Serotonin depletion hampers survival and proliferation in neurospheres derived from adult neural stem cells

Cannabidiol Exerts Sedative and Hypnotic Effects in Normal and Insomnia Model Mice Through Activation of 5-HT1A Receptor

Cannabis sativa has been used for improving sleep for long history. Cannabidiol (CBD) has drown much attention as a non-addictive psychoactive component in Cannabis sativa extract. However, the effects of CBD on sleep architecture and it's acting mechanism remains unclear. In the present study, we evaluated the sedative-hypnotic effect of cannabidiol (CBD), assessed the effects of CBD on sleep using a wireless physiological telemetry system. We further explored the therapeutic effects of CBD using 4-chloro-dl-phenylalanine (PCPA) induced insomnia model and changes in sleep latency, sleep duration and intestinal flora were evaluated. CBD shortened sleep latency and increases sleep duration in both normal and insomnia mice, and those effects were blocked by 5-HT1A receptor antagonist WAY100635. We determined that CBD increases 5-HT1A receptors expression and 5-HT content in the hypothalamus of PCPA-pretreated mice and affects tryptophan metabolism in the intestinal flora. These results showed that activation of 5-HT1A receptors is one of the potential mechanisms underlying the sedative-hypnotic effect of CBD. This study validated the effects of CBD on sleep and evaluated its potential therapeutic effects on insomnia.

SERS detection of dopamine using metal-chelated Ag nanoshell

As the concentrations of different neurotransmitters can indicate the presence of certain disorders affecting brain functions, quantitative analyses of neurotransmitters have attracted increasing attention in various fields. Surface-enhanced Raman scattering (SERS) spectroscopy is an outstanding spectroscopic analytical tool that enables detection at the single molecule level with high specificity. As local field enhancement of surface plasmon is effective within nanometers, active interaction between SERS-active noble metals (gold and silver) and analyte molecules enhances the molecular detection capacity of SERS. However, neurotransmitters and noble metal nanoparticles are often not affinitive, because neurotransmitters generally have a hydroxyl group rather than a thiol group. As a result, the interaction between the two typically remains inactive, which makes detection more difficult. To overcome this limitation, in the present work we utilized metal-chelation to attract dopamine, a neurotransmitter molecule, close to the surface of silver nanoparticles. AgNS was capped with poly(vinyl alcohol) (PVA) and sequentially integrated with copper ion to bind dopamine in the form of chelate bonding between dopamine and copper. The PVA linked AgNS and metal ions through a coordinate bond between hydroxyl groups and metal ions. This metal-chelation-functionalized nanoprobe allowed us to stably detect dopamine in aqueous solution at a concentration of less than 10-6 M. Therefore, this method provides a convenient and easy-to-prepare option for the effective detection of dopamine, thus meaning it has the potential to be applied to other neurotransmitters.

Gut brain interaction theory reveals gut microbiota mediated neurogenesis and traditional Chinese medicine research strategies

Adult neurogenesis is the process of differentiation of neural stem cells (NSCs) into neurons and glial cells in certain areas of the adult brain. Defects in neurogenesis can lead to neurodegenerative diseases, mental disorders, and other maladies. This process is directionally regulated by transcription factors, the Wnt and Notch pathway, the extracellular matrix, and various growth factors. External factors like stress, physical exercise, diet, medications, etc., affect neurogenesis and the gut microbiota. The gut microbiota may affect NSCs through vagal, immune and chemical pathways, and other pathways. Traditional Chinese medicine (TCM) has been proven to affect NSCs proliferation and differentiation and can regulate the abundance and metabolites produced by intestinal microorganisms. However, the underlying mechanisms by which these factors regulate neurogenesis through the gut microbiota are not fully understood. In this review, we describe the recent evidence on the role of the gut microbiota in neurogenesis. Moreover, we hypothesize on the characteristics of the microbiota-gut-brain axis based on bacterial phyla, including microbiota's metabolites, and neuronal and immune pathways while providing an outlook on TCM's potential effects on adult neurogenesis by regulating gut microbiota.

Microbiota-Gut-Brain Axis Regulation of Adult Hippocampal Neurogenesis

The birth, maturation, and integration of new neurons in the adult hippocampus regulates specific learning and memory processes, responses to stress, and antidepressant treatment efficacy. This process of adult hippocampal neurogenesis is sensitive to environmental stimuli, including peripheral signals from certain cytokines, hormones, and metabolites, which can promote or hinder the production and survival of new hippocampal neurons. The trillions of microorganisms resident to the gastrointestinal tract and collectively known as the gut microbiota, also demonstrate the ability to modulate adult hippocampal neurogenesis. In doing so, the microbiota-gut-brain axis can influence brain functions regulated by adult hippocampal neurogenesis. Unlike the hippocampus, the gut microbiota is highly accessible to direct interventions, such as prebiotics, probiotics, and antibiotics, and can be manipulated by lifestyle choices including diet. Therefore, understanding the pathways by which the gut microbiota shapes hippocampal neurogenesis may reveal novel targets for non-invasive therapeutics to treat disorders in which alterations in hippocampal neurogenesis have been implicated. This review first outlines the factors which influence both the gut microbiome and adult hippocampal neurogenesis, with cognizance that these effects might happen either independently or due to microbiota-driven mechanisms. We then highlight approaches for investigating the regulation of adult hippocampal neurogenesis by the microbiota-gut-brain axis. Finally, we summarize the current evidence demonstrating the gut microbiota’s ability to influence adult hippocampal neurogenesis, including mechanisms driven through immune pathways, microbial metabolites, endocrine signalling, and the nervous system, and postulate implications for these effects in disease onset and treatment.

