Adult-born neurons maintain hippocampal cholinergic inputs and support working memory during aging.

Adult neurogenesis is reduced during aging and impaired in disorders of stress, memory, and cognition though its normal function remains unclear. Moreover, a systems level understanding of how a small number of young hippocampal neurons could dramatically influence brain function is lacking. We examined whether adult neurogenesis sustains hippocampal connections cumulatively across the life span. Long-term suppression of neurogenesis as occurs during stress and aging resulted in an accelerated decline in hippocampal acetylcholine signaling and a slow and progressing emergence of profound working memory deficits. These deficits were accompanied by compensatory reorganization of cholinergic dentate gyrus inputs with increased cholinergic innervation to the ventral hippocampus and recruitment of ventrally projecting neurons by the dorsal projection. While increased cholinergic innervation was dysfunctional and corresponded to overall decreases in cholinergic levels and signaling, it could be recruited to correct the resulting memory dysfunction even in old animals. Our study demonstrates that hippocampal neurogenesis supports memory by maintaining the septohippocampal cholinergic circuit across the lifespan.  It also provides a systems level explanation for the progressive nature of memory deterioration during normal and pathological aging and indicates that the brain connectome is malleable by experience.

The Ontogeny of Hippocampus-Dependent Memories.

The formation of memories that contain information about the specific time and place of acquisition, which are commonly referred to as "autobiographical" or "episodic" memories, critically relies on the hippocampus and on a series of interconnected structures located in the medial temporal lobe of the mammalian brain. The observation that adults retain very few of these memories from the first years of their life has fueled a long-standing debate on whether infants can make the types of memories that in adults are processed by the hippocampus-dependent memory system, and whether the hippocampus is involved in learning and memory processes early in life. Recent evidence shows that, even at a time when its circuitry is not yet mature, the infant hippocampus is able to produce long-lasting memories. However, the ability to acquire and store such memories relies on molecular pathways and network-based activity dynamics different from the adult system, which mature with age. The mechanisms underlying the formation of hippocampus-dependent memories during infancy, and the role that experience exerts in promoting the maturation of the hippocampus-dependent memory system, remain to be understood. In this review, we discuss recent advances in our understanding of the ontogeny and the biological correlates of hippocampus-dependent memories.

Altered structure and functional connectivity of the hippocampus are associated with social and mathematical difficulties in nonverbal learning disability.

The hippocampus is known to play a critical role in a variety of complex abilities, including visual-spatial reasoning, social functioning, and math. Nonverbal learning disability (NVLD) is a neurodevelopmental disorder characterized by deficits in visual-spatial reasoning that are accompanied by impairment in social function or mathematics, as well as motor or executive function skills. Despite the overlap between behaviors supported by the hippocampus and impairments in NVLD, the structure and function of the hippocampus in NVLD has not been studied. To address this gap in the literature, we first compared hippocampal volume and resting-state functional connectivity in children with NVLD (n = 24) and typically developing (TD) children (n = 20). We then explored associations between hippocampal structure, connectivity, and performance on measures of spatial, social, and mathematical ability. Relative to TD children, those with NVLD showed significant reductions in left hippocampal volume and greater hippocampal-cerebellar connectivity. In children with NVLD, reduced hippocampal volume associated with worse mathematical problem solving. Although children with NVLD exhibited more social problems (social responsiveness scale [SRS]) and higher hippocampal-cerebellar connectivity relative to TD children, greater connectivity was associated with fewer social problems among children with NVLD but not TD children. Such an effect may suggest a compensatory mechanism. These structural and functional alterations of the hippocampus may disrupt its putative role in organizing conceptual frameworks through cognitive mapping, thus contributing to the cross-domain difficulties that characterize NVLD.

Ablation of proliferating neural stem cells during early life is sufficient to reduce adult hippocampal neurogenesis.

