How widespread is adult neurogenesis in mammals?

The partial 5-HT1A receptor agonist buspirone enhances neurogenesis in the opossum (Monodelphis domestica)

We demonstrate for the first time that neurogenesis in the adult Monodelphis opossum has a typical mammalian pattern and occurs only in the dentate gyrus (DG) and subventricular zone (SVZ) of the lateral ventricles. In these two brain regions neurogenesis is present throughout the lifespan, although its rate is reduced by half in the old age. Treatment with buspirone, a partial 5-HT1A receptor agonist which is used in human clinic as an anxiolytic agent, boosts proliferation in the SVZ and DG in both adult and aged opossums. The neuronal phenotype dominates among newly generated cells in both non-treated and buspirone-treated opossums. We suggest that if functional importance of adult neurogenesis is in improving olfactory discrimination and generation of hippocampus-dependent memory, both spatial and emotional, then administration of drugs increasing the rate of neurogenesis via activation of 5-HT1A receptors may be a valuable aid in combating problems of the advanced age.

Chronic but not acute foot-shock stress leads to temporary suppression of cell proliferation in rat hippocampus

Stressful experiences, especially when prolonged and severe are associated with psychopathology and impaired neuronal plasticity. Among other effects on the brain, stress has been shown to negatively regulate hippocampal neurogenesis, and this effect is considered to be exerted via glucocorticoids. Here, we sought to determine the temporal dynamics of changes in hippocampal neurogenesis after acute and chronic exposure to foot-shock stress. Rats subjected to a foot-shock procedure showed strong activation of the hypothalamic-pituitary-adrenal (HPA) axis, even after exposure to daily stress for 3 weeks. Despite a robust release of corticosterone, acute foot-shock stress did not affect the rate of hippocampal cell proliferation. In contrast, exposure to foot-shock stress daily for 3 weeks led to reduced cell proliferation 2 hours after the stress procedure. Interestingly, this stress-induced effect did not persist and was no longer detected 24 hours later. Also, while chronic foot-shock stress had no impact on survival of hippocampal cells that were born before the stress procedure, it led to a decreased number of doublecortin-positive granule neurons that were born during the chronic stress period. Thus, whereas a strong activation of the HPA axis during acute foot-shock stress is not sufficient to reduce hippocampal cell proliferation, repeated exposure to stressful stimuli for prolonged period of time ultimately results in dysregulated neurogenesis. In sum, this study supports the notion that chronic stress may lead to cumulative changes in the brain that are not seen after acute stress. Such changes may indicate compromised brain plasticity and increased vulnerability to neuropathology.

Identification of flap structure-specific endonuclease 1 as a factor involved in long-term memory formation of aversive learning

We previously proposed that DNA recombination/repair processes play a role in memory formation. Here, we examined the possible role of the fen-1 gene, encoding a flap structure-specific endonuclease, in memory consolidation of conditioned taste aversion (CTA). Quantitative real-time PCR showed that amygdalar fen-1 mRNA induction was associated to the central processing of the illness experience related to CTA and to CTA itself, but not to the central processing resulting from the presentation of a novel flavor. CTA also increased expression of the Fen-1 protein in the amygdala, but not the insular cortex. In addition, double immunofluorescence analyses showed that amygdalar Fen-1 expression is mostly localized within neurons. Importantly, functional studies demonstrated that amygdalar antisense knockdown of fen-1 expression impaired consolidation, but not short-term memory, of CTA. Overall, these studies define the fen-1 endonuclease as a new DNA recombination/repair factor involved in the formation of long-term memories.

Mutant mouse models and antidepressant drug research: focus on serotonin and brain-derived neurotrophic factor

Several lines of knockout (KO) mice have been evaluated as models of depression-related behavioral and neurobiological changes, and used to investigate molecular and cellular mechanisms underlying the activity of antidepressant drugs. Adult neurogenesis and brain 5-hydroxytryptamine (5-HT)/neurotrophic factor interactions have recently attracted great interest in relation to the mechanism of action of antidepressant drugs. The present review focuses primarily on genetic manipulation of the serotoninergic (5-HT) system. Basal neurochemical and behavioral changes occurring in mice lacking the 5-HT transporter (SERT), which is the main target of antidepressant drugs, as well as in those lacking G protein-coupled serotonin receptors (e.g. 5-HT1B, 5-HT1A, and 5-HT4 receptors) are described and evaluated. The importance of KO mice for neurotrophic factors, particularly for brain-derived neurotrophic factor and its high-affinity receptor (R-TrkB), is also addressed. Constitutive KO, tissue specific, or inducible KO mice targeting both 5-HT and brain-derived neurotrophic factor systems may potentially make an important contribution to knowledge of the pathophysiology and treatment of depression.

Novel stem/progenitor cells with neuronal differentiation potential reside in the leptomeningeal niche

Stem cells capable of generating neural differentiated cells are recognized by the expression of nestin and reside in specific regions of the brain, namely, hippocampus, subventricular zone and olfactory bulb. For other brain structures, such as leptomeninges, which contribute to the correct cortex development and functions, there is no evidence so far that they may contain stem/precursor cells. In this work, we show for the first time that nestin-positive cells are present in rat leptomeninges during development up to adulthood. The newly identified nestin-positive cells can be extracted and expanded in vitro both as neurospheres, displaying high similarity with subventricular zone-derived neural stem cells, and as homogeneous cell population with stem cell features. In vitro expanded stem cell population can differentiate with high efficiency into excitable cells with neuronal phenotype and morphology. Once injected into the adult brain, these cells survive and differentiate into neurons, thus showing that their neuronal differentiation potential is operational also in vivo. In conclusion, our data provide evidence that a specific population of immature cells endowed of neuronal differentiation potential is resident in the leptomeninges throughout the life. As leptomeninges cover the entire central nervous system, these findings could have relevant implications for studies on cortical development and for regenerative medicine applied to neurological disorders.

