Dentate gyrus and spatial behaviour

Spatiotemporal analysis of gene expression in the human dentate gyrus reveals age-associated changes in cellular maturation and neuroinflammation

The dentate gyrus of the hippocampus is important for many cognitive functions, including learning, memory, and mood. Here, we investigated age-associated changes in transcriptome-wide spatial gene expression in the human dentate gyrus across the lifespan. Genes associated with neurogenesis and the extracellular matrix were enriched in infants, while gene markers of inhibitory neurons and cell proliferation showed increases and decreases in post-infancy, respectively. While we did not find evidence for neural proliferation post-infancy, we did identify molecular signatures supporting protracted maturation of granule cells. We also identified a wide-spread hippocampal aging signature and an age-associated increase in genes related to neuroinflammation. Our findings suggest major changes to the putative neurogenic niche after infancy and identify molecular foci of brain aging in glial and neuropil enriched tissue.

Variations of the grid and place cells in the entorhinal cortex and dentate gyrus of 6 individuals aged 56 to 87 years

INTRODUCTION

The relationship between the entorhinal cortex (EC) and the hippocampus has been studied by different authors, who have highlighted the importance of grid cells, place cells, and the trisynaptic circuit in the processes that they regulate: the persistence of spatial, explicit, and recent memory and their possible impairment with ageing.

OBJECTIVE

We aimed to determine whether older age causes changes in the size and number of grid cells contained in layer III of the EC and in the granular layer of the dentate gyrus (DG) of the hippocampus.

METHODS

We conducted post-mortem studies of the brains of 6 individuals aged 56-87 years. The brain sections containing the DG and the adjacent EC were stained according to the Klüver-Barrera method, then the ImageJ software was used to measure the individual neuronal area, the total neuronal area, and the number of neurons contained in rectangular areas in layer III of the EC and layer II of the DG. Statistical analysis was subsequently performed.

RESULTS

We observed an age-related reduction in the cell population of the external pyramidal layer of the EC, and in the number of neurons in the granular layer of the DG.

CONCLUSION

Our results indicate that ageing causes a decrease in the size and density of grid cells of the EC and place cells of the DG.

9-cis beta-carotene-enriched diet significantly improved cognition and decreased Alzheimer's disease neuropathology and neuroinflammation in Alzheimer's disease-like mouse models

A significant progressive decline in beta-carotene (βC) levels in the brain is associated with cognitive impairment and a higher prevalence of Alzheimer's disease (AD). In this study, we investigated whether the administration of 9-cis beta-carotene (9CBC)-rich powder of the alga Dunaliella bardawil, the best-known source of βC in nature, inhibits the development of AD-like neuropathology and cognitive deficits. We demonstrated that in 3 AD mouse models, Tg2576, 5xFAD, and apoE4, 9CBC treatment improved long- and short-term memory, decreased neuroinflammation, and reduced the prevalence of β-amyloid plaques and tau hyperphosphorylation. These findings suggest that 9CBC has the potential to be an effective preventive and symptomatic AD therapy.

MiR-93 alleviates DEHP plasticizer-induced neurotoxicity by negatively regulating TNFAIP1 and inhibiting ubiquitin-mediated degradation of CK2β

Di-(2-ethylhexyl) phthalate (DEHP) is a plasticizer that is widely used in various products, such as plastic packaging in food industries. As an environmental endocrine disruptor, it induces adverse effects on brain development and function. However, the molecular mechanisms by which DEHP induces learning and memory impairment remain poorly understood. Herein, we found that DEHP impaired learning and memory in pubertal C57BL/6 mice, decreased the number of neurons, downregulated miR-93 and the β subunit of casein kinase 2 (CK2β), upregulated tumor necrosis factor-induced protein 1 (TNFAIP1), and inhibited Akt/CREB pathway in mouse hippocampi. Coimmunoprecipitation and western blotting assays revealed that TNFAIP1 interacted with CK2β and promoted its degradation by ubiquitination. Bioinformatics analysis showed a miR-93 binding site in the 3'-untranslated region of Tnfaip1. A dual-luciferase reporter assay revealed that miR-93 targeted TNFAIP1 and negatively regulated its expression. MiR-93 overexpression prevented DEHP-induced neurotoxicity by downregulating TNFAIP1 and then activating CK2/Akt/CREB pathway. These data indicate that DEHP upregulates TNFAIP1 expression by downregulating miR-93, thus promoting ubiquitin-mediated degradation of CK2β, subsequently inhibiting Akt/CREB pathway, and finally inducing learning and memory impairment. Therefore, miR-93 can relieve DEHP-induced neurotoxicity and may be used as a potential molecular target for prevention and treatment of related neurological disorders.

Connectome-based prediction of functional impairment in experimental stroke models

Experimental rat models of stroke and hemorrhage are important tools to investigate cerebrovascular disease pathophysiology mechanisms, yet how significant patterns of functional impairment induced in various models of stroke are related to changes in connectivity at the level of neuronal populations and mesoscopic parcellations of rat brains remain unresolved. To address this gap in knowledge, we employed two middle cerebral artery occlusion models and one intracerebral hemorrhage model with variant extent and location of neuronal dysfunction. Motor and spatial memory function was assessed and the level of hippocampal activation via Fos immunohistochemistry. Contribution of connectivity change to functional impairment was analyzed for connection similarities, graph distances and spatial distances as well as the importance of regions in terms of network architecture based on the neuroVIISAS rat connectome. We found that functional impairment correlated with not only the extent but also the locations of the injury among the models. In addition, via coactivation analysis in dynamic rat brain models, we found that lesioned regions led to stronger coactivations with motor function and spatial learning regions than with other unaffected regions of the connectome. Dynamic modeling with the weighted bilateral connectome detected changes in signal propagation in the remote hippocampus in all 3 stroke types, predicting the extent of hippocampal hypoactivation and impairment in spatial learning and memory function. Our study provides a comprehensive analytical framework in predictive identification of remote regions not directly altered by stroke events and their functional implication.

Route-dependent spatial engram tagging in mouse dentate gyrus

The dentate gyrus (DG) of hippocampus is hypothesized to act as a pattern separator that distinguishes between similar input patterns during memory formation and retrieval. Sparse ensembles of DG cells associated with learning and memory, i.e. engrams, have been labeled and manipulated to recall novel context memories. Functional studies of DG cell activity have demonstrated the spatial specificity and stability of DG cells during navigation. To reconcile how the DG contributes to separating global context as well as individual navigational routes, we trained mice to perform a delayed-non-match-to-position (DNMP) T-maze task and labeled DG neurons during performance of this task on a novel T-maze. The following day, mice navigated a second environment: the same T-maze, the same T-maze with one route permanently blocked but still visible, or a novel open field. We found that the degree of engram reactivation across days differed based on the traversal of maze routes, such that mice traversing only one arm had higher ensemble overlap than chance but less overlap than mice running the full two-route task. Mice experiencing the open field had similar ensemble sizes to the other groups but only chance-level ensemble reactivation. Ensemble overlap differences could not be explained by behavioral variability across groups, nor did behavioral metrics correlate to degree of ensemble reactivation. Together, these results support the hypothesis that DG contributes to spatial navigation memory and that partially non-overlapping ensembles encode different routes within the context of an environment.

