GABA regulates synaptic integration of newly generated neurons in the adult brain

Interleukin-6 actions in the hypothalamus protects against obesity and is involved in the regulation of neurogenesis

Background

Interleukin-6 (IL6) produced in the context of exercise acts in the hypothalamus reducing obesity-associated inflammation and restoring the control of food intake and energy expenditure. In the hippocampus, some of the beneficial actions of IL6 are attributed to its neurogenesis-inducing properties. However, in the hypothalamus, the putative neurogenic actions of IL6 have never been explored, and its potential to balance energy intake can be an approach to prevent or attenuate obesity.

Methods

Wild-type (WT) and IL6 knockout (KO) mice were employed to study the capacity of IL6 to induce neurogenesis. We used cell labeling with Bromodeoxyuridine (BrdU), immunofluorescence, and real-time PCR to determine the expression of markers of neurogenesis and neurotransmitters. We prepared hypothalamic neuroprogenitor cells from KO that were treated with IL6 in order to provide an ex vivo model to further characterizing the neurogenic actions of IL6 through differentiation assays. In addition, we analyzed single-cell RNA sequencing data and determined the expression of IL6 and IL6 receptor in specific cell types of the murine hypothalamus.

Results

IL6 expression in the hypothalamus is low and restricted to microglia and tanycytes, whereas IL6 receptor is expressed in microglia, ependymocytes, endothelial cells, and astrocytes. Exogenous IL6 reduces diet-induced obesity. In outbred mice, obesity-resistance is accompanied by increased expression of IL6 in the hypothalamus. IL6 induces neurogenesis-related gene expression in the hypothalamus and in neuroprogenitor cells, both from WT as well as from KO mice.

Conclusion

IL6 induces neurogenesis-related gene expression in the hypothalamus of WT mice. In KO mice, the neurogenic actions of IL6 are preserved; however, the appearance of new fully differentiated proopiomelanocortin (POMC) and neuropeptide Y (NPY) neurons is either delayed or disturbed.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02242-8.

GABAB Receptor-Mediated Impairment of Intermediate Progenitor Maturation During Postnatal Hippocampal Neurogenesis of Newborn Rats

The neurotransmitter GABA and its receptors assume essential functions during fetal and postnatal brain development. The last trimester of a human pregnancy and early postnatal life involves a vulnerable period of brain development. In the second half of gestation, there is a developmental shift from depolarizing to hyperpolarizing in the GABAergic system, which might be disturbed by preterm birth. Alterations of the postnatal GABA shift are associated with several neurodevelopmental disorders. In this in vivo study, we investigated neurogenesis in the dentate gyrus (DG) in response to daily administration of pharmacological GABA_A (DMCM) and GABA_B (CGP 35348) receptor inhibitors to newborn rats. Six-day-old Wistar rats (P6) were daily injected (i.p.) to postnatal day 11 (P11) with DMCM, CGP 35348, or vehicle to determine the effects of both antagonists on postnatal neurogenesis. Due to GABA_B receptor blockade by CGP 35348, immunohistochemistry revealed a decrease in the number of NeuroD1 positive intermediate progenitor cells and a reduction of proliferative Nestin-positive neuronal stem cells at the DG. The impairment of hippocampal neurogenesis at this stage of differentiation is in line with a significantly decreased RNA expression of the transcription factors Pax6, Ascl1, and NeuroD1. Interestingly, the number of NeuN-positive postmitotic neurons was not affected by GABA_B receptor blockade, although strictly associated transcription factors for postmitotic neurons, Tbr1, Prox1, and NeuroD2, displayed reduced expression levels, suggesting impairment by GABA_B receptor antagonization at this stage of neurogenesis. Antagonization of GABA_B receptors decreased the expression of neurotrophins (BDNF, NT-3, and NGF). In contrast to the GABA_B receptor blockade, the GABA_A receptor antagonization revealed no significant changes in cell counts, but an increased transcriptional expression of Tbr1 and Tbr2. We conclude that GABAergic signaling via the metabotropic GABA_B receptor is crucial for hippocampal neurogenesis at the time of rapid brain growth and of the postnatal GABA shift. Differentiation and proliferation of intermediate progenitor cells are dependent on GABA. These insights become more pertinent in preterm infants whose developing brains are prematurely exposed to spostnatal stress and predisposed to poor neurodevelopmental disorders, possibly as sequelae of early disruption in GABAergic signaling.

The intellectual disability PAK3 R67C mutation impacts cognitive functions and adult hippocampal neurogenesis

The link between mutations associated with intellectual disability (ID) and the mechanisms underlying cognitive dysfunctions remains largely unknown. Here, we focused on PAK3, a serine/threonine kinase whose gene mutations cause X-linked ID. We generated a new mutant mouse model bearing the missense R67C mutation of the Pak3 gene (Pak3-R67C), known to cause moderate to severe ID in humans without other clinical signs and investigated hippocampal-dependent memory and adult hippocampal neurogenesis. Adult male Pak3-R67C mice exhibited selective impairments in long-term spatial memory and pattern separation function, suggestive of altered hippocampal neurogenesis. A delayed non-matching to place paradigm testing memory flexibility and proactive interference, reported here as being adult neurogenesis-dependent, revealed a hypersensitivity to high interference in Pak3-R67C mice. Analyzing adult hippocampal neurogenesis in Pak3-R67C mice reveals no alteration in the first steps of adult neurogenesis, but an accelerated death of a population of adult-born neurons during the critical period of 18-28 days after their birth. We then investigated the recruitment of hippocampal adult-born neurons after spatial memory recall. Post-recall activation of mature dentate granule cells in Pak3-R67C mice was unaffected, but a complete failure of activation of young DCX + newborn neurons was found, suggesting they were not recruited during the memory task. Decreased expression of the KCC2b chloride cotransporter and altered dendritic development indicate that young adult-born neurons are not fully functional in Pak3-R67C mice. We suggest that these defects in the dynamics and learning-associated recruitment of newborn hippocampal neurons may contribute to the selective cognitive deficits observed in this mouse model of ID.

GABAA receptors: structure, function, pharmacology, and related disorders

Background

γ-Aminobutyric acid sub-type A receptors (GABA_ARs) are the most prominent inhibitory neurotransmitter receptors in the CNS. They are a family of ligand-gated ion channel with significant physiological and therapeutic implications.

Main body

GABA_ARs are heteropentamers formed from a selection of 19 subunits: six α (alpha1-6), three β (beta1-3), three γ (gamma1-3), three ρ (rho1-3), and one each of the δ (delta), ε (epsilon), π (pi), and θ (theta) which result in the production of a considerable number of receptor isoforms. Each isoform exhibits distinct pharmacological and physiological properties. However, the majority of GABA_ARs are composed of two α subunits, two β subunits, and one γ subunit arranged as γ2β2α1β2α1 counterclockwise around the center. The mature receptor has a central chloride ion channel gated by GABA neurotransmitter and modulated by a variety of different drugs. Changes in GABA synthesis or release may have a significant effect on normal brain function. Furthermore, The molecular interactions and pharmacological effects caused by drugs are extremely complex. This is due to the structural heterogeneity of the receptors, and the existence of multiple allosteric binding sites as well as a wide range of ligands that can bind to them. Notably, dysfunction of the GABAergic system contributes to the development of several diseases. Therefore, understanding the relationship between GABA_A receptor deficits and CNS disorders thus has a significant impact on the discovery of disease pathogenesis and drug development.

