Voluntary Running Exercise-Mediated Enhanced Neurogenesis Does Not Obliterate Retrograde Spatial Memory

Neurobiology and systems biology of stress resilience

Stress resilience is the phenomenon that some people maintain their mental health despite exposure to adversity or show only temporary impairments followed by quick recovery. Resilience research attempts to unravel the factors and mechanisms that make resilience possible and to harness its insights for the development of preventative interventions in individuals at risk for acquiring stress-related dysfunctions. Biological resilience research has been lagging behind the psychological and social sciences but has seen a massive surge in recent years. At the same time, progress in this field has been hampered by methodological challenges related to finding suitable operationalizations and study designs, replicating findings, and modeling resilience in animals. We embed a review of behavioral, neuroimaging, neurobiological, and systems biological findings in adults in a critical methods discussion. We find preliminary evidence that hippocampus-based pattern separation and prefrontal-based cognitive control functions protect against the development of pathological fears in the aftermath of singular, event-type stressors [as found in fear-related disorders, including simpler forms of posttraumatic stress disorder (PTSD)] by facilitating the perception of safety. Reward system-based pursuit and savoring of positive reinforcers appear to protect against the development of more generalized dysfunctions of the anxious-depressive spectrum resulting from more severe or longer-lasting stressors (as in depression, generalized or comorbid anxiety, or severe PTSD). Links between preserved functioning of these neural systems under stress and neuroplasticity, immunoregulation, gut microbiome composition, and integrity of the gut barrier and the blood-brain barrier are beginning to emerge. On this basis, avenues for biological interventions are pointed out.

Voluntary exercise during puberty promotes spatial memory and hippocampal DG/CA3 synaptic transmission in mice

Physical exercise has been shown to have an impact on memory and hippocampal function across different age groups. Nevertheless, the influence and mechanisms underlying how voluntary exercise during puberty affects memory are still inadequately comprehended. This research aims to examine the impacts of self-initiated physical activity throughout adolescence on spatial memory. Developing mice were exposed to a 4-wk voluntary wheel running exercise protocol, commencing at the age of 30 d. After engaging in voluntary wheel running exercise during development, there was an enhancement in spatial memory. Moreover, hippocampal dentate gyrus and CA3 neurons rather than CA1 neurons exhibited an increase in the miniature excitatory postsynaptic currents and miniature inhibitory postsynaptic currents. In addition, there was an increase in the expression of NR2A/NR2B subunits of N-methyl-D-aspartate receptors and α1GABAA subunit of gamma-aminobutyric acid type A receptors, as well as dendritic spine density, specifically within dentate gyrus and CA3 regions rather than CA1 region. The findings suggest that voluntary exercise during development can enhance spatial memory in mice by increasing synapse numbers and improving synaptic transmission in hippocampal dentate gyrus and CA3 regions, but not in CA1 region. This study sheds light on the neural mechanisms underlying how early-life exercise improves cognitive function.

Extracellular vesicles from hiPSC-NSCs can prevent peripheral inflammation-induced cognitive dysfunction with inflammasome inhibition and improved neurogenesis in the hippocampus

Extracellular vesicles (EVs) released by human induced pluripotent stem cell-derived neural stem cells (hiPSC-NSCs) are enriched with miRNAs and proteins capable of mediating robust antiinflammatory activity. The lack of tumorigenic and immunogenic properties and ability to permeate the entire brain to incorporate into microglia following intranasal (IN) administrations makes them an attractive biologic for curtailing chronic neuroinflammation in neurodegenerative disorders. We tested the hypothesis that IN administrations of hiPSC-NSC-EVs can alleviate chronic neuroinflammation and cognitive impairments induced by the peripheral lipopolysaccharide (LPS) challenge. Adult male, C57BL/6J mice received intraperitoneal injections of LPS (0.75 mg/kg) for seven consecutive days. Then, the mice received either vehicle (VEH) or hiPSC-NSC-EVs (~ 10 × 109 EVs/administration, thrice over 6 days). A month later, mice in all groups were investigated for cognitive function with behavioral tests and euthanized for histological and biochemical studies. Mice receiving VEH after LPS displayed deficits in associative recognition memory, temporal pattern processing, and pattern separation. Such impairments were associated with an increased incidence of activated microglia presenting NOD-, LRR-, and pyrin domain containing 3 (NLRP3) inflammasomes, elevated levels of NLRP3 inflammasome mediators and end products, and decreased neurogenesis in the hippocampus. In contrast, the various cognitive measures in mice receiving hiPSC-NSC-EVs after LPS were closer to naive mice. Significantly, these mice displayed diminished microglial activation, NLRP3 inflammasomes, proinflammatory cytokines, and a level of neurogenesis matching age-matched naïve controls. Thus, IN administrations of hiPSC-NSC-EVs are an efficacious approach to reducing chronic neuroinflammation-induced cognitive impairments.

Role of Microglia, Decreased Neurogenesis and Oligodendrocyte Depletion in Long COVID-Mediated Brain Impairments

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of a recent worldwide coronavirus disease-2019 (COVID-19) pandemic. SARS-CoV-2 primarily causes an acute respiratory infection but can progress into significant neurological complications in some. Moreover, patients with severe acute COVID-19 could develop debilitating long-term sequela. Long-COVID is characterized by chronic symptoms that persist months after the initial infection. Common complaints are fatigue, myalgias, depression, anxiety, and "brain fog," or cognitive and memory impairments. A recent study demonstrated that a mild COVID-19 respiratory infection could generate elevated proinflammatory cytokines and chemokines in the cerebral spinal fluid. This commentary discusses findings from this study, demonstrating that even a mild respiratory SARS-CoV-2 infection can cause considerable neuroinflammation with microglial and macrophage reactivity. Such changes could also be gleaned by measuring chemokines and cytokines in the circulating blood. Moreover, neuroinflammation caused by mild SARS-CoV-2 infection can also impair hippocampal neurogenesis, deplete oligodendrocytes, and decrease myelinated axons. All these changes likely contribute to cognitive deficits in long-COVID syndrome. Therefore, strategies capable of restraining neuroinflammation, maintaining better hippocampal neurogenesis, and preserving oligodendrocyte lineage differentiation and maturation may prevent or reduce the incidence of long-COVID after SARS-CoV-2 respiratory infection.