Fluoxetine regulates the neuronal differentiation of neural stem cells transplanted into rat brains after stroke by increasing the 5HT level

Fluoxetine, a 5-HT uptake inhibitor, has been adopted for the treatment of post-stroke depression in recent years. It has been confirmed to induce neuronal regeneration in vivo, but its effect on inducing stem cell differentiation after transplantation has not yet been verified. To evaluate its regulatory effect on stem cell differentiation, fluoxetine was used in this study to treat rats with cerebral ischemia after neural stem cell (NSC) transplantation. The results showed that the proportion of NSCs differentiating into neurons significantly increased after fluoxetine treatment. In NSC adherent culture, the addition of 5-HT but not of fluoxetine significantly increased the neuronal differentiation ratio of NSCs. Moreover, the addition of 5-HT2A or 5-HT3A antagonists inhibited this effect. In addition, Western blotting revealed that the increase in 5-HT inhibited ERK2 phosphorylation and upregulated neurogenin1 expression. In conclusion, fluoxetine increased the 5-HT level and promoted neuronal differentiation, thereby upregulating neurogenin1 expression and downregulating ERK2 phosphorylation.

The Influence of Gut Microbiota on Neurogenesis: Evidence and Hopes

Adult neurogenesis (i.e., the life-long generation of new neurons from undifferentiated neuronal precursors in the adult brain) may contribute to brain repair after damage, and participates in plasticity-related processes including memory, cognition, mood and sensory functions. Among the many intrinsic (oxidative stress, inflammation, and ageing), and extrinsic (environmental pollution, lifestyle, and diet) factors deemed to impact neurogenesis, significant attention has been recently attracted by the myriad of saprophytic microorganismal communities inhabiting the intestinal ecosystem and collectively referred to as the gut microbiota. A growing body of evidence, mainly from animal studies, reveal the influence of microbiota and its disease-associated imbalances on neural stem cell proliferative and differentiative activities in brain neurogenic niches. On the other hand, the long-claimed pro-neurogenic activity of natural dietary compounds endowed with antioxidants and anti-inflammatory properties (such as polyphenols, polyunsaturated fatty acids, or pro/prebiotics) may be mediated, at least in part, by their action on the intestinal microflora. The purpose of this review is to summarise the available information regarding the influence of the gut microbiota on neurogenesis, analyse the possible underlying mechanisms, and discuss the potential implications of this emerging knowledge for the fight against neurodegeneration and brain ageing.

Constitutive Activity of Serotonin Receptor 6 Regulates Human Cerebral Organoids Formation and Depression-like Behaviors

Serotonin receptor 6 (5-HT₆R), a typical G protein-coupled receptor (GPCR) mainly expressed in the neurogenic area with constitutive activity, is of particular interest as a promising target for emotional impairment. Here, we found that 5-HT₆R was highly expressed in human NSCs and activation of the receptor promoted self-renewal of human NSCs, and thus induced the expansion and folding of human cerebral organoids; dysfunction of receptor or inhibition of its constitutive activity resulted in the premature differentiation of NSCs, which ultimately depleted the NSC pool. The following mechanistic study revealed that EPAC-CREB signaling was involved in 5-HT₆R regulation. Furthermore, we showed that mice with genetic deletion of 5-HT₆R or knockin A268R mutant presented depression-like behaviors and impaired hippocampal neurogenesis for progressive decrease of the NSC pool. Thus, this study indicates that the modulation of 5-HT₆R and its constitutive activity may provide a therapeutic alternative to alleviate depression.

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5-HT₆R regulates human neural stem cell proliferation

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The constitutive activity of 5-HT₆R is essential for human neural stem cell's multipotency

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5-HT₆R modulates neurogenesis of human cerebral organoids

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Mice with reduced constitutive activity of 5-HT₆R show depression-like behaviors

Trehalose as glucose surrogate in proliferation and cellular mobility of adult neural progenitor cells derived from mouse hippocampus

The disaccharide trehalose (TRE) represents a natural energy supply for distinct non-mammalian species. Evidence has shown that TRE impacts on various properties including the stabilization of protein structure and cell membranes, which are important neuroprotective features against neurodegeneration. In this study, we tested the specific effect of TRE on cell proliferation and mobilization using an established experimental paradigm of adult neural progenitor cells (NPCs) derived from murine hippocampus. NPC proliferation, both measured by growth curve analysis over 25 days and by bromodeoxyuridine (BrdU) incorporation, was not altered by adding TRE instead of GLC to the culture media. Using Boyden chamber experiments, the mobility in regular glucose-containing media did not differ from glucose-free TRE-supplemented media. Our observation suggests that TRE has the capacity to replace glucose (GLC) as energy source in neural cells in our experimental paradigm.

