Doublecortin expression levels in adult brain reflect neurogenesis

Progress in the field of neurogenesis is currently limited by the lack of tools enabling fast and quantitative analysis of neurogenesis in the adult brain. Doublecortin (DCX) has recently been used as a marker for neurogenesis. However, it was not clear whether DCX could be used to assess modulations occurring in the rate of neurogenesis in the adult mammalian central nervous system following lesioning or stimulatory factors. Using two paradigms increasing neurogenesis levels (physical activity and epileptic seizures), we demonstrate that quantification of DCX-expressing cells allows for an accurate measurement of modulations in the rate of adult neurogenesis. Importantly, we excluded induction of DCX expression during physiological or reactive gliogenesis and excluded also DCX re-expression during regenerative axonal growth. Our data validate DCX as a reliable and specific marker that reflects levels of adult neurogenesis and its modulation. We demonstrate that DCX is a valuable alternative to techniques currently used to measure the levels of neurogenesis. Importantly, in contrast to conventional techniques, analysis of neurogenesis through the detection of DCX does not require in vivo labelling of proliferating cells, thereby opening new avenues for the study of human neurogenesis under normal and pathological conditions.     

NMDA receptor antagonist treatment induces a long-lasting increase in the number of proliferating cells, PSA-NCAM-immunoreactive granule neurons and radial glia in the adult rat dentate gyrus

During adulthood, neural precursors located in the subgranular zone of the dentate gyrus continue to proliferate, leading to the generation of new granule neurons. These recently generated cells transiently express the polysialylated form of the neural cell adhesion molecule, PSA-NCAM, and are supported by radial glia-like cells that are likely to play a role in neuronal migration and differentiation, or even act as their precursors. Previous reports indicate that treatment with NMDA receptor antagonists stimulates adult neurogenesis in the dentate gyrus, and because of the potential therapeutic value of this approach, we were interested in further characterizing the consequences of pharmacologically modulating this process. We treated adult rats with the competitive NMDA receptor antagonist, CGP43487, and examined cell proliferation, PSA-NCAM expression, and changes in the radial glia cell population in the subgranular zone at different time points. In addition, we sought to determine if this treatment led to changes in cell death or gliotic reactions. The number of proliferating cells in the subgranular region of the dentate gyrus was increased significantly 2 days after treatment and it remained elevated 7 days postinjection. PSA-NCAM-immunoreactive granule cells and nestin-expressing radial glia-like cells also increased in number 7 days after the treatment. In contrast, we did not observe any change in granule cell death, and we were unable to detect any microglial or astroglial reaction during the first 7 days after treatment. Thus, NMDA receptor antagonist treatment serves as a valuable tool to increase neurogenesis in the adult hippocampus without undesirable collateral deleterious effects.     

Specific characteristic of radial glia in the human fetal telencephalon

Phenotypic characteristics of cells in the developing human telencephalic wall were analyzed using electron microscopy and immunocytochemistry with various glial and neuronal cell markers. The results suggest that multiple defined cell types emerge in the neocortical proliferative zones and are differentially regulated during embryonic development. At 5-6 weeks gestation, three major cell types are observed. Most proliferating ventricular zone (VZ) cells are labeled with radial glial (RG) markers such as vimentin, glial fibrillary acidic protein (GFAP), and glutamate astrocyte-specific transporter (GLAST) antibodies. A subpopulation of these RG cells also express the neuronal markers beta III-tubulin, MAP-2, and phosphorylated neurofilament SMI-31, in addition to the stem cell marker nestin, indicating their multipotential capacity. In addition, the presence of VZ cells that immunoreact only with neuronal markers indicates the emergence of restricted neuronal progenitors. The number of multipotential progenitors in the VZ gradually decreases, whereas the number of more restricted progenitors increases systematically during the 3-month course of human corticogenesis. These results suggest that multipotential progenitors coexist with restricted neuronal progenitors and RG cells during initial corticogenesis in the human telencephalon. Since the multipotential VZ cells disappear during the major wave of neocortical neurogenesis, the RG and restricted neuronal progenitors appear to serve as the main sources of cortical neurons. Thus, the diversification of cells in human VZ and overlying subventricular zone (SVZ) begins earlier and is more pronounced than in rodents.     

