC) are known to have a synergistic effect on developmental programs across several vertebrate species, and their effects on hippocampal structure and function are well-documented. However, there are few studies that focus on the processes and genes that are cooperatively regulated by the two hormone axes. Cross-regulation of the thyroid and stress axes in the hippocampus occurs on multiple levels such that TH can regulate the expression of the GC receptor (GR) while GC can modulate tissue sensitivity to TH by controlling the expression of TH receptor (TR) and enzymes involved in TH biosynthesis. Thyroid hormone and GC are also known to synergistically regulate the transcription of genes associated with neuronal function and development. Synergistic gene regulation by TH and GC may occur through the direct, cooperative action of TR and GR on common target genes, or by indirect mechanisms involving gene regulatory cascades activated by TR and GR. In this chapter, we describe the known physiological effects and underlying molecular mechanisms of TH and GC synergistic gene regulation in the hippocampus.

Complementary roles of differential medial entorhinal cortex inputs to the hippocampus for the formation and integration of temporal and contextual memory (Systems Neuroscience)

In humans and rodents, the entorhinal cortical (EC)-hippocampal (HPC) circuit is crucial for the formation and recall of memory, preserving both spatial information and temporal information about the occurrence of past events. Both modeling and experimental studies have revealed circuits within this network that play crucial roles in encoding space and context. However, our understanding about the time-related aspects of memory is just beginning to be understood. In this review, we first describe updates regarding recent anatomical discoveries for the EC-HPC network, as several important neural circuits critical for memory formation have been discovered by newly developed neural tracing technologies. Second, we examine the complementary roles of multiple medial entorhinal cortical inputs, including newly discovered circuits, into the hippocampus for the temporal and spatial aspects of memory. Finally, we will discuss how temporal and contextual memory information is integrated in HPC cornu ammonis 1 cells. We provide new insights into the neural circuit mechanisms for anatomical and functional segregation and integration of the temporal and spatial aspects of memory encoding in the EC-HPC networks.

Modulation of Neural Networks by Interleukin-1

Interleukin-1 (IL-1) is an inflammatory cytokine that has been shown to modulate neuronal signaling in homeostasis and diseases. In homeostasis, IL-1 regulates sleep and memory formation, whereas in diseases, IL-1 impairs memory and alters affect. Interestingly, IL-1 can cause long-lasting changes in behavior, suggesting IL-1 can alter neuroplasticity. The neuroplastic effects of IL-1 are mediated via its cognate receptor, Interleukin-1 Type 1 Receptor (IL-1R1), and are dependent on the distribution and cell type(s) of IL-1R1 expression. Recent reports found that IL-1R1 expression is restricted to discrete subpopulations of neurons, astrocytes, and endothelial cells and suggest IL-1 can influence neural circuits directly through neuronal IL-1R1 or indirectly via non-neuronal IL-1R1. In this review, we analyzed multiple mechanisms by which IL-1/IL-1R1 signaling might impact neuroplasticity based upon the most up-to-date literature and provided potential explanations to clarify discrepant and confusing findings reported in the past.

Neurogenesis-dependent transformation of hippocampal engrams

In humans and other mammals, memories of events are encoded by neuronal ensembles (or engrams) in the hippocampus. The mnemonic information stored in these engrams can then be used to guide future behavior, including prediction- and decision-making in dynamic environments. While some hippocampal engrams may be persistently stored, others are modified over time, suggesting that the represented memories may also be transformed. How might hippocampal engrams be modified through time? Adult hippocampal neurogenesis represents one process that continuously rewires hippocampal circuitry, presumably including stored hippocampal engrams. At intermediate stages, we propose that neurogenesis-mediated rewiring of hippocampal engram circuitry induces forgetting of specific stimulus attributes, and this less precise engram allows for generalization. At more advanced stages, we propose that neurogenesis-mediated rewiring of hippocampal engram circuitry leads to silencing of hippocampal engrams, rendering them no longer accessible by natural retrieval cues.

