Evidence for hippocampal role in place avoidance other than merely memory storage

Spatial navigation is used as a popular animal model of higher cognitive functions in people. The data suggest that the hippocampus is important for both storing spatial memories and for performing spatial computations necessary for navigation. Animals use multiple behavioral strategies to solve spatial tasks often using multiple memory systems. We investigated how inactivation of the rat hippocampus affects performance in a place avoidance task to determine if the role of the hippocampus in this task could be attributed to memory storage/retrieval or to the computations needed for navigation. Injecting tetrodotoxin (TTX) into both hippocampi impaired conditioned place avoidance, but after injecting only one hippocampus, the rats learned the place avoidance as well as without any injections. Retention of the place avoidance learned with one hippocampus was not impaired when the injection was switched to the hippocampus that had not been injected during learning. The result suggests that during learning, the hippocampus did not store the place avoidance memory.     

Nestin Null Mice Show Improved Reversal Place Learning

The intermediate filament protein nestin is expressed by neural stem cells, but also by some astrocytes in the neurogenic niche of the hippocampus in the adult rodent brain. We recently reported that nestin-deficient (Nes^?/?) mice showed increased adult hippocampal neurogenesis, reduced Notch signaling from Nes^?/? astrocytes to the neural stem cells, and impaired long-term memory. Here we assessed learning and memory of Nes^?/? mice in a home cage set up using the IntelliCage system, in which the mice learn in which cage corner a nose poke earns access to drinking water. Nes^?/? and wildtype mice showed comparable place learning assessed as the incorrect corner visit ratio and the incorrect nose poke ratio. However, during reversal place learning, a more challenging task, Nes^?/? mice, compared to wildtype mice, showed improved learning over time demonstrated by the incorrect visit ratio and improved memory extinction over time assessed as nose pokes per visit to the previous drinking corner. In addition, Nes^?/? mice showed increased explorative activity as judged by the increased total numbers of corner visits and nose pokes. We conclude that Nes^?/? mice exhibit improved reversal place learning and memory extinction, a finding which together with the previous results supports the concept of the dual role of hippocampal neurogenesis in cognitive functions.

     

Not all water mazes are created equal: cyclin D2 knockout mice with constitutively suppressed adult hippocampal neurogenesis do show specific spatial learning deficits

Studies using the Morris water maze to assess hippocampal function in animals, in which adult hippocampal neurogenesis had been suppressed, have yielded seemingly contradictory results. Cyclin D2 knockout (Ccnd2^?/?) mice, for example, have constitutively suppressed adult hippocampal neurogenesis but had no overt phenotype in the water maze. In other paradigms, however, ablation of adult neurogenesis was associated with specific deficits in the water maze. Therefore, we hypothesized that the neurogenesis‐related phenotype might also become detectable in Ccnd2^?/? mice, if we used the exact setup and protocol that in our previous study had revealed deficits in mice with suppressed adult neurogenesis. Ccnd2^?/? mice indeed learned the task and developed a normal preference for the goal quadrant, but were significantly less precise for the exact goal position and were slower in acquiring efficient and spatially more precise search strategies. Upon goal reversal (when the hidden platform was moved to a new position) Ccnd2^?/? mice showed increased perseverance at the former platform location, implying that they were less flexible in updating the previously learned information. Both with respect to adult neurogenesis and behavioral performance, Ccnd2^+/? mice ranged between wild types and knockouts. Importantly, hippocampus‐dependent learning was not generally impaired by the mutation, but specifically functional aspects relying on precise and flexible encoding were affected. Whether ablation of adult neurogenesis causes a specific behavioral phenotype thus also depends on the actual task demands. The test parameters appear to be important variables influencing whether a task can pick up a contribution of adult neurogenesis to test performance.     

Irradiation induces neural precursor-cell dysfunction

In both pediatric and adult patients, cranial radiation therapy causes a debilitating cognitive decline that is poorly understood and currently untreatable. This decline is characterized by hippocampal dysfunction, and seems to involve a radiation-induced decrease in postnatal hippocampal neurogenesis. Here we show that the deficit in neurogenesis reflects alterations in the microenvironment that regulates progenitor-cell fate, as well as a defect in the proliferative capacity of the neural progenitor-cell population. Not only is hippocampal neurogenesis ablated, but the remaining neural precursors adopt glial fates and transplants of non-irradiated neural precursor cells fail to differentiate into neurons in the irradiated hippocampus. The inhibition of neurogenesis is accompanied by marked alterations in the neurogenic microenvironment, including disruption of the microvascular angiogenesis associated with adult neurogenesis and a marked increase in the number and activation status of microglia within the neurogenic zone. These findings provide clear targets for future therapeutic interventions.     

