Glial alterations from early to late stages in a model of Alzheimer's disease: Evidence of autophagy involvement in Aβ internalization

Alzheimer's disease (AD) is a progressive neurodegenerative disease without effective therapy. Brain amyloid deposits are classical histopathological hallmarks that generate an inflammatory reaction affecting neuronal and glial function. The identification of early cell responses and of brain areas involved could help to design new successful treatments. Hence, we studied early alterations of hippocampal glia and their progression during the neuropathology in PDAPP‐J20 transgenic mice, AD model, at 3, 9, and 15 months (m) of age. At 3 m, before deposits formation, microglial Iba1+ cells from transgenic mice already exhibited signs of activation and larger soma size in the hilus, alterations appearing later on stratum radiatum. Iba1 immunohistochemistry revealed increased cell density and immunoreactive area in PDAPP mice from 9 m onward selectively in the hilus, in coincidence with prominent amyloid Congo red + deposition. At pre‐plaque stages, GFAP+ astroglia showed density alterations while, at an advanced age, the presence of deposits was associated with important glial volume changes and apparently being intimately involved in amyloid degradation. Astrocytes around plaques were strongly labeled for LC3 until 15 m in Tg mice, suggestive of increased autophagic flux. Moreover, β‐Amyloid fibrils internalization by astrocytes in in vitro conditions was dependent on autophagy. Co‐localization of Iba1 with ubiquitin or p62 was exclusively found in microglia contacting deposits from 9 m onward, suggesting torpid autophagy. Our work characterizes glial changes at early stages of the disease in PDAPP‐J20 mice, focusing on the hilus as an especially susceptible hippocampal subfield, and provides evidence that glial autophagy could play a role in amyloid processing at advanced stages. © 2015 Wiley Periodicals, Inc.     

Entorhinal cortex of the monkey: V. Projections to the dentate gyrus, hippocampus, and subicular complex.

The topographic and laminar organization of entorhinal projections to the dentate gyrus, hippocampus, and subicular complex was investigated in the Macaca fascicularis monkey. Injections of 3H-amino acids were placed at various positions within the entorhinal cortex and the distribution of anterogradely labeled fibers and terminals within the other fields of the hippocampal formation was determined. Injections of the retrograde tracers Fast blue, Diamidino yellow, and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were also placed into the dentate gyrus, hippocampus, and subicular complex, and the distribution of retrogradely labeled cells in the entorhinal cortex was plotted using a computer-aided digitizing system. The entorhinal cortex gave rise to projections that terminated in the subiculum, in the CA1, CA2, and CA3 fields of the hippocampus, and in the dentate gyrus. Projections to the dentate gyrus, and fields CA3 and CA2 of the hippocampus, originated preferentially in layers II and VI of the entorhinal cortex whereas projections to CA1 and to the subiculum originated mainly in layers III and V. Anterograde tracing experiments demonstrated that all regions of the entorhinal cortex project to the outer two-thirds of the molecular layer of the dentate gyrus and to much of the radial extent of the stratum lacunosum-moleculare of CA3 and CA2. While the terminal distributions of entorhinal projections to the dentate gyrus, CA3, and CA2 were not as clearly laminated as in the rat, projections from rostral levels of the entorhinal cortex preferentially innervated the outer portion of the molecular layer and stratum lacunosum-moleculare, whereas more caudal levels of the entorhinal cortex projected relatively more heavily to the deeper portions of the entorhinal terminal zones. The entorhinal projection to the CA1 field of the hippocampus and to the subiculum followed a transverse rather than radial gradient of distribution. Rostral levels of the entorhinal cortex terminated most heavily at the border of CA1 and the subiculum. More caudal levels of the entorhinal cortex projected to progressively more distal portions of the subiculum (towards the presubiculum) and more proximal portions of CA1 (towards CA2). Lateral portions of the entorhinal cortex projected to caudal levels of the recipient fields and more medial parts of the entorhinal cortex projected to progressively more rostral portions of the fields.     