Conclusion

To date, few reviews have discussed GABA_A receptors in detail. Accordingly, this review aims to summarize the current understanding of the structural, physiological, and pharmacological properties of GABA_ARs, as well as shedding light on the most common associated disorders.

Within-Trial Persistence of Learned Behavior as a Dissociable Behavioral Component in Hippocampus-Dependent Memory Tasks: A Potential Postlearning Role of Immature Neurons in the Adult Dentate Gyrus

The term “memory strength” generally refers to how well one remembers something. But more precisely it contains multiple modalities, such as how easily, how accurately, how confidently and how vividly we remember it. In human, these modalities of memory strength are dissociable. In this study, we asked whether we can isolate a behavioral component that is dissociable from others in hippocampus-dependent memory tasks in mice, which potentially reflect a modality of memory strength. Using a virus-mediated inducible method, we ablated immature neurons in the dentate gyrus in mice after we trained the mice with hippocampus-dependent memory tasks normally. In memory retrieval tests, these ablated mice initially showed intact performance. However, the ablated mice ceased learned behavior prematurely within a trial compared with control mice. In addition, the ablated mice showed shorter duration of individual episodes of learned behavior. Both affected behavioral measurements point to persistence of learned behavior. Thus, the effect of the postlearning manipulation showed dissociation between initial performance and persistence of learned behavior. These two behavioral components are likely to reflect different brain functions and be mediated by separate mechanisms, which might represent different modalities of memory strength. These simple dissociable measurements in widely used behavioral paradigms would be useful to understand detailed mechanisms underlying the expression of learned behavior and potentially different modalities of memory strength in mice. We also discuss a potential role that immature neurons in the dentate gyrus may play in persistence of learned behavior.

Brain microstructural changes in mice persist in adulthood and are modulated by the palmitoyl acyltransferase ZDHHC7

For a long time, mice have been classified as adults with completely mature brains at 8 weeks of age, but recent research suggests that developmental brain changes occur for up to 6 months. In particular, adolescence coincides with dramatic changes of neuronal structure and function in the brain that influence the connectivity between areas like hippocampus and medial prefrontal cortex (mPFC). Neuronal development and plasticity are regulated in part by the palmitoyl acyltransferase ZDHHC7, which modulates structural connectivity between hippocampus and mPFC. The aim of the current study was to investigate whether developmental changes take place in hippocampus and mPFC microstructure even after 8 weeks of age and whether deficiency of ZDHHC7 impacts such age-dependent alterations. Altogether, 46 mice at 11, 14 or 17 weeks of age with a genetic Zdhhc7 knockout (KO) or wild type (WT) were analysed with neuroimaging and diffusion tensor-based fibre tractography. The hippocampus and mPFC regions were compared regarding fibre metrics, supplemented by volumetric and immunohistological analyses of the hippocampus. In WT animals, we identified age-dependent changes in hippocampal fibre lengths that followed a U-shaped pattern, whereas in mPFC, changes were linear. In Zdhhc7-deficient animals, the fibre statistics were reduced in both regions, whereas the hippocampus volume and the intensities of myelin and neurofilament were higher in 11-week-old KO mice compared to WTs. Our results confirmed ongoing changes of microstructure in mice up to 17 weeks old and demonstrate that deleting the Zdhhc7 gene impairs fibre development, suggesting that palmitoylation is important in this process.

Exercise Improved the Anti-Epileptic Effect of Carbamazepine through GABA Enhancement in Epileptic Rats

Carbamazepine (CBZ) is an anticonvulsant drug that usually is used for the treatment of seizures. The anti-epileptic and the anti-epileptogenic effect of exercise has been reported, as well. This study was aimed to evaluate the synergic effect of combined therapy of exercise and CBZ in epileptic rats, as well as the alternation of the GABA pathway as a possible involved mechanism. The seizure was induced by pentylenetetrazol (PTZ) injection. Animals were divided into sham, seizure, exercise (EX), CBZ (25, 50 and 75), EX + CBZ (25, 50 and 75). Treadmill forced running for 30 min has been considered as the exercise 5 days per week for four weeks. CBZ was injected in doses of 25, 50 and 75 mg/kg, half an hour before seizure induction and 5 h after doing exercise in the animals forced to exercise. Seizure severity reduced and latency increased in the EX + CBZ (25) and EX + CBZ (50) groups compared to the seizure group. The distribution of GAD65 in both hippocampal CA1 and CA3 areas increased in the EX + CBZ (75) group. GABAA receptor α1 was up-regulated in the CA3 area of the EX + CBZ (75) group. The distribution of GAD65 in the cortical area increased in EX, EX + CBZ (50), CBZ (75) and EX + CBZ (75) groups. GABAA receptor α1 was up-regulated in the neocortex of EX + CBZ (50), CBZ (75) and EX + CBZ (75) groups. Our findings suggested that exercise has improved the efficacy of CBZ and reduced the anti-epileptic dose. The enhancement of GABA signaling might be involved in the synergistic effect of exercise and CBZ.

The effect of microglial inhibition on the expression of BDNF, KCC2, and GABAA receptor before and after the establishment of CCI-induced neuropathic pain model

Damage to the peripheral or central nervous system results in neuropathic pain. Based on a complicated mechanism, neuropathic pain has no efficient treatment so far. It has been well-known that the expression of some proteins (BDNF, KCC2, GABA-A) during neuropathic pain changes. Microglial cell activation is considered as a trigger to alter the expression of these proteins. In the current study, the effect of minocycline as a potent microglial activation inhibitor on the gene and protein expression of these neuropathic pain mediators was investigated. This experiment was done in two paradigms, preinjury and postinjury administration of minocycline. In each paradigm, male Wistar rats (weight 150-200 g, n = 6) were allocated to sham, control, and drug groups. Minocycline (30 mg/kg, i.p.) was injected 1 h before or at day seven after nerve injury and continued till day 14 in the preemptive or postinjury part of the study, respectively. After the last injection, the animals were decapitated and the lumbar part of the spinal cord was isolated to assess the expression of genes and proteins of interest. In the preventive study, minocycline increased the expression of KCC2 and GABA-A/γ₂ proteins and decreased BDNF expression. On the other hand, the target gene expression and protein expression were not changed when minocycline was administered after nerve injury. It seems that minocycline was able to change the expression of proteins of interest merely when used before nerve damage.

Learning, Neurogenesis, and Effects of Flavonoids on Learning

Learning and memory are two of our mind's most magical abilities. Different brain regions have roles in processing and storing different types of memories. The hippocampus is the part of the brain responsible for receiving information and storing it in the neocortex. One of the most impressive characteristics of the hippocampus is its capacity for neurogenesis, which is a process in which new neurons are produced and then transformed into mature neurons and finally integrated into neural circuits. The neurogenesis process in the hippocampus, an example of neuroplasticity in the adult brain, is believed to aid hippocampal-dependent learning and memory. New neurons are constantly produced in the hippocampus and integrated into the pre-existing neuronal network; this allows old memories already stored in the neocortex to be removed from the hippocampus and replaced with new ones. Factors affecting neurogenesis in the hippocampus may also affect hippocampal-dependent learning and memory. The flavonoids can particularly exert powerful actions in mammalian cognition and improve hippocampal-dependent learning and memory by positively affecting hippocampal neurogenesis.