Tool Use Specific Adult Neurogenesis and Synaptogenesis in Rodent (Octodon degus) Hippocampus

We previously demonstrated that degus (Octodon degus), which are a species of small caviomorph rodents, could be trained to use a T-shaped rake as a hand tool to expand accessible spaces. To elucidate the neurobiological underpinnings of this higher brain function, we compared this tool use learning task with a simple spatial (radial maze) memory task and investigated the changes that were induced in the hippocampal neural circuits known to subserve spatial perception and learning. With the exposure to an enriched environment in home cage, adult neurogenesis in the dentate gyrus of the hippocampus was augmented by tool use learning, but not radial maze learning, when compared to control conditions. Furthermore, the proportion of new synapses formed in the CA3 region of the hippocampus, the target area for projections of mossy fiber axons emanating from newborn neurons, was specifically increased by tool use learning. Thus, active tool use behavior by rodents, learned through multiple training sessions, requires the hippocampus to generate more novel neurons and synapses than spatial information processing in radial maze learning.     

Adult-Generated Hippocampal Neurons Allow the Flexible Use of Spatially Precise Learning Strategies

Despite enormous progress in the past few years the specific contribution of newly born granule cells to the function of the adult hippocampus is still not clear. We hypothesized that in order to solve this question particular attention has to be paid to the specific design, the analysis, and the interpretation of the learning test to be used. We thus designed a behavioral experiment along hypotheses derived from a computational model predicting that new neurons might be particularly relevant for learning conditions, in which novel aspects arise in familiar situations, thus putting high demands on the qualitative aspects of (re-)learning.In the reference memory version of the water maze task suppression of adult neurogenesis with temozolomide (TMZ) caused a highly specific learning deficit. Mice were tested in the hidden platform version of the Morris water maze (6 trials per day for 5 days with a reversal of the platform location on day 4). Testing was done at 4 weeks after the end of four cycles of treatment to minimize the number of potentially recruitable new neurons at the time of testing. The reduction of neurogenesis did not alter longterm potentiation in CA3 and the dentate gyrus but abolished the part of dentate gyrus LTP that is attributed to the new neurons. TMZ did not have any overt side effects at the time of testing, and both treated mice and controls learned to find the hidden platform. Qualitative analysis of search strategies, however, revealed that treated mice did not advance to spatially precise search strategies, in particular when learning a changed goal position (reversal). New neurons in the dentate gyrus thus seem to be necessary for adding flexibility to some hippocampus-dependent qualitative parameters of learning.Our finding that a lack of adult-generated granule cells specifically results in the animal''s inability to precisely locate a hidden goal is also in accordance with a specialized role of the dentate gyrus in generating a metric rather than just a configurational map of the environment. The discovery of highly specific behavioral deficits as consequence of a suppression of adult hippocampal neurogenesis thus allows to link cellular hippocampal plasticity to well-defined hypotheses from theoretical models.     

BMP Signaling Mediates Effects of Exercise on Hippocampal Neurogenesis and Cognition in Mice

Exposure to exercise or to environmental enrichment increases the generation of new neurons in the adult hippocampus and promotes certain kinds of learning and memory. While the precise role of neurogenesis in cognition has been debated intensely, comparatively few studies have addressed the mechanisms linking environmental exposures to cellular and behavioral outcomes. Here we show that bone morphogenetic protein (BMP) signaling mediates the effects of exercise on neurogenesis and cognition in the adult hippocampus. Elective exercise reduces levels of hippocampal BMP signaling before and during its promotion of neurogenesis and learning. Transgenic mice with decreased BMP signaling or wild type mice infused with a BMP inhibitor both exhibit remarkable gains in hippocampal cognitive performance and neurogenesis, mirroring the effects of exercise. Conversely, transgenic mice with increased BMP signaling have diminished hippocampal neurogenesis and impaired cognition. Exercise exposure does not rescue these deficits, suggesting that reduced BMP signaling is required for environmental effects on neurogenesis and learning. Together, these observations show that BMP signaling is a fundamental mechanism linking environmental exposure with changes in cognitive function and cellular properties in the hippocampus.     

