Adult mammalian neurogenesis and the New Zealand white rabbit

Immunohistochemical analyses of neural stem cell lineage markers in normal feline brains and glial tumors

Neural stem cell (NSC) lineage cells have not been fully identified in feline brains, and the NSC-like nature of feline glial tumors has not been determined. In this study, 6 normal cat brains (3 newborn and 3 older cats) and 13 feline glial tumors were analyzed using immunohistochemical NSC lineage markers. The feline glial tumors were subjected to immunohistochemical scoring followed by hierarchical cluster analysis. In newborn brains, glial acidic fibrillary protein (GFAP)/nestin/sex-determining region Y-box transcription factor 2 (SOX2)-immunopositive NSCs, SOX2-immunopositive intermediate progenitor cells, oligodendrocyte transcription factor 2 (OLIG2)/platelet-derived growth factor receptor-α (PDGFR-α)-immunopositive oligodendrocyte precursor cells (OPCs), OLIG2/GFAP-immunopositive immature astrocytes, and neuronal nuclear (NeuN)/β-3 tubulin-immunopositive mature neuronal cells were observed. The apical membrane of NSCs was also immunopositive for Na+/H+ exchanger regulatory factor 1 (NHERF1). In mature brains, the NSC lineage cells were similar to those of the newborn brains. A total of 13 glial tumors consisted of 2 oligodendrogliomas, 4 astrocytomas, 3 subependymomas, and 4 ependymomas. Astrocytomas, subependymomas, and ependymomas were immunopositive for GFAP, nestin, and SOX2. Subependymomas and ependymomas showed dot-like or apical membrane immunolabeling for NHERF1, respectively. Astrocytomas were immunopositive for OLIG2. Oligodendrogliomas and subependymomas were immunopositive for OLIG2 and PDGFR-α. Feline glial tumors also showed variable immunolabeling for β-3 tubulin, NeuN, and synaptophysin. Based on these results, feline astrocytomas, subependymomas, and ependymomas appear to have an NSC-like immunophenotype. In addition, astrocytomas, subependymomas, and ependymomas have the characteristics of glial, oligodendrocyte precursor, and ependymal cells, respectively. Feline oligodendrogliomas likely have an OPC-like immunophenotype. In addition, feline glial tumors may have multipotential stemness for differentiation into neuronal cells. These preliminary results should be validated by gene expression analyses in future studies with larger case numbers.

Maternal surgery during pregnancy has a transient adverse effect on the developing fetal rabbit brain

BACKGROUND

Recently, the US Food and Drug Administration called for cautious use of anesthetic drugs during pregnancy. In 0.2-2% of pregnancies, nonobstetric surgery is being performed. The consequences of anesthesia during pregnancy on fetal development remain unclear, and preclinical studies in relevant animal models may help to elucidate them.

OBJECTIVE

To assess the effect of maternal anesthesia and surgery during pregnancy on the developing fetal brain, using a rabbit model.

MATERIALS AND METHODS

This is a randomized, sham-controlled study in time-mated pregnant does at 28 days of gestation (term = 31 days), which corresponds to the end of the second trimester in humans. Anesthesia was induced in 14 does (155 pups) with propofol and maintained with 4 vol% (equivalent to 1 minimum alveolar concentration) sevoflurane for 2 hours, and a laparotomy with minimal organ manipulation was performed (surgery group). Maternal vital signs (blood pressure, heart rate, peripheral and cerebral oxygen saturation, temperature, end-tidal CO₂, pH, lactate) were continuously monitored. Sham controls consisted of 7 does (74 pups) undergoing invasive hemodynamic monitoring for 2 hours without sedation. At term, does underwent cesarean delivery under ketamine-medetomidine sedation and local anesthesia. Pups either underwent motor and sensory neurologic testing followed by euthanasia at day 1 or daily neurodevelopment testing for 2 weeks and extensive neurologic assessment at 5 and 7 weeks (open field and object recognition test, T-maze, and radial-arm maze). Brains were harvested for histologic assessment of neuron density and synaptophysin expression.

RESULTS

Blood gases and vital parameters were stable in both groups. On postnatal day 1, surgery pups had significant lower motor (25 ± 1 vs 23 ± 3; P = .004) and sensory (16 ± 2 vs 15 ± 2; P = .005) neurobehavioral scores and lower brain-to-body weight ratios (3.7% ± 0.6% vs 3.4% ± 0.6%; P = .001). This was accompanied by lower neuron density in multiple brain regions (eg, hippocampus 2617 ± 410 vs 2053 ± 492 neurons/mm²; P = .004) with lower proliferation rates and less synaptophysin expression. Furthermore, surgery pups had delayed motor development during the first week of life, for example with hopping appearing later (6 ± 5 vs 12 ± 3 days; P = .011). Yet, by 7 weeks of age, neurobehavioral impairment was limited to a reduced digging behavior, and no differences in neuron density or synaptophysin expression were seen.

CONCLUSION

In rabbits, 2 hours of maternal general anesthesia and laparotomy, with minimal organ and no fetal manipulation, had a measurable impact on neonatal neurologic function and brain morphology. Pups had a slower motoric neurodevelopment, but by 7 weeks the effect became almost undetectable.

