Entorhinal cortex of the human, monkey, and rat: metabolic map as revealed by cytochrome oxidase

Bnip3 expression is strongly associated with reelin-positive entorhinal cortex layer II neurons

In layer II of the entorhinal cortex, the principal neurons that project to the dentate gyrus and the CA3/2 hippocampal fields markedly express the large glycoprotein reelin (Re + ECLII neurons). In rodents, neurons located at the dorsal extreme of the EC, which border the rhinal fissure, express the highest levels, and the expression gradually decreases at levels successively further away from the rhinal fissure. Here, we test two predictions deducible from the hypothesis that reelin expression is strongly correlated with neuronal metabolic rate. Since the mitochondrial turnover rate serves as a proxy for energy expenditure, the mitophagy rate arguably also qualifies as such. Because messenger RNA of the canonical promitophagic BCL2 and adenovirus E1B 19-kDa-interacting protein 3 (Bnip3) is known to be highly expressed in the EC, we predicted that Bnip3 would be upregulated in Re + ECLII neurons, and that the degree of upregulation would strongly correlate with the expression level of reelin in these neurons. We confirm both predictions, supporting that the energy requirement of Re + ECLII neurons is generally high and that there is a systematic increase in metabolic rate as one moves successively closer to the rhinal fissure. Intriguingly, the systematic variation in energy requirement of the neurons that manifest the observed reelin gradient appears to be consonant with the level of spatial and temporal detail by which they encode information about the external environment.

Cell-specific expression of key mitochondrial enzymes limits OXPHOS in astrocytes of the adult human neocortex and hippocampal formation

The astrocyte-to-neuron lactate shuttle model entails that, upon glutamatergic neurotransmission, glycolytically derived pyruvate in astrocytes is mainly converted to lactate instead of being entirely catabolized in mitochondria. The mechanism of this metabolic rewiring and its occurrence in human brain are unclear. Here by using immunohistochemistry (4 brains) and imaging mass cytometry (8 brains) we show that astrocytes of the adult human neocortex and hippocampal formation express barely detectable amounts of mitochondrial proteins critical for performing oxidative phosphorylation (OXPHOS). These data are corroborated by queries of transcriptomes (107 brains) of neuronal versus non-neuronal cells fetched from the Allen Institute for Brain Science for genes coding for a much larger repertoire of entities contributing to OXPHOS, showing that human non-neuronal elements barely expressed mRNAs coding for such proteins. With less OXPHOS, human brain astrocytes are thus bound to produce more lactate to avoid interruption of glycolysis.

Disentangling associations between complex traits and cell types with seismic

Integrating single-cell RNA sequencing (scRNA-seq) with Genome-Wide Association Studies (GWAS) can help reveal GWAS-associated cell types, furthering our understanding of the cell-type-specific biological processes underlying complex traits and disease. However, current methods have technical limitations that hinder them from making systematic, scalable, interpretable disease-cell-type associations. In order to rapidly and accurately pinpoint associations, we develop a novel framework, seismic, which characterizes cell types using a new specificity score. We compare seismic with alternative methods across over 1,000 cell type characterizations at different granularities and 28 traits, demonstrating that seismic both corroborates findings and identifies trait-relevant cell groups which are not apparent through other methodologies. Furthermore, as part of the seismic framework, the specific genes driving cell type-trait associations can easily be accessed and analyzed, enabling further biological insights. The advantages of seismic are particularly salient in neurodegenerative diseases such as Parkinson's and Alzheimer's, where disease pathology has not only cell-specific manifestations, but also brain region-specific differences. Interestingly, a case study of Alzheimer's disease reveals the importance of considering GWAS endpoints, as studies relying on clinical diagnoses consistently identify microglial associations, while GWAS with a tau biomarker endpoint reveals neuronal associations. In general, seismic is a computationally efficient, powerful, and interpretable approach for identifying associations between complex traits and cell type-specific expression.

Alzheimer's Disease and Its Possible Evolutionary Origin: Hypothesis

The enormous, 2-3-million-year evolutionary expansion of hominin neocortices to the current enormity enabled humans to take over the planet. However, there appears to have been a glitch, and it occurred without a compensatory expansion of the entorhinal cortical (EC) gateway to the hippocampal memory-encoding system needed to manage the processing of the increasing volume of neocortical data converging on it. The resulting age-dependent connectopathic glitch was unnoticed by the early short-lived populations. It has now surfaced as Alzheimer's disease (AD) in today's long-lived populations. With advancing age, processing of the converging neocortical data by the neurons of the relatively small lateral entorhinal cortex (LEC) inflicts persistent strain and high energy costs on these cells. This may result in their hyper-release of harmless Aβ1-42 monomers into the interstitial fluid, where they seed the formation of toxic amyloid-β oligomers (AβOs) that initiate AD. At the core of connectopathic AD are the postsynaptic cellular prion protein (PrPC). Electrostatic binding of the negatively charged AβOs to the positively charged N-terminus of PrPC induces hyperphosphorylation of tau that destroys synapses. The spread of these accumulating AβOs from ground zero is supported by Aβ's own production mediated by target cells' Ca2+-sensing receptors (CaSRs). These data suggest that an early administration of a strongly positively charged, AβOs-interacting peptide or protein, plus an inhibitor of CaSR, might be an effective AD-arresting therapeutic combination.

Intracellular Amyloid-β in the Normal Rat Brain and Human Subjects and Its relevance for Alzheimer's Disease

BACKGROUND

Amyloid-β (Aβ) is a normal product of neuronal activity, including that of the aggregation-prone Aβ42 variant that is thought to cause Alzheimer's disease (AD). Much knowledge about AD comes from studies of transgenic rodents expressing mutated human amyloid-β protein precursor (AβPP) to increase Aβ production or the Aβ42/40 ratio. Yet, little is known about the normal expression of Aβ42 in rodent brains.

OBJECTIVE

To characterize the brain-wide expression of Aβ42 throughout the life span of outbred Wistar rats, and to relate these findings to brains of human subjects without neurological disease.

METHODS

Aβ42 immunolabeling of 12 Wistar rat brains (3-18 months of age) and brain sections from six human subjects aged 20-88 years.

RESULTS

In healthy Wistar rats, we find intracellular Aβ42 (iAβ42) in neurons throughout the brain at all ages, but levels vary greatly between brain regions. The highest levels are in neurons of entorhinal cortex layer II, alongside hippocampal neurons at the CA1/subiculum border. Concerning entorhinal cortex layer II, we find similarly high levels of iAβ42 in the human subjects.

CONCLUSION

Expression of iAβ42 in healthy Wistar rats predominates in the same structures where iAβ accumulates and Aβ plaques initially form in the much used, Wistar based McGill-R-Thy1-APP rat model for AD. The difference between wild-type Wistar rats and these AD model rats, with respect to Aβ42, is therefore quantitative rather that qualitative. This, taken together with our human results, indicate that the McGill rat model in fact models the underlying wild-type neuronal population-specific vulnerability to Aβ42 accumulation.