Activation of neural precursors in the adult neurogenic niches

The generation of new neurons within the dentate gyrus of the mature hippocampus is critical for spatial learning, object recognition and memory, whereas new neurons born in the subventricular zone (SVZ) contribute to olfactory function. Adult neurogenesis is a multistep process that begins with the activation and proliferation of a pool of stem/precursor cells. Although the presence of self-renewing and multipotent neural precursors is well established in the SVZ, it is only recently that the existence of such a precursor population has been demonstrated in the hippocampus, the region of the brain involved in learning and memory. Determining how this normally latent pool can be activated therefore offers considerable potential for the development of targeted neurogenic-based therapeutics to ameliorate the cognitive decline associated with hippocampal dysfunction in several neurodegenerative diseases. In this review, we summarize the effects of neural activity, various molecular factors and pharmaceutical agents, as well as voluntary exercise, in activating endogenous neural precursors in the two neurogenic niches of the adult brain, and highlight the role of activation-driven enhancement of neurogenesis for the treatment of psychiatric illness and aging dementia.     

Exposure to Folate Receptor Alpha Antibodies during Gestation and Weaning Leads to Severe Behavioral Deficits in Rats: A Pilot Study

The central nervous system continues to develop during gestation and after birth, and folate is an essential nutrient in this process. Folate deficiency and folate receptor alpha autoantibodies (FRα-AuAb) have been associated with pregnancy-related complications and neurodevelopmental disorders. In this pilot study, we investigated the effect of exposure to FRα antibodies (Ab) during gestation (GST), the pre-weaning (PRW), and the post weaning (POW) periods on learning and behavior in adulthood in a rat model. In the open field test and novel object recognition task, which examine locomotor activity and anxiety-like behavior, deficits in rats exposed to Ab during gestation and pre-weaning (GST+PRW) included more time spent in the periphery or corner areas, less time in the central area, frequent self-grooming akin to stereotypy, and longer time to explore a novel object compared to a control group; these are all indicative of increased levels of anxiety. In the place avoidance tasks that assess learning and spatial memory formation, only 30% of GST+PRW rats were able to learn the passive place avoidance task. None of these rats learned the active place avoidance task indicating severe learning deficits and cognitive impairment. Similar but less severe deficits were observed in rats exposed to Ab during GST alone or only during the PRW period, suggesting the extreme sensitivity of the fetal as well as the neonatal rat brain to the deleterious effects of exposure to Ab during this period. Behavioral deficits were not seen in rats exposed to antibody post weaning. These observations have implications in the pathology of FRα-AuAb associated with neural tube defect pregnancy, preterm birth and neurodevelopmental disorders including autism.     

Short-term environmental enrichment rescues adult neurogenesis and memory deficits in APP(Sw,Ind) transgenic mice

Epidemiological studies indicate that intellectual activity prevents or delays the onset of Alzheimer's disease (AD). Similarly, cognitive stimulation using environmental enrichment (EE), which increases adult neurogenesis and functional integration of newborn neurons into neural circuits of the hippocampus, protects against memory decline in transgenic mouse models of AD, but the mechanisms involved are poorly understood. To study the therapeutic benefits of cognitive stimulation in AD we examined the effects of EE in hippocampal neurogenesis and memory in a transgenic mouse model of AD expressing the human mutant β-amyloid (Aβ) precursor protein (APP(Sw,Ind)). By using molecular markers of new generated neurons (bromodeoxiuridine, NeuN and doublecortin), we found reduced neurogenesis and decreased dendritic length and projections of doublecortin-expressing cells of the dentate gyrus in young APP(Sw,Ind) transgenic mice. Moreover, we detected a lower number of mature neurons (NeuN positive) in the granular cell layer and a reduced volume of the dentate gyrus that could be due to a sustained decrease in the incorporation of new generated neurons. We found that short-term EE for 7 weeks efficiently ameliorates early hippocampal-dependent spatial learning and memory deficits in APP(Sw,Ind) transgenic mice. The cognitive benefits of enrichment in APP(Sw,Ind) transgenic mice were associated with increased number, dendritic length and projections to the CA3 region of the most mature adult newborn neurons. By contrast, Aβ levels and the total number of neurons in the dentate gyrus were unchanged by EE in APP(Sw,Ind) mice. These results suggest that promoting the survival and maturation of adult generated newborn neurons in the hippocampus may contribute to cognitive benefits in AD mouse models.     