Malignant astrocytomas originate from neural stem/progenitor cells in a somatic tumor suppressor mouse model

Malignant astrocytomas are infiltrative and incurable brain tumors. Despite profound therapeutic implications, the identity of the cell (or cells) of origin has not been rigorously determined. We previously reported mouse models based on conditional inactivation of the human astrocytoma-relevant tumor suppressors p53, Nf1, and Pten, wherein through somatic loss of heterozygosity, mutant mice develop tumors with 100% penetrance. In the present study, we show that tumor suppressor inactivation in neural stem/progenitor cells is both necessary and sufficient to induce astrocytoma formation. We demonstrate in vivo that transformed cells and their progeny undergo infiltration and multilineage differentiation during tumorigenesis. Tumor suppressor heterozygous neural stem/progenitor cultures from presymptomatic mice show aberrant growth advantage and altered differentiation, thus identifying a pretumorigenic cell population.

Puzzling challenges in contemporary neuroscience: insights from complexity and emergence in epileptogenic circuits

The brain is a complex system that, in the normal condition, has emergent properties like those associated with activity-dependent plasticity in learning and memory, and in pathological situations, manifests abnormal long-term phenomena like the epilepsies. Data from our laboratory and from the literature were classified qualitatively as sources of complexity and emergent properties from behavior to electrophysiological, cellular, molecular, and computational levels. We used such models as brainstem-dependent acute audiogenic seizures and forebrain-dependent kindled audiogenic seizures. Additionally we used chemical or electrical experimental models of temporal lobe epilepsy that induce status epilepticus with behavioral, anatomical, and molecular sequelae such as spontaneous recurrent seizures and long-term plastic changes. Current computational neuroscience tools will help the interpretation, storage, and sharing of the exponential growth of information derived from those studies. These strategies are considered solutions to deal with the complexity of brain pathologies such as the epilepsies.

Identifying neural progenitor cells in the adult brain

There is incontrovertible evidence that neural progenitor cells (NPC) are found in the adult brain. The ability to identify and track NPC in the adult brain is of considerable importance if the properties of these cells are to be harnessed as potential therapies for degenerative brain disorders. The most commonly used approach of identifying these NPC in experimental studies, bromodeoxyuridine (BrdU) labelling, is outlined in this chapter. Immunohistochemical protocols for detecting endogenous and exogenous (introduced via transplantation) NPC in fresh-frozen and paraffin wax embedded brain tissue are described. Advice on how to label these NPC is also offered and multi-label fluorescence immunochemical staining approaches to determine the differentiation fate of NPC are described.

Doublecortin expression in adult cat and primate cerebral cortex relates to immature neurons that develop into GABAergic subgroups

DCX-immunoreactive (DCX+) cells occur in the piriform cortex in adult mice and rats, but also in the neocortex in adult guinea pigs and rabbits. Here we describe these cells in adult domestic cats and primates. In cats and rhesus monkeys, DCX+ cells existed across the allo- and neocortex, with an overall ventrodorsal high to low gradient at a given frontal plane. Labeled cells formed a cellular band in layers II and upper III, exhibiting dramatic differences in somal size (5-20 microm), shape (unipolar, bipolar, multipolar and irregular), neuritic complexity and labeling intensity. Cell clusters were also seen in this band, and those in the entorhinal cortex extended into deeper layers as chain-like structures. Densitometry revealed a parallel decline of the cells across regions with age in cats. Besides the cellular band, medium-sized cells with weak DCX reactivity resided sparsely in other layers. Throughout the cortex, virtually all DCX+ cells co-expressed polysialylated neural cell adhesion molecule. Medium to large mature-looking DCX+ cells frequently colocalized with neuron-specific nuclear protein and gamma-aminobutyric acid (GABA), and those with a reduced DCX expression also partially co-labeled for glutamic acid decarboxylase, parvalbumin, calbindin, beta-nicotinamide adenine dinucleotide phosphate diaphorase and neuronal nitric oxide synthase. Similar to cats and monkeys, small and larger DCX+ cells were detected in surgically removed human frontal and temporal cortices. These data suggest that immature neurons persist into adulthood in many cortical areas in cats and primates, and that these cells appear to undergo development and differentiation to become functional subgroups of GABAergic interneurons.

Sedative and anticonvulsant drugs suppress postnatal neurogenesis

OBJECTIVE

Sedative and anticonvulsant drugs, which inhibit N-methyl-D-aspartate receptor-mediated excitation or enhance GABA-mediated action, may cause apoptotic neurodegeneration in the developing mammalian brain. Here we explored whether such agents influence early postnatal neurogenesis.

METHODS

The N-methyl-D-aspartate antagonist MK801 and the GABA subtype A agonists phenobarbital and diazepam were administered to infant rats, and cell proliferation and neurogenesis were studied in the brain using 5-bromo-2'-deoxyuridine and doublecortin immunohistochemistry and stereology. Using confocal microscopy, we quantified neurogenesis in the dentate gyrus on postnatal day 15 (P15) after treatment with MK801 or phenobarbital on P6 to P10. Learning and memory were assessed at the age of 6 months after early postnatal treatment with phenobarbital.

RESULTS

MK801, phenobarbital, and diazepam reduced numbers of newly born cells in the brain. We found no evidence that these agents caused apoptosis of 5-bromo-2'-deoxyuridine-positive cells. In the dentate gyrus, many of the newly formed cells differentiated toward a neuronal phenotype. Phenobarbital and MK801 reduced numbers of newly formed neurons in the dentate gyrus. At the age of 6 months, phenobarbital-treated rats had fewer neurons in the dentate gyrus and performed worse than saline-treated littermates in water maze learning and memory task.