Early neuropathological and neurobehavioral consequences of preterm birth in a rabbit model

Preterm birth is the most significant problem in contemporary obstetrics accounting for 5–18% of worldwide deliveries. Encephalopathy of prematurity encompasses the multifaceted diffuse brain injury resulting from preterm birth. Current animal models exploring the underlying pathophysiology of encephalopathy of prematurity employ significant insults to generate gross central nervous system abnormalities. To date the exclusive effect of prematurity was only studied in a non-human primate model. Therefore, we aimed to develop a representative encephalopathy of prematurity small animal model only dependent on preterm birth. Time mated New-Zealand white rabbit does were either delivered on 28 (pre-term) or 31 (term) postconceptional days by caesarean section. Neonatal rabbits underwent neurobehavioral evaluation on 32 days post conception and then were transcardially perfuse fixed. Neuropathological assessments for neuron and oligodendrocyte quantification, astrogliosis, apoptosis and cellular proliferation were performed. Lastly, ex-vivo high-resolution Magnetic Resonance Imaging was used to calculate T1 volumetric and Diffusion Tensor Imaging derived fractional anisotropy and mean diffusivity. Preterm birth was associated with a motoric (posture instability, abnormal gait and decreased locomotion) and partial sensory (less pain responsiveness and failing righting reflex) deficits that coincided with global lower neuron densities, less oligodendrocyte precursors, increased apoptosis and less proliferation. These region-specific histological changes corresponded with Magnetic Resonance Diffusion Tensor Imaging differences. The most significant differences were seen in the hippocampus, caudate nucleus and thalamus of the preterm rabbits. In conclusion this model of preterm birth, in the absence of any other contributory events, resulted in measurable neurobehavioral deficits with associated brain structural and Magnetic Resonance Diffusion Tensor Imaging findings.

The Distribution of Ki-67 and Doublecortin-Immunopositive Cells in the Brains of Three Strepsirrhine Primates: Galago demidoff, Perodicticus potto, and Lemur catta

This study investigated the pattern of adult neurogenesis throughout the brains of three prosimian primate species using immunohistochemical techniques for endogenous markers of this neural process. Two species, Galago demidoff and Perodicticus potto, were obtained from wild populations in the primary rainforest of central Africa, while one species, Lemur catta, was captive-bred. Two brains from each species, perfusion-fixed with 4% paraformaldehyde, were sectioned (50 µm section thickness) in sagittal and coronal planes. Using Ki-67 and doublecortin (DCX) antibodies, proliferating cells and immature neurons were identified in the two canonical neurogenic sites of mammals, the subventricular zone of the lateral ventricle (SVZ) giving rise to the rostral migratory stream (RMS), and the subgranular zone of the dentate gyrus of the hippocampus. In addition a temporal migratory stream (TMS), emerging from the temporal horn of the lateral ventricle to supply the piriform cortex and adjacent brain regions with new neurons, was also evident in the three prosimian species. While no Ki-67-immunoreactive cells were observed in the cerebellum, DCX-immunopositive cells were observed in the cerebellar cortex of all three species. These findings are discussed in a phylogenetic context.

Adult Neurogenesis in Sheep: Characterization and Contribution to Reproduction and Behavior

Sheep have many advantages to study neurogenesis in comparison to the well-known rodent models. Their development and life expectancy are relatively long and they possess a gyrencephalic brain. Sheep are also seasonal breeders, a characteristic that allows studying the involvement of hypothalamic neurogenesis in the control of seasonal reproduction. Sheep are also able to individually recognize their conspecifics and develop selective and lasting bonds. Adult olfactory neurogenesis could be adapted to social behavior by supporting recognition of conspecifics. The present review reveals the distinctive features of the hippocampal, olfactory, and hypothalamic neurogenesis in sheep. In particular, the organization of the subventricular zone and the dynamic of neuronal maturation differs from that of rodents. In addition, we show that various physiological conditions, such as seasonal reproduction, gestation, and lactation differently modulate these three neurogenic niches. Last, we discuss recent evidence indicating that hypothalamic neurogenesis acts as an important regulator of the seasonal control of reproduction and that olfactory neurogenesis could be involved in odor processing in the context of maternal behavior.

Systemic effects after intravitreal injection of bevacizumab in new born rabbit eyes

PURPOSE

To determine the systemic impact of intravitreal injection of bevacizumab (IVB), an anti-vascular endothelium growth factor antibody, in newborn rabbits.

MATERIALS AND METHODS

We used four groups of rabbits. Group 1 rabbits received a single injection of IVB starting from the age of 6 weeks. Group 2 rabbits received a single injection of balanced salt solution (BSS, 0.025 ml) and served as controls for group 1. Group 3 rabbits received two consecutive injections of IVB at the ages of 6 and 10 weeks. Group 4 rabbits received two consecutive injections of BSS at the ages of 6 and 10 weeks and served as controls for group 3. During the experiment, a complete blood count (CBC), clinical biochemistry, weight gain, food intake, body temperature, blood pressure, pulse, and mortality were measured in the animals. Two months after IVB injection, the animals were sacrificed, and histology of the major organs was checked. Immunohistochemistry was assessed to explore the neurons in the central nervous system (CNS).

RESULTS

We found there were no morphological or functional changes in the eyes following IVB injection. Furthermore, there were no differences in CBC, biochemistry, or other measured parameters among the four groups of animals. We checked the histology of the major organs and neurons in the CNS and they did not reveal significant differences among the four groups of animals.