Neural stem cells persist to generate new neurons in the hippocampus of adult and aged human brain - Fiction or accurate?

Adult neurogenesis, comprising the generation, differentiation and integration of new neurons in the mature brain, has emerged as a dynamic area of research over decades. The discovery of adult neurogenesis was a paradigm shift to comprehend mechanisms underlying brain plasticity, cognitive function, and neurological disorders. This review deliberates significant findings from articles published over four decades on adult neurogenesis, highlighting key milestones, methodological advances, and controversies that have shaped our comprehension of the phenomenon of adult neurogenesis. Early skepticism gave way to a rich body of evidence via various reliable approaches. Studies on neurogenic niches, microenvironmental factors, molecular regulators, and functional implications have uncovered the involvement of adult neurogenesis in learning, memory, mood, and even neurological and neurodegenerative conditions. Despite significant progress, several questions still need to be answered, including the exact contributions of new neurons to brain function, their integration into existing circuits, and the impact of enhancing adult neurogenesis in the human hippocampus. While the existence of robust neurogenesis in the adult and aged human hippocampus is yet to be confirmed, this review highlights evidence from a significant number of studies supporting the persistence of hippocampal neurogenesis during adulthood and aging in humans, including in some neurological conditions, such as epilepsy and Alzheimer's disease. Nonetheless, additional large-scale studies using single cell-RNA-seq, single nucleus-RNA-seq, and spatial transcriptomics are critical to validate the presence and contribution of hippocampal neurogenesis in the pathophysiology of various neurological and neurodegenerative conditions at different stages of the disease. There is also a need to develop standardized protocols for analyzing postmortem hippocampal tissues for cellular and molecular analyses.

Pathophysiological basis and promise of experimental therapies for Gulf War Illness, a chronic neuropsychiatric syndrome in veterans

This article describes the pathophysiology and potential treatments for Gulf War Illness (GWI), which is a chronic neuropsychiatric illness linked to a combination of chemical exposures experienced by service personnel during the first Gulf War in 1991. However, there is currently no effective treatment for veterans with GWI. The article focuses on the current status and efficacy of existing therapeutic interventions in preclinical models of GWI, as well as potential perspectives of promising therapies. GWI stems from changes in brain and peripheral systems in veterans, leading to neurocognitive deficits, as well as physiological and psychological effects resulting from multifaceted changes such as neuroinflammation, oxidative stress, and neuronal damage. Aging not only renders veterans more susceptible to GWI symptoms, but also attenuates their immune capabilities and response to therapies. A variety of experimental models are being used to investigate the pathophysiology and develop therapies that have the ability to alleviate devastating symptoms. Over two dozen therapeutic interventions targeting neuroinflammation, mitochondrial dysfunction, neuronal injury, and neurogenesis are being tested, including agents such as curcumin, curcumin nanoparticles, monosodium luminol, melatonin, resveratrol, fluoxetine, rolipram, oleoylethanolamide, ketamine, levetiracetam, nicotinamide riboside, minocycline, pyridazine derivatives, and neurosteroids. Preclinical outcomes show that some agents have promise, including curcumin, resveratrol, and ketamine, which are being tested in clinical trials in GWI veterans. Neuroprotectants and other compounds such as monosodium luminol, melatonin, levetiracetam, oleoylethanolamide, and nicotinamide riboside appear promising for future clinical trials. Neurosteroids have been shown to have neuroprotective and disease-modifying properties, which makes them a promising medicine for GWI. Therefore, accelerated clinical studies are urgently needed to evaluate and launch an effective therapy for veterans displaying GWI.

Intranasally administered human MSC-derived extracellular vesicles inhibit NLRP3-p38/MAPK signaling after TBI and prevent chronic brain dysfunction

Traumatic brain injury (TBI) leads to lasting brain dysfunction with chronic neuroinflammation typified by nucleotide-binding domain leucine-rich repeat and pyrin domain-containing receptor 3 (NLRP3) inflammasome activation in microglia. This study probed whether a single intranasal (IN) administration of human mesenchymal stem cell-derived extracellular vesicles (hMSC-EVs) naturally enriched with activated microglia-modulating miRNAs can avert chronic adverse outcomes of TBI. Small RNA sequencing confirmed the enrichment of miRNAs capable of modulating activated microglia in hMSC-EV cargo. IN administration of hMSC-EVs into adult mice ninety minutes after the induction of a unilateral controlled cortical impact injury resulted in their incorporation into neurons and microglia in both injured and contralateral hemispheres. A single higher dose hMSC-EV treatment also inhibited NLRP3 inflammasome activation after TBI, evidenced by reduced NLRP3, apoptosis-associated speck-like protein containing a CARD, activated caspase-1, interleukin-1 beta, and IL-18 levels in the injured brain. Such inhibition in the acute phase of TBI endured in the chronic phase, which could also be gleaned from diminished NLRP3 inflammasome activation in microglia of TBI mice receiving hMSC-EVs. Proteomic analysis and validation revealed that higher dose hMSC-EV treatment thwarted the chronic activation of the p38 mitogen-activated protein kinase (MAPK) signaling pathway by IL-18, which decreased the release of proinflammatory cytokines. Inhibition of the chronic activation of NLRP3-p38/MAPK signaling after TBI also prevented long-term cognitive and mood impairments. Notably, the animals receiving higher doses of hMSC-EVs after TBI displayed better cognitive and mood function in all behavioral tests than animals receiving the vehicle after TBI. A lower dose of hMSC-EV treatment also partially improved cognitive and mood function. Thus, an optimal IN dose of hMSC-EVs naturally enriched with activated microglia-modulating miRNAs can inhibit the chronic activation of NLRP3-p38/MAPK signaling after TBI and prevent lasting brain dysfunction.

