Novel Psychopharmacological Herbs Relieve Behavioral Abnormalities and Hippocampal Dysfunctions in an Animal Model of Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is an anxiety disorder caused by traumatic or frightening events, with intensified anxiety, fear memories, and cognitive impairment caused by a dysfunctional hippocampus. Owing to its complex phenotype, currently prescribed treatments for PTSD are limited. This study investigated the psychopharmacological effects of novel COMBINATION herbal medicines on the hippocampus of a PTSD murine model induced by combining single prolonged stress (SPS) and foot shock (FS). We designed a novel herbal formula extract (HFE) from Chaenomeles sinensis, Glycyrrhiza uralensis, and Atractylodes macrocephala. SPS+FS mice were administered HFE (500 and 1000 mg/kg) once daily for 14 days. The effects of HFE of HFE on the hippocampus were analyzed using behavioral tests, immunostaining, Golgi staining, and Western blotting. HFE alleviated anxiety-like behavior and fear response, improved short-term memory, and restored hippocampal dysfunction, including hippocampal neurogenesis alteration and aberrant migration and hyperactivation of dentate granule cells in SPS+FS mice. HFE increased phosphorylation of the Kv4.2 potassium channel, extracellular signal-regulated kinase, and cAMP response element-binding protein, which were reduced in the hippocampus of SPS+FS mice. Therefore, our study suggests HFE as a potential therapeutic drug for PTSD by improving behavioral impairment and hippocampal dysfunction and regulating Kv4.2 potassium channel-related pathways in the hippocampus.

Experimental early-life febrile seizures cause a sustained increase in excitatory neurotransmission in newborn dentate granule cells

Prolonged febrile seizures (FS) are a risk factor for the development of hippocampal-associated temporal lobe epilepsy. The dentate gyrus is the major gateway to the hippocampal network and one of the sites in the brain where neurogenesis continues postnatally. Previously, we found that experimental FS increase the survival rate and structural integration of newborn dentate granule cells (DGCs). In addition, mature post-FS born DGCs express an altered receptor panel. Here, we aimed to study if these molecular and structural changes are accompanied by an altered cellular functioning. Experimental FS were induced by hyperthermia in 10-days-old Sprague-Dawley rats. Proliferating progenitor cells were labeled the next day by injecting green fluorescent protein expressing retroviral particles bilaterally in the dentate gyri. Eight weeks later, spontaneous excitatory and inhibitory postsynaptic events (sEPSCs and sIPSCs, respectively) were recorded from labeled DGCs using the whole-cell patch-clamp technique. Experimental FS resulted in a robust decrease of the inter event interval (p < .0001) and a small decrease of the amplitude of sEPSCs (p < .001). Collectively the spontaneous excitatory charge transfer increased (p < .01). Experimental FS also slightly increased the frequency of sIPSCs (p < .05), while the amplitude of these events decreased strongly (p < .0001). The net inhibitory charge transfer remained unchanged. Experimental, early-life FS have a long-term effect on post-FS born DGCs, as they display an increased spontaneous excitatory input when matured. It remains to be established if this presents a mechanism for FS-induced epileptogenesis.

Reelin Is Required for Maintenance of Granule Cell Lamination in the Healthy and Epileptic Hippocampus

One characteristic feature of mesial temporal lobe epilepsy is granule cell dispersion (GCD), a pathological widening of the granule cell layer in the dentate gyrus. The loss of the extracellular matrix protein Reelin, an important positional cue for neurons, correlates with GCD formation in MTLE patients and in rodent epilepsy models. Here, we used organotypic hippocampal slice cultures (OHSC) from transgenic mice expressing enhanced green fluorescent protein (eGFP) in differentiated granule cells (GCs) to monitor GCD formation dynamically by live cell video microscopy and to investigate the role of Reelin in this process. We present evidence that following treatment with the glutamate receptor agonist kainate (KA), eGFP-positive GCs migrated mainly toward the hilar region. In the hilus, Reelin-producing neurons were rapidly lost following KA treatment as shown in a detailed time series. Addition of recombinant Reelin fragments to the medium effectively prevented the KA-triggered movement of eGFP-positive GCs. Placement of Reelin-coated beads into the hilus of KA-treated cultures stopped the migration of GCs in a distance-dependent manner. In addition, quantitative Western blot analysis revealed that KA treatment affects the Reelin signal transduction pathway by increasing intracellular adaptor protein Disabled-1 synthesis and reducing the phosphorylation of cofilin, a downstream target of the Reelin pathway. Both events were normalized by addition of recombinant Reelin fragments. Finally, following neutralization of Reelin in healthy OHSC by incubation with the function-blocking CR-50 Reelin antibody, GCs started to migrate without any direction preference. Together, our findings demonstrate that normotopic position of Reelin is essential for the maintenance of GC lamination in the dentate gyrus and that GCD is the result of a local Reelin deficiency.