Investigating the potential mechanisms of depression induced-by COVID-19 infection in patients

The new coronavirus (COVID-19) has emerged now in the world as a pandemic. The SARS-CoV-2 infection causes variant common symptoms, such as dry cough, tiredness, dyspnea, fever, myalgia, chills, headache, chest pain, and conjunctivitis. Different organs may be affected by COVID-19, such as the respiratory system, gastrointestinal tract, kidneys, and CNS. However, the information about the COVID-19 infection in the CNS is insufficient. We do know that the virus can enter the central nervous system (CNS) via different routes, causing symptoms such as dizziness, headache, seizures, loss of consciousness, and depression. Depression is the most common disorder among all neurological symptoms following COVID-19 infection, although the mechanism of COVID-19-induced depression is not yet clear.

The aim of the present study is to investigate the probable mechanisms of COVID-19-induced depression.

The reasons for depression in infected patients may be due to social and pathological factors including social quarantine, economic problems, stress, changes in the HPA axis, inflammation due to the entry of proinflammatory cytokines into the CNS, production of inflammatory cytokines by microglia, mitochondrial disorders, damage to the hippocampus, and malnutrition.

By evaluating different factors involved in COVID-19-induced depression, we have concluded that depression can be minimized by controlling stress, preventing the cytokine storm with appropriate anti-inflammatory drugs, and proper nutrition.

The potential involvement of cholinergic system in finasteride induced cognitive dysfunction

OBJECTIVE

Neurosteroids are known to exert diverse functions in the brain. 5α-reductase (5α-R), a rate-limiting enzyme involved in the biosynthesis of neurosteroids is inhibited by finasteride. Clinical studies suggest that administration of finasteride causes the emergence of affective symptoms and cognitive dysfunction. Modeling this in rats would provide an opportunity to understand the mechanisms. Accordingly, in the present study, we evaluated the effects of repeated finasteride administration on spatial learning and memory in the partially baited radial arm maze task (RAM) and social cognitive behavior in the social interaction test. Further, to initiate the quest to understand the mechanisms underlying the effects of finasteride, in a separate group of animals, acetylcholinesterase (AChE) activity in the frontal cortex, hippocampus, septum and striatum was estimated.

METHODS

2 months old male Wistar rats were trained to learn a partially baited radial arm maze task (four trials per day till they reach a choice accuracy of 80 %). Following this, rats were administered with either vehicle (HPβCD) or finasteride (30 or 100 mg/Kg, s.c.) for 7 days and then subjected to retention test on the eighth day. To evaluate the social cognition, finasteride was administered for 7 days, followed by social interaction test on the eighth day. All the sessions were video-recorded and analyzed using Noldus Ethovision XT™ software. Following finasteride administration, on the eighth day, rats were euthanized, and AChE activity was estimated by modified Ellman's method.

RESULTS

Finasteride (100 mg/Kg, s.c.) administration decreased the percent correct choice during the retention trial of the RAM task. This was paralleled by an increase in the number of total number of errors and reference memory errors. In the social interaction test, finasteride (100 mg/Kg, s.c.) administration decreased the time spent with the rat compared to the object, implying decreased sociability and diminished social preference evidenced by similar time spent with the novel and familiar rat. Reduced AChE activity was observed in the frontal cortex, hippocampus and septum.

CONCLUSION

Our study provides evidence that repeated administration of finasteride decreases social interaction and results in cognitive deficits, potentially through a cholinergic mechanism. Further studies are required to understand the exact link between the cognitive effects and the cholinergic system. A deeper probe of the current findings holds promise for the development of novel neurosteroid-based therapeutics to treat affective and cognitive disorders.

Adolescent binge-ethanol accelerates cognitive impairment and β-amyloid production and dysregulates endocannabinoid signaling in the hippocampus of APP/PSE mice

