Genetic regulation of dentate gyrus morphogenesis

The dentate gyrus is one of the small number of forebrain areas that have continued adult neurogenesis. During development the dentate gyrus acquires the capacity for neurogenesis by generating a new neurogenic stem cell niche at the border between the hilus and dentate granule cell layer. This is in distinction to the other prominent zone of continued neurogenesis in the subventricular zone where neurons are born in a structure directly descended from the mid-gestation subventricular zone. The ability to generate this newly formed dentate neurogenic niche is controlled by the action of a number of genes during prenatal and early postnatal development that regulate the fate, survival, migration, expansion, and differentiation of the cellular components of the dentate neurogenic niche. In this review, we provide an updated framework discussing the molecular steps and genes involved in these early stages of dentate gyrus formation. We previously described a molecular framework for dentate gyrus morphogenesis that can be associated with specific gene defects (Li, G., Pleasure, S.J. (2005). Dev. Neurosci., 27, 93-99), and here we add additional recently described molecular players and discuss this framework.

Expression of Sox11 and Brn transcription factors during development and following transient forebrain ischemia in the rat

Sox11 is a transcription factor that is proposed to be involved in the development and regeneration of the brain [M.P. Jankowski, P.K. Cornuet, S. Mcllwrath, H.R. Koerber, K.M. Albers, SRY-box containing gene 11 (Sox11) transcription factor is required for neuron survive and neurite growth, Neuroscience 143 (2006) 501-514]. In this study, we compared the expression patterns of Sox11 and its two putative binding partners, Brn1 and Brn2 during development and following transient forebrain ischemia in the rat. The spatiotemporal expression pattern of Brn1 was similar to that of Sox11 from the late embryonic to postnatal development, and they are strongly expressed in the brain regions where neuronal progenitors and immature neurons are enriched. On the other hand, Brn2 was ubiquitously expressed in most tissues including developing nervous system. Neuronal depolarization of cerebral cortex neurons in vitro enhanced both Sox11 and Brn1 expression, whereas the induction of Brn2 was only marginal, further suggesting the similar transcriptional modulation of Sox11 and Brn1. In the hippocampus, however, they showed a little different expression patterns. The expression of Brn1 was not substantial in developing dentate gyrus (DG) where Sox11 expression was strong. The transient forebrain ischemia enhanced Sox11 gene expression moderately in the CA1 and strongly in the DG, whereas Brn1 was selectively induced only in the CA1 of the hippocampal formation. Collectively, overall results suggest that the expression of Sox11 and Brn1 may be modulated by the cell-type specific machinery.

Conditional knock-out of beta-catenin in postnatal-born dentate gyrus granule neurons results in dendritic malformation

Neurons are continuously added to the brain throughout life, and these neurons must develop dendritic arbors and functional connections with existing neurons to be integrated into neuronal circuitry. The molecular mechanisms that regulate dendritic development of newborn neurons in the hippocampal dentate gyrus are still unclear. Here, we show that beta-catenin is expressed in newborn granule neurons and in neural progenitor cells in the hippocampal dentate gyrus. Specific knock-out of beta-catenin in newborn neurons, without affecting beta-catenin expression in neural progenitor cells, led to defects in dendritic morphology of these newborn neurons in vivo. Majority of newborn neurons that cannot extend dendrites survive <1 month after they were born. Our results indicate that beta-catenin plays an important role in dendritic development of postnatal-born neurons in vivo, and is therefore essential for the neurogenesis in the postnatal brain.

Notch regulates cell fate and dendrite morphology of newborn neurons in the postnatal dentate gyrus

The lifelong addition of neurons to the hippocampus is a remarkable form of structural plasticity, yet the molecular controls over proliferation, neuronal fate determination, survival, and maturation are poorly understood. Expression of Notch1 was found to change dynamically depending on the differentiation state of neural precursor cells. Through the use of inducible gain- and loss-of-function of Notch1 mice we show that this membrane receptor is essential to these distinct processes. We found in vivo that activated Notch1 overexpression induces proliferation, whereas gamma-secretase inhibition or genetic ablation of Notch1 promotes cell cycle exit, indicating that the level of activated Notch1 regulates the magnitude of neurogenesis from postnatal progenitor cells. Abrogation of Notch signaling in vivo or in vitro leads to a transition from neural stem or precursor cells to transit-amplifying cells or neurons. Further, genetic Notch1 manipulation modulates survival and dendritic morphology of newborn granule cells. These results provide evidence for the expansive prevalence of Notch signaling in hippocampal morphogenesis and plasticity, suggesting that Notch1 could be a target of diverse traumatic and environmental modulators of adult neurogenesis.