CONCLUSIONS

Conclusively, IVB of either one or two injections (0.625 mg) in newborn rabbit eyes is well tolerated and does not cause noticeable systemic organ pathology.

Non-neurogenic SVZ-like niche in dolphins, mammals devoid of olfaction

Adult neurogenesis has been implicated in brain plasticity and brain repair. In mammals, it is mostly restricted to specific brain regions and specific physiological functions. The function and evolutionary history of mammalian adult neurogenesis has been elusive so far. The largest neurogenic site in mammals (subventricular zone, SVZ) generates neurons destined to populate the olfactory bulb. The SVZ neurogenic activity appears to be related to the dependence of the species on olfaction since it occurs at high rates throughout life in animals strongly dependent on this function for their survival. Indeed, it dramatically decreases in humans, who do not depend so much on it. This study investigates whether the SVZ neurogenic site exists in mammals devoid of olfaction and olfactory brain structures, such as dolphins. Our results demonstate that a small SVZ-like region persists in these aquatic mammals. However, this region seems to have lost its neurogenic capabilities since neonatal stages. In addition, instead of the typical newly generated neuroblasts, some mature neurons were observed in the dolphin SVZ. Since cetaceans evolved from terrestrial ancestors, non-neurogenic SVZ may indicate extinction of adult neurogenesis in the absence of olfactory function, with the retention of an SVZ-like anatomical region either vestigial or of still unknown role.

Identification of novel cellular clusters define a specialized area in the cerebellar periventricular zone

The periventricular zone of cerebellum is a germinative niche during the embryonic development, nevertheless its structural organization and functional implications in adult have not been widely studied. Here we disclose the presence of two novel clusters of cells in that area. The first one was named the subventricular cellular cluster (SVCC) and is composed of cells that express glial and neuronal markers. The second was named the ventromedial cord (VMC) and appears as a streak of biciliated cells with microvillosities facing the ventricle, that includes GFAP⁺ and nestin⁺ cells organized along the periventricular vasculature. The dorsal limit of the SVCC is associated with myelinated axons of neurons of unknown origin. This paper describes the characteristics and organization of these groups of cells. They can be observed from late embryonic development in the transgenic mouse line GFAP-GFP. The SVCC and VMC expand during early postnatal development but are restricted to the central area of the ventricle in adulthood. We did not find evidence of cell proliferation, cell migration or the presence of fenestrated blood vessels. These findings provide new insights into the knowledge of the cellular composition and structural organization of the periventricular zone of cerebellum.

Post-hatching brain morphogenesis and cell proliferation in the pulse-type mormyrid Mormyrus rume proboscirostris

The anatomical organization of African Mormyrids' brain is a clear example of departure from the average brain morphotype in teleosts, probably related to functional specialization associated to electrosensory processing and sensory-motor coordination. The brain of Mormyrids is characterized by a well-developed rhombencephalic electrosensory lobe interconnected with relatively large mesencephalic torus semicircularis and optic tectum, and a huge and complex cerebellum. This unique morphology might imply cell addition from extraventricular proliferation zones up to late developmental stages. Here we studied the ontogeny of these brain regions in Mormyrus rume proboscirostris from embryonic to adult stages by classical histological techniques and 3D reconstruction, and analyzed the spatial-temporal distribution of proliferating cells, using pulse type BrdU labeling. Brain morphogenesis and maturation progressed in rostral-caudal direction, from 4day old free embryos, through larvae, to juveniles whose brain almost attained adult morphological complexity. The change in the relative size of the telencephalon, and mesencephalic and rhombencephalic brain regions suggest a developmental shift in the relative importance of visual and electrosensory modalities. In free embryos, proliferating cells densely populated the lining of the ventricular system. During development, ventricular proliferating cells decreased in density and extension of distribution, constituting ventricular proliferation zones. The first recognizable one was found at the optic tectum of free embryos. Several extraventricular proliferation zones were found in the cerebellar divisions of larvae, persisting along life. Adult M. rume proboscirostris showed scarce ventricular but profuse cerebellar proliferation zones, particularly at the subpial layer of the valvula cerebelli, similar to lagomorphs. This might indicate that adult cerebellar proliferation is a conserved vertebrate feature.

Forebrain neuroanatomy of the neonatal and juvenile dolphin (T. truncatus and S. coeruloalba)

Knowledge of dolphin functional neuroanatomy mostly derives from post-mortem studies and non-invasive approaches (i.e., magnetic resonance imaging), due to limitations in experimentation on cetaceans. As a consequence the availability of well-preserved tissues for histology is scarce, and detailed histological analyses are referred mainly to adults. Here we studied the neonatal/juvenile brain in two species of dolphins, the bottlenose dolphin (Tursiops truncatus) and the striped dolphin (Stenella coeruleoalba), with special reference to forebrain regions. We analyzed cell density in subcortical nuclei, white/gray matter ratio, and myelination in selected regions at different anterior–posterior levels of the whole dolphin brain at different ages, to better define forebrain neuroanatomy and the developmental stage of the dolphin brain around birth. The analyses were extended to the periventricular germinal layer and the cerebellum, whose delayed genesis of the granule cell layer is a hallmark of postnatal development in the mammalian nervous system. Our results establish an atlas of the young dolphin forebrain and, on the basis of occurrence/absence of delayed neurogenic layers, confirm the stage of advanced brain maturation in these animals with respect to most terrestrial mammals.

