Running Improves Pattern Separation during Novel Object Recognition

Exercise and Fear and Safety Learning

Fear conditioning paradigms have been studied for over 100 years and are of great interest to the behavioral and clinical sciences given that several safety learning processes (e.g., extinction learning and recall) are thought to be fundamental to the success of exposure-based therapies for anxiety and related disorders. This chapter provides an overview of preclinical and clinical investigations that examined the effects of exercise on initial fear acquisition, fear extinction learning and consolidation, and return of fear outcomes. This chapter highlights the collective body of evidence suggesting that exercise administered after extinction learning enhances the consolidation and subsequent recall of extinction memories to a greater extent than exercise administered prior to extinction learning. This suggests that the addition of exercise after exposure therapy sessions may improve treatment outcomes for people with anxiety and related disorders. Potential mechanisms are discussed in addition to suggestions for future research to improve our understanding of the effects of exercise on fear conditioning and extinction outcomes.

Face-valid phenotypes in a mouse model of the most common mutation in EEF1A2-related neurodevelopmental disorder

De novo heterozygous missense mutations in EEF1A2, encoding neuromuscular translation-elongation factor eEF1A2, are associated with developmental and epileptic encephalopathies. We used CRISPR/Cas9 to recapitulate the most common mutation, E122K, in mice. Although E122K heterozygotes were not observed to have convulsive seizures, they exhibited frequent electrographic seizures and EEG abnormalities, transient early motor deficits and growth defects. Both E122K homozygotes and Eef1a2-null mice developed progressive motor abnormalities, with E122K homozygotes reaching humane endpoints by P31. The null phenotype is driven by progressive spinal neurodegeneration; however, no signs of neurodegeneration were observed in E122K homozygotes. The E122K protein was relatively stable in neurons yet highly unstable in skeletal myocytes, suggesting that the E122K/E122K phenotype is instead driven by loss of function in muscle. Nevertheless, motor abnormalities emerged far earlier in E122K homozygotes than in nulls, suggesting a toxic gain of function and/or a possible dominant-negative effect. This mouse model represents the first animal model of an EEF1A2 missense mutation with face-valid phenotypes and has provided mechanistic insights needed to inform rational treatment design.

PEA-OXA restores cognitive impairments associated with vitamin D deficiency-dependent alterations of the gut microbiota

There is a growing evidence suggesting the association of vitamin D deficiency (VDD) and cognitive impairment. In this study we evaluated the possible involvement of gut microbiota in the cognitive impairments mediated by VDD and investigated the effects of pharmacological treatment with the oxazoline derivative of the aliamide palmitoylethanolamide, 2-Pentadecyl-2-oxazoline (PEA-OXA). Mice were submitted to behavioural, biochemical and electrophysiological analysis to assess whether their vitamin D status affected cognitive performance together with gut microbiota composition. In VDD mice we found cognitive malfunctioning associated with reduced neuroplasticity, indicated by impaired long term potentiation, and neuroinflammation at the hippocampal level. Importantly, PEA-OXA counteracted the cognitive impairments and modified the biochemical and functional changes induced by VDD. Additionally, PEA-OXA treatment enhanced gut microbiota diversity, which tended to be decreased by VDD only in female mice, elevated the relative abundance of lactic and butyric acid-producing families, i.e. Aerococcaceae and Butyricicoccaceae, and reversed the VDD-induced decrease of butyrate-producing beneficial genera, such as Blautia in female mice, and Roseburia in male mice. These data provide novel insights for a better understanding of the cognitive decline induced by VDD and related gut dysbiosis and its potential therapeutic treatment.

Increased flexibility of CA3 memory representations following environmental enrichment

Environmental enrichment (EE) improves memory, particularly the ability to discriminate similar past experiences.1,2,3,4,5,6 The hippocampus supports this ability via pattern separation, the encoding of similar events using dissimilar memory representations.7 This is carried out in the dentate gyrus (DG) and CA3 subfields.8,9,10,11,12 Upregulation of adult neurogenesis in the DG improves memory through enhanced pattern separation.1,2,3,4,5,6,11,13,14,15,16 Adult-born granule cells (abGCs) in DG are suggested to contribute to pattern separation by driving inhibition in regions such as CA3,13,14,15,16,17,18 leading to sparser, nonoverlapping representations of similar events (although a role for abGCs in driving excitation in the hippocampus has also been reported16). Place cells in the hippocampus contribute to pattern separation by remapping to spatial and contextual alterations to the environment.19,20,21,22,23,24,25,26,27 How spatial responses in CA3 are affected by EE and input from increased numbers of abGCs in DG is, however, unknown. Here, we investigate the neural mechanisms facilitating improved memory following EE using associative recognition memory tasks that model the automatic and integrative nature of episodic memory. We find that EE-dependent improvements in difficult discriminations are related to increased neurogenesis and sparser memory representations across the hippocampus. Additionally, we report for the first time that EE changes how CA3 place cells discriminate similar contexts. CA3 place cells of enriched rats show greater spatial tuning, increased firing rates, and enhanced remapping to contextual changes. These findings point to more precise and flexible CA3 memory representations in enriched rats, which provides a putative mechanism for EE-dependent improvements in fine memory discrimination.

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

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

Exercised blood plasma promotes hippocampal neurogenesis in the Alzheimer's disease rat brain

BACKGROUND

Exercise training promotes brain plasticity and is associated with protection against cognitive impairment and Alzheimer's disease (AD). These beneficial effects may be partly mediated by blood-borne factors. Here we used an in vitro model of AD to investigate effects of blood plasma from exercise-trained donors on neuronal viability, and an in vivo rat model of AD to test whether such plasma impacts cognitive function, amyloid pathology, and neurogenesis.

METHODS

Mouse hippocampal neuronal cells were exposed to AD-like stress using amyloid-β and treated with plasma collected from human male donors 3 h after a single bout of high-intensity exercise. For in vivo studies, blood was collected from exercise-trained young male Wistar rats (high-intensity intervals 5 days/week for 6 weeks). Transgenic AD rats (McGill-R-Thy1-APP) were injected 5 times/fortnight for 6 weeks at 2 months or 5 months of age with either (a) plasma from the exercise-trained rats, (b) plasma from sedentary rats, or (c) saline. Cognitive function, amyloid plaque pathology, and neurogenesis were assessed. The plasma used for the treatment was analyzed for 23 cytokines.

RESULTS

Plasma from exercised donors enhanced cell viability by 44.1% (p = 0.032) and reduced atrophy by 50.0% (p < 0.001) in amyloid-β-treated cells. In vivo exercised plasma treatment did not alter cognitive function or amyloid plaque pathology but did increase hippocampal neurogenesis by ∼3 fold, regardless of pathological stage, when compared to saline-treated rats. Concentrations of 7 cytokines were significantly reduced in exercised plasma compared to sedentary plasma.

CONCLUSION

Our proof-of-concept study demonstrates that plasma from exercise-trained donors can protect neuronal cells in culture and promote adult hippocampal neurogenesis in the AD rat brain. This effect may be partly due to reduced pro-inflammatory signaling molecules in exercised plasma.