LDL receptor deficiency results in decreased cell proliferation and presynaptic bouton density in the murine hippocampus

An aberrant cholesterol metabolism in the brain may contribute to the pathogenesis of Alzheimer's disease (AD). The LDL receptor (LDLR) regulates plasma cholesterol levels and recently we and others obtained evidence that it is also involved in regulating brain cholesterol homeostasis. Moreover, we found that LDLR-deficient mice display impaired spatial memory. Because cholesterol, in part derived from cellular uptake via LDLR, is required for peripheral cell proliferation and growth, we examined the effect of absence of the LDLR on hippocampal proliferation and the density of synaptic connections. Mice deficient for the LDLR displayed a reduced number of proliferating (BrdU-labeled) cells in the hippocampus as compared to wild type control mice. In addition, the number of synaptophysin-immunoreactive presynaptic boutons in the hippocampal CA1 and the dentate gyrus (DG) areas, but not in cortical areas, was lower in the LDLR-knockout mice than in the control mice. In vitro experiments showed that LDLR activity is increased when cell growth is enhanced by the addition of N2 supplement. This further supports a role for the LDLR in the outgrowth of neurites. These findings support the notion that, similar to its role in the periphery, the LDLR is important for the cellular uptake of cholesterol in the brain and that disturbance of this process affects neuronal plasticity.

Functional maturation of developing interneurons in the molecular layer of mouse dentate gyrus

The dentate gyrus is the main target for cortical inputs to the hippocampal formation and is particularly strongly controlled by synaptic inhibition. Many GABAergic interneurons migrate from the dentate molecular layer towards their final position in the hilus during the first two postnatal weeks. During this critical period of development we monitored the intrinsic and synaptic properties of developing interneurons in the molecular layer of mouse hippocampal slices. We focussed on multipolar cells in the middle portion of the molecular layer. With increasing age, input resistance decreased and action potential waveform changed to larger amplitude and shorter duration. Repetitive spiking was scarce at early stages, while trains of action potentials could be readily elicited after the first postnatal week. At all ages, we observed spontaneous postsynaptic currents which were almost exclusively GABA(A) receptor-mediated and increased in frequency with age. All developmental changes in intrinsic and synaptic properties occurred between p 6-8 and p 9-11, indicating a rapid functional maturation at the end of the first postnatal week. Parallel immunohistochemical experiments revealed that calretinin positive cells formed the major part of developing interneurons in the middle molecular layer. Together, the data shows a rapid functional maturation of intrinsic and synaptic properties of interneurons in the dentate molecular layer and an early integration into synaptic networks with clear prevalence of inhibitory synaptic inputs.

Neonatally born granule cells numerically dominate adult mice dentate gyrus

Hippocampal granule cells (GCs) are continuously generated in the subgranular zone of the dentate gyrus (DG) and functionally incorporated to dentate neural circuits even in adulthood. This raises a question about the fate of neonatally born GCs in adult DG. Do they exist until adulthood or are they largely superseded by adult-born GCs? To investigate this question, we examined the contributions of postnatally born GCs to the adult mouse DG. C57BL/6 mice were grouped in three different postnatal (P) ages (group 1: P0, group 2: P7, and group 3: P35) and received a daily bromodeoxyuridine (BrdU) injection for three consecutive days (P0/1/2, P7/8/9, and P35/36/37, respectively) to label dividing cells. At 6 months old, hippocampal sections were prepared from the animals and immunostained with anti-BrdU antibody and an antibody against the homeobox prospero-like protein Prox1, a marker of GCs. We defined BrdU- and Prox1-double positive cells as newborn GCs and analyzed their density and distribution in the granule cell layer (gcl), revealing that newborn GCs of each group still existed 6 months after BrdU injections and that the density of GCs born during P0-2 (group 1) was significantly higher compared with the other groups. Although the density of newborn GCs in the each group did not differ between male and female, the radial distribution of them in gcl showed some differences, that is, male newborn GCs localized toward the molecular layer compared with female ones in group 1, while to the hilus in group 2. These results suggest that GCs born in early postnatal days numerically dominate adult DG and that there exist sex differences in GC localizations which depend on the time when they were born.