Adult-born neurons add flexibility to hippocampal memories

Although most neurons are generated embryonically, neurogenesis is maintained at low rates in specific brain areas throughout adulthood, including the dentate gyrus of the mammalian hippocampus. Episodic-like memories encoded in the hippocampus require the dentate gyrus to decorrelate similar experiences by generating distinct neuronal representations from overlapping inputs (pattern separation). Adult-born neurons integrating into the dentate gyrus circuit compete with resident mature cells for neuronal inputs and outputs, and recruit inhibitory circuits to limit hippocampal activity. They display transient hyperexcitability and hyperplasticity during maturation, making them more likely to be recruited by any given experience. Behavioral evidence suggests that adult-born neurons support pattern separation in the rodent dentate gyrus during encoding, and they have been proposed to provide a temporal stamp to memories encoded in close succession. The constant addition of neurons gradually degrades old connections, promoting generalization and ultimately forgetting of remote memories in the hippocampus. This makes space for new memories, preventing saturation and interference. Overall, a small population of adult-born neurons appears to make a unique contribution to hippocampal information encoding and removal. Although several inconsistencies regarding the functional relevance of neurogenesis remain, in this review we argue that immature neurons confer a unique form of transience on the dentate gyrus that complements synaptic plasticity to help animals flexibly adapt to changing environments.

Promise of irisin to attenuate cognitive dysfunction in aging and Alzheimer's disease

Strategies proficient for relieving cognitive impairments in aging and Alzheimer's disease (AD) have an enormous impact. Regular physical exercise (PE) can prevent age-related dementia and slow down AD progression. However, such a lifestyle change is likely not achievable for individuals displaying age-related frailty. Hence, drugs or biologics that could simulate the benefits of PE have received much attention. Previous studies suggested that the fibronectin-domain III containing 5 (FNDC5) underlies the PE-mediated improved cognitive function. A recent study reports that PE-related cognitive benefits in aging and AD are mediated by irisin, the cleaved form of FNDC5 released into the blood after PE. Such a conclusion was apparent from the deletion of irisin through a global knockout of FNDC5, leading to the loss of PE-induced cognitive benefits or inducing memory impairments in adult or aged models. Furthermore, in AD models, peripherally administered irisin mimicked the cognitive benefits of PE by modulating neuroinflammation. This short review discusses the promise of irisin to simulate the cognitive benefits of PE in age- and AD-related dementia. In addition, critical issues such as how blood-borne irisin acts on neural cells, the role of the brain-derived neurotrophic factor in irisin-mediated cognitive benefits, and irisin's ability to inhibit neuroinflammatory cascades in aging and AD are discussed.

Oral Nano-Curcumin in a Model of Chronic Gulf War Illness Alleviates Brain Dysfunction with Modulation of Oxidative Stress, Mitochondrial Function, Neuroinflammation, Neurogenesis, and Gene Expression

Unrelenting cognitive and mood impairments concomitant with incessant oxidative stress and neuroinflammation are among the significant symptoms of chronic Gulf War Illness (GWI). Curcumin (CUR), an antiinflammatory compound, has shown promise to alleviate brain dysfunction in a model of GWI following intraperitoneal administrations at a high dose. However, low bioavailability after oral treatment has hampered its clinical translation. Therefore, this study investigated the efficacy of low-dose, intermittent, oral polymer nanoparticle encapsulated CUR (nCUR) for improving brain function in a rat model of chronic GWI. Intermittent administration of 10 or 20 mg/Kg nCUR for 8 weeks in the early phase of GWI improved brain function and reduced oxidative stress (OS) and neuroinflammation. We next examined the efficacy of 12-weeks of intermittent nCUR at 10 mg/Kg in GWI animals, with treatment commencing 8 months after exposure to GWI-related chemicals and stress, mimicking treatment for the persistent cognitive and mood dysfunction displayed by veterans with GWI. GWI rats receiving nCUR exhibited better cognitive and mood function associated with improved mitochondrial function and diminished neuroinflammation in the hippocampus. Improved mitochondrial function was evident from normalized expression of OS markers, antioxidants, and mitochondrial electron transport genes, and complex proteins. Lessened neuroinflammation was noticeable from reductions in astrocyte hypertrophy, NF-kB, activated microglia with NLRP3 inflammasomes, and multiple proinflammatory cytokines. Moreover, nCUR treated animals displayed enhanced neurogenesis with a normalized expression of synaptophysin puncta, and multiple genes linked to cognitive dysfunction. Thus, low-dose, intermittent, oral nCUR therapy has promise for improving brain function in veterans with GWI.

The mediating effect of engagement in physical activity over a 24-hour period on chronic disease and depression: Using compositional mediation model

BACKGROUND

Popular mediation method only considers a single activity as a mediator instead of all 24-hour physical activity, such as a four-way decomposition method. We investigated the mediation of 24-hour movement continuum between chronic diseases (cardiovascular disease [CVD] and diabetes) and depression using a novel compositional mediation model.

METHODS

24-hour activity data measured by accelerometer were obtained from NHANES 2005-2006. Adjusted coefficient with 95% confidence interval (95% CI) for PHQ-9 total score and adjusted odds ratio (OR) with 95% CI for depression were computed from compositional mediation models.

RESULTS

In total, 2,375 participants aged ≥ 20 were included in our analysis. Both diabetes and CVD were associated with increased sedentary behavior (SB) and sleep and reduced moderate-to-vigorous physical activity (MVPA) and light-intensity physical activity (LPA), leading to an increased likelihood of depression. Although not all component indirect effects were associated with PHQ-9 total score and depression, the total indirect effect was significantly associated with both PHQ-9 total score (coefficient (95% CI) for diabetes: 0.162 (0.081, 0.261); coefficient (95% CI) for CVD: 1.139 (1.061, 1.240)) and depression (OR (95% CI) for diabetes: 0.235 (0.126, 0.362); OR (95% CI) for CVD: 1.200 (1.088, 1.346)) using the compositional mediation model.

CONCLUSION

We developed a compositional mediation model for continuous and binary outcomes, which can handle entire compositional mediators as a unit. The mediation of 24-hour movement continuum mediated the association between diabetes, CVD, and depression. Our findings present potential interventions for reducing risk of depression among patients with CVD and diabetes.