5-HTTLPR and MTHFR 677C>T polymorphisms and response to yoga-based lifestyle intervention in major depressive disorder: A randomized active-controlled trial

BACKGROUND

There is growing evidence suggesting that both genetic and environmental factors modulate treatment outcome in, a highly heterogeneous, major depressive disorder (MDD). 5-HTTLPR variant of the serotonin transporter gene (SLC6A4) and MTHFR 677C>T polymorphisms have been linked to the pathogenesis of MDD, and antidepressant treatment response. The evidence is lacking on the clinical utility of yoga in patients with MDD who have 5-HTTLPR and MTHFR 677C>T polymorphisms and less likely to respond to medications (SSRIs).

AIMS

We aimed to examine the impact of YBLI in those who have susceptible 5-HTTLPR and MTHFR 677C>T polymorphisms and are less likely to drug therapy with SSRIs.

SETTINGS AND DESIGN

In a 12 week randomized active-controlled trial, MDD patients (n = 178) were randomized to receive YBLI or drug therapy.

METHODS

Genotyping was conducted using PCR-based methods. The clinical remission was defined as BDI-II score ≤ 9.

STATISTICAL ANALYSIS USED

An intent-to-treat analysis was performed, and the association of genotype with treatment remission consisted of the logistic regression model. A P value of <0.05 was considered statistically significant.

RESULTS

Multivariate logistic regression models for remission including either 5-HTTLPR or MTHFR 677C>T genotypes showed statistically significant odds of remission in YOGA arm vs. DRUG arm. Neither 5-HTTLPR nor MTHFR 677C>T genotype showed any influence on remission to YBLI (P = 0.73 and P = 0.64, respectively). Further analysis showed childhood adversity interact with 5-HTTLPR and MTHFR 677C>T polymorphisms to decrease treatment response in DRUG treatment arm, but not in YOGA arm.

CONCLUSIONS

YBLI provides MDD remission in those who have susceptible 5-HTTLPR and MTHFR 677C>T polymorphisms and are resistant to SSRIs treatment. YBLI may be therapeutic for MDD independent of heterogeneity in its etiopathogenesis.

In vivo relationship between serotonin 1A receptor binding and gray matter volume in the healthy brain and in major depressive disorder

Serotonin 1A (5-HT_1A) receptors mediate serotonin trophic role in brain neurogenesis. Gray matter volume (GMV) loss and 5-HT_1A receptor binding alterations have been identified in major depressive disorder (MDD). Here we investigated the relationship between 5-HT_1A receptor binding and GMV in 40 healthy controls (HCs) and, for the first time, 47 antidepressant-free MDD patients using Voxel-Based Morphometry and [¹¹C]WAY100635 Positron Emission Tomography. Values of GMV and 5-HT_1A binding (expressed as BP_F, one of the types of binding potentials that refer to displaceable or specific binding that can be quantified in vivo with PET) were obtained in 13 regions of interest, including raphe, and at the voxel level. We used regression analysis within each group to predict GMV from BP_F, while covarying for age, sex, total gray matter volume and medication status. In the HCs group, we found overall a positive correlation between terminal field 5-HT_1A receptor binding and GMV, which reached statistical significance in regions such as hippocampus, insula, orbital prefrontal cortex, and parietal lobe. We observed a trend towards inverse correlation between raphe 5-HT_1A autoreceptor binding and anterior cingulate GMV in both groups, and a statistically significant positive correlation between raphe 5-HT_1A binding and temporal GMV in MDD. Analysis of covariance at the voxel-level revealed a trend towards interaction between diagnosis and raphe 5-HT_1A binding in predicting GMV in cerebellum and supramarginal gyrus (higher correlation in HCs compared with MDD). Our results replicated previous findings in the normative brain, but did not extend them to the brain in MDD, and indicated a trend towards dissociation between MDD and HCs in the relationship of raphe 5-HT_1A binding with postsynaptic GMV. These results suggest that 5-HT_1A receptors contribute to altered neuroplasticity in MDD, possibly via effects predating depression onset.

Gene-environment interactions informing therapeutic approaches to cognitive and affective disorders

Gene-environment interactions drive experience-dependent changes in the brain that alter cognition, emotion and behaviour. Positive engagement with the environment, through novel experience and physical activity, can improve brain function, although the mechanisms mediating such experience-dependent plasticity remain to be fully elucidated. In this article, we discuss the therapeutic value of environmental stimuli, exercise and environmental enrichment (EE), for cognitive and affective disorders, with implications for the understanding and treatment of depression and anxiety disorders. We demonstrate that environmental manipulations are potential therapeutic strategies for improving outcomes in these psychiatric disorders, including beneficial impacts on cognition. We discuss how EE and exercise are therapeutic environmental interventions impacting both affective and cognitive function. Serotonergic (5-HTergic) signaling is strongly implicated in the manifestation of psychiatric disorders and regulates cognitive and emotional processing that can underpin them. Thus, we focus on evidence implicating the serotonergic system in mediating gene-environment interactions to EE and exercise. Finally, we discuss robust gene-environment interactions associated with EE and exercise interventions, and their impacts on specific brain areas, particularly the hippocampus. We focus on potential mediators of this experience-dependent plasticity, including adult neurogenesis and brain-derived neurotrophic factor (BDNF). Furthermore, we explore molecular and cellular mechanisms of experience-dependent plasticity that potentially underlie the restoration of affective and cognitive phenotypes, thus identifying novel therapeutic targets. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".

