The dentate gyrus: fundamental neuroanatomical organization (dentate gyrus for dummies)

Dependence of the histofluorescently reactive zinc pool on zinc transporter-3 in the normal brain

In the brain, free zinc levels are under the exquisite control of a variety of zinc-regulating systems, in which zinc transporter (ZnT) proteins play a central role. ZnT3, which is prominently expressed in the brain, facilitates the concentration of free zinc in pre-synaptic vesicles. In addition to histochemical staining methods, a variety of zinc-specific fluorescence dyes has been developed to image or analyze zinc in brain tissue. In this study, we demonstrate the close correlations between histofluorescently reactive zinc and ZnT3. We examined the overlapping distribution of the zinc-specific fluorescent dye, N-(6-methoxy-8-quinolyl)-p-toluenesulfonamide (TSQ)-, and ZnT3-immunoreactive fluorescence throughout the normal brain. TSQ and ZnT3-antibody intensely stained the hippocampus, cortex and amygdala, highlighting the characteristic laminar organization of these regions by variably staining the different layers. TSQ fluorescence and ZnT3 immunoreactivity were roughly co-localized with synaptophysin along the neuropil, but were absent in the neuronal soma. However, albeit relatively faint, TSQ fluorescence was also found throughout the brains of ZnT3-knockout mice. Although these results may indicate the presence of very small cerebral free zinc pools distinct from synaptic vesicle zinc, the synaptic vesicle zinc pool is predominant, accounting for more than 95% of the entire histofluorescently reactive zinc pool in the hippocampus and cortex. Thus, the physiological activity of free zinc in the normal brain might largely depend on the pool of synaptic vesicle zinc that is determined by ZnT3.

Resolving new memories: a critical look at the dentate gyrus, adult neurogenesis, and pattern separation

Recently, investigation of new neurons in memory formation has focused on a specific function-pattern separation. However, it has been difficult to reconcile the form of separation tested in behavioral tasks with how it is conceptualized according to computational and electrophysiology perspectives. Here, we propose a memory resolution hypothesis that considers the unique information contributions of broadly tuned young neurons and highly specific mature neurons and describe how the fidelity of memories can relate to spatial and contextual discrimination. See the related Perspective from Sahay, Wilson, and Hen, "Pattern Separation: A Common Function for New Neurons in Hippocampus and Olfactory Bulb," in this issue of Neuron.

Neurogliaform cells in the molecular layer of the dentate gyrus as feed-forward γ-aminobutyric acidergic modulators of entorhinal-hippocampal interplay

Feed-forward inhibition from molecular layer interneurons onto granule cells (GCs) in the dentate gyrus is thought to have major effects regulating entorhinal-hippocampal interactions, but the precise identity, properties, and functional connectivity of the GABAergic cells in the molecular layer are not well understood. We used single and paired intracellular patch clamp recordings from post-hoc-identified cells in acute rat hippocampal slices and identified a subpopulation of molecular layer interneurons that expressed immunocytochemical markers present in members of the neurogliaform cell (NGFC) class. Single NGFCs displayed small dendritic trees, and their characteristically dense axonal arborizations covered significant portions of the outer and middle one-thirds of the molecular layer, with frequent axonal projections across the fissure into the CA1 and subicular regions. Typical NGFCs exhibited a late firing pattern with a ramp in membrane potential prior to firing action potentials, and single spikes in NGFCs evoked biphasic, prolonged GABA(A) and GABA(B) postsynaptic responses in GCs. In addition to providing dendritic GABAergic inputs to GCs, NGFCs also formed chemical synapses and gap junctions with various molecular layer interneurons, including other NGFCs. NGFCs received low-frequency spontaneous synaptic events, and stimulation of perforant path fibers revealed direct, facilitating synaptic inputs from the entorhinal cortex. Taken together, these results indicate that NGFCs form an integral part of the local molecular layer microcircuitry generating feed-forward inhibition and provide a direct GABAergic pathway linking the dentate gyrus to the CA1 and subicular regions through the hippocampal fissure.

Effects of environmental enrichment and voluntary exercise on neurogenesis, learning and memory, and pattern separation: BDNF as a critical variable?

Adult-generated neurons in the dentate gyrus of the hippocampus have been the focus of many studies concerned with learning and memory (L&M). It has been shown that procedures like environmental enrichment (EE) or voluntary physical exercise (Vex) can increase neurogenesis (NG) and also enhance L&M. It is tempting to conclude that improvements in L&M are due to the increased NG; that is, a causal relationship exists between enhancement of NG and enhancement of L&M. However, it remains unclear whether the L&M enhancement observed after these treatments is causally dependent on the increase in newborn neurons in the dentate gyrus. It remains a possibility that some unspecified change--a "third variable"--brought about by EE and/or Vex could be a causal determinant of both NG and L&M. We suggest that this third variable could be neurotrophic and/or plasticity-related factors such as BDNF. Indeed, both EE and Vex can induce expression of such proteins, and BDNF in particular has long been linked with L&M. In addition, we argue that a very likely source of variation in previous experiments was the load on "pattern separation", a process that keeps similar memories distinct, and in which NG has been shown to be critically involved. To attempt to bring these ideas together, we present preliminary evidence that BDNF is also required for pattern separation, which strengthens the case for BDNF as a candidate third variable. Other ways in which BDNF might be involved are also discussed.