Differential synaptic integration of interneurons in the outer and inner molecular layers of the developing dentate gyrus

The dentate gyrus (DG) undergoes continued reorganization and lamination during early postnatal development. Interneurons with anatomically identified synaptic contacts migrate from the outer to the inner regions of the molecular layer (ML) of the DG. By using the 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP)-enhanced green fluorescent protein transgenic mouse, we were able to target and physiologically characterize Dlx2(+) developing ML interneurons. We investigated whether synapses on migrating ML interneurons were functional and defined properties of synaptic inputs onto interneurons that were located in the outer ML (OML) or inner ML (IML). Consistent with ongoing maturation, IML interneurons displayed lower input resistances and more hyperpolarized resting membrane potentials than OML interneurons. Both OML and IML interneurons received a direct excitatory monosynaptic input from the entorhinal cortex via the perforant paths, but this input was differentially sensitive to activation of presynaptic group II and III metabotropic glutamate receptors. Furthermore, only IML interneurons also received significant synaptic input from the CA3/hilar region, especially under conditions of experimentally induced disinhibition. These changes are attributed to a significant reorganization of dendritic fields. GABA(A) receptor-mediated innervation of OML and IML interneurons also displayed significant differences in miniature IPSC amplitude, frequency, and decay kinetics. Finally, cell-attached recordings indicated that GABA(A) receptor activation was depolarizing in OML interneurons but predominantly shunting in IML interneurons. Our data provide evidence that developing ML interneurons receive functional glutamatergic and GABAergic inputs and undergo significant changes in synaptic integration during migration from the OML to the IML.

Morphological and behavioral consequences of recurrent seizures in neonatal rats are associated with glucocorticoid levels

OBJECTIVE

It is well documented that epilepsy can increase neurogenesis in certain brain regions and cause behavioral alternations in patients and different epileptic animal models. A series of experimental studies have demonstrated that neurogenesis is regulated by various factors including glucocorticoid (CORT), which can reduce neurogenesis. Most of studies in animal have been focused on adulthood stage, while the effect of recurrent seizures to immature brain in neonatal period has not been well established. This study was designed to investigate how the recurrent seizures occurred in the neonatal period affected the immature brain and how CORT regulated neurogenesis in immature animals.

METHODS

Neonatal rats were subjected to 3 pilocarpine-induced seizures from postnatal day 1 to day 7. Then neurogenesis at different postnatal ages (i.e. P8, P12, P22, P50) was observed. Behavioral performance was tested when the rats were mature (P40), and plasma CORT levels following recurrent seizures were simultaneously monitored.

RESULTS

Rats with neonatal seizures had a significant reduction in the number of Bromodeoxyuridine (BrdU) labeled cells in the dentate gyrus compared with the control groups when the animals were euthanized on P8 or P12 (P<0.05); whereas there was no difference between the two groups on P22. Until P50, rats with neonatal seizures had increased number of BrdU-labeled cells compared with the control group (P<0.05). In Morris water maze task, pilocarpine-treated rats were significantly slower than the control rats at the first and second day, and there were no differences at other days. In probe trial, there was no significant difference in time spent in the goal quadrant between the two groups. Endocrine studies showed a correlation between the number of BrdU positive cells and the CORT level. Sustained increase in circulating CORT levels was observed following neonatal seizures on P8 and P12.

CONCLUSION

Neonatal recurrent seizures can biphasely modulate neurogenesis over different time windows with a down-regulation at early time and up-regulation afterwards, cause persistent deficits in cognitive functions of adults, and increase the circulating CORT levels. CORT levels are related with the morphological and behavioral consequences of recurrent seizures.