Decoding behavior from global cerebrovascular activity using neural networks

Functional Ultrasound (fUS) provides spatial and temporal frames of the vascular activity in the brain with high resolution and sensitivity in behaving animals. The large amount of resulting data is underused at present due to the lack of appropriate tools to visualize and interpret such signals. Here we show that neural networks can be trained to leverage the richness of information available in fUS datasets to reliably determine behavior, even from a single fUS 2D image after appropriate training. We illustrate the potential of this method with two examples: determining if a rat is moving or static and decoding the animal's sleep/wake state in a neutral environment. We further demonstrate that our method can be transferred to new recordings, possibly in other animals, without additional training, thereby paving the way for real-time decoding of brain activity based on fUS data. Finally, the learned weights of the network in the latent space were analyzed to extract the relative importance of input data to classify behavior, making this a powerful tool for neuroscientific research.

Inactivation of the dorsal CA1 hippocampus impairs the consolidation of discriminative avoidance memory by modulating the intrinsic and extrinsic hippocampal circuitry

Despite progress in understanding the role of the dorsal hippocampus in the acquisition, consolidation and retrieval of episodic-like memory, plastic changes within the intra- and extrahippocampal circuits for aversive memory formation and anxiety-like behaviours must still be identified since both processes contribute to multiple aspects of flexible decision-making. Here, we investigated the effect of reversible inactivation induced by a muscimol microinfusion into the dorsal CA1 subfield (dCA1) either prior to acquisition or to retrieval testing of a discriminative avoidance task performed in a plus-maze apparatus (PM-DAT). Differential cAMP-response-element-binding protein 1 (CREB-1) expression in the dorsal and ventral CA1 and CA3 of the hippocampus (dCA1, dCA3, vCA1, and vCA3), dorsal dentate gyrus (dDG), and infralimbic (IL) and prelimbic (PrL) regions of the medial prefrontal cortex was also assessed to investigate the molecular changes associated with the consolidation or retrieval of episodic-like memory and anxiety. Adult male Wistar rats were assigned to two control groups, learning (no surgery/no microinfusion, n = 7) and sham-operated (sham surgery/no microinfusion, n = 6) groups, or four experimental groups, in which the vehicle (0.5 µl per side, n = 8/per group) or a GABA_A receptor agonist (0.5 µg/0.5 µl muscimol/per side) was bilaterally microinfused in the dCA1 30 min prior to training (n = 9) or prior to testing sessions (n = 6) with a 24 h intertrial interval. Memory was evaluated using the percentage of time spent in the nonaversive enclosed arms, whereas anxiety was measured by calculating the percentages of time spent and entries into open arms and the percentage of time spent self-grooming. Our findings corroborated previous data showing that the dCA1 is required for discriminative avoidance consolidation. Furthermore, additional information indicated that impaired long-term memory was associated with downregulated CREB-1 expression in the dDG and vCA3. Moreover, memory retrieval was not impaired by dCA1 inactivation prior to the testing session, which was associated with the upregulation of CREB-1 in the dCA3 and vCA1 and downregulation in the dCA1 and vCA3. Differential expression of CREB was not identified in the IL or PrL areas. These results improve our understanding of how the hippocampal circuitry mediates the acquisition and retrieval of aversive memory and anxiety.

Hypomagnetic Field Induces the Production of Reactive Oxygen Species and Cognitive Deficits in Mice Hippocampus

Previous studies have found that hypomagnetic field (HMF) exposure impairs cognition behaviors in animals; however, the underlying neural mechanisms of cognitive dysfunction are unclear. The hippocampus plays important roles in magnetoreception, memory, and spatial navigation in mammals. Therefore, the hippocampus may be the key region in the brain to reveal its neural mechanisms. We recently reported that long-term HMF exposure impairs adult hippocampal neurogenesis and cognition through reducing endogenous reactive oxygen species (ROS) levels in adult neural stem cells that are confined in the subgranular zone (SGZ) of the hippocampus. In addition to adult neural stem cells, the redox state of other cells in the hippocampus is also an important factor affecting the functions of the hippocampus. However, it is unclear whether and how long-term HMF exposure affects ROS levels in the entire hippocampus (i.e., the dentate gyrus (DG) and ammonia horn (CA) regions). Here, we demonstrate that male C57BL/6J mice exposed to 8-week HMF exhibit cognitive impairments. We then found that the ROS levels of the hippocampus were significantly higher in these HMF-exposed mice than in the geomagnetic field (GMF) group. PCR array analysis revealed that the elevated ROS levels were due to HMF-regulating genes that maintain the redox balance in vivo, such as Nox4, Gpx3. Since high levels of ROS may cause hippocampal oxidative stress, we suggest that this is another reason why HMF exposure induces cognitive impairment, besides the hippocampal neurogenesis impairments. Our study further demonstrates that GMF plays an important role in maintaining hippocampal function by regulating the appropriate endogenous ROS levels.

Teratogenic Effect of Aqueous Leaf Extract of Aspilia africana on The Dentate Gyrus of Wistar Rat Fetuses

Aspilia africana is an herbal plant widespread in Africa used for medicinal purposes and also used by pregnant women for health related issues. This study was aimed at investigating the teratogenic effect of aqueous leaf extract of Aspilia africana on the dentate gyrus of albino wistar rat fetuses. Twenty (20) female adult rats weighing between 190-205g were used for this study. The rats were divided into four groups; control, low dose, medium dose and high dose with each group containing five rats. Pregnancy was induced by caging the female rats with sexually matured males. The presence of vaginal plug and tail structure in the vaginal smear the following morning confirmed coition, and it was regarded as day 0 of pregnancy. The control group was given distilled water. The low dose, medium dose, and the high dose groups received 750mg/kg, 1000mg/kg, and 1250mg/kg body weight of aqueous leaf extract of Aspilia africana through an orogastric tube from day 7-11 of gestation. On the 20th day of gestation, the animals were sacrificed using chloroform-inhalation method. Their fetuses were harvested via uterectomy, the brain was excised and fixed in 10% buffered formalin, and then routine histological processes were carried out. Staining was done using Haematoxylin and Eosin method. Histological observation of the dentate gyri of experimental groups revealed marked distortion, reduction of the polymorphic layer, hyperplasia and hypertrophy of cells in the molecular and granular layer especially in the high dose group whose mothers received 1250mg/kg of the extracts. The result suggests high doses of aqueous leaf extract of Aspilia africana may be teratogenic to the dentate gyrus of Wistar rat fetuses.

Neurobehavioral alternations of the female offspring born to polycystic ovary syndrome model rats administered by Chinese herbal medicine

Background

Chinese herbal medicine (CHM) has significant effects that improve the reproductive functions of patients with polycystic ovary syndrome (PCOS). However, the intergenerational effects of CHM on offspring and the underlying mechanism of CHM remain unclear. This study aimed to explore the effects and the underlying mechanism of CHM, specifically the Bu-Shen-Tian-Jing formula (BSTJF), on model rats with polycystic ovary syndrome (PCOS) and the neurobehavioral alterations of female offspring born to PCOS rats administered BSTJF.