Environmental exposures during early life, but not during adolescence or adulthood, lead to persistent reductions in neurogenesis in the adult hippocampal dentate gyrus (DG). The mechanisms by which early life exposures lead to long-term deficits in neurogenesis remain unclear. Here, we investigated whether targeted ablation of dividing neural stem cells during early life is sufficient to produce long-term decreases in DG neurogenesis. Having previously found that the stem cell lineage is resistant to long-term effects of transient ablation of dividing stem cells during adolescence or adulthood (Kirshenbaum, Lieberman, Briner, Leonardo, & Dranovsky, ), we used a similar pharmacogenetic approach to target dividing neural stem cells for elimination during early life periods sensitive to environmental insults. We then assessed the Nestin stem cell lineage in adulthood. We found that the adult neural stem cell reservoir was depleted following ablation during the first postnatal week, when stem cells were highly proliferative, but not during the third postnatal week, when stem cells were more quiescent. Remarkably, ablating proliferating stem cells during either the first or third postnatal week led to reduced adult neurogenesis out of proportion to the changes in the stem cell pool, indicating a disruption of the stem cell function or niche following stem cell ablation in early life. These results highlight the first three postnatal weeks as a series of sensitive periods during which elimination of dividing stem cells leads to lasting alterations in adult DG neurogenesis and stem cell function. These findings contribute to our understanding of the relationship between DG development and adult neurogenesis, as well as suggest a possible mechanism by which early life experiences may lead to lasting deficits in adult hippocampal neurogenesis.

Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation.

Adult hippocampal neurogenesis is a unique form of neural circuit plasticity that results in the generation of new neurons in the dentate gyrus throughout life. Neurons that arise in adults (adult-born neurons) show heightened synaptic plasticity during their maturation and can account for up to ten per cent of the entire granule cell population. Moreover, levels of adult hippocampal neurogenesis are increased by interventions that are associated with beneficial effects on cognition and mood, such as learning, environmental enrichment, exercise and chronic treatment with antidepressants. Together, these properties of adult neurogenesis indicate that this process could be harnessed to improve hippocampal functions. However, despite a substantial number of studies demonstrating that adult-born neurons are necessary for mediating specific cognitive functions, as well as some of the behavioural effects of antidepressants, it is unknown whether an increase in adult hippocampal neurogenesis is sufficient to improve cognition and mood. Here we show that inducible genetic expansion of the population of adult-born neurons through enhancing their survival improves performance in a specific cognitive task in which two similar contexts need to be distinguished. Mice with increased adult hippocampal neurogenesis show normal object recognition, spatial learning, contextual fear conditioning and extinction learning but are more efficient in differentiating between overlapping contextual representations, which is indicative of enhanced pattern separation. Furthermore, stimulation of adult hippocampal neurogenesis, when combined with an intervention such as voluntary exercise, produces a robust increase in exploratory behaviour. However, increasing adult hippocampal neurogenesis alone does not produce a behavioural response like that induced by anxiolytic agents or antidepressants. Together, our findings suggest that strategies that are designed to increase adult hippocampal neurogenesis specifically, by targeting the cell death of adult-born neurons or by other mechanisms, may have therapeutic potential for reversing impairments in pattern separation and dentate gyrus dysfunction such as those seen during normal ageing.

5-HT 1A Agonist Properties Contribute to a Robust Response to Vilazodone in the Novelty Suppressed Feeding Paradigm.

BACKGROUND: Differences in 5-HT 1A receptor function have been implicated in vulnerability to depression and in response to treatment. Adding 5-HT 1A partial agonists to selective serotonin reuptake inhibitors has been touted as a strategy to increase their efficacy. Here we use the novelty suppressed feeding paradigm to compare the effects of vilazodone, a high-potency selective serotonin reuptake inhibitor, with high affinity for 5-HT 1A receptors to the reference selective serotonin reuptake inhibitor fluoxetine across several mouse strains that differ in their response to selective serotonin reuptake inhibitors. METHODS: To confirm 5-HT 1A agonist activity, body temperature was measured after acute administration of vilazodone or fluoxetine, as administration of 5-HT 1A agonists induces hypothermia. We next used 3 strains of mice to examine the effects of the drugs on latency in the novelty suppressed feeding, a paradigm generally sensitive to chronic but not acute effects of antidepressants. RESULTS: Vilazodone induces robust hypothermia and blocks stress-induced hyperthermia in a 5-HT 1A -dependent manner, consistent with agonist effects at 5-HT 1A autoreceptors. In 129SvEv mice, vilazodone (10mg/kg/d) reduces the latency to eat in the novelty suppressed feeding test within 8 days, while no effect of fluoxetine (20mg/kg/d) was detected at that time. In contrast, both vilazodone and fluoxetine are effective at decreasing latency to eat in the novelty suppressed feeding paradigm in a strain with low autoreceptor levels. In mice with higher autoreceptor levels, no significant difference was detected between fluoxetine and vehicle ( P=. 8) or vilazodone and vehicle ( P =.06). CONCLUSION: In mice, vilazodone may offer advantages in time of onset and efficacy over a reference selective serotonin reuptake inhibitor in the novelty suppressed feeding test.

