Role of apolipoprotein E and estrogen in mossy fiber sprouting in hippocampal slice cultures.

A role for apolipoprotein E is implicated in regeneration of synaptic circuitry after neural injury. The in vitro mouse organotypic hippocampal slice culture system shows Timm's stained mossy fiber sprouting into the dentate gyrus molecular layer in response to deafferentation of the entorhinal cortex. We show that cultures derived from apolipoprotein E knockout mice are defective in this sprouting response; specifically, they show no sprouting in the dorsal region of the dentate gyrus, yet retain sprouting in the ventral region. Dorsal but not ventral sprouting in cultures from C57B1/6J mice is increased 75% by treatment with 100 pM 17beta-estradiol; this response is blocked by both progesterone and tamoxifen. These results show that neuronal sprouting is increased by estrogen in the same region where sprouting is dependent on apolipoprotein E. Sprouting may be stimulated by estrogen through its up-regulation of apolipoprotein E expression leading to increased recycling of membrane lipids for use by sprouting neurons. Estrogen and apolipoprotein E may therefore interact in their modulation of both Alzheimer's disease risk and recovery from CNS injury.     

Functional consequences of the lack of amyloid precursor protein in the mouse dentate gyrus in vivo

The amyloid precursor protein (APP) plays a crucial role in the pathogenesis of Alzheimer's disease. Here, we studied whether the lack of APP affects the synaptic properties in the dentate gyrus by measuring granule cell field potentials evoked by perforant path stimulation in anesthetized 9-11-month-old APP-deficient mice in vivo. We found decreased paired-pulse facilitation, indicating altered presynaptic short-term plasticity in the APP-deficient dentate gyrus. In contrast, excitatory synaptic strength and granule cell firing were unchanged in APP knockout mice. Likewise, long-term potentiation (LTP) induced by a theta-burst stimulation protocol was not impaired in the absence of APP. These findings suggest that the deletion of APP may affect presynaptic plasticity of synaptic transmission at the perforant path-granule cell synapse but leaves synaptic efficacy intact and LTP preserved, possibly due to functional redundancy within the APP gene family.     

Comparison of synaptic changes in the precentral and postcentral cerebral cortex of aging humans: a quantitative ultrastructural study

Ultrastructural quantitative analysis was undertaken to determine whether any age-related synaptic changes occur in cortical layer 1 of the human precentral motor gyrus (Brodmann's area 4) and postcentral somatosensory gyrus (Brodmann's area 3). Immersion fixed, osmicated, uranyl acetate/lead citrate stained (OsUL) preparations of autopsied brains were taken from patients aged 45 to 84 years, with no prior history of neurological or intellectual abnormalities. In the precentral gyrus there was a significant decrease in the number of synapses, which was primarily due to a decrease in asymmetrical axospinous synapses. Symmetrical synapses remained constant in number, while axodendritic synapses showed a small increase with age. Accompanying the decline in synapse number was an increase in mean length of the postsynaptic contact zone. In the postcentral gyrus there were no significant changes in synaptic number or in any of the synaptic parameters measured. The results suggest that the motor cortex of the human brain is capable of synaptic plasticity in response to aging-induced synaptic loss. This plasticity is not apparent in the somatosensory cortex, where there is no age-related synapse loss.     

Alterations in synaptic curvature in the dentate gyrus following induction of long-term potentiation,long-term depression,and treatment with the N-methyl-d-aspartate receptor antagonist CPP

Alterations in curvature of the post synaptic density (PSD) and apposition zone (AZ), are believed to play an important role in determining synaptic efficacy. In the present study we have examined curvature of PSDs and AZs 24 h following homosynaptic long-term potentiation (LTP), and heterosynaptic long-term depression (LTD) in vivo, in awake adult rats. High frequency stimulation (HFS) applied to the medial perforant path to the dentate gyrus induced LTP while HFS stimulation of the lateral perforant path induced LTD in the middle molecular layer of the dentate gyrus (DG). Curvature changes were analysed in this area using three dimensional (3-D) reconstructions of electron microscope images of ultrathin serial sections. Very large and significant changes in 3-D measurements of AZ and PSD curvature occurred 24 h following both LTP and LTD, with a flattening of the normal concavity of mushroom spine heads and a change to convexity for thin spines. An N-methyl-d-aspartate (NMDA) receptor antagonist CPP (3-[(R)-2-Carboxypiperazin-4-yl]-propyl-1-phosphonic acid) blocked the changes in curvature of mushroom and thin spine PSDs and apposition zones, actually increasing the concavity of mushroom spines as the spine engulfed the presynaptic bouton. In order to establish whether these changes resulted from the effect of the NMDA antagonist or from its coincidence with synaptic activation during testing we examined the effects of CPP alone on PSD and apposition zone curvature. It was found that CPP alone also caused a small decrease in curvature of both PSD and apposition zone of mushroom and thin spines.     