INTERPRETATION

These findings show that blockade of N-methyl-D-aspartate receptor-mediated excitation and enhancement of GABA subtype A receptor activation impair cell proliferation and inhibit neurogenesis in the immature rat brain. Because many sedative and antiepileptic drugs used in pediatric medicine act via these mechanisms, our findings raise concerns about their potential impact on human brain development.

Flow cytometric analysis of BrdU incorporation as a high-throughput method for measuring adult neurogenesis in the mouse

INTRODUCTION

The generation of new neurons occurs throughout adulthood in discrete brain regions, and may be regulated by neuropsychiatric diseases and therapeutic drug treatments. Most current methods that study this process measure the labeling of newborn cells by 5-bromo-2-deoxyuridine (BrdU) using immunohistochemical methods followed by the microscopic counting of BrdU positive cells. This method is time consuming and labor intensive, typically taking several weeks to analyze.

METHODS

Therefore, we characterized a method to measure BrdU incorporation in the adult mouse hippocampus in vivo by using flow cytometry, which normally allows analysis of data within a single day.

RESULTS

The present study compared multiple BrdU dosing and loading protocols to determine a dosing strategy that produced the best signal to noise ratio. BrdU incorporation was also compared across different brain regions. The method was sensitive to a number of experimental disease manipulations. Induction of type-1 diabetes and depletion of norepinephrine reduced hippocampal cell proliferation. In contrast, chronic administration of electroconvulsive shock, a somatic treatment for depression, as well as chronic treatment with the antidepressant fluoxetine elevated hippocampal cell proliferation. This increase in cell proliferation with fluoxetine was detected as early as 14 days into treatment. Moreover, comparing measures of cell proliferation obtained by immunohistochemical and flow cytometric methods within the same animals were convergent and significantly correlated to each other. Flow cytometry was also sufficiently sensitive to quantify the survival of newly born cells.

DISCUSSION

These experiments validate the utility of flow cytometry in analyzing hippocampal cell proliferation and survival in a reliable and high-throughput fashion. The speedy analysis afforded by flow cytometry lends itself to be utilized in novel drug discovery and physiology.

Ubiquitin, the proteasome and protein degradation in neuronal function and dysfunction

Eukaryotic protein degradation by the proteasome and the lysosome is a dynamic and complex process in which ubiquitin has a key regulatory role. The distinctive morphology of the postmitotic neuron creates unique challenges for protein degradation systems with respect to cell-surface protein turnover and substrate delivery to proteolytic machineries that are required for both synaptic plasticity and self-renewal. Moreover, the discovery of ubiquitin-positive protein aggregates in a wide spectrum of neurodegenerative diseases underlines the importance and vulnerability of the degradative system in neurons. In this article, we discuss the molecular mechanism of protein degradation in the neuron with respect to both its function and its dysfunction.

Cell proliferation and survival in the mating circuit of adult male hamsters: effects of testosterone and sexual behavior

The transient actions of gonadal steroids on the adult brain facilitate social behaviors, including reproduction. In male rodents, testosterone acts in the posterior medial amygdala (MeP) and medial preoptic area (MPOA) to promote mating. Adult neurogenesis occurs in both regions. The current study determined if testosterone and/or sexual behavior promote cell proliferation and survival in MeP and MPOA. Two experiments were conducted using the thymidine analog BrdU. First, gonad-intact and castrated male hamsters (n=6/group) were compared 24 h or 7 weeks after BrdU. In MeP, testosterone-stimulated cell proliferation 24 h after BrdU (intact: 22.8+/-3.9 cells/mm(2), castrate: 13.2+/-1.4 cells/mm(2)). Testosterone did not promote cell proliferation in MPOA. Seven weeks after BrdU, cell survival was sparse in both regions (MeP: 2.5+/-0.6 and MPOA: 1.7+/-0.2 cells/mm(2)), and was not enhanced by testosterone. In Experiment 2, gonad-intact sexually-experienced animals were mated weekly to determine if regular neural activation enhances cell survival 7 weeks after BrdU in MeP and MPOA. Weekly mating failed to increase cell survival in MeP (8.1+/-1.6 vs. 9.9+/-3.2 cells/mm(2)) or MPOA (3.9+/-0.7 vs. 3.4+/-0.3 cells/mm(2)). Furthermore, mating at the time of BrdU injection did not stimulate cell proliferation in MeP (8.9+/-1.7 vs. 8.1+/-1.6 cells/mm(2)) or MPOA (3.6+/-0.5 vs. 3.9+/-0.7 cells/mm(2)). Taken together, our results demonstrate a limited capacity for neurogenesis in the mating circuitry. Specifically, cell proliferation in MeP and MPOA are differentially influenced by testosterone, and the birth and survival of new cells in either region are not enhanced by reproductive activity.

