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

Hippocampal laminar distribution of tau relates to Alzheimer's disease and age of onset

BACKGROUND

Cerebral deposition of phospho-tau in Alzheimer's disease (AD) occurs with varying patterns within hippocampus. Lamina-specific tau changes in AD may reflect trans-synaptic propagation of phospho-tau along neuroanatomical pathways.

OBJECTIVE

To study patterns of tau deposition within inner (IML) and outer (OML) molecular layers of dentate gyrus and their clinical and neuropathological correlates.

METHODS

98 consecutive autopsied brains from the Columbia University Brain Bank were stained for phospho-tau using AT-8. Staining density was rated as High versus Low within IML and OML. Four patterns were observed among the 98 brains: High IML&OML, n = 44; High OML Only (n = 35); High IML Only (n = 5); and Low IML&OML (n = 14). Demographic, clinical, and neuropathological characteristics of these four groups were compared.

RESULTS

High IML&OML subjects, versus High OML Only, were more likely to fulfill CERAD criteria for Definite AD (93% versus 66%, p < 0.01) and to have higher median Braak stage (6 versus 5, p < 0.01) and earlier mean age of onset (65.9 versus 73.7 y, p = 0.02), with similar symptom duration. Using logistic regression, the association between High IML&OML and AD remained significant after adjustment for demographics but not symptom duration. In the 70 subjects with Definite AD, High IML&OML was associated with younger age of onset (mean difference 3.7 years, 95%CI -6.7 to -0.7, p < 0.01), after adjustment for demographics and symptom duration.

CONCLUSIONS

Phospho-tau pathology, when prominent within both IML and OML, is associated with CERAD diagnosis of Definite AD and earlier age of onset in AD. Laminar patterns of tau deposition reflect regional involvements during disease course.

Dentate gyrus and hilar region revisited

It is suggested that the dentate gyrus and hilar region in the hippocampus perform memory selection and that the selectivity of the gating of memory by this circuit is modulated by the norepinephrine–glutamate loop described by Mather et al.

Gastrin-releasing peptide contributes to the regulation of adult hippocampal neurogenesis and neuronal development

In the postnatal hippocampus, newly generated neurons contribute to learning and memory. Disruptions in neurogenesis and neuronal development have been linked to cognitive impairment and are implicated in a broad variety of neurological and psychiatric disorders. To identify putative factors involved in this process, we examined hippocampal gene expression alterations in mice possessing a heterozygous knockout of the calcium/calmodulin-dependent protein kinase II alpha heterozygous knockout gene (CaMK2α-hKO), an established model of cognitive impairment that also displays altered neurogenesis and neuronal development. Using this approach, we identified gastrin-releasing peptide (GRP) as the most dysregulated gene. In wild-type mice, GRP labels NeuN-positive neurons, the lone exception being GRP-positive, NeuN-negative cells in the subgranular zone, suggesting GRP expression may be relevant to neurogenesis and/or neuronal development. Using a model of in vitro hippocampal neurogenesis, we determined that GRP signaling is essential for the continued survival and development of newborn neurons, both of which are blocked by transient knockdown of GRP's cognate receptor (GRPR). Furthermore, GRP appears to negatively regulate neurogenesis-associated proliferation in neural stem cells both in vitro and in vivo. Intracerebroventricular infusion of GRP resulted in a decrease in immature neuronal markers, increased cAMP response element-binding protein (CREB) phosphorylation, and decreased neurogenesis. Despite increased levels of GRP mRNA, CaMK2α-hKO mutant mice expressed reduced levels of GRP peptide. This lack of GRP may contribute to the elevated neurogenesis and impaired neuronal development, which are reversed following exogenous GRP infusion. Based on these findings, we hypothesize that GRP modulates neurogenesis and neuronal development and may contribute to hippocampus-associated cognitive impairment.

Maturation and Functional Integration of New Granule Cells into the Adult Hippocampus

The adult hippocampus generates functional dentate granule cells (GCs) that release glutamate onto target cells in the hilus and cornus ammonis (CA)3 region, and receive glutamatergic and γ-aminobutyric acid (GABA)ergic inputs that tightly control their spiking activity. The slow and sequential development of their excitatory and inhibitory inputs makes them particularly relevant for information processing. Although they are still immature, new neurons are recruited by afferent activity and display increased excitability, enhanced activity-dependent plasticity of their input and output connections, and a high rate of synaptogenesis. Once fully mature, new GCs show all the hallmarks of neurons generated during development. In this review, we focus on how developing neurons remodel the adult dentate gyrus and discuss key aspects that illustrate the potential of neurogenesis as a mechanism for circuit plasticity and function.

Interactions between Inhibitory Interneurons and Excitatory Associational Circuitry in Determining Spatio-Temporal Dynamics of Hippocampal Dentate Granule Cells: A Large-Scale Computational Study

This paper reports on findings from a million-cell granule cell model of the rat dentate gyrus that was used to explore the contributions of local interneuronal and associational circuits to network-level activity. The model contains experimentally derived morphological parameters for granule cells, which each contain approximately 200 compartments, and biophysical parameters for granule cells, basket cells, and mossy cells that were based both on electrophysiological data and previously published models. Synaptic input to cells in the model consisted of glutamatergic AMPA-like EPSPs and GABAergic-like IPSPs from excitatory and inhibitory neurons, respectively. The main source of input to the model was from layer II entorhinal cortical neurons. Network connectivity was constrained by the topography of the system, and was derived from axonal transport studies, which provided details about the spatial spread of axonal terminal fields, as well as how subregions of the medial and lateral entorhinal cortices project to subregions of the dentate gyrus. Results of this study show that strong feedback inhibition from the basket cell population can cause high-frequency rhythmicity in granule cells, while the strength of feedforward inhibition serves to scale the total amount of granule cell activity. Results furthermore show that the topography of local interneuronal circuits can have just as strong an impact on the development of spatio-temporal clusters in the granule cell population as the perforant path topography does, both sharpening existing clusters and introducing new ones with a greater spatial extent. Finally, results show that the interactions between the inhibitory and associational loops can cause high frequency oscillations that are modulated by a low-frequency oscillatory signal. These results serve to further illustrate the importance of topographical constraints on a global signal processing feature of a neural network, while also illustrating how rich spatio-temporal and oscillatory dynamics can evolve from a relatively small number of interacting local circuits.

High spatial resolution compressed sensing (HSPARSE) functional MRI

PURPOSE

To propose a novel compressed sensing (CS) high spatial resolution functional MRI (fMRI) method and demonstrate the advantages and limitations of using CS for high spatial resolution fMRI.

METHODS

A randomly undersampled variable density spiral trajectory enabling an acceleration factor of 5.3 was designed with a balanced steady state free precession sequence to achieve high spatial resolution data acquisition. A modified k-t SPARSE method was then implemented and applied with a strategy to optimize regularization parameters for consistent, high quality CS reconstruction.

RESULTS

The proposed method improves spatial resolution by six-fold with 12 to 47% contrast-to-noise ratio (CNR), 33 to 117% F-value improvement and maintains the same temporal resolution. It also achieves high sensitivity of 69 to 99% compared the original ground-truth, small false positive rate of less than 0.05 and low hemodynamic response function distortion across a wide range of CNRs. The proposed method is robust to physiological noise and enables detection of layer-specific activities in vivo, which cannot be resolved using the highest spatial resolution Nyquist acquisition.

CONCLUSION

The proposed method enables high spatial resolution fMRI that can resolve layer-specific brain activity and demonstrates the significant improvement that CS can bring to high spatial resolution fMRI. Magn Reson Med 76:440-455, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Hippocampome.org: a knowledge base of neuron types in the rodent hippocampus

Hippocampome.org is a comprehensive knowledge base of neuron types in the rodent hippocampal formation (dentate gyrus, CA3, CA2, CA1, subiculum, and entorhinal cortex). Although the hippocampal literature is remarkably information-rich, neuron properties are often reported with incompletely defined and notoriously inconsistent terminology, creating a formidable challenge for data integration. Our extensive literature mining and data reconciliation identified 122 neuron types based on neurotransmitter, axonal and dendritic patterns, synaptic specificity, electrophysiology, and molecular biomarkers. All ∼3700 annotated properties are individually supported by specific evidence (∼14,000 pieces) in peer-reviewed publications. Systematic analysis of this unprecedented amount of machine-readable information reveals novel correlations among neuron types and properties, the potential connectivity of the full hippocampal circuitry, and outstanding knowledge gaps. User-friendly browsing and online querying of Hippocampome.org may aid design and interpretation of both experiments and simulations. This powerful, simple, and extensible neuron classification endeavor is unique in its detail, utility, and completeness.

