Adult neurogenesis and pattern separation in rodents: A critical evaluation of data, tasks and interpretation

Oscillations in the dentate gyrus as a tool for the performance of the hippocampal functions: Healthy and epileptic brain

The dentate gyrus (DG) is part of the hippocampal formation and is essential for important cognitive processes such as navigation and memory. The oscillatory activity of the DG network is believed to play a critical role in cognition. DG circuits generate theta, beta, and gamma rhythms, which participate in the specific information processing performed by DG neurons. In the temporal lobe epilepsy (TLE), cognitive abilities are impaired, which may be due to drastic alterations in the DG structure and network activity during epileptogenesis. The theta rhythm and theta coherence are especially vulnerable in dentate circuits; disturbances in DG theta oscillations and their coherence may be responsible for general cognitive impairments observed during epileptogenesis. Some researchers suggested that the vulnerability of DG mossy cells is a key factor in the genesis of TLE, but others did not support this hypothesis. The aim of the review is not only to present the current state of the art in this field of research but to help pave the way for future investigations by highlighting the gaps in our knowledge to completely appreciate the role of DG rhythms in brain functions. Disturbances in oscillatory activity of the DG during TLE development may be a diagnostic marker in the treatment of this disease.

Use of an Automated Mouse Touchscreen Platform for Quantification of Cognitive Deficits After Central Nervous System Injury

Analyzing cognitive performance is an important aspect of assessing physiological deficits after stroke or other central nervous system (CNS) injuries in both humans and in basic science animal models. Cognitive testing on an automated touchscreen operant platform began in humans but is now increasingly popular in preclinical studies as it enables testing in many cognitive domains in a highly reproducible way while minimizing stress to the laboratory animal. Here, we describe the step-by-step setup and application of four operant touchscreen tests used on adult mice. In brief, mice are trained to touch a graphical image on a lit screen and initiate subsequent trials for a reward. Following initial training, mice can be tested on tasks that probe performance in many cognitive domains and thus infer the integrity of brain circuits and regions. There are already many outstanding published protocols on touchscreen cognitive testing. This chapter is designed to add to the literature in two specific ways. First, this chapter provides in a single location practical, behind-the-scenes tips for setup and testing of mice in four touchscreen tasks that are useful to assess in CNS injury models: Paired Associates Learning (PAL), a task of episodic, associative (object-location) memory; Location Discrimination Reversal (LDR), a test for mnemonic discrimination (also called behavioral pattern separation) and cognitive flexibility; Autoshaping (AUTO), a test of Pavlovian or classical conditioning; and Extinction (EXT), tasks of stimulus-response and response inhibition, respectively. Second, this chapter summarizes issues to consider when performing touchscreen tests in mouse models of CNS injury. Quantifying gross and fine aspects of cognitive function is essential to improved treatment for brain dysfunction after stroke or CNS injury as well as other brain diseases, and touchscreen testing provides a sensitive, reliable, and robust way to achieve this.

Targeting hippocampal neurogenesis to protect astronauts' cognition and mood from decline due to space radiation effects

Neurogenesis is an essential, lifelong process during which neural stem cells generate new neurons within the hippocampus, a center for learning, memory, and mood control. Neural stem cells are vulnerable to environmental insults spanning from chronic stress to radiation. These insults reduce their numbers and diminish neurogenesis, leading to memory decline, anxiety, and depression. Preserving neural stem cells could thus help prevent these neurogenesis-associated pathologies, an outcome particularly important for long-term space missions where environmental exposure to radiation is significantly higher than on Earth. Multiple developments, from mechanistic discoveries of radiation injury on hippocampal neurogenesis to new platforms for the development of selective, specific, effective, and safe small molecules as neurogenesis-protective agents hold great promise to minimize radiation damage on neurogenesis. In this review, we summarize the effects of space-like radiation on hippocampal neurogenesis. We then focus on current advances in drug discovery and development and discuss the nuclear receptor TLX/NR2E1 (oleic acid receptor) as an example of a neurogenic target that might rescue neurogenesis following radiation.