New neurons in the adult brain: the role of sleep and consequences of sleep loss

Research over the last few decades has firmly established that new neurons are generated in selected areas of the adult mammalian brain, particularly the dentate gyrus of the hippocampal formation and the subventricular zone of the lateral ventricles. The function of adult-born neurons is still a matter of debate. In the case of the hippocampus, integration of new cells in to the existing neuronal circuitry may be involved in memory processes and the regulation of emotionality. In recent years, various studies have examined how the production of new cells and their development into neurons is affected by sleep and sleep loss. While disruption of sleep for a period shorter than one day appears to have little effect on the basal rate of cell proliferation, prolonged restriction or disruption of sleep may have cumulative effects leading to a major decrease in hippocampal cell proliferation, cell survival and neurogenesis. Importantly, while short sleep deprivation may not affect the basal rate of cell proliferation, one study in rats shows that even mild sleep restriction may interfere with the increase in neurogenesis that normally occurs with hippocampus-dependent learning. Since sleep deprivation also disturbs memory formation, these data suggest that promoting survival, maturation and integration of new cells may be an unexplored mechanism by which sleep supports learning and memory processes. Most methods of sleep deprivation that have been employed affect both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Available data favor the hypothesis that decreases in cell proliferation are related to a reduction in REM sleep, whereas decreases in the number of cells that subsequently develop into adult neurons may be related to reductions in both NREM and REM sleep. The mechanisms by which sleep loss affects different aspects of adult neurogenesis are unknown. It has been proposed that adverse effects of sleep disruption may be mediated by stress and glucocorticoids. However, a number of studies clearly show that prolonged sleep loss can inhibit hippocampal neurogenesis independent of adrenal stress hormones. In conclusion, while modest sleep restriction may interfere with the enhancement of neurogenesis associated with learning processes, prolonged sleep disruption may even affect the basal rates of cell proliferation and neurogenesis. These effects of sleep loss may endanger hippocampal integrity, thereby leading to cognitive dysfunction and contributing to the development of mood disorders.

Motor neuron regeneration in adult zebrafish

The mammalian spinal cord does not regenerate motor neurons that are lost as a result of injury or disease. Here we demonstrate that adult zebrafish, which show functional spinal cord regeneration, are capable of motor neuron regeneration. After a spinal lesion, the ventricular zone shows a widespread increase in proliferation, including slowly proliferating olig2-positive (olig2+) ependymo-radial glial progenitor cells. Lineage tracing in olig2:green fluorescent protein transgenic fish indicates that these cells switch from a gliogenic phenotype to motor neuron production. Numbers of undifferentiated small HB9+ and islet-1+ motor neurons, which are double labeled with the proliferation marker 5-bromo-2-deoxyuridine (BrdU), are transiently strongly increased in the lesioned spinal cord. Large differentiated motor neurons, which are lost after a lesion, reappear at 6-8 weeks after lesion, and we detected ChAT+/BrdU+ motor neurons that were covered by contacts immunopositive for the synaptic marker SV2. These observations suggest that, after a lesion, plasticity of olig2+ progenitor cells may allow them to generate motor neurons, some of which exhibit markers for terminal differentiation and integration into the existing adult spinal circuitry.

Roles of continuous neurogenesis in the structural and functional integrity of the adult forebrain

Neurogenesis occurs continuously in the forebrain of adult mammals, but the functional importance of adult neurogenesis is still unclear. Here, using a genetic labeling method in adult mice, we found that continuous neurogenesis results in the replacement of the majority of granule neurons in the olfactory bulb and a substantial addition of granule neurons to the hippocampal dentate gyrus. Genetic ablation of newly formed neurons in adult mice led to a gradual decrease in the number of granule cells in the olfactory bulb, inhibition of increases in the granule cell number in the dentate gyrus and impairment of behaviors in contextual and spatial memory, which are known to depend on hippocampus. These results suggest that continuous neurogenesis is required for the maintenance and reorganization of the whole interneuron system in the olfactory bulb, the modulation and refinement of the existing neuronal circuits in the dentate gyrus and the normal behaviors involved in hippocampal-dependent memory.

Behavior of neural stem cells in the Alzheimer brain

Alzheimer's disease (AD) is characterized by the deposition of beta-amyloid peptides (Abeta) and a progressive loss of neurons leading to dementia. Because hippocampal neurogenesis is linked to functions such as learning, memory and mood, there has been great interest in examining the effects of AD on hippocampal neurogenesis. This article reviews the pertinent studies and tries to unite them in one possible disease model. Early in the disease, oligomeric Abeta may transiently promote the generation of immature neurons from neural stem cells (NSCs). However, reduced concentrations of multiple neurotrophic factors and higher levels of fibroblast growth factor-2 seem to induce a developmental arrest of newly generated neurons. Furthermore, fibrillary Abeta and down-regulation of oligodendrocyte-lineage transcription factor-2 (OLIG2) may cause the death of these nonfunctional neurons. Therefore, altering the brain microenvironment for fostering apt maturation of graft-derived neurons may be critical for improving the efficacy of NSC transplantation therapy for AD.

Deletion of lysophosphatidic acid receptor LPA1 reduces neurogenesis in the mouse dentate gyrus

Neurogenesis persists in certain regions of the adult brain including the subgranular zone of the hippocampal dentate gyrus wherein its regulation is essential, particularly in relation to learning, stress and modulation of mood. Lysophosphatidic acid (LPA) is an extracellular signaling phospholipid with important neural regulatory properties mediated by specific G protein-coupled receptors, LPA(1-5). LPA(1) is highly expressed in the developing neurogenic ventricular zone wherein it is required for normal embryonic neurogenesis, and, by extension may play a role in adult neurogenesis as well. By means of the analyses of a variant of the original LPA(1)-null mutant mouse, termed the Malaga variant or "maLPA(1)-null," which has recently been reported to have defective neurogenesis within the embryonic cerebral cortex, we report here a role for LPA(1) in adult hippocampal neurogenesis. Proliferation, differentiation and survival of newly formed neurons are defective in the absence of LPA(1) under normal conditions and following exposure to enriched environment and voluntary exercise. Furthermore, analysis of trophic factors in maLPA(1)-null mice demonstrated alterations in brain-derived neurotrophic factor and insulin growth factor 1 levels after enrichment and exercise. Morphological analyses of doublecortin positive cells revealed the anomalous prevalence of bipolar cells in the subgranular zone, supporting the operation of LPA(1) signaling pathways in normal proliferation, maturation and differentiation of neuronal precursors.