DOI:http://dx.doi.org/10.7554/eLife.09960.001

The hippocampus is a seahorse-shaped region of the brain that is responsible for learning, emotions, and memory. Like other regions of the brain, it contains many types of neurons that send information to each other by releasing chemicals called neurotransmitters across junctions known as synapses. Identifying all the different neuron types in the hippocampus is an important step towards understanding in detail how this brain region works.

Thousands of articles have been published that attempt to characterize the neurons in the hippocampus, but many of these studies report only some of the properties of a new neuron type. It is also often difficult to compare the results of different studies, as many different approaches have been used to investigate neuron types, and different studies often use different terms to describe similar features.

Wheeler et al. have now created a resource called Hippocampome.org that combines approximately 14,000 pieces of experimental evidence about neuron types in the rat hippocampus into a unified database. Analyzing these data has revealed about 3700 different neuron properties. By primarily considering the pattern formed by the branched axons and dendrites, the outputs and inputs that extend out of a neuron, Wheeler et al. have identified over a hundred different neuron types. This classification system also considers how selectively the neuron forms synapses with other cells and the identity of the neurotransmitter released by the neuron. In the future, other features of the neurons will also be incorporated into the system to help refine the classifications.

All of this information is online and freely available at Hippocampome.org. This resource is expected to provide a solid basis for analyzing how the hippocampus works, by helping researchers to design and interpret experiments and simulations.

DOI:http://dx.doi.org/10.7554/eLife.09960.002

Chronic Alcohol Exposure is Associated with Decreased Neurogenesis, Aberrant Integration of Newborn Neurons, and Cognitive Dysfunction in Female Mice

BACKGROUND

Neurological deficits of alcohol use disorder (AUD) have been attributed to dysfunctions of specific brain structures. Studies of alcoholic patients and chronic alcohol exposure animal models consistently identify reduced hippocampal mass and cogntive dysfunctions as a key alcohol-induced brain adaptation. However, the precise substrate of chronic alcohol exposure that leads to structural and functional impairments of the hippocampus is largely unknown.

METHODS

Using a calorie-matched alcohol feeding method, we tested whether chronic alcohol exposure targets neural stem cells and neurogenesis in the adult hippocampus. The effect of alcohol on proliferation of neural stem cells as well as cell fate determination and survival of newborn cells was evaluated via bromodeoxyuridine pulse and chase methods. A retrovirus-mediated single-cell labeling method was used to determine the effect of alcohol on the morphological development and circuitry incorporation of individual hippocampal newborn neurons. Finally, novel object recognition (NOR) and Y-maze tests were performed to examine whether disrupted neurogenesis is associated with hippocampus-dependent functional deficits in alcohol-fed mice.

RESULTS

Chronic alcohol exposure reduced proliferation of neural stem cells and survival rate of newborn neurons; however, the fate determination of newborn cells remained unaltered. Moreover, the dendritic spine density of newborn neurons significantly decreased in alcohol-fed mice. Impaired spine formation indicates that alcohol interfered the synaptic connectivity of newborn neurons with excitatory neurons originating from various areas of the brain. In the NOR test, alcohol-fed mice displayed deficits in the ability to discriminate the novel object.

CONCLUSIONS

Our study revealed that chronic alcohol exposure disrupted multiple steps of neurogenesis, including the production and development of newborn neurons. In addition, chronic alcohol exposure altered connectivity of newborn neurons with other input neurons. Decreased neurogenesis and aberrant integration of newborn neurons into hippocampal networks are closely associated with deficits in hippocampus-dependent cognitive functions of alcohol-fed mice.

Observations on hippocampal mossy cells in mink (Neovison vison) with special reference to dendrites ascending to the granular and molecular layers

Detailed knowledge about the neural circuitry connecting the hippocampus and entorhinal cortex is necessary to understand how this system contributes to spatial navigation and episodic memory. The two principal cell types of the dentate gyrus, mossy cells and granule cells, are interconnected in a positive feedback loop, by which mossy cells can influence information passing from the entorhinal cortex via granule cells to hippocampal pyramidal cells. Mossy cells, like CA3 pyramidal cells, are characterized by thorny excrescences on their proximal dendrites, postsynaptic to giant terminals of granule cell axons. In addition to disynaptic input from the entorhinal cortex and perforant path via granule cells, mossy cells may also receive monosynaptic input from the perforant path via special dendrites ascending to the molecular layer. We here report qualitative and quantitative descriptions of Golgi-stained hippocampal mossy cells in mink, based on light microscopic observations and three-dimensional reconstructions. The main focus is on the location, branching pattern, and length of dendrites, particularly those ascending to the granular and molecular layers. In mink, the latter dendrites are more numerous than in rat, but fewer than in primates. They form on average 12% (and up to 29%) of the total dendritic length, and appear to cover the terminal fields of both the lateral and medial perforant paths. In further contrast to rat, the main mossy cell dendrites in mink branch more extensively with distal dendrites encroaching upon the CA3 field. The dendritic arbors extend both along and across the septotemporal axis of the dentate gyrus, not conforming to the lamellar pattern of the hippocampus. The findings suggest that the afferent input to the mossy cells becomes more complex in species closer to primates.

Prostaglandin E2 EP2 activation reduces memory decline in R6/1 mouse model of Huntington's disease by the induction of BDNF-dependent synaptic plasticity

Huntington's disease (HD) patients and mouse models show learning and memory impairment even before the onset of motor symptoms. Deficits in hippocampal synaptic plasticity have been involved in the HD memory impairment. Several studies show that prostaglandin E2 (PGE2) EP2 receptor stimulates synaptic plasticity and memory formation. However, this role was not explored in neurodegenerative diseases. Here, we investigated the capacity of PGE2 EP2 receptor to promote synaptic plasticity and memory improvements in a model of HD, the R6/1 mice, by administration of the agonist misoprostol. We found that misoprostol increases dendritic branching in cultured hippocampal neurons in a brain-derived neurotrophic factor (BDNF)-dependent manner. Then, we implanted an osmotic mini-pump system to chronically administrate misoprostol to R6/1 mice from 14 to 18weeks of age. We observed that misoprostol treatment ameliorates the R6/1 long-term memory deficits as analyzed by the T-maze spontaneous alternation task and the novel object recognition test. Importantly, administration of misoprostol promoted the expression of hippocampal BDNF. Moreover, the treatment with misoprostol in R6/1 mice blocked the reduction in the number of PSD-95 and VGluT-1 positive particles observed in hippocampus of vehicle-R6/1 mice. In addition, we observed an increase of cAMP levels in the dentate ` of WT and R6/1 mice treated with misoprostol. Accordingly, we showed a reduction in the number of mutant huntingtin nuclear inclusions in the dentate gyrus of R6/1 mice. Altogether, these results suggest a putative therapeutic effect of PGE2 EP2 receptor in reducing cognitive deficits in HD.

Homeostatic regulation of KCC2 activity by the zinc receptor mZnR/GPR39 during seizures

The aim of this study was to investigate the role of the synaptic metabotropic zinc receptor mZnR/GPR39 in physiological adaptation to epileptic seizures. We previously demonstrated that synaptic activation of mZnR/GPR39 enhances inhibitory drive in the hippocampus by upregulating neuronal K(+)/Cl(-) co-transporter 2 (KCC2) activity. Here, we first show that mZnR/GPR39 knockout (KO) adult mice have dramatically enhanced susceptibility to seizures triggered by a single intraperitoneal injection of kainic acid, when compared to wild type (WT) littermates. Kainate also substantially enhances seizure-associated gamma oscillatory activity in juvenile mZnR/GPR39 KO hippocampal slices, a phenomenon that can be reproduced in WT tissue by extracellular Zn(2+) chelation. Importantly, kainate-induced synaptic Zn(2+) release enhances surface expression and transport activity of KCC2 in WT, but not mZnR/GPR39 KO hippocampal neurons. Kainate-dependent upregulation of KCC2 requires mZnR/GPR39 activation of the Gαq/phospholipase C/extracellular regulated kinase (ERK1/2) signaling cascade. We suggest that mZnR/GPR39-dependent upregulation of KCC2 activity provides homeostatic adaptation to an excitotoxic stimulus by increasing inhibition. As such, mZnR/GPR39 may provide a novel pharmacological target for dampening epileptic seizure activity.