Voluntary Exercise Increases Neurogenesis and Mediates Forgetting of Complex Paired Associates Memories

The hippocampus is a critical structure involved in many forms of learning and memory. It is also one of the only regions in the mammalian brain that continues to generate new neurons throughout adulthood. This process of adult neurogenesis may increase the plasticity of the hippocampus which could be beneficial for learning but has also been demonstrated to decrease the stability of previously acquired memories. Here we test whether exposure to voluntary running (which increases the production of new neurons) following the formation of a gradually acquired paired associates task will result in forgetting of this type of memory. We trained mice in a touchscreen-based object/location task and then increased neurogenesis using voluntary running. Our results indicate that running increased neurogenesis and resulted in poor recall of the previously established memory. When subsequently exposed to a reversal task we also show that running reduced the number of correction trials required to acquire the new task contingencies. This suggests that prior forgetting reduces perseveration on the now outdated memory. Together our results add to a growing body of literature which indicates the important role of adult neurogenesis in destabilizing previously acquired memories to allow for flexible encoding of new memories.

Moderate, intermittent voluntary exercise in a model of Gulf War Illness improves cognitive and mood function with alleviation of activated microglia and astrocytes, and enhanced neurogenesis in the hippocampus

Persistent cognitive and mood impairments in Gulf War Illness (GWI) are associated with chronic neuroinflammation, typified by hypertrophied astrocytes, activated microglia, and increased proinflammatory mediators in the brain. Using a rat model, we investigated whether a simple lifestyle change such as moderate voluntary physical exercise would improve cognitive and mood function in GWI. Because veterans with GWI exhibit fatigue and post-exertional malaise, we employed an intermittent voluntary running exercise (RE) regimen, which prevented exercise-induced stress. The GWI rats were provided access to running wheels three days per week for 13 weeks, commencing ten weeks after the exposure to GWI-related chemicals and stress (GWI-RE group). Groups of age-matched sedentary GWI rats (GWI-SED group) and naïve rats were maintained parallelly. Interrogation of rats with behavioral tests after the 13-week RE regimen revealed improved hippocampus-dependent object location memory and pattern separation function and reduced anxiety-like behavior in the GWI-RE group compared to the GWI-SED group. Moreover, 13 weeks of RE in GWI rats significantly reversed activated microglia with short and less ramified processes into non-inflammatory/antiinflammatory microglia with highly ramified processes and reduced the hypertrophy of astrocytes. Moreover, the production of new neurons in the hippocampus was enhanced when examined eight weeks after the commencement of RE. Notably, increased neurogenesis continued even after the cessation of RE. Collectively, the results suggest that even a moderate, intermittent physical exercise has the promise to improve brain function in veterans with GWI in association with suppression of neuroinflammation and enhancement of hippocampal neurogenesis.

Adult neurogenesis mediates forgetting of multiple types of memory in the rat

The formation and retention of hippocampus-dependent memories is impacted by neurogenesis, a process that involves the production of new neurons in the dentate gyrus of the hippocampus. Recent studies demonstrate that increasing neurogenesis after memory formation induces forgetting of previously acquired memories. Neurogenesis-induced forgetting was originally demonstrated in mice, but a recent report suggests that the same effect may be absent in rats. Although a general species difference is possible, other potential explanations for these incongruent findings are that memories which are more strongly reinforced become resilient to forgetting or that perhaps only certain types of memories are affected. Here, we investigated whether neurogenesis-induced forgetting occurs in rats using several hippocampus-dependent tasks including contextual fear conditioning (CFC), the Morris Water Task (MWT), and touchscreen paired associates learning (PAL). Neurogenesis was increased following training using voluntary exercise for 4 weeks before recall of the previous memory was assessed. We show that voluntary running causes forgetting of context fear memories in a neurogenesis-dependent manner, and that neurogenesis-induced forgetting is present in rats across behavioral tasks despite differences in complexity or reliance on spatial, context, or object memories. In addition, we asked whether stronger memories are less susceptible to forgetting by varying the strength of training. Even with a very strong training protocol in the CFC task, we still observed enhanced forgetting related to increased neurogenesis. These results suggest that forgetting due to neurogenesis is a conserved mechanism that aids in the clearance of memories.

Angiogenin in the Neurogenic Subventricular Zone After Stroke

Ischemic stroke is a leading cause of death and disability worldwide with effective acute thrombolytic treatments. However, brain repair mechanisms related to spontaneous or rehabilitation-induced recovery are still under investigation, and little is known about the molecules involved. The present study examines the potential role of angiogenin (ANG), a known regulator of cell function and metabolism linked to neurological disorders, focusing in the neurogenic subventricular zone (SVZ). Angiogenin expression was examined in the mouse SVZ and in SVZ-derived neural stem cells (NSCs), which were exposed to exogenous ANG treatment during neurosphere formation as well as in other neuron-like cells (SH-SY5Y). Additionally, male C57Bl/6 mice underwent a distal permanent occlusion of the middle cerebral artery to study endogenous and exercise-induced expression of SVZ-ANG and neuroblast migration. Our results show that SVZ areas are rich in ANG, primarily expressed in DCX+ neuroblasts but not in nestin+NSCs. In vitro, treatment with ANG increased the number of SVZ-derived NSCs forming neurospheres but could not modify SH-SY5Y neurite differentiation. Finally, physical exercise rapidly increased the amount of endogenous ANG in the ipsilateral SVZ niche after ischemia, where DCX-migrating cells increased as part of the post-stroke neurogenesis process. Our findings position for the first time ANG in the SVZ during post-stroke recovery, which could be linked to neurogenesis.

Melatonin improves brain function in a model of chronic Gulf War Illness with modulation of oxidative stress, NLRP3 inflammasomes, and BDNF-ERK-CREB pathway in the hippocampus