Mossy Cells in the Dorsal and Ventral Dentate Gyrus Differ in Their Patterns of Axonal Projections

Mossy cells (MCs) of the dentate gyrus (DG) are a major group of excitatory hilar neurons that are important for regulating activity of dentate granule cells. MCs are particularly intriguing because of their extensive longitudinal connections within the DG. It has generally been assumed that MCs in the dorsal and ventral DG have similar patterns of termination in the inner one-third of the dentate molecular layer. Here, we demonstrate that axonal projections of MCs in these two regions are considerably different. MCs in dorsal and ventral regions were labeled selectively with Cre-dependent eYFP or mCherry, using two transgenic mouse lines (including both sexes) that express Cre-recombinase in MCs. At four to six weeks following unilateral labeling of MCs in the ventral DG, a dense band of fibers was present in the inner one-fourth of the molecular layer and extended bilaterally throughout the rostral-caudal extent of the DG, replicating the expected distribution of MC axons. In contrast, following labeling of MCs in the dorsal DG, the projections were more diffusely distributed. At the level of transfection, fibers were present in the inner molecular layer, but they progressively expanded into the middle molecular layer and, most ventrally, formed a distinct band in this region. Optical stimulation of these caudal fibers expressing ChR2 demonstrated robust EPSCs in ipsilateral granule cells and enhanced the effects of perforant path stimulation in the ventral DG. These findings suggest that MCs in the dorsal and ventral DG differ in the distribution of their axonal projections and possibly their function.SIGNIFICANCE STATEMENT Mossy cells (MCs), a major cell type in the hilus of the dentate gyrus (DG), are unique in providing extensive longitudinal and commissural projections throughout the DG. Although it has been assumed that all MCs have similar patterns of termination in the inner molecular layer of the DG, we discovered that the axonal projections of dorsal and ventral MCs differ. While ventral MC projections exhibit the classical pattern, with dense innervation in the inner molecular layer, dorsal MCs have a more diffuse distribution and expand into the middle molecular layer where they overlap and interact with innervation from the perforant path. These distinct locations and patterns of axonal projections suggest that dorsal and ventral MCs may have different functional roles.

Development of Depotentiation in Adult-Born Dentate Granule Cells

Activity-dependent synaptic plasticity, i.e., long-term potentiation (LTP), long-term depression (LTD) and LTP reversal, is generally thought to make up the cellular mechanism underlying learning and memory in the mature brain, in which N-methyl-D-aspartate subtype of glutamate (NMDA) receptors and neurogenesis play important roles. LTP reversal may be the mechanism of forgetting and may mediate many psychiatric disorders, such as schizophrenia, but the specific mechanisms underlying these disorders remain unclear. In addition, LTP reversal during the development of adult-born dentate granule cells (DGCs) remains unknown. We found that the expression of the NMDA receptor subunits NR2A and NR2B displayed dynamic changes during the development of postnatal individuals and the maturation of adult-born neurons and was coupled with the change in LTP reversal. The susceptibility of LTP reversal progressively increases with the rise in the expression of NR2A during the development of postnatal individual and adult-born neurons. In addition, NMDA receptor subunits NR2A, but not NR2B, mediated LTP reversal in the DGCs of the mouse hippocampus.