Cytochrome oxidase "blobs": a call for more anatomy

An ordered relation of structure and function has been a cornerstone in thinking about brain organization. Like the brain itself, however, this is not straightforward and is confounded both by functional intricacy and structural plasticity (many routes to a given outcome). As a striking case of putative structure-function correlation, this mini-review focuses on the relatively well-characterized pattern of cytochrome oxidase (CO) blobs (aka "patches" or "puffs") in the supragranular layers of macaque monkey visual cortex. The pattern is without doubt visually compelling, and the semi-dichotomous array of CO+ blobs and CO- interblobs is consistent with multiple studies reporting compartment-specific preferential connectivity and distinctive physiological response properties. Nevertheless, as briefly reviewed here, the finer anatomical organization of this system is surprisingly under-investigated, and the relation to functional aspects, therefore, unclear. Microcircuitry, cell type, and three-dimensional spatiotemporal level investigations of the CO+ CO- pattern are needed and may open new views to structure-function organization of visual cortex, and to phylogenetic and ontogenetic comparisons across nonhuman primates (NHP), and between NHP and humans.

Insights into the Pathogenesis of Neurodegenerative Diseases: Focus on Mitochondrial Dysfunction and Oxidative Stress

As the population ages, the incidence of neurodegenerative diseases is increasing. Due to intensive research, important steps in the elucidation of pathogenetic cascades have been made and significantly implicated mitochondrial dysfunction and oxidative stress. However, the available treatment in Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis is mainly symptomatic, providing minor benefits and, at most, slowing down the progression of the disease. Although in preclinical setting, drugs targeting mitochondrial dysfunction and oxidative stress yielded encouraging results, clinical trials failed or had inconclusive results. It is likely that by the time of clinical diagnosis, the pathogenetic cascades are full-blown and significant numbers of neurons have already degenerated, making it impossible for mitochondria-targeted or antioxidant molecules to stop or reverse the process. Until further research will provide more efficient molecules, a healthy lifestyle, with plenty of dietary antioxidants and avoidance of exogenous oxidants may postpone the onset of neurodegeneration, while familial cases may benefit from genetic testing and aggressive therapy started in the preclinical stage.

Selective Neuron Vulnerability in Common and Rare Diseases-Mitochondria in the Focus

Mitochondrial dysfunction is a central feature of neurodegeneration within the central and peripheral nervous system, highlighting a strong dependence on proper mitochondrial function of neurons with especially high energy consumptions. The fitness of mitochondria critically depends on preservation of distinct processes, including the maintenance of their own genome, mitochondrial dynamics, quality control, and Ca²⁺ handling. These processes appear to be differently affected in common neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease, as well as in rare neurological disorders, including Huntington’s disease, Amyotrophic Lateral Sclerosis and peripheral neuropathies. Strikingly, particular neuron populations of different morphology and function perish in these diseases, suggesting that cell-type specific factors contribute to the vulnerability to distinct mitochondrial defects. Here we review the disruption of mitochondrial processes in common as well as in rare neurological disorders and its impact on selective neurodegeneration. Understanding discrepancies and commonalities regarding mitochondrial dysfunction as well as individual neuronal demands will help to design new targets and to make use of already established treatments in order to improve treatment of these diseases.

The Causal Role of Lipoxidative Damage in Mitochondrial Bioenergetic Dysfunction Linked to Alzheimer's Disease Pathology

Current shreds of evidence point to the entorhinal cortex (EC) as the origin of the Alzheimer’s disease (AD) pathology in the cerebrum. Compared with other cortical areas, the neurons from this brain region possess an inherent selective vulnerability derived from particular oxidative stress conditions that favor increased mitochondrial molecular damage with early bioenergetic involvement. This alteration of energy metabolism is the starting point for subsequent changes in a multitude of cell mechanisms, leading to neuronal dysfunction and, ultimately, cell death. These events are induced by changes that come with age, creating the substrate for the alteration of several neuronal pathways that will evolve toward neurodegeneration and, consequently, the development of AD pathology. In this context, the present review will focus on description of the biological mechanisms that confer vulnerability specifically to neurons of the entorhinal cortex, the changes induced by the aging process in this brain region, and the alterations at the mitochondrial level as the earliest mechanism for the development of AD pathology. Current findings allow us to propose the existence of an altered allostatic mechanism at the entorhinal cortex whose core is made up of mitochondrial oxidative stress, lipid metabolism, and energy production, and which, in a positive loop, evolves to neurodegeneration, laying the basis for the onset and progression of AD pathology.

Deep Brain Stimulation in Alzheimer's Disease: Targeting the Nucleus Basalis of Meynert

Alzheimer's disease (AD) is becoming a prevalent disease in the elderly population. Past decades have witnessed the development of drug therapies with varying targets. However, all drugs with a single molecular target fail to reverse or ameliorate AD progression, which ultimately results in cortical and subcortical network dysregulation. Deep brain stimulation (DBS) has been proven effective for the treatment of Parkinson's disease, essential tremor, and other neurological diseases. As such, DBS has also been gradually acknowledged as a potential therapy for AD. The current review focuses on DBS of the nucleus basalis of Meynert (NBM). As a critical component of the cerebral cholinergic system and the Papez circuit in the basal ganglia, the NBM plays an indispensable role in the subcortical regulation of memory, attention, and arousal state, which makes the NBM a promising target for modulation of neural network dysfunction and AD treatment. We summarized the intricate projection relations and functionality of the NBM, current approaches for stereotactic localization and evaluation of the NBM, and the therapeutic effects of NBM-DBS both in patients and animal models. Furthermore, the current shortcomings of NBM-DBS, such as variations in cortical blood flow, increased temperature in the target area, and stimulation-related neural damage, were presented.

Structural modularity and grid activity in the medial entorhinal cortex

Following the groundbreaking discovery of grid cells, the medial entorhinal cortex (MEC) has become the focus of intense anatomical, physiological, and computational investigations. Whether and how grid activity maps onto cell types and cortical architecture is still an open question. Fundamental similarities in microcircuits, function, and connectivity suggest a homology between rodent MEC and human posteromedial entorhinal cortex. Both are specialized for spatial processing and display similar cellular organization, consisting of layer 2 pyramidal/calbindin cell patches superimposed on scattered stellate neurons. Recent data indicate the existence of a further nonoverlapping modular system (zinc patches) within the superficial MEC layers. Zinc and calbindin patches have been shown to receive largely segregated inputs from the presubiculum and parasubiculum. Grid cells are also clustered in the MEC, and we discuss possible structure-function schemes on how grid activity could map onto cortical patch systems. We hypothesize that in the superficial layers of the MEC, anatomical location can be predictive of function; thus relating functional properties and neuronal morphologies to the cortical modules will be necessary for resolving how grid activity maps onto cortical architecture. Imaging or cell identification approaches in freely moving animals will be required for testing this hypothesis.

The Human Periallocortex: Layer Pattern in Presubiculum, Parasubiculum and Entorhinal Cortex. A Review

The cortical mantle is not homogeneous, so that three types of cortex can be distinguished: allocortex, periallocortex and isocortex. The main distinction among those three types is based on morphological differences, in particular the number of layers, overall organization, appearance, etc., as well as its connectivity. Additionally, in the phylogenetic scale, this classification is conserved among different mammals. The most primitive and simple cortex is the allocortex, which is characterized by the presence of three layers, with one cellular main layer; it is continued by the periallocortex, which presents six layers, although with enough differences in the layer pattern to separate three different fields: presubiculum (PrS), parasubiculum (PaS), and entorhinal cortex (EC). The closest part to the allocortex (represented by the subiculum) is the PrS, which shows outer (layers I-III) and inner (V-VI) principal layers (lamina principalis externa and lamina principalis interna), both separated by a cell poor band, parallel to the pial surface (layer IV or lamina dissecans). This layer organization is present throughout the anterior-posterior axis. The PaS continues the PrS, but its rostrocaudal extent is shorter than the PrS. The organization of the PaS shows the layer pattern more clearly than in the PrS. Up to six layers are recognizable in the PaS, with layer IV as lamina dissecans between superficial (layers I-III) and deep (V-VI) layers, as in the PrS. The EC presents even more clearly the layer pattern along both mediolateral and rostrocaudal extent. The layer pattern is a thick layer I, layer II in islands, layer III medium pyramids, layer IV as lamina dissecans (not present throughout the EC extent), layer V with dark and big pyramids and a multiform layer VI. The EC borders laterally the proisocortex (incomplete type of isocortex). Variations in the appearance of its layers justify the distinction of subfields in the EC, in particular in human and nonhuman primates. EC layers are not similar to those in the neocortex. The transition between the periallocortical EC and isocortex is not sharp, so that the proisocortex forms an intervening cortex, which fills the gap between the periallocortex and the isocortex.