Scopolamine-Induced Memory Impairment in Mice: Effects of PEA-OXA on Memory Retrieval and Hippocampal LTP

Transient global amnesia, both persistent and transient, is a very common neuropsychiatric syndrome. Among animal models for amnesia and testing new drugs, the scopolamine test is the most widely used for transient global amnesia (TGA). This study examined the scopolamine-induced deficits in working memory, discriminative memory, anxiety, and motor activity in the presence of intranasal PEA-OXA, a dual antagonist of presynaptic α2 and H3 receptors. Male C57BL/6 mice were treated with intraperitoneal scopolamine (1 mg/kg) with or without pre-treatment (15 min) or post-treatment (15 min) with intranasal PEA-OXA (10 mg/kg). It was seen that scopolamine induced deficits of discriminative and spatial memory and motor deficit. These changes were associated with a loss of synaptic plasticity in the hippocampal dentate gyrus: impaired LTP after lateral entorhinal cortex/perforant pathway tetanization. Furthermore, hippocampal Ach levels were increased while ChA-T expression was reduced following scopolamine administration. PEA-OXA either prevented or restored the scopolamine-induced cognitive deficits (discriminative and spatial memory). However, the same treatment did not affect the altered motor activity or anxiety-like behavior induced by scopolamine. Consistently, electrophysiological analysis showed LTP recovery in the DG of the hippocampus, while the Ach level and ChoA-T were normalized. This study confirms the neuroprotective and pro-cognitive activity of PEA-OXA (probably through an increase in the extracellular levels of biogenic amines) in improving transient memory disorders for which the available pharmacological tools are obsolete or inadequate and not directed on specific pathophysiological targets.

Effects of acute aerobic exercise on mnemonic discrimination performance in older adults

OBJECTIVES

Ample evidence suggests exercise is beneficial for hippocampal function. Furthermore, a single session of aerobic exercise provides immediate benefits to mnemonic discrimination performance, a highly hippocampal-specific memory process, in healthy younger adults. However, it is unknown if a single session of aerobic exercise alters mnemonic discrimination in older adults, who generally exhibit greater hippocampal deterioration and deficits in mnemonic discrimination performance.

METHODS

We conducted a within subject acute exercise study in 30 cognitively healthy and physically active older adults who underwent baseline testing and then completed two experimental visits in which they performed a mnemonic discrimination task before and after either 30 min of cycling exercise or 30 min of seated rest. Linear mixed-effects analyses were conducted in which condition order and age were controlled, time (pre vs. post) and condition (exercise vs. rest) were modeled as fixed effects, and subject as a random effect.

RESULTS

No significant time by condition interaction effect was found for object recognition (p = .254, η2=.01), while a significant reduction in interference was found for mnemonic discrimination performance following the exercise condition (p = .012, η2=.07). A post-intervention only analysis indicated that there was no difference between condition for object recognition (p = .186, η2=.06), but that participants had better mnemonic discrimination performance (p < .001, η2=.22) following the exercise.

CONCLUSIONS

Our results suggest a single session of moderate-intensity aerobic exercise may reduce interference and elicit better mnemonic discrimination performance in healthy older adults, suggesting benefits for hippocampal-specific memory function.

Protective role of cod liver oil on hippocampal oxidative damage and neuronal count in Wistar rat model of comorbid depression

Background

Proper nutrition and balanced diet have a profound influence on mental well-being. Nutritional psychiatry plays an important role in influencing a healthy mind and body. The animal model of chronic unpredictable stress has been considered the effective model to explore research on anxiety and depression.

Aim

The present study aimed to explore the protective role of cod liver oil on various biochemical and neuronal analyses in the hippocampus tissue of the Wistar rat model of comorbid depression.

Methods

Healthy adult albino rats of Wistar strain weighing (120-160 g) were divided into control groups and experimental groups. These groups were further categorized into various subgroups based on stress exposure, cod liver oil, and antidepressant treatment. Six animals were taken in each group. The duration of stress exposure was for 15 days. After the experimentation procedure, the animals were anesthetized and hippocampus was dissected for the estimations of various biochemical and neurological parameters.

Results

The combination of cod liver oil with the antidepressant significantly (p < 0.001) decreased the lipid peroxidation level. Total antioxidant (TAO) and superoxide dismutase (SOD) levels significantly increased (p < 0.001) in the hippocampus. Treatment of cod liver oil during the stress exposure increased (p < 0.001) the neuronal count.

Conclusion

Cod liver oil proved to be an effective antidepressant agent by increasing the antioxidants and promoting neurogenesis in the hippocampus.

Aerobic exercise in the treatment of PTSD: An examination of preclinical and clinical laboratory findings, potential mechanisms, clinical implications, and future directions

Posttraumatic stress disorder (PTSD) is associated with heightened emotional responding, avoidance of trauma related stimuli, and physical health concerns (e.g., metabolic syndrome, type 2 diabetes, cardiovascular disease). Existing treatments such as exposure-based therapies (e.g., prolonged exposure) aim to reduce anxiety symptoms triggered by trauma reminders, and are hypothesized to work via mechanisms of extinction learning. However, these conventional gold standard psychotherapies do not address physical health concerns frequently presented in PTSD. In addition to widely documented physical and mental health benefits of exercise, emerging preclinical and clinical evidence supports the hypothesis that precisely timed administration of aerobic exercise can enhance the consolidation and subsequent recall of fear extinction learning. These findings suggest that aerobic exercise may be a promising adjunctive strategy for simultaneously improving physical health while enhancing the effects of exposure therapies, which is desirable given the suboptimal efficacy and remission rates. Accordingly, this review 1) encompasses an overview of preclinical and clinical exercise and fear conditioning studies which form the basis for this claim; 2) discusses several plausible mechanisms for enhanced consolidation of fear extinction memories following exercise, and 3) provides suggestions for future research that could advance the understanding of the potential importance of incorporating exercise into the treatment of PTSD.

Sex-dependent changes of hippocampal synaptic plasticity and cognitive performance in C57BL/6J mice exposed to neonatal repeated maternal separation

The repeated maternal separation (RMS) is a useful experimental model useful in rodents to study the long-term influence of early-life stress on brain neurophysiology. We here investigated the influence of RMS exposure on hippocampal inhibitory and excitatory synaptic transmission, long-term synaptic plasticity and the related potential alterations in learning and memory performance in adult male and female C57Bl/6J mice. Mice were separated daily from their dam for 360 min, from postnatal day 2 (PND2) to PND17, and experiments were performed at PND 60. Patch-clamp recordings in hippocampal CA1 pyramidal neurons revealed a significant enhancement of GABAergic miniature IPSC (mIPSC) frequency, and a decrease in the amplitude of glutamatergic mEPSCs in male mice exposed to RMS. Only a slight but significant reduction in the amplitude of GABAergic mIPSCs was observed in females exposed to RMS compared to the relative controls. A marked increase in long-term depression (LTD) at CA3-CA1 glutamatergic synapses and in the response to the CB1r agonist win55,212 were detected in RMS male, but not female mice. An impaired spatial memory and a reduced preference for novelty was observed in males exposed to RMS but not in females. A single injection of β-ethynyl estradiol at PND2, prevented the changes observed in RMS male mice, suggesting that estrogens may play a protective role early in life against the exposure to stressful conditions. Our findings strengthen the idea of a sex-dependent influence of RMS on long-lasting modifications in synaptic transmission, effects that may be relevant for cognitive performance.