Doublecortin expression in the normal and epileptic adult human brain

Mesial temporal lobe epilepsy (MTLE) is a neurological disorder associated with spontaneous recurrent complex partial seizures and hippocampal sclerosis. Although increased hippocampal neurogenesis has been reported in animal models of MTLE, increased neurogenesis has not been reported in the hippocampus of adult human MTLE cases. Here we showed that cells expressing doublecortin (Dcx), a microtubule‐associated protein expressed in migrating neuroblasts, were present in the hippocampus and temporal cortex of the normal and MTLE adult human brain. In particular, increased numbers of Dcx‐positive cells were observed in the epileptic compared with the normal temporal cortex. Importantly, 56% of Dcx‐expressing cells in the epileptic temporal cortex coexpressed both the proliferative cell marker, proliferating cell nuclear antigen and early neuronal marker, TuJ1, suggesting that they may be newly generated neurons. A subpopulation of Dcx‐positive cells in the epileptic temporal cortex also coexpressed the mature neuronal marker, NeuN, suggesting that epilepsy may promote the generation of new neurons in the temporal cortex. This study has identified, for the first time, a novel population of Dcx‐positive cells in the adult human temporal cortex that can be upregulated by epilepsy and thus, raises the possibility that these cells may have functional significance in the pathophysiology of epilepsy.     

Cells expressing markers of immature neurons in the amygdala of adult humans

The polysialylated form of the neuronal cell adhesion molecule (PSA‐NCAM) is expressed by immature neurons in the amygdala of adult mammals, including non‐human primates. In a recent report we have also described the presence of PSA‐NCAM‐expressing cells in the amygdala of adult humans. Although many of these cells have been classified as mature interneurons, some of them lacked mature neuronal markers, suggesting the presence of immature neurons. We have studied, using immunohistochemistry, the existence and distribution of these immature neurons using post mortem material. We have also analysed the presence of proliferating cells and the association between immature neurons and specialised astrocytes. These parameters have also been studied for comparative purposes in the amygdalae of cats and squirrel monkeys. Our results demonstrate that cells coexpressing doublecortin and PSA‐NCAM, but lacking neuronal nuclear antigen expression, were present in the amygdala of adult humans. These cells were organised in elongated clusters, which were located between the white matter of the dorsal hippocampus and the basolateral amygdaloid nucleus. These clusters were not associated with astroglial specialised structures. No cells expressing the proliferative marker Ki67 were observed in the amygdaloid parenchyma, although some of them were found in the vicinity of the lateral ventricle. Immature neurons were also present in the amygdala of squirrel monkeys and cats. These cells also appeared clustered in monkeys, although not as organised as in humans. In cats these cells are scarce, appear isolated and most of the PSA‐NCAM‐expressing structures corresponded to processes apparently originating from the paleocortical layer II.     

Roles of astrocytes and microglia in seizure‐induced aberrant neurogenesis in the hippocampus of adult rats

Recent evidence showed that epileptic seizures increase hippocampal neurogenesis in the adult rat, but prolonged seizures result in the aberrant hippocampal neurogenesis that often leads to a recurrent excitatory circuitry and thus contributes to epileptogenesis. However, the mechanism underlying the aberrant neurogenesis after prolonged seizures remains largely unclear. In this study, we examined the role of activated astrocytes and microglia in the aberrant hippocampal neurogenesis induced by status epilepticus. Using a lithium‐pilocarpine model to mimic human temporal lobe epilepsy, we found that status epilepticus induced a prominent activation of astrocytes and microglia in the dentate gyrus 3, 7, 14, and 20 days after the initial seizures. Then, we injected fluorocitrate stereotaxicly into the dentate hilus to inhibit astrocytic metabolism and found that fluorocitrate failed to prevent the seizure‐induced formation of ectopic hilar basal dendrites but instead promoted the degeneration of dentate granule cells after seizures. In contrast, a selective inhibitor of microglia activation, minocycline, inhibited the aberrant migration of newborn neurons at 14 days after status epilepticus. Furthermore, with stereotaxic injection of lipopolysaccharide into the intact dentate hilus to activate local microglia, we found that lipopolysaccharide promoted the development of ectopic hilar basal dendrites in the hippocampus. These results indicate that the activated microglia in the epileptic hilus may guide the aberrant migration of newborn neurons and that minocycline could be a potential drug to impede seizure‐induced aberrant migration of newborn neurons. © 2009 Wiley‐Liss, Inc.     