Glycogen distribution in the microwave-fixed mouse brain reveals heterogeneous astrocytic patterns

In the brain, glycogen metabolism has been implied in synaptic plasticity and learning, yet the distribution of this molecule has not been fully described. We investigated cerebral glycogen of the mouse by immunohistochemistry (IHC) using two monoclonal antibodies that have different affinities depending on the glycogen size. The use of focused microwave irradiation yielded well-defined glycogen immunoreactive signals compared with the conventional periodic acid-Schiff method. The IHC signals displayed a punctate distribution localized predominantly in astrocytic processes. Glycogen immunoreactivity (IR) was high in the hippocampus, striatum, cortex, and cerebellar molecular layer, whereas it was low in the white matter and most of the subcortical structures. Additionally, glycogen distribution in the hippocampal CA3-CA1 and striatum had a 'patchy' appearance with glycogen-rich and glycogen-poor astrocytes appearing in alternation. The glycogen patches were more evident with large-molecule glycogen in young adult mice but they were hardly observable in aged mice (1-2 years old). Our results reveal brain region-dependent glycogen accumulation and possibly metabolic heterogeneity of astrocytes. GLIA 2016;64:1532-1545.

fMRI at High Spatial Resolution: Implications for BOLD-Models

As high-resolution functional magnetic resonance imaging (fMRI) and fMRI of cortical layers become more widely used, the question how well high-resolution fMRI signals reflect the underlying neural processing, and how to interpret laminar fMRI data becomes more and more relevant. High-resolution fMRI has shown laminar differences in cerebral blood flow (CBF), volume (CBV), and neurovascular coupling. Features and processes that were previously lumped into a single voxel become spatially distinct at high resolution. These features can be vascular compartments such as veins, arteries, and capillaries, or cortical layers and columns, which can have differences in metabolism. Mesoscopic models of the blood oxygenation level dependent (BOLD) response therefore need to be expanded, for instance, to incorporate laminar differences in the coupling between neural activity, metabolism and the hemodynamic response. Here we discuss biological and methodological factors that affect the modeling and interpretation of high-resolution fMRI data. We also illustrate with examples from neuropharmacology and the negative BOLD response how combining BOLD with CBF- and CBV-based fMRI methods can provide additional information about neurovascular coupling, and can aid modeling and interpretation of high-resolution fMRI.

Functional Connectome Analysis of Dopamine Neuron Glutamatergic Connections in Forebrain Regions

In the ventral tegmental area (VTA), a subpopulation of dopamine neurons express vesicular glutamate transporter 2 and make glutamatergic connections to nucleus accumbens (NAc) and olfactory tubercle (OT) neurons. However, their glutamatergic connections across the forebrain have not been explored systematically. To visualize dopamine neuron forebrain projections and to enable photostimulation of their axons independent of transmitter status, we virally transfected VTA neurons with channelrhodopsin-2 fused to enhanced yellow fluorescent protein (ChR2-EYFP) and used DAT(IREScre) mice to restrict expression to dopamine neurons. ChR2-EYFP-expressing neurons almost invariably stained for tyrosine hydroxylase, identifying them as dopaminergic. Dopamine neuron axons visualized by ChR2-EYFP fluorescence projected most densely to the striatum, moderately to the amygdala and entorhinal cortex (ERC), sparsely to prefrontal and cingulate cortices, and rarely to the hippocampus. Guided by ChR2-EYFP fluorescence, we recorded systematically from putative principal neurons in target areas and determined the incidence and strength of glutamatergic connections by activating all dopamine neuron terminals impinging on recorded neurons with wide-field photostimulation. This revealed strong glutamatergic connections in the NAc, OT, and ERC; moderate strength connections in the central amygdala; and weak connections in the cingulate cortex. No glutamatergic connections were found in the dorsal striatum, hippocampus, basolateral amygdala, or prefrontal cortex. These results indicate that VTA dopamine neurons elicit widespread, but regionally distinct, glutamatergic signals in the forebrain and begin to define the dopamine neuron excitatory functional connectome.

SIGNIFICANCE STATEMENT

Dopamine neurons are important for the control of motivated behavior and are involved in the pathophysiology of several major neuropsychiatric disorders. Recent studies have shown that some ventral midbrain dopamine neurons are capable of glutamate cotransmission. With conditional expression of channelrhodopsin in dopamine neurons, we systematically explored dopamine neuron connections in the forebrain and identified regionally specific dopamine neuron excitatory connections. Establishing that only a subset of forebrain regions receive excitatory connections from dopamine neurons will help to determine the function of dopamine neuron glutamate cotransmission, which likely involves transmission of precise temporal signals and enhancement of the dynamic range of dopamine neuron signals.

Conserved size and periodicity of pyramidal patches in layer 2 of medial/caudal entorhinal cortex

To understand the structural basis of grid cell activity, we compare medial entorhinal cortex architecture in layer 2 across five mammalian species (Etruscan shrews, mice, rats, Egyptian fruit bats, and humans), bridging ∼100 million years of evolutionary diversity. Principal neurons in layer 2 are divided into two distinct cell types, pyramidal and stellate, based on morphology, immunoreactivity, and functional properties. We confirm the existence of patches of calbindin-positive pyramidal cells across these species, arranged periodically according to analyses techniques like spatial autocorrelation, grid scores, and modifiable areal unit analysis. In rodents, which show sustained theta oscillations in entorhinal cortex, cholinergic innervation targeted calbindin patches. In bats and humans, which only show intermittent entorhinal theta activity, cholinergic innervation avoided calbindin patches. The organization of calbindin-negative and calbindin-positive cells showed marked differences in entorhinal subregions of the human brain. Layer 2 of the rodent medial and the human caudal entorhinal cortex were structurally similar in that in both species patches of calbindin-positive pyramidal cells were superimposed on scattered stellate cells. The number of calbindin-positive neurons in a patch increased from ∼80 in Etruscan shrews to ∼800 in humans, only an ∼10-fold over a 20,000-fold difference in brain size. The relatively constant size of calbindin patches differs from cortical modules such as barrels, which scale with brain size. Thus, selective pressure appears to conserve the distribution of stellate and pyramidal cells, periodic arrangement of calbindin patches, and relatively constant neuron number in calbindin patches in medial/caudal entorhinal cortex.