Noncanonical Sites of Adult Neurogenesis in the Mammalian Brain

Two decades after the discovery that neural stem cells (NSCs) populate some regions of the mammalian central nervous system (CNS), deep knowledge has been accumulated on their capacity to generate new neurons in the adult brain. This constitutive adult neurogenesis occurs throughout life primarily within remnants of the embryonic germinal layers known as "neurogenic sites." Nevertheless, some processes of neurogliogenesis also occur in the CNS parenchyma commonly considered as "nonneurogenic." This "noncanonical" cell genesis has been the object of many claims, some of which turned out to be not true. Indeed, it is often an "incomplete" process as to its final outcome, heterogeneous by several measures, including regional location, progenitor identity, and fate of the progeny. These aspects also strictly depend on the animal species, suggesting that persistent neurogenic processes have uniquely adapted to the brain anatomy of different mammals. Whereas some examples of noncanonical neurogenesis are strictly parenchymal, others also show stem cell niche-like features and a strong link with the ventricular cavities. This work will review results obtained in a research field that expanded from classic neurogenesis studies involving a variety of areas of the CNS outside of the subventricular zone (SVZ) and subgranular zone (SGZ). It will be highlighted how knowledge concerning noncanonical neurogenic areas is still incomplete owing to its regional and species-specific heterogeneity, and to objective difficulties still hampering its full identification and characterization.

Adult neurogenesis and brain remodelling after brain injury: From bench to bedside?

OBJECTIVE

Brain trauma and stroke cause important disabilities. The mechanisms involved are now well described, but all therapeutics developed thus far for neuro-protection are currently unsuccessful at improving neurologic prognosis. The recently studied neuro-restorative time following brain injury may point towards a promising therapeutic approach. The purpose of this paper is to explain the mechanisms of this revolutionary concept, give an overview of related knowledge and discuss its transfer into clinical practice.

DATA SOURCES AND SYNTHESIS

An overview of the neurogenesis concept using MEDLINE, EMBASE and CENTRAL databases was carried out in May 2014. The clinicaltrials.gov registry was used to search for ongoing clinical trials in this domain.

CONCLUSION

The concept of brain remodelling upset fundamental ideas concerning the neurologic system and opened new fields of research. Therapies currently under evaluation hold promising results and could have a real prognostic impact in future years, but the translation of these therapies from the laboratory to the clinic is still far from completion.

Adult hippocampal neurogenesis in natural populations of mammals

This review will discuss adult hippocampal neurogenesis in wild mammals of different taxa and outline similarities with and differences from laboratory animals. It begins with a review of evidence for hippocampal neurogenesis in various mammals, and shows the similar patterns of age-dependent decline in cell proliferation in wild and domesticated mammals. In contrast, the pool of immature neurons that originate from proliferative activity varies between species, implying a selective advantage for mammals that can make use of a large number of these functionally special neurons. Furthermore, rapid adaptation of hippocampal neurogenesis to experimental challenges appears to be a characteristic of laboratory rodents. Wild mammals show species-specific, rather stable hippocampal neurogenesis, which appears related to demands that characterize the niche exploited by a species rather than to acute events in the life of its members. Studies that investigate adult neurogenesis in wild mammals are not numerous, but the findings of neurogenesis under natural conditions can provide new insights, and thereby also address the question to which cognitive demands neurogenesis may respond during selection.

Regulation and function of adult neurogenesis: from genes to cognition

Adult neurogenesis in the hippocampus is a notable process due not only to its uniqueness and potential impact on cognition but also to its localized vertical integration of different scales of neuroscience, ranging from molecular and cellular biology to behavior. This review summarizes the recent research regarding the process of adult neurogenesis from these different perspectives, with particular emphasis on the differentiation and development of new neurons, the regulation of the process by extrinsic and intrinsic factors, and their ultimate function in the hippocampus circuit. Arising from a local neural stem cell population, new neurons progress through several stages of maturation, ultimately integrating into the adult dentate gyrus network. The increased appreciation of the full neurogenesis process, from genes and cells to behavior and cognition, makes neurogenesis both a unique case study for how scales in neuroscience can link together and suggests neurogenesis as a potential target for therapeutic intervention for a number of disorders.

Allopregnanolone and neurogenesis in the nigrostriatal tract

Reinstalling the neurobiological circuits to effectively change the debilitating course of neurodegenerative diseases is of utmost importance. This reinstallation requires generation of new cells which are able to differentiate into specific types of neurons and modification of the local environment suitable for integration of these new neurons into the neuronal circuits. Allopregnanolone (APα) seems to be involved in both of these processes, and therefore, is a potential neurotrophic agent. Loss of dopamine neurons in the substantia nigra (SN) is one of the main pathological features of Parkinson’s and also in, at least, a subset of Alzheimer’s patients. Therefore, reinstallation of the dopamine neurons in nigrostriatal tract is of unique importance for these neurodegenerative diseases. However, for the neurogenic status and the roles of allopregnanolone in the nigrostriatal tract, the evidence is accumulating and debating. This review summarizes recent studies regarding the neurogenic status in the nigrostriatal tract. Furthermore, special attention is placed on evidence suggesting that reductions in allopregnenalone levels are one of the major pathological features in PD and AD. This evidence has also been confirmed in brains of mice that were lesioned with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or those bearing neurodegenerative mutations. Lastly, we highlight studies showing that allopregnanalone can augment the number of total cells and dopaminergic neurons via peripheral exogenous administration.