Adult neurogenesis in the mammalian dentate gyrus

Earlier observations in neuroscience suggested that no new neurons form in the mature central nervous system. Evidence now indicates that new neurons do form in the adult mammalian brain. Two regions of the mature mammalian brain generate new neurons: (a) the border of the lateral ventricles of the brain (subventricular zone) and (b) the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. This review focuses only on new neuron formation in the dentate gyrus of the hippocampus. During normal prenatal and early postnatal development, neural stem cells (NSCs) give rise to differentiated neurons. NSCs persist in the dentate gyrus SGZ, undergoing cell division, with some daughter cells differentiating into functional neurons that participate in learning and memory and general cognition through integration into pre-existing neural networks. Axons, which emanate from neurons in the entorhinal cortex, synapse with dendrites of the granule cells (small neurons) of the dentate gyrus. Axons from granule cells synapse with pyramidal cells in the hippocampal CA3 region, which send axons to synapse with CA1 hippocampal pyramidal cells that send their axons out of the hippocampus proper. Adult neurogenesis includes proliferation, differentiation, migration, the death of some newly formed cells and final integration of surviving cells into neural networks. We summarise these processes in adult mammalian hippocampal neurogenesis and discuss the roles of major signalling molecules that influence neurogenesis, including neurotransmitters and some hormones. The recent controversy raised concerning whether or not adult neurogenesis occurs in humans also is discussed.

Opposing Morphogenetic Defects on Dendrites and Mossy Fibers of Dentate Granular Neurons in CRMP3-Deficient Mice

Collapsin response mediator proteins (CRMPs) are highly expressed in the brain during early postnatal development and continue to be present in specific regions into adulthood, especially in areas with extensive neuronal plasticity including the hippocampus. They are found in the axons and dendrites of neurons wherein they contribute to specific signaling mechanisms involved in the regulation of axonal and dendritic development/maintenance. We previously identified CRMP3’s role on the morphology of hippocampal CA1 pyramidal dendrites and hippocampus-dependent functions. Our focus here was to further analyze its role in the dentate gyrus where it is highly expressed during development and in adults. On the basis of our new findings, it appears that CRMP3 has critical roles both in axonal and dendritic morphogenesis of dentate granular neurons. In CRMP3-deficient mice, the dendrites become dystrophic while the infrapyramidal bundle of the mossy fiber shows aberrant extension into the stratum oriens of CA3. This axonal misguided projection of granular neurons suggests that the mossy fiber-CA3 synaptic transmission, important for the evoked propagation of the activity of the hippocampal trisynaptic circuitry, may be altered, whereas the dystrophic dendrites may impair the dynamic interactions with the entorhinal cortex, both expected to affect hippocampal function.

Clonal Analysis of Newborn Hippocampal Dentate Granule Cell Proliferation and Development in Temporal Lobe Epilepsy

Hippocampal dentate granule cells are among the few neuronal cell types generated throughout adult life in mammals. In the normal brain, new granule cells are generated from progenitors in the subgranular zone and integrate in a typical fashion. During the development of epilepsy, granule cell integration is profoundly altered. The new cells migrate to ectopic locations and develop misoriented “basal” dendrites. Although it has been established that these abnormal cells are newly generated, it is not known whether they arise ubiquitously throughout the progenitor cell pool or are derived from a smaller number of “bad actor” progenitors. To explore this question, we conducted a clonal analysis study in mice expressing the Brainbow fluorescent protein reporter construct in dentate granule cell progenitors. Mice were examined 2 months after pilocarpine-induced status epilepticus, a treatment that leads to the development of epilepsy. Brain sections were rendered translucent so that entire hippocampi could be reconstructed and all fluorescently labeled cells identified. Our findings reveal that a small number of progenitors produce the majority of ectopic cells following status epilepticus, indicating that either the affected progenitors or their local microenvironments have become pathological. By contrast, granule cells with “basal” dendrites were equally distributed among clonal groups. This indicates that these progenitors can produce normal cells and suggests that global factors sporadically disrupt the dendritic development of some new cells. Together, these findings strongly predict that distinct mechanisms regulate different aspects of granule cell pathology in epilepsy.

HIV encephalopathy: glial activation and hippocampal neuronal apoptosis, but limited neural repair

OBJECTIVES

HIV infection affects the central nervous system (CNS), frequently causing cognitive impairment. Hippocampal injury impedes the ability to transfer information into memory. Therefore, we aimed to examine neuronal injury and repair in the hippocampal formation in HIV encephalopathy.

METHODS

We compared neuropathological findings in 14 autopsy cases after death from systemic complications of HIV infection and in 15 age-matched HIV-negative control cases after sudden death from nonneurological causes using immunohistochemistry.