Laminar spongiosis of the dentate gyrus: a sign of disconnection, present in cases of severe Alzheimer’s disease

An extensive laminar spongiosis was found in the outer part of the dentate gyrus in an 84-year-old patient. An old cavitary infarct in the parahippocampal gyrus disconnected the dentate gyrus from the entorhinal area. This finding prompted us to seek laminar spongiosis in Alzheimer’s disease, where the neuronal loss in the entorhinal cortex might be severe. The dentate gyrus was systematically examined in a series of prospectively assessed cases either intellectually normal or affected by mental impairment of graded severity. Laminar spongiosis was present in the most severely affected patients. The neuritic crown of the senile plaques seen in the laminar band of spongiosis contained only a few tau- and Bodian-positive fibers, a sign that was taken as evidence of “plaque denervation”. By contrast, deposits of Aβ peptide remained abundant but lacked a dense core. These data suggest that dendritic and axonal processes are intermingled in the senile plaque and that the amyloid core is at least partially dependent on the presence of the axonal component. Received: 24 July 1997 / Revised, accepted: 17 October 1997     

Activation of perforant path neurons to field CA1 by hippocampal projections

Previous evidence showed that single-shock stimulation of dorsal hippocampal commissure (PSD) fibers to the entorhinal cortex led to sequential activation of perforant path neurons to the dentate gyrus, dentate granule cells, pyramidal neurons of hippocampal fields CA3 and CA1, and, through reentrant hippocampal impulses, neurons of deep and superficial layers of the entorhinal cortex. The aim of the present study was to ascertain whether perforant path neurons to CA1 are activated by the PSD input and/or by the reentrant hippocampal impulses in this model. Field potentials evoked by single-shock (0.1-Hz) or repetitive (1-4 Hz) PSD stimulation were recorded in anesthetized guinea pigs from the entorhinal cortex, dentate gyrus, fields CA1 and CA3, and subiculum. A current source-density analysis of the evoked potentials was used to localize the input to field CA1 and dentate gyrus. After either single-shock or repetitive PSD stimulation, an early current sink was found in the molecular layer of the dentate gyrus, but no sink was present in CA1. With low-frequency PSD stimulation, a late (approximately 40-ms) surface positive wave occurred in field CA1 alone. During this wave, a current sink was found in the stratum lacunosum-moleculare of CA1, but no sink was present in the dentate gyrus. The late wave had threshold and magnitude related to the building up of the response evoked by reentrant hippocampal impulses in layer III of the entorhinal cortex and was abolished by selective interruption of the perforant path to CA1. The results show that the commissural input to the entorhinal cortex activates perforant path neurons to the dentate gyrus, but not those to field CA1 which are recruited by repetitive hippocampal impulses. These findings show different frequency-dependent patterns of loop operation that might be related to different behaviors.     

Entorhinal cortex of the rat: Organization of intrinsic connections

Two sets of experiments were carried out to examine the organization of associational connections within the rat entorhinal cortex. First, a comprehensive analysis of the areal and laminar distribution of intrinsic projections was performed by using the anterograde tracers Phaseolus vulgaris–leuocoagglutinin (PHA-L) and biotinylated dextran amine (BDA). Second, retrograde tracers were injected into the dentate gyrus and PHA-L and BDA were injected into the entorhinal cortex to determine the extent to which entorhinal neurons that project to different septotemporal levels of the dentate gyrus are linked by intrinsic connections. The regional distribution of intrinsic projections within the entorhinal cortex was related to the location of the cells of origin along the mediolateral axis of the entorhinal cortex. Cells located in the lateral regions of the entorhinal cortex gave rise to intrinsic connections that largely remained within the lateral reaches of the entorhinal cortex, i.e., within the rostrocaudally situated entorhinal band of cells that projected to septal levels of the dentate gyrus. Cells located in the medial regions of the entorhinal cortex gave rise to intrinsic projections confined to the medial portion of the entorhinal cortex. Injections made into mid-mediolateral regions of the entorhinal cortex mainly gave rise to projections to mid-mediolateral levels, although some fibers did enter either lateral or medial portions of the entorhinal cortex. These patterns were the same regardless of whether the projections originated from the superficial (II–III) or deep (V–VI) layers of the entorhinal cortex. This organizational scheme indicates, and our combined retrograde/anterograde labeling studies confirmed, that laterally situated entorhinal neurons that project to septal levels of the dentate gyrus are not in direct communication with neurons projecting to the temporal portions of the dentate gyrus. These results suggest that entorhinal intrinsic connections allow for both integration (within a band) and segregation (across bands) of entorhinal cortical information processing. J. Comp. Neurol. 398:49–82, 1998. © 1998 Wiley-Liss, Inc.     

