The Temporal Expression Pattern of Alpha-Synuclein Modulates Olfactory Neurogenesis in Transgenic Mice

Background

Adult neurogenesis mirrors the brain´s endogenous capacity to generate new neurons throughout life. In the subventricular zone/ olfactory bulb system adult neurogenesis is linked to physiological olfactory function and has been shown to be impaired in murine models of neuronal alpha-Synuclein overexpression. We analyzed the degree and temporo-spatial dynamics of adult olfactory bulb neurogenesis in transgenic mice expressing human wild-type alpha-Synuclein (WTS) under the murine Thy1 (mThy1) promoter, a model known to have a particularly high tg expression associated with impaired olfaction.

Results

Survival of newly generated neurons (NeuN-positive) in the olfactory bulb was unchanged in mThy1 transgenic animals. Due to decreased dopaminergic differentiation a reduction in new dopaminergic neurons within the olfactory bulb glomerular layer was present. This is in contrast to our previously published data on transgenic animals that express WTS under the control of the human platelet-derived growth factor β (PDGF) promoter, that display a widespread decrease in survival of newly generated neurons in regions of adult neurogenesis, resulting in a much more pronounced neurogenesis deficit. Temporal and quantitative expression analysis using immunofluorescence co-localization analysis and Western blots revealed that in comparison to PDGF transgenic animals, in mThy1 transgenic animals WTS is expressed from later stages of neuronal maturation only but at significantly higher levels both in the olfactory bulb and cortex.

Conclusions

The dissociation between higher absolute expression levels of alpha-Synuclein but less severe impact on adult olfactory neurogenesis in mThy1 transgenic mice highlights the importance of temporal expression characteristics of alpha-Synuclein on the maturation of newborn neurons.     

Adult Neurogenesis in the Olfactory System: Improving Performance for Difficult Discrimination Tasks?

What is the function of new neurons entering the olfactory bulb? Many insights regarding the molecular control of adult neurogenesis have been uncovered, but the purpose of new neurons entering the olfactory bulb has been difficult to ascertain. Here, studies investigating the role of adult neurogenesis in olfactory discrimination in mice are reviewed. Studies in which adult neurogenesis is affected are highlighted, with a focus on the role of environment enrichment and what happens during ageing. There is evidence for a role of adult neurogenesis in fine discrimination tasks, as underscored by studies that enhance adult neurogenesis. This is also observed in ageing studies, where older mice with reduced levels of adult neurogenesis perform poorly in olfactory discrimination. Differences in methodology that could account for alternative conclusions, and the importance of specificity in methods being used to investigate the effect of adult neurogenesis in olfactory performance are emphasized.     

侧脑室室管膜下区神经再生的研究进展

神经再生(Neurogenesis)是指具有自我更新能力的神经干细胞(Neural Stem Cells,NSCs)经过迁移、增殖,最终分化为具有特定功能的神经细胞的过程。以往人们认为,神经再生只存在于胚胎期或外周神经系统,近几年发现,在成年动物的中枢神经系统也存在神经再生,研究发现侧脑室室管膜下区(SVZ)是神经再生发生的主要区域之一,产生新的神经元和神经胶质细胞通过RMS通路运输至嗅球进而对嗅觉损伤部分进行修复。本文主要从成年神经再生的发展、神经再生与疾病的关系、神经再生的过程等方面进行综述。     

Neurogenesis in the adult brain. Functional consequences

In the adult mammalian brain, neuroblasts are continuously produced within the subgranular zone of the hippocampus and the subventricular zone (SVZ) of the forebrain. In this review we describe how some physiological and environmental factors play important roles in regulating neurogenesis in the hippocampus. Neuroblasts in the SVZ network migrate rostrally into the olfactory bulb where they differentiate into local interneurons. We focus on the production, survival and functional consequences of these newly generated interneurons. We show that enriched odor-exposure enhances the number of newborn neurons in the adult olfactory bulb but not in the hippocampus. This effect did not result from changes in cell proliferation but rather was due to greater neuronal survival. Furthermore, the enriched condition was found to dramatically extend the olfactory memory. By maintaining a constitutive turnover of interneurons subjected to regulation by bulbar activity, ongoing neurogenesis plays a key role in olfactory memory.     