Proneurogenic Effects of Trazodone in Murine and Human Neural Progenitor Cells

Several antidepressants increase adult hippocampal neurogenesis (ahNG) in rodents, primates, and, potentially, humans. This effect may at least partially account for their therapeutic activity. The availability of antidepressants whose mechanism of action involves different neurotransmitter receptors represents an opportunity for increasing our knowledge on their distinctive peculiarities and for dissecting the contribution of receptor subtypes in ahNG modulation. The aim of this study was to evaluate, in vitro, the effects of the antidepressant trazodone (TZD) on ahNG by using primary cultures of murine adult hippocampal neural progenitor cells (ahNPCs) and human induced pluripotent stem cell (iPSC)-derived NPCs. We demonstrated that TZD enhances neuronal differentiation of murine as well as human NPCs. TZD is a multimodal antidepressant, which binds with high affinity to 5-HT_2a, α₁, and 5-HT_1a and with lower affinity to 5-HT_2c, α₂ and 5-HTT. We demonstrated that TZD proneurogenic effects were mediated by 5-HT_2a antagonism both in murine and in human NPCs and by 5-HT_2c antagonism in murine cells. Moreover NF-κB p50 nuclear translocation appeared to be required for TZD-mediated proneurogenic effects. Interestingly, TZD had no proneurogenic effects in 5-HT depleted ahNPCs. The TDZ bell-shaped dose-response curve suggested additional effects. However, in our model 5-HT_1a and α₁/α₂ receptors had no role in neurogenesis. Overall, our data also demonstrated that serotoninergic neurotransmission may exert both positive and negative effects on neuronal differentiation of ahNPCs in vitro.

Disinhibition of the rat prelimbic cortex promotes serotonergic activation of the dorsal raphe nucleus and panicolytic-like behavioral effects

Several studies have shown that serotonin plays a dual role in the modulation of defensive behaviors related to anxiety and panic. A major source of serotonergic projections to limbic structures responsible for this modulation is the dorsal raphe nucleus (DR). Anatomical studies indicate that the prelimbic (PL) cortex sends dense glutamatergic projections to the DR, leading to stimulation or inhibition of serotonin release in structures innervated by the DR. The objective of the present study was to investigate if GABAergic disinhibition of the PL by means of local administration of picrotoxin (PIC), a chloride channel blocker, can affect serotonergic tone and the expression of defensive behaviors related to anxiety and panic. We used the elevated T-maze model and Vogel conflict test to evaluate defensive responses associated with anxiety or panic. The results showed that intra-PL PIC caused an increase in c-Fos activation in serotonergic cells in DR subregions. Furthermore, the intra-PL injection of PIC induced a panicolytic-like effect without affecting behaviors associated with anxiety. Our findings suggest that the PL-DR pathway, through DR serotonergic stimulation, is involved in the control of panic-related behaviors by control of serotonin release in structures that modulate panic responses, such as the dorsal periaqueductal gray.

Recent Advances in Neurogenic Small Molecules as Innovative Treatments for Neurodegenerative Diseases

The central nervous system of adult mammals has long been considered as a complex static structure unable to undergo any regenerative process to refurbish its dead nodes. This dogma was challenged by Altman in the 1960s and neuron self-renewal has been demonstrated ever since in many species, including humans. Aging, neurodegenerative, and some mental diseases are associated with an exponential decrease in brain neurogenesis. Therefore, the controlled pharmacological stimulation of the endogenous neural stem cells (NSCs) niches might counteract the neuronal loss in Alzheimer’s disease (AD) and other pathologies, opening an exciting new therapeutic avenue. In the last years, druggable molecular targets and signalling pathways involved in neurogenic processes have been identified, and as a consequence, different drug types have been developed and tested in neuronal plasticity. This review focuses on recent advances in neurogenic agents acting at serotonin and/or melatonin systems, Wnt/β-catenin pathway, sigma receptors, nicotinamide phosphoribosyltransferase (NAMPT) and nuclear erythroid 2-related factor (Nrf2).

Control of adult neurogenesis by programmed cell death in the mammalian brain

The presence of neural stem cells (NSCs) and the production of new neurons in the adult brain have received great attention from scientists and the public because of implications to brain plasticity and their potential use for treating currently incurable brain diseases. Adult neurogenesis is controlled at multiple levels, including proliferation, differentiation, migration, and programmed cell death (PCD). Among these, PCD is the last and most prominent process for regulating the final number of mature neurons integrated into neural circuits. PCD can be classified into apoptosis, necrosis, and autophagic cell death and emerging evidence suggests that all three may be important modes of cell death in neural stem/progenitor cells. However, the molecular mechanisms that regulate PCD and thereby impact the intricate balance between self-renewal, proliferation, and differentiation during adult neurogenesis are not well understood. In this comprehensive review, we focus on the extent, mechanism, and biological significance of PCD for the control of adult neurogenesis in the mammalian brain. The role of intrinsic and extrinsic factors in the regulation of PCD at the molecular and systems levels is also discussed. Adult neurogenesis is a dynamic process, and the signals for differentiation, proliferation, and death of neural progenitor/stem cells are closely interrelated. A better understanding of how adult neurogenesis is influenced by PCD will help lead to important insights relevant to brain health and diseases.