Species differences in the projections from the entorhinal cortex to the hippocampus

Both differences and similarities exist between mammalian species in the projections from entorhinal cortex to the hippocampal formation. In most species, layer II cells of the entorhinal cortex project to the dentate gyrus, and they terminate in the outer two-thirds of the molecular layer of the dentate gyrus. The axons from layer III cells project bilaterally to areas CA(1) and CA(3) of the hippocampus, terminating in the stratum lacunosum moleculare. We have analyzed these projections in mice, and in general, the entorhinal cortex-to-hippocampus projections are similar to those in rats. Axons from layer II neurons terminate in the outer and middle thirds of the molecular layer of the dentate gyrus, and axons from layer III neurons terminate bilaterally in the stratum lacunosum moleculare of areas CA(1) and CA(3), and in the molecular layer of the subiculum. However, in contrast to rat, mouse entorhinal cortex neurons do not appreciably project to the contralateral dentate gyrus. Most species, including mice, show a similar topographical organization of the entorhinal-hippocampal projections, with neurons in the lateral part of both the lateral and medial entorhinal cortex projecting to the dorsal part or septal pole of the hippocampus, whereas the projection to the ventral hippocampus originates primarily from neurons in medial parts of the entorhinal cortex.     

Two reentrant pathways in the hippocampal-entorhinal system

The entorhinal cortex has long been recognized as an important interface between the hippocampal formation and the neocortex. The notion of bidirectional connections between the entorhinal cortex and the hippocampal formation have led to the suggestion that hippocampal output originating in CA1 and subiculum may reenter hippocampal subfields via the entorhinal cortex. To investigate this, we used simultaneous multi-site field potential recordings and current source density analysis in the entorhinal cortex and hippocampal formation of the rat in vivo. Under ketamine/xylazine anesthesia, we found that repetitive stimulation of subiculum or Schaffer collaterals facilitated entorhinal responses, such that a population spike appeared in layer III. In addition, a current sink in stratum lacunosum-moleculare of area CA1 was found, that followed responses in the entorhinal cortex, indicating reentrance into this area. Responses indicating reentrance in the dentate gyrus were not found under ketamine/xylazine anesthesia, but were readily evoked under urethane anesthesia. Reentrance into CA1 was also encountered under urethane anesthesia. These results suggest that parallel, but possibly functionally distinct, connections are present between the output of the hippocampal formation and cells in layers III and II of the entorhinal cortex that project to area CA1 and the dentate gyrus, respectively.     

Neuronal inputs to hippocampal formation in Alzheimer's disease and in parkinsonism-dementia complex on Guam

Summary This study concerns the expression of synaptophysin in the hippocampal formation of normal controls, of patients with Alzheimer's disease (AD) and of patients with parkinsonism-dementia complex on Guam (P-D complex). A monoclonal antibody was used to visualize synaptophysin, an integral component of presynaptic vesicle membranes. In the normal controls, a strong synaptophysin immunoreactivity was seen in the stratum pyramidale, stratum radiatum and stratum lacunosum-moleculare of the hippocampus proper, in the subiculum and in the molecular layer of the dentate gyrus. In the dentate gyrus molecular layer, the reaction product was distributed in a laminar fashion. By contrast, in AD and in P-D complex a significant decrease in immunoreactivity was observed in all hippocampal strata, and especially of the hippocampal subfield CA1 and the subiculum. In both diseases, synaptophysin expression was also diminished in the molecular layer of the dentate gyrus of all patients examined, with the inner portion exhibiting almost normal and the outer portion a strikingly reduced synaptophysin immunoreactivity.     

Alz-50 immunoreactive neuropil differentiates hippocampal complex subfields in Alzheimer's disease