A neurogenic microenvironment defined by excitatory-inhibitory neuronal circuits in adult dentate gyrus

Adult neurogenesis in the dentate gyrus plays a role in adaptive brain functions such as memory formation. Adding new neurons to a specific locus of a neural circuit with functional needs is an efficient way to achieve such an adaptive function. However, it is unknown whether neurogenesis is linked to local functional demands potentially specified by the activity of neuronal circuits. By examining the distribution of neurogenesis and different types of neuronal activity in the dentate gyrus of freely moving adult rats, we find that neurogenesis is positionally associated with active excitatory neurons, some of which show place-cell activity, but is positionally dissociated from a type of interneuron with high-burst tendency. Our finding suggests that the behaviorally relevant activity of excitatory-inhibitory neuronal circuits can define a microenvironment stimulating/inhibiting neurogenesis. Such local regulation of neurogenesis may contribute to strategic recruitment of new neurons to modify functionally relevant neural circuits.

A functional model of adult dentate gyrus neurogenesis

In adult dentate gyrus neurogenesis, the link between maturation of newborn neurons and their function, such as behavioral pattern separation, has remained puzzling. By analyzing a theoretical model, we show that the switch from excitation to inhibition of the GABAergic input onto maturing newborn cells is crucial for their proper functional integration. When the GABAergic input is excitatory, cooperativity drives the growth of synapses such that newborn cells become sensitive to stimuli similar to those that activate mature cells. When GABAergic input switches to inhibitory, competition pushes the configuration of synapses onto newborn cells toward stimuli that are different from previously stored ones. This enables the maturing newborn cells to code for concepts that are novel, yet similar to familiar ones. Our theory of newborn cell maturation explains both how adult-born dentate granule cells integrate into the preexisting network and why they promote separation of similar but not distinct patterns.

The interplay of neurovasculature and adult hippocampal neurogenesis

The subgranular zone of the dentate gyrus provides a local microenvironment (niche) for neural stem cells. In the adult brain, it has been established that the vascular compartment of such niches has a significant role in regulating adult hippocampal neurogenesis. More recently, evidence showed that neurovascular coupling, the relationship between blood flow and neuronal activity, also regulates hippocampal neurogenesis. Here, we review the most recent articles on addressing the intricate relationship between neurovasculature and adult hippocampal neurogenesis and a novel pathway where functional hyperemia enhances hippocampal neurogenesis. In the end, we have further reviewed recent research showing that impaired neurovascular coupling may cause declined neurogenesis and contribute to brain damage in neurodegenerative diseases.

Congenital hypothyroidism impairs spine growth of dentate granule cells by downregulation of CaMKIV

Congenital hypothyroidism (CH), a common neonatal endocrine disorder, can result in cognitive deficits if delay in diagnose and treatment. Dentate gyrus (DG) is the severely affected subregion of the hippocampus by the CH, where the dentate granule cells (DGCs) reside in. However, how CH impairs the cognitive function via affecting DGCs and the underlying mechanisms are not fully elucidated. In the present study, the CH model of rat pups was successfully established, and the aberrant dendrite growth of the DGCs and the impaired cognitive behaviors were observed in the offspring. Transcriptome analysis of hippocampal tissues following rat CH successfully identified that calcium/calmodulin-dependent protein kinase IV (CaMKIV) was the prominent regulator involved in mediating deficient growth of DGC dendrites. CaMKIV was shown to be dynamically regulated in the DG subregion of the rats following drug-induced CH. Interference of CaMKIV expression in the primary DGCs significantly reduced the spine density of dendrites, while addition of T3 to the primary DGCs isolated from CH pups could facilitate the spine growth of dendrites. Insights into relevant mechanisms revealed that CH-mediated CaMKIV deficiency resulted in the significant decrease of phosphorylated CREB in DGCs, in association with the abnormality of dendrites. Our results have provided a distinct cell type in hippocampus that is affected by CH, which would be beneficial for the treatment of CH-induced cognitive deficiency.

Structural and Functional Modulation of Perineuronal Nets: In Search of Important Players with Highlight on Tenascins

The extracellular matrix (ECM) of the brain plays a crucial role in providing optimal conditions for neuronal function. Interactions between neurons and a specialized form of ECM, perineuronal nets (PNN), are considered a key mechanism for the regulation of brain plasticity. Such an assembly of interconnected structural and regulatory molecules has a prominent role in the control of synaptic plasticity. In this review, we discuss novel ways of studying the interplay between PNN and its regulatory components, particularly tenascins, in the processes of synaptic plasticity, mechanotransduction, and neurogenesis. Since enhanced neuronal activity promotes PNN degradation, it is possible to study PNN remodeling as a dynamical change in the expression and organization of its constituents that is reflected in its ultrastructure. The discovery of these subtle modifications is enabled by the development of super-resolution microscopy and advanced methods of image analysis.

Propofol modulates inhibitory inputs in paraventricular thalamic nucleus of mice

The mechanisms of general anaesthetics such as propofol have drawn substantial attention. The effects of propofol on inhibitory postsynaptic currents are not exactly the same in different brain nuclei. Recent studies revealed that the paraventricular thalamic nucleus (PVT) is a critical nucleus modulating wakefulness. However, the effects of propofol on PVT neurons and the mechanisms underlying such effects remain unknown. Here, we performed the whole-cell recording of the PVT neurons in acute brain slices and bath application of propofol. We found that propofol hyperpolarized the membrane potentials of the PVT neurons and suppressed the action potentials induced by step-current injection. Propofol did not affect the spontaneous inhibitory postsynaptic currents (sIPSCs) amplitude or frequency, but prolonged the sIPSCs half-width. Besides, propofol increased miniature inhibitory synaptic currents (mIPSCs) frequency and half-width. Furthermore, propofol could induce GABA_A receptors-mediated tonic inhibitory currents dose-dependently. Thus, our results demonstrate that propofol hyperpolarizes PVT neurons by modulating inhibitory currents via GABA_A receptors in mice.

RGS6 Mediates Effects of Voluntary Running on Adult Hippocampal Neurogenesis

Voluntary running enhances adult hippocampal neurogenesis, with consequences for hippocampal-dependent learning ability and mood regulation. However, the underlying mechanism remains unclear. Here, we show that voluntary running induces unique and dynamic gene expression changes specifically within the adult-born hippocampal neurons, with significant impact on genes involved in neuronal maturation and human diseases. We identify the regulator of G protein signaling 6 (RGS6) as a key factor that mediates running impact on adult-born neurons. RGS6 overexpression mimics the positive effects of voluntary running on morphological and physiological maturation of adult new neurons and reduced sensitivity of adult-born neurons to the inhibitory effect of GABA_B (γ-Aminobutyric acid B) receptor activation. Knocking down RGS6 abolishes running-enhanced neuronal maturation and hippocampal neurogenesis-dependent learning and anxiolytic effect. Our study provides a data resource showing genome-wide intrinsic molecular changes in adult-born hippocampal neurons that contribute to voluntary running-induced neurogenesis.