Potential implications of a monosynaptic pathway from mossy cells to adult-born granule cells of the dentate gyrus

The dentate gyrus (DG) is important to many aspects of hippocampal function, but there are many aspects of the DG that are incompletely understood. One example is the role of mossy cells (MCs), a major DG cell type that is glutamatergic and innervates the primary output cells of the DG, the granule cells (GCs). MCs innervate the GCs as well as local circuit neurons that make GABAergic synapses on GCs, so the net effect of MCs on GCs – and therefore the output of the DG – is unclear. Here we first review fundamental information about MCs and the current hypotheses for their role in the normal DG and in diseases that involve the DG. Then we review previously published data which suggest that MCs are a source of input to a subset of GCs that are born in adulthood (adult-born GCs). In addition, we discuss the evidence that adult-born GCs may support the normal inhibitory ‘gate’ functions of the DG, where the GCs are a filter or gate for information from the entorhinal cortical input to area CA3. The implications are then discussed in the context of seizures and temporal lobe epilepsy (TLE). In TLE, it has been suggested that the DG inhibitory gate is weak or broken and MC loss leads to insufficient activation of inhibitory neurons, causing hyperexcitability. That idea was called the “dormant basket cell hypothesis.” Recent data suggest that loss of normal adult-born GCs may also cause disinhibition, and seizure susceptibility. Therefore, we propose a reconsideration of the dormant basket cell hypothesis with an intervening adult-born GC between the MC and basket cell and call this hypothesis the “dormant immature granule cell hypothesis.”

Irradiation of Neurons with High-Energy Charged Particles: An In Silico Modeling Approach

In this work, a stochastic computational model of microscopic energy deposition events is used to study for the first time damage to irradiated neuronal cells of the mouse hippocampus. An extensive library of radiation tracks for different particle types is created to score energy deposition in small voxels and volume segments describing a neuron's morphology that later are sampled for given particle fluence or dose. Methods included the construction of in silico mouse hippocampal granule cells from neuromorpho.org with spine and filopodia segments stochastically distributed along the dendritic branches. The model is tested with high-energy (56)Fe, (12)C, and (1)H particles and electrons. Results indicate that the tree-like structure of the neuronal morphology and the microscopic dose deposition of distinct particles may lead to different outcomes when cellular injury is assessed, leading to differences in structural damage for the same absorbed dose. The significance of the microscopic dose in neuron components is to introduce specific local and global modes of cellular injury that likely contribute to spine, filopodia, and dendrite pruning, impacting cognition and possibly the collapse of the neuron. Results show that the heterogeneity of heavy particle tracks at low doses, compared to the more uniform dose distribution of electrons, juxtaposed with neuron morphology make it necessary to model the spatial dose painting for specific neuronal components. Going forward, this work can directly support the development of biophysical models of the modifications of spine and dendritic morphology observed after low dose charged particle irradiation by providing accurate descriptions of the underlying physical insults to complex neuron structures at the nano-meter scale.

The hippocampus in aging and disease: From plasticity to vulnerability

The hippocampus has a pivotal role in learning and in the formation and consolidation of memory and is critically involved in the regulation of emotion, fear, anxiety, and stress. Studies of the hippocampus have been central to the study of memory in humans and in recent years, the regional specialization and organization of hippocampal functions have been elucidated in experimental models and in human neurological and psychiatric diseases. The hippocampus has long been considered a classic model for the study of neuroplasticity as many examples of synaptic plasticity such as long-term potentiation and -depression have been identified and demonstrated in hippocampal circuits. Neuroplasticity is the ability to adapt and reorganize the structure or function to internal or external stimuli and occurs at the cellular, population, network or behavioral level and is reflected in the cytological and network architecture as well as in intrinsic properties of hippocampal neurons and circuits. The high degree of hippocampal neuroplasticity might, however, be also negatively reflected in the pronounced vulnerability of the hippocampus to deleterious conditions such as ischemia, epilepsy, chronic stress, neurodegeneration and aging targeting hippocampal structure and function and leading to cognitive deficits. Considering this framework of plasticity and vulnerability, we here review basic principles of hippocampal anatomy and neuroplasticity on various levels as well as recent findings regarding the functional organization of the hippocampus in light of the regional vulnerability in Alzheimer's disease, ischemia, epilepsy, neuroinflammation and aging.

GABAB receptor subtypes differentially modulate synaptic inhibition in the dentate gyrus to enhance granule cell output

BACKGROUND AND PURPOSE

Activation of GABAB receptors in the dentate gyrus (DG) enhances granule cell (GC) activity by reducing synaptic inhibition imposed by hilar interneurons. This disinhibitory action facilitates signal transfer from the perforant path to the hippocampus. However, as the two main molecular subtypes, GABA(B(1a,2)) and GABA(B(1b,2)) receptors, prefer axonal terminal and dendritic compartments, respectively, they may modulate the hilar pathways at different synaptic localizations. We examined their relative expression and functions in the DG.

EXPERIMENTAL APPROACH

The localization of GABAB subtypes was revealed immunohistochemically using subunit-selective antibodies in GABA(B1a)(-/-) and GABA(B1b)(-/-) mice. Effects of subtype activation by the GABAB receptor agonist, baclofen, were examined on the perforant path-stimulated GC population activities in brain slices.

KEY RESULTS

GABA(B(1a,2)) receptors were concentrated in the inner molecular layer, the neuropil of the hilus and hilar neurons at the border zone; while GABA(B(1b,2)) receptors dominated the outer molecular layer and hilar neurons in the deep layer, showing their differential localization on GC dendrite and in the hilus. Baclofen enhanced the GC population spike to a larger extent in the GABA(B1b)(-/-) mice, demonstrating exclusively disinhibitory roles of the GABA(B(1a,2)) receptors. Conversely, in the GABA(B1a)(-/-) mice baclofen not only enhanced but also inhibited the population spike during GABAA blockade, revealing both disinhibitory and inhibitory effects of GABA(B(1b,2)) receptors.

CONCLUSIONS AND IMPLICATIONS

The GABA(B(1a,2)) and GABA(B(1b,2)) receptor subtypes differentially modulate GC outputs via selective axonal terminal and dendritic locations in the hilar pathways. The GABA(B(1a,2)) receptors exclusively mediate disinhibition, thereby playing a greater role in gating signal transfer for hippocampal spatial and pattern learning.

A Million-Plus Neuron Model of the Hippocampal Dentate Gyrus: Critical Role for Topography in Determining Spatiotemporal Network Dynamics

GOAL

This paper describes a million-plus granule cell compartmental model of the rat hippocampal dentate gyrus, including excitatory, perforant path input from the entorhinal cortex, and feedforward and feedback inhibitory input from dentate interneurons.

METHODS

The model includes experimentally determined morphological and biophysical properties of granule cells, together with glutamatergic AMPA-like EPSP and GABAergic GABAA-like IPSP synaptic excitatory and inhibitory inputs, respectively. Each granule cell was composed of approximately 200 compartments having passive and active conductances distributed throughout the somatic and dendritic regions. Modeling excitatory input from the entorhinal cortex was guided by axonal transport studies documenting the topographical organization of projections from subregions of the medial and lateral entorhinal cortex, plus other important details of the distribution of glutamatergic inputs to the dentate gyrus. Information contained within previously published maps of this major hippocampal afferent were systematically converted to scales that allowed the topographical distribution and relative synaptic densities of perforant path inputs to be quantitatively estimated for inclusion in the current model.