Cognitive role of neurogenesis in depression and antidepressant treatment

The discovery of newborn neurons in the adult brain has generated enormous interest over the past decade. Although this process is well documented in the hippocampus and olfactory bulb, the possibility of neuron formation in other brain regions is under vigorous debate. Neurogenesis within the adult hippocampus is suppressed by factors that predispose to major depression and stimulated by antidepressant interventions. This pattern has generated the hypothesis that impaired neurogenesis is pathoetiological in depression and stimulation of newborn neurons essential for effective antidepressant action. This review critically evaluates the evidence in support of and in conflict with this theory. The literature is divided into three areas: neuronal maturation, factors that influence neurogenesis rates, and function of newborn neurons. Unique elements in each of these areas allow for the refinement of the hypothesis. Newborn hippocampal neurons appear to be necessary for detecting subtle environmental changes and coupling emotions to external context. Thus speculatively, stress-induced suppression of neurogenesis would uncouple emotions from external context leading to a negative mood state. Persistence of negative mood beyond the duration of the initial stressor can be defined as major depression. Antidepressant-induced neurogenesis therefore would restore coupling of mood with environment, leading to the resolution of depression. This conceptual framework is provisional and merits evaluation in further experimentation. Critically, manipulation of newborn hippocampal neurons may offer a portal of entry for more effective antidepressant treatment strategies.

Intact neurogenesis is required for benefits of exercise on spatial memory but not motor performance or contextual fear conditioning in C57BL/6J mice

The mammalian hippocampus continues to generate new neurons throughout life. Experiences such as exercise, anti-depressants, and stress regulate levels of neurogenesis. Exercise increases adult hippocampal neurogenesis and enhances behavioral performance on rotarod, contextual fear and water maze in rodents. To directly test whether intact neurogenesis is required for gains in behavioral performance from exercise in C57BL/6J mice, neurogenesis was reduced using focal gamma irradiation (3 sessions of 5 Gy). Two months after treatment, mice (total n=42 males and 42 females) (Irradiated or Sham), were placed with or without running wheels (Runner or Sedentary) for 54 days. The first 10 days mice received daily injections of bromodeoxyuridine (BrdU) to label dividing cells. The last 14 days mice were tested on water maze (two trials per day for 5 days, then 1 h later probe test), rotarod (four trials per day for 3 days), and contextual fear conditioning (2 days), then measured for neurogenesis using immunohistochemical detection of BrdU and neuronal nuclear protein (NeuN) mature neuronal marker. Consistent with previous studies, in Sham animals, running increased neurogenesis fourfold and gains in performance were observed for the water maze (spatial learning and memory), rotarod (motor performance), and contextual fear (conditioning). These positive results provided the reference to determine whether gains in performance were blocked by irradiation. Irradiation reduced neurogenesis by 50% in both groups, Runner and Sedentary. Irradiation did not affect running or baseline performance on any task. Minimal changes in microglia associated with inflammation (using immunohistochemical detection of cd68) were detected at the time of behavioral testing. Irradiation did not reduce gains in performance on rotarod or contextual fear, however it eliminated gain in performance on the water maze. Results support the hypothesis that intact exercise-induced hippocampal neurogenesis is required for improved spatial memory, but not motor performance or contextual fear in C57BL/6J mice.

The neuroscience of primate intellectual evolution: natural selection and passive and intentional niche construction

We trained Japanese macaque monkeys to use tools, an advanced cognitive function monkeys do not exhibit in the wild, and then examined their brains for signs of modification. Following tool-use training, we observed neurophysiological, molecular genetic and morphological changes within the monkey brain. Despite being 'artificially' induced, these novel behaviours and neural connectivity patterns reveal overlap with those of humans. Thus, they may provide us with a novel experimental platform for studying the mechanisms of human intelligence, for revealing the evolutionary path that created these mechanisms from the 'raw material' of the non-human primate brain, and for deepening our understanding of what cognitive abilities are and of those that are not uniquely human. On these bases, we propose a theory of 'intentional niche construction' as an extension of natural selection in order to reveal the evolutionary mechanisms that forged the uniquely intelligent human brain.

The genomic determinants of alcohol preference in mice

Searches for the identity of genes that influence the levels of alcohol consumption by humans and other animals have often been driven by presupposition of the importance of particular gene products in determining positively or negatively reinforcing effects of ethanol. We have taken an unbiased approach and performed a meta-analysis across three types of mouse populations to correlate brain gene expression with levels of alcohol intake. Our studies, using filtering procedures based on QTL analysis, produced a list of eight candidate genes with highly heritable expression, which could explain a significant amount of the variance in alcohol preference in mice. Using the Allen Brain Atlas for gene expression, we noted that the candidate genes' expression was localized to the olfactory and limbic areas as well as to the orbitofrontal cortex. Informatics techniques and pathway analysis illustrated the role of the candidate genes in neuronal migration, differentiation, and synaptic remodeling. The importance of olfactory cues, learning and memory formation (Pavlovian conditioning), and cortical executive function, for regulating alcohol intake by animals (including humans), is discussed.

Antidepressants and Cdk inhibitors: releasing the brake on neurogenesis?