Adult hypothalamic neurogenesis and sleep-wake dysfunction in aging

In the mammalian brain, adult neurogenesis has been extensively studied in the hippocampal sub-granular zone and the sub-ventricular zone of the anterolateral ventricles. However, growing evidence suggests that new cells are not only "born" constitutively in the adult hypothalamus, but many of these cells also differentiate into neurons and glia and serve specific functions. The preoptic-hypothalamic area plays a central role in the regulation of many critical functions, including sleep-wakefulness and circadian rhythms. While a role for adult hippocampal neurogenesis in regulating hippocampus-dependent functions, including cognition, has been extensively studied, adult hypothalamic neurogenic process and its contributions to various hypothalamic functions, including sleep-wake regulation are just beginning to unravel. This review is aimed at providing the current understanding of the hypothalamic adult neurogenic processes and the extent to which it affects hypothalamic functions, including sleep-wake regulation. We propose that hypothalamic neurogenic processes are vital for maintaining the proper functioning of the hypothalamic sleep-wake and circadian systems in the face of regulatory challenges. Sleep-wake disturbance is a frequent and challenging problem of aging and age-related neurodegenerative diseases. Aging is also associated with a decline in the neurogenic process. We discuss a hypothesis that a decrease in the hypothalamic neurogenic process underlies the aging of its sleep-wake and circadian systems and associated sleep-wake disturbance. We further discuss whether neuro-regenerative approaches, including pharmacological and non-pharmacological stimulation of endogenous neural stem and progenitor cells in hypothalamic neurogenic niches, can be used for mitigating sleep-wake and other hypothalamic dysfunctions in aging.

Dynamic interplay between GABAergic networks and developing neurons in the adult hippocampus

Neurogenesis is a powerful mechanism for structural and functional remodeling that occurs in restricted areas of the adult brain. Although different neurotransmitters regulate various aspects of the progression from neural stem cell quiescence to neuronal maturation, GABA is the main player. The developmental switch from excitation to inhibition combined with a heterogeneous population of GABAergic interneurons that target different subcellular compartments provides multiple points for the regulation of development and function of new neurons. This complexity is enhanced by feedback and feedforward networks that act as sensors and controllers of circuit activity, impinging directly or indirectly onto developing granule cells and, subsequently, on mature neurons. Newly generated granule cells ultimately connect with input and output partners in a manner that is largely sculpted by the activity of local circuits.

Formation and integration of new neurons in the adult hippocampus

Neural stem cells (NSCs) generate new neurons throughout life in the mammalian brain. Adult-born neurons shape brain function, and endogenous NSCs could potentially be harnessed for brain repair. In this Review, focused on hippocampal neurogenesis in rodents, we highlight recent advances in the field based on novel technologies (including single-cell RNA sequencing, intravital imaging and functional observation of newborn cells in behaving mice) and characterize the distinct developmental steps from stem cell activation to the integration of newborn neurons into pre-existing circuits. Further, we review current knowledge of how levels of neurogenesis are regulated, discuss findings regarding survival and maturation of adult-born cells and describe how newborn neurons affect brain function. The evidence arguing for (and against) lifelong neurogenesis in the human hippocampus is briefly summarized. Finally, we provide an outlook of what is needed to improve our understanding of the mechanisms and functional consequences of adult neurogenesis and how the field may move towards more translational relevance in the context of acute and chronic neural injury and stem cell-based brain repair.

An Interesting Molecule: γ-Aminobutyric Acid. What Can We Learn from Hydra Polyps?

Neuronal excitability is controlled primarily by γ-aminobutyric acid (GABA) in the central and peripheral nervous systems of vertebrate as well as invertebrate organisms. Besides its recognized neurotransmitter functions, GABA also plays a fundamental role in neurogenesis and synaptogenesis during embryonic development. In addition, GABAergic mechanisms are also involved in disorders of various peripheral tissues, ranging from diabetes to hypothyroidism to inflammatory responses. The discovery of the molecule and the history of its biosynthetic pathways in vertebrate and invertebrate phyla are summarized here. The occurrence and distribution of GABA, GABA-synthesizing enzymes, and receptors to GABA in the freshwater polyp Hydra vulgaris (Cnidaria: Hydrozoa), endowed with an early evolved nervous system, are discussed in relation to possible interactions with the microbiota, a stable component of Hydra polyps; their contribution to the evolution of nervous systems through microbe-neuronal interactions is proposed.

Plasticity in the Hippocampus, Neurogenesis and Drugs of Abuse

Synaptic plasticity in the hippocampus assists with consolidation and storage of long-lasting memories. Decades of research has provided substantial information on the cellular and molecular mechanisms underlying synaptic plasticity in the hippocampus, and this review discusses these mechanisms in brief. Addiction is a chronic relapsing disorder with loss of control over drug taking and drug seeking that is caused by long-lasting memories of drug experience. Relapse to drug use is caused by exposure to context and cues associated with the drug experience, and is a major clinical problem that contributes to the persistence of addiction. This review also briefly discusses some evidence that drugs of abuse alter plasticity in the hippocampus, and that development of novel treatment strategies that reverse or prevent drug-induced synaptic alterations in the hippocampus may reduce relapse behaviors associated with addiction.

Adult Neurogenesis in Epileptogenesis: An Update for Preclinical Finding and Potential Clinical Translation

Epileptogenesis refers to the process in which a normal brain becomes epileptic, and is characterized by hypersynchronous spontaneous recurrent seizures involving a complex epileptogenic network. Current available pharmacological treatment of epilepsy is generally symptomatic in controlling seizures but is not disease-modifying in epileptogenesis. Cumulative evidence suggests that adult neurogenesis, specifically in the subgranular zone of the hippocampal dentate gyrus, is crucial in epileptogenesis. In this review, we describe the pathological changes that occur in adult neurogenesis in the epileptic brain and how adult neurogenesis is involved in epileptogenesis through different interventions. This is followed by a discussion of some of the molecular signaling pathways involved in regulating adult neurogenesis, which could be potential druggable targets for epileptogenesis. Finally, we provide perspectives on some possible research directions for future studies.

A critical period of neuronal activity results in aberrant neurogenesis rewiring hippocampal circuitry in a mouse model of epilepsy

In the mammalian hippocampus, adult-born granule cells (abGCs) contribute to the function of the dentate gyrus (DG). Disruption of the DG circuitry causes spontaneous recurrent seizures (SRS), which can lead to epilepsy. Although abGCs contribute to local inhibitory feedback circuitry, whether they are involved in epileptogenesis remains elusive. Here, we identify a critical window of activity associated with the aberrant maturation of abGCs characterized by abnormal dendrite morphology, ectopic migration, and SRS. Importantly, in a mouse model of temporal lobe epilepsy, silencing aberrant abGCs during this critical period reduces abnormal dendrite morphology, cell migration, and SRS. Using mono-synaptic tracers, we show silencing aberrant abGCs decreases recurrent CA3 back-projections and restores proper cortical connections to the hippocampus. Furthermore, we show that GABA-mediated amplification of intracellular calcium regulates the early critical period of activity. Our results demonstrate that aberrant neurogenesis rewires hippocampal circuitry aggravating epilepsy in mice.