Sex Differences in Cognition Across Aging

Sex and gender differences are seen in cognitive disturbances in a variety of neurological and psychiatry diseases. Men are more likely to have cognitive symptoms in schizophrenia whereas women are more likely to have more severe cognitive symptoms with major depressive disorder and Alzheimer's disease. Thus, it is important to understand sex and gender differences in underlying cognitive abilities with and without disease. Sex differences are noted in performance across various cognitive domains - with males typically outperforming females in spatial tasks and females typically outperforming males in verbal tasks. Furthermore, there are striking sex differences in brain networks that are activated during cognitive tasks and in learning strategies. Although rarely studied, there are also sex differences in the trajectory of cognitive aging. It is important to pay attention to these sex differences as they inform researchers of potential differences in resilience to age-related cognitive decline and underlying mechanisms for both healthy and pathological cognitive aging, depending on sex. We review literature on the progressive neurodegenerative disorder, Alzheimer's disease, as an example of pathological cognitive aging in which human females show greater lifetime risk, neuropathology, and cognitive impairment, compared to human males. Not surprisingly, the relationships between sex and cognition, cognitive aging, and Alzheimer's disease are nuanced and multifaceted. As such, this chapter will end with a discussion of lifestyle factors, like education and diet, as modifiable factors that can alter cognitive aging by sex. Understanding how cognition changes across age and contributing factors, like sex differences, will be essential to improving care for older adults.

Disruption of NREM sleep and sleep-related spatial memory consolidation in mice lacking adult hippocampal neurogenesis

Cellular plasticity at the structural level and sleep at the behavioural level are both essential for memory formation. The link between the two is not well understood. A functional connection between adult neurogenesis and hippocampus-dependent memory consolidation during NREM sleep has been hypothesized but not experimentally shown. Here, we present evidence that during a three-day learning session in the Morris water maze task a genetic knockout model of adult neurogenesis (Cyclin D2^-/-) showed changes in sleep macro- and microstructure. Sleep EEG analyses revealed a lower total sleep time and NREM fraction in Cyclin D2^-/- mice as well as an impairment of sleep specific neuronal oscillations that are associated with memory consolidation. Better performance in the memory task was associated with specific sleep parameters in wild-type, but not in Cyclin D2^-/- mice. In wild-type animals the number of proliferating cells correlated with the amount of NREM sleep. The lack of adult neurogenesis led to changes in sleep architecture and oscillations that represent the dialog between hippocampus and neocortex during sleep. We suggest that adult neurogenesis-as a key event of hippocampal plasticity-might play an important role for sleep-dependent memory consolidation and modulates learning-induced changes of sleep macro- and microstructure.

An investigation into the effective role of astrocyte in the hippocampus pattern separation process: A computational modeling study

A physiologically realistic three layer neuron-astrocyte network model is used to evaluate the biological mechanism in pattern separation. The innovative feature of the model is the use of a combination of three elements: neuron, interneuron and astrocyte. In the input layer, a pyramidal neuron receives input patterns from stimulus current, while in the middle layer there are two pyramidal neurons coupled with two inhibitory interneurons and an astrocyte. Finally, in the third layer, a pyramidal neuron produces the output of the model by integrating the output of two neurons from the middle layer resulting from inhibitory and excitatory connections among neurons, interneurons and the astrocyte. Results of computer simulations show that the neuron-astrocyte network within the hippocampal dentate gyrus can generate diverse, complex and different output patterns to given inputs. It is concluded that astrocytes within the dentate gyrus play an important role in the pattern separation process.

Born this way: Hippocampal neurogenesis across the lifespan

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

TLX knockdown in the dorsal dentate gyrus of juvenile rats differentially affects adolescent and adult behaviour

The orphan nuclear receptor TLX is predominantly expressed in the central nervous system and is an important factor regulating the maintenance and self-renewal of neural stem cells from embryonic development through adulthood. In adolescence and adulthood, TLX expression is restricted to the neurogenic niches of the brain: the dentate gyrus of the hippocampus and the subventricular zone. The adolescent period is critical for maturation of the hippocampus with heightened levels of neurogenesis observed in rodents. Therefore, we investigated whether lentiviral silencing of TLX expression (TLX knockdown) in the dorsal dentate gyrus of juvenile rats incurred differential impairments in behaviour during late adolescence and adulthood. Our results showed that knockdown of TLX in the dorsal dentate gyrus led to a decrease in cell proliferation in the dorsal but not ventral dentate gyrus. At a behavioural level we observed differential effects in adolescence and adulthood across a number of parameters. A hyperactive phenotype was present in adolescent but not adult TLX knockdown rats, and an increase in immobility during adolescence and in swimming frequency during adulthood was observed in the forced swim test. There was an increased defecation frequency in the open field during adulthood but not adolescence. There were no changes in cognitive performance on hippocampus-dependent tasks or in anxiety-related behaviours. In conclusion, silencing of TLX in the dorsal dentate gyrus led to impairments in hippocampal-independent behaviours which either did not persist or were reversed during adulthood. The current data highlight the temporal importance and function of the nuclear receptor TLX during development.