Persistent cognitive and mood dysfunction is the primary CNS symptom in veterans afflicted with Gulf War Illness (GWI). This study investigated the efficacy of melatonin (MEL) for improving cognitive and mood function with antioxidant, antiinflammatory, and pro-cognitive effects in a rat model of chronic GWI. Six months after exposure to GWI-related chemicals and stress, rats were treated with vehicle or MEL (5, 10, 20, 40, and 80 mg/kg) for eight weeks. Behavioral tests revealed cognitive and mood dysfunction in GWI rats receiving vehicle, which were associated with elevated oxidative stress, reduced NRF2, catalase and mitochondrial complex proteins, astrocyte hypertrophy, activated microglia with NLRP3 inflammasomes, elevated proinflammatory cytokines, waned neurogenesis, and synapse loss in the hippocampus. MEL at 10 mg/kg alleviated simple and associative recognition memory dysfunction and anhedonia, along with reduced oxidative stress, enhanced glutathione and complex III, and reduced NLRP3 inflammasomes, IL-18, TNF-α, and IFN-γ. MEL at 20 mg/kg also normalized NRF2 and catalase and increased microglial ramification. MEL at 40 mg/kg, in addition, reduced astrocyte hypertrophy, activated microglia, NF-kB-NLRP3-caspase-1 signaling, IL-1β, MCP-1, and MIP-1α. Moreover, MEL at 80 mg/kg activated the BDNF-ERK-CREB signaling pathway, enhanced neurogenesis and diminished synapse loss in the hippocampus, and improved a more complex hippocampus-dependent cognitive function. Thus, MEL therapy is efficacious for improving cognitive and mood function in a rat model of chronic GWI, and MEL's effect was dose-dependent. The study provides the first evidence of MEL's promise for alleviating neuroinflammation and cognitive and mood impairments in veterans with chronic GWI.

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A low dose of Melatonin alleviated recognition memory dysfunction and anhedonia in a model of chronic GWI.

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A moderate dose of Melatonin improved more complex cognitive function in a model of chronic GWI.

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Melatonin treatment reduced oxidative stress and enhanced mitochondrial complex proteins in the GWI brain.

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Melatonin inhibited NLRP3 inflammasomes and proinflammatory cytokines in the GWI brain.

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Melatonin activated the BDNF-ERK-CREB signaling pathway and enhanced neurogenesis in the GWI brain.

On the novel mechanisms for social memory and the emerging role of neurogenesis

Social memory (SM) is a key element in social cognition and it encompasses the neural representation of conspecifics, an essential information to guide behavior in a social context. Here we evaluate classical and cutting-edge studies on neurobiology of SM, using as a guiding principle behavioral tasks performed in adult rodents. Our review highlights the relevance of the hippocampus, especially the CA2 region, as a neural substrate for SM and suggest that neural ensembles in the olfactory bulb may also encode SM traces. Compared to other hippocampus-dependent memories, much remains to be done to describe the neurobiological foundations of SM. Nonetheless, we argue that special attention should be paid to neurogenesis. Finally, we pinpoint the remaining open question on whether the hippocampal adult neurogenesis acts through pattern separation to permit the discrimination of highly similar stimuli during behavior.

Exercise-induced Nitric Oxide Contributes to Spatial Memory and Hippocampal Capillaries in Rats

Exercise has been argued to improve cognitive function in both humans and rodents. Angiogenesis significantly contributes to brain health, including cognition. The hippocampus is a crucial brain region for cognitive function. However, studies quantifying the capillary changes in the hippocampus after running exercise are lacking. Moreover, the molecular details underlying the effects of running exercise remain poorly understood. We show that endogenous nitric oxide contributes to the beneficial effects of running exercise on cognition and hippocampal capillaries. Four weeks of running exercise significantly improved spatial memory ability and increased the number of capillaries in the cornu ammonis 1 subfield and dentate gyrus of Sprague-Dawley rats. Running exercise also significantly increased nitric oxide synthase activity and nitric oxide content in the rat hippocampus. After blocking the synthesis of endogenous nitric oxide by lateral ventricular injection of NG-nitro-L-arginine methyl ester, a nonspecific nitric oxide synthase inhibitor, the protective effect of running exercise on spatial memory was eliminated. The protective effect of running exercise on angiogenesis in the cornu ammonis 1 subfield and dentate gyrus of rats was also absent after nitric oxide synthase inhibition. Therefore, during running excise, endogenous nitric oxide may contribute to regulating spatial memory ability and angiogenesis in cornu ammonis 1 subfield and dentate gyrus of the hippocampus.

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.

Memory Traces Diminished by Exercise Affect New Learning as Proactive Facilitation

Exercise enhances cognitive function through increased neurogenesis but can also cause neurogenesis-induced forgetting. It remains unclear whether the diminished memory traces are completely forgotten. Our goals were to determine whether spatial memory is diminished by exercise, and if so, whether the memory is completely gone or whether only the local details disappear but not the acquired strategy. Two-month-old male C57BL/6J mice were trained on a spatial memory task using the Morris water maze and tested to determine that they had learned the platform location. Another mouse group received no training. Half the mice in each group then exercised on a running wheel, while the other half remained sedentary in home cages. After 4 weeks of this, previously trained mice were tested for their retention of the platform location. All mice were then subjected to the task, but the platform was located in a different position (reversal learning for previously trained mice). We found that exercise significantly facilitated the forgetting of the first platform location (i.e., diminished spatial memory) but also significantly enhanced reversal learning. Compared with mice that received no pre-exercise training, mice that had been previously trained, even those in the exercise group that had decreased recall, showed significantly better performance in the reversal learning test. Activation of new adult-born neurons was also examined. Although newborn neuron activation between groups that had or had not received prior task training was not different, activation was significantly higher in exercise groups than in sedentary groups after the probe test for reversal learning. These results indicated that the experience of pre-exercise training equally facilitated new learning in the sedentary and exercise groups, even though significantly lower memory retention was found in the exercise group, suggesting rule-based learning in mice. Furthermore, newborn neurons equally participated in similar and novel memory acquisition.