Early behavioral changes and quantitative analysis of neuropathological features in murine prion disease: stereological analysis in the albino Swiss mice model

Behavioral and neuropathological changes have been widely investigated in murine prion disease but stereological based unbiased estimates of key neuropathological features have not been carried out. After injections of ME7 infected (ME7) or normal brain homogenates (NBH) into dorsal CA1 of albino Swiss mice and C57BL6, we assessed behavioral changes on hippocampal-dependent tasks. We also estimated by optical fractionator at 15 and 18 weeks post-injections (w.p.i.) the total number of neurons, reactive astrocytes, activated microglia and perineuronal nets (PN) in the polymorphic layer of dentate gyrus (PolDG), CA1 and septum in albino Swiss mice. On average, early behavioral changes in albino Swiss mice start four weeks later than in C57BL6. Cluster and discriminant analysis of behavioral data in albino Swiss mice revealed that four of nine subjects start to change their behavior at 12 w.p.i. and reach terminal stage at 22 w.p.i and the remaining subjects start at 22 w.p.i. and reach terminal stage at 26 w.p.i. Biotinylated dextran-amine BDA-tracer experiments in mossy fiber pathway confirmed axonal degeneration, and stereological data showed that early astrocytosis, microgliosis and reduction in the perineuronal nets are independent of a change in the number of neuronal cell bodies. Statistical analysis revealed that the septal region had greater levels of neuroinflammation and extracellular matrix damage than CA1. This stereological and multivariate analysis at early stages of disease in an outbred model of prion disease provided new insights connecting behavioral changes and neuroinflammation and seems to be important to understand the mechanisms of prion disease progression.

Therapeutic window of opportunity for the neuroprotective effect of valproate versus the competitive AMPA receptor antagonist NS1209 following status epilepticus in rats

Epileptogenesis, i.e., the process leading to epilepsy, is a presumed consequence of brain insults including head trauma, stroke, infections, tumors, status epilepticus (SE), and complex febrile seizures. Typically, brain insults produce morphological and functional alterations in the hippocampal formation, including neurodegeneration in CA1, CA3, and, most consistently, the dentate hilus. Most of these alterations develop gradually, over several days, after the insult, providing a therapeutic window of opportunity for neuroprotective agents in the immediate post-injury period. We have previously reported that prolonged (four weeks) treatment with the antiepileptic drug valproate (VPA) after SE prevents hippocampal damage and most of the behavioral alterations that occur after brain insult, but not the development of spontaneously occurring seizures. These data indicated that VPA, although not preventing epilepsy, might be an effective disease-modifying treatment following brain insult. The present study was designed to (1) determine the therapeutic window for the neuroprotective effect of VPA after SE; (2) compare the efficacy of different intermittent i.p. versus continuous i.v. VPA treatment protocols; and (3) compare VPA with the glutamate (AMPA) receptor antagonist NS1209. As in our previous study with VPA, SE was induced by sustained electrical stimulation of the basolateral amygdala in rats and terminated after 4 h by diazepam. In vehicle controls, >90% of the animals developed significant neurodegeneration in the dentate hilus, whereas damage in CA1 and CA3 was more variable. Hilar parvalbumin-expressing interneurons were more sensitive to the effects of seizures than somatostatin-stained hilar interneurons or hilar mossy cells. Among the various VPA treatment protocols, continuous infusion of VPA for 24 immediately following the SE was the most effective neuroprotective treatment, preventing most of the neuronal damage. Infusion with NS1209 for 24 h exhibited similar neuroprotective efficacy. These data demonstrate that short treatment after SE with either VPA or NS1209 is powerfully neuroprotective, and may be disease-modifying treatments following brain insult.

Cocaine and MDMA Induce Cellular and Molecular Changes in Adult Neurogenic Systems: Functional Implications

The capacity of the brain to generate new adult neurons is a recent discovery that challenges the old theory of an immutable adult brain. A new and fascinating field of research now focuses on this regenerative process. The two brain systems that constantly produce new adult neurons, known as the adult neurogenic systems, are the dentate gyrus (DG) of the hippocampus and the lateral ventricules/olfactory bulb system. Both systems are involved in memory and learning processes. Different drugs of abuse, such as cocaine and MDMA, have been shown to produce cellular and molecular changes that affect adult neurogenesis. This review summarizes the effects that these drugs have on the adult neurogenic systems. The functional relevance of adult neurogenesis is obscured by the functions of the systems that integrate adult neurons. Therefore, we explore the effects that cocaine and MDMA produce not only on adult neurogenesis, but also on the DG and olfactory bulbs. Finally, we discuss the possible role of new adult neurons in cocaine- and MDMA-induced impairments. We conclude that, although harmful drug effects are produced at multiple physiological and anatomical levels, the specific consequences of reduced hippocampus neurogenesis are unclear and require further exploration.