CDK2 is Dispensable for Adult Hippocampal Neurogenesis

Granule neurons of the dentate gyrus (DG) of the hippocampus undergo continuous renewal throughout life. Among cell-cycle regulators, cyclin-dependent kinase 2 (Cdk2) is considered as a major regulator of S-phase entry. We used Cdk2-deficient mice to decipher the requirement of Cdk2 for the generation of new neurons in the adult hippocampus. The quantification of cell cycle markers first revealed that the lack of Cdk2 activity does not influence spontaneous or seizure-induced proliferation of neural progenitor cells (NPC) in the adult DG. Using bromodeoxyuridine incorporation assays, we showed that the number of mature newborn granule neurons generated de novo was similar in both wild-type (WT) and Cdk2-deficient adult mice. Moreover, the apparent lack of cell output reduction in Cdk2-/- mice DG did not result from a reduction in apoptosis of newborn granule cells as analyzed by TUNEL assays. Our results therefore suggest that Cdk2 is dispensable for NPC proliferation, differentiation and survival of adult-born DG granule neurons in vivo. These data emphasize that functional redundancies between Cdks also occur in the adult brain at the level of neural progenitor cell cycle regulation during hippocampal neurogenesis.     

Photoperiod Mediated Changes in Olfactory Bulb Neurogenesis and Olfactory Behavior in Male White-Footed Mice (Peromyscus leucopus)

Brain plasticity, in relation to new adult mammalian neurons generated in the subgranular zone of the hippocampus, has been well described. However, the functional outcome of new adult olfactory neurons born in the subventricular zone of the lateral ventricles is not clearly defined, as manipulating neurogenesis through various methods has given inconsistent and conflicting results in lab mice. Several small rodent species, including Peromyscus leucopus, display seasonal (photoperiodic) brain plasticity in brain volume, hippocampal function, and hippocampus-dependent behaviors; plasticity in the olfactory system of photoperiodic rodents remains largely uninvestigated. We exposed adult male P. leucopus to long day lengths (LD) and short day lengths (SD) for 10 to 15 weeks and then examined olfactory bulb cell proliferation and survival using the thymidine analog BrdU, olfactory bulb granule cell morphology using Golgi-Cox staining, and behavioral investigation of same-sex conspecific urine. SD mice did not differ from LD counterparts in granular cell morphology of the dendrites or in dendritic spine density. Although there were no differences due to photoperiod in habituation to water odor, SD mice rapidly habituated to male urine, whereas LD mice did not. In addition, short day induced changes in olfactory behavior were associated with increased neurogenesis in the caudal plexiform and granule cell layers of the olfactory bulb, an area known to preferentially respond to water-soluble odorants. Taken together, these data demonstrate that photoperiod, without altering olfactory bulb neuronal morphology, alters olfactory bulb neurogenesis and olfactory behavior in Peromyscus leucopus.     

Loss of STOP Protein Impairs Peripheral Olfactory Neurogenesis

Background

STOP (Stable Tubulin-Only Polypeptide) null mice show behavioral deficits, impaired synaptic plasticity, decrease in synaptic vesicular pools and disturbances in dopaminergic transmission, and are considered a neurodevelopmental model of schizophrenia. Olfactory neurons highly express STOP protein and are continually generated throughout life. Experimentally-induced loss of olfactory neurons leads to epithelial regeneration within two months, providing a useful model to evaluate the role played by STOP protein in adult olfactory neurogenesis.

Methodology/Principal Findings

Immunocytochemistry and electron microscopy were used to study the structure of the glomerulus in the main olfactory bulb and neurogenesis in the neurosensorial epithelia. In STOP null mice, olfactory neurons showed presynaptic swellings with tubulovesicular profiles and autophagic-like structures. In olfactory and vomeronasal epithelia, there was an increase in neurons turnover, as shown by the increase in number of proliferating, apoptotic and immature cells with no changes in the number of mature neurons. Similar alterations in peripheral olfactory neurogenesis have been previously described in schizophrenia patients. In STOP null mice, regeneration of the olfactory epithelium did not modify these anomalies; moreover, regeneration resulted in abnormal organisation of olfactory terminals within the olfactory glomeruli in STOP null mice.

Conclusions/Significance

In conclusion, STOP protein seems to be involved in the establishment of synapses in the olfactory glomerulus. Our results indicate that the olfactory system of STOP null mice is a well-suited experimental model (1) for the study of the mechanism of action of STOP protein in synaptic function/plasticity and (2) for pathophysiological studies of the mechanisms of altered neuronal connections in schizophrenia.     