Identification of neurotoxic cytokines by profiling Alzheimer's disease tissues and neuron culture viability screening

Alzheimer's disease (AD) therapeutics based on the amyloid hypothesis have shown minimal efficacy in patients, suggesting that the activity of amyloid beta (Aβ) represents only one aspect of AD pathogenesis. Since neuroinflammation is thought to play an important role in AD, we hypothesized that cytokines may play a direct role in promoting neuronal death. Here, we profiled cytokine expression in a small cohort of human AD and control brain tissues. We identified AD-associated cytokines using partial least squares regression to correlate cytokine expression with quantified pathologic disease state and then used neuron cultures to test whether cytokines up-regulated in AD tissues could affect neuronal viability. This analysis identified cytokines that were associated with the pathological severity. Of the top correlates, only TNF-α reduced viability in neuron culture when applied alone. VEGF also reduced viability when applied together with Aβ, which was surprising because VEGF has been viewed as a neuro-protective protein. We found that this synthetic pro-death effect of VEGF in the context of Aβ was commensurate with VEGFR-dependent changes in multiple signaling pathways that govern cell fate. Our findings suggest that profiling of tissues combined with a culture-based screening approach can successfully identify new mechanisms driving neuronal death.

Entorhinal cortical defects in Tg2576 mice are present as early as 2-4 months of age

The entorhinal cortex (EC) is one of the first brain areas to display neuropathology in Alzheimer's disease. A mouse model which simulates amyloid-β (Aβ) neuropathology, the Tg2576 mouse, was used to address these early changes. Here, we show EC abnormalities occur in 2- to 4-month-old Tg2576 mice, an age before Aβ deposition and where previous studies suggest that there are few behavioral impairments. First we show, using a sandwich enzyme-linked immunosorbent assay, that soluble human Aβ40 and Aβ42 are detectable in the EC of 2-month-old Tg2576 mice before Aβ deposition. We then demonstrate that 2- to 4-month-old Tg2576 mice are impaired at object placement, an EC-dependent cognitive task. Next, we show that defects in neuronal nuclear antigen expression and myelin uptake occur in the superficial layers of the EC in 2- to 4-month-old Tg2576 mice. In slices from Tg2576 mice that contained the EC, there were repetitive field potentials evoked by a single stimulus to the underlying white matter, and a greater response to reduced extracellular magnesium ([Mg(2+)]o), suggesting increased excitability. However, deep layer neurons in Tg2576 mice had longer latencies to antidromic activation than wild type mice. The results show changes in the EC at early ages and suggest that altered excitability occurs before extensive plaque pathology.

Island cells control temporal association memory

Episodic memory requires associations of temporally discontiguous events. In the entorhinal-hippocampal network, temporal associations are driven by a direct pathway from layer III of the medial entorhinal cortex (MECIII) to the hippocampal CA1 region. However, the identification of neural circuits that regulate this association has remained unknown. In layer II of entorhinal cortex (ECII), we report clusters of excitatory neurons called island cells, which appear in a curvilinear matrix of bulblike structures, directly project to CA1, and activate interneurons that target the distal dendrites of CA1 pyramidal neurons. Island cells suppress the excitatory MECIII input through the feed-forward inhibition to control the strength and duration of temporal association in trace fear memory. Together, the two EC inputs compose a control circuit for temporal association memory.

Molecular drivers and cortical spread of lateral entorhinal cortex dysfunction in preclinical Alzheimer's disease

The entorhinal cortex has been implicated in the early stages of Alzheimer's disease, which is characterized by changes in the tau protein and in the cleaved fragments of the amyloid precursor protein (APP). We used a high-resolution functional magnetic resonance imaging (fMRI) variant that can map metabolic defects in patients and mouse models to address basic questions about entorhinal cortex pathophysiology. The entorhinal cortex is divided into functionally distinct regions, the medial entorhinal cortex (MEC) and the lateral entorhinal cortex (LEC), and we exploited the high-resolution capabilities of the fMRI variant to ask whether either of them was affected in patients with preclinical Alzheimer's disease. Next, we imaged three mouse models of disease to clarify how tau and APP relate to entorhinal cortex dysfunction and to determine whether the entorhinal cortex can act as a source of dysfunction observed in other cortical areas. We found that the LEC was affected in preclinical disease, that LEC dysfunction could spread to the parietal cortex during preclinical disease and that APP expression potentiated tau toxicity in driving LEC dysfunction, thereby helping to explain regional vulnerability in the disease.

An isomorphic mapping hypothesis of the grid representation

We introduce a grid cell microcircuit hypothesis. We propose the ‘grid in the world’ (evident in grid cell discharges) is generated by a ‘grid in the cortex’. This cortical grid is formed by patches of calbindin-positive pyramidal neurons in layer 2 of medial entorhinal cortex (MEC). Our isomorphic mapping hypothesis assumes three types of isomorphism: (i) metric correspondence of neural space (the two-dimensional cortical sheet) and the external two-dimensional space within patches; (ii) isomorphism between cellular connectivity matrix and firing field; (iii) isomorphism between single cell and population activity. Each patch is a grid cell lattice arranged in a two-dimensional map of space with a neural : external scale of approximately 1 : 2000 in the dorsal part of rat MEC. The lattice behaves like an excitable medium with neighbouring grid cells exciting each other. Spatial scale is implemented as an intrinsic scaling factor for neural propagation speed. This factor varies along the dorsoventral cortical axis. A connectivity scheme of the grid system is described. Head direction input specifies the direction of activity propagation. We extend the theory to neurons between grid patches and predict a rare discharge pattern (inverted grid cells) and the relative location and proportion of grid cells and spatial band cells.

The entorhinal grid map is discretized

The medial entorhinal cortex (MEC) is part of the brain's circuit for dynamic representation of self-location. The metric of this representation is provided by grid cells, cells with spatial firing fields that tile environments in a periodic hexagonal pattern. Limited anatomical sampling has obscured whether the grid system operates as a unified system or a conglomerate of independent modules. Here we show with recordings from up to 186 grid cells in individual rats that grid cells cluster into a small number of layer-spanning anatomically overlapping modules with distinct scale, orientation, asymmetry and theta-frequency modulation. These modules can respond independently to changes in the geometry of the environment. The discrete topography of the grid-map, and the apparent autonomy of the modules, differ from the graded topography of maps for continuous variables in several sensory systems, raising the possibility that the modularity of the grid map is a product of local self-organizing network dynamics.

Microcircuits of functionally identified neurons in the rat medial entorhinal cortex

VIDEO ABSTRACT

Extracellular recordings have elucidated spatial neural representations without identifying underlying microcircuits. We labeled neurons juxtacellularly in medial entorhinal cortex of freely moving rats with a friction-based, pipette-stabilization system. In a linear maze novel to the animals, spatial firing of superficial layer neurons was reminiscent of grid cell activity. Layer 2 stellate cells showed stronger theta modulation than layer 3 neurons, and both fired during the ascending phase of field potential theta. Deep-layer neurons showed little or no activity. Layer 2 stellate cells resided in hundreds of small patches. At the dorsomedial entorhinal border, we identified larger (putative parasubicular) patches, which contained polarized head-direction selective neurons firing during the descending theta phase. Three axon systems interconnected patches: centrifugal axons from superficial cells to single large patches, centripetal axons from large-patch cells to single small patches, and circumcurrent axons interconnecting large patches. Our microcircuit analysis during behavior reveals modularity of entorhinal processing.