Segmentation of the mouse hippocampal formation in magnetic resonance images

The hippocampal formation plays an important role in cognition, spatial navigation, learning, and memory. High resolution magnetic resonance (MR) imaging makes it possible to study in vivo changes in the hippocampus over time and is useful for comparing hippocampal volume and structure in wild type and mutant mice. Such comparisons demand a reliable way to segment the hippocampal formation. We have developed a method for the systematic segmentation of the hippocampal formation using the perfusion-fixed C57BL/6 mouse brain for application in longitudinal and comparative studies. Our aim was to develop a guide for segmenting over 40 structures in an adult mouse brain using 30 μm isotropic resolution images acquired with a 16.4 T MR imaging system and combined using super-resolution reconstruction.

Differential transition metal uptake and fluorescent probe localization in hippocampal slices

Metals are taken up by the combined action of metal transporters and ion channels. In this communication we have measured the uptake of the biologically important transition metals Mn, Fe, Co, Ni, Cu, Zn and Cd by rat and mouse hippocampal slices using the fluorescent probes FluoZin-3 (FZ3) and Newport Green (NPG), introduced by acetoxymethyl ester (AM) loading. The combination of metals and probes is also used to attempt to localize cellular sites into which metals translocate. We show that FZ3 and NPG partition into different cellular compartments; FZ3 into neuropil, whereas NPG localizes in neuropil and compartments within the cell bodies of neurons. Ni, Zn and Cd pass across the plasma membrane and then accumulate in intracellular vesicles and within intracellular membranes of cell bodies. The latter accumulate Cd, while synaptic vesicles take up Co. The passage of Mn, Cu and Fe into cells can be detected but there is some uncertainty about their disposition within the cell. All of our experiments are consistent with metals accumulating in intracellular compartments rather than the cytoplasm. Whether and to what extent there are transient elevations of free zinc levels in the cytoplasm remains unclear.

Postnatal development of the hippocampal formation: a stereological study in macaque monkeys

We performed a stereological analysis of neuron number, neuronal soma size, and volume of individual regions and layers of the macaque monkey hippocampal formation during early postnatal development. We found a protracted period of neuron addition in the dentate gyrus throughout the first postnatal year and a concomitant late maturation of the granule cell population and individual dentate gyrus layers that extended beyond the first year of life. Although the development of CA3 generally paralleled that of the dentate gyrus, the distal portion of CA3, which receives direct entorhinal cortex projections, matured earlier than the proximal portion of CA3. CA1 matured earlier than the dentate gyrus and CA3. Interestingly, CA1 stratum lacunosum-moleculare, in which direct entorhinal cortex projections terminate, matured earlier than CA1 strata oriens, pyramidale, and radiatum, in which the CA3 projections terminate. The subiculum developed earlier than the dentate gyrus, CA3, and CA1, but not CA2. However, similarly to CA1, the molecular layer of the subiculum, in which the entorhinal cortex projections terminate, was overall more mature in the first postnatal year compared with the stratum pyramidale in which most of the CA1 projections terminate. Unlike other hippocampal fields, volumetric measurements suggested regressive events in the structural maturation of presubicular neurons and circuits. Finally, areal and neuron soma size measurements revealed an early maturation of the parasubiculum. We discuss the functional implications of the differential development of distinct hippocampal circuits for the emergence and maturation of different types of "hippocampus-dependent" memory processes, including spatial and episodic memories.

Morphometry of hilar ectopic granule cells in the rat

Granule cell (GC) neurogenesis in the dentate gyrus (DG) does not always proceed normally. After severe seizures (e.g., status epilepticus [SE]) and some other conditions, newborn GCs appear in the hilus. Hilar ectopic GCs (EGCs) can potentially provide insight into the effects of abnormal location and seizures on GC development. Additionally, hilar EGCs that develop after SE may contribute to epileptogenesis and cognitive impairments that follow SE. Thus, it is critical to understand how EGCs differ from normal GCs. Relatively little morphometric information is available on EGCs, especially those restricted to the hilus. This study quantitatively analyzed the structural morphology of hilar EGCs from adult male rats several months after pilocarpine-induced SE, when they are considered to have chronic epilepsy. Hilar EGCs were physiologically identified in slices, intracellularly labeled, processed for light microscopic reconstruction, and compared to GC layer GCs, from both the same post-SE tissue and the NeuroMorpho database (normal GCs). Consistently, hilar EGC and GC layer GCs had similar dendritic lengths and field sizes, and identifiable apical dendrites. However, hilar EGC dendrites were topologically more complex, with more branch points and tortuous dendritic paths. Three-dimensional analysis revealed that, remarkably, hilar EGC dendrites often extended along the longitudinal DG axis, suggesting increased capacity for septotemporal integration. Axonal reconstruction demonstrated that hilar EGCs contributed to mossy fiber sprouting. This combination of preserved and aberrant morphological features, potentially supporting convergent afferent input to EGCs and broad, divergent efferent output, could help explain why the hilar EGC population could impair DG function.

Is there a role for young hippocampal neurons in adaptation to stress?

The hippocampus has been implicated in many cognitive and emotional behaviors and in the physiology of the stress response. Within the hippocampus, the dentate gyrus has been implicated in the detection of novelty. The dentate is also a major target for stress hormones and modulates the hypothalamic-pituitary-adrenal (HPA) axis response to stress. Whether these functions of the dentate integrate or segregate remains unknown, as most investigations of its role in stress and learning are separate. Since the exciting discovery of adult neurogenesis in the dentate gyrus, adult-born neurons have been implicated in both novelty detection and the stress response. In this perspective we will discuss the literature that implicates the hippocampus, and potentially, adult-born neurons in these two functions. We will attempt to reconcile the seemingly contradictory behavioral results for the function of adult-born neurons. Finally, we will speculate that a key function of adult-born neurons within hippocampal function may be to modulate the stress response and perhaps assign stress salience to the sensory context.