No association between schizophrenia susceptibility variants and macroscopic structural brain volume variation in healthy subjects

Previous studies have suggested that genetic variants for schizophrenia susceptibility might contribute to structural brain volume variations in schizophrenia patients, including total brain volume, hippocampal volume, and amygdalar volume. However, whether these schizophrenia susceptibility variants are associated with macroscopic structural brain volume (i.e., intracranial volume, total brain volume, and hippocampal volume) in healthy subjects is still unclear. In this study, we investigated the associations between 47 schizophrenia susceptibility variants (from 25 well-characterized schizophrenia susceptibility genes) and cranial volume variation in a healthy Chinese sample (N = 1,013). We also extracted the association between these 47 schizophrenia risk variants and the macroscopic structural brain volume (intracranial volume, total brain volume and hippocampal volume) in a large healthy sample of European ancestry (ENIGMA sample, N = 5,775). We identified several single-nucleotide polymorphisms (SNPs) nominally associated with intracranial volume, total brain volume, and hippocampal volume at P < 0.05 (uncorrected). However, after Bonferroni corrections for multiple testing, no SNP showed significant association. Hence, our results do not support previous observations that schizophrenia susceptibility variants are associated with brain structure (e.g., hippocampal volume) in healthy individuals, and indicate that single schizophrenia risk variant may not contribute significantly to macroscopic brain structure (e.g., intracranial volume or hippocampal volume) variation in healthy subjects.

5-HT2 receptors mediate functional modulation of GABAa receptors and inhibitory synaptic transmissions in human iPS-derived neurons

Neural progenitors differentiated from induced pluripotent stem cells (iPS) hold potentials for treating neurological diseases. Serotonin has potent effects on neuronal functions through multiple receptors, underlying a variety of neural disorders. Glutamate and GABA receptors have been proven functional in neurons differentiated from iPS, however, little is known about 5-HT receptor-mediated modulation in such neuronal networks. In the present study, human iPS were differentiated into cells possessing featured physiological properties of cortical neurons. Whole-cell patch-clamp recording was used to examine the involvement of 5-HT₂ receptors in functional modulation of GABAergic synaptic transmission. We found that serotonin and DOI (a selective agonist of 5-HT_2A/C receptor) reversibly reduced GABA-activated currents, and this 5-HT_2A/C receptor mediated inhibition required G protein, PLC, PKC, and Ca²⁺ signaling. Serotonin increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs), which could be mimicked by α-methylserotonin, a 5-HT₂ receptor agonist. In contrast, DOI reduced both frequency and amplitude of mIPSCs. These findings suggested that in iPS-derived human neurons serotonin postsynaptically reduced GABAa receptor function through 5-HT_2A/C receptors, but presynaptically other 5-HT₂ receptors counteracted the action of 5-HT_2A/C receptors. Functional expression of serotonin receptors in human iPS-derived neurons provides a pre-requisite for their normal behaviors after grafting.

Fluoxetine regulates neurogenesis in vitro through modulation of GSK-3β/β-catenin signaling

BACKGROUND

It is generally accepted that chronic treatment with antidepressants increases hippocampal neurogenesis, but the molecular mechanisms underlying their effects are unknown. Recently, glycogen synthase kinase-3 beta (GSK-3β)/β-catenin signaling was shown to be involved in the mechanism of how antidepressants might influence hippocampal neurogenesis.

METHODS

The aim of this study was to determine whether GSK-3β/β-catenin signaling is involved in the alteration of neurogenesis as a result of treatment with fluoxetine, a selective serotonin reuptake inhibitor. The mechanisms involved in fluoxetine's regulation of GSK-3β/β-catenin signaling pathway were also examined.

RESULTS

Our results demonstrated that fluoxetine increased the proliferation of embryonic neural precursor cells (NPCs) by up-regulating the phosphorylation of Ser9 on GSK-3β and increasing the level of nuclear β-catenin. The overexpression of a stabilized β-catenin protein (ΔN89 β-catenin) significantly increased NPC proliferation, while inhibition of β-catenin expression in NPCs led to a significant decrease in the proliferation and reduced the proliferative effects induced by fluoxetine. The effects of fluoxetine-induced up-regulation of both phosphorylation of Ser9 on GSK-3β and nuclear β-catenin were significantly prevented by the 5-hydroxytryptamine-1A (5-HT1A) receptor antagonist WAY-100635.

CONCLUSIONS

The results demonstrate that fluoxetine may increase neurogenesis via the GSK-3β/β-catenin signaling pathway that links postsynaptic 5-HT1A receptor activation.

Targeting melanocyte and melanoma stem cells by 8-hydroxy-2-dipropylaminotetralin

Monobenzyl ether of hydroquinone (MBEH) is cytotoxic towards melanocytes. Its treatment efficacy is limited by an inability to eradicate stem cells. By contrast, 8-hydroxy-N,N-dipropyl-2-aminotetralin (8-DPAT) affects melanocyte stem cell survival. MBEH and 8-DPAT were added to melanocytes and melanoma cells to compare cytotoxicity. Stem cell content among viable cells was determined by fluorocytometry using markers CD34, Pax3, and CD271. Immunostaining was used to identify stem cells in skin explants treated with MBEH or 8-DPAT ex vivo. Mice were exposed to MBEH or 8-DPAT and scanned for depigmentation before harvesting skin. MBEH exposure prompted a relative increase in stem cells among cultured melanocytes and melanoma cells, as treatment preferentially eliminated differentiated cells and spared the stem cells. Viability of this remaining, enriched stem cell population was however rapidly reduced by exposure to 8-DPAT within melanocyte and melanoma cell cultures. In human skin explants, the abundance of melanocyte stem cells was also visibly reduced after 8-DPAT treatment, in contrast to tissue exposed to MBEH. Meanwhile, significant depigmentation of the mouse pelage and loss of differentiated melanocytes was observed in vivo in response to topical application of MBEH, but not 8-DPAT. Prolonged application of the latter agent instead appeared to effectively reduce the abundance of melanocyte stem cells in the dermis. This furthers the idea that MBEH and 8-DPAT target complementary cell populations. Results indicate that combination treatment may demonstrate superior therapeutic activity by eliminating both differentiated and tumor initiating populations.