The topographic distribution of Alz-50 containing profiles was determined within the hippocampal formation and anterior parahippocampal gyrus by using a monoclonal antibody directed against the A68 protein in normal and Alzheimer's diseased (AD) brains. Although there was a paucity of immunoreactive neuropil in the normal hippocampal complex, there were a few Alz-50 positive neurons that occupied the hippocampal subfield, CA2. In most AD cases, Alz-50 immunoreactive neuropil was prominent in the outer two-thirds of the molecular layer of the dentate gyrus, although a few cases exhibited staining in the inner third of the molecular layer. CA2 was characterized by an increased density of neuropil staining within stratum pyramidale. The neuropil in subfield CA1 was stained densely with Alz-50 in strata oriens, pyramidale, and at the border between strata lacunosum-moleculare and radiatum. Alz-50 immunostained neurites occupied primarily the lateral two-thirds of the subiculum proper, whereas only sparse staining was seen in the adjacent presubiculum. Alz-50 neuropil and neuronal staining displayed three distinct laminar patterns along the mediolateral extent of the entorhinal cortex, whereas the perirhinal cortex exhibited a bilaminar pattern of immunoreactivity involving heavy staining in layers 1-3 as compared to layer 5. In general, the density of Alz-50 neurite staining in the neuropil appeared inversely proportional to the distribution of Alz-50 immunoreactivity within dendritic and somal compartments. Interestingly, the patterns of Alz-50 staining observed in the hippocampal complex in AD coincides with patterns of well-characterized afferent fiber pathways to these regions, thus further supporting the suggestion that hippocampal subfield specific pathology effectively disconnects medial temporal structures from adjacent neocortex in AD.     

The entorhinal cortex of the mouse: organization of the projection to the hippocampal formation

The origin and the terminations of the projections from the entorhinal cortex to the hippocampal formation of the mouse (C57BL/6J strain) have been studied using anterogradely and retrogradely transported tracers. The entorhinal cortex is principally divided into two areas, the lateral entorhinal area (LEA) and the medial entorhinal area (MEA). LEA is the origin of the lateral perforant path that terminates in the outer one-third of the molecular layer of the dentate gyrus, and MEA is the origin of the medial perforant path that ends in the middle one-third of the molecular layer of the dentate gyrus. This projection is mostly to the ispsilateral dentate gyrus; only a few labeled axons and terminals are found in the contralateral dentate gyrus. The projection to the dentate gyrus originates predominantly from neurons in layer II of the entorhinal cortex. The entorhinal cortex also projects to CA3 and CA1 and to subiculum; in both CA3 and CA1, the terminals are present in stratum lacunosum-moleculare, whereas in the subiculum the terminals are in the outer part of the molecular layer. The projection from the entorhinal cortex to CA3, CA1, and subiculum is bilateral, and it originates predominantly from neurons in layer III, but a small number of neurons in the deeper layers of the entorhinal cortex contributes to this projection. The projection of entorhinal cortex to the hippocampus is topographically organized, neurons in the lateral part of both LEA and MEA project to the dorsal part (i.e., septal pole) of the hippocampus, whereas the projection to the ventral (i.e., temporal pole) hippocampus originates from neurons in medial parts of the entorhinal cortex.     

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

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

Decreased adult hippocampal neurogenesis in the PDAPP mouse model of Alzheimer's disease

Abnormal subgranular zone (SGZ) neurogenesis is proposed to contribute to Alzheimer's disease (AD)-related decreases in hippocampal function. Our goal was to examine hippocampal neurogenesis in the PDAPP mouse, a model of AD with age-dependent accumulation of amyloid-beta(42) (Abeta(42))-containing plaques that is well studied with regard to AD therapies. A secondary goal was to determine whether altered neurogenesis in the PDAPP mouse is associated with abnormal maturation or number of mature cells. A tertiary goal was to provide insight into why hippocampal neurogenesis appears to be increased in AD post-mortem tissue and decreased in most AD mouse models. We report an age-dependent decrease in SGZ proliferation in homozygous PDAPP mice. At 1 year of age, PDAPP mice also had new dentate gyrus granule neurons with abnormal maturation and fewer dying cells relative to control mice. In contrast to decreased SGZ cell birth, PDAPP mice had increased birth of immature neurons in the outer portion of the granule cell layer (oGCL), providing insight into why some studies link AD with increased neurogenesis. However, these ectopic oGCL cells were still rare compared with SGZ proliferating cells, emphasizing that the primary characteristic of PDAPP mice is decreased neurogenesis. The decrease in SGZ neurogenesis was not associated with an age-dependent loss of dentate granule neurons. The altered neurogenesis in the PDAPP mouse may contribute to the age-related cognitive deficits reported in this model of AD and may be a useful adjunct target for assessing the impact of AD therapies.     