A selective role for ARMS/Kidins220 scaffold protein in spatial memory and trophic support of entorhinal and frontal cortical neurons

Progressive cortical pathology is common to several neurodegenerative and psychiatric disorders. The entorhinal cortex (EC) and frontal cortex (FC) are particularly vulnerable, and neurotrophins have been implicated because they appear to be protective. A downstream signal transducer of neurotrophins, the ankyrin repeat-rich membrane spanning scaffold protein/Kidins 220 (ARMS) is expressed in the cortex, where it could play an important role in trophic support. To test this hypothesis, we evaluated mice with a heterozygous deletion of ARMS (ARMS(+/-) mice). Remarkably, the EC and FC were the regions that demonstrated the greatest defects. Many EC and FC neurons became pyknotic in ARMS(+/-) mice, so that large areas of the EC and FC were affected by 12 months of age. Areas with pyknosis in the EC and FC of ARMS(+/-) mice were also characterized by a loss of immunoreactivity to a neuronal antigen, NeuN, which has been reported after insult or injury to cortical neurons. Electron microscopy showed that there were defects in mitochondria, myelination, and multilamellar bodies in the EC and FC of ARMS(+/-) mice. Although primarily restricted to the EC and FC, pathology appeared to be sufficient to cause functional impairments, because ARMS(+/-) mice performed worse than wild-type on the Morris water maze. Comparisons of males and females showed that female mice were the affected sex in all comparisons. Taken together, the results suggest that the expression of a prominent neurotrophin receptor substrate normally protects the EC and FC, and that ARMS may be particularly important in females.

Selective vulnerability of neurons in layer II of the entorhinal cortex during aging and Alzheimer's disease

All neurons are not created equal. Certain cell populations in specific brain regions are more susceptible to age-related changes that initiate regional and system-level dysfunction. In this respect, neurons in layer II of the entorhinal cortex are selectively vulnerable in aging and Alzheimer's disease (AD). This paper will cover several hypotheses that attempt to account for age-related alterations among this cell population. We consider whether specific developmental, anatomical, or biochemical features of neurons in layer II of the entorhinal cortex contribute to their particular sensitivity to aging and AD. The entorhinal cortex is a functionally heterogeneous environment, and we will also review data suggesting that, within the entorhinal cortex, there is subregional specificity for molecular alterations that may initiate cognitive decline. Taken together, the existing data point to a regional cascade in which entorhinal cortical alterations directly contribute to downstream changes in its primary afferent region, the hippocampus.

Accurate path integration in continuous attractor network models of grid cells

Grid cells in the rat entorhinal cortex display strikingly regular firing responses to the animal's position in 2-D space and have been hypothesized to form the neural substrate for dead-reckoning. However, errors accumulate rapidly when velocity inputs are integrated in existing models of grid cell activity. To produce grid-cell-like responses, these models would require frequent resets triggered by external sensory cues. Such inadequacies, shared by various models, cast doubt on the dead-reckoning potential of the grid cell system. Here we focus on the question of accurate path integration, specifically in continuous attractor models of grid cell activity. We show, in contrast to previous models, that continuous attractor models can generate regular triangular grid responses, based on inputs that encode only the rat's velocity and heading direction. We consider the role of the network boundary in the integration performance of the network and show that both periodic and aperiodic networks are capable of accurate path integration, despite important differences in their attractor manifolds. We quantify the rate at which errors in the velocity integration accumulate as a function of network size and intrinsic noise within the network. With a plausible range of parameters and the inclusion of spike variability, our model networks can accurately integrate velocity inputs over a maximum of ∼10–100 meters and ∼1–10 minutes. These findings form a proof-of-concept that continuous attractor dynamics may underlie velocity integration in the dorsolateral medial entorhinal cortex. The simulations also generate pertinent upper bounds on the accuracy of integration that may be achieved by continuous attractor dynamics in the grid cell network. We suggest experiments to test the continuous attractor model and differentiate it from models in which single cells establish their responses independently of each other.

Even in the absence of external sensory cues, foraging rodents maintain an estimate of their position, allowing them to return home in a roughly straight line. This computation is known as dead reckoning or path integration. A discovery made three years ago in rats focused attention on the dorsolateral medial entorhinal cortex (dMEC) as a location in the rat's brain where this computation might be performed. In this area, so-called grid cells fire whenever the rat is on any vertex of a triangular grid that tiles the plane. Here we propose a model that could generate grid-cell-like responses in a neural network. The inputs to the model network convey information about the rat's velocity and heading, consistent with known inputs projecting into the dMEC. The network effectively integrates these inputs to produce a response that depends on the rat's absolute position. We show that such a neural network can integrate position accurately and can reproduce grid-cell-like responses similar to those observed experimentally. We then suggest a set of experiments that could help identify whether our suggested mechanism is responsible for the emergence of grid cells and for path integration in the rat's brain.

Architectonic subdivisions of neocortex in the gray squirrel (Sciurus carolinensis)

Squirrels are highly visual mammals with an expanded cortical visual system and a number of well-differentiated architectonic fields. To describe and delimit cortical fields, subdivisions of cortex were reconstructed from serial brain sections cut in the coronal, sagittal, or horizontal planes. Architectonic characteristics of cortical areas were visualized after brain sections were processed with immunohistochemical and histochemical procedures for revealing parvalbumin, calbindin, neurofilament protein, vesicle glutamate transporter 2, limbic-associated membrane protein, synaptic zinc, cytochrome oxidase, myelin or Nissl substance. In general, these different procedures revealed similar boundaries between areas, suggesting that functionally relevant borders were being detected. The results allowed a more precise demarcation of previously identified areas as well as the identification of areas that had not been previously described. Primary sensory cortical areas were characterized by sparse zinc staining of layer 4, as thalamocortical terminations lack zinc, as well as by layer 4 terminations rich in parvalbumin and vesicle glutamate transporter 2. Primary areas also expressed higher levels of cytochrome oxidase and myelin. Primary motor cortex was associated with large SMI-32 labeled pyramidal cells in layers 3 and 5. Our proposed organization of cortex in gray squirrels includes both similarities and differences to the proposed of cortex in other rodents such as mice and rats. The presence of a number of well-differentiated cortical areas in squirrels may serve as a guide to the identification of homologous fields in other rodents, as well as a useful guide in further studies of cortical organization and function.

Retromer sorting: a pathogenic pathway in late-onset Alzheimer disease

During the tail end of the 20th century, a "golden period" in Alzheimer disease (AD) research, many of the pathogenic molecules of the autosomal dominant form of the disease were isolated. These molecular defects, however, do not exist in "sporadic" late-onset AD, the form of the disease that accounts for more than 95% of all cases. Pinpointing the pathogenic molecules of late-onset AD has, therefore, become an urgent goal, both for understanding disease mechanisms and for opening up novel therapeutic avenues. The retromer sorting pathway transports cargo along the endosome-trans-Golgi network, and retromer defects were first implicated in late-onset AD by a study that combined brain imaging with microarray. A range of studies have confirmed that defects in this pathway can play a pathogenic role in the disease. Herein, these findings will be reviewed, the details of the retromer sorting pathway will be discussed, and a biological model that can account for the disease's regional selectivity will be elaborated.