Protein kinase Mζ is involved in the modulatory effect of fluoxetine on hippocampal neurogenesis in vitro

The efficacy of chronic selective serotonin reuptake inhibitors (SSRIs) on depression is paralleled by the recovery of deficits in hippocampal neurogenesis related to sustained stress and elevated glucocorticoids. Previous studies have shown that atypical protein kinase C (aPKC) is implicated in the regulation of neurogenesis and the antidepressant response. Whether the specific aPKC isoforms (PKCζ, PKMζ and PKCι) are involved in SSRI-induced hippocampal neurogenesis and the underlying mechanisms is unknown. The present study shows that PKMζ and PKCι but not PKCζ are expressed in rat embryonic hippocampal neural stem cells (NSCs), whereas PKMζ but not PKCι expression is increased by the SSRI fluoxetine both in the absence and presence of the glucocorticoid receptor agonist dexamethasone. PKMζ shRNA significantly decreased neuronal proliferation and neuron-oriented differentiation, increased NSC apoptosis, and blocked the stimulatory effect of fluoxetine on NSC neurogenesis. Fluoxetine significantly increased PKMζ expression in hippocampal NSCs in a 5-hydroxytryptamine-1A (5-HT1A) receptor-dependent manner in both the absence and presence of dexamethasone. The PKMζ peptide blocker ZIP and MEK inhibitor U0126 significantly inhibited the increase in extracellular signal-regulated kinase 1/2 and cyclic adenosine monophosphate response element binding protein phosphorylation in the mitogen-activated protein kinase (MAPK) pathway and hippocampal NSC neurogenesis in response to fluoxetine and the 5-HT1A receptor agonist 8-OH DPAT. Collectively, our results suggest that the SSRI fluoxetine increases hippocampal NSC neurogenesis via a PKMζ-mediated mechanism that links 5-HT1A receptor activation with the phosphorylation of the downstream MAPK signaling pathway.

The role of serotonin in adult hippocampal neurogenesis

Serotonin is probably best known for its role in conveying a sense of contentedness and happiness. It is one of the most unique and pharmacologically complex monoamines in both the peripheral and central nervous system (CNS). Serotonin has become in focus of interest for the treatment of depression with multiple serotonin-mimetic and modulators of adult neurogenesis used clinically. Here we will take a broad view of serotonin from development to its physiological role as a neurotransmitter and its contribution to homeostasis of the adult rodent hippocampus. This chapter reflects the most significant findings on cellular and molecular mechanisms from neuroscientists in the field over the last two decades. We illustrate the action of serotonin by highlighting basic receptor targeting studies, and how receptors impact brain function. We give an overview of recent genetically modified mouse models that differ in serotonin availability and focus on the role of the monoamine in antidepressant response. We conclude with a synthesis of the most recent data surrounding the role of serotonin in activity and hippocampal neurogenesis. This synopsis sheds light on the mechanisms and potential therapeutic model by which serotonin plays a critical role in the maintenance of mood.

Fluoxetine induces proliferation and inhibits differentiation of hypothalamic neuroprogenitor cells in vitro

A significant number of children undergo maternal exposure to antidepressants and they often present low birth weight. Therefore, it is important to understand how selective serotonin reuptake inhibitors (SSRIs) affect the development of the hypothalamus, the key center for metabolism regulation. In this study we investigated the proliferative actions of fluoxetine in fetal hypothalamic neuroprogenitor cells and demonstrate that fluoxetine induces the proliferation of these cells, as shown by increased neurospheres size and number of proliferative cells (Ki-67⁺ cells). Moreover, fluoxetine inhibits the differentiation of hypothalamic neuroprogenitor cells, as demonstrated by decreased number of mature neurons (Neu-N⁺ cells) and increased number of undifferentiated cells (SOX-2⁺ cells). Additionally, fluoxetine-induced proliferation and maintenance of hypothalamic neuroprogenitor cells leads to changes in the mRNA levels of appetite regulator neuropeptides, including Neuropeptide Y (NPY) and Cocaine-and-Amphetamine-Regulated-Transcript (CART). This study provides the first evidence that SSRIs affect the development of hypothalamic neuroprogenitor cells in vitro with consequent alterations on appetite neuropeptides.