It is now clear that neurogenesis occurs in the brain of adult mammals. Many studies have attempted to establish relationships among neurogenesis, depression and the mechanism of action of antidepressant drugs. Therapeutic effects of antidepressants appear to be linked to increased neurogenesis in the hippocampus. Cdk inhibitors are expressed in multiple brain regions, presumably maintaining quiescence in differentiated neurons. Recently, the abundant expression of p21(Cip1) was found in neuroblasts and in newly developing neurons in the subgranular zone of the hippocampus, a region where adult neurogenesis occurs. Chronic treatment with the tricyclic antidepressant imipramine markedly decreased p21(Cip1) mRNA and protein levels and stimulated neurogenesis in this region. These results suggest that p21(Cip1) restrains neurogenesis in the hippocampus, and antidepressant-induced stimulation of neurogenesis might be a consequence of decreased p21(Cip1) expression, with the subsequent release of neuronal progenitor cells from the blockade of proliferation. These findings suggest the potential for new therapeutic strategies for the treatment of depression that target cell cycle proteins. However, there is a possibility that long-term stimulation of neurogenesis might exhaust the proliferation potentials of neuronal progenitors.

Genesis of neuronal and glial progenitors in the cerebellar cortex of peripuberal and adult rabbits

Adult neurogenesis in mammals is restricted to some brain regions, in contrast with other vertebrates in which the genesis of new neurons is more widespread in different areas of the nervous system. In the mammalian cerebellum, neurogenesis is thought to be limited to the early postnatal period, coinciding with end of the granule cell genesis and disappearance of the external granule cell layer (EGL). We recently showed that in the rabbit cerebellum the EGL is replaced by a proliferative layer called ‘subpial layer’ (SPL) which persists beyond puberty on the cerebellar surface. Here we investigated what happens in the cerebellar cortex of peripuberal rabbits by using endogenous and exogenously-administered cell proliferation antigens in association with a cohort of typical markers for neurogenesis. We show that cortical cell progenitors extensively continue to be generated herein. Surprisingly, this neurogenic process continues to a lesser extent in the adult, even in the absence of a proliferative SPL. We describe two populations of newly generated cells, involving neuronal cells and multipolar, glia-like cells. The genesis of neuronal precursors is restricted to the molecular layer, giving rise to cells immunoreactive for GABA, and for the transcription factor Pax2, a marker for GABAergic cerebellar interneuronal precursors of neuroepithelial origin that ascend through the white matter during early postnatal development. The multipolar cells are Map5+, contain Olig2 and Sox2 transcription factors, and are detectable in all cerebellar layers. Some dividing Sox2+ cells are Bergmann glia cells. All the cortical newly generated cells are independent from the SPL and from granule cell genesis, the latter ending before puberty. This study reveals that adult cerebellar neurogenesis can exist in some mammals. Since rabbits have a longer lifespan than rodents, the protracted neurogenesis within its cerebellar parenchyma could be a suitable model for studying adult nervous tissue permissiveness in mammals.

Aromatase in the brain: not just for reproduction anymore

Aromatase, the enzyme that synthesises oestrogens from androgen precursors, is expressed in the brain, where it has been classically associated with the regulation of neuroendocrine events and behaviours linked with reproduction. Recent findings, however, have revealed new unexpected roles for brain aromatase, indicating that the enzyme regulates synaptic activity, synaptic plasticity, neurogenesis and the response of neural tissue to injury, and may contribute to control nonreproductive behaviours, mood and cognition. Therefore, the function of brain aromatase is not restricted to the regulation of reproduction as previously thought.

CXCR4 signaling in the regulation of stem cell migration and development

The regulated migration of stem cells is a feature of the development of all tissues and also of a number of pathologies. In the former situation the migration of stem cells over large distances is required for the correct formation of the embryo. In addition, stem cells are deposited in niche like regions in adult tissues where they can be called upon for tissue regeneration and repair. The migration of cancer stem cells is a feature of the metastatic nature of this disease. In this article we discuss observations that have demonstrated the important role of chemokine signaling in the regulation of stem cell migration in both normal and pathological situations. It has been demonstrated that the chemokine receptor CXCR4 is expressed in numerous types of embryonic and adult stem cells and the chemokine SDF-1/CXCL12 has chemoattractant effects on these cells. Animals in which SDF-1/CXCR4 signaling has been interrupted exhibit numerous phenotypes that can be explained as resulting from inhibition of SDF-1 mediated chemoattraction of stem cells. Hence, CXCR4 signaling is a key element in understanding the functions of stem cells in normal development and in diverse pathological situations.

Changes in connectivity after visual cortical brain damage underlie altered visual function

The full extent of the brain's ability to compensate for damage or changed experience is yet to be established. One question particularly important for evaluating and understanding rehabilitation following brain damage is whether recovery involves new and aberrant neural connections or whether any change in function is due to the functional recruitment of existing pathways, or both. Blindsight, a condition in which subjects with complete destruction of part of striate cortex (V1) retain extensive visual capacities within the clinically blind field, is an excellent example of altered visual function. Since the main pathway to the visual cortex is destroyed, the spared or recovered visual ability must arise from either an existing alternative pathway, or the formation of a new pathway. Using diffusion-weighted MRI, we show that both controls and blindsight subject GY, whose left V1 is destroyed, show an ipsilateral pathway between LGN (lateral geniculate nucleus) and human motion area MT+/V5 (bypassing V1). However, in addition, GY shows two major features absent in controls: (i) a contralateral pathway from right LGN to left MT+/V5, (ii) a substantial cortico-cortical connection between MT+/V5 bilaterally. Both observations are consistent with previous functional MRI data from GY showing enhanced ipsilateral activation in MT+/V5. There is also evidence for a pathway in GY from left LGN to right MT+/V5, although the lesion makes its quantification difficult. This suggests that employing alternative brain regions for processing of information following cortical damage in childhood may strengthen or establish specific connections.