Parkinson's disease, aging and adult neurogenesis: Wnt/β-catenin signalling as the key to unlock the mystery of endogenous brain repair

A common hallmark of age-dependent neurodegenerative diseases is an impairment of adult neurogenesis. Wingless-type mouse mammary tumor virus integration site (Wnt)/β-catenin (WβC) signalling is a vital pathway for dopaminergic (DAergic) neurogenesis and an essential signalling system during embryonic development and aging, the most critical risk factor for Parkinson's disease (PD). To date, there is no known cause or cure for PD. Here we focus on the potential to reawaken the impaired neurogenic niches to rejuvenate and repair the aged PD brain. Specifically, we highlight WβC-signalling in the plasticity of the subventricular zone (SVZ), the largest germinal region in the mature brain innervated by nigrostriatal DAergic terminals, and the mesencephalic aqueduct-periventricular region (Aq-PVR) Wnt-sensitive niche, which is in proximity to the SNpc and harbors neural stem progenitor cells (NSCs) with DAergic potential. The hallmark of the WβC pathway is the cytosolic accumulation of β-catenin, which enters the nucleus and associates with T cell factor/lymphoid enhancer binding factor (TCF/LEF) transcription factors, leading to the transcription of Wnt target genes. Here, we underscore the dynamic interplay between DAergic innervation and astroglial-derived factors regulating WβC-dependent transcription of key genes orchestrating NSC proliferation, survival, migration and differentiation. Aging, inflammation and oxidative stress synergize with neurotoxin exposure in "turning off" the WβC neurogenic switch via down-regulation of the nuclear factor erythroid-2-related factor 2/Wnt-regulated signalosome, a key player in the maintenance of antioxidant self-defense mechanisms and NSC homeostasis. Harnessing WβC-signalling in the aged PD brain can thus restore neurogenesis, rejuvenate the microenvironment, and promote neurorescue and regeneration.

Monosodium luminol reinstates redox homeostasis, improves cognition, mood and neurogenesis, and alleviates neuro- and systemic inflammation in a model of Gulf War Illness

Enduring brain dysfunction is amid the highly manifested symptoms in veterans with Gulf War Illness (GWI). Animal studies have established that lasting brain dysfunction in GWI is concomitant with augmented oxidative stress, inflammation, and declined neurogenesis in the brain, and systemic inflammation. We hypothesize that drugs capable of restoring redox homeostasis in GWI will improve cognitive and mood function with modulation of neuroinflammation and neurogenesis. We examined the efficacy of monosodium luminol-GVT (MSL), a drug that promotes redox homeostasis, for improving cognitive and mood function in GWI rats. Young rats were exposed to GWI-related chemicals and moderate restraint stress for four weeks. Four months later, GWI rats received different doses of MSL or vehicle for eight weeks. Behavioral analyses in the last three weeks of treatment revealed that GWI rats receiving higher doses of MSL displayed better cognitive and mood function associated with reinstatement of redox homeostasis. Such restoration was evident from the normalized expression of multiple genes encoding proteins involved in combating oxidative stress in the brain and the return of several oxidative stress markers to control levels in the brain and the circulating blood. Sustained redox homeostasis by MSL also resulted in antiinflammatory and pro-neurogenic effects, which were apparent from reduced densities of hypertrophied astrocytes and activated microglia, and increased neurogenesis with augmented neural stem cell proliferation. Moreover, MSL treatment normalized the concentration of multiple proinflammatory markers in the circulating blood. Thus, MSL treatment reinstated redox homeostasis in an animal model of GWI, which resulted in alleviation of both brain and systemic inflammation, improved neurogenesis, and better cognitive and mood function.

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Brain dysfunction in an animal model of Gulf War Illness is linked with persistently elevated oxidative stress.

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Monosodium Luminol treatment reinstated redox homeostasis in a model of Gulf War Illness.

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Reinstatement of redox balance improved cognitive and mood function.

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Restoration of redox balance modulated reactive astrocytes and activated microglia in the brain.

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Return of redox homeostasis enhanced neurogenesis and suppressed systemic inflammation.

Voluntary exercise reduces plasma cortisol levels and improves transitory memory impairment in young and aged Octodon degus

Sleep deprivation (SD) has been reported to induce transient cognitive impairment in functional domains commonly affected in dementia, including memory. Indeed, sleep disturbance has been proposed as an early marker for Alzheimer's disease (AD). SD emulates many aging-related modifications, including important memory dysfunctions possibly caused by triggers of stress such as cortisol. Although exercise is widely assumed to be beneficial for overall health, only recently has the research community focused its attention on its possible effects on brain functions such as cognition. Octodon degus (O. degus) is a recent rodent model considered suitable for the study of neurodegenerative diseases, since it spontaneously develops several histopathological hallmarks observed in AD. We aimed to uncover the interaction between stress, exercise, age and transient memory impairments after SD insult. In this study, animals had free individual access to wheels to practice voluntary exercise. The Barnes Maze (BM) task was conducted with young and aged O. degus animals after combining voluntary exercise and either normal sleep or SD. Plasma cortisol levels were measured after each condition. SD impaired hippocampus-dependent memory in both young and old animals, while cortisol levels did not significantly differ between non-SD and SD animals. However, voluntary exercise for 45 days improved the cognitive impairment caused by SD compared with the control condition. Moreover, voluntary exercise decreased plasma cortisol levels in both conditions, independently of the age.

Validation of hippocampal biomarkers of cumulative affective experience

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Recent knowledge on hippocampal structural plasticity is reviewed.

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This knowledge is harnessed to develop biomarkers of cumulative experience.

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Hippocampal plasticity is shown to have construct, content and criterion validity in mammals.

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The biomarkers require further validation to be used in birds and fish.

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We discuss some practical considerations to implement the biomarkers.

Progress in improving the welfare of captive animals has been hindered by a lack of objective indicators to assess the quality of lifetime experience, often called cumulative affective experience. Recent developments in stress biology and psychiatry have shed new light on the role of the mammalian hippocampus in affective processes. Here we review these findings and argue that structural hippocampal biomarkers demonstrate criterion, construct and content validity as indicators of cumulative affective experience in mammals. We also briefly review emerging findings in birds and fish, which have promising implications for applying the hippocampal approach to these taxa, but require further validation. We hope that this review will motivate welfare researchers and neuroscientists to explore the potential of hippocampal biomarkers of cumulative affective experience.

Hippocampal neurogenesis regulates recovery of defensive responses by recruiting threat- and extinction-signalling brain networks

Safe exposure to a context that was previously associated with threat leads to extinction of defensive responses. Such contextual fear extinction involves the formation of a new memory that inhibits a previously acquired contextual fear memory. However, fear-related responses often return with the simple passage of time (spontaneous fear recovery). Given that contextual fear and extinction memories are hippocampus-dependent and hippocampal neurogenesis has been reported to modify preexisting memories, we hypothesized that neurogenesis-mediated modification of preexisting extinction memory would modify spontaneous fear recovery. To test this, rats underwent contextual fear conditioning followed by extinction. Subsequently, we exposed rats to an enriched environment or focal X-irradiation to enhance or ablate hippocampal neurogenesis, respectively. Over a month later, rats were tested to evaluate spontaneous fear recovery. We found that enhancing neurogenesis after, but not before, extinction prevented fear recovery. In contrast, neurogenesis ablation after, but not before, extinction promoted fear recovery. Using the neuronal activity marker c-Fos, we identified brain regions recruited in these opposing neurogenesis-mediated changes during fear recovery. Together, our findings indicate that neurogenesis manipulation after extinction learning modifies fear recovery by recruiting brain network activity that mediates the expression of preexisting contextual fear and extinction memories.