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.

Moderate effect of early-life experience on dentate gyrus function

The development, maturation, and plasticity of neural circuits are strongly influenced by experience and the interaction of an individual with their environment can have a long-lasting effect on cognitive function. Using an enriched environment (EE) paradigm, we have recently demonstrated that enhancing social, physical, and sensory activity during the pre-weaning time in mice led to an increase of inhibitory and excitatory synapses in the dentate gyrus (DG) of the hippocampus. The structural plasticity induced by experience may affect information processing in the circuit. The DG performs pattern separation, a computation that enables the encoding of very similar and overlapping inputs into dissimilar outputs. In the presented study, we have tested the hypothesis that an EE in juvenile mice will affect DG's functions that are relevant for pattern separation: the decorrelation of the inputs from the entorhinal cortex (EC) and the recruitment of the principal excitatory granule cell (GC) during behavior. First, using a novel slice electrophysiology protocol, we found that the transformation of the incoming signal from the EC afferents by individual GC is moderately affected by EE. We further show that EE does not affect behaviorally induced recruitment of principal excitatory GC. Lastly, using the novel object recognition task, a hippocampus-dependent memory test, we show that the ontogeny of this discrimination task was similar among the EE mice and the controls. Taken together, our work demonstrates that pre-weaning enrichment moderately affects DG function.

Long-term neuropathic pain behaviors correlate with synaptic plasticity and limbic circuit alteration: a comparative observational study in mice

ABSTRACT

Neuropathic pain has long-term consequences in affective and cognitive disturbances, suggesting the involvement of supraspinal mechanisms. In this study, we used the spared nerve injury (SNI) model to characterize the development of sensory and aversive components of neuropathic pain and to determine their electrophysiological impact across prefrontal cortex and limbic regions. Moreover, we evaluated the regulation of several genes involved in immune response and inflammation triggered by SNI. We showed that SNI led to sensorial hypersensitivity (cold and mechanical stimuli) and depressive-like behavior lasting 12 months after nerve injury. Of interest, changes in nonemotional cognitive tasks (novel object recognition and Y maze) showed in 1-month SNI mice were not evident normal in the 12-month SNI animals. In vivo electrophysiology revealed an impaired long-term potentiation at prefrontal cortex-nucleus accumbens core pathway in both the 1-month and 12-month SNI mice. On the other hand, a reduced neural activity was recorded in the lateral entorhinal cortex-dentate gyrus pathway in the 1-month SNI mice, but not in the 12-month SNI mice. Finally, we observed the upregulation of specific genes involved in immune response in the hippocampus of 1-month SNI mice, but not in the 12-month SNI mice, suggesting a neuroinflammatory response that may contribute to the SNI phenotype. These data suggest that distinct brain circuits may drive the psychiatric components of neuropathic pain and pave the way for better investigation of the long-term consequences of peripheral nerve injury for which most of the available drugs are to date unsatisfactory.

Short-Term, Voluntary Exercise Affects Morpho-Functional Maturation of Adult-Generated Neurons in Rat Hippocampus

Physical exercise is a well-proven neurogenic stimulus, promoting neuronal progenitor proliferation and affecting newborn cell survival. Besides, it has beneficial effects on brain health and cognition. Previously, we found that three days of physical activity in a very precocious period of adult-generated granule cell life is able to antedate the appearance of the first GABAergic synaptic contacts and increase T-type Ca²⁺ channel expression. Considering the role of GABA and Ca²⁺ in fostering neuronal maturation, in this study, we used short-term, voluntary exercise on a running wheel to investigate if it is able to induce long-term morphological and synaptic changes in newborn neurons. Using adult male rats, we found that: (i) three days of voluntary physical exercise can definitively influence the morpho-functional maturation process of newborn granule neurons when applied very early during their development; (ii) a significant percentage of new neurons show more mature morphological characteristics far from the end of exercise protocol; (iii) the long-term morphological effects result in enhanced synaptic plasticity. Present findings demonstrate that the morpho-functional changes induced by exercise on very immature adult-generated neurons are permanent, affecting the neuron maturation and integration in hippocampal circuitry. Our data contribute to underpinning the beneficial potential of physical activity on brain health, also performed for short times.

Galanin and Neuropeptide Y Interaction Enhances Proliferation of Granule Precursor Cells and Expression of Neuroprotective Factors in the Rat Hippocampus with Consequent Augmented Spatial Memory

Dysregulation of hippocampal neurogenesis is linked to several neurodegenereative diseases, where boosting hippocampal neurogenesis in these patients emerges as a potential therapeutic approach. Accumulating evidence for a neuropeptide Y (NPY) and galanin (GAL) interaction was shown in various limbic system regions at molecular-, cellular-, and behavioral-specific levels. The purpose of the current work was to evaluate the role of the NPY and GAL interaction in the neurogenic actions on the dorsal hippocampus. We studied the Y1R agonist and GAL effects on: hippocampal cell proliferation through the proliferating cell nuclear antigen (PCNA), the expression of neuroprotective and anti-apoptotic factors, and the survival of neurons and neurite outgrowth on hippocampal neuronal cells. The functional outcome was evaluated in the object-in-place task. We demonstrated that the Y1R agonist and GAL promote cell proliferation and the induction of neuroprotective factors. These effects were mediated by the interaction of NPYY1 (Y1R) and GAL2 (GALR2) receptors, which mediate the increased survival and neurites' outgrowth observed on neuronal hippocampal cells. These cellular effects are linked to the improved spatial-memory effects after the Y1R agonist and GAL co-injection at 24 h in the object-in-place task. Our results suggest the development of heterobivalent agonist pharmacophores, targeting Y1R-GALR2 heterocomplexes, therefore acting on the neuronal precursor cells of the DG in the dorsal hippocampus for the novel therapy of neurodegenerative cognitive-affecting diseases.

A Feed-Forward Neural Network for Increasing the Hopfield-Network Storage Capacity

In the hippocampal dentate gyrus (DG), pattern separation mainly depends on the concepts of 'expansion recoding', meaning random mixing of different DG input channels. However, recent advances in neurophysiology have challenged the theory of pattern separation based on these concepts. In this study, we propose a novel feed-forward neural network, inspired by the structure of the DG and neural oscillatory analysis, to increase the Hopfield-network storage capacity. Unlike the previously published feed-forward neural networks, our bio-inspired neural network is designed to take advantage of both biological structure and functions of the DG. To better understand the computational principles of pattern separation in the DG, we have established a mouse model of environmental enrichment. We obtained a possible computational model of the DG, associated with better pattern separation ability, by using neural oscillatory analysis. Furthermore, we have developed a new algorithm based on Hebbian learning and coupling direction of neural oscillation to train the proposed neural network. The simulation results show that our proposed network significantly expands the storage capacity of Hopfield network, and more effective pattern separation is achieved. The storage capacity rises from 0.13 for the standard Hopfield network to 0.32 using our model when the overlap in patterns is 10%.