Estradiol does not influence strategy choice but place strategy choice is associated with increased cell proliferation in the hippocampus of female rats

Adult neurogenesis occurs in the hippocampus of most mammals. While the function of adult hippocampal neurogenesis is not known, there is a relationship between neurogenesis and hippocampus-dependent learning and memory. Ovarian hormones can influence learning and memory and strategy choice. In competitive memory tasks, higher levels of estradiol shift female rats towards the use of the place strategy. Previous studies using a cue-competition paradigm find that 36% of male rats will use a hippocampus-dependent place strategy and place strategy users had lower levels of cell proliferation in the hippocampus. Here, we used the same paradigm to test whether endogenous or exogenous ovarian hormones influence strategy choice in the cue-competition paradigm and whether cell proliferation was related to strategy choice. We tested ovariectomized estradiol-treated (10 μg of estradiol benzoate) or sham-operated female rats on alternating blocks of hippocampus-dependent and hippocampus-independent versions of the Morris water task. Rats were then given a probe session with the platform visible and in a novel location. Preferred strategy was classified as place strategy (hippocampus-dependent) if they swam to the old platform location or cue strategy (hippocampus-independent) if they swam to the visible platform. All groups showed a preference for the cue strategy. However, proestrous rats were more likely to be place strategy users than rats not in proestrus. Female place strategy users had increased cell proliferation in the dentate gyrus compared to cue strategy users. Our study suggests that 78% of female rats chose the cue strategy instead of the place strategy. In summary the present results suggest that estradiol does not shift strategy use in this paradigm and that cell proliferation is related to strategy use with greater cell proliferation seen in place strategy users in female rats.     

Differences in inhibitory avoidance,cortisol and brain gene expression in TL and AB zebrafish

Recently, we established an inhibitory avoidance paradigm in Tupfel Long‐Fin (TL) zebrafish. Here, we compared task performance of TL fish and fish from the AB strain; another widely used strain and shown to differ genetically and behaviourally from TL fish. Whole‐body cortisol and telencephalic gene expression related to stress, anxiety and fear were measured before and 2 h post‐task. Inhibitory avoidance was assessed in a 3‐day paradigm: fish learn to avoid swimming from a white to a black compartment where a 3V‐shock is given: day 1 (first shock), day 2 (second shock) and day 3 (no shock, sampling). Tupfel Long‐Fin fish rapidly learned to avoid the black compartment and showed an increase in avoidance‐related spatial behaviour in the white compartment across days. In contrast, AB fish showed no inhibitory avoidance learning. AB fish had higher basal cortisol levels and expression levels of stress‐axis related genes than TL fish. Tupfel Long‐Fin fish showed post‐task learning‐related changes in cortisol and gene expression levels, but these responses were not seen in AB fish. We conclude that AB fish show higher cortisol levels and no inhibitory avoidance than TL fish. The differential learning responses of these Danio strains may unmask genetically defined risks for stress‐related disorders.     

Ablation of Fmrp in adult neural stem cells disrupts hippocampus-dependent learning

Deficiency in fragile X mental retardation protein (FMRP) results in fragile X syndrome (FXS), an inherited form of intellectual disability. Despite extensive research, it is unclear how FMRP deficiency contributes to the cognitive deficits in FXS. Fmrp-null mice show reduced adult hippocampal neurogenesis. As Fmrp is also enriched in mature neurons, we investigated the function of Fmrp expression in neural stem and progenitor cells (aNSCs) and its role in adult neurogenesis. Here we show that ablation of Fmrp in aNSCs by inducible gene recombination leads to reduced hippocampal neurogenesis in vitro and in vivo, as well as markedly impairing hippocampus-dependent learning in mice. Conversely, restoration of Fmrp expression specifically in aNSCs rescues these learning deficits in Fmrp-deficient mice. These data suggest that defective adult neurogenesis may contribute to the learning impairment seen in FXS, and these learning deficits can be rectified by delayed restoration of Fmrp specifically in aNSCs.     