RESULTS

Results showed that when medial and lateral entorhinal cortical neurons maintained Poisson random firing, dentate granule cells expressed, throughout the million-cell network, a robust nonrandom pattern of spiking best described as a spatiotemporal "clustering." To identify the network property or properties responsible for generating such firing "clusters," we progressively eliminated from the model key mechanisms, such as feedforward and feedback inhibition, intrinsic membrane properties underlying rhythmic burst firing, and/or topographical organization of entorhinal afferents.

CONCLUSION

Findings conclusively identified topographical organization of inputs as the key element responsible for generating a spatiotemporal distribution of clustered firing. These results uncover a functional organization of perforant path afferents to the dentate gyrus not previously recognized: topography-dependent clusters of granule cell activity as "functional units" or "channels" that organize the processing of entorhinal signals. This modeling study also reveals for the first time how a global signal processing feature of a neural network can evolve from one of its underlying structural characteristics.

Fluoxetine induces input-specific hippocampal dendritic spine remodeling along the septotemporal axis in adulthood and middle age

Fluoxetine, a selective serotonin-reuptake inhibitor (SSRI), is known to induce structural rearrangements and changes in synaptic transmission in hippocampal circuitry. In the adult hippocampus, structural changes include neurogenesis, dendritic, and axonal plasticity of pyramidal and dentate granule neurons, and dedifferentiation of dentate granule neurons. However, much less is known about how chronic fluoxetine affects these processes along the septotemporal axis and during the aging process. Importantly, studies documenting the effects of fluoxetine on density and distribution of spines along different dendritic segments of dentate granule neurons and CA1 pyramidal neurons along the septotemporal axis of hippocampus in adulthood and during aging are conspicuously absent. Here, we use a transgenic mouse line in which mature dentate granule neurons and CA1 pyramidal neurons are genetically labeled with green fluorescent protein (GFP) to investigate the effects of chronic fluoxetine treatment (18 mg/kg/day) on input-specific spine remodeling and mossy fiber structural plasticity in the dorsal and ventral hippocampus in adulthood and middle age. In addition, we examine levels of adult hippocampal neurogenesis, maturation state of dentate granule neurons, neuronal activity, and glutamic acid decarboxylase-67 expression in response to chronic fluoxetine in adulthood and middle age. Our studies reveal that while chronic fluoxetine fails to augment adult hippocampal neurogenesis in middle age, the middle-aged hippocampus retains high sensitivity to changes in the dentate gyrus (DG) such as dematuration, hypoactivation, and increased glutamic acid decarboxylase 67 (GAD67) expression. Interestingly, the middle-aged hippocampus shows greater sensitivity to fluoxetine-induced input-specific synaptic remodeling than the hippocampus in adulthood with the stratum-oriens of CA1 exhibiting heightened structural plasticity. The input-specific changes and circuit-level modifications in middle-age were associated with modest enhancement in contextual fear memory precision, anxiety-like behavior and antidepressant-like behavioral responses.

Opioid receptor-dependent sex differences in synaptic plasticity in the hippocampal mossy fiber pathway of the adult rat

The mossy fiber (MF) pathway is critical to hippocampal function and influenced by gonadal hormones. Physiological data are limited, so we asked whether basal transmission and long-term potentiation (LTP) differed in slices of adult male and female rats. The results showed small sex differences in basal transmission but striking sex differences in opioid receptor sensitivity and LTP. When slices were made from females on proestrous morning, when serum levels of 17β-estradiol peak, the nonspecific opioid receptor antagonist naloxone (1 μm) enhanced MF transmission but there was no effect in males, suggesting preferential opioid receptor-dependent inhibition in females when 17β-estradiol levels are elevated. The μ-opioid receptor (MOR) antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP; 300 nm) had a similar effect but the δ-opioid receptor (DOR) antagonist naltrindole (NTI; 1 μm) did not, implicating MORs in female MF transmission. The GABAB receptor antagonist saclofen (200 μm) occluded effects of CTOP but the GABAA receptor antagonist bicuculline (10 μm) did not. For LTP, a low-frequency (LF) protocol was used because higher frequencies elicited hyperexcitability in females. Proestrous females exhibited LF-LTP but males did not, suggesting a lower threshold for synaptic plasticity when 17β-estradiol is elevated. NTI blocked LF-LTP in proestrous females, but CTOP did not. Electron microscopy revealed more DOR-labeled spines of pyramidal cells in proestrous females than males. Therefore, we suggest that increased postsynaptic DORs mediate LF-LTP in proestrous females. The results show strong MOR regulation of MF transmission only in females and identify a novel DOR-dependent form of MF LTP specific to proestrus.

Oxidative stress and redox regulation on hippocampal-dependent cognitive functions

Hippocampal-dependent cognitive functions rely on production of new neurons and maintenance of dendritic structures to provide the synaptic plasticity needed for learning and formation of new memories. Hippocampal formation is exquisitely sensitive to patho-physiological changes, and reduced antioxidant capacity and exposure to low dose irradiation can significantly impede hippocampal-dependent functions of learning and memory by reducing the production of new neurons and alter dendritic structures in the hippocampus. Although the mechanism leading to impaired cognitive functions is complex, persistent oxidative stress likely plays an important role in the SOD-deficient and radiation-exposed hippocampal environment. Aging is associated with increased production of pro-oxidants and accumulation of oxidative end products. Similar to the hippocampal defects observed in SOD-deficient mice and mice exposed to low dose irradiation, reduced capacity in learning and memory, diminishing hippocampal neurogenesis, and altered dendritic network are universal in the aging brains. Given the similarities in cellular and structural changes in the aged, SOD-deficient, and radiation-exposed hippocampal environment and the corresponding changes in cognitive decline, understanding the shared underlying mechanism will provide more flexible and efficient use of SOD deficiency or irradiation to model age-related changes in cognitive functions and identify potential therapeutic or intervention methods.

Presynaptic D2 dopamine receptors control long-term depression expression and memory processes in the temporal hippocampus

BACKGROUND

Dysfunctional mesocorticolimbic dopamine signaling has been linked to alterations in motor and reward-based functions associated with psychiatric disorders. Converging evidence from patients with psychiatric disorders and use of antipsychotics suggests that imbalance of dopamine signaling deeply alters hippocampal functions. However, given the lack of full characterization of a functional mesohippocampal pathway, the precise role of dopamine transmission in memory deficits associated with these disorders and their dedicated therapies is unknown. In particular, the positive outcome of antipsychotic treatments, commonly antagonizing D2 dopamine receptors (D2Rs), on cognitive deficits and memory impairments remains questionable.

METHODS

Following pharmacologic and genetic manipulation of dopamine transmission, we performed anatomic, neurochemical, electrophysiologic, and behavioral investigations to uncover the role of D2Rs in hippocampal-dependent plasticity and learning. Naïve mice (n = 4-21) were used in the different procedures.

RESULTS

Dopamine modulated both long-term potentiation and long-term depression in the temporal hippocampus as well as spatial and recognition learning and memory in mice through D2Rs. Although genetic deletion or pharmacologic blockade of D2Rs led to the loss of long-term potentiation expression, the specific genetic removal of presynaptic D2Rs impaired long-term depression and performances on spatial memory tasks.

CONCLUSIONS

Presynaptic D2Rs in dopamine fibers of the temporal hippocampus tightly modulate long-term depression expression and play a major role in the regulation of hippocampal learning and memory. This direct role of mesohippocampal dopamine input as uncovered here adds a new dimension to dopamine involvement in the physiology underlying deficits associated with neuropsychiatric disorders.

Stem cell therapy for Alzheimer's disease and related disorders: current status and future perspectives

Underlying cognitive declines in Alzheimer's disease (AD) are the result of neuron and neuronal process losses due to a wide range of factors. To date, all efforts to develop therapies that target specific AD-related pathways have failed in late-stage human trials. As a result, an emerging consensus in the field is that treatment of AD patients with currently available drug candidates might come too late, likely as a result of significant neuronal loss in the brain. In this regard, cell-replacement therapies, such as human embryonic stem cell- or induced pluripotent stem cell-derived neural cells, hold potential for treating AD patients. With the advent of stem cell technologies and the ability to transform these cells into different types of central nervous system neurons and glial cells, some success in stem cell therapy has been reported in animal models of AD. However, many more steps remain before stem cell therapies will be clinically feasible for AD and related disorders in humans. In this review, we will discuss current research advances in AD pathogenesis and stem cell technologies; additionally, the potential challenges and strategies for using cell-based therapies for AD and related disorders will be discussed.