Complicated Role of Exercise in Modulating Memory: A Discussion of the Mechanisms Involved

Evidence has shown the beneficial effects of exercise on learning and memory. However, many studies have reported controversial results, indicating that exercise can impair learning and memory. In this article, we aimed to review basic studies reporting inconsistent complicated effects of exercise on memory in rodents. Also, we discussed the mechanisms involved in the effects of exercise on memory processes. In addition, we tried to find scientific answers to justify the inconsistent results. In this article, the role of brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (involved in synaptic plasticity and neurogenesis), and vascular endothelial growth factor, nerve growth factor, insulin-like growth factor 1, inflammatory markers, apoptotic factors, and antioxidant system was discussed in the modulation of exercise effects on memory. The role of intensity and duration of exercise, and type of memory task was also investigated. We also mentioned to the interaction of exercise with the function of neurotransmitter systems, which complicates the prediction of exercise effect via altering the level of BDNF. Eventually, we suggested that changes in the function of neurotransmitter systems following different types of exercise (depending on exercise intensity or age of onset) should be investigated in further studies. It seems that exercise-induced changes in the function of neurotransmitter systems may have a stronger role than age, type of memory task, or exercise intensity in modulating memory. Importantly, high levels of interactions between neurotransmitter systems and BDNF play a critical role in the modulation of exercise effects on memory performance.

Exercise to spot the differences: a framework for the effect of exercise on hippocampal pattern separation in humans

Exercise has a beneficial effect on mental health and cognitive functioning, but the exact underlying mechanisms remain largely unknown. In this review, we focus on the effect of exercise on hippocampal pattern separation, which is a key component of episodic memory. Research has associated exercise with improvements in pattern separation. We propose an integrated framework mechanistically explaining this relationship. The framework is divided into three pathways, describing the pro-neuroplastic, anti-inflammatory and hormonal effects of exercise. The pathways are heavily intertwined and may result in functional and structural changes in the hippocampus. These changes can ultimately affect pattern separation through direct and indirect connections. The proposed framework might guide future research on the effect of exercise on pattern separation in the hippocampus.

Enriched Environment Modulates Sharp Wave-Ripple (SPW-R) Activity in Hippocampal Slices

Behavioral flexibility depends on neuronal plasticity which forms and adapts the central nervous system in an experience-dependent manner. Thus, plasticity depends on interactions between the organism and its environment. A key experimental paradigm for studying this concept is the exposure of rodents to an enriched environment (EE), followed by studying differences to control animals kept under standard conditions (SC). While multiple changes induced by EE have been found at the cellular-molecular and cognitive-behavioral levels, little is known about EE-dependent alterations at the intermediate level of network activity. We, therefore, studied spontaneous network activity in hippocampal slices from mice which had previously experienced EE for 10–15 days. Compared to control animals from standard conditions (SC) and mice with enhanced motor activity (MC) we found several differences in sharp wave-ripple complexes (SPW-R), a memory-related activity pattern. Sharp wave amplitude, unit firing during sharp waves, and the number of superimposed ripple cycles were increased in tissue from the EE group. On the other hand, spiking precision with respect to the ripple oscillations was reduced. Recordings from single pyramidal cells revealed a reduction in synaptic inhibition during SPW-R together with a reduced inhibition-excitation ratio. The number of inhibitory neurons, including parvalbumin-positive interneurons, was unchanged. Altered activation or efficacy of synaptic inhibition may thus underlie changes in memory-related network activity patterns which, in turn, may be important for the cognitive-behavioral effects of EE exposure.

5 Years of Exercise Intervention Did Not Benefit Cognition Compared to the Physical Activity Guidelines in Older Adults, but Higher Cardiorespiratory Fitness Did. A Generation 100 Substudy

Background: Aerobic exercise is proposed to attenuate cognitive decline in aging. We investigated the effect of different aerobic exercise interventions and cardiorespiratory fitness (CRF) upon cognition throughout a 5-year exercise intervention in older adults. Methods: 106 older adults (52 women, age 70-77 years) were randomized into high-intensity interval training (HIIT; ∼90% peak heart rate), moderate-intensity continuous training (MICT; ∼70% peak heart rate), or control for 5 years. The HIIT and MICT groups performed supervised training twice weekly, while the control group was asked to follow the national physical activity guidelines (30 min of physical activity/day). At baseline, 1-, 3-, and 5-year follow-up, participants partook in cognitive testing (spatial memory, verbal memory, pattern separation, processing speed, working memory, and planning ability), underwent clinical testing, and filled out health-related questionnaires. Linear mixed models were used to assess the effects of the exercise group and CRF (measured as peak and max oxygen uptake) on each cognitive test. The effects of changes in CRF on changes in each cognitive test score throughout the intervention were also assessed. The associations between baseline CRF and cognitive abilities at the follow-ups were investigated using linear regressions. Results: There was no group-by-time interaction on the cognitive measures, and neither HIIT nor MICT participation was associated with better cognitive performance than control at any time point during the 5-year intervention. All groups increased their CRF similarly during the 1st year and subsequently declined back to baseline levels after 5 years. A higher CRF was associated with higher processing speed throughout the intervention while increasing CRF during the intervention was associated with better working memory and worse pattern separation. Higher CRF at baseline predicted consistently better processing speed and verbal memory performance. Conclusion: In this first 5-year randomized controlled trial investigating the effects of HIIT, MICT, and physical activity according to national guidelines on cognition, we observed no effect of exercise intervention group on cognition when compared to following the national physical activity guidelines. Still, the results showed that higher CRF and increasing CRF benefited multiple, but not all, cognitive abilities in older adults. Clinical Trial Registration: www.ClinicalTrials.gov, identifier [NCT01666340].

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.