Neural Circuitry-Neurogenesis Coupling Model of Depression

Depression is characterized by the disruption of both neural circuitry and neurogenesis. Defects in hippocampal activity and volume, indicative of reduced neurogenesis, are associated with depression-related behaviors in both humans and animals. Neurogenesis in adulthood is considered an activity-dependent process; therefore, hippocampal neurogenesis defects in depression can be a result of defective neural circuitry activity. However, the mechanistic understanding of how defective neural circuitry can induce neurogenesis defects in depression remains unclear. This review highlights the current findings supporting the neural circuitry-regulated neurogenesis, especially focusing on hippocampal neurogenesis regulated by the entorhinal cortex, with regard to memory, pattern separation, and mood. Taken together, these findings may pave the way for future progress in neural circuitry-neurogenesis coupling studies of depression.

The unique plasticity of hippocampal adult-born neurons: Contributing to a heterogeneous dentate

The dentate gyrus (DG) of the hippocampus is evolutionarily conserved as one of the few sites of adult neurogenesis in mammals. Although there is clear evidence that neurogenesis is necessary for healthy hippocampal function, whether adult-born neurons are simply integrated into existing hippocampal networks to serve a similar purpose to that of developmentally born neurons or whether they represent a discrete cell population with unique functions remains less clear. In this review, we consider evidence for discrete cellular, synaptic, and structural features of adult-born DG neurons, suggesting that neurogenesis contributes to the formation of a heterogeneous DG. We therefore propose that hippocampal neurogenesis creates a specialized neuronal subpopulation that may play a key role in hippocampal functions like episodic memory. We note critical gaps in this extensive body of work, including a general failure to include female animals in relevant research and a need for more precise consideration of intrahippocampal neuroanatomy.

The postnatal GABA shift: A developmental perspective

GABA is the major inhibitory neurotransmitter that counterbalances excitation in the mature brain. The inhibitory action of GABA relies on the inflow of chloride ions (Cl^-), which hyperpolarizes the neuron. In early development, GABA signaling induces outward Cl^- currents and is depolarizing. The postnatal shift from depolarizing to hyperpolarizing GABA is a pivotal event in brain development and its timing affects brain function throughout life. Altered timing of the postnatal GABA shift is associated with several neurodevelopmental disorders. Here, we argue that the postnatal shift from depolarizing to hyperpolarizing GABA represents the final shift in a sequence of GABA shifts, regulating proliferation, migration, differentiation, and finally plasticity of developing neurons. Each developmental GABA shift ensures that the instructive role of GABA matches the circumstances of the developing network. Sensory input may be a crucial factor in determining proper timing of the postnatal GABA shift. A developmental perspective is necessary to interpret the full consequences of a mismatch between connectivity, activity and GABA signaling during brain development.

Rational approaches for the design of various GABA modulators and their clinical progression

GABA (γ-amino butyric acid) is an important inhibitory neurotransmitter in the central nervous system. Attenuation of GABAergic neurotransmission plays an important role in the etiology of several neurological disorders including epilepsy, Alzheimer's disease, Huntington's chorea, migraine, Parkinson's disease, neuropathic pain, and depression. Increase in the GABAergic activity may be achieved through direct agonism at the GABAA receptors, inhibition of enzymatic breakdown of GABA, or by inhibition of the GABA transport proteins (GATs). These functionalities make GABA receptor modulators and GATs attractive drug targets in brain disorders associated with decreased GABA activity. There have been several reports of development of GABA modulators (GABA receptors, GABA transporters, and GABAergic enzyme inhibitors) in the past decade. Therefore, the focus of the present review is to provide an overview on various design strategies and synthetic approaches toward developing GABA modulators. Furthermore, mechanistic insights, structure-activity relationships, and molecular modeling inputs for the biologically active derivatives have also been discussed. Summary of the advances made over the past few years in the clinical translation and development of GABA receptor modulators is also provided. This compilation will be of great interest to the researchers working in the field of neuroscience. From the light of detailed literature, it can be concluded that numerous molecules have displayed significant results and their promising potential, clearly placing them ahead as potential future drug candidates.

5-HT/GABA interaction in neurodevelopment and plasticity

The adult brain is the result of a multistages complex neurodevelopmental process involving genetic, molecular and microenvironmental factors as well as diverse patterns of electrical activity. In the postnatal life, immature neuronal circuits undergo an experience-dependent maturation during critical periods of plasticity, but the brain still retains plasticity during adult life. In all these stages, the neurotransmitter GABA plays a pivotal role. In this chapter, we will describe the interaction of 5-HT with GABA in regulating neurodevelopment and plasticity.

Multiple Roles for Cholinergic Signaling from the Perspective of Stem Cell Function

Stem cells have extensive proliferative potential and the ability to differentiate into one or more mature cell types. The mechanisms by which stem cells accomplish self-renewal provide fundamental insight into the origin and design of multicellular organisms. These pathways allow the repair of damage and extend organismal life beyond that of component cells, and they probably preceded the evolution of complex metazoans. Understanding the true nature of stem cells can only come from discovering how they are regulated. The concept that stem cells are controlled by particular microenvironments, also known as niches, has been widely accepted. Technical advances now allow characterization of the zones that maintain and control stem cell activity in several organs, including the brain, skin, and gut. Cholinergic neurons release acetylcholine (ACh) that mediates chemical transmission via ACh receptors such as nicotinic and muscarinic receptors. Although the cholinergic system is composed of organized nerve cells, the system is also involved in mammalian non-neuronal cells, including stem cells, embryonic stem cells, epithelial cells, and endothelial cells. Thus, cholinergic signaling plays a pivotal role in controlling their behaviors. Studies regarding this signal are beginning to unify our understanding of stem cell regulation at the cellular and molecular levels, and they are expected to advance efforts to control stem cells therapeutically. The present article reviews recent findings about cholinergic signaling that is essential to control stem cell function in a cholinergic niche.

Alterations in the behavior, cognitive function, and BDNF level in adult male rats following neonatal blockade of GABA-A receptors

Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in the mature brain. At an early developmental period, it acts in an excitatory manner that influences many processes of proliferation, migration, and differentiation of the neurons. Previous evidence indicated that manipulation of the GABAergic system function by activation or blockade of its receptors during developmental periods leads to behavioral and cognitive abnormality in adulthood. Therefore, we examined the effects of neonatal blockade of GABA-A receptors by bicuculline on behavior, cognitive function, and hippocampal and prefrontal cortex (PFC) brain-derived neurotrophic factors level (BDNF) in adulthood. As a result, neonatal rats were treated with either bicuculline (75,150, and 300 μg/kg) or DMSO on postnatal days 7,9, and 11. These groups underwent the behavioral (open field, elevated plus maze, and hot plate) and learning and memory (passive avoidance learning and memory) tests in postnatal days (PNDs) 61-70. After the ending of the behavioral tests, the rats were sacrificed under deep anesthesia and the hippocampi and prefrontal cortex (PFC) of the brain were removed for assessing the BDNF mRNA expression. Our results indicated that neonatal administration of bicuculline at the highest dose increased passive avoidance memory and hippocampal BDNF level. Meanwhile, this drug at a low dose impaired this type of memory and increased PFC BDNF level. Besides, treatment with bicuculline during postnatal days increased anxiety and pain sensitivity in a dose-dependent manner. Taken together, these findings confirmed the notion that GABA-A receptors during the developmental period are important for programming neurobehavioral phenotypes in adult life.