Hard-Diet Feeding Recovers Neurogenesis in the Subventricular Zone and Olfactory Functions of Mice Impaired by Soft-Diet Feeding

The subventricular zone (SVZ) generates an immense number of neurons even during adulthood. These neurons migrate to the olfactory bulb (OB) and differentiate into granule cells and periglomerular cells. The information broadcast by general odorants is received by the olfactory sensory neurons and transmitted to the OB. Recent studies have shown that a reduction of mastication impairs both neurogenesis in the hippocampus and brain functions. To examine these effects, we first measured the difference in Fos-immunoreactivity (Fos-ir) at the principal sensory trigeminal nucleus (Pr5), which receives intraoral touch information via the trigeminal nerve, when female adult mice ingested a hard or soft diet to explore whether soft-diet feeding could mimic impaired mastication. Ingestion of a hard diet induced greater expression of Fos-ir cells at the Pr5 than did a soft diet or no diet. Bromodeoxyuridine-immunoreactive (BrdU-ir) structures in sagittal sections of the SVZ and in the OB of mice fed a soft or hard diet were studied to explore the effects of changes in mastication on newly generated neurons. After 1 month, the density of BrdU-ir cells in the SVZ and OB was lower in the soft-diet-fed mice than in the hard-diet-fed mice. The odor preferences of individual female mice to butyric acid were tested in a Y-maze apparatus. Avoidance of butyric acid was reduced by the soft-diet feeding. We then explored the effects of the hard-diet feeding on olfactory functions and neurogenesis in the SVZ of mice impaired by soft-diet feeding. At 3 months of hard-diet feeding, avoidance of butyric acid was reversed and responses to odors and neurogenesis were recovered in the SVZ. The present results suggest that feeding with a hard diet improves neurogenesis in the SVZ, which in turn enhances olfactory function at the OB.     

Consolidation of an Olfactory Memory Trace in the Olfactory Bulb Is Required for Learning-Induced Survival of Adult-Born Neurons and Long-Term Memory

Background

It has recently been proposed that adult-born neurons in the olfactory bulb, whose survival is modulated by learning, support long-term olfactory memory. However, the mechanism used to select which adult-born neurons following learning will participate in the long-term retention of olfactory information is unknown. We addressed this question by investigating the effect of bulbar consolidation of olfactory learning on memory and neurogenesis.

Methodology/Principal Findings

Initially, we used a behavioral ecological approach using adult mice to assess the impact of consolidation on neurogenesis. Using learning paradigms in which consolidation time was varied, we showed that a spaced (across days), but not a massed (within day), learning paradigm increased survival of adult-born neurons and allowed long-term retention of the task. Subsequently, we used a pharmacological approach to block consolidation in the olfactory bulb, consisting in intrabulbar infusion of the protein synthesis inhibitor anisomycin, and found impaired learning and no increase in neurogenesis, while basic olfactory processing and the basal rate of adult-born neuron survival remained unaffected. Taken together these data indicate that survival of adult-born neurons during learning depends on consolidation processes taking place in the olfactory bulb.

Conclusion/Significance

We can thus propose a model in which consolidation processes in the olfactory bulb determine both survival of adult-born neurons and long-term olfactory memory. The finding that adult-born neuron survival during olfactory learning is governed by consolidation in the olfactory bulb strongly argues in favor of a role for bulbar adult-born neurons in supporting olfactory memory.     

Postnatal Loss of Hap1 Reduces Hippocampal Neurogenesis and Causes Adult Depressive-Like Behavior in Mice