Early motherhood in rats is associated with a modification of hippocampal function

The transition to motherhood results in a number of hormonal, neurological and behavioral changes necessary to ensure offspring survival. However, little attention has been paid to changes not directly linked to reproductive function in the early mother. In this study, we demonstrate that spatial performances during the learning phase were impaired after the delivery in rats, while spatial retention ability was improved 2 weeks later. In addition, we also report that early motherhood reduced the cell proliferation in the dentate gyrus of the hippocampus without inducing a decrease in the newborn cells 2 weeks later. The decrease of estradiol levels and high levels of glucocorticoids after delivery could in part explain the changes in the hippocampal function. In summary, our findings suggest that early postpartum period is associated with a modification of hippocampal function. This may reflect a homeostatic form of hippocampal plasticity in response to the onset of the maternal experience.

Coordinated expression of HuD and GAP-43 in hippocampal dentate granule cells during developmental and adult plasticity

Previous work from our laboratory demonstrated that the RNA-binding protein HuD binds to and stabilizes the GAP-43 mRNA. In this study, we characterized the expression of HuD and GAP-43 mRNA in the hippocampus during two forms of neuronal plasticity. During post-natal development, maximal expression of both molecules was found at P5 and their levels steadily decreased thereafter. At P5, HuD was also present in the subventricular zone, where it co-localized with doublecortin. In the adult hippocampus, the basal levels of HuD and GAP-43 were lower than during development but were significantly increased in the dentate gyrus after seizures. The function of HuD in GAP-43 gene expression was confirmed using HuD-KO mice, in which the GAP-43 mRNA was significantly less stable than in wild type mice. Altogether, these results demonstrate that HuD plays a role in the post-transcriptional control of GAP-43 mRNA in dentate granule cells during developmental and adult plasticity.

Ontogeny of cocaine‐ and amphetamine‐regulated transcript (CART) peptide and calbindin immunoreactivity in granule cells of the dentate gyrus in the rat

Cocaine- and amphetamine-regulated transcript (CART) peptide was first discovered in the rat striatum following cocaine and amphetamine administration. However, even without psychostimulant treatment, many neuronal groups of the central nervous system, including granule cells of the dentate gyrus, express CART peptide. Earlier studies, based on the prenatal expression of CART peptide in the mesencephalon, suggest that it exerts neurotrophic effects. In the present study, ontogenetic expression of CART peptide in dentate gyrus granule cells was studied using immunohistochemistry in rats from 5 days to 3 months old. Expression was correlated with the expression of another neurochemical marker of granule cells, the calcium binding protein, calbindin. Calbindin was already present in granule cells at postnatal day 5 (P5), whereas CART peptide was first observed at P12. The first CART peptide- and calbindin-immunoreactive cells were localized to the lateral end of the dorsal blade, to the outer part of granule cell layer adjacent to the molecular layer, which agrees with the localization of the first-generated granule cells in the dentate gyrus. The first calbindin-immnunoreactive mossy fibers were seen at P9 in the stratum lucidum of CA3, and the entire projection path of mossy fibers expressed calbindin at P18. Mossy fibers were CART peptide-immunopositive at P12, and they were visible in the most distal part of CA3, in CA3a next to CA2. This localization fits with the known spatial organization of mossy fiber axon terminals. An adult-like expression of CART peptide and calbindin in the hippocampal formation was detectable at P30. The late postnatal appearance of CART peptide in dentate granule cells, and their axonal terminals, indicates that CART peptide may play a neurotrophic role in late developmental events, such as synaptogenesis. However, this does not exclude the possibility of a neuromodulatory role for this peptide.

Mecp2 deficiency leads to delayed maturation and altered gene expression in hippocampal neurons

It is well known that Rett Syndrome, a severe postnatal childhood neurological disorder, is mostly caused by mutations in the MECP2 gene. However, how deficiencies in MeCP2 contribute to the neurological dysfunction of Rett Syndrome is not clear. We aimed to resolve the role of MeCP2 epigenetic regulation in postnatal brain development in an Mecp2-deficient mouse model. We found that, while Mecp2 was not critical for the production of immature neurons in the dentate gyrus (DG) of the hippocampus, the newly generated neurons exhibited pronounced deficits in neuronal maturation, including delayed transition into a more mature stage, altered expression of presynaptic proteins and reduced dendritic spine density. Furthermore, analysis of gene expression profiles of isolated DG granule neurons revealed abnormal expression levels of a number of genes previously shown to be important for synaptogenesis. Our studies suggest that MeCP2 plays a central role in neuronal maturation, which might be mediated through epigenetic control of expression pathways that are instrumental in both dendritic development and synaptogenesis.