Order-specific quantitative patterns of cortical gyrification

Why the extent of cortical gyrification varies across mammals of different brain sizes is a problem that is not clearly understood. The aim of the present study was to test a hypothesis indicating that the order is a significant phylogenetic grouping in terms of quantifiable gyrification indices (GIs) and thus variation between mammals. The GI was determined from serial sections of the brain of 25 different mammalian species, representing four different orders: primates, carnivores, ungulates and rodents. Image J analysis was used to measure the contours of the cerebral cortex, and the GI was calculated using three different methods of analysis: complete vs outer; gyral vs sulcal; and outer vs inner surface contours. The measurements were then computed against the brain weights of each species within the order. An increasing GI correlates with an increasing brain weight in all the mammalian orders. Each order has its own specific allometric pattern that is significantly different from the other orders. The ungulates were the mammals with the most gyrencephalic brains, these species being significantly more gyrencephalic than all other mammals when species of similar brain weights are compared. The North American beaver has an atypically lissencephalic brain for its size, differing from the trend for increased gyrencephaly found in the other rodent species examined. Our results show definite trends and patterns specific to each order; thus, it would seem that the order is a significant phylogenetic grouping in terms of this neural parameter, from which we can predict with a reasonable degree of certainty the GI of any species of a particular order given the brain weight.

Spatial representation and the architecture of the entorhinal cortex

It has recently been recognized that the entorhinal cortex has a crucial role in spatial representation and navigation. How the position of an animal is computed within the entorhinal circuitry remains to be determined, but the architectural organization of this brain area might provide some clues. Here, we review three organizational principles--recurrent connectivity, interlaminar connectivity and modular organization--and propose how each of them might contribute to the emergence and maintenance of positional representations in entorhinal neural networks.

Populations of hippocampal inhibitory neurons express different levels of cytochromec

Cytochrome c (CC) immunoreactivity was quantified in functionally distinct rat hippocampal inhibitory neuron populations using double immunocytochemistry and laser scanning confocal microscopy to measure the CC expression level as well as the amount of mitochondria within the cells, which is a sign of neuronal activity. The CC signal showed a similar distribution to cytochrome c oxidase histochemical staining. Strongly stained somata, dendrites and axon terminal clouds were dispersed over the low intensity neuropil staining. The staining was granular and electron microscopic investigation confirmed that the signal was localized in mitochondria. Intensively labeled neurons, showing the morphological features of inhibitory cells, were most frequently found in the principal cell layers, stratum oriens of the CA1-3 areas, stratum lucidum and hilus. These neurons contained not only a higher number of mitochondria than the principal cells but the intensity of the mitochondrial staining was evidently stronger. Among the examined interneuronal populations, parvalbumin-immunoreactive neurons were intensively labeled for CC. Calbindin D28k- (CB), somatostatin- and cholecystokinin-labeled cells showed heterogeneous CC levels, whereas calretinin-immunoreactive cells never showed a strong CC signal. CB cells in stratum oriens and alveus layers, lucidum and the hilus were strongly labeled for CC. CB cells in such regions are known to project to the medial septum and contain somatostatin. We have demonstrated that the CA1 interneurons that project to the medial septum (hippocampo-septal neurons) express a high level of CC. Thus, similar to the parvalbumin-containing basket and axo-axonic cells, the hippocampo-septal neurons potentially have a high average activity level.

Glycogen phosphorylase reactivity in the entorhinal complex in familiar and novel environments: Evidence for labile glycogenolytic modules in the rat

Active and total glycogen phosphorylase were measured histochemically in the entorhinal complex of male Sprague-Dawley rats. Rats were sacrificed from their home cage, or after 5 min in a novel holeboard. Hemispheres from each group were paired, sectioned and processed together. Glycogen phosphorylase reactivity highlighted entorhinal cortex in contrast to less densely stained perirhinal cortex or neocortex. The presubiculum, but not parasubiculum, was strongly reactive for glycogen phosphorylase. Within medial and lateral entorhinal cortex, modularity of active glycogen phosphorylase reactivity was apparent. In inner Layer I there were small ( approximately 50 microm) intense patches of active glycogen phosphorylase. In Layer III there were both small and larger ( approximately 200 microm), patches of active glycogen phosphorylase. Lamina dessicans was reactive. Layers V and VI were relatively unreactive. Exposure to a holeboard intensified the small patches of active glycogen phosphorylase in inner Layer I, while attenuating active glycogen phosphorylase reactivity in Layer III. Total glycogen phosphorylase was unaffected by exposure to the novel environment and exhibited a pattern of continuous dense reactivity suggesting enzyme reserves, particularly in superficial layers of entorhinal cortex. These patterns confirm earlier evidence that glycogenolytic demand in Layers I and III of rat entorhinal cortex is organized in a modular fashion and show that such demand can be modified by brief exposure to a novel holeboard.

Detection of entorhinal layer II using 7Tesla [corrected] magnetic resonance imaging

The entorhinal cortex lies in the mediotemporal lobe and has major functional, structural, and clinical significance. The entorhinal cortex has a unique cytoarchitecture with large stellate neurons in layer II that form clusters. The entorhinal cortex receives vast sensory association input, and its major output arises from the layer II and III neurons that form the perforant pathway. Clinically, the neurons in layer II are affected with neurofibrillary tangles, one of the two pathological hallmarks of Alzheimer's disease. We describe detection of the entorhinal layer II islands using magnetic resonance imaging. We scanned human autopsied temporal lobe blocks in a 7T human scanner using a solenoid coil. In 70 and 100 microm isotropic data, the entorhinal islands were clearly visible throughout the anterior-posterior extent of entorhinal cortex. Layer II islands were prominent in both the magnetic resonance imaging and corresponding histological sections, showing similar size and shape in two types of data. Area borders and island location based on cytoarchitectural features in the mediotemporal lobe were robustly detected using the magnetic resonance images. Our ex vivo results could break ground for high-resolution in vivo scanning that could ultimately benefit early diagnosis and treatment of neurodegenerative disease.

Dendritic morphology, local circuitry, and intrinsic electrophysiology of principal neurons in the entorhinal cortex of macaque monkeys

Little is known about the neuroanatomical or electrophysiological properties of individual neurons in the primate entorhinal cortex. We have used intracellular recording and biocytin-labeling techniques in the entorhinal slice preparation from macaque monkeys to investigate the morphology and intrinsic electrophysiology of principal neurons. These neurons have previously been studied most extensively in rats. In monkeys, layer II neurons are usually stellate cells, as in rats, but they occasionally have a pyramidal shape. They tend to discharge trains, not bursts, of action potentials, and some display subthreshold membrane potential oscillations. Layer III neurons are pyramidal, and they do not appear to display membrane potential oscillations. The distribution of dendrites and of axon collaterals suggests that neurons in layers II and III are interconnected by a network of associational fibers. Layer V and VI neurons are pyramidal and tend to discharge trains of action potentials. The distribution of dendrites and axon collaterals suggests that there is an associative network of principal neurons in layers V and VI, and they also project axon collaterals toward superficial layers. Importantly, entorhinal cortical neurons in monkeys appear to exhibit significant differences from those in rats. Morphologically, neurons in monkey entorhinal layers II and III have more primary dendrites, more dendritic branches, and greater total dendritic length than in rats. Electrophysiologically, layer II neurons in monkeys exhibit less sag, and subthreshold oscillations are less robust and slower. Some monkey layer III neurons discharge bursts of action potentials that are not found in rats. The interspecies differences revealed by this study may influence information processing and pathophysiological processes in the primate entorhinal cortex. J. Comp. Neurol. 470:317-329, 2004.