Differential expression of nuclear lamin, the major component of nuclear lamina, during neurogenesis in two germinal regions of adult rat brain

Lamins are major structural proteins of the nuclear envelope. Three lamin subtypes, A/C, B1 and B2, predominate in mammalian somatic cells. While the expression levels of lamins in several tissues are known to change during cell differentiation, lamin expression is poorly understood in the nervous system. To investigate the expression of lamins during neuronal differentiation in the mammalian adult brain, we performed immunohistochemical studies on lamins A/C, B1 and B2 in two neurogenic regions of rat brain: the subgranular zone of the dentate gyrus and the subventricular zone of the lateral ventricle. In particular, three types of cells were analysed using confocal microscopy: GFAP-positive cells as primary progenitor (stem) cells, PSA-NCAM-positive cells as subsequent neuronal progenitor cells, and NeuN-positive mature neurons. GFAP-positive cells possesed lamin A/C (++), B1 (++) and B2 (++), PSA-NCAM-positive cells possessed lamin A/C (-), B1 (+++) and B2 (+), and mature neurons possessed lamin A/C (++), B1 (+) and B2 (+++), in both neurogenic regions. These observations showed that the compositions of expressing lamin subtypes are distinct in particular differentiation stages during neurogenesis in the adult rat brain. Our results suggest that the alteration of nuclear lamina structure is coupled with the progression of neuronal differentiation.     

Lesion induced new neuron incorporation in the adult hypothalamus of the avian brain

Cell loss in most adult vertebrate brain regions is thought to be irreversible. Here, we explore the effects of electrolytic lesions on the induction of cell proliferation and newborn neurons in the ventromedial nuclei (VMN) of the hypothalamus in young and adult ring doves. The hypothalamus does not normally recruit new neurons. Bromodeoxyuridine (BrdU) and tritiated thymidine ([3H]Thy) were used to identify cells born before and after bilateral electrolytic lesions. Hu and NeuN were used to identify neurons. TUNEL test for apoptosis and 3A7 antibodies were used to identify morphological changes of pre-existing cells. Lesions produced significantly more newborn cells in the subventricular zone (SVZ). The rate of cell proliferation peaked at 7-14 days postlesion. A fraction of these newborn cells were neuronal precursor and began to migrate away along the radial glial fibers 2 weeks after lesion. During this period, the outer area of the lesion site was marked with massive apoptosis and re-expression of radial glial-like fibers. In birds that survived 5 months, we found newly differentiated neurons in the outer area of the lesion site. We conclude that electrolytic lesion can invoke neuronal recruitment in the adult hypothalamus. We further suggest that lesion-induced apoptosis and re-expression of developmental mechanisms might be involved in the recruitment process.     

Neurotransmitter phenotype plasticity in cultured dissociated adult rat dorsal root ganglia: an immunocytochemical study