Stage-dependent alterations of progenitor cell proliferation and neurogenesis in an animal model of Wernicke-Korsakoff syndrome

Alcohol-induced Wernicke-Korsakoff syndrome (WKS) culminates in bilateral diencephalic lesion and severe amnesia. Using the pyrithiamine-induced thiamine deficiency (PTD) animal paradigm of WKS, our laboratory has demonstrated hippocampal dysfunction in the absence of gross anatomical pathology. Extensive literature has revealed reduced hippocampal neurogenesis following a neuropathological insult, which might contribute to hippocampus-based learning and memory impairments. Thus, the current investigation was conducted to determine whether PTD treatment altered hippocampal neurogenesis in a stage-dependent fashion. Male Sprague-Dawley rats were assigned to one of 4 stages of thiamine deficiency based on behavioral symptoms: pre-symptomatic stage, ataxic stage, early post-opisthotonus stage, or the late post-opisthotonus stage. The S-phase mitotic marker 5'-bromo-2'-deoxyuridine (BrdU) was administered at the conclusion of each stage following thiamine restoration and subjects were perfused 24 hours or 28 days after BrdU to assess cellular proliferation or neurogenesis and survival, respectively. Dorsal hippocampal sections were immunostained for BrdU (proliferating cell marker), NeuN (neurons), GFAP (astrocytes), Iba-1 (microglia), and O4 (oligodendrocytes). The PTD treatment increased progenitor cell proliferation and survival during the early post-opisthotonus stage. However, levels of neurogenesis were reduced during this stage as well as the late post-opisthotonus stage where there was also an increase in astrocytogenesis. The diminished numbers of newly generated neurons (BrdU/NeuN co-localization) was paralleled by increased BrdU cells that did not co-localize with any of the phenotypic markers during these later stages. These data demonstrate that long-term alterations in neurogenesis and gliogenesis might contribute to the observed hippocampal dysfunction in the PTD model and human WKS.

From network structure to network reorganization: implications for adult neurogenesis

Networks can be dynamical systems that undergo functional and structural reorganization. One example of such a process is adult hippocampal neurogenesis, in which new cells are continuously born and incorporate into the existing network of the dentate gyrus region of the hippocampus. Many of these introduced cells mature and become indistinguishable from established neurons, joining the existing network. Activity in the network environment is known to promote birth, survival and incorporation of new cells. However, after epileptogenic injury, changes to the connectivity structure around the neurogenic niche are known to correlate with aberrant neurogenesis. The possible role of network-level changes in the development of epilepsy is not well understood. In this paper, we use a computational model to investigate how the structural and functional outcomes of network reorganization, driven by addition of new cells during neurogenesis, depend on the original network structure. We find that there is a stable network topology that allows the network to incorporate new neurons in a manner that enhances activity of the persistently active region, but maintains global network properties. In networks having other connectivity structures, new cells can greatly alter the distribution of firing activity and destroy the initial activity patterns. We thus find that new cells are able to provide focused enhancement of network only for small-world networks with sufficient inhibition. Network-level deviations from this topology, such as those caused by epileptogenic injury, can set the network down a path that develops toward pathological dynamics and aberrant structural integration of new cells.

Attenuated long-term Arc expression in the aged fascia dentata

One prominent component of aging is a defect in memory stabilization. To understand how the formation of enduring memories is altered in the aged brain, long-term markers of the biological events that may mediate memory consolidation were used to examine the activity dynamics of hippocampal circuits over extended intervals. The immediate early gene Arc, which is implicated in both durable memory and synaptic plasticity, is expressed in the fascia dentata (FD) for long periods following behavioral experience. To test the hypothesis that aging alters long-term Arc transcription in the FD, a region critical for spatial memory and impaired with progressive age, young and aged rats explored a novel environment twice, separated by an 8-hour interval, and FD Arc transcription was assessed. Relative to young rats, (a) fewer granule cells in the aged FD transcribe arc 8 hours after spatial exploration, and (b) this decrease is correlated with impaired spatial memory. These findings are consistent with behavioral evidence of age-related decline in hippocampal-dependent memory processing long after an event is to be remembered, and reaffirm the integral role of the FD in the neural circuits supporting durable memory.

Adult neurogenesis: integrating theories and separating functions

The continuous incorporation of new neurons in the dentate gyrus of the adult hippocampus raises exciting questions about memory and learning, and has inspired new computational models to understand the function of adult neurogenesis. These theoretical approaches suggest distinct roles for new neurons as they slowly integrate into the existing dentate gyrus network: immature adult-born neurons seem to function as pattern integrators of temporally adjacent events, thereby enhancing pattern separation for events separated in time; whereas maturing adult-born neurons possibly contribute to pattern separation by being more amenable to learning new information, leading to dedicated groups of granule cells that respond to experienced environments. We review these hypothesized functions and supporting empirical research and point to new directions for future theoretical efforts.