Architectonics and cortical connections of the upper bank of the superior temporal sulcus in the rhesus monkey: an analysis in the tangential plane

Area TPO in the upper bank of the superior temporal sulcus (STS) of macaque monkeys is thought to correspond to the superior temporal polysensory (STP) cortex, but has been shown to have neurochemical/connectional subdivisions. To examine directly the relationship between chemoarchitecture and cortical connections of area TPO, the upper bank of the STS was sectioned tangential to the cortical surface. Three subdivisions of area TPO (TPOr, TPOi, and TPOc) were examined with cytochrome oxidase (CO) histochemistry and neurofilament protein (NF) immunoreactivity and architectonic patterns were compared with connections on the same or adjacent sections. Area TPOc, which may partly overlap with the location of the medial superior temporal area MST, exhibited regular patchy staining for CO in layers III/IV and a complementary pattern in the NF stain. Area TPOr, but not TPOi, also had a patchy pattern of complementary staining in CO and neurofilament similar to TPOc, although not as distinct. Tracer injections within cortex including the frontal eye fields (areas 46 and 8) labeled areas TPOc, TPOi, and TPOr. The caudal inferior parietal lobule (IPL) projected to all three areas. The projections from prearcuate and posterior parietal cortices showed both overlap and nonoverlap with each other within TPOc, TPOi, and TPOr. Projections were to all neurochemical components within the subdivisions of TPO. The findings support the parcellation of area TPO into three subdivisions and extend findings of chemoarchitectonic modules within high-order association cortices.

Physiological patterns in the hippocampo-entorhinal cortex system

The anatomical connectivity and intrinsic properties of entorhinal cortical neurons give rise to ordered patterns of ensemble activity. How entorhinal ensembles form, interact, and accomplish emergent processes such as memory formation is not well-understood. We lack sufficient understanding of how neuronal ensembles in general can function transiently and distinctively from other neuronal ensembles. Ensemble interactions are bound, foremost, by anatomical connectivity and temporal constraints on neuronal discharge. We present an overview of the structure of neuronal interactions within the entorhinal cortex and the rest of the hippocampal formation. We wish to highlight two principle features of entorhinal-hippocampal interactions. First, large numbers of entorhinal neurons are organized into at least two distinct high-frequency population patterns: gamma (40-100 Hz) frequency volleys and ripple (140-200 Hz) frequency volleys. These patterns occur coincident with other well-defined electrophysiological patterns. Gamma frequency volleys are modulated by the theta cycle. Ripple frequency volleys occur on each sharp wave event. Second, these patterns occur dominantly in specific layers of the entorhinal cortex. Theta/gamma frequency volleys are the principle pattern observed in layers I-III, in the neurons that receive cortical inputs and project to the hippocampus. Ripple frequency volleys are the principle population pattern observed in layers V-VI, in the neurons that receive hippocampal output and project primarily to the neocortex. Further, we will highlight how these ensemble patterns organize interactions within distributed forebrain structures and support memory formation.

The parahippocampal gyrus in Alzheimer's disease. Clinical and preclinical neuroanatomical correlates

The human parahippocampal gyrus forms a large part of the limbic lobe along the ventromedial part of the temporal cortical mantle. It is a variable and complicated cortex in terms of structure, and the latter is aggravated further by interfaces with the anterior insula anteriorly and the cingulate gyrus and occipital lobe posteriorly. Additional complications relate to its lateral border with the temporal cortex and especially the sulcal configurations that define this junction. The rhinal sulcus, which separates parahippocampal and temporal cortices in other species, including the anthropoid apes, is either lacking or rudimentary in the human brain. Thus, defining this junction requires cytoarchitectural examination and precludes the use of mere inspection of sulcal existing patterns. The cortical areas that form the parahippocampal gyrus are vulnerable to pathological changes in Alzheimer's disease (AD), and its entorhinal and perirhinal subdivisions are both the most heavily damaged cortical areas and the focus for disease onset. The neurons that acquire neurofibrillary tangles (NFTs) occupy the junction of the isocortical mantle with the limbic cortical mantle, but share, or partially share, a vulnerability phenotype with large neurons in both domains. The differential expression of this phenotype across time creates the false impression of NFT spread in cross-sectional comparisons of AD brains. The questions of what this phenotype is and why it is expressed first in the perirhinal and entorhinal cortices of the parahippocampal gyrus are the central molecular biological/neuroanatomical questions in understanding the etiology of AD.

Histochemical mapping of the substrate for brain-stimulation reward with glycogen phosphorylase

Glycogen phosphorylase is the enzyme that regulates glycogenolysis and it appears that there is a relationship between central levels of glycogen and neuronal activity, which is influenced by a variety of neurotransmitters. In the present study, glycogen phosphorylase histochemistry was used to correlate changes in metabolic activity in response to rewarding lateral hypothalamic stimulation. Rats were allowed to self-stimulate for 1 h per day for ten consecutive days following which postmortem phosphorylase a activity was examined. Significant differences in optical density between the stimulated and contralateral hemispheres were found in three of the eight analyzed structures, two of which, the diagonal band of Broca and the caudate nucleus, showed a greater density of glycogen phosphorylase a on the stimulated side and the third, the habenula, had greater contralateral activity. In conclusion, our data suggest that glycogen phosphorylase activity is a viable but not weighty marker of energy alterations induced by chronic exposure to intracranial self-stimulation, and that it is generally consistent with the patterns revealed by other metabolic indices such as cytochrome oxidase and 2-deoxyglucose autoradiography.

Control of resolution and perception in working memory

Mechanisms underlying and controlling resolution and perception in working memory are studied by means of a pulse-coupled network model. It is shown that the adaptivity, i.e. the degree to which previous activity affects the ability to fire, of the excitatory units can control several aspects of the network dynamics in a coordinated way to enable multiple items to be resolved and perceived in working memory. One basic aspect is the complexity of the dynamics that regulates the temporal resolution of several items. The slow NMDA-receptor-mediated component of synaptic couplings to excitatory units facilitates successive activations of a given item. The dimension of the activated subspace of the complete available neural representation space is gradually decreased as adaptivity is reduced. It is also shown that the formation of perception by sufficiently intense and coherent activation of different features of an object can be controlled concurrently with resolution by the adaptivity. The mechanisms derived can account for the observed capacity of working memory with respect to number of items consciously resolved and also for the observed temporal separation of different items. Numerous observations link neuromodulators to cognitive functions and to various brain disorders involving working memory. Based on the influence of various neuromodulators on neuronal adaptivity, the model can also account for neuromodulatory regulation of working memory functions.

Modular subdivisions of dolphin insular cortex: does evolutionary history repeat itself?

The structural organization of the insular cortex in the bottlenose dolphin was investigated by examining Nissl- and myelin-stained tissue that was sectioned coronally and tangentially. An uneven distribution of cell clusters that coincided with myelin-light zones was observed in layer II. When the present observations were compared to descriptions of modules in other animals, we found that the range of module size is restricted, while the size of the brain, particularly the neocortex, varies dramatically. Indeed, despite the tremendous expansion of the cetacean neocortex, the size of the modules in the insular cortex is similar to that described for small-brained mammals like the mouse, suggesting that module size is evolutionarily stable across species. Selection for optimal-size processing units, in terms of the lengths of connections within and between them, is a likely source of this stability.

Cytochrome oxidase in Alzheimer's disease: biochemical, histochemical, and immunohistochemical analyses of the visual and other systems

Defects in oxidative metabolism have been implicated in Alzheimer's disease (AD). The present study evaluated the level of cytochrome oxidase (C.O.), an indicator of neuronal oxidative capacity, in various brain regions of post-mortem AD and control patients. We found a statistically significant reduction in C.O. levels in all cortical areas examined, including the primary and secondary visual cortices. In addition, all layers of the dorsal lateral geniculate nucleus and sublaminae of the primary visual cortex in AD cases examined suffered a reduction in their relative C.O. activity and protein amount. Our results suggest a generalized suppression of oxidative metabolism throughout the cortex, as well as in a major subcortical visual center in AD. Such hypometabolism may form the basis for not only deficits in higher cortical functions, but also a variety of visual dysfunctions known to occur in AD.