Reduced hippocampal neurogenesis and skill reaching performance in adult Emx1 mutant mice

Mammalian homeobox gene Emx family is involved in the development of the rostral brain. Loss-of-function studies suggest that, despite the agenesis of corpus callosum, the Emx1 mutants display relatively modest defects compared to the Emx2 mutants. However, the role of the Emx1 in neurogenesis and brain function has never been explored. We used unbiased stereology to determine the number of proliferating progenitors and immature neurons in the adult neurogenic zones. Although previous studies have established that the formation of the dentate gyrus (DG) requires Emx2, we found that the adult Emx1 mutants also exhibited a smaller DG, reduced number of proliferating progenitor cells and immature neurons in the DG, in contrast to the indistinguishable level of neurogenesis in the subventricular zone when compared to the wild type mice. In view of the involvement of callosal projection neurons in mediating interhemispheric crosstalk and spatial coupling between the limbs, and the importance of DG in hippocampus-dependent function in learning and memory, we assessed motor and cognitive functions. Emx1 deletion impaired performance on a forelimb skill reaching task and attenuated training induced hippocampal neurogenesis, but it did not affect motor activity or basic motor function as evaluated in the open field, wire hanging and rotor rod tests. Unexpectedly, the adult Emx1 mutant mice did not exhibit impairment in spatial learning and memory in the Barnes maze test. Our data suggest that deletion of the Emx1 gene reduces hippocampal neurogenesis and affects higher motor function that requires extensive learning.

Impaired postnatal development of hippocampal dentate gyrus in Sp4 null mutant mice

Sp4, a member of the Sp1 family of transcription factors, is expressed restrictively in the developing nervous system and abundantly in the hippocampus. Previously, we demonstrated that hypomorphic Sp4 mice display hippocampal vacuolization and concomitant deficits in memory and sensorimotor gating. Here, we report further analyses of Sp4 functions during postnatal development of the dentate gyrus in Sp4 null mutant mice. A reduced cell proliferation restrictively in hippocampus, but not cerebellum, was observed in the first week of postnatal development of Sp4 null mutant mice. The dendritic growth and arborization of dentate granule cells was decreased in hippocampal cultures in vitro from mutant neonatal mice. The adult Sp4 null mutant mice displayed decreased dentate granule cell density with reduced width of both dentate gyrus and the molecular layer. The abnormality of the molecular layer was indicated by a reduced level of synaptophysin expression in the mutant mice. The Sp4 transcription factor therefore appears to predominantly regulate the development of dentate granule cells.

Undernutrition during early life increases the level of apoptosis in the dentate gyrus but not in the CA2+CA3 region of the hippocampal formation

We have previously shown that undernutrition during early life causes a permanent deficit in the total number of dentate granule cells. However, it is unknown whether this deficit is due to neuronal cell death and/or to fewer cells being born during the period of neurogenesis. We have therefore used stereological methods combined with specific labeling techniques to examine the numbers of apoptotic cells in specific regions of the hippocampal formation. Rats were undernourished by restricting their daily food intake to about half that eaten by well-fed controls. Control and undernourished rats were killed on postnatal day 21, and their brains fixed in 4% paraformaldehyde. Serial sections through the hippocampal formation were labeled with the TUNEL technique to distinguish apoptotic cells. All care and animal handling procedures were approved by the institutional Animal Ethics Committee in line with Australian NHMRC procedures. There were about 21,500 and 57,000 TUNEL-positive cells in the dentate gyrus granule cell layer of control and undernourished rats, respectively. The difference between these values was statistically significant. In the CA3+CA2 region, there were about 22,000 and 19,500 TUNEL-positive cells in control and undernourished rats, respectively. The difference between these values was not statistically significant. Furthermore, it was observed that the majority of the TUNEL-positive cells in the dentate gyrus were located close to the border between the dentate gyrus granule cells and hilus of the hippocampal formation. Our results show that undernutrition during gestation and lactation can result in an increase in the level of TUNEL-positive apoptotic cells in the rat dentate gyrus.