Emerging Anti-Aging Strategies - Scientific Basis and Efficacy

The prevalence of age-related diseases is in an upward trend due to increased life expectancy in humans. Age-related conditions are among the leading causes of morbidity and death worldwide currently. Therefore, there is an urgent need to find apt interventions that slow down aging and reduce or postpone the incidence of debilitating age-related diseases. This review discusses the efficacy of emerging anti-aging approaches for maintaining better health in old age. There are many anti-aging strategies in development, which include procedures such as augmentation of autophagy, elimination of senescent cells, transfusion of plasma from young blood, intermittent fasting, enhancement of adult neurogenesis, physical exercise, antioxidant intake, and stem cell therapy. Multiple pre-clinical studies suggest that administration of autophagy enhancers, senolytic drugs, plasma from young blood, drugs that enhance neurogenesis and BDNF are promising approaches to sustain normal health during aging and also to postpone age-related neurodegenerative diseases such as Alzheimer's disease. Stem cell therapy has also shown promise for improving regeneration and function of the aged or Alzheimer's disease brain. Several of these approaches are awaiting critical appraisal in clinical trials to determine their long-term efficacy and possible adverse effects. On the other hand, procedures such as intermittent fasting, physical exercise, intake of antioxidants such as resveratrol and curcumin have shown considerable promise for improving function in aging, some of which are ready for large-scale clinical trials, as they are non-invasive, and seem to have minimal side effects. In summary, several approaches are at the forefront of becoming mainstream therapies for combating aging and postponing age-related diseases in the coming years.

Neurogenesis: remembering all or forgetting some

Research over the last 20 years has begun to elucidate the importance of adult neurogenesis in cognition. Three studies recently asked what might be happening to memories formed before neurogenesis in the hippocampus (Akers KG, Martinez-Canabal A, Restivo L, Yiu AP, De Cristofaro A, Hsiang HL, Wheeler AL, Guskjolen A, Niibori Y, Shoji H, Ohira K, Richards BA, Miyakawa T, Josselyn SA, Frankland PW. Science 344: 598–602, 2014; Epp JR, Silva Mera R, Köhler S, Josselyn SA, Frankland PW. Nat Commun 7: 10838, 2016; Kodali M, Megahed T, Mishra V, Shuai B, Hattiangady B, Shetty AK. J Neurosci 36: 8112–8122, 2016). These studies found conflicting results: running (which increases neurogenesis) induced forgetting in two studies, but there was no difference in memory retrieval after exercise in another. To reconcile these studies, one must understand the processes behind memory maintenance and recall and consider context, species, and other factors.

On the Run for Hippocampal Plasticity

Accumulating research in rodents and humans indicates that exercise benefits brain function and may prevent or delay onset of neurodegenerative conditions. In particular, exercise modifies the structure and function of the hippocampus, a brain area important for learning and memory. This review addresses the central and peripheral mechanisms underlying the beneficial effects of exercise on the hippocampus. We focus on running-induced changes in adult hippocampal neurogenesis, neural circuitry, neurotrophins, synaptic plasticity, neurotransmitters, and vasculature. The role of peripheral factors in hippocampal plasticity is also highlighted. We discuss recent evidence that systemic factors released from peripheral organs such as muscle (myokines), liver (hepatokines), and adipose tissue (adipokines) during exercise contribute to hippocampal neurotrophin and neurogenesis levels, and memory function. A comprehensive understanding of the body-brain axis is needed to elucidate how exercise improves hippocampal plasticity and cognition.

Curcumin treatment leads to better cognitive and mood function in a model of Gulf War Illness with enhanced neurogenesis, and alleviation of inflammation and mitochondrial dysfunction in the hippocampus

Diminished cognitive and mood function are among the most conspicuous symptoms of Gulf War Illness (GWI). Our previous studies in a rat model of GWI have demonstrated that persistent cognitive and mood impairments are associated with substantially declined neurogenesis, chronic low-grade inflammation, increased oxidative stress and mitochondrial dysfunction in the hippocampus. We tested the efficacy of curcumin (CUR) to maintain better cognitive and mood function in a rat model of GWI because of its neurogenic, antiinflammatory, antioxidant, and memory and mood enhancing properties. Male rats were exposed daily to low doses of GWI-related chemicals, pyridostigmine bromide, N,N-diethyl-m-toluamide (DEET) and permethrin, and 5-minutes of restraint stress for 28 days. Animals were next randomly assigned to two groups, which received daily CUR or vehicle treatment for 30 days. Animals also received 5'-bromodeoxyuridine during the last seven days of treatment for analysis of neurogenesis. Behavioral studies through object location, novel object recognition and novelty suppressed feeding tests performed sixty days after treatment revealed better cognitive and mood function in CUR treated GWI rats. These rats also displayed enhanced neurogenesis and diminished inflammation typified by reduced astrocyte hypertrophy and activated microglia in the hippocampus. Additional studies showed that CUR treatment to GWI rats enhanced the expression of antioxidant genes and normalized the expression of multiple genes related to mitochondrial respiration. Thus, CUR therapy is efficacious for maintaining better memory and mood function in a model of GWI. Enhanced neurogenesis, restrained inflammation and oxidative stress with normalized mitochondrial respiration may underlie better memory and mood function mediated by CUR treatment.