Differential Coupling of Adult-Born Granule Cells to Parvalbumin and Somatostatin Interneurons

A strong GABAergic tone imposes sparse levels of activity in the dentate gyrus of the hippocampus. This balance is challenged by the addition of new granule cells (GCs) with high excitability. How developing GCs integrate within local inhibitory networks remains unknown. We used optogenetics to study synaptogenesis between new GCs and GABAergic interneurons expressing parvalbumin (PV-INs) and somatostatin (SST-INs). PV-INs target the soma, and synapses become mature after 6 weeks. This transition is accelerated by exposure to an enriched environment. PV-INs exert efficient control of GC spiking and participate in both feedforward and feedback loops, a mechanism that would favor lateral inhibition and sparse coding. SST-INs target the dendrites, and synapses mature after 8 weeks. Outputs from GCs onto PV-INs develop faster than those onto SST-INs. Our results reveal a long-lasting transition wherein adult-born neurons remain poorly coupled to inhibition, which might enhance activity-dependent plasticity of input and output synapses.

Groisman et al. examine the integration of adult-born granule cells (GCs) to inhibitory networks of the adult hippocampus. Synapse maturation is remarkably slow for parvalbumin and somatostatin interneurons, both for connections toward and from GCs. Inhibition controls the activity of new GCs late in development.

Entacapone promotes hippocampal neurogenesis in mice

Entacapone, a catechol-O-methyltransferase inhibitor, can strengthen the therapeutic effects of levodopa on the treatment of Parkinson’s disease. However, few studies are reported on whether entacapone can affect hippocampal neurogenesis in mice. To investigate the effects of entacapone, a modulator of dopamine, on proliferating cells and immature neurons in the mouse hippocampal dentate gyrus, 60 mice (7 weeks old) were randomly divided into a vehicle-treated group and the groups treated with 10, 50, or 200 mg/kg entacapone. The results showed that 50 and 200 mg/kg entacapone increased the exploration time for novel object recognition. Immunohistochemical staining results revealed that after entacapone treatment, the numbers of Ki67-positive proliferating cells, doublecortin-positive immature neurons, and phosphorylated cAMP response element-binding protein (pCREB)-positive cells were significantly increased. Western blot analysis results revealed that treatment with tyrosine kinase receptor B (TrkB) receptor antagonist significantly decreased the exploration time for novel object recognition and inhibited the expression of phosphorylated TrkB and brain-derived neurotrophic factor (BDNF). Entacapone treatment antagonized the effects of TrkB receptor antagonist. These results suggest that entacapone treatment promoted hippocampal neurogenesis and improved memory function through activating the BDNF-TrkB-pCREB pathway. This study was approved by the Institutional Animal Care and Use Committee of Seoul National University (approval No. SNU-130730-1) on February 24, 2014.

An Overview of Adult Neurogenesis

Neurogenesis is maintained in the mammalian brain throughout adulthood in two main regions: the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the hippocampal dentate gyrus. Adult neurogenesis is a process composed of multiple steps by which neurons are generated from dividing adult neural stem cells and migrate to be integrated into existing neuronal circuits. Alterations in any of these steps impair neurogenesis and may compromise brain function, leading to cognitive impairment and neurodegenerative diseases. Therefore, understanding the cellular and molecular mechanisms that modulate adult neurogenesis is the centre of attention of regenerative research. In this chapter, we review the main properties of the adult neurogenic niches.

Role of Hippocampal Neurogenesis in Alcohol Withdrawal Seizures

Chronic alcohol consumption results in alcohol use disorder (AUD). Interestingly, however, sudden alcohol withdrawal (AW) after chronic alcohol exposure also leads to a devastating series of symptoms, referred to as alcohol withdrawal syndromes. One key feature of AW syndromes is to produce phenotypes that are opposite to AUD. For example, while the brain is characterized by a hypoactive state in the presence of alcohol, AW induces a hyperactive state, which is manifested as seizure expression. In this review, we discuss the idea that hippocampal neurogenesis and neural circuits play a key role in neuroadaptation and establishment of allostatic states in response to alcohol exposure and AW. The intrinsic properties of dentate granule cells (DGCs), and their contribution to the formation of a potent feedback inhibitory loop, endow the dentate gyrus with a "gate" function, which can limit the entry of excessive excitatory signals from the cortex into the hippocampus. We discuss the possibility that alcohol exposure and withdrawal disrupts structural development and circuitry integration of hippocampal newborn neurons, and that this altered neurogenesis impairs the gate function of the hippocampus. Failure of this gate function is expected to alter the ratio of excitatory to inhibitory (E/I) signals in the hippocampus and to induce seizure expression during AW. Recent functional studies have shown that specific activation and inhibition of hippocampal newborn DGCs are both necessary and sufficient for the expression of AW-associated seizures, further supporting the concept that neurogenesis-induced neuroadaptation is a critical target to understand and treat AUD and AW-associated seizures.

Adult-Born Neurons in the Hippocampus Are Essential for Social Memory Maintenance

Throughout adulthood, the dentate gyrus continues to produce new granule cells, which integrate into the hippocampal circuitry. New neurons have been linked to several known functions of the hippocampus, including learning and memory, anxiety and stress regulation, and social behavior. We explored whether transgenic reduction of adult-born neurons in mice would impair social memory and the formation of social dominance hierarchies. We used a conditional transgenic mouse strain [thymidine kinase (TK) mice] that selectively reduces adult neurogenesis by treatment with the antiviral drug valganciclovir (VGCV). TK mice treated with VGCV were unable to recognize conspecifics as familiar 24 h after initial exposure. We then explored whether reducing new neurons completely impaired their ability to acquire or retrieve a social memory and found that TK mice treated with VGCV were able to perform at control levels when the time between exposure (acquisition) and reexposure (retrieval) was brief. We next explored whether adult-born neurons are involved in dominance hierarchy formation by analyzing their home cage behavior as well as their performance in the tube test, a social hierarchy test, and did not find any consistent alterations in behavior between control and TK mice treated with VGCV. These data suggest that adult neurogenesis is essential for social memory maintenance, but not for acquisition nor retrieval over a short time frame, with no effect on social dominance hierarchy. Future work is needed to explore whether the influence of new neurons on social memory is mediated through connections with the CA2, an area involved in social recognition.

Study on the therapeutic material basis and effect of Acanthopanax senticosus (Rupr. et Maxim.) Harms leaves in the treatment of ischemic stroke by PK-PD analysis based on online microdialysis-LC-MS/MS method

Leaves of Acanthopanax senticosus (Rupr. et Maxim.) Harms (ASL) have revealed significant biological activity in the treatment of ischemic stroke diseases. However, there was no in-depth study of the therapeutic material basis and effect of ASL from the pharmacokinetics-pharmacodynamics (PK-PD) analysis level. In this study, a method based on microdialysis coupled with ultra-performance liquid chromatography combined with triple quadruple mass spectrometry (MD-UPLC-QQQ-MS) was established to simultaneously and continuously collect and quantify the active compounds and endogenous neuroactive substances related to therapeutic effect in plasma and hippocampus of fully awake ischemic stroke rats. The acquired data were analyzed by the PK-PD analysis method. It was found that hyperoside, quercitrin, quercetin, and caffeic acid could pass through the blood-brain barrier, and quercetin needed a longer intake time than quercitrin and hyperoside, but the passage rate was higher. The exposure of the four compounds in the hippocampus affected the contents of seven neuroactive substances in different ways and was depicted graphically (concentration-time effect). In addition, the study found that the brain index and brain water content of ischemic stroke rats were significantly reduced after the oral administration of ASL. ASL observably regulated the content or activity of six important biochemical indexes in rats. On the one hand, this study verified that ASL could regulate ischemic stroke in many aspects. On the other hand, a visualized method to express the relationship between pharmacokinetics and pharmacodynamics in the hippocampus of cerebral ischemic areas was established. This research gives a hand to the study on the therapeutic material basis and effect of traditional Chinese medicine mechanism.