Previous research in rodents suggests that the long-term neurobehavioral disturbances induced by chronic ethanol (EtOH) exposure could be due to endocannabinoid system (ECS) alterations. Moreover, ECS failure has been proposed to mediate the cognitive impairment and β-amyloid production in Alzheimer disease (AD). Thus, in the present study, we evaluated the effects of adolescent EtOH binge drinking on the cognitive disturbances, hippocampal β-amyloid levels, and in the ECS expression on a transgenic mouse model (APP/PSEN, AZ) of AD. We exposed AZ and wild-type mice to a binge-drinking treatment during adolescence. At 6 and 12 months of age, we evaluated hippocampal-dependent learning and memory: β-amyloid concentrations and RNA and protein levels of cannabinoid type-2 receptors (CB2), diacylglycerol lipase-α (DAGLα), and monoacylglycerol lipase (MAGL) in the hippocampus. The results showed that binge-EtOH treatment worsens cognitive function and increases β-amyloid levels in AZ. At 6 months, EtOH heightens CB2 (RNA and protein) and DAGLα (RNA) expression in wild type but not in AZ. On the contrary, EtOH enhances MAGL RNA expression only in AZ. At 12 months, AZ displays increased levels of CB2 (RNA and protein) and DAGLα (protein) compared with control. Similar to what happens at 6 months, EtOH induces an increase in CB2 gene expression in wild type but not in AZ; however, it augments CB2 and DAGLα protein levels in both genotypes. Therefore, we propose that adolescent binge drinking accelerates cognitive deficits associated with aging and AD. It also accelerates hippocampal β-amyloid accumulation in AZ and affects differently the ECS response in wild type and AZ.

Cell-Type-Specific Optogenetic Techniques Reveal Neural Circuits Crucial for Episodic Memories

The formation and maintenance of episodic memories are important for our daily life. Accumulating evidence from extensive studies with pharmacological, electrophysiological, and molecular biological approaches has shown that both entorhinal cortex (EC) and hippocampus (HPC) are crucial for the formation and recall of episodic memory. However, to further understand the neural mechanisms of episodic memory processes in the EC-HPC network, cell-type-specific manipulation of neural activity with high temporal resolution during memory process has become necessary. Recently, the technological innovation of optogenetics combined with pharmacological, molecular biological, and electrophysiological approaches has significantly advanced our understanding of the circuit mechanisms for learning and memory. Optogenetic techniques with transgenic mice and/or viral vectors enable us to manipulate the neural activity of specific cell populations as well as specific neural projections with millisecond-scale temporal control during animal behavior. Integrating optogenetics with drug-regulatable activity-dependent gene expression systems has identified memory engram cells, which are a subpopulation of cells that encode a specific episode. Finally, millisecond pulse stimulation of neural activity by optogenetics has further achieved (a) identification of synaptic connectivity between targeted pairs of neural populations, (b) cell-type-specific single-unit electrophysiological recordings, and (c) artificial induction and modification of synaptic plasticity in targeted synapses. In this chapter, we summarize technological and conceptual advancements in the field of neurobiology of learning and memory as revealed by optogenetic approaches in the rodent EC-HPC network for episodic memories.

α2-Antiplasmin as a potential regulator of the spatial memory process and age-related cognitive decline

α2-Antiplasmin (α2AP), a principal physiological plasmin inhibitor, is mainly produced by the liver and kidneys, but it is also expressed in several parts of the brain, including the hippocampus and cerebral cortex. Our previous study demonstrated that α2AP knockout mice exhibit spatial memory impairment in comparison to wild-type mice, suggesting that α2AP is necessary for the fetal and/or neonatal development of the neural network for spatial memory. However, it is still unclear whether α2AP plays a role in the memory process. The present study demonstrated that adult hippocampal neurogenesis and remote spatial memory were enhanced by the injection of an anti-α2AP neutralizing antibody in WT mice, while the injection of α2AP reduced hippocampal neurogenesis and impaired remote spatial memory, suggesting that α2AP is a negative regulator in memory processing. The present study also found that the levels of α2AP in the brains of old mice were higher than those in young mice, and a negative correlation between the α2AP level and spatial working memory. In addition, aging-dependent brain oxidative stress and hippocampal inflammation were attenuated by α2AP deficiency. Thus, an age-related increase in α2AP might cause cognitive decline accompanied by brain oxidative stress and neuroinflammation. Taken together, our findings suggest that α2AP is a key regulator of the spatial memory process, and that it may represent a promising target to effectively regulate healthy brain aging.