Aerobic Exercise: Evidence for a Direct Brain Effect to Slow Parkinson Disease Progression

No medications are proven to slow the progression of Parkinson disease (PD). Of special concern with longer-standing PD is cognitive decline, as well as motor symptoms unresponsive to dopamine replacement therapy. Not fully recognized is the substantial accumulating evidence that long-term aerobic exercise may attenuate PD progression. Randomized controlled trial proof will not be forthcoming due to many complicating methodological factors. However, extensive and diverse avenues of scientific investigation converge to argue that aerobic exercise and cardiovascular fitness directly influence cerebral mechanisms mediating PD progression. To objectively assess the evidence for a PD exercise benefit, a comprehensive PubMed literature search was conducted, with an unbiased focus on exercise influences on parkinsonism, cognition, brain structure, and brain function. This aggregate literature provides a compelling argument for regular aerobic-type exercise and cardiovascular fitness attenuating PD progression.

Elevation of Hippocampal Neurogenesis Induces a Temporally Graded Pattern of Forgetting of Contextual Fear Memories

Throughout life neurons are continuously generated in the subgranular zone of the hippocampus. The subsequent integration of newly generated neurons alters patterns of dentate gyrus input and output connectivity, potentially rendering memories already stored in those circuits harder to access. Consistent with this prediction, we previously showed that increasing hippocampal neurogenesis after training induces forgetting of hippocampus-dependent memories, including contextual fear memory. However, the brain regions supporting contextual fear memories change with time, and this time-dependent memory reorganization might regulate the sensitivity of contextual fear memories to fluctuations in hippocampal neurogenesis. By virally expressing the inhibitory designer receptor exclusively activated by designer drugs, hM4Di, we first confirmed that chemogenetic inhibition of dorsal hippocampal neurons impairs retrieval of recent (day-old) but not remote (month-old) contextual fear memories in male mice. We then contrasted the effects of increasing hippocampal neurogenesis at recent versus remote time points after contextual fear conditioning in male and female mice. Increasing hippocampal neurogenesis immediately following training reduced conditioned freezing when mice were replaced in the context 1 month later. In contrast, when hippocampal neurogenesis was increased time points remote to training, conditioned freezing levels were unaltered when mice were subsequently tested. These temporally graded forgetting effects were observed using both environmental and genetic interventions to increase hippocampal neurogenesis. Our experiments identify memory age as a boundary condition for neurogenesis-mediated forgetting and suggest that, as contextual fear memories mature, they become less sensitive to changes in hippocampal neurogenesis levels because they no longer depend on the hippocampus for their expression.SIGNIFICANCE STATEMENT New neurons are generated in the hippocampus throughout life. As they integrate into the hippocampus, they remodel neural circuitry, potentially making information stored in those circuits harder to access. Consistent with this, increasing hippocampal neurogenesis after learning induces forgetting of the learnt information. The current study in mice asks whether these forgetting effects depend on the age of the memory. We found that post-training increases in hippocampal neurogenesis only impacted recently acquired, and not remotely acquired, hippocampal memories. These experiments identify memory age as a boundary condition for neurogenesis-mediated forgetting, and suggest remote memories are less sensitive to changes in hippocampal neurogenesis levels because they no longer depend critically on the hippocampus for their expression.

Maturation, Behavioral Activation, and Connectivity of Adult-Born Medium Spiny Neurons in a Striatal Song Nucleus

Neurogenesis continues in the adult songbird brain. Many telencephalic song control regions incorporate new neurons into their existing circuits in adulthood. One song nucleus that receives many new neurons is Area X. Because this striatal region is crucial for song learning and song maintenance the recruitment of new neurons into Area X could influence these processes. As an entry point into addressing this possibility, we investigated the maturation and connectivity within the song circuit and behavioral activation of newly generated Area X neurons. Using BrdU birth dating and virally mediated GFP expression we followed adult-generated neurons from their place of birth in the ventricle to their place of incorporation into Area X. We show that newborn neurons receive glutamatergic input from pallial/cortical song nuclei. Additionally, backfills revealed that the new neurons connect to pallidal-like projection neurons that innervate the thalamus. Using in situ hybridization, we found that new neurons express the mRNA for D1- and D2-type dopamine receptors. Employing DARPP-32 (dopamine and cAMP-regulated phosphoprotein of 32 kDa) and EGR-1 (early growth response protein 1) as markers for neural maturation and activation, we established that at 42 days after labeling approximately 80% of new neurons were mature medium spiny neurons (MSNs) and could be activated by singing behavior. Finally, we compared the MSN density in Area X of birds up to seven years of age and found a significant increase with age, indicating that new neurons are constantly added to the nucleus. In summary, we provide evidence that newborn MSNs in Area X constantly functionally integrate into the circuit and are thus likely to play a role in the maintenance and regulation of adult song.

Running wheel training does not change neurogenesis levels or alter working memory tasks in adult rats

BACKGROUND

Exercise can change cellular structure and connectivity (neurogenesis or synaptogenesis), causing alterations in both behavior and working memory. The aim of this study was to evaluate the effect of exercise on working memory and hippocampal neurogenesis in adult male Wistar rats using a T-maze test.

METHODS

An experimental design with two groups was developed: the experimental group (n = 12) was subject to a forced exercise program for five days, whereas the control group (n = 9) stayed in the home cage. Six to eight weeks after training, the rats' working memory was evaluated in a T-maze test and four choice days were analyzed, taking into account alternation as a working memory indicator. Hippocampal neurogenesis was evaluated by means of immunohistochemistry of BrdU positive cells.

RESULTS

No differences between groups were found in the behavioral variables (alternation, preference index, time of response, time of trial or feeding), or in the levels of BrdU positive cells.

DISCUSSION

Results suggest that although exercise may have effects on brain structure, a construct such as working memory may require more complex changes in networks or connections to demonstrate a change at behavioral level.

Mechanisms of Hippocampal Aging and the Potential for Rejuvenation

The past two decades have seen remarkable progress in our understanding of the multifactorial drivers of hippocampal aging and cognitive decline. Recent findings have also raised the possibility of functional rejuvenation in the aged hippocampus. In this review, we aim to synthesize the mechanisms that drive hippocampal aging and evaluate critically the potential for rejuvenation. We discuss the functional changes in synaptic plasticity and regenerative potential of the aged hippocampus, followed by mechanisms of microglia aging, and assess the cross talk between these proaging processes. We then examine proyouth interventions that demonstrate significant promise in reversing age-related impairments in the hippocampus and, finally, attempt to look ahead toward novel therapeutics for brain aging.