A novel population of calretinin-positive neurons comprises reelin-positive Cajal-Retzius cells in the hippocampal formation of the adult domestic pig

Calretinin-containing neurons in the hippocampal formation, including the subiculum, presubiculum, parasubiculum, and entorhinal cortex, were visualized with immunocytochemistry. Calretinin immunoreactivity was present exclusively in non-principal cells. The largest immunoreactive cell population was found in the outer half of the molecular layer of the dentate gyrus and in the stratum lacunosum-moleculare of Ammon's horn. A proportion of these cells were also immunoreactive for reelin, a Cajal-Retzius cell marker. Similar calretinin-positive cells were found in the molecular layer of the subicular complex and entorhinal cortex. In the parasubiculum, a few immunoreactive bipolar and multipolar cells could be observed in the superficial and deep pyramidal cell layers. In the entorhinal cortex, bipolar and multipolar calretinin-positive cells were frequent in layer II, and large numbers of multipolar cells in layer V were immunoreactive. Electron microscopic analysis showed that somata of calretinin-positive cells contained either round nuclei with smooth nuclear envelopes or nuclei with multiple deep infoldings. Immunoreactive dendrites were smooth varicose, and the apposing axon terminals formed both symmetric and asymmetric synapses. Zonula adherentia were observed between calretinin-positive dendrites. Calretinin-positive axon terminals formed two types of synapses. Axon terminals with asymmetric synapses were found close to the hippocampal fissure, whereas axon terminals forming symmetric synapses innervated spiny dendrites in both the molecular layer of the dentate gyrus and in stratum lacunosum-moleculare of Ammon's horn. Calretinin-positive axon terminals formed both symmetric and asymmetric synapses with calretinin-positive dendrites. In conclusion, calretinin-positive neurons form two major subpopulations in the adult domestic pig hippocampus: (1) a gamma-aminobutyric acid (GABA)ergic subpopulation of local circuit neurons that innervates distal dendrites of principal cells in both the dentate gyrus and in Ammon's horn; and (2) Cajal-Retzius type cells close to the hippocampal fissure, as well as in the molecular layer of the subicular complex and entorhinal cortex.     

Autoradiographic evidence that NMDA receptor-coupled channels are located postsynaptically and not presynaptically in the perforant path-dentate granule cell system of the rat hippocampal formation

In vitro quantitative autoradiography with [3H]MK-801 was used to determine Kd and Bmax values for the NMDA receptor-coupled channel in subregions of the rat hippocampal formation. A single form of the channel with an apparent Kd in the 15-20 nM range was found for [3H]MK-801 binding in the presence of both 1 microM glutamate and 1 microM glycine. Specific binding was highest in the molecular layer of the dentate gyrus, followed by CA1 stratum radiatum and CA1 stratum oriens. Fewer binding sites were observed in the hilus of the dentate gyrus, cerebral cortex, CA1 stratum pyramidale, CA3 subregion (stratum oriens, stratum pyramidale, stratum radiatum), and thalamus. Selective destruction of dentate granule cells by colchicine microinjections reduced the amount of specific [3H]MK-801 binding by half in the molecular layer of the dentate, compared to intact tissue. [3H]MK-801 binding did not change in other hippocampal subregions as a consequence of colchicine injection. Electrolytic entorhinal cortical lesions produced no changes in regional MK-801 binding site density in any of the regions under study. To address the tissue shrinkage following entorhinal cortex lesions, detailed analysis of the binding site density per fixed (16 microns) length of granule cell dendrite, and of the aggregate density across the entire molecular layer revealed no change in the number of MK-801 binding sites per unit length of dendrite in the molecular layer of the dentate gyrus. These findings indicate that NMDA receptor-coupled channels are confined to a postsynaptic location in the perforant path-dentate granule cell system of the adult rat.     

Aberrant trajectory of entorhino-dentate axons in the mutant Shaking Rat Kawasaki: a Dil-labelling study