Physiology, pathology and relatedness of human tissues from gene expression meta-analysis

BACKGROUND

Development and maintenance of the identity of tissues is of central importance for multicellular organisms. Based on gene expression profiles, it is possible to divide genes in housekeeping genes and those whose expression is preferential in one or a few tissues and which provide specialized functions that have a strong effect on the physiology of the whole organism.

RESULTS

We have surveyed the gene expression in 78 normal human tissues integrating publicly available microarray gene expression data. A total amount of 1601 genes were identified as selectively expressed in one or more tissues. The tissue-selective genes covered a wide range of cellular and molecular functions, and could be linked to 361 human diseases with Mendelian inheritance. Based on the gene expression profiles, we were able to form a network of tissues reflecting their functional relatedness and, to certain extent, their development. Using co-citation driven gene network technique and promoter analysis, we predicted a transcriptional module where the co-operation of the transcription factors E2F and NF-kappaB can possibly regulate a number of genes involved in the neurogenesis that takes place in the adult hippocampus.

CONCLUSIONS

Here we propose that integration of gene expression data from Affymetrix GeneChip experiments is possible through re-annotation and commonly used pre-processing methods. We suggest that some functional aspects of the tissues can be explained by the co-operation of multiple transcription factors that regulate the expression of selected groups of genes.

Curcumin stimulates proliferation of embryonic neural progenitor cells and neurogenesis in the adult hippocampus

Curcumin is a natural phenolic component of yellow curry spice, which is used in some cultures for the treatment of diseases associated with oxidative stress and inflammation. Curcumin has been reported to be capable of preventing the death of neurons in animal models of neurodegenerative disorders, but its possible effects on developmental and adult neuroplasticity are unknown. In the present study, we investigated the effects of curcumin on mouse multi-potent neural progenitor cells (NPC) and adult hippocampal neurogenesis. Curcumin exerted biphasic effects on cultured NPC; low concentrations stimulated cell proliferation, whereas high concentrations were cytotoxic. Curcumin activated extracellular signal-regulated kinases (ERKs) and p38 kinases, cellular signal transduction pathways known to be involved in the regulation of neuronal plasticity and stress responses. Inhibitors of ERKs and p38 kinases effectively blocked the mitogenic effect of curcumin in NPC. Administration of curcumin to adult mice resulted in a significant increase in the number of newly generated cells in the dentate gyrus of hippocampus, indicating that curcumin enhances adult hippocampal neurogenesis. Our findings suggest that curcumin can stimulate developmental and adult hippocampal neurogenesis, and a biological activity that may enhance neural plasticity and repair.

Changes in adult neurogenesis in neurodegenerative diseases: cause or consequence?

This review addresses the role of adult hippocampal neurogenesis and stem cells in some of the most common neurodegenerative disorders and their related animal models. We discuss recent literature in relation to Alzheimer's disease and dementia, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, alcoholism, ischemia, epilepsy and major depression.

Neurogenesis and neuronal commitment following ischemia in a new mouse model for neonatal stroke

Stroke in the neonatal brain is an important cause of neurologic morbidity. To characterize the dynamics of neural progenitor cell proliferation and maturation after survival delays in the neonatal brain following ischemia, we utilized unilateral carotid ligation alone to produce infarcts in postnatal day 12 CD1 mice. We investigated the neurogenesis derived from the sub-ventricular zone and the sub-granular zone of the dentate gyrus subsequent to injury. Newly produced cells were labeled by bromodeoxyuridine at approximately 1 week (P18-20) after the insult by 5 i.p. injections (each 50 mg/kg). Subsequent migration and differentiation of the newborn cells was investigated at postnatal day 40 by immunohistochemistry for molecular neuronal and glial cell-lineage markers and BrdU incorporation. Cresyl violet stain demonstrated massive loss of neurons in the ipsilateral septal hippocampus in the CA3 and CA1 regions associated with atrophy. Total counts of new cells were significantly lowered not only in the ipsilateral injured but also the contralateral uninjured hippocampi and correlated with the lesion induced atrophy. Bilateral percent neuronal commitments in the dentate gyri however, were not significantly different from control. New cell densities in the neocortex and striatum increased bilaterally after neonatal stroke. The predominantly non-neuronal commitment of the SVZ-derived new cells was similar to the percentage of non-neuronal commitment in controls. In conclusion, neurogenesis occurring at 1 week after neonatal ischemia in the model maintained cell-lineage commitment patterns similar to sham controls. However, the total number of hippocampal SGZ-derived new neurons was reduced bilaterally; in contrast, the SVZ-derived neurogenesis was amplified.

Mechanisms and functional implications of adult neurogenesis

The generation of new neurons is sustained throughout adulthood in the mammalian brain due to the proliferation and differentiation of adult neural stem cells. In this review, we discuss the factors that regulate proliferation and fate determination of adult neural stem cells and describe recent studies concerning the integration of newborn neurons into the existing neural circuitry. We further address the potential significance of adult neurogenesis in memory, depression, and neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

Adult neurogenesis in primate and rodent spinal cord: comparing a cervical dorsal rhizotomy with a dorsal column transection