Increased pattern separation in the aged fascia dentata

One prominent impairment associated with aging is a deficit in the ability of the hippocampus to form stable contextual representations. Place-specific firing in granule cells of the fascia dentata (FD) is thought to aid the formation of multiple stable memory representations by disambiguating similar experiences (a process termed pattern separation), such as when an animal repeatedly enters similar environments or contexts. Using zif268/egr1 as a marker of cellular activity, we show that aged animals, which have altered place maps in other areas of the hippocampal formation, also show altered granule cell activity during multiple visits to similar environments. That is, the FD of aged animals is more likely to recruit distinct granule cell populations, and thus show greater pattern separation, during two visits to similar (or even the same) environments. However, if two highly distinct environments are visited, this age-related increase in pattern separation is no longer apparent. Moreover, increased pattern separation in similar environments correlates with decline in the ability of aged animals to disambiguate similar contexts in a sequential spatial recognition task.

New neurons and new memories: how does adult hippocampal neurogenesis affect learning and memory?

The integration of adult-born neurons into the circuitry of the adult hippocampus suggests an important role for adult hippocampal neurogenesis in learning and memory, but its specific function in these processes has remained elusive. In this article, we summarize recent progress in this area, including advances based on behavioural studies and insights provided by computational modelling. Increasingly, evidence suggests that newborn neurons might be involved in hippocampal functions that are particularly dependent on the dentate gyrus, such as pattern separation. Furthermore, newborn neurons at different maturation stages may make distinct contributions to learning and memory. In particular, computational studies suggest that, before newborn neurons are fully mature, they might function as a pattern integrator by introducing a degree of similarity to the encoding of events that occur closely in time.

Structural plasticity of dentate granule cell mossy fibers during the development of limbic epilepsy

Altered granule cell>>CA3 pyramidal cell synaptic connectivity may contribute to the development of limbic epilepsy. To explore this possibility, granule cell giant mossy fiber bouton plasticity was examined in the kindling and pilocarpine models of epilepsy using green fluorescent protein-expressing transgenic mice. These studies revealed significant increases in the frequency of giant boutons with satellite boutons 2 days and 1 month after pilocarpine status epilepticus, and increases in giant bouton area at 1 month. Similar increases in giant bouton area were observed shortly after kindling. Finally, both models exhibited plasticity of mossy fiber giant bouton filopodia, which contact GABAergic interneurons mediating feedforward inhibition of CA3 pyramids. In the kindling model, however, all changes were fleeting, having resolved by 1 month after the last evoked seizure. Together, these findings demonstrate striking structural plasticity of granule cell mossy fiber synaptic terminal structure in two distinct models of adult limbic epileptogenesis. We suggest that these plasticities modify local connectivities between individual mossy fiber terminals and their targets, inhibitory interneurons, and CA3 pyramidal cells potentially altering the balance of excitation and inhibition during the development of epilepsy.

Ongoing interplay between the neural network and neurogenesis in the adult hippocampus

As a unique form of structural plasticity in the central nervous system, adult neurogenesis in the hippocampus alters network functions by continuously adding new neurons to the mature network, while at the same time is subjected to regulation by surrounding network activity. Here, we review the recently identified mechanisms through which network activity exerts its impacts on multiple steps of adult neurogenesis in rodents and culminates in the selective recruitment of new neurons. We also review recent progress on the study of cellular connectivity modified by new neurons in the dentate gyrus and its physiological functions in rodents. We believe that understanding these processes will allow eventual elucidation of the mechanisms controlling the development of balanced inputs and outputs for the adult-born neurons and reveal important insights into the cellular organization of learning and memory.

Quantitative analysis of postnatal neurogenesis and neuron number in the macaque monkey dentate gyrus

The dentate gyrus is one of only two regions of the mammalian brain where substantial neurogenesis occurs postnatally. However, detailed quantitative information about the postnatal structural maturation of the primate dentate gyrus is meager. We performed design-based, stereological studies of neuron number and size, and volume of the dentate gyrus layers in rhesus macaque monkeys (Macaca mulatta) of different postnatal ages. We found that about 40% of the total number of granule cells observed in mature 5-10-year-old macaque monkeys are added to the granule cell layer postnatally; 25% of these neurons are added within the first three postnatal months. Accordingly, cell proliferation and neurogenesis within the dentate gyrus peak within the first 3 months after birth and remain at an intermediate level between 3 months and at least 1 year of age. Although granule cell bodies undergo their largest increase in size during the first year of life, cell size and the volume of the three layers of the dentate gyrus (i.e. the molecular, granule cell and polymorphic layers) continue to increase beyond 1 year of age. Moreover, the different layers of the dentate gyrus exhibit distinct volumetric changes during postnatal development. Finally, we observe significant levels of cell proliferation, neurogenesis and cell death in the context of an overall stable number of granule cells in mature 5-10-year-old monkeys. These data identify an extended developmental period during which neurogenesis might be modulated to significantly impact the structure and function of the dentate gyrus in adulthood.