The Shaking Rat Kawasaki (SRK) is a neurological mutant that exhibits abnormalities of cell migration and lamination, with many similarities to the mouse reeler mutant. We recently used lamina-specific antibody staining to show that despite severe aberrations in the laminar organization of the SRK dentate gyrus, the entorhinal terminal field in the outer dentate molecular layer appeared relatively normal (Woodhams & Terashima, 1999, J. Comp. Neurol. 409 p57). However, neurofilament immunostaining suggested that entorhino-dentate afferents take an abnormal trajectory in reaching their appropriate targets, the granule cells dendrites. In the present study, anterograde tracing with the carbocyanine dye 1, 1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) has been used to delineate directly the path that entorhinal axons take to the dentate gyrus, confirming that in SRK entorhinal axons do indeed reach their appropriate terminal fields in the molecular layer, with laminar segregation between projections from the lateral and medial entorhinal cortices. However, these fibres fail to cross the hippocampal fissure between the subiculum and the dentate gyrus, coursing instead parallel to it until they curve round the deepest point of the fissure in field CA3. Similar findings were seen in the murine reeler mutant. Insertion of DiI crystals into the entorhinal cortex of neonatal rats also retrogradely labelled the developmentally transient Cajal-Retzius cells at the hippocampal fissure; these survive for longer in SRK than in normal littermates. The presence of a marked astrogliosis at the SRK hippocampal fissure may play a part in determining the abnormal trajectory taken by entorhino-dentate afferents in this mutant.     

Gene expression of growth-associated proteins,GAP-43 and SCG10, in the hippocampal formation of the Macaque monkey: Nonradioactive in situ hybridization study

We performed nonradioactive in situ hybridization histochemistry in the monkey hippocampal formation that includes the hippocampus, the subicular complex, and the entorhinal cortex to detect the expression of mRNA for two growth-associated proteins: GAP-43 and SCG10. Overall, the distribution patterns overlapped but were partially distinct. In the hippocampus, the intense hybridization signals for both GAP-43 and SCG10 mRNAs were observed in the pyramidal cell layer of Ammon's horn, especially in CA3 subfields. The intense hybridization signals were also observed in the stratum oriens of Ammon's horn and the polymorphic layer of the dentate gyrus. In the granule cell layer of the dentate gyrus, many GAP-43 mRNA-positive cells were observed, whereas a few positive cells with weak signals were observed for SCG10 mRNA. Throughout the subicular complex, the hybridization signals for both mRNAs were weak. In the entorhinal cortex, both mRNAs were abundant in the caudal field. These subregion-specific expression of the growth-associated proteins may reflect the functional specialization regarding plasticity in each region of the monkey hippocampal formation. Hippocampus 1998;8:533–547. © 1998 Wiley-Liss, Inc.     

Stereologic analysis of hippocampal Alzheimer's disease pathology in the oldest-old: evidence for sparing of the entorhinal cortex and CA1 field

Several neuropathologic analyses postulate that Alzheimer disease (AD) in the oldest-old is associated with substantial neurofibrillary tangle (NFT) formation in the CA fields of the hippocampus and neuronal loss confined to the entorhinal cortex. All of these studies have measured densities, rather than absolute numbers, and most do not take into account the potential interaction between the above pathological hallmarks in a global multivariate analysis. We present here a stereologic analysis of AD-related pathology in 12 oldest-old individuals including a complete assessment of total NFT, neuron numbers and amyloid volume in entorhinal cortex, CA fields, and dentate gyrus. The progression of NFT numbers and amyloid volume across the different Clinical Dementia Rating (CDR) groups was significantly slower in these cases compared to previously reported younger cases. Although patients with mild and moderate dementia showed significantly lower mean neuron numbers compared to CDR 0-0.5 cases, there was a marked overlap in individual values among CDR groups. A modest proportion of the variability in CDR scores was explained by NFT numbers in the CA2 field (18.1%) and the dentate gyrus (17.3%). In contrast, neither Nissl-stained neuron numbers nor total amyloid volume in the areas studied significantly predicted cognitive status. These data indicate that the occurrence and progression of AD-related pathologic changes are not an unavoidable consequence of aging. They also suggest that dementia in extreme aging depends more on the damage of hippocampal subdivisions commonly less affected than on severe NFT formation and neuronal loss in the CA1 field and entorhinal cortex.     