Adolescent Binge-Type Ethanol Exposure in Rats Mirrors Age-Related Cognitive Decline by Suppressing Cholinergic Tone and Hippocampal Neurogenesis

Heavy alcohol consumption followed by periods of abstinence (i.e., binge drinking) during adolescence is a concern for both acute and chronic health issues. Persistent brain damage after adolescent intermittent ethanol exposure in rodents, a model of binge drinking, includes reduced hippocampal neurogenesis and a loss of neurons in the basal forebrain that express the cholinergic phenotype. The circuit formed between those regions, the septohippocampal pathway, is critical for learning and memory. Furthermore, this circuit is also altered during the aging process. Thus, we examined whether pathology in septohippocampal circuit and impairments in spatial behaviors are amplified during aging following adolescent intermittent ethanol exposure. Female and male rats were exposed to intermittent intragastric gavage of water (control) or 20% ethanol (dose of 5 g/kg) for a 2 days on/off cycle from postnatal days 25-55. Either 2 (young adult) or 12-14 (middle-age) months post exposure, rats were tested on two spatial tasks: spontaneous alternation and novel object in place. Acetylcholine efflux was assessed in the hippocampus during both tasks. There was no adolescent ethanol-induced deficit on spontaneous alternation, but middle-aged male rats displayed lower alternation rates. Male rats exposed to ethanol during adolescence had blunted behavioral evoked acetylcholine during spontaneous alternation testing. All ethanol-exposed rats displayed suppression of the cholinergic neuronal phenotype. On the novel object in place task, regardless of sex, ethanol-exposed rats performed significantly worse than control-treated rats, and middle aged-rats, regardless of sex or ethanol exposure, were significantly impaired relative to young adult rats. These results indicate that male rats display earlier age-related cognitive impairment on a working memory task. Furthermore, male rats exposed to ethanol during adolescence have blunted behavior-evoked hippocampal acetylcholine efflux. In addition, middle-aged and ethanol-exposed rats, regardless of sex, are impaired at determining discrete spatial relationship between objects. This type of pattern separation impairment was associated with a loss of neurogenesis. Thus, binge-type adolescent ethanol exposure does affect the septohippocampal circuit, and can accelerate age-related cognitive impairment on select spatial tasks.

Orexin-A and endocannabinoids are involved in obesity-associated alteration of hippocampal neurogenesis, plasticity, and episodic memory in mice

The mammalian brain stores and distinguishes among episodic memories, i.e. memories formed during the personal experience, through a mechanism of pattern separation computed in the hippocampal dentate gyrus. Decision-making for food-related behaviors, such as the choice and intake of food, might be affected in obese subjects by alterations in the retrieval of episodic memories. Adult neurogenesis in the dentate gyrus regulates the pattern separation. Several molecular factors affect adult neurogenesis and exert a critical role in the development and plasticity of newborn neurons. Orexin-A/hypocretin-1 and downstream endocannabinoid 2-arachidonoylglycerol signaling are altered in obese mice. Here, we show that excessive orexin-A/2-arachidonoylglycerol/cannabinoid receptor type-1 signaling leads to the dysfunction of adult hippocampal neurogenesis and the subsequent inhibition of plasticity and impairment of pattern separation. By inhibiting orexin-A action at orexin-1 receptors we rescued both plasticity and pattern separation impairment in obese mice, thus providing a molecular and functional mechanism to explain alterations in episodic memory in obesity.

The authors show that adult hippocampal neurogenesis is altered in the dentate gyrus of obese mice with subsequent inhibition of long-term potentiation and impairment of pattern separation. Inhibition of orexin-A action at orexin-1 receptors rescued both impairments in obese mice.

Sparsification of AP firing in adult-born hippocampal granule cells via voltage-dependent α5-GABAA receptors

GABA can depolarize immature neurons close to the action potential (AP) threshold in development and adult neurogenesis. Nevertheless, GABAergic synapses effectively inhibit AP firing in newborn granule cells of the adult hippocampus as early as two weeks post-mitosis. The underlying mechanisms are largely unclear. Here, we analyze GABAergic inputs in newborn hippocampal granule cells mediated by soma-targeting parvalbumin and dendrite-targeting somatostatin interneurons. Surprisingly, both interneuron subtypes activate α5-subunit-containing GABA_A receptors (α5-GABA_ARs) in young neurons, showing a nonlinear voltage dependence with increasing conductance around the AP threshold. By contrast, in mature cells, parvalbumin interneurons mediate linear GABAergic synaptic currents lacking α5-subunits, while somatostatin interneurons continue to target nonlinear α5-GABA_ARs. Computational modeling shows that the voltage-dependent amplification of α5-GABA_AR opening in young neurons is crucial for inhibition of AP firing to generate balanced and sparse firing activity, even with depolarized GABA reversal potential.

Spiking Neural Networks and Hippocampal Function: A Web-Accessible Survey of Simulations, Modeling Methods, and Underlying Theories

Computational modeling has contributed to hippocampal research in a wide variety of ways and through a large diversity of approaches, reflecting the many advanced cognitive roles of this brain region. The intensively studied neuron type circuitry of the hippocampus is a particularly conducive substrate for spiking neural models. Here we present an online knowledge base of spiking neural network simulations of hippocampal functions. First, we overview theories involving the hippocampal formation in subjects such as spatial representation, learning, and memory. Then we describe an original literature mining process to organize published reports in various key aspects, including: (i) subject area (e.g., navigation, pattern completion, epilepsy); (ii) level of modeling detail (Hodgkin-Huxley, integrate-and-fire, etc.); and (iii) theoretical framework (attractor dynamics, oscillatory interference, self-organizing maps, and others). Moreover, every peer-reviewed publication is also annotated to indicate the specific neuron types represented in the network simulation, establishing a direct link with the Hippocampome.org portal. The web interface of the knowledge base enables dynamic content browsing and advanced searches, and consistently presents evidence supporting every annotation. Moreover, users are given access to several types of statistical reports about the collection, a selection of which is summarized in this paper. This open access resource thus provides an interactive platform to survey spiking neural network models of hippocampal functions, compare available computational methods, and foster ideas for suitable new directions of research.

Transcriptome Analysis in a Mouse Model of Premature Aging of Dentate Gyrus: Rescue of Alpha-Synuclein Deficit by Virus-Driven Expression or by Running Restores the Defective Neurogenesis

The dentate gyrus of the hippocampus and the subventricular zone are neurogenic niches where neural stem and progenitor cells replicate throughout life to generate new neurons. The Btg1 gene maintains the stem cells of the neurogenic niches in quiescence. The deletion of Btg1 leads to an early transient increase of stem/progenitor cells division, followed, however, by a decrease during adulthood of their proliferative capability, accompanied by apoptosis. Since a physiological decrease of neurogenesis occurs during aging, the Btg1 knockout mouse may represent a model of neural aging. We have previously observed that the defective neurogenesis of the Btg1 knockout model is rescued by the powerful neurogenic stimulus of physical exercise (running). To identify genes responsible for stem and progenitor cells maintenance, we sought here to find genes underlying this premature neural aging, and whose deregulated expression could be rescued by running. Through RNA sequencing we analyzed the transcriptomic profiles of the dentate gyrus isolated from Btg1 wild-type or Btg1 knockout adult (2-month-old) mice submitted to physical exercise or sedentary. In Btg1 knockout mice, 545 genes were deregulated, relative to wild-type, while 2081 genes were deregulated by running. We identified 42 genes whose expression was not only down-regulated in the dentate gyrus of Btg1 knockout, but was also counter-regulated to control levels by running in Btg1 knockout mice, vs. sedentary. Among these 42 counter-regulated genes, alpha-synuclein (Snca), Fos, Arc and Npas4 showed significantly greater differential regulation. These genes control neural proliferation, apoptosis, plasticity and memory and are involved in aging. In particular, Snca expression decreases during aging. We tested, therefore, whether an Snca-expressing lentivirus, by rescuing the defective Snca levels in the dentate gyrus of Btg1 knockout mice, could also reverse the aging phenotype, in particular the defective neurogenesis. We found that the exogenous expression of Snca reversed the Btg1 knockout-dependent decrease of stem cell proliferation as well as the increase of progenitor cell apoptosis. This indicates that Snca has a functional role in the process of neural aging observed in this model, and also suggests that Snca acts as a positive regulator of stem cell maintenance.