Transiently lowering tumor necrosis factor-α synthesis ameliorates neuronal cell loss and cognitive impairments induced by minimal traumatic brain injury in mice

Background

The treatment of traumatic brain injury (TBI) represents an unmet medical need, as no effective pharmacological treatment currently exists. The development of such a treatment requires a fundamental understanding of the pathophysiological mechanisms that underpin the sequelae resulting from TBI, particularly the ensuing neuronal cell death and cognitive impairments. Tumor necrosis factor-alpha (TNF-α) is a cytokine that is a master regulator of systemic and neuroinflammatory processes. TNF-α levels are reported to become rapidly elevated post TBI and, potentially, can lead to secondary neuronal damage.

Methods

To elucidate the role of TNF-α in TBI, particularly as a drug target, the present study evaluated (i) time-dependent TNF-α levels and (ii) markers of apoptosis and gliosis within the brain and related these to behavioral measures of ‘well being’ and cognition in a mouse closed head 50 g weight drop mild TBI (mTBI) model in the presence and absence of post-treatment with an experimental TNF-α synthesis inhibitor, 3,6′-dithiothalidomide.

Results

mTBI elevated brain TNF-α levels, which peaked at 12 h post injury and returned to baseline by 18 h. This was accompanied by a neuronal loss and an increase in astrocyte number (evaluated by neuronal nuclei (NeuN) and glial fibrillary acidic protein (GFAP) immunostaining), as well as an elevation in the apoptotic death marker BH3-interacting domain death agonist (BID) at 72 h. Selective impairments in measures of cognition, evaluated by novel object recognition and passive avoidance paradigms - without changes in well being, were evident at 7 days after injury. A single systemic treatment with the TNF-α synthesis inhibitor 3,6′-dithiothalidomide 1 h post injury prevented the mTBI-induced TNF-α elevation and fully ameliorated the neuronal loss (NeuN), elevations in astrocyte number (GFAP) and BID, and cognitive impairments. Cognitive impairments evident at 7 days after injury were prevented by treatment as late as 12 h post mTBI but were not reversed when treatment was delayed until 18 h.

Conclusions

These results implicate that TNF-α in mTBI induced secondary brain damage and indicate that pharmacologically limiting the generation of TNF-α post mTBI may mitigate such damage, defining a time-dependent window of up to 12 h to achieve this reversal.

Revisiting hippocampal sclerosis in mesial temporal lobe epilepsy according to the "two-hit" hypothesis

Hippocampal sclerosis (HS) is the most common neuropathological pattern observed in pharmacoresistant epilepsy and represents a critical feature in mesial temporal lobe epilepsy syndrome. However, its pathophysiological mechanisms and neuropathological consequences on seizures remain mostly unresolved. The new international classification of hippocampal sclerosis aims at standardizing its description to allow comparisons between different clinical studies. However, several aspects are not considered in this classification (granule cell dispersion, sprouting, glial modifications…). In this chapter, we discuss these different features associated with hippocampal sclerosis in perspective with the "two-hit" hypothesis and propose mechanisms that could be involved in the modulation of some specific neuropathological aspects like early life stress, hyperthermic seizures, brain lesions or hormonal modifications.

PTEN deletion from adult-generated dentate granule cells disrupts granule cell mossy fiber axon structure

Dysregulation of the mTOR-signaling pathway is implicated in the development of temporal lobe epilepsy. In mice, deletion of PTEN from hippocampal dentate granule cells leads to mTOR hyperactivation and promotes the rapid onset of spontaneous seizures. The mechanism by which these abnormal cells initiate epileptogenesis, however, is unclear. PTEN-knockout granule cells develop abnormally, exhibiting morphological features indicative of increased excitatory input. If these cells are directly responsible for seizure genesis, it follows that they should also possess increased output. To test this prediction, dentate granule cell axon morphology was quantified in control and PTEN-knockout mice. Unexpectedly, PTEN deletion increased giant mossy fiber bouton spacing along the axon length, suggesting reduced innervation of CA3. Increased width of the mossy fiber axon pathway in stratum lucidum, however, which likely reflects an unusual increase in mossy fiber axon collateralization in this region, offsets the reduction in boutons per axon length. These morphological changes predict a net increase in granule cell innervation of CA3. Increased diameter of axons from PTEN-knockout cells would further enhance granule cell communication with CA3. Altogether, these findings suggest that amplified information flow through the hippocampal circuit contributes to seizure occurrence in the PTEN-knockout mouse model of temporal lobe epilepsy.

Functional and structural deficits of the dentate gyrus network coincide with emerging spontaneous seizures in an Scn1a mutant Dravet Syndrome model during development

Dravet syndrome (DS) is characterized by severe infant-onset myoclonic epilepsy along with delayed psychomotor development and heightened premature mortality. A primary monogenic cause is mutation of the SCN1A gene, which encodes the voltage-gated sodium channel subunit Nav1.1. The nature and timing of changes caused by SCN1A mutation in the hippocampal dentate gyrus (DG) network, a core area for gating major excitatory input to hippocampus and a classic epileptogenic zone, are not well known. In particularly, it is still not clear whether the developmental deficit of this epileptogenic neural network temporally matches with the progress of seizure development. Here, we investigated the emerging functional and structural deficits of the DG network in a novel mouse model (Scn1a(E1099X/+)) that mimics the genetic deficit of human DS. Scn1a(E1099X/+) (Het) mice, similarly to human DS patients, exhibited early spontaneous seizures and were more susceptible to hyperthermia-induced seizures starting at postnatal week (PW) 3, with seizures peaking at PW4. During the same period, the Het DG exhibited a greater reduction of Nav1.1-expressing GABAergic neurons compared to other hippocampal areas. Het DG GABAergic neurons showed altered action potential kinetics, reduced excitability, and generated fewer spontaneous inhibitory inputs into DG granule cells. The effect of reduced inhibitory input to DG granule cells was exacerbated by heightened spontaneous excitatory transmission and elevated excitatory release probability in these cells. In addition to electrophysiological deficit, we observed emerging morphological abnormalities of DG granule cells. Het granule cells exhibited progressively reduced dendritic arborization and excessive spines, which coincided with imbalanced network activity and the developmental onset of spontaneous seizures. Taken together, our results establish the existence of significant structural and functional developmental deficits of the DG network and the temporal correlation between emergence of these deficits and the onset of seizures in Het animals. Most importantly, our results uncover the developmental deficits of neural connectivity in Het mice. Such structural abnormalities likely further exacerbate network instability and compromise higher-order cognitive processing later in development, and thus highlight the multifaceted impacts of Scn1a deficiency on neural development.

Distribution of neurotransmitter receptors and zinc in the pigeon (Columba livia) hippocampal formation: A basis for further comparison with the mammalian hippocampus

The avian hippocampal formation (HF) and mammalian hippocampus share a similar functional role in spatial cognition, but the underlying neuronal mechanisms allowing the functional similarity are incompletely understood. To understand better the organization of the avian HF and its transmitter receptors, we analyzed binding site densities for glutamatergic AMPA, NMDA, and kainate receptors; GABAA receptors; muscarinic M1 , M2 and nicotinic (nACh) acetylcholine receptors; noradrenergic α1 and α2 receptors; serotonergic 5-HT1A receptors; dopaminergic D1/5 receptors by using quantitative in vitro receptor autoradiography. Additionally, we performed a modified Timm staining procedure to label zinc. The regionally different receptor densities mapped well onto seven HF subdivisions previously described. Several differences in receptor expression highlighted distinct HF subdivisions. Notable examples include 1) high GABAA and α1 receptor expression, which rendered distinctive ventral subdivisions; 2) high α2 receptor expression, which rendered distinctive a dorsomedial subdivision; 3) distinct kainate, α2 , and muscarinic receptor densities that rendered distinctive the two dorsolateral subdivisions; and 4) a dorsomedial region characterized by high kainate receptor density. We further observed similarities in receptor binding densities between subdivisions of the avian and mammalian HF. Despite the similarities, we propose that 300 hundred million years of independent evolution has led to a mosaic of similarities and differences in the organization of the avian HF and mammalian hippocampus and that thinking about the avian HF in terms of the strict organization of the mammalian hippocampus is likely insufficient to understand the HF of birds.