Culturing sympathetic ganglion neurons in vitro may modify phenotypic expression of some neurotransmitters. For dorsal root ganglia (DRG), contradictory results have been reported; most studies have used immature material. We have therefore performed a detailed immunocytochemical analysis of the transmitter content of cultured adult rat DRG neurons. To demonstrate possible modifications of neurotransmitter phenotypes, we have compared the results obtained with the same techniques on neurons cultured for 3 days and on freshly dissociated DRG cells. Also, the transmitter profile of cultured neurons was compared with that known from in situ studies. Out of 22 antigens studied, 20 were detected in cultured DRG neurons. All of them were expressed in small and/or intermediate-sized cells. Large neurons only contained CGRP, VIP, NPY, beta-END, ENK, and GABA. The percentage of immunostained neurons varied for the various antisera: less than 10% of cultured neurons were positive for ENK, beta-LPH, beta-END, DYN, VASO, and OXY; 10-30% for SOM, CCK, CAT, and SP; and greater than 30% for NPY, CRF, GLU, NT, VIP, GABA, GRP, CGRP, 5-HT, and TRH. In the latter two groups of transmitters (except CGRP), the proportion of immunoreactive neurons was by far larger in cultured than in freshly dissociated DRG. The most pronounced (greater than 25%) increase in the proportion of positively stained neurons after culturing was observed for the GRP, CRF, TRH, and 5-HT antisera. Serotonin was the only transmitter identified in cultured but not in freshly dissociated cells. These data indicate, on one hand, that various antigens, for example, CAT, GABA, NT, TRH, NPY, beta-LPH, and beta-END, which up to now have not been described in DRG in situ, can be detected immunocytochemically a few hours after dissociation of adult rat DRG. On the other hand, several transmitters, for example, VIP, NPY, SP, GABA, GLU, NT, GRP, CRF, TRH, and 5-HT, are expressed in a significantly higher proportion of cells in cultured than in freshly dissociated preparations. This might reflect a change in the phenotypic expression of transmitters due to the new environment generated by the culture conditions, a hypothesis that can be tested by measuring specific mRNA levels. Moreover, considering the plasticity and multipotentiality of their transmitter phenotype, cultured adult DRG neurons might represent an interesting material for autografts into the injured central nervous system.     

Adrenalectomy increases neurogenesis but not PSA-NCAM expression in aged dentate gyrus

Ageing is accompanied by a decline in neurogenesis and in polysialylated isoforms of neural cell adhesion molecule (PSA-NCAM) expression within the hippocampus and by elevated basal levels of circulating corticosterone. In a companion study, we demonstrated that suppression of corticosterone by adrenalectomy increased neurogenesis and PSA-NCAM expression in the dentate gyrus of adult rats. Here we show that adrenalectomy increased neurogenesis in this structure in old rats, as measured by the incorporation of 5-bromo-2'-deoxyuridine in neuronal progenitors. This effect was prevented by corticosterone replacement. In contrast, PSA-NCAM expression remained unchanged in comparison with controls. Thus, in the aged brain, stem cells are still present and able to enter the cell cycle. This may point to ways of protecting or treating age-related cognitive impairments.     

Heterogeneity in progenitor cell subtypes in the ventricular zone of the zebrafish adult telencephalon

The zebrafish has become a new model for adult neurogenesis, owing to its abundant neurogenic areas in most brain subdivisions. Radial glia‐like cells, actively proliferating cells, and label‐retaining progenitors have been described in these areas. In the telencephalon, this complexity is enhanced by an organization of the ventricular zone (VZ) in fast and slow‐dividing domains, suggesting the existence of heterogeneous progenitor types. In this work, we studied the expression of various transgenic or immunocytochemical markers for glial cells (gfap:gfp, cyp19a1b:gfp, BLBP, and S100β), progenitors (nestin:gfp and Sox2), and neuroblasts (PSA‐NCAM) in cycling progenitors of the adult zebrafish telencephalon (identified by expression of proliferating cell nuclear antigen (PCNA), MCM5, or bromodeoxyuridine incorporation). We demonstrate the existence of distinct populations of dividing cells at the adult telencephalic VZ. Progenitors of the overall slow‐cycling domains express high levels of Sox2 and nestin:gfp as well as all glial markers tested. In contrast, domains with an overall fast division rate are characterized by low or missing expression of glial markers. PCNA‐positive cells in fast domains further display a morphology distinct from radial glia and co‐express PSA‐NCAM, suggesting that they are early neuronal precursors. In addition, the VZ contains cycling progenitors that express neither glial markers nor nestin:gfp, but are positive for Sox2 and PSA‐NCAM, identifying them as committed neuroblasts. On the basis of the marker gene expression and distinct cell morphologies, we propose a classification for the dividing cell states at the zebrafish adult telencephalic VZ. © 2010 Wiley‐Liss, Inc.     

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. © 2010 Wiley‐Liss, Inc.     