Methods

High-performance liquid chromatography-mass spectrometry (HPLC–MS) and network pharmacology analysis were performed to identify the active ingredients and potential targets of BSTJF. Moreover, PCOS model rats were used to validate the role of BSTJF in reproduction and progeny neural development and to confirm the network pharmacological targets.

Results

A total of 91 constituents were characterized from BSTJF. The 20 most significant KEGG pathways and the high-frequency genes of these pathways were predicted to be putative targets of these molecules. The rat experiment showed that the downregulation of FOS protein expression in the ovarian granulosa cells of the PCOS group was reversed by BSTJF. The target residence time of the 5-week-old female offspring of the BSTJF group was higher than that of the PCOS group in the water maze experiment. Compared to the PCOS group, the changes in dendritic spine density, ultrastructure of neurons and synapses, and Gabrb1 and Grin2b protein expression levels in the hippocampus of female offspring were partially reversed in the BSTJF group.

Conclusions

BSTJF can effectively improve ovarian follicle development in PCOS rats and has positive effects on pubertal neurobehavioral alterations in the female offspring of these rats by reversing dendritic spine density, the ultrastructure of neurons and synapses, and the Gabrb1 and Grin2b protein expression levels in the hippocampus.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13020-021-00512-4.

Variations of the grid and place cells in the entorhinal cortex and dentate gyrus of 6 individuals aged 56 to 87 years

INTRODUCTION

The relationship between the entorhinal cortex and the hippocampus has been studied by different authors, who have highlighted the importance of grid cells, place cells, and the trisynaptic circuit in the processes that they regulate: the persistence of spatial, explicit, and recent memory and their possible impairment with ageing.

OBJECTIVE

We aimed to determine whether older age causes changes in the size and number of grid cells contained in layer III of the entorhinal cortex and in the granular layer of the dentate gyrus of the hippocampus.

METHODS

We conducted post-mortem studies of the brains of 6 individuals aged 56-87 years. The brain sections containing the dentate gyrus and the adjacent entorhinal cortex were stained according to the Klüver-Barrera method, then the Image J software was used to measure the individual neuronal area, the total neuronal area, and the number of neurons contained in rectangular areas in layer III of the entorhinal cortex and layer II of the dentate gyrus. Statistical analysis was subsequently performed.

RESULTS

We observed an age-related reduction in the cell population of the external pyramidal layer of the entorhinal cortex, and in the number of neurons in the granular layer of the dentate gyrus.

CONCLUSION

Our results indicate that ageing causes a decrease in the size and density of grid cells of the entorhinal cortex and place cells of the dentate gyrus.

Immediate and cumulative effects of high-frequency repetitive transcranial magnetic stimulation on cognition and neuronal excitability in mice

This study primarily explored the potential effects of high-frequency (20 Hz) repetitive transcranial magnetic stimulation (rTMS) with different intervention protocols on cognition and neuronal excitability in mice. Mice were randomly divided into 4 groups: a control group that received sham stimulation, an rTMS in vitro group whose acute brain slices received high-frequency stimulation, an rTMS 1 d group that received high-frequency stimulation for only 1 d, and an rTMS 15 d group that received high-frequency stimulation for 15 d. The novel object recognition and step-down tests were used to assess cognitive ability. The patch-clamp technique was used to record the membrane potentials and neural discharges of dentate gyrus granule cells to evaluate neuronal excitability. Results revealed that cognition and neuronal excitability in the rTMS 15 d group were significantly increased than that in the control and rTMS 1 d groups. The neuronal excitability in the rTMS in vitro group was also significantly increased than that in the control and rTMS 1 d groups. No significant changes were observed between the control and rTMS 1 d groups. These results suggested that high-frequency rTMS applied to the acute brain slices of mice in vitro exerted an immediate effect on increasing neuronal excitability. Chronic high-frequency rTMS applied to the brain of mice in vivo exerted a cumulative effect in improving cognition and increasing neuronal excitability.

Effect of Acetamiprid on spatial memory and hippocampal glutamatergic system

Acetamiprid (ACE) is one of the widely used neonicotinoid insecticides. In mammals, in spite of the low-affinity nAChRs, neurotoxic effects following the Acetamiprid exposure have recently been reported, which suggests some concerns regarding the impacts on the nervous system of mammals. This study aims to investigate the effect of Acetamiprid on spatial memory and possible vulnerability of hippocampal glutamatergic system following the Acetamiprid exposure. 10, 20, and 40 mg/kg doses of Acetamiprid were administered to male rats by gavage once per day for 28 days. The spatial memory was examined with the Morris water maze apparatus. The amount of Acetamiprid in the serum and hippocampus was measured. In addition, glutamate level and changes in the expression of NR1, NR2, and NR2B genes were measured in the hippocampus; also, the hippocampus tissue was histologically evaluated. A significant increase in training parameters which consist of escape latency and traveled distance was observed on the first and second day of training in Acetamiprid-treated groups (20 and 40 mg/kg) compared to the control group (p < 0.001). In the probe test, rats in all Acetamiprid-treated groups significantly spent less time in the target quadrant compared to the control group (p < 0.001). Acetamiprid concentration dose dependently increased in the serum and in the hippocampus followed by Acetamiprid exposure. In all Acetamiprid-treated groups, a significant reduction of glutamate level in the hippocampus was observed (p < 0.05). The reduction of NR1, NR2A, and NR2B gene expression in the hippocampus was observed at a dose of 20 mg/kg. The histological evaluation showed neural degeneration in the dentate gyrus area of the hippocampus at a dose of 40 mg/kg in the Acetamiprid-treated group. The results of the present study indicate that Acetamiprid impairs memory consolidation through the reduction of glutamate and the expression of NMDA receptor subunits in the hippocampus at low doses, along with the loss of neural cells in dentate gyrus at high dose.

Working and Reference Memory Tasks Trigger Opposed Long-Term Synaptic Changes in the Rat Dentate Gyrus

Long-term storage of information into memory is supposed to rely on long-term synaptic plasticity processes. The detection of such synaptic changes after training in long-term/reference memory (RM) tasks has yet been scarce, variable and only studied on a short time scale. Short-term or working memory (WM) is largely known to depend on persistent neuronal activity or short-term plasticity. However, processing information into WM could also involve long-term synaptic changes that could be responsible for the erasure/forgetting of items previously stored in WM and acting as proactive interference. In order to study long-term synaptic changes associated with RM or WM, we trained chronically implanted rats in 3 different radial maze tasks: a classical RM task and 2 WM tasks involving different levels of proactive interference. Synaptic responses in the dentate gyrus were recorded during 2 × 24 h in freely moving rats after training. We found that consolidation of long-term information leads first to a delayed synaptic potentiation, occurring 9 h after RM training that is replaced by a synaptic depression once the RM rule is fully acquired. In contrast, optimal information processing into WM triggers a synaptic depression immediately after training and lasting 3 h that could act as a mechanism for interference erasure/forgetting.