Exercise hormone irisin is a critical regulator of cognitive function

Identifying secreted mediators that drive the cognitive benefits of exercise holds great promise for the treatment of cognitive decline in ageing or Alzheimer's disease (AD). Here, we show that irisin, the cleaved and circulating form of the exercise-induced membrane protein FNDC5, is sufficient to confer the benefits of exercise on cognitive function. Genetic deletion of Fndc5/irisin (global Fndc5 knock-out (KO) mice; F5KO) impairs cognitive function in exercise, ageing and AD. Diminished pattern separation in F5KO mice can be rescued by delivering irisin directly into the dentate gyrus, suggesting that irisin is the active moiety. In F5KO mice, adult-born neurons in the dentate gyrus are morphologically, transcriptionally and functionally abnormal. Importantly, elevation of circulating irisin levels by peripheral delivery of irisin via adeno-associated viral overexpression in the liver results in enrichment of central irisin and is sufficient to improve both the cognitive deficit and neuropathology in AD mouse models. Irisin is a crucial regulator of the cognitive benefits of exercise and is a potential therapeutic agent for treating cognitive disorders including AD.

Recruitment of parvalbumin and somatostatin interneuron inputs to adult born dentate granule neurons

GABA is a key regulator of adult-born dentate granule cell (abDGC) maturation so mapping the functional connectivity between abDGCs and local interneurons is required to understand their development and integration into the hippocampal circuit. We recorded from birthdated abDGCs in mice and photoactivated parvalbumin (PV) and somatostatin (SST) interneurons to map the timing and strength of inputs to abDGCs during the first 4 weeks after differentiation. abDGCs received input from PV interneurons in the first week, but SST inputs were not detected until the second week. Analysis of desynchronized quantal events established that the number of GABAergic synapses onto abDGCs increased with maturation, whereas individual synaptic strength was constant. Voluntary wheel running in mice scaled the GABAergic input to abDGCs by increasing the number of synaptic contacts from both interneuron types. This demonstrates that GABAergic innervation to abDGCs develops during a prolonged post-mitotic period and running scales both SST and PV synaptic afferents.

The effects of exercise treatment on learning and memory ability, and cognitive performance in diet-induced prediabetes animals

Changes associated with cognitive function in the high-fat high-carbohydrate diet-induced prediabetes animal model and effect of exercise remain unclear. Rats were randomly assigned to the following groups (n = 6): non-diabetic (ND), prediabetic (PD), intermittent exercising PD (PD + IE) and regular exercising PD (PD + RE). After exercise cessation, oral glucose tolerance (OGT), Novel Object Recognition Test (NORT) and Morris-Water Maze (MWM) tests were performed to assess cognitive function. After sacrifice, malonaldehyde, glutathione peroxidase, interleukin-1β and dopamine concentration in the prefrontal cortex (PFC) and hippocampus were measured. Impaired OGT response in PD animals was accompanied by poor performance on behavioural tasks. This was associated with increased oxidative stress markers and impaired dopamine neurotransmission as evidence by elevated dopamine concentration in the PFC and hippocampal tissue. Improved OGT response by exercise was coupled with improved performance on behavioural tasks, oxidative stress markers and increased interleukin-1β concentration. In regular exercise, this was further coupled with improved dopamine neurotransmission. Cognitive function was affected during prediabetes in animals. This was partly due to oxidative stress and impaired dopamine neurotransmission. Both intermittent and regular exercise improved cognitive function. This was partly mediated by improved glucose tolerance and oxidative stress as well as a subclinical increase in interleukin-1β concentration. In regular exercise, this was further mediated by improved dopamine neurotransmission.

Steps towards standardized quantification of adult neurogenesis

New neurons are generated in adult mammals. Adult hippocampal neurogenesis is considered to play an important role in cognition and mental health. The number and properties of newly born neurons are regulatable by a broad range of physiological and pathological conditions. To begin to understand the underlying cellular mechanisms and functional relevance of adult neurogenesis, many studies rely on quantification of adult-born neurons. However, lack of standardized methods to quantify new neurons is impeding research reproducibility across laboratories. Here, we review the importance of stereology, and propose why and how it should be applied to the study of adult neurogenesis.

Hypothesized Mechanisms Through Which Exercise May Attenuate Memory Interference

In this paper we introduce a mechanistic model through which exercise may enhance episodic memory, specifically via attenuating proactive and retroactive memory interference. We discuss the various types of memory, different stages of memory function, review the mechanisms behind forgetting, and the mechanistic role of exercise in facilitating pattern separation (to attenuate memory interference).

Treatment with gut bifidobacteria improves hippocampal plasticity and cognitive behavior in adult healthy rats

At the present time, gut microbiota inspires great interest in the field of neuroscience as a function of its role in normal physiology and involvement in brain function. This aspect suggests a specific gut-brain pathway, mainly modulated by gut microbiota activity. Among the multiple actions controlled by microbiota at the brain level, neuronal plasticity and cognitive function represent two of the most interesting aspects of this cross-talk communication. We address the possible action of two-months implementation of gut Bifidobacteria using a mixture of three different strains (B-MIX) on hippocampal plasticity and related cognitive behavior in adult healthy Sprague Dawley rats. B-MIX treatment increases the hippocampal BDNF with a parallel gain in dendritic spines' density of hippocampal CA1 pyramidal neurons. Electrophysiological experiments revealed a significant increment of HFS-induced LTP formation on the CA1 hippocampal region in B-MIX treated rats. All these effects are accompanied by a better cognitive performance observed in B-MIX treated animals with no impairments in locomotion activity. Therefore, in adult rats, the treatment with different strains of bifidobacteria is able to markedly enhance neuronal plasticity and the CNS function influencing cognitive behavior, an effect that may suggest a potential therapeutic treatment in brain diseases associated with cognitive functions.

Physical Activity and Alzheimer's Disease: A Narrative Review

Although age is a dominant risk factor for Alzheimer’s disease (AD), epidemiological studies have shown that physical activity may significantly decrease age-related risks for AD, and indeed mitigate the impact in existing diagnosis. The aim of this study was to perform a narrative review on the preventative, and mitigating, effects of physical activity on AD onset, including genetic factors, mechanism of action and physical activity typology. In this article, we conducted a narrative review of the influence physical activity and exercise have on AD, utilising key terms related to AD, physical activity, mechanism and prevention, searching the online databases; Web of Science, PubMed and Google Scholar, and, subsequently, discuss possible mechanisms of this action. On the basis of this review, it is evident that physical activity and exercise may be incorporated in AD, notwithstanding, a greater number of high-quality randomised controlled trials are needed, moreover, physical activity typology must be acutely considered, primarily due to a dearth of research on the efficacy of physical activity types other than aerobic.