Proliferation, migration and differentiation of neuronal progenitor cells in the adult mouse subventricular zone surgically separated from its olfactory bulb

The subventricular zone of the adult mammalian forebrain contains progenitor cells that, by migrating along a restricted pathway called the ‘rostral migratory stream’ (RMS), add new neurons to the olfactory bulb throughout life. To determine the influence of the olfactory bulb on the development of these progenitor cells, we performed lesions that interrupt this pathway and separate the olfactory bulb from the rest of the forebrain. By labelling cells born at several survival times after the lesions with the thymidine analogue bromodeoxyuridine (BrdU), we found that disconnection from the bulb influences the rate of BrdU incorporation by the progenitor cells. The number of labelled cells in lesioned mice was almost half that found in control mice. In the disconnected migratory pathway, the number of neurons expressing calretinin was increased indicating that neuronal differentiation was enhanced: newly born neurons occurred within and around the RMS, most of them expressed calretinin and left the pathway starting about 2 weeks after the lesion. Thereafter, these neurons preserving their phenotype, spread for long distances, and accumulated ectopically in dorsal regions of the anterior olfactory nucleus and the frontal cortex. Finally, transplantation of adult subventricular cells into the lesioned pathway showed that the lesion neither prevents neuronal migration nor alters its direction. Thus, although the olfactory bulb appears to regulate the pace of the developmental processes, its disconnection does not prevent the proliferation, migration and phenotypic acquisition of newly generated bulbar interneurons that, since they cannot reach their terminal domains, populate some precise regions of the lesioned adult forebrain.     

脑出血后微管相关蛋白Doublecortin的表达及其意义

目的 研究成年大鼠脑出血模型中发生的神经细胞再生现象及其特点.方法 应用雄性Sprague-Dawley大鼠制作脑出血模型,采用蛋白印迹分析、免疫组化染色等方法 对5-溴脱氧尿嘧啶核苷(Bromodeoxyuridine,Brdu)及未成熟神经元标记doublecortin(DCX)等进行分析.结果 DCX的表达于脑出血后7d开始增加,2周后同侧脑室下区(SVZ)及基底节区可见大量DCX阳性细胞,其表达达到高峰,为对照组的(793±192)%(P＜0.01).结论 成年大鼠中SVZ的神经元前体细胞在脑出血后明显增生,并向受损伤区域移行.     

Migration of neuronal precursors from the telencephalic ventricular zone into the olfactory bulb in adult zebrafish

In the brain of adult mammals, neuronal precursors are generated in the subventricular zone in the lateral wall of the lateral ventricles and migrate into the olfactory bulbs (OBs) through a well-studied route called the rostral migratory stream (RMS). Recent studies have revealed that a comparable neural stem cell niche is widely conserved at the ventricular wall of adult vertebrates. However, little is known about the migration route of neuronal precursors in nonmammalian adult brains. Here, we show that, in the adult zebrafish, a cluster of neuronal precursors generated in the telencephalic ventricular zone migrates into the OB via a route equivalent to the mammalian RMS. Unlike the mammalian RMS, these neuronal precursors are not surrounded by glial tubes, although radial glial cells with a single cilium lined the telencephalic ventricular wall, much as in embryonic and neonatal mammals. To observe the migrating neuronal precursors in living brain tissue, we established a brain hemisphere culture using a zebrafish line carrying a GFP transgene driven by the neurogenin1 (ngn1) promoter. In these fish, GFP was observed in the neuronal precursors migrating in the RMS, some of which were aligned with blood vessels. Numerous ngn1:gfp-positive cells were observed migrating tangentially in the RMS-like route medial to the OB. Taken together, our results suggest that the RMS in the adult zebrafish telencephalon is a functional migratory pathway. This is the first evidence for the tangential migration of neuronal precursors in a nonmammalian adult telencephalon.     