Impact of neonatal anoxia and hypothermic treatment on development and memory of rats

Therapeutic hypothermia (TH) is well established as a standard treatment for term and near-term infants. However, therapeutic effects of hypothermia following neonatal anoxia in very premature babies remains inconclusive. The present rodent model of preterm neonatal anoxia has been shown to alter developmental milestones and hippocampal neurogenesis, and to disrupt spatial learning and memory in adulthood. These effects seem to be reduced by post-insult hypothermia. Epigenetic-related mechanisms have been postulated as valuable tools for developing new therapies. Dentate gyrus neurogenesis is regulated by epigenetic factors. This study evaluated whether TH effects in a rodent model of preterm oxygen deprivation are based on epigenetic alterations. The effects of TH on both developmental features (somatic growth, maturation of physical characteristics and early neurological reflexes) and performance of behavioral tasks at adulthood (spatial reference and working memory, and fear conditioning) were investigated in association with the possible involvement of the epigenetic operator Enhancer of zeste homolog 2 (Ezh2), possibly related to long-lasting effects on hippocampal neurogenesis. Results showed that TH reduced both anoxia-induced hippocampal neurodegeneration and anoxia-induced impairments on risk assessment behavior, acquisition of spatial memory, and extinction of auditory and contextual fear conditioning. In contrast, TH did not prevent developmental alterations caused by neonatal anoxia and did not restore hippocampal neurogenesis or cause changes in EZH2 levels. In conclusion, despite the beneficial effects of TH in hippocampal neurodegeneration and in reversing disruption of performance of behavioral tasks following oxygen deprivation in prematurity, these effects seem not related to developmental alterations and hippocampal neurogenesis and, apparently, is not caused by Ezh2-mediated epigenetic alteration.

Evaluation of the dentate gyrus in adult mice exposed to acetaminophen (paracetamol) on postnatal day 10

Acetaminophen (AAP; or paracetamol) is a widely used nonprescription drug with antipyretic and analgesic properties. Alarmingly, there is an increasing body of evidence showing that developmental exposure to AAP is associated with adverse behavioural outcomes later in life. We have previously shown that relevant doses of AAP in 10-day-old mice affected memory, learning and locomotor activity in the adult animals. Interestingly, the neurons of the dentate gyrus (DG) have a relatively late time of origin as they are generated during the first two weeks of postnatal life in rodents. Since the generation of these cells, which are important for memory processing, coincides with our AAP exposure, we aim to investigate if the cytoarchitecture of the DG is affected by postnatal day 10 AAP treatment. In addition, we investigate if markers for differentiation and migration in the hippocampus were affected by the same treatment. We did not observe any visual effects in adult DG cytoarchitecture, nor any changes of markers for differentiation/migration in the hippocampus in 24 hr after exposure. Even though a large effect size was estimated on adult DG thickness following AAP exposure, the estimated 95% CIs around the differences of the means reveal no significant effect. Hence, larger sample sizes are warranted to clarify if neonatal AAP exposure affects adult DG thickness in mice.

Developmental, cellular, and behavioral phenotypes in a mouse model of congenital hypoplasia of the dentate gyrus

In the hippocampus, a widely accepted model posits that the dentate gyrus improves learning and memory by enhancing discrimination between inputs. To test this model, we studied conditional knockout mice in which the vast majority of dentate granule cells (DGCs) fail to develop – including nearly all DGCs in the dorsal hippocampus – secondary to eliminating Wntless (Wls) in a subset of cortical progenitors with Gfap-Cre. Other cells in the Wls^fl/-;Gfap-Cre hippocampus were minimally affected, as determined by single nucleus RNA sequencing. CA3 pyramidal cells, the targets of DGC-derived mossy fibers, exhibited normal morphologies with a small reduction in the numbers of synaptic spines. Wls^fl/-;Gfap-Cre mice have a modest performance decrement in several complex spatial tasks, including active place avoidance. They were also modestly impaired in one simpler spatial task, finding a visible platform in the Morris water maze. These experiments support a role for DGCs in enhancing spatial learning and memory.

Dentate gyrus circuits for encoding, retrieval and discrimination of episodic memories

The dentate gyrus (DG) has a key role in hippocampal memory formation. Intriguingly, DG lesions impair many, but not all, hippocampus-dependent mnemonic functions, indicating that the rest of the hippocampus (CA1-CA3) can operate autonomously under certain conditions. An extensive body of theoretical work has proposed how the architectural elements and various cell types of the DG may underlie its function in cognition. Recent studies recorded and manipulated the activity of different neuron types in the DG during memory tasks and have provided exciting new insights into the mechanisms of DG computational processes, particularly for the encoding, retrieval and discrimination of similar memories. Here, we review these DG-dependent mnemonic functions in light of the new findings and explore mechanistic links between the cellular and network properties of, and the computations performed by, the DG.

Oral treatment with royal jelly improves memory and presents neuroprotective effects on icv-STZ rat model of sporadic Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive decline in cognitive function. Intracerebroventricular injection of streptozotocin (icv-STZ) has been used as an experimental model of Sporadic AD (SAD) in rodents and represents a promising tool for etiopathogenic analysis and evaluation of new therapeutic proposals for AD. The icv-STZ model shows many aspects of SAD abnormalities, resulting in decreased brain glucose and energy metabolism, cognitive impairment, oxidative stress, neuronal loss, and amyloid angiopathy. Royal jelly (RJ), a substance produced by worker honeybees of the Apis mellifera species, has been popularly used for more than 30 years in areas related to health eating and natural medicine. Researches indicate that RJ has a several pharmacological activities, including neuroprotective and improvement of cognitive function. The objective of this study was to investigate the effects of oral treatment with royal jelly during 2 weeks in Wistar rats submitted to icv-STZ on a working memory and neuroprotection, as evaluated by neurogenesis, neurodegeneration and oxidative stress. In this study, icv-STZ injection induced deleterious effects in the hippocampus, associated with cognitive impairments, and developed marked neurodegeneration, besides the reduction of neurogenesis and increased oxidative stress. On the other hand, RJ long-term oral administration induced beneficial effects in animals injured by icv-STZ injection, increasing retention time for working spatial memory, reducing neurodegeneration and oxidative stress level and increasing the proliferation of new neurons in the hippocampus. Thus, RJ promotes beneficial effects on cognitive functions and exhibits a neuroprotective action in the STZ experimental model of SAD.