Beneficial in vivo effect of aripiprazole on neuronal regeneration following neuronal loss in the dentate gyrus: evaluation using a mouse model of trimethyltin-induced neuronal loss/self-repair in the dentate gyrus

Aripiprazole is used clinically as an atypical antipsychotic. We evaluated the effect of in vivo treatment with aripiprazole on the proliferation and differentiation of neural stem/progenitor cells in a mouse model, trimethyltin-induced neuronal loss/self-repair in the hippocampal dentate gyrus (referred as "impaired animals") [Ogita et al., J Neurosci Res. 82, 609 - 621 (2005)]. In the impaired animals, an increased number of 5-bromo-2'-deoxyuridine (BrdU)-positive cells was seen in the dentate gyrus at the initial time window of the self-repair stage. At the same time window, a single treatment with aripiprazole significantly increased the number of cells positive for both BrdU and nestin in the dentate gyrus of the impaired animals. Chronic treatment with aripiprazole promoted the proliferation/survival and neuronal differentiation of the cells newly-generated following the neuronal loss in the dentate gyrus of the impaired animals. The chronic treatment with aripiprazole improved depression-like behavior seen in the impaired animals. Taken together, our data suggest that aripiprazole had a beneficial effect on neuronal regeneration following neuronal loss in the dentate gyrus through indirectly promoted proliferation/survival and neuronal differentiation of neural stem/progenitor cells in the subgranular zone of the dentate gyrus.

5-HTT deficiency affects neuroplasticity and increases stress sensitivity resulting in altered spatial learning performance in the Morris water maze but not in the Barnes maze

The purpose of this study was to evaluate whether spatial hippocampus-dependent learning is affected by the serotonergic system and stress. Therefore, 5-HTT knockout (-/-), heterozygous (+/-) and wildtype (+/+) mice were subjected to the Barnes maze (BM) and the Morris water maze (WM), the latter being discussed as more aversive. Additionally, immediate early gene (IEG) expression, hippocampal adult neurogenesis (aN), and blood plasma corticosterone were analyzed.

While the performance of 5-HTT-/- mice in the BM was undistinguishable from both other genotypes, they performed worse in the WM. However, in the course of the repeated WM trials 5-HTT-/- mice advanced to wildtype level. The experience of a single trial of either the WM or the BM resulted in increased plasma corticosterone levels in all genotypes. After several trials 5-HTT-/- mice exhibited higher corticosterone concentrations compared with both other genotypes in both tests. Corticosterone levels were highest in 5-HTT-/- mice tested in the WM indicating greater aversiveness of the WM and a greater stress sensitivity of 5-HTT deficient mice.

Quantitative immunohistochemistry in the hippocampus revealed increased cell counts positive for the IEG products cFos and Arc as well as for proliferation marker Ki67 and immature neuron marker NeuroD in 5-HTT-/- mice compared to 5-HTT+/+ mice, irrespective of the test. Most differences were found in the suprapyramidal blade of the dentate gyrus of the septal hippocampus. Ki67-immunohistochemistry revealed a genotype x environment interaction with 5-HTT genotype differences in naïve controls and WM experience exclusively yielding more Ki67-positive cells in 5-HTT+/+ mice. Moreover, in 5-HTT-/- mice we demonstrate that learning performance correlates with the extent of aN.

Overall, higher baseline IEG expression and increased an in the hippocampus of 5-HTT-/- mice together with increased stress sensitivity may constitute the neurobiological correlate of raised alertness, possibly impeding optimal learning performance in the more stressful WM.

Cholinergic impact on neuroplasticity drives muscarinic M1 receptor mediated differentiation into neurons

OBJECTIVES

Increasing evidence indicates that canonical neurotransmitters act as regulatory signals during neuroplasticity. Here, we report that muscarinic cholinergic neurotransmission stimulates differentiation of adult neural stem cells in vitro.

METHODS

Adult neural stem cells (ANSC) dissociated from the adult mouse hippocampus were expanded in culture with basic fibroblast growth factor (BFGF) and epidermal growth factor (EGF).

RESULTS

Carbachol (CCh), an analog of acetylcholine (ACh) significantly enhanced de novo differentiation into neurons on bFGF- and EGF-deprived stem cells as shown by the percentage of TUJ1 positive cells. By contrast, pirenzepine (PIR), a muscarinic M1 receptor antagonist, reduced the generation of neurons.

CONCLUSION

Activation of cholinergic signaling drives the de novo differentiation of uncommitted stem cells into neurons. These effects appear to be predominantly mediated via the muscarinic M1 receptor subtype.

Neural stem cell survival factors

Neural stem and progenitor cells (NSCs and NPs) give rise to the central nervous system (CNS) during embryonic development. NSCs and NPs differentiate into three main cell-types of the CNS; astrocytes, oligodendrocytes, and neurons. NSCs are present in the adult CNS and are important in maintenance and repair. Adult NSCs hold great promise for endogenous or self-repair of the CNS. Intriguingly, NSCs have been implicated as the cells that give rise to brain tumors. Thus, the balance between survival, growth and differentiation is a critical aspect of NSC biology, during development, in the adult, and in disease processes. In this review, we survey what is known about survival factors that control both embryonic and adult NSCs. We discuss the neurosphere culture system as this is widely used to measure NSC activity and behavior in vitro and emphasize the importance of clonality. We define here NSC survival factors in their broadest sense to include any factor that influences survival and proliferation of NSCs and NPs. NSC survival factors identified to date include growth factors, morphogens, proteoglycans, cytokines, hormones, and neurotransmitters. Understanding NSC and NP interaction in response to these survival factors will provide insight to CNS development, disease and repair.