Preschool externalizing behavior predicts gender-specific variation in adolescent neural structure

Dysfunction in the prefrontal cortex, amygdala, and hippocampus is believed to underlie the development of much psychopathology. However, to date only limited longitudinal data relate early behavior with neural structure later in life. Our objective was to examine the relationship of early life externalizing behavior with adolescent brain structure. We report here the first longitudinal study linking externalizing behavior during preschool to brain structure during adolescence. We examined the relationship of preschool externalizing behavior with amygdala, hippocampus, and prefrontal cortex volumes at age 15 years in a community sample of 76 adolescents followed longitudinally since their mothers’ pregnancy. A significant gender by externalizing behavior interaction revealed that males—but not females—with greater early childhood externalizing behavior had smaller amygdala volumes at adolescence (t = 2.33, p = .023). No significant results were found for the hippocampus or the prefrontal cortex. Greater early externalizing behavior also related to smaller volume of a cluster including the angular gyrus and tempoparietal junction across genders. Results were not attributable to the impact of preschool anxiety, preschool maternal stress, school-age internalizing or externalizing behaviors, or adolescent substance use. These findings demonstrate a novel, gender-specific relationship between early-childhood externalizing behavior and adolescent amygdala volume, as well as a cross-gender result for the angular gyrus and tempoparietal junction.

Dentate Gyrus circuitry features improve performance of sparse approximation algorithms

Memory-related activity in the Dentate Gyrus (DG) is characterized by sparsity. Memory representations are seen as activated neuronal populations of granule cells, the main encoding cells in DG, which are estimated to engage 2–4% of the total population. This sparsity is assumed to enhance the ability of DG to perform pattern separation, one of the most valuable contributions of DG during memory formation. In this work, we investigate how features of the DG such as its excitatory and inhibitory connectivity diagram can be used to develop theoretical algorithms performing Sparse Approximation, a widely used strategy in the Signal Processing field. Sparse approximation stands for the algorithmic identification of few components from a dictionary that approximate a certain signal. The ability of DG to achieve pattern separation by sparsifing its representations is exploited here to improve the performance of the state of the art sparse approximation algorithm “Iterative Soft Thresholding” (IST) by adding new algorithmic features inspired by the DG circuitry. Lateral inhibition of granule cells, either direct or indirect, via mossy cells, is shown to enhance the performance of the IST. Apart from revealing the potential of DG-inspired theoretical algorithms, this work presents new insights regarding the function of particular cell types in the pattern separation task of the DG.

A three-plane architectonic atlas of the rat hippocampal region

The hippocampal region, comprising the hippocampal formation and the parahippocampal region, has been one of the most intensively studied parts of the brain for decades. Better understanding of its functional diversity and complexity has led to an increased demand for specificity in experimental procedures and manipulations. In view of the complex 3D structure of the hippocampal region, precisely positioned experimental approaches require a fine-grained architectural description that is available and readable to experimentalists lacking detailed anatomical experience. In this paper, we provide the first cyto- and chemoarchitectural description of the hippocampal formation and parahippocampal region in the rat at high resolution and in the three standard sectional planes: coronal, horizontal and sagittal. The atlas uses a series of adjacent sections stained for neurons and for a number of chemical marker substances, particularly parvalbumin and calbindin. All the borders defined in one plane have been cross-checked against their counterparts in the other two planes. The entire dataset will be made available as a web-based interactive application through the Rodent Brain WorkBench (http://www.rbwb.org) which, together with this paper, provides a unique atlas resource.

Parvalbumin and neuropeptide Y expressing hippocampal GABA-ergic inhibitory interneuron numbers decline in a model of Gulf War illness

Cognitive dysfunction is amongst the most conspicuous symptoms in Gulf War illness (GWI). Combined exposure to the nerve gas antidote pyridostigmine bromide (PB), pesticides and stress during the Persian Gulf War-1 (PGW-1) are presumed to be among the major causes of GWI. Indeed, our recent studies in rat models have shown that exposure to GWI-related (GWIR) chemicals and mild stress for 4 weeks engenders cognitive impairments accompanied with several detrimental changes in the hippocampus. In this study, we tested whether reduced numbers of hippocampal gamma-amino butyric acid (GABA)-ergic interneurons are among the pathological changes induced by GWIR-chemicals and stress. Animals were exposed to low doses of GWIR-chemicals and mild stress for 4 weeks. Three months after this exposure, subpopulations of GABA-ergic interneurons expressing the calcium binding protein parvalbumin (PV), the neuropeptide Y (NPY) and somatostatin (SS) in the hippocampus were stereologically quantified. Animals exposed to GWIR-chemicals and stress for 4 weeks displayed reduced numbers of PV-expressing GABA-ergic interneurons in the dentate gyrus and NPY-expressing interneurons in the CA1 and CA3 subfields. However, no changes in SS+ interneuron population were observed in the hippocampus. Furthermore, GABA-ergic interneuron deficiency in these animals was associated with greatly diminished hippocampus neurogenesis. Because PV+ and NPY+ interneurons play roles in maintaining normal cognitive function and neurogenesis, and controlling the activity of excitatory neurons in the hippocampus, reduced numbers of these interneurons may be one of the major causes of cognitive dysfunction and reduced neurogenesis observed in GWI. Hence, strategies that improve inhibitory neurotransmission in the hippocampus may prove beneficial for reversing cognitive dysfunction in GWI.

LDL receptor knock-out mice show impaired spatial cognition with hippocampal vulnerability to apoptosis and deficits in synapses

Background

Evidence from clinical studies support the fact that abnormal cholesterol metabolism in the brain leads to progressive cognitive dysfunction. The low-density lipoprotein receptor (LDLR) is well-known for its role in regulating cholesterol metabolism. Whether LDLR involved in this impaired cognition and the potential mechanisms that underlie this impairment are unknown.

Methods

Twelve-month-old Ldlr-/- mice (n = 10) and wild-type littermates C57BL/6 J (n = 14) were subjected to the Morris water maze test. At 1 week after completion of the behavioural testing, all of the animals were sacrificed for analysis of synaptic and apoptotic markers.

Results

The plasma cholesterol concentration of Ldlr-/- mice was increased moderately when compared with C57BL/6 J mice (P < 0.05). Behavioural testing revealed that Ldlr-/- mice displayed impaired spatial memory, and moreover, the expression levels of synaptophysin and the number of synaptophysin-immunoreactive presynaptic boutons in the hippocampal CA1 and dentate gyrus were decreased (all P < 0.05). Ultrastructural changes in the dentate gyrus were observed using transmission electron microscopy. Furthermore, apoptosis in the hippocampus of Ldlr-/- mice was revealed based on elevation, at both the mRNA and protein levels, of the ratio of Bax/Bcl-2 expression (all P < 0.05)and an increase in activated-caspase3 protein level (P < 0.05).

Conclusion

LDLR deficiency contributes to impaired spatial cognition. This most likely occurs via negative effects that promote apoptosis and synaptic deficits in the hippocampus.

[Cellular and molecular mechanisms underlying aberrant network reorganization in the epileptic brain]

Well-refined wiring of neural circuits is fundamental to proper brain function. Aberrantly formed neural circuits may induce epileptiform discharges of neurons. Therefore, elucidating the cellular and molecular mechanisms that underlie the development of aberrant neural circuitry will advance the understanding and prevention of epilepsy. The dentate gyrus has been suggested to serve as a gate that prevents the propagation of epileptiform activity from the entorhinal cortex to the hippocampus. Within the dentate gyrus is the dentate granule cell layer, which consists of densely packed granule cells that maintain intrinsically low-firing properties and rarely exhibit burst discharges synchronized with other neurons. Additionally, granule cells form abundant synaptic inputs to inhibitory interneurons in the dentate hilus, a fraction of which provide feedback inhibition back to the granule cells. Network reorganization of the dentate gyrus in patients with temporal lobe epilepsy and in corresponding animal models was reported. Specifically, mossy fiber sprouting and the emergence of ectopic granule cells contribute to the observed phenotypes. This paper reviews the expanding literature on the cellular and molecular mechanisms underlying the formation of aberrant hippocampal networks and their role in epileptogenesis.