Born this way: Hippocampal neurogenesis across the lifespan

The capability of the mammalian brain to generate new neurons through the lifespan has gained much attention for the promise of new therapeutic possibilities especially for the aging brain. One of the brain regions that maintains a neurogenesis-permissive environment is the dentate gyrus of the hippocampus. Here, new neurons are generated from a pool of multipotent neural progenitor cells to become fully functional neurons that are integrated into the brain circuitry. A growing body of evidence points to the fact that neurogenesis in the adult hippocampus is necessary for certain memory processes, and in mood regulation, while alterations in hippocampal neurogenesis have been associated with a myriad of neurological and psychiatric disorders. More recently, evidence has come to light that new neurons may differ in their vulnerability to environmental and disease-related influences depending on the time during the life course at which they are exposed. Thus, it has been the topic of intense research in recent years. In this review, we will discuss the complex process and associated functional relevance of hippocampal neurogenesis during the embryonic/postnatal period and in adulthood. We consider the implications of hippocampal neurogenesis during the developmentally critical periods of adolescence and older age. We will further consider the literature surrounding hippocampal neurogenesis and its functional role during these critical periods with a view to providing insight into the potential of harnessing neurogenesis for health and therapeutic benefit.

Exercise-induced enhancement of synaptic function triggered by the inverse BAR protein, Mtss1L

Exercise is a potent enhancer of learning and memory, yet we know little of the underlying mechanisms that likely include alterations in synaptic efficacy in the hippocampus. To address this issue, we exposed mice to a single episode of voluntary exercise, and permanently marked activated mature hippocampal dentate granule cells using conditional Fos-TRAP mice. Exercise-activated neurons (Fos-TRAPed) showed an input-selective increase in dendritic spines and excitatory postsynaptic currents at 3 days post-exercise, indicative of exercise-induced structural plasticity. Laser-capture microdissection and RNASeq of activated neurons revealed that the most highly induced transcript was Mtss1L, a little-studied I-BAR domain-containing gene, which we hypothesized could be involved in membrane curvature and dendritic spine formation. shRNA-mediated Mtss1L knockdown in vivo prevented the exercise-induced increases in spines and excitatory postsynaptic currents. Our results link short-term effects of exercise to activity-dependent expression of Mtss1L, which we propose as a novel effector of activity-dependent rearrangement of synapses.

Exercise and Hippocampal Memory Systems

No medications prevent or reverse age-related cognitive decline. Physical activity (PA) enhances memory in rodents, but findings are mixed in human studies. As a result, exercise guidelines specific for brain health are absent. Here, we re-examine results from human studies, and suggest the use of more sensitive tasks to evaluate PA effects on age-related changes in the hippocampus, such as relational memory and mnemonic discrimination. We discuss recent advances from rodent and human studies into the underlying mechanisms at both the central and peripheral levels, including neurotrophins and myokines that could contribute to improved memory. Finally, we suggest guidelines for future research to help expedite well-founded PA recommendations for the public.

Effects of Voluntary Exercise on Cell Proliferation and Neurogenesis in the Dentate Gyrus of Adult FMR1 Knockout Mice

Fragile X syndrome (FXS) is the most common cause of inherited intellectual disability that can be traced to a single gene mutation. This disorder is caused by the hypermethylation of the Fmr1 gene, which impairs translation of Fragile X Mental Retardation Protein (FMRP). In Fmr1 knockout (KO) mice, the loss of FMRP has been shown to negatively impact adult hippocampal neurogenesis, and to contribute to learning, memory, and emotional deficits. Conversely, physical exercise has been shown to enhance cognitive performance, emotional state, and increase adult hippocampal neurogenesis. In the current experiments, we used two different voluntary running paradigms to examine how exercise impacts adult neurogenesis in the dorsal and ventral hippocampal dentate gyrus (DG) of Fmr1 KO mice. Immunohistochemical analyses showed that short-term (7 day) voluntary running enhanced cell proliferation in both wild-type (WT) and Fmr1 KO mice. In contrast, long-term (28 day) running only enhanced cell proliferation in the whole DG of WT mice, but not in Fmr1 KO mice. Interestingly, cell survival was enhanced in both WT and Fmr1 KO mice following exercise. Interestingly we found that running promoted cell proliferation and survival in the ventral DG of WTs, but promoted cell survival in the dorsal DG of Fmr1 KOs. Our data indicate that long-term exercise has differential effects on adult neurogenesis in ventral and dorsal hippocampi in Fmr1 KO mice. These results suggest that physical training can enhance hippocampal neurogenesis in the absence of FMRP, may be a potential intervention to enhance learning and memory and emotional regulation in FXS.

Differential effects of adolescent and adult-initiated exercise on cognition and hippocampal neurogenesis

Adolescence is a critical period for postnatal brain maturation and thus a time when environmental influences may affect cognitive processes in later life. Exercise during adulthood has been shown to increase hippocampal neurogenesis and enhance cognition. However, the impact of exercise initiated in adolescence on the brain and behavior in adulthood is not fully understood. The aim of this study was to compare the impact of voluntary exercise that is initiated during adolescence or early adulthood on cognitive performance in hippocampal-dependent and -independent processes using both object-based and touchscreen operant paradigms. Adult (8 week) and adolescent (4 week) male Sprague-Dawley rats had access to a running wheel (exercise) or were left undisturbed (sedentary control) for 4 weeks prior to behavioral testing and for the duration of the experiment. Results from touchscreen-based tasks showed that reversal learning was enhanced by both adult and adolescent-initiated exercise, while only exercise that began in adolescence induced a subtle but transient increase in performance on a location discrimination task. Spontaneous alternation in the Y-maze was impaired following adolescent onset exercise, while object memory was unaffected by either adult or adolescent-initiated exercise. Adolescent-initiated exercise increased the number of hippocampal DCX cells, an indicator of neurogenesis. It also promoted the complexity of neurites on DCX cells, a key process for synaptic integration, to a greater degree than adult-initiated exercise. Together the data here show that exercise during the adolescent period compared to adulthood differentially affects cognitive processes and the development of new hippocampal neurons in later life.

Depression and adult neurogenesis: Positive effects of the antidepressant fluoxetine and of physical exercise

Of wide interest for health is the relation existing between depression, a very common psychological illness, accompanied by anxiety and reduced ability to concentrate, and adult neurogenesis. We will focus on two neurogenic stimuli, fluoxetine and physical exercise, both endowed with the ability to activate adult neurogenesis in the dentate gyrus of the hippocampus, known to be required for learning and memory, and both able to counteract depression. Fluoxetine belongs to the class of selective serotonin reuptake inhibitor (SSRI) antidepressants, which represent the most used pharmacological therapy; physical exercise has also been shown to effectively counteract depression symptoms in rodents as well as in humans. While there is evidence that the antidepressant effect of fluoxetine requires its pro-neurogenic action, exerted by promoting proliferation, differentiation and survival of progenitor cells of the hippocampus, on the other hand fluoxetine exerts also neurogenesis-independent antidepressant effects by influencing the plasticity of the new neurons generated. Similarly, the antidepressant action of running also correlates with an increase of hippocampal neurogenesis and plasticity, although the gene pathways involved are only partially coincident with those of fluoxetine, such as those involved in serotonin metabolism and synapse formation. We further discuss how extra-neurogenic actions are also suggested by the fact that, unlike running, fluoxetine is unable to stimulate neurogenesis during aging, but still displays antidepressant effects. Moreover, in specific conditions, fluoxetine or running activate not only progenitor but also stem cells, which normally are not stimulated; this fact reveals how stem cells have a long-term, hidden ability to self-renew and, more generally, that neurogenesis is subject to complex controls that may play a role in depression, such as the type of neurogenic stimulus or the state of the local niche. Finally, we discuss how fluoxetine or running are effective in counteracting depression originated from stress or neurodegenerative diseases.