Diethylstilbestrol alters the population dynamic of neural precursor cells in the neonatal male rat dentate gyrus

Little is known about how estrogens influence neurogenesis in the newborn male rodent. Herein, we examined the effects of neonatal diethylstilbestrol (DES) exposure on the proliferation and survival of type-1 and type-2 neural precursor cells (NPC) in the dentate gyrus of male rats. This was achieved by exposing newborn male pups to DES on postnatal day (PND) 1, PND3, PND5, and PND7, sacrificed at PND8 or PND21, followed by double immunohistochemistry and morphometric analysis of hippocampal dentate gyrus. Furthermore, vascular endothelial growth factor (VEGF) and brain-derived neurotrophic factor (BDNF) mRNA expression was evaluated in hippocampal tissue blocks by real time RT-PCR. At PND8, the density of total proliferating NPC decreased in DES-treated animals. This reduction was due to a significant decrease in the mitotic rate of type-2 cells only, since type-1 NPCs did not show changes in the proliferation index. Type-2 NPCs expressed the cell-cycle inhibitor p27(kip1) and its expression was clearly augmented in the DES-treated group. Furthermore, the number of apoptotic cells in the dentate gyrus of DES-treated rats decreased. Surprisingly, DES treatment enhanced cell survival and increased NPCs proliferation when animals were examined 14 days after treatment. VEGF mRNA expression showed a positive correlation with NPCs proliferation and BDNF mRNA levels were higher in DES-treated animals at both time points examined. Collectively, these results indicate that hippocampal NPCs proliferation and survival is a critical target of DES exposure during the early postnatal period. VEGF and BDNF are proposed as key mediators of DES-induced NPC mitotic response.

Changes of hippocampal neurons after perinatal exposure to ethanol

The effect of ethanol on the structural development of the central nervous system was studied in offspring of Wistar rats, drinking 20 % ethanol during pregnancy and till the 28th day of their postnatal life. The structural changes in the hippocampus and dentate gyrus were analyzed at the age of 18, 35 and 90 days. A lower width of pyramidal and granular cell layers, cell extinction and fragmentation of numerous nuclei were found in all experimental animals compared to control animals. The extent of neural cell loss was similar in all monitored areas and in all age groups. At the age of 18 and 35 days, the degenerating cells were observed in the CA1 and CA3 area of the hippocampus and in the ventral and dorsal blade of the dentate gyrus. Numerous glial cells replaced the neuronal population of this region. Some degenerating cells with fragmented nuclei were observed at the age of 90 days. Our experiments confirmed the vulnerability of the developing central nervous system by ethanol intake during the perinatal period and revealed a long-lasting degeneration process in the hippocampus and dentate gyrus.

Expression of SDF-1 and CXCR4 during reorganization of the postnatal dentate gyrus

Previous studies have demonstrated that stromal cell-derived factor 1 (SDF-1) is crucial for early dentate development; however, the mouse mutants for this chemokine and its only receptor, CXCR4, are neonatally lethal, making conclusions about the role of these molecules in postnatal development difficult to sustain. Previous expression analyses have used single labeling, but the distribution of CXCR4 is complex and to determine the cell types expressing CXCR4 requires multiple marker labeling. In this study, we examined the distribution of SDF-1 and CXCR4 mRNAs during the first postnatal weeks, combining these markers with several other cell-type-specific markers. We found that SDF-1 has three sites of expression: (1) continuation of prenatal expression in the meninges; (2) expression in Cajal-Retzius cells occupying the molecular layer of the upper and lower blades of the dentate, and (3) the maturing dentate granule neurons themselves. The timing of expression in these three sites corresponds to alterations in the distribution of the primary cell types expressing CXCR4 during the same periods, notably the expression of CXCR4 in radial-glial-like GFAP-expressing dentate precursors and immature dentate granule neurons. Taken together, our data suggest potential ongoing roles for SDF-1/CXCR4 signaling in the dentate gyrus during the early postnatal period that will be tested in the future with more precise genetic approaches.