Functional deficit in hippocampal activity during fear extinction recall in the single prolonged-stress model of PTSD in male rats

To interrogate whether altered function of the hippocampal-mPFC circuit underlies the deficit in fear extinction recall in rats subjected to single-prolonged stress (SPS), changes in brain region-specific metabolic rate were measured in male rats (control and SPS treated). Brain region metabolic rates were quantified using uptake of ¹⁴C-2-deoxyglucose (¹⁴C-2DG) during fear memory formation, fear memory extinction and extinction recall. Control and SPS rats had similar regional brain activities at baseline. During extinction recall, ¹⁴C-2DG uptake decreased in hippocampal regions in control rats, but not in SPS rats. SPS rats also exhibited a significant deficiency in fear extinction recall, replicating a previously reported finding. Reduced hippocampal activity during fear extinction recall in control animals may reflect reduction in fear overgeneralization, thereby enabling discrimination between distinct contexts. In contrast, persistent levels of hippocampal activity in SPS-exposed male animals during fear extinction recall may reflect the dysfunctional persistence of fear overgeneralization. Future studies in females can test gender-specificity of these effects, with appropriate attention to luteal dependent effects on extinction of fear learning. Detailed knowledge of regional brain activities underlying stress-induced deficits in extinction recall may help identify therapeutic targets in PTSD.

Metabolomics Analysis of Hippocampus and Cortex in a Rat Model of Traumatic Brain Injury in the Subacute Phase

Traumatic brain injury (TBI) is a complex and serious disease as its multifaceted pathophysiological mechanisms remain vague. The molecular changes of hippocampal and cortical dysfunction in the process of TBI are poorly understood, especially their chronic effects on metabolic profiles. Here we utilize metabolomics-based liquid chromatography coupled with tandem mass spectrometry coupled with bioinformatics method to assess the perturbation of brain metabolism in rat hippocampus and cortex on day 7. The results revealed a signature panel which consisted of 13 identified metabolites to facilitate targeted interventions for subacute TBI discrimination. Purine metabolism change in cortical tissue and taurine and hypotaurine metabolism change in hippocampal tissue were detected. Furthermore, the associations between the metabolite markers and the perturbed pathways were analyzed based on databases: 64 enzyme and one pathway were evolved in TBI. The findings represented significant profiling changes and provided unique metabolite-protein information in a rat model of TBI following the subacute phase. This study may inspire scientists and doctors to further their studies and provide potential therapy targets for clinical interventions.

Effects of neonatal isoflurane anesthesia exposure on learning-specific and sensory systems in adults

Millions of children undergo general anesthesia each year, and animal and human studies have indicated that exposure to anesthesia at an early age can impact neuronal development, leading to behavioral and learning impairments that manifest later in childhood and adolescence. Here, we examined the effects of isoflurane, a commonly-used general anesthetic, which was delivered to newborn rabbits. Trace eyeblink classical conditioning was used to assess the impact of neonatal anesthesia exposure on behavioral learning in adolescent subjects, and a variety of MRI techniques including fMRI, MR volumetry, spectroscopy and DTI captured functional, metabolic, and structural changes in key regions of the learning and sensory systems associated with anesthesia-induced learning impairment. Our results demonstrated a wide array of changes that were specific to anesthesia-exposed subjects, which supports previous studies that have pointed to a link between early anesthesia exposure and the development of learning and behavioral deficiencies. These findings point to the need for caution in avoiding excessive use of general anesthesia in young children and neonates.

eEF2/eEF2K Pathway in the Mature Dentate Gyrus Determines Neurogenesis Level and Cognition

Levels of adult neurogenesis in the dentate gyrus (DG) of the hippocampus are correlated with unique cognitive functions. However, the molecular pathways controlling it are poorly understood. Here, we found that the known physiological ways to enhance neurogenesis converged on the eEF2/eEF2K pathway via AMPK in the DG. Enhancing the elongation phase of mRNA translation in eEF2K-knockout (eEF2K-KO) mice induced the expression of neurogenesis-related proteins in the hippocampus. We thus tested the hypothesis that inducing eEF2K-KO in mature neurons of the DG controls neurogenesis. Indeed, both general eEF2K-KO and targeted KO in DG excitatory mature neurons resulted in enhanced neurogenesis levels and upregulation of neurogenesis-related proteins. Increased neurogenesis was correlated with enhanced performance in DG-dependent learning. Moreover, general and local eEF2K-KO in old mice rejuvenated the DG, paving the way for better mechanistic understanding of how neurogenesis is controlled in the mature DG and possible treatments for incurable aging-associated diseases.