Dissociation of exteroceptive and idiothetic orientation cues: effect on hippocampal place cells and place navigation.

Navigation by means of cognitive maps appears to require the hippocampus; hippocampal place cells (PCs) appear to store spatial memories because their discharge is confined to cell-specific places called firing fields (FFs). Experiments with rats manipulated idiothetic and landmark-related information to understand the relationship between PC activity and spatial cognition. Rotating a circular arena in the light caused a discrepancy between these cues. This discrepancy caused most FFs to disappear in both the arena and room reference frames. However, FFs persisted in the rotating arena frame when the discrepancy was reduced by darkness or by a card in the arena. The discrepancy was increased by ''field clamping'' the rat in a room-defined FF location by rotations that countered its locomotion. Most FFs dissipated and reappeared an hour or more after the clamp. Place-avoidance experiments showed that navigation uses independent idiothetic and exteroceptive memories. Rats learned to avoid the unmarked footshock region within a circular arena. When acquired on the stable arena in the light, the location of the punishment was learned by using both room and idiothetic cues; extinction in the dark transferred to the following session in the light. If, however, extinction occurred during rotation, only the arena-frame avoidance was extinguished in darkness; the room-defined location was avoided when the lights were turned back on. Idiothetic memory of room-defined avoidance was not formed during rotation in light; regardless of rotation, there was no avoidance when the lights were turned off, but room-frame avoidance reappeared when the lights were turned back on. The place-preference task rewarded visits to an allocentric target location with a randomly dispersed pellet. The resulting behaviour alternated between random pellet searching and target-directed navigation, making it possible to examine PC correlates of these two classes of spatial behaviour. The independence of idiothetic and exteroceptive spatial memories and the disruption of PC firing during rotation suggest that PCs may not be necessary for spatial cognition; this idea can be tested by recordings during the place-avoidance and preference tasks.     

Development of neural stem cell in the adult brain

New neurons are continuously generated in the dentate gyrus of the mammalian hippocampus and in the subventricular zone of the lateral ventricles throughout life. The origin of these new neurons is believed to be from multipotent adult neural stem cells. Aided by new methodologies, significant progress has been made in the characterization of neural stem cells and their development in the adult brain. Recent studies have also begun to reveal essential extrinsic and intrinsic molecular mechanisms that govern sequential steps of adult neurogenesis in the hippocampus and subventricular zone/olfactory bulb, from proliferation and fate specification of neural progenitors to maturation, navigation, and synaptic integration of the neuronal progeny. Future identification of molecular mechanisms and physiological functions of adult neurogenesis will provide further insight into the plasticity and regenerative capacity of the mature central nervous system.     

Transgenic mouse models for studying adult neurogenesis

The mammalian hippocampus shows a remarkable capacity for continued neurogenesis throughout life. Newborn neurons, generated by the radial neural stem cells (NSCs), are important for learning and memory as well as mood control. During aging, the number and responses of NSCs to neurogenic stimuli diminish, leading to decreased neurogenesis and age-associated cognitive decline and psychiatric disorders. Thus, adult hippocampal neurogenesis has garnered significant interest because targeting it could be a novel potential therapeutic strategy for these disorders. However, if we are to use neurogenesis to halt or reverse hippocampal-related pathology, we need to understand better the core molecular machinery that governs NSC and their progeny. In this review, we summarize a wide variety of mouse models used in adult neurogenesis field, present their advantages and disadvantages based on specificity and efficiency of labeling of different cell types, and review their contribution to our understanding of the biology and the heterogeneity of different cell types found in adult neurogenic niches.     

proNGF Inhibits Neurogenesis and Induces Glial Activation in Adult Mouse Dentate Gyrus