RESULTS

The density of apoptotic granule cells in the dentate gyrus was higher in HIV-infected than in control cases (P = 0.048). Proliferation of neural progenitor cells in the dentate gyrus was increased in HIV infection (P = 0.028), whereas the density of recently generated TUC-4 [TOAD (turned on after division)/Ulip/CRMP family 4]-expressing neurons in this region was not significantly elevated in HIV-infected cases (P = 0.13). HIV infection caused microglial activation and astrocytosis in the neocortex and hippocampal formation. Conversely, we were unable to detect more pronounced axonal injury in HIV-infected than in control cases.

CONCLUSIONS

As in other infections involving the CNS, apoptosis of hippocampal neurons accompanied by microglial activation and astrocytosis is a prominent feature of HIV encephalopathy. The regenerative potential, assessed using the density of young neurons in the hippocampal dentate gyrus, in HIV infection appears to be lower than in acute bacterial meningitis and septic encephalitis.

Neurotoxic Effects of Platinum Compounds: Studies in vivo on Intracellular Calcium Homeostasis in the Immature Central Nervous System

Platinum compounds cause significant clinical neurotoxicity. Several studies highlight neurological complications especially in paediatric oncology patients with Central Nervous System (CNS) and non-CNS malignancies. To understand the toxicity mechanisms of platinum drugs at cellular and molecular levels in the immature brain, which appears more vulnerable to injury than in the adult one, we compared the effects in vivo of the most used platinum compounds, i.e., cisdichlorodiammineplatinum (cisplatin, cisPt), and the new [Pt(O,O′-acac)(γ-acac)(DMS)] (PtAcacDMS). As models of developing brain areas, we have chosen the cerebellum and hippocampus dentate gyrus. Both areas show the neurogenesis events, from proliferation to differentiation and synaptogenesis, and therefore allow comparing the action of platinum compounds with DNA and non-DNA targets. Here, we focused on the changes in the intracellular calcium homeostasis within CNS architecture, using two immunohistochemical markers, the calcium buffer protein Calbindin and Plasma Membrane Calcium ATPase. From the comparison of the cisPt and PtAcacDMS effects, it emerges how essential the equilibrium and synergy between CB and PMCA1 is or how important the presence of at least one of them is to warrant the morphology and function of nervous tissue and limit neuroarchitecture damages, depending on the peculiar and intrinsic properties of the developing CNS areas.

Expansion of the dentate mossy fiber-CA3 projection in the brain-derived neurotrophic factor-enriched mouse hippocampus

Structural changes that alter hippocampal functional circuitry are implicated in learning impairments, mood disorders and epilepsy. Reorganization of mossy fiber (MF) axons from dentate granule cells is one such form of plasticity. Increased neurotrophin signaling is proposed to underlie MF plasticity, and there is evidence to support a mechanistic role for brain-derived neurotrophic factor (BDNF) in this process. Transgenic mice overexpressing BDNF in the forebrain under the α-calcium/calmodulin-dependent protein kinase II promoter (TgBDNF mice) exhibit spatial learning deficits at 2-3months of age, followed by the emergence of spontaneous seizures at ∼6months. These behavioral changes suggest that chronic increases in BDNF progressively disrupt hippocampal functional organization. To determine if the dentate MF pathway is structurally altered in this strain, the present study employed Timm staining and design-based stereology to compare MF distribution and projection volumes in transgenic and wild-type mice at 2-3months, and at 6-7months. Mice in the latter age group were assessed for seizure vulnerability with a low dose of pilocarpine given 2h before euthanasia. At 2-3months, TgBDNF mice showed moderate expansion of CA3-projecting MFs (∼20%), with increased volumes measured in the suprapyramidal (SP-MF) and intra/infrapyramidal (IIP-MF) compartments. At 6-7months, a subset of transgenic mice exhibited increased seizure susceptibility, along with an increase in IIP-MF volume (∼30%). No evidence of MF sprouting was seen in the inner molecular layer. Additional stereological analyses demonstrated significant increases in molecular layer (ML) volume in TgBDNF mice at both ages, as well as an increase in granule cell number by 8months of age. Collectively, these results indicate that sustained increases in endogenous BDNF modify dentate structural organization over time, and may thereby contribute to the development of pro-epileptic circuitry.

Making connections

Deleting a gene called Sema5A, which has been linked to autism in humans, causes neurons to form more connections in mice, and also alters how these mutant mice interact with other mice.