Lgl1 deficiency disrupts hippocampal development and impairs cognitive performance in mice

Cellular polarity is crucial for brain development and morphogenesis. Lethal giant larvae 1 (Lgl1) plays a crucial role in the establishment of cell polarity from Drosophila to mammalian cells. Previous studies have found the importance of Lgl1 in the development of cerebellar, olfactory bulb, and cerebral cortex. However, the role of Lgl1 in hippocampal development during the embryonic stage and function in adult mice is still unknown. In our study, we created Lgl1-deficient hippocampus mice by using Emx1-Cre mice. Histological analysis showed that the Emx1-Lgl1^-/- mice exhibited reduced size of the hippocampus with severe malformations of hippocampal cytoarchitecture. These defects mainly originated from the disrupted hippocampal neuroepithelium, including increased cell proliferation, abnormal interkinetic nuclear migration, reduced differentiation, increased apoptosis, gradual disruption of adherens junctions, and abnormal neuronal migration. The radial glial scaffold was disorganized in the Lgl1-deficient hippocampus. Thus, Lgl1 plays a distinct role in hippocampal neurogenesis. In addition, the Emx1-Lgl1^-/- mice displayed impaired behavioral performance in the Morris water maze and fear conditioning test.

Reduced hippocampal subfield volumes and memory function in school-aged children born preterm with very low birthweight (VLBW)

BACKGROUND

The hippocampus, an essential structure for learning and memory, has a reduced volume in preterm born (gestational age < 37 weeks) individuals with very low birth weight (VLBW: birth weight < 1500 g), which may affect memory function. However, the hippocampus is a complex structure with distinct subfields related to specific memory functions. These subfields are differentially affected by a variety of neuropathological conditions, but it remains unclear how these subfields may be affected by medical complications following preterm birth which may cause aberrant brain development, and the consequences of this on learning and memory function in children with VLBW.

METHODS

Children born preterm with VLBW (n = 34) and term-born controls from the Norwegian Mother and Child Cohort Study (MoBa) (n = 104) underwent structural MRI and a neuropsychological assessment of memory function at primary school age. FreeSurfer 6.0 was used to analyze the volumes of hippocampal subfields which were compared between groups, as was memory performance. Correlations between abnormal hippocampal subfields and memory performance were explored in the VLBW group.

RESULTS

All absolute hippocampal subfield volumes were lower in the children with VLBW compared to MoBa term-born controls, and the volumes of the left and right dentate gyrus and the right subiculum remained significantly lower after correcting for total intracranial volume. The VLBW group had inferior working memory performance and the score on the subtest Spatial Span backwards was positively correlated to the volume of the right dentate gyrus.

CONCLUSIONS

Hippocampal subfield volumes seem to be differently affected by early brain development related to preterm birth. The dentate gyrus appears particularly susceptible to adverse effects of preterm birth. Reduced working memory function among children with VLBW was associated with smaller volume of right dentate gyrus. This finding demonstrates alterations in hippocampal structure-function relationships associated with early brain development related to preterm birth.

Functional Brain Connectivity Revealed by Sparse Coding of Large-Scale Local Field Potential Dynamics

Exploration of brain dynamics patterns has attracted increasing attention due to its fundamental significance in understanding the working mechanism of the brain. However, due to the lack of effective modeling methods, how the simultaneously recorded LFP can inform us about the brain dynamics remains a general challenge. In this paper, we propose a novel sparse coding based method to investigate brain dynamics of freely-behaving mice from the perspective of functional connectivity, using super-long local field potential (LFP) recordings from 13 distinct regions of the mouse brain. Compared with surrogate datasets, six and four reproducible common functional connectivities were discovered to represent the space of brain dynamics in the frequency bands of alpha and theta respectively. Modeled by a finite state machine, temporal transition framework of functional connectivities was inferred for each frequency band, and evident preference was discovered. Our results offer a novel perspective for analyzing neural recording data at such high temporal resolution and recording length, as common functional connectivities and their transition framework discovered in this work reveal the nature of the brain dynamics in freely behaving mice.

The absence of maternal pineal melatonin rhythm during pregnancy and lactation impairs offspring physical growth, neurodevelopment, and behavior

Maternal melatonin provides photoperiodic information to the fetus and thus influences the regulation and timing of the offspring's internal rhythms and preparation for extra-uterine development. There is clinical evidence that melatonin deprivation of both mother and fetus during pregnancy, and of the neonate during lactation, results in negative long-term health outcomes. As a consequence, we hypothesized that the absence of maternal pineal melatonin might determine abnormal brain programming in the offspring, which would lead to long-lasting implications for behavior and brain function. To test our hypothesis, we investigated in rats the effects of maternal melatonin deprivation during gestation and lactation (MMD) to the offspring and the effects of its therapeutic replacement. The parameters evaluated were: (1) somatic, physical growth and neurobehavioral development of pups of both sexes; (2) hippocampal-dependent spatial learning and memory of the male offspring; (3) adult hippocampal neurogenesis of the male offspring. Our findings show that MMD significantly delayed male offspring's onset of fur development, pinna detachment, eyes opening, eruption of superior incisor teeth, testis descent and the time of maturation of palmar grasp, righting reflex, free-fall righting and walking. Conversely, female offspring neurodevelopment was not affected. Later on, male offspring show that MMD was able to disrupt both spatial reference and working memory in the Morris Water Maze paradigm and these deficits correlate with changes in the number of proliferative cells in the hippocampus. Importantly, all the observed impairments were reversed by maternal melatonin replacement therapy. In summary, we demonstrate that MMD delays the appearance of physical features, neurodevelopment and cognition in the male offspring, and points to putative public health implications for night shift working mothers.

Dentate network activity is necessary for spatial working memory by supporting CA3 sharp-wave ripple generation and prospective firing of CA3 neurons

Complex spatial working memory tasks have been shown to require both hippocampal sharp-wave ripple (SWR) activity and dentate gyrus (DG) neuronal activity. We therefore asked whether DG inputs to CA3 contribute to spatial working memory by promoting SWR generation. Recordings from DG and CA3 while rats performed a dentate-dependent working memory task on an eight-arm radial maze revealed that the activity of dentate neurons and the incidence rate of SWRs both increased during reward consumption. We then found reduced reward-related CA3 SWR generation without direct input from dentate granule neurons. Furthermore, CA3 cells with place fields in not-yet-visited arms preferentially fired during SWRs at reward locations, and these prospective CA3 firing patterns were more pronounced for correct trials and were dentate-dependent. These results indicate that coordination of CA3 neuronal activity patterns by DG is necessary for the generation of neuronal firing patterns that support goal-directed behavior and memory.

Spatial learning and neurogenesis: Effects of cessation of wheel running and survival of novel neurons by engagement in cognitive tasks

Physical exercise stimulates cell proliferation in the adult dentate gyrus and facilitates acquisition and/or retention of hippocampal-dependent tasks. It is established that regular physical exercise improves cognitive performance. However, it is unclear for how long these benefits last after its interruption. Independent groups of rats received both free access to either unlocked (EXE Treatment) or locked (No-EXE Treatment) running wheels for 7 days, and daily injections of bromodeoxyuridine (BrdU) in the last 3 days. After a time delay period of either 1, 3, or 6 weeks without training, the animals were tested in the Morris water maze (MWM) either in a working memory task dependent on hippocampal function (MWM-HD) or in a visible platform searching task, independent on hippocampal function (MWM-NH). Data confirmed that exposure of rats to 7 days of spontaneous wheel running increases cell proliferation and neurogenesis. In contrast, neurogenesis was not accompanied by significant improvements of performance in the working memory version of the MWM. Longer time delays between the end of exercise and the beginning of cognitive training in the MWM resulted in lower cell survival; that is, the number of novel surviving mature neurons was decreased when this delay was 6 weeks as compared with when it was 1 week. In addition, data showed that while exposure to the MWM-HD working memory task substantially increased survival of novel neurons, exposure to the MWM-NH task did not, thus indicating that survival of novel dentate gyrus neurons depends on the engagement of this brain region in performance of cognitive tasks. © 2015 Wiley Periodicals, Inc.