A novel unbiased counting method for the quantification of synapses in the mouse brain

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We describe a novel method for synapse quantification using electron microscopy.

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Nissl-stained vibratome sections allowed accurate brain region identification.

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Automatic microscope stage shifts using custom-made software excluded observer bias.

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The method showed altered synaptic features after environmental enrichment.

Background

The numerical density of synapses and their ultrastructural features are best assessed with electron microscopy. Counting is done within counting frames placed on a pair of sections (disector technique). But this requires that the thin sections are taken from comparable brain regions and the disectors are placed in a uniform random fashion. Small brain areas like the polymorph layer of the mouse dentate gyrus are difficult to encounter, and manually moving the microscope stage for placing the micrographs seems arbitrary.

New method

Here the polymorph layer was approximated with 20 μm thin, Nissl-stained vibratome sections. The subsequent vibratome section was processed for electron microscopy and serially thin sectioned. The microscope stage was moved using a random number generator, placing at least 20 disectors onto a pair of sections. The numerical synapse density, the numerical density of dense-core vesicles, and other ultrastructural features were compared between mice that had been kept in an enriched environment and mice kept under standard housing conditions.

Results

Environmental enrichment significantly decreased the numerical density of dense-core vesicles and synaptic cleft widths within the polymorph layer, associated with behavioral improvement in the Morris water maze, a hippocampus-dependent task of spatial learning and memory.

Comparison with existing methods

This procedure was easy to handle and enabled us to produce thin sections in small, defined brain areas. Furthermore, placing the disectors with random numbers excluded observer bias.

Conclusions

Our procedure provides an uncomplicated way of assessing numerical densities in small brain areas in an unbiased manner.

Immunofluorescently labeling glutamic acid decarboxylase 65 coupled with confocal imaging for identifying GABAergic somata in the rat dentate gyrus-A comparison with labeling glutamic acid decarboxylase 67

As γ-aminobutyric acid (GABA) is synthesized by two isoforms of glutamic acid decarboxylase (GAD), namely, GAD65 and GAD67, immunohistochemically targeting either isoform of GAD is theoretically useful for identifying GABAergic cell bodies. In practice, targeting GAD67 remains to be a popular choice. However, identifying GABAergic cell bodies with GAD67 immunoreactivity in the hippocampal dentate gyrus, especially in the hilus, is not without pitfalls. In the present study, we compared the characteristics of GAD65 immunoreactivity to GAD67 immunoreactivity in the rat dentate gyrus and examined perikaryal expression of GAD65 in four neurochemically prevalent subgroups of interneurons in the hilus. Experiments were done in normal adult Sprague-Dawley rats and GAD67-GFP knock-in mice. Horizontal hippocampal slices cut from the ventral portion of hippocampi were immunofluorescently stained and scanned using a confocal microscope. Immunoreactivity for both GAD67 and GAD65 was visible throughout the dentate gyrus. Perikaryal GAD67 immunoreactivity was denser but variable in terms of distribution pattern and intensity among cells whereas perikaryal GAD65 immunoreactivity displayed similar distribution pattern and staining intensity. Among different layers of the dentate gyrus, GAD67 immunoreactivity was densest in the hilus despite GAD65 immunoreactivity being more intense in the granule cell layer. Co-localization experiments showed that GAD65, but not GAD67, was expressed in all hilar calretinin (CR)-, neuronal nitric oxide synthase (nNOS)-, parvalbumin (PV)- or somatostatin (SOM)-positive somata. Labeling CR, nNOS, PV, and SOM in sections obtained from GAD67-GFP knock-in mice revealed that a large portion of SOM-positive cells had weak GFP expression. In addition, double labeling of GAD65/GABA and GAD67/GABA showed that nearly all of GABA-immunoreactive cells had perikaryal GAD65 expression whereas more than one-tenth of GABA-immunoreactive cells lacked perikaryal GAD67 immunoreactivity. Inhibition of axonal transport with colchicine dramatically improved perikaryal GAD65 immunoreactivity in GABAergic cells without significant augmentation to be seen in granule cells. Double labeling GAD65 and GAD67 in the sections obtained from colchicine-pretreated animals confirmed that a portion of GAD65-immunoreactive cells had weak or even no GAD67 immunoreactivity. We conclude that for confocal imaging, immunofluorescently labeling GAD65 for identifying GABAergic somata in the hilus of the dentate gyrus has advantages over labeling GAD67 in terms of easier recognition of perikaryal labeling and more consistent expression in GABAergic somata. Inhibition of axonal transport with colchicine further improves perikaryal GAD65 labeling, making GABAergic cells more distinguishable.

Association between toll-like receptor 4 expression and neural stem cell proliferation in the hippocampus following traumatic brain injury in mice

Whether or how neural stem cells (NSCs) respond to toll-like receptor 4 (TLR4) in an inflammatory environment caused by traumatic brain injury (TBI) has not been understood. In the present study, association between TLR4 expression and NSCs proliferation in the hippocampus was investigated in a mouse model of TBI using controlled cortical impact (CCI). Hippocampal proliferating cells were labeled with the thymidine analog 5-bromo-2-deoxyuridine (BrdU). In order to identify NSCs, the proliferating cells were further co-labeled with BrdU/sex determination region of Y chromosome related high mobility group box gene 2 (SOX2). Morphological observation on the expression of BrdU, SOX2, and TLR4 in the hippocampus was performed by inmmunofluorescence (IF). Relative quantification of TLR4 expression at the protein and mRNA level was performed using Western blotting and real-time polymerase chain reaction (PCR). It was observed that BrdU⁺/SOX2⁺ cells accounted for 95.80% ± 7.91% among BrdU⁺ cells; several BrdU⁺ cells and SOX2⁺ cells in the hippocampus were also TLR4-positive post injury, and that BrdU⁺ cell numbers, together with TLR4 expression at either protein or mRNA level, increased significantly in TBI mice over 1, 3, 7, 14, and 21 days survivals and changed in a similar temporal pattern with a peak at 3 day post-injury. These results indicate that hippocampal proliferating cells (suggestive of NSCs) expressed TLR4, and that there was a potential association between increased expression of TLR4 and the proliferation of NSCs post TBI. It is concluded that hippocampal TLR4 may play a potential role in endogenous neurogenesis after TBI.

Ethanol alters local cellular levels of (3α,5α)-3-hydroxypregnan-20-one (3α,5α-THP) independent of the adrenals in subcortical brain regions

The neuroactive steroid (3α,5α)-3-hydroxypregnan-20-one (3α,5α-THP or allopregnanolone) is a positive modulator of GABAA receptors synthesized in the brain, adrenal glands, and gonads. In rats, ethanol activates the hypothalamic-pituitary-adrenal axis and elevates 3α,5α-THP in plasma, cerebral cortex, and hippocampus. In vivo, these effects are dependent on both the pituitary and adrenal glands. In vitro, however, ethanol locally increases 3α,5α-THP in hippocampal slices, in the absence of adrenal influence. Therefore, it is not known whether ethanol can change local brain levels of 3α,5α-THP in vivo, independent of the adrenals. To directly address this controversy, we administered ethanol (2 g/kg) or saline to rats that underwent adrenalectomy (ADX) or received sham surgery and performed immunohistochemistry for 3α,5α-THP. In the medial prefrontal cortex (mPFC), ethanol increased 3α,5α-THP after sham surgery, compared with saline controls, with no ethanol-induced change in 3α,5α-THP following ADX. In subcortical regions, 3α,5α-THP was increased independent of adrenals in the CA1 pyramidal cell layer, dentate gyrus polymorphic layer, bed nucleus of the stria terminalis, and paraventricular nucleus of the hypothalamus. Furthermore, ethanol decreased 3α,5α-THP labeling in the nucleus accumbens shore and central nucleus of the amygdala, independent of the adrenal glands. These data indicate that ethanol dynamically regulates local 3α,5α-THP levels in several subcortical regions; however, the adrenal glands contribute to 3α,5α-THP elevations in the mPFC. Using double immunofluorescent labeling we determined that adrenal dependence of 3α,5α-THP induction by ethanol is not due to a lack of colocalization of 3α,5α-THP with the cholesterol transporters steroidogenic acute regulatory protein (StAR) or translocator protein (TSPO).