Neurogenesis decreases during brain maturation from adolescence to adulthood

Adolescence is an important stage of brain development. Recent studies have indicated that neurogenesis in the brain occurs throughout life prompting comparisons of adolescent and adult neurogenesis. Since insulin-like growth factor 1 (IGF-1) has been implicated in promoting neurogenesis we investigated the levels of neurogenesis in adolescents (PND30) and adults (PND120) using IGF-1 over-expressing mice and IGFBP-1 (IGF binding protein-1) over-expressing mice. Proliferation and differentiation of neuroprogenitors were determined using bromodeoxyuridine (BrdU)- and doublecortin (DCX)-labeling. High levels of neurogenesis were found in both the hippocampal dentate gyrus (DG) and the forebrain subventricular zone (SVZ) of the adolescents as compared with the adults. Both adolescent IGF-1 and IGFBP-1 transgenic mice as well as their wildtype controls have significantly higher expression of BrdU and DCX in the hippocampus and SVZ when compared with their adult counterparts. However, no significant differences on BrdU-labeling were found when either of transgenic mice were compared with their wildtype littermates in both age groups. These studies indicate that adolescent mice have high levels of neurogenesis compared to adults suggesting a dramatic loss of neurogenesis during the transition from adolescence to adulthood. However, the role of IGF-1 during adolescent development is still unclear.

Morphological development and maturation of granule neuron dendrites in the rat dentate gyrus

The first granule neurons in the dentate gyrus are born during late embryogenesis in the rodent, and the primary period of granule cell neurogenesis continues into the second postnatal week. On the day of birth in the rat, the oldest granule neurons are visible in the suprapyramidal blade and exhibit rudimentary dendrites extending into the molecular layer. Here we describe the morphological development of the dendritic trees between birth and day 14, and we then review the process of dendritic remodeling that occurs after the end of the second week. Data indicate that the first adult-like granule neurons are present on day 7, and, furthermore, physiological recordings demonstrate that some granule neurons are functional at this time. Taken together, these results suggest that the dentate gyrus may be incorporated into the hippocampal circuit as early as the end of the first week. The dendritic trees of the granule neurons, however, continue to increase in size until day 14. After that time, the dendritic trees of the oldest granule neurons are sculpted and refined. Some dendrites elongate while others are lost, resulting in a conservation of total dendritic length. We end this chapter with a review of the quantitative aspects of granule cell dendrites in the adult rat and a discussion of the relationship between the morphology of a granule neuron and the location of its cell body within stratum granulosum and along the transverse axis of the dentate gyrus.

The dentate mossy fibers: structural organization, development and plasticity

Hippocampal mossy fibers are the axons of the dentate granule cells and project to hippocampal CA3 pyramidal cells and mossy cells of the dentate hilus (CA4) as well as a number of interneurons in the two areas. Besides their role in hippocampal function, studies of which are still evolving and taking interesting turns, the mossy fibers display a number of unique features with regard to axonal projections, terminal structures and synaptic contacts, development and variations among species and strains, as well as to normal occurring and lesion-induced plasticity and neural transplantation. These features are the topic of this review, which will use the mossy fiber system of the rat as basis and reference in its aim to provide an up-to-date, yet historically based guide to students in the field.

Development of cell and fiber layers in the dentate gyrus

This chapter deals with the laminated organization of the dentate gyrus, particularly with the molecular signals controlling its development. First, sites of granule cell generation, their modes and routes of migration are described. This is followed by an analysis of the molecular determinants governing the formation of a tightly packed granule cell layer that is normal in rodents and primates. Reelin, a protein of the extracellular matrix, plays an important role for the proper migration and lamination of the granule cells during development and for the maintenance of a laminated dentate gyrus in adulthood. Granule cell positioning is crucial for the laminated termination of commissural/associational fibers to the dentate gyrus, suggesting that the granule cells carry positional signals for these fibers. In contrast, not signals of the target cells but molecules of the extracellular matrix, such as hyaluronan, underlie the layer-specific termination of fibers from the entorhinal cortex. The molecular determinants controlling axonal pathfinding and target recognition of the profusely terminating cholinergic and GABAergic subcortical afferents still need to be elucidated.

Comparative anatomy of the hippocampal dentate gyrus in adult and developing rodents, non-human primates and humans

There has been substantial progress in our understanding of the hippocampus in the past 70 years. During this time, it has become clear that the hippocampus is not an olfactory-related structure alone, but plays critical roles in other functions that do not necessarily depend on olfaction, such as learning and memory. In addition, it has become clear how important the hippocampus is to a wide variety of neurological disorders and psychiatric illness. Animal models have provided a great resource in such studies, but a frequent question is whether the data from laboratory animals is relevant to man.