Mice lacking the PSD-95-interacting E3 ligase, Dorfin/Rnf19a, display reduced adult neurogenesis, enhanced long-term potentiation, and impaired contextual fear conditioning

Protein ubiquitination has a significant influence on diverse aspects of neuronal development and function. Dorfin, also known as Rnf19a, is a RING finger E3 ubiquitin ligase implicated in amyotrophic lateral sclerosis and Parkinson's disease, but its in vivo functions have not been explored. We report here that Dorfin is a novel binding partner of the excitatory postsynaptic scaffolding protein PSD-95. Dorfin-mutant (Dorfin(-/-)) mice show reduced adult neurogenesis and enhanced long-term potentiation in the hippocampal dentate gyrus, but normal long-term potentiation in the CA1 region. Behaviorally, Dorfin(-/-) mice show impaired contextual fear conditioning, but normal levels of cued fear conditioning, fear extinction, spatial learning and memory, object recognition memory, spatial working memory, and pattern separation. Using a proteomic approach, we also identify a number of proteins whose ubiquitination levels are decreased in the Dorfin(-/-) brain. These results suggest that Dorfin may regulate adult neurogenesis, synaptic plasticity, and contextual fear memory.

Prostaglandin E2 EP2 activation reduces memory decline in R6/1 mouse model of Huntington's disease by the induction of BDNF-dependent synaptic plasticity

Huntington's disease (HD) patients and mouse models show learning and memory impairment even before the onset of motor symptoms. Deficits in hippocampal synaptic plasticity have been involved in the HD memory impairment. Several studies show that prostaglandin E2 (PGE2) EP2 receptor stimulates synaptic plasticity and memory formation. However, this role was not explored in neurodegenerative diseases. Here, we investigated the capacity of PGE2 EP2 receptor to promote synaptic plasticity and memory improvements in a model of HD, the R6/1 mice, by administration of the agonist misoprostol. We found that misoprostol increases dendritic branching in cultured hippocampal neurons in a brain-derived neurotrophic factor (BDNF)-dependent manner. Then, we implanted an osmotic mini-pump system to chronically administrate misoprostol to R6/1 mice from 14 to 18weeks of age. We observed that misoprostol treatment ameliorates the R6/1 long-term memory deficits as analyzed by the T-maze spontaneous alternation task and the novel object recognition test. Importantly, administration of misoprostol promoted the expression of hippocampal BDNF. Moreover, the treatment with misoprostol in R6/1 mice blocked the reduction in the number of PSD-95 and VGluT-1 positive particles observed in hippocampus of vehicle-R6/1 mice. In addition, we observed an increase of cAMP levels in the dentate ` of WT and R6/1 mice treated with misoprostol. Accordingly, we showed a reduction in the number of mutant huntingtin nuclear inclusions in the dentate gyrus of R6/1 mice. Altogether, these results suggest a putative therapeutic effect of PGE2 EP2 receptor in reducing cognitive deficits in HD.

Glibenclamide for the treatment of ischemic and hemorrhagic stroke

Ischemic and hemorrhagic strokes are associated with severe functional disability and high mortality. Except for recombinant tissue plasminogen activator, therapies targeting the underlying pathophysiology of central nervous system (CNS) ischemia and hemorrhage are strikingly lacking. Sur1-regulated channels play essential roles in necrotic cell death and cerebral edema following ischemic insults, and in neuroinflammation after hemorrhagic injuries. Inhibiting endothelial, neuronal, astrocytic and oligodendroglial sulfonylurea receptor 1-transient receptor potential melastatin 4 (Sur1-Trpm4) channels and, in some cases, microglial KATP (Sur1-Kir6.2) channels, with glibenclamide is protective in a variety of contexts. Robust preclinical studies have shown that glibenclamide and other sulfonylurea agents reduce infarct volumes, edema and hemorrhagic conversion, and improve outcomes in rodent models of ischemic stroke. Retrospective studies suggest that diabetic patients on sulfonylurea drugs at stroke presentation fare better if they continue on drug. Additional laboratory investigations have implicated Sur1 in the pathophysiology of hemorrhagic CNS insults. In clinically relevant models of subarachnoid hemorrhage, glibenclamide reduces adverse neuroinflammatory and behavioral outcomes. Here, we provide an overview of the preclinical studies of glibenclamide therapy for CNS ischemia and hemorrhage, discuss the available data from clinical investigations, and conclude with promising preclinical results that suggest glibenclamide may be an effective therapeutic option for ischemic and hemorrhagic stroke.

Adult hippocampal neurogenesis and its role in cognition

Adult hippocampal neurogenesis (AHN) has intrigued neuroscientists for decades. Several lines of evidence show that adult-born neurons in the hippocampus are functionally integrated and contribute to cognitive function, in particular learning and memory processes. Biological properties of immature hippocampal neurons indicate that these cells are more easily excitable compared with mature neurons, and demonstrate enhanced structural plasticity. The structure in which adult-born hippocampal neurons are situated-the dentate gyrus-is thought to contribute to hippocampus function by disambiguating similar input patterns, a process referred to as pattern separation. Several ideas about AHN function have been put forward; currently there is good evidence in favor of a role for AHN in pattern separation. This function of AHN may be understood within a 'representational-hierarchical' view of brain organization. WIREs Cogn Sci 2014, 5:573-587. doi: 10.1002/wcs.1304 For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Physical exercise-induced adult neurogenesis: a good strategy to prevent cognitive decline in neurodegenerative diseases?

Cumulative evidence has indicated that there is an important role for adult hippocampal neurogenesis in cognitive function. With the increasing prevalence of cognitive decline associated with neurodegenerative diseases among the ageing population, physical exercise, a potent enhancer of adult hippocampal neurogenesis, has emerged as a potential preventative strategy/treatment to reduce cognitive decline. Here we review the functional role of adult hippocampal neurogenesis in learning and memory, and how this form of structural plasticity is altered in neurodegenerative diseases known to involve cognitive impairment. We further discuss how physical exercise may contribute to cognitive improvement in the ageing brain by preserving adult neurogenesis, and review the recent approaches for measuring changes in neurogenesis in the live human brain.

Adult neurogenesis in the mammalian hippocampus: why the dentate gyrus?