Dopamine D2 receptor controls hilar mossy cells excitability

Hippocampal control of memory formation is regulated by dopaminergic signaling. Whereas the role of dopamine D1 receptors is well documented in such regulations, functions of dopamine D2 receptors (DRD2) are not fully understood. Using fluorescence in situ hybridization we demonstrate that Drd2 expression in the hippocampus of wild-type mice is limited to glutamatergic hilar mossy cells. Using whole cell electrophysiological recordings in hippocampal slice preparations, we provide evidence that unlike in basal ganglia, activation of DRD2 by the selective agonist, quinpirole, induces a long-lasting increase in excitability of hilar mossy cells, which can be blocked by the DRD2 antagonist raclopride. Such activity is mediated by the Akt/GSK pathway, as application of specific inhibitors such as A1070722 or SB216763 prevented quinpirole activity. Long-term effects of acute DRD2 activation in vitro suggest that volume transmission of dopamine may modulate mossy cell activities in vivo. This is supported by the presence of dense tyrosine hydroxylase positive varicosities in the hilus, which are rarely seen in the vicinity of mossy cell dendrites. From these data we discuss how dopamine could control mossy cell activity and thus dentate gyrus functions.

Long-term effects of an acute and systemic administration of LPS on adult neurogenesis and spatial memory

The cognitive reserve is the capacity of the brain to maintain normal performance while exposed to insults or ageing. Increasing evidences point to a role for the interaction between inflammatory conditions and cognitive reserve status during Alzheimer's disease (AD) progression. The production of new neurons along adult life can be considered as one of the components of the cognitive reserve. Interestingly, adult neurogenesis is decreased in mouse models of AD and following inflammatory processes. The aim of this work is to reveal the long-term impact of a systemic inflammatory event on memory and adult neurogenesis in wild type (WT) and triple transgenic mouse model of AD (3xTg-AD). Four month-old mice were intraperitoneally injected once with saline or lipopolysaccharide (LPS) and their performance on spatial memory analyzed with the Morris water maze (MWM) test 7 weeks later. Our data showed that a single intraperitoneal injection with LPS has a long-term impact in the production of hippocampal neurons. Consistently, LPS-treated WT mice showed less doublecortin-positive neurons, less synaptic contacts in newborn neurons, and decreased dendritic volume and complexity. These surprising observations were accompanied with memory deficits. 3xTg-AD mice showed a decrease in new neurons in the dentate gyrus compatible with, although exacerbated, the pattern observed in WT LPS-treated mice. In 3xTg-AD mice, LPS injection did not significantly affected the production of new neurons but reduced their number of synaptic puncta and impaired memory performance, when compared to the observations made in saline-treated 3xTg-AD mice. These data indicate that LPS treatment induces a long-term impairment on hippocampal neurogenesis and memory. Our results show that acute neuroinflammatory events influence the production of new hippocampal neurons, affecting the cognitive reserve and leading to the development of memory deficits associated to AD pathology.

Developmental and adult GAP-43 deficiency in mice dynamically alters hippocampal neurogenesis and mossy fiber volume

Growth-associated protein-43 (GAP-43) is a presynaptic protein that plays key roles in axonal growth and guidance and in modulating synapse formation. Previous work has demonstrated that mice lacking one allele of this gene (GAP-43+/- mice) exhibit hippocampal structural abnormalities, impaired spatial learning and stress-induced behavioral withdrawal and anxiety, behaviors that are dependent on proper hippocampal circuitry and function. Given the correlation between hippocampal function, synaptic connectivity and neurogenesis, we tested if behaviorally naïve GAP-43+/- mice had alterations in either neurogenesis or synaptic connectivity in the hippocampus during early postnatal development and young adulthood, and following behavior testing in older adults. To test our hypothesis, we examined hippocampal cell proliferation (Ki67), number of immature neuroblasts (doublecortin, DCX) and mossy fiber volume (synaptoporin) in behaviorally naïve postnatal day 9 (P9) and P26, and behaviorally experienced 5- to 7-month-old GAP-43+/- and +/+ littermate mice. P9 GAP-43+/- mice had fewer Ki67+ and DCX+ cells compared to +/+ mice, particularly in the posterior dentate gyrus, and smaller mossy fiber volume in the same region. In young adulthood, however, male GAP-43+/- mice had more Ki67+ and DCX+ cells and greater mossy fiber volume in the posterior dentate gyrus relative to male +/+ mice. These increases were not seen in females. In 5- to 7-month-old GAP-43+/- mice (whose behaviors were the focus of our prior publication), there was no global change in the number of proliferating or immature neurons relative to +/+ mice. However, more detailed analysis revealed fewer proliferative DCX+ cells in the anterior dentate gyrus of male GAP-43+/- mice compared to male +/+ mice. This reduction was not observed in females. These results suggest that young GAP-43+/- mice have decreased hippocampal neurogenesis and synaptic connectivity, but slightly older mice have greater hippocampal neurogenesis and synaptic connectivity. In conjunction with our previous study, these findings suggest that GAP-43 is dynamically involved in early postnatal and adult hippocampal neurogenesis and synaptic connectivity, possibly contributing to the GAP-43+/- behavioral phenotype.

Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?

Adult neurogenesis, the birth of new neurons in the mature brain, has attracted considerable attention in the last decade. One of the earliest identified and most profound factors that affect adult neurogenesis both positively and negatively is stress. Here, we review the complex interplay between stress and adult neurogenesis. In particular, we review the role of the glucocorticoid receptor, the main mediator of the stress response in the proliferation, differentiation, migration, and functional integration of newborn neurons in the hippocampus. We review a multitude of mechanisms regulating glucocorticoid receptor activity in relationship to adult neurogenesis. We postulate a novel concept in which the level of glucocorticoid receptor expression directly regulates the excitation-inhibition balance, which is key for proper neurogenesis. We furthermore argue that an excitation-inhibition dis-balance may underlie aberrant functional integration of newborn neurons that is associated with psychiatric and paroxysmal brain disorders.

Input integration around the dendritic branches in hippocampal dentate granule cells

Recent studies have shown that the dendrites of several neurons are not simple translators but are crucial facilitators of excitatory postsynaptic potential (EPSP) propagation and summation of synaptic inputs to compensate for inherent voltage attenuation. Granule cells (GCs)are located at the gateway for valuable information arriving at the hippocampus from the entorhinal cortex. However, the underlying mechanisms of information integration along the dendrites of GCs in the hippocampus are still unclear. In this study, we investigated the input integration around dendritic branches of GCs in the rat hippocampus. We applied differential spatiotemporal stimulations to the dendrites using a high-speed glutamate-uncaging laser. Our results showed that when two sites close to and equidistant from a branching point were simultaneously stimulated, a nonlinear summation of EPSPs was observed at the soma. In addition, nonlinear summation (facilitation) depended on the stimulus location and was significantly blocked by the application of a voltage-dependent Ca(2+) channel antagonist. These findings suggest that the nonlinear summation of EPSPs around the dendritic branches of hippocampal GCs is a result of voltage-dependent Ca(2+) channel activation and may play a crucial role in the integration of input information.

Is plasticity of GABAergic mechanisms relevant to epileptogenesis?

Numerous changes in GABAergic neurons, receptors, and inhibitory mechanisms have been described in temporal lobe epilepsy (TLE), either in humans or in animal models. Nevertheless, there remains a common assumption that epilepsy can be explained by simply an insufficiency of GABAergic inhibition. Alternatively, investigators have suggested that there is hyperinhibition that masks an underlying hyperexcitability. Here we examine the status epilepticus (SE) models of TLE and focus on the dentate gyrus of the hippocampus, where a great deal of data have been collected. The types of GABAergic neurons and GABAA receptors are summarized under normal conditions and after SE. The role of GABA in development and in adult neurogenesis is discussed. We suggest that instead of "too little or too much" GABA there is a complexity of changes after SE that makes the emergence of chronic seizures (epileptogenesis) difficult to understand mechanistically, and difficult to treat. We also suggest that this complexity arises, at least in part, because of the remarkable plasticity of GABAergic neurons and GABAA receptors in response to insult or injury.