NKCC1, an Elusive Molecular Target in Brain Development: Making Sense of the Existing Data

Ionotropic GABA transmission is mediated by anion (mainly Cl⁻)-permeable GABA_A receptors (GABA_ARs). In immature neurons, GABA exerts depolarizing and sometimes functionally excitatory actions, based on active uptake of Cl⁻ by the Na-K-2Cl cotransporter NKCC1. While functional evidence firmly shows NKCC1-mediated ion transport in immature and diseased neurons, molecular detection of NKCC1 in the brain has turned out to be extremely difficult. In this review, we describe the highly inconsistent data that are available on the cell type-specific expression patterns of the NKCC1 mRNA and protein in the CNS. We discuss the major technical caveats, including a lack of knock-out-controlled immunohistochemistry in the forebrain, possible effects of alternative splicing on the binding of antibodies and RNA probes, and the wide expression of NKCC1 in different cell types, which make whole-tissue analyses of NKCC1 useless for studying its neuronal expression. We also review novel single-cell RNAseq data showing that most of the NKCC1 in the adult CNS may, in fact, be expressed in non-neuronal cells, especially in glia. As future directions, we suggest single-cell NKCC1 mRNA and protein analyses and the use of genetically tagged endogenous proteins or systematically designed novel antibodies, together with proper knock-out controls, for the visualization of endogenous NKCC1 in distinct brain cell types and their subcellular compartments.

Extracellular Vesicles, Influential Players of Intercellular Communication within Adult Neurogenic Niches

Adult neurogenesis, involving the generation of functional neurons from adult neural stem cells (NSCs), occurs constitutively in discrete brain regions such as hippocampus, sub-ventricular zone (SVZ) and hypothalamus. The intrinsic structural plasticity of the neurogenic process allows the adult brain to face the continuously changing external and internal environment and requires coordinated interplay between all cell types within the specialized microenvironment of the neurogenic niche. NSC-, neuronal- and glia-derived factors, originating locally, regulate the balance between quiescence and self-renewal of NSC, their differentiation programs and the survival and integration of newborn cells. Extracellular Vesicles (EVs) are emerging as important mediators of cell-to-cell communication, representing an efficient way to transfer the biologically active cargos (nucleic acids, proteins, lipids) by which they modulate the function of the recipient cells. Current knowledge of the physiological role of EVs within adult neurogenic niches is rather limited. In this review, we will summarize and discuss EV-based cross-talk within adult neurogenic niches and postulate how EVs might play a critical role in the regulation of the neurogenic process.

Traumatic brain injury modifies synaptic plasticity in newly-generated granule cells of the adult hippocampus

The hippocampus is vulnerable to traumatic brain injury (TBI), and hippocampal damage is associated with cognitive deficits that are often the hallmark of TBI. Recent studies have found that TBI induces enhanced neurogenesis in the dentate gyrus (DG) of the hippocampus, and this cellular response is related to innate cognitive recovery. However, cellular mechanisms of the role of DG neurogenesis in post-TBI recovery remain unclear. This study investigated changes in long-term potentiation (LTP) within the DG in relation to TBI-induced neurogenesis. Adult male rats received a moderate TBI or sham injury and were sacrificed for brain slice recordings at 30 or 60 days post-injury. Recordings were taken from the medial perforant path input to DG granule cells in the presence or absence of the GABAergic antagonist picrotoxin, reflecting activity of either all DG granule cells or predominately newborn granule cells, respectively. Measurements of LTP observed in the total granule cell population (with picrotoxin) showed a prolonged impairment which worsened between 30 and 60 days post-TBI. Under conditions which predominantly reflected the LTP elicited in newly born granule cells (no picrotoxin), a strikingly different pattern of post-TBI changes was observed, with a time-dependent cycle of functional impairment and recovery. At 30 days after injury this cell population showed little or no LTP, but by 60 days the capacity for LTP of the newly born granule cells was no different from that of sham controls. The time-frame of LTP improvements in the newborn cell population, comparable to that of behavioral recovery reported previously, suggests the unique functional properties of newborn granule cells enable them to contribute to restorative change following brain injury.

Transcriptional Effects of Psychoactive Drugs on Genes Involved in Neurogenesis

Although neurogenesis is affected in several psychiatric diseases, the effects and mechanisms of action of psychoactive drugs on neurogenesis remain unknown and/or controversial. This study aims to evaluate the effects of psychoactive drugs on the expression of genes involved in neurogenesis. Neuronal-like cells (NT2-N) were treated with amisulpride (10 µM), aripiprazole (0.1 µM), clozapine (10 µM), lamotrigine (50 µM), lithium (2.5 mM), quetiapine (50 µM), risperidone (0.1 µM), or valproate (0.5 mM) for 24 h. Genome wide mRNA expression was quantified and analysed using gene set enrichment analysis, with the neurogenesis gene set retrieved from the Gene Ontology database and the Mammalian Adult Neurogenesis Gene Ontology (MANGO) database. Transcription factors that are more likely to regulate these genes were investigated to better understand the biological processes driving neurogenesis. Targeted metabolomics were performed using gas chromatography-mass spectrometry. Six of the eight drugs decreased the expression of genes involved in neurogenesis in both databases. This suggests that acute treatment with these psychoactive drugs negatively regulates the expression of genes involved in neurogenesis in vitro. SOX2 and three of its target genes (CCND1, BMP4, and DKK1) were also decreased after treatment with quetiapine. This can, at least in part, explain the mechanisms by which these drugs decrease neurogenesis at a transcriptional level in vitro. These results were supported by the finding of increased metabolite markers of mature neurons following treatment with most of the drugs tested, suggesting increased proportions of mature relative to immature neurons consistent with reduced neurogenesis.

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

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

Neural Stem Cells and Cannabinoids in the Spotlight as Potential Therapy for Epilepsy

Epilepsy is one of the most common brain diseases worldwide, having a huge burden in society. The main hallmark of epilepsy is the occurrence of spontaneous recurrent seizures, having a tremendous impact on the lives of the patients and of their relatives. Currently, the therapeutic strategies are mostly based on the use of antiepileptic drugs, and because several types of epilepsies are of unknown origin, a high percentage of patients are resistant to the available pharmacotherapy, continuing to experience seizures overtime. Therefore, the search for new drugs and therapeutic targets is highly important. One key aspect to be targeted is the aberrant adult hippocampal neurogenesis (AHN) derived from Neural Stem Cells (NSCs). Indeed, targeting seizure-induced AHN may reduce recurrent seizures and shed some light on the mechanisms of disease. The endocannabinoid system is a known modulator of AHN, and due to the known endogenous antiepileptic properties, it is an interesting candidate for the generation of new antiepileptic drugs. However, further studies and clinical trials are required to investigate the putative mechanisms by which cannabinoids can be used to treat epilepsy. In this manuscript, we will review how cannabinoid-induced modulation of NSCs may promote neural plasticity and whether these drugs can be used as putative antiepileptic treatment.