In the adult mammalian brain, newly generated neurons are continuously incorporated into two networks: interneurons born in the subventricular zone migrate to the olfactory bulb, whereas the dentate gyrus (DG) of the hippocampus integrates locally born principal neurons. That the rest of the mammalian brain loses significant neurogenic capacity after the perinatal period suggests that unique aspects of the structure and function of DG and olfactory bulb circuits allow them to benefit from the adult generation of neurons. In this review, we consider the distinctive features of the DG that may account for it being able to profit from this singular form of neural plasticity. Approaches to the problem of neurogenesis are grouped as "bottom-up," where the phenotype of adult-born granule cells is contrasted to that of mature developmentally born granule cells, and "top-down," where the impact of altering the amount of neurogenesis on behavior is examined. We end by considering the primary implications of these two approaches and future directions.

Delayed amyloid plaque deposition and behavioral deficits in outcrossed AβPP/PS1 mice

Alzheimer's disease (AD) is a progressive neurodegenerative dementia characterized by amyloid plaque accumulation, synapse/dendrite loss, and cognitive impairment. Transgenic mice expressing mutant forms of amyloid-β precursor protein (AβPP) and presenilin-1 (PS1) recapitulate several aspects of this disease and provide a useful model system for studying elements of AD progression. AβPP/PS1 mice have been previously shown to exhibit behavioral deficits and amyloid plaque deposition between 4-9 months of age. We crossed AβPP/PS1 animals with mice of a mixed genetic background (C57BL/6 × 129/SvJ) and investigated the development of AD-like features in the resulting outcrossed mice. The onset of memory-based behavioral impairment is delayed considerably in outcrossed AβPP/PS1 mice relative to inbred mice on a C57BL/6 background. While inbred AβPP/PS1 mice develop deficits in radial-arm water maze performance and novel object recognition as early as 8 months, outcrossed AβPP/PS1 mice do not display defects until 18 months. Within the forebrain, we find that inbred AβPP/PS1 mice have significantly higher amyloid plaque burden at 12 months than outcrossed AβPP/PS1 mice of the same age. Surprisingly, inbred AβPP/PS1 mice at 8 months have low plaque burden, suggesting that plaque burden alone cannot explain the accompanying behavioral deficits. Analysis of AβPP processing revealed that elevated levels of soluble Aβ correlate with the degree of behavioral impairment in both strains. Taken together, these findings suggest that animal behavior, amyloid plaque deposition, and AβPP processing are sensitive to genetic differences between mouse strains.

Inhibition of the Sur1-Trpm4 channel reduces neuroinflammation and cognitive impairment in subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Subarachnoid hemorrhage (SAH) can leave patients with memory impairments that may not recover fully. Molecular mechanisms are poorly understood, and no treatment is available. The sulfonylurea receptor 1-transient receptor potential melastatin 4 (Sur1-Trpm4) channel plays an important role in acute central nervous system injury. We evaluated upregulation of Sur1-Trpm4 in humans with SAH and, in rat models of SAH, we examined Sur1-Trpm4 upregulation, its role in barrier dysfunction and neuroinflammation, and its consequences on spatial learning.

METHODS

We used Förster resonance energy transfer to detect coassociated Sur1 and Trpm4 in human autopsy brains with SAH. We studied rat models of SAH involving filament puncture of the internal carotid artery or injection of blood into the subarachnoid space of the entorhinal cortex. In rats, we used Förster resonance energy transfer and coimmunoprecipitation to detect coassociated Sur1 and Trpm4, we measured immunoglobulin G extravasation and tumor necrosis α overexpression as measures of barrier dysfunction and neuroinflammation, and we assessed spatial learning and memory on days 7 to 19.

RESULTS

Sur1-Trpm4 channels were upregulated in humans and rats with SAH. In rats, inhibiting Sur1 using antisense or the selective Sur1 inhibitor glibenclamide reduced SAH-induced immunoglobulin G extravasation and tumor necrosis α overexpression. In models with entorhinal SAH, rats treated with glibenclamide for 7 days after SAH exhibited better platform search strategies and better performance on incremental and rapid spatial learning than vehicle-treated controls.

CONCLUSIONS

Sur1-Trpm4 channels are upregulated in humans and rats with SAH. Channel inhibition with glibenclamide may reduce neuroinflammation and the severity of cognitive deficits after SAH.

Hippocampal neurogenesis levels predict WATERMAZE search strategies in the aging brain

The hippocampus plays a crucial role in the formation of spatial memories, and it is thought that adult hippocampal neurogenesis may participate in this form of learning. To better elucidate the relationship between neurogenesis and spatial learning, we examined both across the entire life span of mice. We found that cell proliferation, neuronal differentiation, and neurogenesis significantly decrease with age, and that there is an abrupt reduction in these processes early on, between 1.5-3 months of age. After this, the neurogenic capacity continues to decline steadily. The initial abrupt decline in adult neurogenesis was paralleled by a significant reduction in Morris Water Maze performance, however overall learning and memory remained constant thereafter. Further analysis of the search strategies employed revealed that reductions in neurogenesis in the aging brain were strongly correlated with the adoption of spatially imprecise search strategies. Overall, performance measures of learning and memory in the Morris Water Maze were maintained at relatively constant levels in aging animals due to an increase in the use of spatially imprecise search strategies.

Melatonin increases dendritogenesis in the hilus of hippocampal organotypic cultures

Neuropsychiatric disorders are characterized by hippocampus decreased volume and loss of dendrite arborizations in the subiculum and prefrontal cortex. These structural changes are associated with diminished memory performance. Hilar neurons of the hippocampus integrate spatial memory and are lost in dementia. They receive information from dentate gyrus neurons through dendrites, while they send axonal tracts to the CA3 region. Dendrites are complex structures of neurons that receive chemical information from presynaptic and postsynaptic terminals. Melatonin, the main product of the pineal gland, has neuroprotective actions through its free radical-scavenging properties and decreases neuronal apoptosis. Recently, we found that melatonin increases dendrite maturation and complexity in new neurons formed in the dentate gyrus of mice. In addition, in N1E-115 cultured cells, the indole stimulates early stages of neurite formation, a process that is known to antecede dendrite formation and maturation. Thus, in this study, we explored whether melatonin stimulates dendrite formation and complexity in the adult rat hippocampus in organotypic slice cultures, which is a model that preserves the hippocampal circuitry and their tridimensional organizations of connectivity. The effects of melatonin were studied in nonpathological conditions and in the absence of harmful agents. The results showed that the indole at nocturnal concentrations reached in the cerebrospinal fluid stimulates dendritogenesis at formation, growth, and maturation stages. Also, data showed that dendrites formed became competent to form presynaptic specializations. Evidence strongly suggests that melatonin may be useful in the treatment of neuropsychiatric diseases to repair the loss of dendrites and re-establish lost synaptic connections.

A role for dorsal and ventral hippocampus in response learning

The hippocampus and the striatum have been traditionally considered as part of different and independent memory systems despite growing evidence supporting that both brain regions may even compete for behavioral control in particular learning tasks. In this regard, it has been reported that the hippocampus could be necessary for the use of idiothetic cues in several types of spatial learning tasks. Accordingly, the ventral striatum receives strong anatomical projections from the hippocampus, suggesting a participation of both regions in goal-directed behavior. Our work examined the role of the dorsal and ventral hippocampus on a response learning task. Cytochrome c oxidase (C.O.) quantitative histochemistry was used as an index of brain oxidative metabolism. In addition, determination of C.O. subunit I levels in the hippocampus by western blot analysis was performed to assess the contribution of this subunit to overall C.O. activity. Increased brain oxidative metabolism was found in most of the studied hippocampal subregions when experimental group was compared with a swim control group. However, no differences were found in the amount of C.O. subunit I expressed in the hippocampus by western blot analysis. Our results support that both the dorsal and ventral hippocampus are associated with the use of response strategies during response learning.