Increased Synthesis of Chondroitin Sulfate Proteoglycan Promotes Adult Hippocampal Neurogenesis in Response to Enriched Environment

Chondroitin sulfate proteoglycan (CSPG) is a candidate regulator of embryonic neurogenesis. The aim of this study was to specify the functional significance of CSPG in adult hippocampal neurogenesis using male mice. Here, we showed that neural stem cells and neuronal progenitors in the dentate gyrus were covered in part by CSPG. Pharmacological depletion of CSPG in the dentate gyrus reduced the densities of neuronal progenitors and newborn granule cells. 3D reconstruction of newborn granule cells showed that their maturation was inhibited by CSPG digestion. The novel object recognition test revealed that CSPG digestion caused cognitive memory impairment. Western blot analysis showed that expression of β-catenin in the dentate gyrus was decreased by CSPG digestion. The amount of CSPG in the dentate gyrus was increased by enriched environment (EE) and was decreased by forced swim stress. In addition, EE accelerated the recovery of CSPG expression in the dentate gyrus from the pharmacological depletion and promoted the restoration of granule cell production. Conversely, the densities of newborn granule cells were also decreased in mice that lacked chondroitin sulfate N-acetylgalactosaminyltransferase 1 (CSGalNAcT1), a key enzyme for CSPG synthesis (T1KO mice). The capacity of EE to promote granule cell production and improve cognitive memory was impaired in T1KO mice. These findings indicate that CSPG is involved in the regulation of adult hippocampal neurogenesis and suggest that increased synthesis of CSPG by CSGalNacT1 may mediate promotion of granule cell production and improvement of cognitive memory in response to EE.SIGNIFICANCE STATEMENT Chondroitin sulfate proteoglycan (CSPG) is a candidate regulator of embryonic neurogenesis. Here, we specified the role of CSPG in adult neurogenesis in the mouse hippocampus. Digestion of CSPG in the dentate gyrus impaired granule cell production and cognitive memory. Enriched environment (EE) promoted the recovery of CSPG expression and granule cell production from the CSPG digestion. Additionally, adult neurogenesis was impaired in mice that lacked a key enzyme for CSPG synthesis (T1KO mice). The capacity of EE to promote granule cell production and cognitive memory was impaired in T1KO mice. Altogether, these findings indicate that CSPG underlies adult hippocampal neurogenesis and suggest that increased synthesis of CSPG may mediate promotion of granule cell production in response to EE.

Intact memory for local and distal cues in male and female rats that lack adult neurogenesis

The dentate gyrus is essential for remembering the fine details of experiences that comprise episodic memory. Dentate gyrus granule cells receive highly-processed sensory information and are hypothesized to perform a pattern separation function, whereby similar sensory inputs are transformed into orthogonal neural representations. Behaviorally, this is believed to enable distinct memory for highly interfering stimuli. Since the dentate gyrus is comprised of a large number of adult-born neurons, which have unique synaptic wiring and neurophysiological firing patterns, it has been proposed that neurogenesis may contribute to this process in unique ways. Some behavioral evidence exists to support this role, whereby neurogenesis-deficient rodents are impaired at discriminating the fine visuospatial details of experiences. However, the extent to which newborn neurons contribute to dentate gyrus-dependent learning tasks is unclear. Furthermore, since most studies of dentate gyrus function are conducted in male rats, little is known about how females perform in similar situations, and whether there might be sex differences in the function of adult neurogenesis. To address these issues, we examined spatial discrimination memory in transgenic male and female rats that lacked adult neurogenesis. The first task probed memory for the position of local objects in an open field, assessed by behavioral responses to novel object locations. The second task examined memory for distal environmental cues. All rats were able to successfully discriminate local and distal cue changes. Males and females also performed comparably, although females displayed higher levels of rearing and locomotion. Collectively, our results indicate that rats are capable of learning about local and distal cues in the absence of adult neurogenesis.

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.

Integrating new findings and examining clinical applications of pattern separation

Pattern separation, the ability to independently represent and store similar experiences, is a crucial facet of episodic memory. Growing evidence suggests that the hippocampus possesses unique circuitry that is computationally capable of resolving mnemonic interference by using pattern separation. In this Review, we discuss recent advances in the understanding of this process and evaluate the caveats and limitations of linking across animal and human studies. We summarize clinical and translational studies using methods that are sensitive to pattern separation impairments, an approach that stems from the fact that the hippocampus is a major site of disruption in many brain disorders. We critically evaluate the assumptions that guide fundamental and translational studies in this area. Finally, we suggest guidelines for future research and offer ways to overcome potential interpretational challenges to increase the utility of pattern separation as a construct that can further understanding of both memory processes and brain disease.

Soluble Tau has devastating effects on the structural plasticity of hippocampal granule neurons

Tau is a neuronal microtubule-associated protein with countless physiological functions. Although the detrimental effects of insoluble aggregated Tau have been widely studied, recent evidence supports the notion that soluble Tau (composed mostly of monomers and dimers) is also toxic for neurons. Here we evaluated the long-term impact of a single stereotaxic injection of human soluble Tau on hippocampal granule neurons in mice. At the ultrastructural level, soluble Tau reduced the number of afferent synapses and caused a dramatic depletion of synaptic vesicles both in afferent and efferent synapses. Furthermore, the use of an RFP-expressing retrovirus revealed that soluble Tau altered the morphology of newborn granule neurons and reduced their afferent (dendritic spines) and efferent (mossy fiber terminals) connectivity. Finally, soluble Tau caused specific impairment of behavioral pattern separation capacity. Our results thus demonstrate for the first time that soluble Tau causes long-term detrimental effects on the morphology and connectivity of newborn granule neurons and that these effects correlate with impaired behavioral pattern separation skills. These data might be relevant for the field of neurodegenerative disorders, since they contribute to reinforcing the pathological roles played by distinct Tau species in vivo.

Running Changes the Brain: the Long and the Short of It

Exercise is a simple intervention that profoundly benefits cognition. In rodents, running increases neurogenesis in the hippocampus, a brain area important for memory. We describe the dynamic changes in new neuron number and afferent connections throughout their maturation. We highlight the effects of exercise on the neurotransmitter systems involved, with a focus on the role of glutamate and acetylcholine in the initial development of new neurons in the adult brain.