The dynamics of adult neurogenesis in human hippocampus

The phenomenon of adult neurogenesis is now an accepted occurrence in mammals and also in humans.At least two discrete places house stem cells for generation of neurons in adult brain. These are olfactory system and the hippocampus. In animals, newly generated neurons have been directly or indirectly demonstrated to generate a significant amount of new neurons to have a functional role. However, the data in humans on the extent of this process is still scanty and such as difficult to comprehend its functional role in humans. This paper explores the available data on as extent of adult hippocampal neurogenesis in humans and makes comparison to animal data.     

Newly generated granule cells show rapid neuroplastic changes in the adult rat dentate gyrus during the first five days following pilocarpine-induced seizures

Long-term neuroplastic changes to dentate granule cells have been reported after seizures and were shown to contribute to recurrent excitatory circuitry. These changes include increased numbers of newborn granule cells, sprouted mossy fibers, granule cell layer dispersion, increased hilar ectopic granule cells and formation of hilar basal dendrites on granule cells. The goal of the current study was to determine the acute progression of neuroplastic changes involving newly generated granule cells after pilocarpine-induced seizures. Doublecortin (DCX) immunocytochemical preparations were used to examine the newly generated granule cells 1-5 days after seizures were induced. The results showed that there are rapid neuroplastic changes to the DCX-labeled cells. At 1 day after seizures were induced, there were significant increases in the percentage of DCX-labeled cells with hilar basal dendrites and in the progenitor cell population. At 2 days after seizures were induced, an increase in the thickness of the layer of DCX-labeled cells occurred. At 3 days after seizures were induced, the number of DCX-labeled cells was significantly increased. At 4 days after seizures were induced, developing synapses were observed on DCX-labeled hilar basal dendrites. Thus, newly generated granule cells in the adult dentate gyrus display neuroplastic changes by 1 day after pilocarpine-induced seizures and further changes occur to this population of cells in the subsequent 4 days. The presence of synapses, albeit developing ones, on hilar basal dendrites during this period indicates that newly generated granule cells become rapidly incorporated into dentate gyrus circuitry following seizures.     

Expression of synapsin III in nerve terminals and neurogenic regions of the adult brain

We have examined the distribution of synapsin III in the adult mouse brain. Expression of synapsin III was observed in puncta throughout the brain, but demonstrated greater regional variation than that of synapsins I or II. This punctate staining is typical for synaptic vesicle proteins located at nerve terminals. These findings are also consistent with the well-established role for synapsins in regulating neurotransmitter release. However, unexpectedly, synapsin III was also highly expressed in the cell body and processes of immature neurons in neurogenic regions of the adult brain, such as the hippocampal dentate gyrus, rostral migratory stream, and olfactory bulb. Many synapsin III-positive neurons also reacted with an antibody directed toward polysialylated-neuronal cell adhesion molecule, a marker of immature, migrating neurons. These results suggest that synapsin III may also play a role in adult neurogenesis.     

Long-term survival and cell death of newly generated neurons in the adult rat olfactory bulb

In the adult rat olfactory bulb, neurons are continually generated from progenitors that reside in the lateral ventricle wall. This study investigates long-term survival and cell death of newly generated cells within the adult olfactory bulb. After injecting rats at 2 months of age with 5-bromodeoxyuridine (BrdU), the newly generated cells were quantified over a period of 19 months. A peak of BrdU-positive cells was reached in the olfactory bulb 1 month after BrdU injection, when all new cells have finished migrating from the ventricle wall. Thereafter, a reduction of BrdU-positive cells to about 50% was observed and it was confirmed by dUTP-nick end-labelling (TUNEL) that progenitors and young neurons undergo programmed cell death. However, cells that survived the first 3 months after BrdU injection persisted for up to 19 months. The majority of the BrdU-positive cells that reach the olfactory bulb differentiate into granule cells, but a small fraction migrate further into the glomerular layer. These newborn cells differentiate more slowly into periglomerular interneurons, with a delay of more than 1 month when compared to the granule cells. The newly generated periglomerular neurons, among them a significant fraction of dopaminergic cells, showed a similar decline in number compared to the granule cell layer and long-term survival for the remaining new neurons of up to 19 months. Rather than replacing old neurons, this data suggests that adult olfactory bulb neurogenesis utilizes the overproduction and turnover of young neurons, which is reminiscent of the cellular dynamics observed during brain development.