Serotonin-1A receptor binding is positively associated with gray matter volume -- a multimodal neuroimaging study combining PET and structural MRI

Animal models revealed that the serotonin-1A (5-HT(1A)) receptor modulates gray matter structure. However, there is a lack of evidence showing the relationship between 5-HT(1A) receptor concentration and gray matter in the human brain in vivo. Here, to demonstrate an association between the 5-HT(1A) receptor binding potential, an index for receptor concentration, and the local gray matter volume (GMV), an index for gray matter structure, we measured 35 healthy subjects with both positron emission tomography (PET) and structural magnetic resonance imaging (MRI). We found that regional heteroreceptor binding was positively associated with GMV in distinctive brain regions such as the hippocampi and the temporal cortices in both hemispheres (R(2) values ranged from 0.308 to 0.503, p<0.05 cluster-level FDR-corrected). Furthermore, autoreceptor binding in the midbrain raphe region was positively associated with GMV in forebrain projection sites (R(2)=0.656, p=0.001). We also observed a broad range between 5-HT(1A) receptor binding and GMV. Given the congruence of altered 5-HT(1A) receptor concentrations and GMV reduction in depression or Alzheimer's disease as reported by numerous studies, these results might provide new insights towards understanding the mechanisms behind GMV alterations observed in these brain disorders.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Serotonin of mast cell origin contributes to hippocampal function

In the central nervous system, serotonin, an important neurotransmitter and trophic factor, is synthesized by both mast cells and neurons. Mast cells, like other immune cells, are born in the bone marrow and migrate to many tissues. We show that they are resident in the mouse brain throughout development and adulthood. Measurements based on capillary electrophoresis with native fluorescence detection indicate that a significant contribution of serotonin to the hippocampal milieu is associated with mast cell activation. Compared with their littermates, mast cell-deficient C57BL/6 Kit(W-sh/W-sh) mice have profound deficits in hippocampus-dependent spatial learning and memory and in hippocampal neurogenesis. These deficits are associated with a reduction in cell proliferation and in immature neurons in the dentate gyrus, but not in the subventricular zone - a neurogenic niche lacking mast cells. Chronic treatment with fluoxetine, a selective serotonin reuptake inhibitor, reverses the deficit in hippocampal neurogenesis in mast cell-deficient mice. In summary, the present study demonstrates that mast cells are a source of serotonin, that mast cell-deficient C57BL/6 Kit(W-sh/W-sh) mice have disrupted hippocampus-dependent behavior and neurogenesis, and that elevating serotonin in these mice, by treatment with fluoxetine, reverses these deficits. We conclude that mast cells contribute to behavioral and physiological functions of the hippocampus and note that they play a physiological role in neuroimmune interactions, even in the absence of inflammatory responses.

The complex role of the serotonin transporter in adult neurogenesis and neuroplasticity. A critical review

OBJECTIVES

Studies on the serotonin transporter (SERT) with regard to neurogenesis and neuroplastic effects on the adult brain are scarce. This is intriguing since neurogenesis is believed to play a decisive role in modulating the effect of selective serotonin reuptake inhibitors (SSRI), which are targeting SERT.

METHODS

Therefore, we reviewed the current scientific literature about the influence of serotonin on neurogenesis with particular emphasis on SERT in various settings, both in vivo and in vitro.

RESULTS

Experiments using SERT KO (knock-out) animal models showed that SERT does not directly or indirectly influence neurogenesis in vitro, whereas compensatory mechanism seem to participate in vivo.

CONCLUSION

At least with regard to adult neural stem cells, the impact of serotonin (5-HT) on neuroplasticity and neurogenesis is not due to SERT-mediated effcts. Instead, serotonergic fine-tuning may be exerted by a number of other different mechanisms including endogenous production of 5-HT in adult neural stem cells, uptake of 5-HT into adult neural stem cells by other monoamine transporters, and actions of the 5-HT1A receptors present on these cells.

Noradrenaline increases neural precursor cells derived from adult rat dentate gyrus through β2 receptor

Several preclinical researches indicate that increased neurogenesis in the adult hippocampus might underlie the therapeutic effect of antidepressant treatment. Most antidepressant drugs have ability to increase serotonin (5-HT) and/or noradrenaline (NA) in brain, and chronic treatment with antidepressant drugs increases the number of proliferating neural precursor cells and neurogenesis in hippocampus. However, the direct effects of antidepressant drugs, 5-HT and NA on the neural precursor cells remain largely unknown. Neural precursor cells in adult hippocampus are divided into stem/progenitor cells of four types based on stages of neural development. We recently established a culture system of adult rat dentate gyrus-derived neural precursor cells (ADPs), which correspond to be type 2a early progenitor cells. Here the direct effects of antidepressant drugs of four types (fluoxetine, imipramine, reboxetine, and tranylcypromine) and two neurotransmitters (5-HT and NA) on the proliferation of ADPs were investigated. Neither antidepressant drugs of all types nor 5-HT increased the number of ADPs. On the other hand, NA increased the number and the DNA synthesis of ADPs. The effect of NA on ADP proliferation was antagonized by propranolol and timolol (β-adrenergic receptor (AR) antagonists), but not by phentolamine (α-AR antagonist), prazosin (α1-AR antagonist), or yohimbine (α2-AR antagonist). Moreover, it was antagonized by ICI 118, 551 (β2-AR selective antagonist) and salmeterol (β2-AR selective agonist) promoted ADP proliferation. These results suggest that NA might increase the proliferation of early progenitor cells in adult hippocampus via β2-AR directly, but antidepressant drugs and 5-HT do not.