The Role of Environmental Chemicals in the Etiology of Learning Difficulties: A Novel Theoretical Framework.

Children from economically disadvantaged communities have a disproportionate risk of exposure to chemicals, social stress, and learning difficulties. Although animal models and epidemiologic studies link exposures and neurodevelopment, little focus has been paid to academic outcomes in environmental health studies. Similarly, in the educational literature, environmental chemical exposures are overlooked as potential etiologic factors in learning difficulties. We propose a theoretical framework for the etiology of learning difficulties that focuses on these understudied exogenous factors. We discuss findings from animal models and longitudinal, prospective birth cohort studies that support this theoretical framework. Studies reviewed point to the effects of prenatal exposure to polycyclic aromatic hydrocarbons on reading comprehension and math skills via effects on inhibitory control processes. Long term, this work will help close the achievement gap in the United States by identifying behavioral and neural pathways from prenatal exposures to learning difficulties in children from economically disadvantaged families.

Inhibition of norepinephrine signaling during a sensitive period disrupts locus coeruleus circuitry and emotional behaviors in adulthood.

Deficits in arousal and stress responsiveness are a feature of numerous psychiatric disorders including depression and anxiety. Arousal is supported by norepinephrine (NE) released from specialized brainstem nuclei, including the locus coeruleus (LC) neurons into cortical and limbic areas. During development, the NE system matures in concert with increased exploration of the animal's environment. While several psychiatric medications target the NE system, the possibility that its modulation during discreet developmental periods can have long-lasting consequences has not yet been explored. We used a chemogenetic strategy in mice to reversibly inhibit NE signaling during brief developmental periods and then evaluated any long-lasting impact of our intervention on adult NE circuit function and on emotional behavior. We also tested whether developmental exposure to the α2 receptor agonist guanfacine, which is commonly used in the pediatric population and is not contraindicated during pregnancy and nursing, recapitulates the effect seen with the chemogenetic strategy. Our results reveal that postnatal days 10-21 constitute a sensitive period during which alterations in NE signaling lead to changes in baseline anxiety, increased anhedonia, and passive coping behaviors in adulthood. Disruption of NE signaling during this sensitive period also caused altered LC autoreceptor function, along with circuit specific changes in LC-NE target regions at baseline, and in response to stress. Our findings indicate an early critical role for NE in sculpting brain circuits that support adult emotional function. Interfering with this role by guanfacine and similar clinically used drugs can have lasting implications for mental health.

Adult-born neurons maintain hippocampal cholinergic inputs and support working memory during aging.

Adult neurogenesis is reduced during aging and impaired in disorders of stress, memory, and cognition though its normal function remains unclear. Moreover, a systems level understanding of how a small number of young hippocampal neurons could dramatically influence brain function is lacking. We examined whether adult neurogenesis sustains hippocampal connections cumulatively across the life span. Long-term suppression of neurogenesis as occurs during stress and aging resulted in an accelerated decline in hippocampal acetylcholine signaling and a slow and progressing emergence of profound working memory deficits. These deficits were accompanied by compensatory reorganization of cholinergic dentate gyrus inputs with increased cholinergic innervation to the ventral hippocampus and recruitment of ventrally projecting neurons by the dorsal projection. While increased cholinergic innervation was dysfunctional and corresponded to overall decreases in cholinergic levels and signaling, it could be recruited to correct the resulting memory dysfunction even in old animals. Our study demonstrates that hippocampal neurogenesis supports memory by maintaining the septohippocampal cholinergic circuit across the lifespan. It also provides a systems level explanation for the progressive nature of memory deterioration during normal and pathological aging and indicates that the brain connectome is malleable by experience.

Serotonin-1A autoreceptors are necessary and sufficient for the normal formation of circuits underlying innate anxiety.

Identifying the factors contributing to the etiology of anxiety and depression is critical for the development of more efficacious therapies. Serotonin (5-HT) is intimately linked to both disorders. The inhibitory serotonin-1A (5-HT(1A)) receptor exists in two separate populations with distinct effects on serotonergic signaling: (1) an autoreceptor that limits 5-HT release throughout the brain and (2) a heteroreceptor that mediates inhibitory responses to released 5-HT. Traditional pharmacologic and transgenic strategies have not addressed the distinct roles of these two receptor populations. Here we use a recently developed genetic mouse system to independently manipulate 5-HT(1A) autoreceptor and heteroreceptor populations. We show that 5-HT(1A) autoreceptors act to affect anxiety-like behavior. In contrast, 5-HT(1A) heteroreceptors affect responses to forced swim stress, without effects on anxiety-like behavior. Together with our previously reported work, these results establish distinct roles for the two receptor populations, providing evidence that signaling through endogenous 5-HT(1A) autoreceptors is necessary and sufficient for the establishment of normal anxiety-like behavior.