Cerebellar stem cells do not produce neurons and astrocytes in adult mouse

Although previous studies implied that cerebellar stem cells exist in some adult mammals, little is known about whether these stem cells can produce new neurons and astrocytes. In this study by bromodeoxyuridine (BrdU) intraperitoneal (i.p.) injection, we found that there are abundant BrdU(+) cells in adult mouse cerebellum, and their quantity and density decreases significantly over time. We also found cell proliferation rate is diversified in different cerebellar regions. Among these BrdU(+) cells, very few are mash1(+) or nestin(+) stem cells, and the vast majority of cerebellar stem cells are quiescent. Data obtained by in vivo retrovirus injection indicate that stem cells do not produce neurons and astrocytes in adult mouse cerebellum. Instead, some cells labeled by retrovirus are Iba1(+) microglia. These results indicate that very few stem cells exist in adult mouse cerebellum, and none of these stem cells contribute to neurogenesis and astrogenesis under physiological condition.

Adult neural stem cells from the subventricular zone: a review of the neurosphere assay

The possibility of obtaining large numbers of cells with potential to become functional neurons implies a great advance in regenerative medicine. A source of cells for therapy is the subventricular zone (SVZ) where adult neural stem cells (NSCs) retain the ability to proliferate, self-renew, and differentiate into several mature cell types. The neurosphere assay, a method to isolate, maintain, and expand these cells has been extensively utilized by research groups to analyze the biological properties of aNSCs and to graft into injured brains from animal models. In this review we briefly describe the neurosphere assay and its limitations, the methods to optimize culture conditions, the identity and the morphology of aNSC-derived neurospheres (including new ultrastructural data). The controversy regarding the identity and "stemness" of cells within the neurosphere is revised. The fine morphology of neurospheres, described thoroughly, allows for phenotypical characterization of cells in the neurospheres and may reveal slight changes that indirectly inform about cell integrity, cell damage, or oncogenic transformation. Along this review we largely highlight the critical points that researchers have to keep in mind before extrapolating results or translating experimental transplantation of neurosphere-derived cells to the clinical setting.

Adult neurogenesis in eight Megachiropteran species

The present study evaluated, using immunohistochemical methods, the presence and characteristics of proliferating and newly generated neurons in the brain of eight wild-caught adult Megachiropteran species. For the neurogenic patterns observed, direct homologies are evident in other mammalian species; however, there were several distinctions in the presence or absence of proliferating and immature neurons, and migratory streams that provide important clues regarding the use of the brain in the analysis of Chiropteran phylogenetic affinities. In all eight species studied, numerous Ki-67- and doublecortin (DCX)-immunopositive cells were identified in the subventricular zone (SVZ). These cells migrated to the olfactory bulb through a Primate-like rostral migratory stream (RMS) that is composed of dorsal and ventral substreams which merge before entering the olfactory bulb. Some cells were observed emerging from the RMS coursing caudally and dorsally to the rostral neocortex. In the dentate gyrus of all species, Ki-67- and DCX-expressing cells were observed in the granular cell layer and hilus. Similar to Primates, proliferating cells and immature neurons were identified in the SVZ of the temporal horn of Megachiropterans. These cells migrated to the rostral and caudal piriform cortex through a Primate-like temporal migratory stream. Sparsely distributed Ki-67-immunopositive, but DCX-immunonegative, cells were identified in the tectum, brainstem and cerebellum. The observations from this study add to a number of neural characteristics that phylogenetically align Megachiropterans to Primates.

Neurogenesis and progenitor cell distribution in the subgranular zone and subventricular zone of the adult sheep brain

Progenitor cell proliferation is ubiquitous in the subventricular zone (SVZ) and subgranular zone (SGZ) of adult mammalian brains, however, the abundance and distribution of proliferation are surprisingly heterogeneous between species. In rodents, proliferation is high in both the SVZ and SGZ, while in humans proliferation is prominent in the SVZ but limited in the SGZ. To accurately study proliferation and how it changes in human disease, we should focus on animals in which the patterns of proliferation are consistent with the human brain. In this study, we characterized the neurogenic niches of the adult sheep, an animal model with a longer lifespan than rodents and a highly gyrencephalic brain, using 5-bromo-2'-deoxyuridine (BrdU) as a mitotic marker and neuronal nuclear antigen to identify neuronal lineage cells. Our study demonstrates that the sheep SVZ is organized into the same distinct layers that are comparable to what has been described in humans. The rate of maturation of new neurons was slower in sheep than in previous reports in rodents, with only 20% of BrdU-positive cells showing neuronal phenotype after 4 months survival following BrdU administration. Most importantly, as in the human, there was much greater proliferation in the sheep SVZ than in the SGZ. These results suggest that the sheep is a better basis for comparisons with human SVZ and SGZ neurogenesis than rodents.