Morphological characteristics of layer II projection neurons in the rat medial entorhinal cortex

The entorhinal cortex receives inputs from a variety of neocortical regions. Neurons in layer II of the entorhinal cortex originate one component of the perforant path which conveys this information to the dentate gyrus and hippocampus. The current study extends our previous work on the electro-responsive properties of layer II neurons of the medial entorhinal cortex in which we distinguished two categories of layer II neurons based on their electrophysiological attributes (Alonso and Klink [1993] J Neurophysiol 70: 128-143). Here we report on the morphological features of layer II projection neurons, as revealed by in vitro intracellular injection of biocytin. We now report that the two electrophysiologically distinct types of neurons correspond to morphologically distinct types of cells. All neurons (65% of the total cells recorded) that developed sustained, subthreshold, sinusoidal membrane potential oscillations were found to have a stellate appearance. Neurons that did not exhibit oscillatory behavior had either a pyramidal-like (32%) or a horizontal cell morphology (3%). Stellate cells had multiple, thick, primary dendrites. Their widely diverging upper dendritic domain expanded mediolaterally over a distance of around 500 microns close to the pial surface. This mediolateral extent was more than double that of the pyramidal-like cells. Dendrites of stellate cells demonstrated long dendritic appendages, and their dendritic spines had a more complex morphology than those of nonstellates. The stellate cell axons emerged from a primary dendrite and were more than double the thickness (approximately 1.4 microns) of the axons of nonstellate cells. Recurrent axonal collaterization appeared more extensive in axons arising from stellate cells than from pyramidal-like cells.

Reciprocal entorhinal-hippocampal connections established by human fetal midgestation

Little is known about the timing or sequence of genesis of connections between different areas of the developing human cerebral cortex. It has been shown that connections between areas V1 and V2 of the visual isocortex are established at about 37 weeks of gestation (Burkhalter [1993] Cerebr. Cortex 3:476-487), suggesting that cortico-cortical connections appear late in the 40-week human gestational period. However, there are indications from other studies that connections between subdivisions of the hippocampal formation may be established much earlier, by about 20 weeks of human gestation. To investigate this possibility, the lipophilic bidirectional tracer 1,1' dioctadecyl-3,3,3',3-tetramethylindocarbocyanine perchlorate (DiI) was used to study connections between the entorhinal cortex, hippocampus, and temporal lobe neocortex in paraformaldehyde-fixed postmortem fetal tissue. The DiI transport revealed robust reciprocal connections between the entorhinal cortex, hippocampus, and subiculum, which were consistently present at 19 weeks of gestation (the earliest age studied), and which were anatomically similar to those in adult primates. Specifically, projections to the hippocampus and subiculum originated from neurons in the entorhinal cortex (EC) layers 2 and 3, whereas reciprocal projections to the EC originated from pyramidal neurons in the cornu ammonis region CA1 and the subiculum. In contrast, the perforant pathway projection from EC to the dentate gyrus, and all connections with the neocortex, reached only rudimentary stages of development by 22 weeks of gestation (the latest age studied). These findings suggest that hippocampal pathways develop prior to isocortical pathways, and that reciprocal entorhinal-hippocampal projections may be among the first cortico-cortical connections to be established in the human brain.

Laminin stimulates expression of two mitochondrial proteins during neurite outgrowth

Differential hybridization was utilized with mRNA from NG108-15 cells cultured on either tissue culture plastic or laminin for 4 hr to identify genes whose mRNA was increased by laminin, a potent stimulator of neurite outgrowth. Two of the 16 laminin-induced clones, cytochrome b and chargerin II, are mitochondrial proteins. Northern blotting confirmed that laminin increased the mRNA levels of cytochrome b and chargerin II several fold. Antibody to chargerin II stained both processes and cell bodies of the cerebellar Purkinje cells and localized in the mitochondria of NG108-15 cells, which also showed increased protein levels in the presence of laminin. In addition, higher levels of chargerin II protein were detected in the newborn brain compared to the adult. However, inhibitors of mitochondrial protein synthesis did not affect laminin-mediated neurite outgrowth. These data suggest that the increased synthesis of mitochondrial enzymes observed with laminin treatment may not be necessary for the formation of neurites.

Gene expression of ND4, a subunit of complex I of oxidative phosphorylation in mitochondria, is decreased in temporal cortex of brains of Alzheimer's disease patients

Gene expression of mitochondrial DNA-encoded ND4 in brains of Alzheimer's disease (AD) patients and age-matched controls was measured using Northern blot. The level of ND4 message in temporal cortex of control subjects was higher than in motor cortex, whereas the level of ND4 gene expression in temporal cortex of AD brains was decreased compared with that in temporal cortex of controls. A control probe showed no difference in expression between the two areas of AD and control brains. These and previous data suggest that neurons vulnerable to AD express higher levels of enzymes of oxidative phosphorylation than do spared neurons, and that this difference may promote selective neuronal vulnerability of AD.

The distribution of two calcium binding proteins, calbindin D-28K and parvalbumin, in the entorhinal cortex of the adult mouse

The immunohistochemical localization of two specific calcium binding proteins, parvalbumin (PV) and calbindin D-28K (CB), were examined in the entorhinal cortex (EC) of the adult mouse. The PV and CB immunoreactivities exhibited a conspicuous regional and laminar distribution in the EC. The overall immunostaining pattern of PV and CB appeared to be complementary in the EC, especially in the medial entorhinal area (MEA). In the dorsal MEA, although layer 2 showed intense PV and CB immunostaining, the PV immunoreactivity was denser in layers 3, 5 and 6a than in layers 4 and 6b, whereas the CB immunoreactivity was denser in layers 4 and 6b than in layers 3, 5 and 6a. Moreover, we recognized the dorsoventral gradation of the PV and CB staining that is, in the dorsal to ventral direction, the intensity of the PV immunostaining in layers 2, 3, 5 and 6a gradually decreased whereas that of the CB immunostaining in those layers gradually increased. In addition, a similar dorsoventral gradation was also observed in the number of PV immunoreactive (PV-IR) and CB-IR neurons in layer 3. In layer 2 of the MEA, the CB-IR neurons were clustered, while displaying a patch-like pattern which could not be recognized in either Nissl staining or PV staining. In contrast, layer 2 of the LEA was separated into two sublayers, the superficial sublayer 2a and the deeper sublayer 2b; both of these sublayers consisted of cell clusters recognized by Nissl staining. These sublayers showed a prominent difference in their CB immunoreactivity; the cells in the layer 2a clusters were CB negative, whereas the cells in the layer 2b clusters were CB-IR. Furthermore, we also recognized a particular region at the most medial part of the MEA, where layer 2 was different from the other portion of the MEA regarding CB immunoreactivity and the cells containing another calcium binding protein, calretinin, were clustered in layer 3. Both the adjacent section technique and the fluorescent double-staining technique clearly revealed that a relatively large number of presumable interneurons contained both PV and CB immunoreactivities. Furthermore, the three neuron groups that were immunoreactive for both PV and CB, immunoreactive for PV alone and immunoreactive for CB alone were heterogeneous in their structural features such as shape and size, and no particular difference was found in their structural features among these three groups.