Depression is a serious mental disorder that affects a person’s mood, thoughts, behavior, physical health, and life in general. Despite our continuous efforts to understand the disease, the etiology of depressive behavior remains perplexing. Recently, aberrant early life or postnatal neurogenesis has been linked to adult depressive behavior; however, genetic evidence for this is still lacking. Here we genetically depleted the expression of huntingtin-associated protein 1 (Hap1) in mice at various ages or in selective brain regions. Depletion of Hap1 in the early postnatal period, but not later life, led to a depressive-like phenotype when the mice reached adulthood. Deletion of Hap1 in adult mice rendered the mice more susceptible to stress-induced depressive-like behavior. Furthermore, early Hap1 depletion impaired postnatal neurogenesis in the dentate gyrus (DG) of the hippocampus and reduced the level of c-kit, a protein expressed in neuroproliferative zones of the rodent brain and that is stabilized by Hap1. Importantly, stereotaxically injected adeno-associated virus (AAV) that directs the expression of c-kit in the hippocampus promoted postnatal hippocampal neurogenesis and ameliorated the depressive-like phenotype in conditional Hap1 KO mice, indicating a link between postnatal-born hippocampal neurons and adult depression. Our results demonstrate critical roles for Hap1 and c-kit in postnatal neurogenesis and adult depressive behavior, and also suggest that genetic variations affecting postnatal neurogenesis may lead to adult depression.     

Targeted Deletion of the ERK5 MAP Kinase Impairs Neuronal Differentiation,Migration, and Survival during Adult Neurogenesis in the Olfactory Bulb

Recent studies have led to the exciting idea that adult-born neurons in the olfactory bulb (OB) may be critical for complex forms of olfactory behavior in mice. However, signaling mechanisms regulating adult OB neurogenesis are not well defined. We recently reported that extracellular signal-regulated kinase (ERK) 5, a MAP kinase, is specifically expressed in neurogenic regions within the adult brain. This pattern of expression suggests a role for ERK5 in the regulation of adult OB neurogenesis. Indeed, we previously reported that conditional deletion of erk5 in adult neurogenic regions impairs several forms of olfactory behavior in mice. Thus, it is important to understand how ERK5 regulates adult neurogenesis in the OB. Here we present evidence that shRNA suppression of ERK5 in adult neural stem/progenitor cells isolated from the subventricular zone (SVZ) reduces neurogenesis in culture. By contrast, ectopic activation of endogenous ERK5 signaling via expression of constitutive active MEK5, an upstream activating kinase for ERK5, stimulates neurogenesis. Furthermore, inducible and conditional deletion of erk5 specifically in the neurogenic regions of the adult mouse brain interferes with cell cycle exit of neuroblasts, impairs chain migration along the rostral migratory stream and radial migration into the OB. It also inhibits neuronal differentiation and survival. These data suggest that ERK5 regulates multiple aspects of adult OB neurogenesis and provide new insights concerning signaling mechanisms governing adult neurogenesis in the SVZ-OB axis.     

Circadian Clock Genes Are Essential for Normal Adult Neurogenesis,Differentiation, and Fate Determination

Adult neurogenesis creates new neurons and glia from stem cells in the human brain throughout life. It is best understood in the dentate gyrus (DG) of the hippocampus and the subventricular zone (SVZ). Circadian rhythms have been identified in the hippocampus, but the role of any endogenous circadian oscillator cells in hippocampal neurogenesis and their importance in learning or memory remains unclear. Any study of stem cell regulation by intrinsic circadian timing within the DG is complicated by modulation from circadian clocks elsewhere in the brain. To examine circadian oscillators in greater isolation, neurosphere cultures were prepared from the DG of two knockout mouse lines that lack a functional circadian clock and from mPer1::luc mice to identify circadian oscillations in gene expression. Circadian mPer1 gene activity rhythms were recorded in neurospheres maintained in a culture medium that induces neurogenesis but not in one that maintains the stem cell state. Although the differentiating neural stem progenitor cells of spheres were rhythmic, evidence of any mature neurons was extremely sparse. The circadian timing signal originated in undifferentiated cells within the neurosphere. This conclusion was supported by immunocytochemistry for mPER1 protein that was localized to the inner, more stem cell-like neurosphere core. To test for effects of the circadian clock on neurogenesis, media conditions were altered to induce neurospheres from BMAL1 knockout mice to differentiate. These cultures displayed unusually high differentiation into glia rather than neurons according to GFAP and NeuN expression, respectively, and very few BetaIII tubulin-positive, immature neurons were observed. The knockout neurospheres also displayed areas visibly devoid of cells and had overall higher cell death. Neurospheres from arrhythmic mice lacking two other core clock genes, Cry1 and Cry2, showed significantly reduced growth and increased astrocyte proliferation during differentiation, but they generated normal percentages of neuronal cells. Neuronal fate commitment therefore appears to be controlled through a non-clock function of BMAL1. This study provides insight into how cell autonomous circadian clocks and clock genes regulate adult neural stem cells with implications for treating neurodegenerative disorders and impaired brain functions by manipulating neurogenesis.     