The (real) neurogenic/gliogenic potential of the postnatal and adult brain parenchyma

During the last two decades basic research in neuroscience has remarkably expanded due to the discovery of neural stem cells (NSCs) and adult neurogenesis in the mammalian central nervous system (CNS). The existence of such unexpected plasticity triggered hopes for alternative approaches to brain repair, yet deeper investigation showed that constitutive mammalian neurogenesis is restricted to two small "neurogenic sites" hosting NSCs as remnants of embryonic germinal layers and subserving homeostatic roles in specific neural systems. The fact that in other classes of vertebrates adult neurogenesis is widespread in the CNS and useful for brain repair sometimes creates misunderstandings about the real reparative potential in mammals. Nevertheless, in the mammalian CNS parenchyma, which is commonly considered as "nonneurogenic," some processes of gliogenesis and, to a lesser extent, neurogenesis also occur. This "parenchymal" cell genesis is highly heterogeneous as to the position, identity, and fate of the progenitors. In addition, even the regional outcomes are different. In this paper the heterogeneity of mammalian parenchymal neurogliogenesis will be addressed, also discussing the most common pitfalls and misunderstandings of this growing and promising research field.

Altered cerebellar organization and function in monoamine oxidase A hypomorphic mice

Monoamine oxidase A (MAO-A) is the key enzyme for the degradation of brain serotonin (5-hydroxytryptamine, 5-HT), norepinephrine (NE) and dopamine (DA). We recently generated and characterized a novel line of MAO-A hypormorphic mice (MAO-A(Neo)), featuring elevated monoamine levels, social deficits and perseverative behaviors as well as morphological changes in the basolateral amygdala and orbitofrontal cortex. Here we showed that MAO-A(Neo) mice displayed deficits in motor control, manifested as subtle disturbances in gait, motor coordination, and balance. Furthermore, magnetic resonance imaging of the cerebellum revealed morphological changes and a moderate reduction in the cerebellar size of MAO-A(Neo) mice compared to wild type (WT) mice. Histological and immunohistochemical analyses using calbindin-D-28k (CB) expression of Purkinje cells revealed abnormal cerebellar foliation with vermal hypoplasia and decreased in Purkinje cell count and their dendritic density in MAO-A(Neo) mice compared to WT. Our current findings suggest that congenitally low MAO-A activity leads to abnormal development of the cerebellum.

Allopregnanolone reinstates tyrosine hydroxylase immunoreactive neurons and motor performance in an MPTP-lesioned mouse model of Parkinson's disease

Restorative/protective therapies to restore dopamine neurons in the substantia nigra pars compacta (SNpc) are greatly needed to effectively change the debilitating course of Parkinson's disease. In this study, we tested the therapeutic potential of a neurogenic neurosteroid, allopregnanolone, in the restoration of the components of the nigrostriatal pathway in MPTP-lesioned mice by measuring striatal dopamine levels, total and tyrosine hydroxylase immunoreactive neuron numbers and BrdU-positive cells in the SNpc. An acute treatment (once/week for two weeks) with allopregnanolone restored the number of tyrosine hydroxylase-positive and total cell numbers in the SNpc of MPTP-lesioned mice, even though this did not increase striatal dopamine. It was also noted that MPTP treated mice to which allopregnanolone was administered had an increase in BrdU-positive cells in the SNpc. The effects of allopregnanolone in MPTP-lesioned mice were more apparent in mice that underwent behavioral tests. Interestingly, mice treated with allopregnanolone after MPTP lesion were able to perform at levels similar to that of non-lesioned control mice in a rotarod test. These data demonstrate that allopregnanolone promotes the restoration of tyrosine hydroxylase immunoreactive neurons and total cells in the nigrostriatal tract, improves the motor performance in MPTP-treated mice, and may serve as a therapeutic strategy for Parkinson's disease.

Higher gamma-aminobutyric acid neuron density in the white matter of orbital frontal cortex in schizophrenia

BACKGROUND

In the orbitofrontal cortex (OFC), reduced gray matter volume and reduced glutamic acid decarboxylase 67kDa isoform (GAD67) messenger (m)RNA are found in schizophrenia; however, how these alterations relate to developmental pathology of interneurons is unclear. The present study therefore aimed to determine if increased interstitial white matter neuron (IWMN) density exists in the OFC; whether gamma-aminobutyric acid (GABA)ergic neuron density in OFC white matter was altered; and how IWMN density may be related to an early-expressed inhibitory neuron marker, Dlx1, in OFC gray matter in schizophrenia.

METHODS

IWMN densities were determined (38 schizophrenia and 38 control subjects) for neuronal nuclear antigen (NeuN+) and 65/67 kDa isoform of glutamic acid decarboxylase immunopositive (GAD65/67+) neurons. In situ hybridization was performed to determine Dlx1 and GAD67 mRNA expression in the OFC gray matter.

RESULTS

NeuN and GAD65/67 immunopositive cell density was significantly increased in the superficial white matter in schizophrenia. Gray matter Dlx1 and GAD67 mRNA expression were reduced in schizophrenia. Dlx1 mRNA levels were negatively correlated with GAD65/67 IWMN density.

CONCLUSIONS

Our study provides evidence that pathology of IWMNs in schizophrenia includes GABAergic interneurons and that increased IWMN density may be related to GABAergic deficits in the overlying gray matter. These findings provide evidence at the cellular level that the OFC is a site of pathology in schizophrenia and support the hypothesis that inappropriate migration of cortical inhibitory interneurons occurs in schizophrenia.