Neurogenesis has not been shown in the primate spinal cord and the conditions for its induction following spinal injury are not known. In the first part of this study, we report neurogenesis in the cervical spinal dorsal horn in adult monkeys 6-8 weeks after receiving a well-defined cervical dorsal rhizotomy (DRL). 5-bromo-2-deoxyuridine (BrdU) was administered 2-4 weeks following the lesion. Cells colabeled with BrdU and five different neuronal markers were observed in the peri-lesion dorsal horn 4-5 weeks after BrdU injection. Those colabeled with BrdU and neuron-specific nuclear protein, and BrdU and glial fibrillary acidic protein were quantified in the dorsal horn peri-lesion region, and the ipsi- and contralateral sides were compared. A significantly greater number of BrdU/neuron-specific nuclear protein- and BrdU/glial fibrillary acidic protein-colabeled cells were found on the lesion side (P<0.01). These findings led us to hypothesize that neurogenesis can occur within the spinal cord following injury, when the injury does not involve direct trauma to the cord and glial scar formation. This was tested in rats. Neurogenesis and astrocytic proliferation were compared between animals receiving a DRL and those receiving a dorsal column lesion. In DRL rats, neurogenesis was observed in the peri-lesion dorsal horn. In dorsal column lesion rats, no neurogenesis was observed but astrocytic activation was intense. The rat data support our hypothesis and findings in the monkey, and show that the response is not primate specific. The possibility that new neurons contribute to recovery following DRL now needs further investigation.

A population of prenatally generated cells in the rat paleocortex maintains an immature neuronal phenotype into adulthood

New neurons in the adult brain transiently express molecules related to neuronal development, such as the polysialylated form of neural cell adhesion molecule, or doublecortin (DCX). These molecules are also expressed by a cell population in the rat paleocortex layer II, whose origin, phenotype, and function are not clearly understood. We have classified most of these cells as a new cell type termed tangled cell. Some cells with the morphology of semilunar-pyramidal transitional neurons were also found among this population, as well as some scarce cells resembling semilunar, pyramidal. and fusiform neurons. We have found that none of these cells in layer II express markers of glial cells, mature, inhibitory, or principal neurons. They appear to be in a prolonged immature state, confirmed by the coexpression of DCX, TOAD/Ulip/CRMP-4, A3 subunit of the cyclic nucleotide-gated channel, or phosphorylated cyclic adenosine monophosphate response element-binding protein. Moreover, most of them lack synaptic contacts, are covered by astroglial lamellae, and fail to express cellular activity markers, such as c-Fos or Arc, and N-methyl-d-aspartate or glucocorticoid receptors. We have found that none of these cells appear to be generated during adulthood or early youth and that most of them have been generated during embryonic development, mainly in E15.5.

Evidence for constitutive neural cell proliferation in the adult murine hypothalamus

Compelling evidence suggests that the mammalian brain is capable of generating new neurons throughout adult life. While neurogenesis can be induced at various brain sites by exogenous cues, constitutive birth of new neurons has been unambiguously demonstrated within the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus. The lack of strong evidence that constitutive neurogenesis occurs elsewhere in the adult brain could be due to its exclusive restriction to the SVZ and SGZ or, for instance, to the inadequacy of the methods used to reveal new-born neurons at other brain sites. By using intracerebroventricular (icv) delivery of the mitotic marker bromodeoxyuridine (BrdU) we demonstrate that new cells are born continuously and in substantial numbers in the adult murine hypothalamus and that many of these cells appear to differentiate into neurons as assessed by the expression of doublecortin (Dcx) and other neuronal fate markers. As compared to intraperitoneal (ip) BrdU injections, central BrdU infusion also uncovers a higher-fold induction of hypothalamic cell proliferation by ciliary neurotrophic factor (CNTF). It appears that new cells are born throughout the hypothalamic parenchyma without an apparent restriction to a specific neurogenic layer, as seen in the SVZ. Thus, we provide evidence that the adult hypothalamus is constitutively neurogenic and that hypothalamic cell proliferation is highly responsive to mitogen action.

Adult neural stem cells: The promise of the future

Stem cells are self-renewing undifferentiated cells that give rise to multiple types of specialized cells of the body. In the adult, stem cells are multipotents and contribute to homeostasis of the tissues and regeneration after injury. Until recently, it was believed that the adult brain was devoid of stem cells, hence unable to make new neurons and regenerate. With the recent evidences that neurogenesis occurs in the adult brain and neural stem cells (NSCs) reside in the adult central nervous system (CNS), the adult brain has the potential to regenerate and may be amenable to repair. The function(s) of NSCs in the adult CNS remains the source of intense research and debates. The promise of the future of adult NSCs is to redefine the functioning and physiopathology of the CNS, as well as to treat a broad range of CNS diseases and injuries.

Is exposure to enriched environment beneficial for functional post-lesional recovery in temporal lobe epilepsy?

Exposure to enriched environment has been shown to induce robust neuronal plasticity in both intact and injured adult central nervous system, including up-regulation of multiple neurotrophic factors, enhanced neurogenesis in the dentate gyrus of the hippocampus, and improved spatial learning and memory function. Neuronal plasticity, though mostly adaptive and abnormal, also occurs during certain neurodegenerative conditions such as the temporal lobe epilepsy (TLE). The TLE is characterized by hippocampal neurodegeneration, aberrant mossy fiber sprouting, spontaneous recurrent motor seizures, cognitive deficits, and abnormally enhanced neurogenesis during the early phase and dramatically declined neurogenesis during the chronic phase of the disease. As environmental enrichment has been found to be beneficial for treating animal models of Alzheimer's, Parkinson's, and Huntington's diseases, there is considerable interest in determining the efficacy of this strategy for preventing or treating chronic TLE after the initial precipitating brain injury. This review first discusses the proof of principle behind the potential application of the environmental enrichment strategy for preventing or treating TLE after brain injury. The subsequent chapters confer the portrayed beneficial effects of enrichment for functional post-lesional recovery in TLE and the possible complications which may arise from housing epilepsy-prone or epileptic rats in enriched environmental conditions. The final segment discusses studies that are essential for further understanding the efficacy of this approach for preventing or treating TLE.