A fast, aqueous, reversible three-day tissue clearing method for adult and embryonic mouse brain and whole body.

Optical clearing methods serve as powerful tools to study intact organs and neuronal circuits. We developed an aqueous clearing protocol, Fast 3D Clear, that relies on tetrahydrofuran for tissue delipidation and iohexol for clearing, such that tissues can be imaged under immersion oil in light-sheet imaging systems. Fast 3D Clear requires 3 days to achieve high transparency of adult and embryonic mouse tissues while maintaining their anatomical integrity and preserving a vast array of transgenic and viral/dye fluorophores. A unique advantage of Fast 3D Clear is its complete reversibility and thus compatibility with tissue sectioning and immunohistochemistry. Fast 3D Clear can be easily and quickly applied to a wide range of biomedical studies, facilitating the acquisition of high-resolution two- and three-dimensional images.

Early life stress delays hippocampal development and diminishes the adult stem cell pool in mice.

Early life stress predisposes to mental illness and behavioral dysfunction in adulthood, but the mechanisms underlying these persistent effects are poorly understood. Stress throughout life impairs the structure and function of the hippocampus, a brain system undergoing considerable development in early life. The long-term behavioral consequences of early life stress may therefore be due in part to interference with hippocampal development, in particular with assembly of the dentate gyrus (DG) region of the hippocampus. We investigated how early life stress produces long-term alterations in DG structure by examining DG assembly and the generation of a stable adult stem cell pool in routine housing and after stress induced by the limited bedding/nesting paradigm in mice. We found that early life stress leads to a more immature, proliferative DG than would be expected for the animal's age immediately after stress exposure, suggesting that early life stress delays DG development. Adult animals exposed to early life stress exhibited a reduction in the number of DG stem cells, but unchanged neurogenesis suggesting a depletion of the stem cell pool with compensation in the birth and survival of adult-born neurons. These results suggest a developmental mechanism by which early life stress can induce long-term changes in hippocampal function by interfering with DG assembly and ultimately diminishing the adult stem cell pool.

RbAp48 Protein Is a Critical Component of GPR158/OCN Signaling and Ameliorates Age-Related Memory Loss.

Precisely deciphering the molecular mechanisms of age-related memory loss is crucial to create appropriate therapeutic interventions. We have previously shown that the histone-binding protein RbAp48/Rbbp4 is a molecular determinant of Age-Related Memory Loss. By exploring how this protein regulates the genomic landscape of the hippocampal circuit, we find that RbAp48 controls the expression of BDNF and GPR158 proteins, both critical components of osteocalcin (OCN) signaling in the mouse hippocampus. We show that inhibition of RbAp48 in the hippocampal formation inhibits OCN's beneficial functions in cognition and causes deficits in discrimination memory. In turn, disruption of OCN/GPR158 signaling leads to the downregulation of RbAp48 protein, mimicking the discrimination memory deficits observed in the aged hippocampus. We also show that activation of the OCN/GPR158 pathway increases the expression of RbAp48 in the aged dentate gyrus and rescues age-related memory loss.

Serotonin Signaling through Prefrontal Cortex 5-HT1A Receptors during Adolescence Can Determine Baseline Mood-Related Behaviors.

Lifelong homeostatic setpoints for mood-related behaviors emerge during adolescence. Serotonin (5-HT) plays an important role in refining the formation of brain circuits during sensitive developmental periods. In rodents, the role of 5-HT1A receptors in general and autoreceptors in particular has been characterized in anxiety. However, less is known about the role of 5-HT1A receptors in depression-related behavior. Here, we show that whole-life suppression of heteroreceptor expression results in a broad depression-like behavioral phenotype accompanied by physiological and cellular changes within medial prefrontal cortex-dorsal raphe proper (mPFC-DRN) circuitry. These changes include increased basal 5-HT in a mPFC that is hyporesponsive to stress and decreased basal 5-HT levels and firing rates in a DRN hyperactivated by the same stressor. Remarkably, loss of heteroreceptors in the PFC at adolescence is sufficient to recapitulate this depression-like behavioral syndrome. Our results suggest that targeting mPFC 5-HT1A heteroreceptors during adolescence in humans may have lifelong ramifications for depression and its treatment.