Traumatic brain injury and hippocampal neurogenesis: Functional implications

In the adult brain, self-renewing radial-glia like (RGL) progenitor cells have been shown to reside in the subventricular zone and the subgranular zone of the hippocampus. A large body of evidence shows that experiences such as learning, enriched environment and stress can alter proliferation and differentiation of RGL progenitor cells. The progenitor cells present in the subgranular zone of the hippocampus divide to give rise to newborn neurons that migrate to the dentate gyrus where they differentiate into adult granule neurons. These newborn neurons have been found to have a unique role in certain types of hippocampus-dependent learning and memory, including goal-directed behaviors that require pattern separation. Experimental traumatic brain injury (TBI) in rodents has been shown to alter hippocampal neurogenesis, including triggering the acute loss of newborn neurons, as well as progenitor cell hyper-proliferation. In this review, we discuss the role of hippocampal neurogenesis in learning and memory. Furthermore, we review evidence for the molecular mechanisms that contribute to newborn neuron loss, as well as increased progenitor cell proliferation after TBI. Finally, we discuss strategies aimed at enhancing neurogenesis after TBI and their possible therapeutic benefits.

How TRPC Channels Modulate Hippocampal Function

Transient receptor potential canonical (TRPC) proteins constitute a group of receptor-operated calcium-permeable nonselective cationic membrane channels of the TRP superfamily. They are largely expressed in the hippocampus and are able to modulate neuronal functions. Accordingly, they have been involved in different hippocampal functions such as learning processes and different types of memories, as well as hippocampal dysfunctions such as seizures. This review covers the mechanisms of activation of these channels, how these channels can modulate neuronal excitability, in particular the after-burst hyperpolarization, and in the persistent activity, how they control synaptic plasticity including pre- and postsynaptic processes and how they can interfere with cell survival and neurogenesis.

Functional neurogenesis over the years

There has been interest in the function of adult neurogenesis since its discovery, by Joseph Altman, nearly 60 years ago. While controversy curtailed follow up studies, in the 1990s a second wave of research validated many of Altman's original claims and revealed that factors such as stress and environmental stimulation altered the production of new neurons in the hippocampus. However, only with the advent of tools for manipulating neurogenesis did it become possible to perform causal tests of the function of newborn neurons. Here, we identify approximately 100 studies in which adult neurogenesis was manipulated to study its function. A majority of these studies demonstrate functions for adult neurogenesis in classic hippocampal behaviors such as context learning and spatial memory, as well as emotional behaviors related to stress, anxiety and depression. However, a closer look reveals a number of other, arguably understudied, functions in decision making, temporal association memory, and addiction. In this special issue, we present 16 new studies and review articles that continue to address and clarify the function of adult neurogenesis in behaviors as diverse as memory formation, consolidation and forgetting, pattern separation and discrimination behaviors, addiction, and attention. Reviews of stem cell dynamics and regenerative properties provide insights into the mechanisms by which neurogenesis may be controlled to offset age- and disease-related brain injury. Finally, translation-oriented reviews identify next steps for minimizing the gap between discoveries made in animals and applications for human health. The articles in this issue synthesize and extend what we have learned in the last half century of functional neurogenesis research and identify themes that will define its future.

Increasing neurogenesis refines hippocampal activity rejuvenating navigational learning strategies and contextual memory throughout life

Functional plasticity of the brain decreases during ageing causing marked deficits in contextual learning, allocentric navigation and episodic memory. Adult neurogenesis is a prime example of hippocampal plasticity promoting the contextualisation of information and dramatically decreases during ageing. We found that a genetically-driven expansion of neural stem cells by overexpression of the cell cycle regulators Cdk4/cyclinD1 compensated the age-related decline in neurogenesis. This triggered an overall inhibitory effect on the trisynaptic hippocampal circuit resulting in a changed profile of CA1 sharp-wave ripples known to underlie memory consolidation. Most importantly, increased neurogenesis rescued the age-related switch from hippocampal to striatal learning strategies by rescuing allocentric navigation and contextual memory. Our study demonstrates that critical aspects of hippocampal function can be reversed in old age, or compensated throughout life, by exploiting the brain’s endogenous reserve of neural stem cells.

Ageing affects several brain areas causing a decrease in cognitive abilities and memory. We find that increasing the endogenous potential of the hippocampus to generate new neurons throughout life rejuvenates learning and memory, indicating that neural reserves can be exploited during ageing to compensate for age- or disease-related cognitive impairments.