Selective lesions of the dentate gyrus produce disruptions in place learning for adjacent spatial locations

The hippocampus (HPP) plays a known role in learning novel spatial information. More specifically, the dentate gyrus (DG) hippocampal subregion is thought to support pattern separation, a mechanism for encoding and separating spatially similar events into distinct representations. Several studies have shown that lesions of the dorsal DG (dDG) in rodents result in inefficient spatial pattern separation for working memory; however, it is unclear whether selective dDG lesions disrupt spatial pattern separation for reference memory. Therefore, the current study investigated the role of the dDG in pattern separation using a spatial reference memory paradigm to determine whether the dDG is necessary for acquiring spatial discriminations for adjacent locations. Male Long-Evans rats were randomly assigned to receive bilateral intracranial infusions of colchicine or saline (control) into the dDG. Following recovery from surgery, each rat was pseudo-randomly assigned to an adjacent arm or separate arm condition and subsequently tested on a place-learning task using an eight-arm radial maze. Rats were trained to discriminate between a rewarded arm and a nonrewarded arm that were either adjacent to one another or separated by a distance of two arm positions. Each rat received 10 trials per day and was tested until the animal reached a criterion of nine correct choices out of 10 consecutive trials across 2 consecutive days of testing. Both groups acquired spatial discriminations for the separate condition at similar rates. However, in the adjacent condition, dDG lesioned animals required significantly more trials to reach the learning criterion than controls. The results suggest that dDG lesions decrease efficiency in pattern separation resulting in impairments in the adjacent condition involving greater overlap among the distal cues. Conversely, in the separate condition, there was less overlap among distal cues during encoding and less need for pattern separation. These findings provide further support for a critical role for the dDG in spatial pattern separation by demonstrating the importance of a processing mechanism that is capable of reducing interference among overlapping spatial inputs across a variety of memory demands.

Voluntary wheel running enhances contextual but not trace fear conditioning

Exercise improves performance on a number of hippocampus involved cognitive tasks including contextual fear conditioning, but whether exercise enhances contextual fear when the retention interval is longer than 1 day is not known. Also unknown is whether exercise improves trace conditioning, a task that requires the hippocampus to bridge the time interval between stimuli. Hence, 4-month-old male C57BL/6J mice were housed with or without running wheels. To assess whether hippocampal neurogenesis was associated with behavioral outcomes, during the initial 10 days, mice received Bromodeoxyuridine to label dividing cells. After 30 days, one group of mice was trained in a contextual fear conditioning task. Freezing to context was assessed 1, 7, or 21 days post-training. A separate group was trained on a trace procedure, in which a tone and footshock were separated by a 15, 30, or 45s interval. Freezing to the tone was measured 24h later in a novel environment, and freezing to the training context was measured 48h later. Running enhanced freezing to context when the retention interval was 1, but not 7 or 21 days. Running had no effect on trace conditioning even though runners displayed enhanced freezing to the training context 48h later. Wheel running increased survival of new neurons in the hippocampus. Collectively, findings indicate that wheel running enhances cognitive performance on some tasks but not others and that enhanced neurogenesis is not always associated with improved performance on hippocampus tasks, one example of which is trace conditioning.

Functions for adult neurogenesis in memory: an introduction to the neurocomputational approach and to its contribution

Until recently, it was believed that the introduction of new neurons in neuronal networks was incompatible with memory function. Since the rediscovery of adult hippocampal neurogenesis, behavioral data demonstrate that adult neurogenesis is required for memory processing. We examine neurocomputational studies to identify which basic mechanisms involved in memory might be mediated by adult neurogenesis. Mainly, adult neurogenesis might be involved in the reduction of catastrophic interference and in a time-related pattern separation function. Artificial neuronal networks suggest that the selective recruitment of new-born or old neurons is not stochastic, but depends on environmental requirements. This leads us to propose the novel concept of "soft-supervision". Soft-supervision would be a biologically plausible process, by which the environment is able to influence activation and learning rules of neurons differentially.

Pathway-specific alteration of synaptic plasticity in Tg2576 mice

Various animal models of Alzheimer disease (AD) are characterized by deficits in spatial memory that are causally related to altered synaptic function and impairment of long-term potentiation (LTP) in the hippocampus. In Tg2576 AD mice, we compared LTP in 2 major hippocampal pathways, Schaffer collateral (SC) and mossy fiber (MF) pathways. Whereas LTP was completely abolished in the SC pathway of Tg2576 mice, we found no decrease in LTP induced by stimulation of the MF pathway. In fact, we found that in the MF pathway, LTP was slightly, but significantly, enhanced compared with that in the MF pathway of WT littermates. This pathway-specific impairment of LTP is not attributable to alterations in transmitter release, as indicated by an unaltered paired-pulse ratio. These results suggest that the spatial memory deficits normally seen in AD models arise primarily from LTP impairment at the SC pathway.

Critical maturational period of new neurons in adult dentate gyrus for their involvement in memory formation

Adult dentate gyrus produces new neurons continuously throughout life. Multiple lines of evidence have pointed to the possibility that young neurons during a certain maturational stage mediate an important role in memory processing. In this review, we highlight the existing evidence of a 'critical period' for new neurons in their involvement in memory formation, describe the unique properties of young neurons as potential mechanisms underlying the critical period, and discuss the implications of the critical period for the function of adult neurogenesis.

A new chapter in the field of memory: adult hippocampal neurogenesis

Understanding the cellular mechanisms underlying learning and memory is a major challenge in neurobiology. Structural and functional changes occurring in the hippocampus such as synaptic remodeling and long-term potentiation are key signatures of long-term memory processes. The discovery of a de novo hippocampal production of neurons in the adult brain has been a breakthrough in the field of plasticity and memory, introducing a new actor that could sustain memory processes. Here we will review our current knowledge on the role of these adult new neurons in memory. In particular we will provide evidence showing that they are required for learning and memory and that an alteration in their production rate or maturation leads to memory impairments. Through a thorough survey of the literature, we will also acknowledge that there are many controversies regarding the specific role played by newborn neurons. The emerging picture is that they are involved in the establishment of spatiotemporal relationships among multiple environmental cues for the flexible use of the acquired information. Indeed, newborn neurons have been found to be required for separating events based on their spatial and temporal characteristics, a process that preserves the uniqueness of a memory representation. Thus, adult-born neurons are required for allocentric space representation, for long-term memory retention and for flexible inferential memory expression. Finally, we will conclude by highlighting directions for future research, emphasizing that the exact participation of newborn neurons in memory processes will not be approached without considering the hippocampal network in general.