Behavioral and structural adaptations to stress

Unpredictable aversive experiences, or stressors, lead to changes in depression- and anxiety-related behavior and to changes in hippocampal structure including decreases in adult neurogenesis, granule cell and pyramidal cell dendritic morphology, and volume. Here we review the relationship between these behavioral and structural changes and discuss the possibility that these changes may be largely adaptive. Specifically, we suggest that new neurons in the dentate gyrus enhance behavioral adaptability to changes in the environment, biasing behavior in novel situations based on previous experience with stress. Conversely, atrophy-like changes in the hippocampus and decreased adult neurogenesis following chronic stress may serve to limit stress responses and stabilize behavior during chronic stress.

The Cholinergic System Modulates Memory and Hippocampal Plasticity via Its Interactions with Non-Neuronal Cells

Degeneration of central cholinergic neurons impairs memory, and enhancement of cholinergic synapses improves cognitive processes. Cholinergic signaling is also anti-inflammatory, and neuroinflammation is increasingly linked to adverse memory, especially in Alzheimer's disease. Much of the evidence surrounding cholinergic impacts on the neuroimmune system focuses on the α7 nicotinic acetylcholine (ACh) receptor, as stimulation of this receptor prevents many of the effects of immune activation. Microglia and astrocytes both express this receptor, so it is possible that some cholinergic effects may be via these non-neuronal cells. Though the presence of microglia is required for memory, overactivated microglia due to an immune challenge overproduce inflammatory cytokines, which is adverse for memory. Blocking these exaggerated effects, specifically by decreasing the release of tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6), has been shown to prevent inflammation-induced memory impairment. While there is considerable evidence that cholinergic signaling improves memory, fewer studies have linked the "cholinergic anti-inflammatory pathway" to memory processes. This review will summarize the current understanding of the cholinergic anti-inflammatory pathway as it relates to memory and will argue that one mechanism by which the cholinergic system modulates hippocampal memory processes is its influence on neuroimmune function via the α7 nicotinic ACh receptor.

Pattern Separation: A Potential Marker of Impaired Hippocampal Adult Neurogenesis in Major Depressive Disorder

Adult neurogenesis involves the generation of new neurons, particularly in the dentate gyrus of the hippocampus. Decreased hippocampal neurogenesis has been implicated in both animal models of depression and in patients with major depressive disorder (MDD), despite some inconsistency in the literature. Here, we build upon current models to generate a new testable hypothesis, linking impaired neurogenesis to downstream psychological outcomes commonly observed in MDD. We contend that disruption in adult neurogenesis impairs pattern separation, a hippocampus-dependent function requiring the careful discrimination and storage of highly similar, but not identical, sensory inputs. This, in turn, can affect downstream processing and response selection, of relevance to emotional wellbeing. Specifically, disrupted pattern separation leads to misperceived stimuli (i.e., stimulus confusion), triggering the selection and deployment of established responses inappropriate for the actual stimuli. We speculate that this may be akin to activation of automatic thoughts, described in the Cognitive Behavior Theory of MDD. Similarly, this impaired ability to discriminate information at a fundamental sensory processing level (e.g., impaired pattern separation) could underlie impaired psychological flexibility, a core component of Acceptance and Commitment Therapy of MDD. We propose that research is needed to test this model by examining the relationship between cognitive functioning (e.g., pattern separation ability), psychological processes (e.g., perseveration and psychological inflexibility), and neurogenesis, taking advantage of emerging magnetic resonance spectroscopy-based imaging that measures neurogenesis in-vivo.

POU3F2 participates in cognitive function and adult hippocampal neurogenesis via mammalian-characteristic amino acid repeats

POU3F2/BRN-2 is a transcription factor that is mainly expressed in the central nervous system and plays an important role in brain development. The transactivation domain of POU3F2 includes multiple mammalian-characteristic tandem amino acid repeats (homopolymeric amino acid repeats). We previously generated knock-in mice (Pou3f2^Δ/Δ mice) in which all three homopolymeric amino acid repeats were deleted from the Pou3f2 transactivation domain and identified phenotypic impairments in maternal behavior and pup recognition. Yet, the exact biological implications of homopolymeric repeats are not completely understood. In this study, we investigated cognitive function and hippocampal neurogenesis in Pou3f2^Δ/Δ mice. Pou3f2^Δ/Δ mice exhibited cognitive impairment in object recognition and object location tests. Immunohistochemistry for doublecortin, a marker of immature neurons, showed a lower number of newborn neurons in the dentate gyrus of adult Pou3f2^Δ/Δ mice compared with wild-type mice. Consistent with this observation, adult Pou3f2^Δ/Δ mice had lower numbers of 5-bromo-2'-deoxyuridine (BrdU) and NeuN double-positive cells at 4 weeks after BrdU injection compared with control mice, indicating the decreased generation of mature granule cells in Pou3f2^Δ/Δ mice. Taken together, these results suggest that POU3F2 is involved in cognitive function as well as adult hippocampal neurogenesis, and that homopolymeric amino acid repeats in this gene play a functional role.