Semaphorin 5A inhibits synaptogenesis in early postnatal- and adult-born hippocampal dentate granule cells

Human SEMAPHORIN 5A (SEMA5A) is an autism susceptibility gene; however, its function in brain development is unknown. In this study, we show that mouse Sema5A negatively regulates synaptogenesis in early, developmentally born, hippocampal dentate granule cells (GCs). Sema5A is strongly expressed by GCs and regulates dendritic spine density in a cell-autonomous manner. In the adult mouse brain, newly born Sema5A-/- GCs show an increase in dendritic spine density and increased AMPA-type synaptic responses. Sema5A signals through PlexinA2 co-expressed by GCs, and the PlexinA2-RasGAP activity is necessary to suppress spinogenesis. Like Sema5A-/- mutants, PlexinA2-/- mice show an increase in GC glutamatergic synapses, and we show that Sema5A and PlexinA2 genetically interact with respect to GC spine phenotypes. Sema5A-/- mice display deficits in social interaction, a hallmark of autism-spectrum-disorders. These experiments identify novel intra-dendritic Sema5A/PlexinA2 interactions that inhibit excitatory synapse formation in developmentally born and adult-born GCs, and they provide support for SEMA5A contributions to autism-spectrum-disorders.

Developmental neurotoxicity of 3,3',4,4'-tetrachloroazobenzene with thyroxine deficit: Sensitivity of glia and dentate granule neurons in the absence of behavioral changes

Thyroid hormones (TH) regulate biological processes implicated in neurodevelopmental disorders and can be altered with environmental exposures. Developmental exposure to the dioxin-like compound, 3,3',4,4'-tetrachloroazobenzene (TCAB), induced a dose response deficit in serum T4 levels with no change in 3,5,3'- triiodothyronine or thyroid stimulating hormone. Female Sprague-Dawley rats were orally gavaged (corn oil, 0.1, 1.0, or 10 mg TCAB/kg/day) two weeks prior to cohabitation until post-partum day 3 and male offspring from post-natal day (PND)4-21. At PND21, the high dose showed a deficit in body weight gain. Conventional neuropathology detected no neuronal death, myelin disruption, or gliosis. Astrocytes displayed thinner and less complex processes at 1.0 and 10 mg/kg/day. At 10 mg/kg/day, microglia showed less complex processes, unbiased stereology detected fewer hippocampal CA1 pyramidal neurons and dentate granule neurons (GC) and Golgi staining of the cerebellum showed diminished Purkinje cell dendritic arbor. At PND150, normal maturation of GC number and Purkinje cell branching area was not observed in the 1.0 mg/kg/day dose group with a diminished number and branching suggestive of effects initiated during developmental exposure. No effects were observed on post-weaning behavioral assessments in control, 0.1 and 1.0mg/kg/day dose groups. The demonstrated sensitivity of hippocampal neurons and glial cells to TCAB and T4 deficit raises support for considering additional anatomical features of brain development in future DNT evaluations.

Increased neurosteroid sensitivity of hippocampal GABAA receptors during postnatal development

To investigate developmental changes in neurosteroid modulation of GABA(A) receptors, whole-cell currents were elicited by applying GABA with allopregnanolone or pregnenolone sulfate (PS) to dentate granule cells (DGCs), acutely isolated from 7-14-day-old and adult rats. GABA evoked larger currents from dentate granule cells acutely isolated from adult rats (adult DGCs) than from neonatal DGCs, due to increased efficacy (1662+/-267 pA in adult DGCs versus 1094+/-198 pA in neonatal DGCs, P=0.004), and current density (0.072+/-0.01 pA/microm(2) in neonatal rat DGCs to 0.178+/-0.02 pA/microm(2) in adult DGCs), but unchanged potency (EC(50) was 18.5+/-2 microm in adult DGCs, and 26.6+/-7.9 microm in neonatal DGCs, P=0.21). Allopregnanolone sensitivity of GABA(A) receptor currents increased during development due to an increased potency (21.1+/-4.7 nM in adult DGCs versus 94.6+/-9 nM in neonatal DGCs, P=0.0002). The potency and efficacy of PS inhibition of GABA(A) receptor currents were remained unchanged during development (13+/-6 microm and 13.2+/-5.9 microm, P=0.71 and 85.5%+/-3.5% and 83.6%+/-0.8%, P=0.29, respectively). To investigate possible mechanism of developmental changes in GABA(A) receptor properties, in situ hybridization for alpha1, alpha4 and gamma2 subunit mRNAs was performed in dentate gyrus of the two age groups. Qualitatively, alpha1 subunit mRNA was expressed at low levels in neonatal rats while it was well expressed in adult rats. The alpha4 and gamma2 subunits were well expressed in the dentate gyrus of adult and neonatal rats. Immunohistochemical staining for alpha1 subunit in hippocampal slices from neonatal and adult rats was examined under confocal laser scanning microscope. This demonstrated that cell bodies and dendrites of granule cells are moderately positive for the alpha1 staining in adult rats but weakly so in neonatal rats. Higher-magnification images demonstrate large number of clusters of alpha1-subunit in the cell bodies of dentate granule cells of adult rat but rare clusters in granule cells of neonatal rats. Maturation of GABA(A) receptors in DGCs is characterized by increased number of GABA(A) receptors that are more sensitive to endogenous neurosteroid allopregnanolone, which might be related to increased expression of alpha1 subunit.