The ratio of 'deleted in colorectal cancer' to 'uncoordinated-5A' netrin-1 receptors on the growth cone regulates mossy fibre directionality

Proper axonal targeting is fundamental to the establishment of functional neural circuits. The hippocampal mossy fibres normally project towards the CA3 region. In the hippocampi of patients with temporal lobe epilepsy and related animal models, however, mossy fibres project towards the molecular layer and produce the hyperexcitable recurrent networks. The cellular and molecular mechanisms underlying this aberrant axonal targeting, known as mossy fibre sprouting, remain unclear. Netrin-1 attracts or repels axons depending on the composition of its attraction-mediating receptor, deleted in colorectal cancer, and its repulsion-mediating receptor, uncoordinated-5, on the growth cone; but the roles of netrin-1-dependent guidance in pathological conditions are largely unknown. In this study, we examined the role of netrin-1 and its receptors in mossy fibre guidance and report that enhanced neuronal activity changes netrin-1-mediated cell targeting by the axons under hyperexcitable conditions. Netrin-1 antibody or Dcc ribonucleic acid interference attenuated mossy fibre growth towards CA3 in slice overlay assays. The axons were repelled from CA3 and ultimately innervated the molecular layer when hyperactivity was pharmacologically introduced. We first hypothesized that a reduction in netrin-1 expression in CA3 underlies the phenomenon, but found that its expression was increased. We then examined two possible activity-dependent changes in netrin-1 receptor expression: a reduction in the deleted in colorectal cancer receptor and induction of uncoordinated-5 receptor. Hyperactivity did not affect the surface expression of the deleted in colorectal cancer receptor on the growth cone, but it increased that of uncoordinated-5A, which was suppressed by blocking cyclic adenosine monophosphate signalling. In addition, Dcc knockdown did not affect hyperactivity-induced mossy fibre sprouting in the slice cultures, whereas Unc5a knockdown rescued the mistargeting. Thus, netrin-1 appears to attract mossy fibres via the deleted in colorectal cancer receptor, while it repels them via cyclic adenosine monophosphate-induced uncoordinated-5A under hyperexcitable conditions, resulting in mossy fibre sprouting.

Inhibition of VEGF receptor 2 increased cell death of dentate hilar neurons after traumatic brain injury

Post-traumatic epilepsy, partly due to the loss of hilar neurons of the hippocampus, is a frequent long-term consequence of traumatic brain injury (TBI). We and others found that the levels of vascular endothelial growth factor (VEGF) that can act as a neuroprotectant increase after TBI. Here we tested whether VEGF and its receptor VEGFR2 are involved in mediating the death or survival of hilar neurons after injury. We demonstrated that VEGFR2 is expressed by most, if not all, hilar neurons and that these neurons are dying in large numbers as indicated by Fluoro-Jade B histology after fluid percussion TBI. To directly test the involvement of VEGFR2 and VEGF in the injury-induced apoptotic death of hilar neurons, we delivered SU5416, an inhibitor to VEGFR2, or recombinant VEGF into the ipsilateral cerebral ventricle of injured animals. We found that blocking VEGFR2 by SU5416 significantly increased the number of apoptotic (TUNEL-positive) cells in the hilus. Infusion of VEGF, however, failed to reduce the number of TUNEL-positive cells. Our results suggest that VEGFR2 is involved in mediating death or survival of hilar neurons after injury but delivering additional exogenous VEGF does not provide further protection from TBI-induced death of hilar neurons.

Reduced excitability in the dentate gyrus network of betaIV-spectrin mutant mice in vivo

The submembrane cytoskeletal meshwork of the axon contains the scaffolding protein betaIV-spectrin. It provides mechanical support for the axon and anchors membrane proteins. Quivering (qv(3j)) mice lack functional betaIV-spectrin and have reduced voltage-gated sodium channel (VGSC) immunoreactivity at the axon initial segment and nodes of Ranvier. Because VGSCs are critically involved in action potential generation and conduction, we hypothesized that qv(3j) mice should also show functional deficits at the network level. To test this hypothesis, we investigated granule cell function in the dentate gyrus of anesthetized qv(3j) mice after electrical stimulation of the perforant path in vivo. This revealed an impaired input-output relationship between stimulus intensity and granule cell population spikes and an enhanced paired-pulse inhibition of population spikes, indicating a reduced ability of granule cells to generate action potentials and decreased network excitability. In contrast, the input-output curve for evoked field excitatory postsynaptic potentials (fEPSPs) and paired-pulse facilitation of fEPSPs were unchanged, suggesting normal excitatory synaptic transmission at perforant path-granule cell synapses in qv(3j) mutants. To corroborate our findings, we analyzed the influence of VGSC density reduction on dentate network activity using an established computational model of the dentate gyrus network. This in silico approach confirmed that the loss of VGSCs is sufficient to explain the electrophysiological changes observed in qv(3j) mice. Taken together, our findings demonstrate that betaIV-spectrin is required for normal granule cell firing and for physiological levels of network excitability in the mouse dentate gyrus in vivo.