The Role of Notch Signaling in Adult Neurogenesis

Neurogenesis occurs throughout adulthood in the mammalian brain. Newly born neurons are incorporated into the functional networks of both the olfactory bulb and the hippocampal dentate gyrus, and there is growing evidence that adult neurogenesis is important for various brain functions. Continuous neurogenesis is achieved by the coordinated proliferation and differentiation of adult neural stem cells. In this review, we discuss the recent findings concerning the roles of Notch signaling in adult neural stem cells.     

Neurogenesis in mouse models of Alzheimer's disease

The brains of the adult mouse and human possess neural stem cells (NSCs) that retain the capacity to generate new neurons through the process of neurogenesis. They share the same anatomical locations of stem cell niches in the brain, as well as the prominent feature of rostral migratory stream formed by neuroblasts migrating from the lateral ventricles towards the olfactory bulb. Therefore the mouse possesses some fundamental features that may qualify it as a relevant model for adult human neurogenesis. Adult born young hippocampal neurons in the mouse display the unique property of enhanced plasticity, and can integrate physically and functionally into existing neural circuits in the brain. Such crucial properties of neurogenesis may at least partially underlie the improved learning and memory functions observed in the mouse when hippocampal neurogenesis is augmented, leading to the suggestion that neurogenesis induction may be a novel therapeutic approach for diseases with cognitive impairments such as Alzheimer's disease (AD). Research towards this goal has benefited significantly from the use of AD mouse models to facilitate the understanding in the impact of AD pathology on neurogenesis. The present article reviews the growing body of controversial data on altered neurogenesis in mouse models of AD and attempts to assess their relative relevance to humans.     

Neurogenesis in the Adult Avian Song-Control System

New neurons are added throughout the forebrain of adult birds. The song-control system is a model to investigate the addition of new long-projection neurons to a cortical circuit that regulates song, a learned sensorimotor behavior. Neuroblasts destined for the song nucleus HVC arise in the walls of the lateral ventricle, and wander through the pallium to reach HVC. The survival of new HVC neurons is supported by gonadally secreted testosterone and its downstream effectors including neurotrophins, vascularization, and electrical activity of postsynaptic neurons in nucleus RA (robust nucleus of the arcopallium). In seasonal species, the HVC→RA circuit degenerates in nonbreeding birds, and is reconstructed by the incorporation of new projection neurons in breeding birds. There is a functional linkage between the death of mature HVC neurons and the birth of new neurons. Various hypotheses for the function of adult neurogenesis in the song system can be proposed, but this remains an open question.Song behavior in oscine birds is regulated by a network of pallial and striatal nuclei. The song-control system shows extensive plasticity in adults, including ongoing neurogenesis in several nuclei (Brenowitz 2008). The addition of new neurons to the adult brain of higher vertebrates was first suggested by the pioneering studies of Altman and Das (1965) and Kaplan and Hinds (1977). They reported that labeled cells were present in the dentate gyrus (DG) of rats following the injection of ³H-thymidine. Their claims, however, met with skepticism and the neuronal identity of the new cells that they observed was called into question (Gross 2000). In an influential study, Rakic (1985) injected adult rhesus monkeys with ³H-thymidine and reported that, “all neurons of the rhesus monkey brain are generated during prenatal and early postnatal life.” The study of neuronal addition to the adult brain, was subsequently dropped for ∼20 years in the face of the dogma that neurogenesis was largely completed by birth (Gross 2000). This prevailing view only started to be overturned when Nottebohm and colleagues published a series of studies showing that new cells are added to the cortical-like song nucleus HVC (Fig. 1) of adult canaries (Serinus canarius) (Goldman and Nottebohm 1983). These new cells have neuronal morphology, some of these cells fire action potentials in response to sound (Paton and Nottebohm 1984), receive synaptic input (Burd and Nottebohm 1985), may synapse on neurons in the efferent robust nucleus of the arcopallium (RA) (Alvarez-Buylla et al. 1990), and express neuron-specific proteins (Barami et al. 1995). Together, these studies in songbirds showed that new neurons are born and incorporated into functional circuits in the brains of adults of higher vertebrates (Nottebohm 2004). This research on adult neurogenesis in songbirds stimulated investigators to re-examine this topic in mammals. It soon became clear that new neurons are added throughout life to the DG and olfactory bulb of mammals including humans (Cameron and Gould 1994; Gould et al. 1997, 1999a; Lim et al. 1997; Eriksson et al. 1998). Because of these initial confirmatory reports, there has been explosive growth in study of the mechanisms and functions of adult neurogenesis in the mammalian DG and olfactory bulb.Open in a separate windowFigure 1.A schematic of the neurogenic regions in the avian brain overlaid on the avian song circuits. Neurogenic regions are shown in red. Note the proximity of HVC (and hippocampus [HC]) to the ventricular zone (VZ). A schematic version of the motor pathway for song production is shown in blue. A schematic of the ascending auditory pathway is shown in green. The dotted line indicates an indirect route through many nuclei of the ascending auditory pathway leading to field L in the telencephalon. The anterior forebrain circuit for song learning and plasticity is shown in yellow. NCM, Caudomedial nidopallium; RA, arcopallium; LMAN, lateral magnocellular nucleus of the anterior neostriatum; OB, olfactory bulb; DLM, dorsolateral medial; PAm, parambigualis; RAm, retroambigualisBirds continue to be a productive model for the study of neurogenesis in the adult brain, as discussed below. In this article, we will focus on neurogenesis in the song-control system as this is the most intensively studied model in birds. (For a review of neurogenesis in the avian hippocampus [HC], see Barnea and Pravosudov 2011.) We will discuss the mechanisms of neurogenesis in the song system, intrinsic and extrinsic factors that influence neuronal addition, a linkage between cell death and neurogenesis, seasonal plasticity, and consider potential functions of adult neurogenesis.     