Recent studies have shown that the precursor of nerve growth factor (proNGF) is highly elevated in aging brains and in the brains of patients with Alzheimer’s Disease. proNGF accumulates in hippocampus which is an important neurogenic region related to learning and memory. However, it remains unclear whether proNGF has an influence on hippocampal neurogenesis. In this study, we demonstrated that the high-affinity receptor of proNGF, p75 neurotrophic factor (p75NTR), was expressed both on cells undergoing mitosis and postmitotic mature cells in mouse hippocampus. proNGF infusion into adult mouse hippocampus significantly reduced the density of BrdU-incorporating cells and the density of BrdU/Doublecortin double positive cells in the subgranular zone of hippocampus, indicating an inhibitory effect of proNGF on hippocampal neurogenesis. proNGF infusion also induced prominent cell apoptosis and activated residential astrocyte and microglia, which might further impair the hippocampal neurogenesis. These results implied that proNGF played a pivotal role in regulating the hippocampal neurogenesis and might account for the memory deficit and cognitive impairment.     

Neural aromatization accelerates the acquisition of spatial memory via an influence on the songbird hippocampus

Circulating estrogens affect the neural circuits that underlie learning and memory in several vertebrates via an influence on the hippocampus. In the songbird hippocampus local estrogen synthesis due to the abundant expression of aromatase may modulate hippocampal function including spatial memory performance. Here, we examined the effect of estradiol, testosterone, and dihydrotestosterone on the structure and function of the songbird hippocampus. Adult male zebra finches were castrated, implanted with one of these steroids or a blank implant, and trained on a spatial memory task. The rate of acquisition and overall performance on this task was recorded by direct observation. The size and density of cells in the hippocampus and its volume were measured. Estradiol-treated birds learned the task more rapidly than any other group. Although testosterone- and blank-implanted birds did learn the task, we found no evidence of learning in dihydrotestosterone-implanted subjects. Cells in the rostral hippocampus were larger in estradiol- and testosterone-treated birds relative to other groups. A corresponding decrease in the density of cells was apparent in estradiol-implanted subjects relative to all other groups. These data suggest that estradiol may accelerate the acquisition of a spatial memory task and increase the size of neurons in the rostral hippocampus. Since testosterone-mediated changes in acquisition and cell size were similar to those of estradiol, but not dihydrotestosterone, we conclude that neural aromatization of testosterone to estrogen is responsible for effects on the structure and function of the songbird hippocampus.     

Prenatal Stress Inhibits Hippocampal Neurogenesis but Spares Olfactory Bulb Neurogenesis

The dentate gyrus (DG) and the olfactory bulb (OB) are two regions of the adult brain in which new neurons are integrated daily in the existing networks. It is clearly established that these newborn neurons are implicated in specific functions sustained by these regions and that different factors can influence neurogenesis in both structures. Among these, life events, particularly occurring during early life, were shown to profoundly affect adult hippocampal neurogenesis and its associated functions like spatial learning, but data regarding their impact on adult bulbar neurogenesis are lacking. We hypothesized that prenatal stress could interfere with the development of the olfactory system, which takes place during the prenatal period, leading to alterations in adult bulbar neurogenesis and in olfactory capacities. To test this hypothesis we exposed pregnant C57Bl/6J mice to gestational restraint stress and evaluated behavioral and anatomic consequences in adult male offspring.We report that prenatal stress has no impact on adult bulbar neurogenesis, and does not alter olfactory functions in adult male mice. However, it decreases cell proliferation and neurogenesis in the DG of the hippocampus, thus confirming previous reports on rats. Altogether our data support a selective and cross-species long-term impact of prenatal stress on neurogenesis.     

Experience dictates stem cell fate in the adult hippocampus

Adult hippocampal neurogenesis has been implicated in cognitive and emotional processes, as well as in response to antidepressant treatment. However, little is known about how the adult stem cell lineage contributes to hippocampal structure and function and how this process is modulated by the animal's experience. Here we perform an indelible lineage analysis and report that neural stem cells can produce expanding and persisting populations of not only neurons, but also stem cells in the adult hippocampus. Furthermore, the ratio of stem cells to neurons depends on experiences of the animal or the location of the stem cell. Surprisingly, social isolation facilitated accumulation of stem cells, but not neurons. These results show that neural stem cells accumulate in the adult hippocampus and that the stem cell-lineage relationship is under control of anatomic and experiential niches. Our findings suggest that, in the hippocampus, fate specification may act as a form of cellular plasticity for adapting to environmental changes.