Hippocampal neurogenesis: regulation by stress and antidepressants.

Accumulating evidence implicates hippocampal neurogenesis in the pathophysiology of depression. Psychosocial stress reduces neurogenesis in rodents, whereas chronic treatment with antidepressants increases neurogenesis and blocks the effects of stress. The effects of stress and antidepressant treatment on hippocampal neurogenesis parallel behavioral changes in animal models. Moreover, ablating hippocampal neurogenesis renders antidepressants inactive in behavioral paradigms used to model antidepressant response and anxiety-like behavior in mice. In humans, monoamine-modulating antidepressants demonstrate clinical efficacy in treating depression and anxiety, which are often precipitated by psychosocial stress. This review examines the mounting evidence that stress and antidepressant treatment regulate neurogenesis in animals. Special attention is paid to the cellular and molecular mechanisms by which this regulation takes place. An analysis of current animal models used to study response to stress and antidepressants indicates the importance of modeling chronic treatment, which reflects both changes in neurogenesis and clinical response. Exploring responses of hippocampal neurogenesis to experimental challenges in appropriate animal models should delineate the role of adult-born neurons in hippocampal physiology. Focusing on neurogenic response to experimental paradigms of stress and antidepressant treatment is particularly interesting for understanding the pathophysiology of major depressive disorder.

5-HT1A receptors on mature dentate gyrus granule cells are critical for the antidepressant response.

Selective serotonin reuptake inhibitors (SSRIs) are widely used antidepressants, but the mechanisms by which they influence behavior are only partially resolved. Adult hippocampal neurogenesis is necessary for some of the responses to SSRIs, but it is not known whether mature dentate gyrus granule cells (DG GCs) also contribute. We deleted the serotonin 1A receptor (5HT1AR, a receptor required for the SSRI response) specifically from DG GCs and found that the effects of the SSRI fluoxetine on behavior and the hypothalamic-pituitary-adrenal (HPA) axis were abolished. By contrast, mice lacking 5HT1ARs only in young adult-born GCs (abGCs) showed normal fluoxetine responses. Notably, 5HT1AR-deficient mice engineered to express functional 5HT1ARs only in DG GCs responded to fluoxetine, indicating that 5HT1ARs in DG GCs are sufficient to mediate an antidepressant response. Taken together, these data indicate that both mature DG GCs and young abGCs must be engaged for an antidepressant response.

Adolescent but not adult-born neurons are critical for susceptibility to chronic social defeat.

Recent evidence implicates adult hippocampal neurogenesis in regulating behavioral and physiologic responses to stress. Hippocampal neurogenesis occurs across the lifespan, however the rate of cell birth is up to 300% higher in adolescent mice compared to adults. Adolescence is a sensitive period in development where emotional circuitry and stress reactivity undergo plasticity establishing life-long set points. Therefore neurogenesis occurring during adolescence may be particularly important for emotional behavior. However, little is known about the function of hippocampal neurons born during adolescence. In order to assess the contribution of neurons born in adolescence to the adult stress response and depression-related behavior, we transiently reduced cell proliferation either during adolescence, or during adulthood in GFAP-Tk mice. We found that the intervention in adolescence did not change adult baseline behavioral response in the forced swim test, sucrose preference test or social affiliation test, and did not change adult corticosterone responses to an acute stressor. However following chronic social defeat, adult mice with reduced adolescent neurogenesis showed a resilient phenotype. A similar transient reduction in adult neurogenesis did not affect depression-like behaviors or stress induced corticosterone. Our study demonstrates that hippocampal neurons born during adolescence, but not in adulthood are important to confer susceptibility to chronic social defeat.

Progressive paranoid psychosis in a 20-year-old with central congenital hypoventilation syndrome.

A 20-year-old man with a history of congenital central hypoventilation syndrome presented with recent-onset psychosis, catatonia, and a diagnosis of schizophrenia. Psychiatric symptoms were resistant to conventional treatment. A fluorodeoxyglucose positron emission tomography scan of the brain obtained during the hospitalization revealed a hypometabolism distribution more consistent with hypoperfusion than with primary central nervous system disease. Increased mechanical ventilation was successfully used to treat the psychiatric symptoms.