Protein phosphatase-1 regulation in the induction of long-term potentiation: heterogeneous molecular mechanisms

Protein phosphatase inhibitor-1 (I-1) has been proposed as a regulatory element in the signal transduction cascade that couples postsynaptic calcium influx to long-term changes in synaptic strength. We have evaluated this model using mice lacking I-1. Recordings made in slices prepared from mutant animals and also in anesthetized mutant animals indicated that long-term potentiation (LTP) is deficient at perforant path-dentate granule cell synapses. In vitro, this deficit was restricted to synapses of the lateral perforant path. LTP at Schaffer collateral-CA1 pyramidal cell synapses remained normal. Thus, protein phosphatase-1-mediated regulation of NMDA receptor-dependent synaptic plasticity involves heterogeneous molecular mechanisms, in both different dendritic subregions and different neuronal subtypes. Examination of the performance of I-1 mutants in spatial learning tests indicated that intact LTP at lateral perforant path-granule cell synapses is either redundant or is not involved in this form of learning.

Human neuronal gamma-aminobutyric acid(A) receptors: coordinated subunit mRNA expression and functional correlates in individual dentate granule cells

gamma-Aminobutyric acid(A) receptors (GABARs) are heteromeric proteins composed of multiple subunits. Numerous subunit subtypes are expressed in individual neurons, which assemble in specific preferred GABAR configurations. Little is known, however, about the coordination of subunit expression within individual neurons or the impact this may have on GABAR function. To investigate this, it is necessary to profile quantitatively the expression of multiple subunit mRNAs within individual cells. In this study, single-cell antisense RNA amplification was used to examine the expression of 14 different GABAR subunit mRNAs simultaneously in individual human dentate granule cells (DGCs) harvested during hippocampectomy for intractable epilepsy. alpha4, beta2, and delta-mRNA levels were tightly correlated within individual DGCs, indicating that these subunits are expressed coordinately. Levels of alpha3- and beta2-mRNAs, as well as epsilon- and beta1-mRNAs, also were strongly correlated. No other subunit correlations were identified. Coordinated expression could not be explained by the chromosomal clustering of GABAR genes and was observed in control and epileptic rats as well as in humans, suggesting that it was not species-specific or secondary to epileptogenesis. Benzodiazepine augmentation of GABA-evoked currents also was examined to determine whether levels of subunit mRNA expression correlated with receptor pharmacology. This analysis delineated two distinct cell populations that differed in clonazepam modulation and patterns of alpha-subunit expression. Clonazepam augmentation correlated positively with the relative expression of alpha1- and gamma2-mRNAs and negatively with alpha4- and delta-mRNAs. These data demonstrate that specific GABAR subunit mRNAs exhibit coordinated control of expression in individual DGCs, which has significant impact on inhibitory function.