Extracellular Signal-regulated Kinase 5 (ERK5) Mediates Prolactin-stimulated Adult Neurogenesis in the Subventricular Zone and Olfactory Bulb

Prolactin-stimulated adult neurogenesis in the subventricular zone (SVZ) and olfactory bulb (OB) mediates several reproductive behaviors including mating/pregnancy, dominant male pheromone preference in females, and paternal recognition of offspring. However, downstream signaling mechanisms underlying prolactin-induced adult neurogenesis are completely unknown. We report here for the first time that prolactin activates extracellular signal-regulated kinase 5 (ERK5), a MAP kinase that is specifically expressed in the neurogenic regions of the adult mouse brain. Knockdown of ERK5 by retroviral infection of shRNA attenuates prolactin-stimulated neurogenesis in SVZ-derived adult neural stem/progenitor cells (aNPCs). Inducible erk5 deletion in adult neural stem cells of transgenic mice inhibits neurogenesis in the SVZ and OB following prolactin infusion or mating/pregnancy. These results identify ERK5 as a novel and critical signaling mechanism underlying prolactin-induced adult neurogenesis.     

Neurogenesis of the cholinergic medial septum in female and male C57BL/6J mice

Sex differences exist in the structure and function of the cholinergic septo-hippocampal system throughout the lifespan of mammals. How and when these sex differences originate is unclear. Because estrogen modulates sexual differentiation of several brain regions during development and influences neurogenesis in adult mammals, we hypothesized that sexual dimorphism of the cholinergic septo-hippocampal system would extend to its neurogenesis. A birthdating agent 5'-bromo-2'-deoxyuridine (BrdU) was injected into pregnant dams on one of eight gestational days, ranging from embryonic day (E)10 to E17. The offspring were euthanized at 2 months of age, and brains were processed for BrdU and choline acetyltransferase (ChAT) immunoreactivity to label cholinergic neurons that became postmitotic on a given embryonic day and survived to adulthood. Unbiased stereology was used to compare the number of double-labeled neurons in the medial septum (MS) of female and male offspring. Cholinergic neurons in the MS were generated primarily between E11 and E14, similar to other published reports. We found sex differences in the pattern of peak neurogenesis but not in the length of neurogenesis, or in total number of neurons generated in the MS. Additionally, in adult female and male mice, we estimated the total number of cholinergic neurons using unbiased stereology and found no sex differences in the number of cholinergic neurons or in the volume of the MS in adulthood. These results suggest that sex differences noted in the function of the postnatal cholinergic septo-hippocampal system may originate from its neurogenesis.