Hippocampal neurogenesis and pattern separation: A meta-analysis of behavioral data

The generation of new neurons in the hippocampus of adult mammals has become a widely accepted phenomenon, but the functional significance of the adult neurogenesis in the hippocampus is not fully understood. One of the main hypotheses currently investigated suggests that neurogenesis contributes to pattern separation in the dentate gyrus. Many behavioral studies were conducted aiming to test this hypothesis using rodents as animal model. In those studies, researches ablated neurogenesis in the animals and subsequently evaluate them in tests of behavioral pattern separation, that is, behaviors that are thought to rely on the computational process of pattern separation. The results of these studies are varied, with most supporting a role for neurogenesis in pattern separation, but some others not. To address this controversy we performed a systematic review and meta-analysis of studies evaluating the effect of neurogenesis ablation on behavioral pattern separation. Analysis results indicated that most of the literature in the topic is surprisingly consistent and, although there are two studies with divergent results, the bulk of the literature supports an effect of hippocampal neurogenesis on behavioral pattern separation. We discuss those findings in light of other behavioral effects of hippocampal neurogenesis ablation, limitations of behavioral data and other lines of evidence about the effect of hippocampal neurogenesis in the dentate gyrus.

Effects of aging and depression on mnemonic discrimination ability

Aging and depression have been found to be associated with poorer performance in mnemonic discrimination. In the current study, a two-response format mnemonic similarity test, Cognitive Drug Research MST, was used to compare these effects. Seventy-six participants were tested; with 52 participants in the young group, aged 18-35 years, and 24 participants in the elderly group, aged 55 years or older. Twenty-two young participants and 10 elderly participants met DSM-IV criteria for MDD or dysthymia. Age-related deficits were found for lure identification and speed of response. Differences in speed of responses to lure images were found for younger depressed participants, and depressive symptom severity was found to be negatively associated with lure identification accuracy in the elderly. These findings may be viewed as putative behavioral correlates of decreased pattern separation ability, which may be indicative of altered hippocampal neurogenesis in aging and depression.

Adult hippocampal neurogenesis and cognitive flexibility - linking memory and mood

Adult hippocampal neurogenesis has been implicated in cognitive processes, such as pattern separation, and in the behavioural effects of stress and antidepressants. Young adult-born neurons have been shown to inhibit the overall activity of the dentate gyrus by recruiting local interneurons, which may result in sparse contextual representations and improved pattern separation. We propose that neurogenesis-mediated inhibition also reduces memory interference and enables reversal learning both in neutral situations and in emotionally charged ones. Such improved cognitive flexibility may in turn help to decrease anxiety-like and depressive-like behaviour.

Stress and adolescent hippocampal neurogenesis: diet and exercise as cognitive modulators

Adolescence is a critical period for brain maturation. Deciphering how disturbances to the central nervous system at this time affect structure, function and behavioural outputs is important to better understand any long-lasting effects. Hippocampal neurogenesis occurs during development and continues throughout life. In adulthood, integration of these new cells into the hippocampus is important for emotional behaviour, cognitive function and neural plasticity. During the adolescent period, maturation of the hippocampus and heightened levels of hippocampal neurogenesis are observed, making alterations to neurogenesis at this time particularly consequential. As stress negatively affects hippocampal neurogenesis, and adolescence is a particularly stressful time of life, it is important to investigate the impact of stressor exposure at this time on hippocampal neurogenesis and cognitive function. Adolescence may represent not only a time for which stress can have long-lasting effects, but is also a critical period during which interventions, such as exercise and diet, could ameliorate stress-induced changes to hippocampal function. In addition, intervention at this time may also promote life-long behavioural changes that would aid in fostering increased hippocampal neurogenesis and cognitive function. This review addresses both the acute and long-term stress-induced alterations to hippocampal neurogenesis and cognition during the adolescent period, as well as changes to the stress response and pubertal hormones at this time which may result in differential effects than are observed in adulthood. We hypothesise that adolescence may represent an optimal time for healthy lifestyle changes to have a positive and long-lasting impact on hippocampal neurogenesis, and to protect against stress-induced deficits. We conclude that future research into the mechanisms underlying the susceptibility of the adolescent hippocampus to stress, exercise and diet and the consequent effect on cognition may provide insight into why adolescence may be a vital period for correct conditioning of future hippocampal function.