Inhibition of dentate granule cell neurogenesis with brain irradiation does not prevent seizure-induced mossy fiber synaptic reorganization in the rat

Aberrant reorganization of dentate granule cell axons, the mossy fibers, occurs in human temporal lobe epilepsy and rodent epilepsy models. Whether this plasticity results from the remodeling of preexisting mossy fibers or instead reflects an abnormality of developing dentate granule cells is unknown. Because these neurons continue to be generated in the adult rodent and their production increases after seizures, mossy fibers that arise from either developing or mature granule cells are potential substrates for this network plasticity. Therefore, to determine whether seizure-induced, mossy fiber synaptic reorganization arises from either developing or mature granule cell populations, we used low-dose, whole-brain x-irradiation to eliminate proliferating dentate granule cell progenitors in adult rats. A single dose of 5 Gy irradiation blocked cell proliferation and eliminated putative progenitor cells in the dentate subgranular proliferative zone. Irradiation 1 d before pilocarpine-induced status epilepticus significantly attenuated dentate granule cell neurogenesis after seizures. Two irradiations, 1 d before and 4 d after status epilepticus, essentially abolished dentate granule cell neurogenesis but failed to prevent mossy fiber reorganization in the dentate molecular layer. These results indicate that dentate granule cell neurogenesis in the mature hippocampal formation is vulnerable to the effects of low-dose ionizing irradiation. Furthermore, the development of aberrant mossy fiber remodeling in the absence of neurogenesis suggests that mature dentate granule cells contribute substantially to seizure-induced network reorganization.

Granule cells in aging rats are sexually dimorphic in their response to estradiol

Normal aging comprises cognitive decline, including deterioration of memory. It has been suggested that this decline in memory is sexually dimorphic because of the cessation in gonadal steroid secretion that occurs during reproductive aging in female, but not male, mammals. We wondered whether neurons in brain regions associated with learning and memory underwent morphological changes that were dimorphic as well and whether cessation of the secretion of gonadal steroids influenced these morphological changes. To explore these questions, we deprived and restored estrogens to young and old gonadectomized females and males and studied the morphology of dentate granule cells by intracellular dye filling in a lightly fixed slice preparation. We found the following: (1) Aged female dentate granule cells deprived of gonadal steroids long-term have a paucity of dendritic spines compared with young females deprived short-term; however, aged male dentate granule cells deprived of gonadal steroids long-term have no decrease in dendritic spines compared with young males deprived short-term. (2) Aged female dentate granule cells with long-term estrogen replacement at either high or low levels still had a decline in spine density. (3) Aged female dentate granule cells with short-term estradiol replacement had spine density increased to levels normally observed in young adults, whereas aged males with short-term estradiol replacement had decreased spine density. These data suggest that the response of rat dentate granule cells to aging and estradiol is sexually dimorphic and that, in females, the responsiveness of granule cells depends on the temporal pattern of estradiol replacement.

Dentate granule cell neurogenesis is increased by seizures and contributes to aberrant network reorganization in the adult rat hippocampus

The dentate granule cell layer of the rodent hippocampal formation has the distinctive property of ongoing neurogenesis that continues throughout adult life. In both human temporal lobe epilepsy and rodent models of limbic epilepsy, this same neuronal population undergoes extensive remodeling, including reorganization of mossy fibers, dispersion of the granule cell layer, and the appearance of granule cells in ectopic locations within the dentate gyrus. The mechanistic basis of these abnormalities, as well as their potential relationship to dentate granule cell neurogenesis, is unknown. We used a systemic chemoconvulsant model of temporal lobe epilepsy and bromodeoxyuridine (BrdU) labeling to investigate the effects of prolonged seizures on dentate granule cell neurogenesis in adult rats, and to examine the contribution of newly differentiated dentate granule cells to the network changes seen in this model. Pilocarpine-induced status epilepticus caused a dramatic and prolonged increase in cell proliferation in the dentate subgranular proliferative zone (SGZ), an area known to contain neuronal precursor cells. Colocalization of BrdU-immunolabeled cells with the neuron-specific markers turned on after division, 64 kDa, class III beta-tubulin, or microtubule-associated protein-2 showed that the vast majority of these mitotically active cells differentiated into neurons in the granule cell layer. Newly generated dentate granule cells also appeared in ectopic locations in the hilus and inner molecular layer of the dentate gyrus. Furthermore, developing granule cells projected axons aberrantly to both the CA3 pyramidal cell region and the dentate inner molecular layer. Induction of hippocampal seizure activity by perforant path stimulation resulted in an increase in SGZ mitotic activity similar to that seen with pilocarpine administration. These observations indicate that prolonged seizure discharges stimulate dentate granule cell neurogenesis, and that hippocampal network plasticity associated with epileptogenesis may arise from aberrant connections formed by newly born dentate granule cells.