New neurons for 'survival of the fittest'

Adult neurogenesis is often considered an archaic trait that has undergone a 'phylogenetic reduction' from amphibian ancestors to humans. However, adult neurogenesis in the hippocampal dentate gyrus might actually be a late-evolved trait. In non-mammals, adult hippocampal neurogenesis is not restricted to the equivalents of the dentate gyrus, which also show different connectivity and functionality compared to their mammalian counterpart. Moving actively in a changing world and dealing with novelty and complexity regulate adult neurogenesis. New neurons might thus provide the cognitive adaptability to conquer ecological niches rich with challenging stimuli.

Adult neurogenesis in mammals--a theme with many variations

Investigations of adult neurogenesis in recent years have revealed numerous differences among mammalian species, reflecting the remarkable diversity in brain anatomy and function of mammals. As a mechanism of brain plasticity, adult neurogenesis might also differ due to behavioural specialization or adaptation to specific ecological niches. Because most research has focused on rodents and only limited data are available on other mammalian orders, it is hotly debated whether, in some species, adult neurogenesis also takes place outside of the well-characterized subventricular zone of the lateral ventricle and subgranular zone of the dentate gyrus. In particular, evidence for the functional integration of new neurons born in 'non-neurogenic' zones is controversial. Considering the promise of adult neurogenesis for regenerative medicine, we posit that differences in the extent, regional occurrence and completion of adult neurogenesis need to be considered from a species-specific perspective. In this review, we provide examples underscoring that the mechanisms of adult neurogenesis cannot simply be generalized to all mammalian species. Despite numerous similarities, there are distinct differences, notably in neuronal maturation, survival and functional integration in existing synaptic circuits, as well as in the nature and localization of neural precursor cells. We also propose a more appropriate use of terminology to better describe these differences and their relevance for brain plasticity under physiological and pathophysiological conditions. In conclusion, we emphasize the need for further analysis of adult neurogenesis in diverse mammalian species to fully grasp the spectrum of variation of this adaptative mechanism in the adult CNS.

Restrictions in time and space--new insights into generation of specific neuronal subtypes in the adult mammalian brain

Key questions in regard to neuronal repair strategies are which cells are best suited to regenerate specific neuronal subtypes and how much of a neuronal circuit needs to persist in order to allow its functional repair. Here we discuss recent findings in the field of adult neurogenesis, which shed new light on these questions. Neural stem cells in the adult brain generate very distinct types of neurons depending on their regional and temporal specification. Moreover, distinct brain regions differ in the mode of neuron addition in adult neurogenesis, suggesting that different brain circuits may be able to cope differently with the incorporation of new neurons. These new insights are then considered in regard to the choice of cells with the appropriate region-specific identity for repair strategies.

Heterogeneity and Fgf dependence of adult neural progenitors in the zebrafish telencephalon

Adult telencephalic neurogenesis is a conserved trait of all vertebrates studied. It has been investigated in detail in rodents, but very little is known about the composition of neurogenic niches and the cellular nature of progenitors in nonmammalian vertebrates. To understand the components of the progenitor zones in the adult zebrafish telencephalon and the link between glial characteristics and progenitor state, we examined whether canonical glial markers are colocalized with proliferation markers. In the adult zebrafish telencephalon, we identify heterogeneous progenitors that reside in two distinct glial domains. We find that the glial composition of the progenitor zone is linked to its proliferative behavior. Analyzing both fast-cycling proliferating cells as well as slowly cycling progenitors, we find four distinct progenitor types characterized by differential expression of glial markers. Importantly, a significant proportion of progenitors do not display typical radial glia characteristics. By blocking or activating Fgf signaling by misexpression of a dominant negative Fgf-receptor 1 or Fgf8a, respectively, we find that ventral and dorsal progenitors in the telencephalon also differ in their requirement for Fgf signaling. Together with data on the expression of Fgf signaling components in the ventricular zone of the telencephalon, this suggests that Fgf signaling directly regulates proliferation of specific subsets of adult telencephalic progenitors in vivo. Taken together our results show that adult neural progenitor cells are heterogeneous with their respect to distribution into two distinct glial domains and their dependence upon Fgf signaling as a proliferative cue in the zebrafish telencephalon.

Differential expression of FABP 3, 5, 7 in infantile and adult monkey cerebellum

To clarify the involvement of fatty acid binding proteins (FABPs) in cerebellar development and function, we explored the distribution of three brain-expressed FABPs, FABP 3, 5 and 7, by comparing three animal groups--infantile, normal and postischemic adult monkeys. Immunoblotting analysis revealed intense expression of FABP 3 and 7, but not of FABP5, in the control and postischemic adult cerebellum. The protein levels of FABP7, but not of FABP 3 or 5, gradually increased until 2 weeks after the insult. Immunohistochemical analysis showed that cerebellar FABP3-positive cells were Purkinje cells and Bergmann glia. FABP5-positive cells were found only in the postischemic cerebellum, and were identified as activated microglia. Interestingly, in the infantile cerebellum, both the granule cell progenitors in the external granular layer (EGL) and the oligodendrocyte progenitors in the internal granular layer (IGL) expressed FABP5. In the adult cerebellum, FABP7 was expressed in Purkinje cells and basket interneurons, while in the infantile cerebellum it was also found in Bergmann glia. These results showed differential expression of FABPs in cerebellar neuronal and glial cell types; FABP 3 and 7 were predominantly expressed in normal cerebellum, FABP5 after ischemic injury, while FABP 3, 5 and 7 were expressed during cerebellar development.