Postnatal neurogenesis as a therapeutic target in temporal lobe epilepsy

After it was first identified that seizures increase neurogenesis in the adult brain of laboratory animals, the idea that postnatal neurogenesis may be involved in epilepsy became a topic of widespread interest. Since that time, two perspectives have developed. They primarily address temporal lobe epilepsy (TLE), because the data have either been based on animal models of TLE or patients with intractable TLE. The first perspective is that postnatal neurogenesis contributes to the predisposition for seizures in TLE. This premise is founded in the observations showing that there is a dramatic rise in neurogenesis after many types of insults or injuries which ultimately lead to TLE. As a result of the increase in neurogenesis, several changes in the dentate gyrus occur, and the net effect appears to be an increase in excitability. One of the changes is the formation of a population of granule cells (GCs) that mismigrate, leading to ectopic granule cells in the hilus (hilar EGCs) that exhibit periodic bursts of action potentials, and contribute to recurrent excitatory circuitry. Atypical dendrites also form on a subset of GCs, and project into the hilus (hilar basal dendrites). Hilar basal dendrites appear to preferentially increase the glutamatergic input relative to GABAergic synapses, increasing excitability of the subset of GCs that form hilar basal dendrites. The alternate view is that postnatal neurogenesis is a homeostatic mechanism in epilepsy that maintains normal excitability. This idea is supported by studies showing that some of the new GCs that are born after seizures, and migrate into the correct location, have normal or reduced excitability. Here we suggest that both perspectives may be important when considering a therapeutic strategy. It would seem advantageous to limit the numbers of mismigrating GCs and hilar basal dendrites, but maintain normal neurogenesis because it is potentially homeostatic. Maintaining normal neurogenesis is also important because it has been suggested that a decrease in dentate gyrus neurogenesis contributes to depression. It is challenging to design a strategy that would achieve these goals, and it is also difficult to propose how one could administer such a therapy prophylactically, that is, as an "antiepileptogenic" approach. Another issue to address is how a therapeutic intervention with these goals could be successful if it were administered after chronic seizures develop, when most patients seek therapy. Although difficult, a number of approaches are possible, and technical advances suggest that there are more on the horizon.

The partial 5-HT1A receptor agonist buspirone enhances neurogenesis in the opossum (Monodelphis domestica)

We demonstrate for the first time that neurogenesis in the adult Monodelphis opossum has a typical mammalian pattern and occurs only in the dentate gyrus (DG) and subventricular zone (SVZ) of the lateral ventricles. In these two brain regions neurogenesis is present throughout the lifespan, although its rate is reduced by half in the old age. Treatment with buspirone, a partial 5-HT1A receptor agonist which is used in human clinic as an anxiolytic agent, boosts proliferation in the SVZ and DG in both adult and aged opossums. The neuronal phenotype dominates among newly generated cells in both non-treated and buspirone-treated opossums. We suggest that if functional importance of adult neurogenesis is in improving olfactory discrimination and generation of hippocampus-dependent memory, both spatial and emotional, then administration of drugs increasing the rate of neurogenesis via activation of 5-HT1A receptors may be a valuable aid in combating problems of the advanced age.

A role for hilar cells in pattern separation in the dentate gyrus: a computational approach

We present a simple computational model of the dentate gyrus to evaluate the hypothesis that pattern separation, defined as the ability to transform a set of similar input patterns into a less-similar set of output patterns, is dynamically regulated by hilar neurons. Prior models of the dentate gyrus have generally fallen into two categories: simplified models that have focused on a single granule cell layer and its ability to perform pattern separation, and large-scale and biophysically realistic models of dentate gyrus, which include hilar cells, but which have not specifically addressed pattern separation. The present model begins to bridge this gap. The model includes two of the major subtypes of hilar cells: excitatory hilar mossy cells and inhibitory hilar interneurons that receive input from and project to the perforant path terminal zone (HIPP cells). In the model, mossy cells and HIPP cells provide a mechanism for dynamic regulation of pattern separation, allowing the system to upregulate and downregulate pattern separation in response to environmental and task demands. Specifically, pattern separation in the model can be strongly decreased by decreasing mossy cell function and/or by increasing HIPP cell function; pattern separation can be increased by the opposite manipulations. We propose that hilar cells may similarly mediate dynamic regulation of pattern separation in the dentate gyrus in vivo, not only because of their connectivity within the dentate gyrus, but also because of their modulation by brainstem inputs and by the axons that "backproject" from area CA3 pyramidal cells.

Computational influence of adult neurogenesis on memory encoding

Adult neurogenesis in the hippocampus leads to the incorporation of thousands of new granule cells into the dentate gyrus every month, but its function remains unclear. Here, we present computational evidence that indicates that adult neurogenesis may make three separate but related contributions to memory formation. First, immature neurons introduce a degree of similarity to memories learned at the same time, a process we refer to as pattern integration. Second, the extended maturation and change in excitability of these neurons make this added similarity a time-dependent effect, supporting the possibility that temporal information is included in new hippocampal memories. Finally, our model suggests that the experience-dependent addition of neurons results in a dentate gyrus network well suited for encoding new memories in familiar contexts while treating novel contexts differently. Taken together, these results indicate that new granule cells may affect hippocampal function in several unique and previously unpredicted ways.