Density and spatial pattern of β-amyloid (Aβ) deposits in corticobasal degeneration

Corticobasal degeneration (CBD) is a rare, progressive movement disorder characterized neuropathologically by widespread neuronal and glial pathology including tau-immunoreactive neuronal cytoplasmic inclusions (NCI), oligodendroglial inclusions (GI), and astrocytic plaques (AP). However, β -amyloid (A β) deposits have been observed in the cerebral cortex and/or hippocampus in some cases of CBD. To clarify the role of Aβ deposition in CBD, the densities and spatial patterns of the Aβ deposits were studied in three cases. In two cases, expressing apolipoprotein E (APOE) genotypes 2/3 or 3/3, the densities of the Aβ deposits were similar to those in normal elderly brain. In the remaining case, expressing APOE genotype 3/4, Aβ deposition was observed throughout the cerebral cortex, sectors CA1 and CA2 of the hippocampus, and the molecular layer of the dentate gyrus. The densities of the Aβ deposits in this case were typical of those observed in Alzheimer's disease (AD). In the three cases, clustering of Aβ deposits, with clusters ranging in size from 200 to >6400 μm in diameter, was evident in 25/27 (93%) of analyses. In addition, the clusters of Aβ deposits were regularly distributed parallel to the tissue boundary in 52% of analyses, a spatial pattern similar to that observed in AD. These results suggest: (1) in some CBD cases, Aβ pathology is age-related, (2) more extensive Aβ deposition is observed in some cases, the density and spatial patterns of the Aβ deposits being similar to AD, and (3) extensive deposition of Aβ in CBD may be associated with APOE allele e4.     

Calcium channel blockade attenuates abnormal synaptic transmission in the dentate gyrus elicited by entorhinal amyloidopathy

Entorhinal‐hippocampal network is one of the earliest circuits which is affected by Alzheimer's disease (AD). There are numerous data providing the evidence of synaptic deficit in the dentate gyrus (DG) of AD animal model. However, there is little known about how entorhinal cortex (EC) amyloidophaty affects each excitatory and/or inhibitory transmission in the early stage of AD. On the other hand, it is believed that calcium dyshomeostasis has a critical role in the etiology of AD. Here, the effect of the EC amyloid pathogenesis on excitatory or inhibitory post synaptic currents (EPSC and IPSC, respectively) in the DG granule cells and then the possible neuroprotective action of L‐type calcium channel blockers (CCBs), nimodipine and isradipine, were examined. The amyloid beta (Aβ) 1–42 was injected bilaterally into the EC of male rats and one week later, synaptic currents in the DG granule cells were assessed by whole cell patch clamp. EPSCs were evoked by stimulating the perforant pathway. Voltage clamp recording showed profound decrease of evoked EPSC amplitude and paired pulse facilitation in the DG granule cells of Aβ treated rats. Furthermore, AMPA/NMDA ratio was significantly decreased in the Aβ treated animals. On the other hand, amplitude of IPSC currents was significantly increased in the DG granule cells of these animals. These modifications of synaptic currents were partially reversed by daily intracerebroventricular administration of isradipine or nimodipine. In conclusion, our results suggest that Aβ in the EC triggers decreased excitatory transmission in the DG with substantial decrement in AMPA currents, leading to a prominent activity of inhibitory circuits and increased inhibition of granule cells which may contribute to the development of AD‐related neurological deficits in AD and treatment by CCBs could preserve normal synaptic transmission against Aβ toxicity.     

Astrocyte hypertrophy in the Alzheimer's disease hippocampal formation

In Alzheimer's disease (AD), neuritic plaques are often found in the hippocampal dentate gyrus along the boundary between inner and outer molecular layers. The dentate outer molecular layer in AD also exhibits axon sprouting in response to an early loss of entorhinal neurons. The relationship between the laminar arrangement of plaques and the sprouting remains unclear. In experimental entorhinal lesions in the rat, the denervated dentate outer molecular layer demonstrates hypertrophic astrocytes which may provide trophic support for the sprouting response. It is not known whether an equivalent astrocyte response occurs in AD or whether this response is related to the distribution of plaques. We used immunohistochemical staining for glial fibrillary acidic protein (GFAP) to demonstrate reactive astrocytes in the hippocampus in AD patients and age-matched controls. These results were compared to the astrocyte response to an experimental entorhinal lesion in the rat. Quantitative and qualitative analyses demonstrated a significant increase in GFAP-positive hypertrophic astrocytes in the dentate outer molecular layer in AD compared to controls. These astrocytes were randomly distributed within the outer layer and did not parallel the distribution of neuritic plaques. In the entorhinal-lesioned rat, reactive hypertrophied astrocytes also showed a selective distribution within the denervated outer molecular layer. Our results further support the similarity of the hippocampal response in AD and experimental entorhinal lesion but do not explain the laminar distribution of neuritic plaques along the denervated zone.