Anatomic changes in multiple brainstem nuclei after incremental, near-complete neurotoxic destruction of the pre-Bötzinger Complex in adult goats

Abrupt, bilateral destruction of the pre-Bötzinger Complex (preBötC) leads to terminal apnea in unanesthetized goats and rats. In contrast, respiratory rhythm and pattern and arterial blood gases in goats during wakefulness and sleep are normal after incremental (over a month) destruction of > 90% of the preBötC. Here, we tested the hypothesis that the difference in effects between abrupt and incremental destruction of the preBötC are a result of time-dependent plasticity, which manifests as anatomic changes at sites within the respiratory network. Accordingly, we report data from histological analyses comparing the brainstems of control goats, and goats that had undergone bilateral, incremental, ibotenic acid (IA)-induced preBötC lesioning. A major focus was on the parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN) and the pontine respiratory group (PRG), which are sites thought to contribute to respiratory rhythmogenesis. We also studied the facial (FN), rostral nucleus ambiguus (NA), medullary raphé (MRN), hypoglossal (HN), and the dorsal motor vagal (DMV) nuclei. Neuronal counts, count region area (mm²), and neuronal densities were calculated using computer-assisted analyses and/or manual microscopy to compare control and preBötC-lesioned animals. We found that within the ventral and lateral medulla 2mm rostral to the caudal pole of the FN (presumed pFRG/RTN), there were 25% and 65% more (P < 0.001) neurons, respectively, in preBötC-lesioned compared to control goats. Lesioned goats also showed 14% and 13% more (P < 0.001) neurons in the HN and medial parabrachialis nucleus, but 46%, 28%, 7%, and 17% fewer (P < 0.001) neurons in the FN, NA, DMV, and Kölliker-Fuse nuclei, respectively. In the remaining sites analyzed, there were no differences between groups. We conclude that anatomic changes at multiple sites within the respiratory network may contribute to the time-dependent plasticity in breathing following incremental and near-complete destruction of the preBötC.

DCX and PSA-NCAM expression identifies a population of neurons preferentially distributed in associative areas of different pallial derivatives and vertebrate species

In adult rodents, doublecortin (DCX) and polysialylated neural cell adhesion molecule (PSA-NCAM) expression is mostly restricted to newly generated neurons. These molecules have also been described in prenatally generated cells of the piriform cortex and, to a lesser extent, neocortex (NC) of the rat. In addition, PSA-NCAM+ cells have been identified in several telencephalic regions of the lizard. Here, through immunohistochemistry and 3-dimensional reconstruction, we have investigated distribution, morphology, and phenotype of DCX/PSA-NCAM-expressing cells in the pallium of different mammals and in lizard. In all species, a population of nonnewly-generated pallial DCX+/PSA-NCAM+ cells shows common morphological and phenotypic characteristics, including expression of Tbr-1, a transcription factor expressed in pallial projection neurons, and preferential distribution in associative areas. In the guinea pig and rabbit, DCX+/PSA-NCAM+ elements are also abundant in the NC, particularly in areas implicated in nonspatial learning and memory networks. In reptiles, DCX+/PSA-NCAM+ cells are located in the lateral and medial cortex and dorsal ventricular ridge but not in the dorsal cortex. These data support the fact that coexpression of DCX+/PSA-NCAM+/Tbr-1+ in the adult brain identifies evolutionary conserved cell populations shared by different pallial derivatives including the mammalian NC.

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.

Fluoxetine-induced proliferation and differentiation of neural progenitor cells isolated from rat postnatal cerebellum

Previous studies have shown that the serotonin-reuptake inhibitor (SSRI) fluoxetine affects neural progenitors derived from postnatal cerebellum or adult hippocampus and stimulates their proliferation. In the human cerebellum, the proliferation of cerebellar granule cells (CGC) continues until the 11th postnatal month and could be influenced in infants by breastfeeding-delivered SSRIs. Current information about fluoxetine effects on postnatal cerebellar neural progenitors is limited. Here we report the characterization of fluoxetine actions on rat postnatal cerebellar neural progenitors. RT-PCR and immunostaining revealed the expression of serotonin transporter (SERT), 5HT(1A) receptors, tryptophan hydroxylase (TPH), and serotonin (5HT). Protracted in vitro fluoxetine treatment increased cell proliferation and differentiation. The proliferative effects of fluoxetine, 5HT, and the selective agonist of 5HT(1A) receptors trans-8-hydroxy-2-(N-n-propyl-N-3'-iodo-2'-propenyl)aminotetralin (8-OH-PIPAT) were abolished by the selective antagonist of 5HT(1A) receptors, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY-100635). Furthermore, fluoxetine-induced activation of both the cAMP-response element-binding (CREB) protein and extracellular signal-regulated protein kinases (ERK1/2), which was abolished by the selective inhibitor of MAP kinase kinase (MEK) 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (U0126), and increased cyclin D1 expression. All these effects were prevented by WAY-100635. Collectively, our results demonstrate that rat postnatal cerebellum contains neural progenitors capable of proliferating and differentiating in response to fluoxetine exposure, possibly through the activation of 5HT(1A) receptors. The relevance of these findings for possible SSRI effects on the developing postnatal/infant human cerebellum should be explored.