Nitric oxide regulates activity-dependent neuroprotective protein (ADNP) in the dentate gyrus of the rodent model of kainic acid-induced seizure

The dentate gyrus (DG) of the normal rat brain contains activity-dependent neuroprotective protein (ADNP) which is widely distributed in the cytoplasm of neurons and astrocytes. Treatment with nitric oxide (NO) synthase (NOS) inhibitor N(G)-nitro-L: -arginine methyl ester (L: -NAME) caused a decrease in ADNP expression in granule cells which persisted 3 days post-treatment. However, treatment with neuronal-specific NOS inhibitor, 7-nitroindazole (7-NI), or soluble guanylyl cyclase inhibitor, ODQ, did not change ADNP expression in the DG. We have previously shown that kainic acid (KA)-induced seizure increases neuronal NOS in neurons and inducible NOS in glia cells and suppresses ADNP in the hippocampus (Cosgrave et al., Neurobiol Dis 30(3):281-292, 2008). In the DG, L: -NAME treatment prior to KA causes ADNP synthesis in granule cells by 3 h which was later restricted to the subgranular zone by 3 days. 7-NI and ODQ had no effect. Double immunostaining for neuronal marker NeuN and ADNP revealed a significant decrease of both ADNP(+) neurons and of total neuron numbers (NeuN(+)) in the hilus of animals having KA-induced seizure that had been pretreated with L: -NAME implying that NO and ADNP may act together to protect hilar neurons. Overall, these observations suggest that NO regulates ADNP in the DG under both basal and pathophysiological conditions.

Intrinsic connections of the macaque monkey hippocampal formation: I. Dentate gyrus

We have carried out a detailed analysis of the intrinsic connectivity of the Macaca fascicularis monkey hippocampal formation. Here we report findings on the topographical organization of the major connections of the dentate gyrus. Localized anterograde tracer injections were made at various rostrocaudal levels of the dentate gyrus, and we investigated the three-dimensional organization of the mossy fibers, the associational projection, and the local projections. The mossy fibers travel throughout the transverse extent of CA3 at the level of the cells of origin. Once the mossy fibers reach the distal portion of CA3, they change course and travel for 3-5 mm rostrally. The associational projection, originating from cells in the polymorphic layer, terminates in the inner one-third of the molecular layer. The associational projection, though modest at the level of origin, travels both rostrally and caudally from the injection site for as much as 80% of the rostrocaudal extent of the dentate gyrus. The caudally directed projection is typically more extensive and denser than the rostrally directed projection. Cells in the polymorphic layer originate local projections that terminate in the outer two-thirds of the molecular layer. These projections are densest at the level of the cells of origin but also extend several millimeters rostrocaudally. Overall, the topographic organization of the intrinsic connections of the monkey dentate gyrus is largely similar to that of the rat. Such extensive longitudinal connections have the potential for integrating information across much of the rostrocaudal extent of the dentate gyrus.

Shape-based alignment of hippocampal subfields: evaluation in postmortem MRI

This paper estimates the accuracy of hippocampal subfield alignment via shape-based normalization. Evaluation takes place in postmortem MRI dataset acquired at 9.4 Tesla with many averages and approximately 0.01 mm3 voxel resolution. Continuous medial representations (cm-reps) are used to establish geometrical correspondences between hippocampal formations in different images; the extent to which these correspondences match up subfields is evaluated and compared to normalization driven by image forces. Shape-based normalization is shown to perform only slightly worse than image-based normalization; this is encouraging because the former is more applicable to in vivo MRI, which typically lacks features that distinguish hippocampal subfields.

Protective effect of BDNF against beta-amyloid induced neurotoxicity in vitro and in vivo in rats

We examined the potential protective effect of BDNF against beta-amyloid-induced neurotoxicity in vitro and in vivo in rats. In neuronal cultures, BDNF had specific and dose-response protective effects on neuronal toxicity induced by Abeta(1-42) and Abeta(25-35). It completely reversed the toxic action induced by Abeta(1-42) and partially that induced by Abeta(25-35). These effects involved TrkB receptor activation since they were inhibited by K252a. Catalytic BDNF receptors (TrkB.FL) were localized in vitro in cortical neurons (mRNA and protein). In in vivo experiments, Abeta(25-35) was administered into the indusium griseum or the third ventricle and several parameters were measured 7 days later to evaluate potential Abeta(25-35)/BDNF interactions, i.e. local measurement of BDNF release, number of hippocampal hilar cells expressing SRIH mRNA and assessment of the corpus callosum damage (morphological examination, pyknotic nuclei counting and axon labeling with anti-MBP antibody). We conclude that BDNF possesses neuroprotective properties against toxic effects of Abeta peptides.

High threshold, proximal initiation, and slow conduction velocity of action potentials in dentate granule neuron mossy fibers

Dentate granule neurons give rise to some of the smallest unmyelinated fibers in the mammalian CNS, the hippocampal mossy fibers. These neurons are also key regulators of physiological and pathophysiological information flow through the hippocampus. We took a comparative approach to studying mossy fiber action potential initiation and propagation in hippocampal slices from juvenile rats. Dentate granule neurons exhibited axonal action potential initiation significantly more proximal than CA3 pyramidal neurons. This conclusion was suggested by phase plot analysis of somatic action potentials and by local tetrodotoxin application to the axon and somatodendritic compartments. This conclusion was also verified by immunostaining for voltage-gated sodium channel alpha subunits and by direct dual soma/axonal recordings. Dentate neurons exhibited a significantly higher action potential threshold and slower axonal conduction velocity than CA3 neurons. We conclude that while the electrotonically proximal axon location of action potential initiation allows granule neurons to sensitively detect and integrate synaptic inputs, the neurons are sluggish to initiate and propagate an action potential.