Woo E, Patten A, Yau SY, Gil-Mohapel J. Beyond the Hippocampus and the SVZ: Adult Neurogenesis Throughout the Brain

Convincing evidence has repeatedly shown that new neurons are produced in the mammalian brain into adulthood. Adult neurogenesis has been best described in the hippocampus and the subventricular zone (SVZ), in which a series of distinct stages of neuronal development has been well characterized. However, more recently, new neurons have also been found in other brain regions of the adult mammalian brain, including the hypothalamus, striatum, substantia nigra, cortex, and amygdala. While some studies have suggested that these new neurons originate from endogenous stem cell pools located within these brain regions, others have shown the migration of neurons from the SVZ to these regions. Notably, it has been shown that the generation of new neurons in these brain regions is impacted by neurologic processes such as stroke/ischemia and neurodegenerative disorders. Furthermore, numerous factors such as neurotrophic support, pharmacologic interventions, environmental exposures, and stem cell therapy can modulate this endogenous process. While the presence and significance of adult neurogenesis in the human brain (and particularly outside of the classical neurogenic regions) is still an area of debate, this intrinsic neurogenic potential and its possible regulation through therapeutic measures present an exciting alternative for the treatment of several neurologic conditions. This review summarizes evidence in support of the classic and novel neurogenic zones present within the mammalian brain and discusses the functional significance of these new neurons as well as the factors that regulate their production. Finally, it also discusses the potential clinical applications of promoting neurogenesis outside of the classical neurogenic niches, particularly in the hypothalamus, cortex, striatum, substantia nigra, and amygdala.

GDNF and GFRα1 Are Required for Proper Integration of Adult-Born Hippocampal Neurons

The glial cell line-derived neurotrophic factor (GDNF) is required for the survival and differentiation of diverse neuronal populations during nervous system development. Despite the high expression of GDNF and its receptor GFRα1 in the adult hippocampus, the functional role of this system remains unknown. Here, we show that GDNF, acting through its GFRα1 receptor, controls dendritic structure and spine density of adult-born granule cells, which reveals that GFRα1 is required for their integration into preexisting circuits. Moreover, conditional mutant mice for GFRα1 show deficits in behavioral pattern separation, a task in which adult neurogenesis is known to play a critical role. We also find that running increases GDNF in the dentate gyrus and promotes GFRα1-dependent CREB (cAMP response element-binding protein) activation and dendrite maturation. Together, these findings indicate that GDNF/GFRα1 signaling plays an essential role in the plasticity of adult circuits, controlling the integration of newly generated neurons.

How Do Electric Fields Coordinate Neuronal Migration and Maturation in the Developing Cortex?

During development the vast majority of cells that will later compose the mature cerebral cortex undergo extensive migration to reach their final position. In addition to intrinsically distinct migratory behaviors, cells encounter and respond to vastly different microenvironments. These range from axonal tracts to cell-dense matrices, electrically active regions and extracellular matrix components, which may all change overtime. Furthermore, migrating neurons themselves not only adapt to their microenvironment but also modify the local niche through cell-cell contacts, secreted factors and ions. In the radial dimension, the developing cortex is roughly divided into dense progenitor and cortical plate territories, and a less crowded intermediate zone. The cortical plate is bordered by the subplate and the marginal zone, which are populated by neurons with high electrical activity and characterized by sophisticated neuritic ramifications. Neuronal migration is influenced by these boundaries resulting in dramatic changes in migratory behaviors as well as morphology and electrical activity. Modifications in the levels of any of these parameters can lead to alterations and even arrest of migration. Recent work indicates that morphology and electrical activity of migrating neuron are interconnected and the aim of this review is to explore the extent of this connection. We will discuss on one hand how the response of migrating neurons is altered upon modification of their intrinsic electrical properties and whether, on the other hand, the electrical properties of the cellular environment can modify the morphology and electrical activity of migrating cortical neurons.

Metabolic tuning of inhibition regulates hippocampal neurogenesis in the adult brain

Hippocampus-engaged behaviors stimulate neurogenesis in the adult dentate gyrus by largely unknown means. To explore the underlying mechanisms, we used tetrode recording to analyze neuronal activity in the dentate gyrus of freely moving adult mice during hippocampus-engaged contextual exploration. We found that exploration induced an overall sustained increase in inhibitory neuron activity that was concomitant with decreased excitatory neuron activity. A mathematical model based on energy homeostasis in the dentate gyrus showed that enhanced inhibition and decreased excitation resulted in a similar increase in neurogenesis to that observed experimentally. To mechanistically investigate this sustained inhibitory regulation, we performed metabolomic and lipidomic profiling of the hippocampus during exploration. We found sustainably increased signaling of sphingosine-1-phosphate, a bioactive metabolite, during exploration. Furthermore, we found that sphingosine-1-phosphate signaling through its receptor 2 increased interneuron activity and thus mediated exploration-induced neurogenesis. Taken together, our findings point to a behavior-metabolism circuit pathway through which experience regulates adult hippocampal neurogenesis.

Role of Wnt Signaling in Adult Hippocampal Neurogenesis in Health and Disease

Neurogenesis persists during adulthood in the dentate gyrus of the hippocampus. Signals provided by the local hippocampal microenvironment support neural stem cell proliferation, differentiation, and maturation of newborn neurons into functional dentate granule cells, that integrate into the neural circuit and contribute to hippocampal function. Increasing evidence indicates that Wnt signaling regulates multiple aspects of adult hippocampal neurogenesis. Wnt ligands bind to Frizzled receptors and co-receptors to activate the canonical Wnt/β-catenin signaling pathway, or the non-canonical β-catenin-independent signaling cascades Wnt/Ca2+ and Wnt/planar cell polarity. Here, we summarize current knowledge on the roles of Wnt signaling components including ligands, receptors/co-receptors and soluble modulators in adult hippocampal neurogenesis. Also, we review the data suggesting distinctive roles for canonical and non-canonical Wnt signaling cascades in regulating different stages of neurogenesis. Finally, we discuss the evidence linking the dysfunction of Wnt signaling to the decline of neurogenesis observed in aging and Alzheimer's disease.

β-catenin signaling modulates the tempo of dendritic growth of adult-born hippocampal neurons

In adult hippocampal neurogenesis, stem/progenitor cells generate dentate granule neurons that contribute to hippocampal plasticity. The establishment of a morphologically defined dendritic arbor is central to the functional integration of adult‐born neurons. We investigated the role of canonical Wnt/β‐catenin signaling in dendritogenesis of adult‐born neurons. We show that canonical Wnt signaling follows a biphasic pattern, with high activity in stem/progenitor cells, attenuation in immature neurons, and reactivation during maturation, and demonstrate that this activity pattern is required for proper dendrite development. Increasing β‐catenin signaling in maturing neurons of young adult mice transiently accelerated dendritic growth, but eventually produced dendritic defects and excessive spine numbers. In middle‐aged mice, in which protracted dendrite and spine development were paralleled by lower canonical Wnt signaling activity, enhancement of β‐catenin signaling restored dendritic growth and spine formation to levels observed in young adult animals. Our data indicate that precise timing and strength of β‐catenin signaling are essential for the correct functional integration of adult‐born neurons and suggest Wnt/β‐catenin signaling as a pathway to ameliorate deficits in adult neurogenesis during aging.