Stability of synaptic density and spine volume in dentate gyrus of aged rats

The number of synapses per unit volume and per granule cell and the size of dendritic spines were studied in the dentate gyrus of Sprague-Dawley rats 6, 24, and 30 months of age. Neither synaptic density nor mean spine volume showed any age-related trends. An increase in granule cell packing density at 24 months and concomitant stability of the height of the granule cell layer is consistent with the idea that postnatal generation of granule cells may continue late into life. Possible explanations for the discrepancies in the literature regarding synaptic loss in this area include differences in morphometric techniques, age of animals used, regional differences within dentate gyrus, and sampling variability. Generalized synapse loss in the senescent rodent brain remains to be established.     

构建不同周龄载脂蛋白E基因敲除动脉粥样硬化模型小鼠血脂和病理学观察

背景：载脂蛋白E基因敲除小鼠形成的动脉粥样硬化病变与人类全身动脉粥样硬化好发处相近,是目前建立动脉粥样硬化理想的动物模型。 目的：研究载脂蛋白E基因敲除小鼠不同周龄动脉粥样硬化的病理进程,探讨不同饮食对载脂蛋白E基因敲除小鼠动脉粥样硬化发生发展的影响。 方法：将8周龄雄性载脂蛋白E基因敲除小鼠,随机分为2组,分别给予高脂饮食和普通饮食喂养8,12,16,20,24周。 结果与结论：血清学指标检测显示,不同周龄的高脂饮食组血清中总胆固醇、三酰甘油和低密度脂蛋白胆固醇水平显著高于普通饮食组(P < 0.05),呈时间依赖性。大体和冰冻切片油红O染色结果显示,高脂饮食组动脉粥样硬化管腔斑块面积显著高于普通饮食组(P < 0.05),呈时间依赖性,此时两组各周龄小鼠管腔斑块面积相比均有显著性意义(P < 0.05),小鼠在高脂饮食16周时主动脉可见明显的脂质斑块。结果表明,实验成功构建了载脂蛋白E基因敲除动脉粥样硬化模型小鼠,此模型形成脂质条纹和纤维增生病变的时间较普通饮食组更快。 中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程全文链接：     

Reduced plasticity and mild cognitive impairment-like deficits after entorhinal lesions in hAPP/APOE4 mice

Mild cognitive impairment (MCI) is a clinical condition that often precedes Alzheimer disease (AD). Compared with apolipoprotein E-ε3 (APOE3), the apolipoprotein E-ε4 (APOE4) allele is associated with an increased risk of developing MCI and spatial navigation impairments. In MCI, the entorhinal cortex (EC), which is the main innervation source of the dentate gyrus, displays partial neuronal loss. We show that bilateral partial EC lesions lead to marked spatial memory deficits and reduced synaptic density in the dentate gyrus of APOE4 mice compared with APOE3 mice. Genotype and lesion status did not affect the performance in non-navigational tasks. Thus, partial EC lesions in APOE4 mice were sufficient to induce severe spatial memory impairments and synaptic loss in the dentate gyrus. In addition, lesioned APOE4 mice showed no evidence of reactional increase in cholinergic terminals density as opposed to APOE3 mice, suggesting that APOE4 interferes with the ability of the cholinergic system to respond to EC input loss. These findings provide a possible mechanism underlying the aggravating effect of APOE4 on the cognitive outcome of MCI patients.     

Does a unique type of CA3 pyramidal cell in primates bypass the dentate gate?

The predominant excitatory synaptic input to the hippocampus arises from entorhinal cortical axons that synapse with dentate granule cells, which in turn synapse with CA3 pyramidal cells.Thus two highly excitable brain areas--the entorhinal cortex and the CA3 field--are separated by dentate granule cells, which have been proposed to function as a gate or filter. However, unlike rats, primates have "dentate" CA3 pyramidal cells with an apical dendrite that extends into the molecular layer of the dentate gyrus, where they could receive strong, monosynaptic, excitatory synaptic input from the entorhinal cortex. To test this possibility, the dentate gyrus molecular layer was stimulated while intracellular recordings were obtained from CA3 pyramidal cells in hippocampal slices from neurologically normal macaque monkeys. Stimulus intensity of the outer molecular layer of the dentate gyrus was standardized by the threshold intensity for evoking a dentate gyrus field potential population spike. Recorded proximal CA3 pyramidal cells were labeled with biocytin, processed with diaminobenzidine for visualization, and classified according to their dendritic morphology. In response to stimulation of the dentate gyrus molecular layer, action potential thresholds were similar in proximal CA3 pyramidal cells with different dendritic morphologies. These findings do not support the hypothesis that dentate CA3 pyramidal cells receive stronger synaptic input from the entorhinal cortex than do other proximal CA3 pyramidal cells.     

Impaired synaptic plasticity and learning in aged amyloid precursor protein transgenic mice

We investigated synaptic communication and plasticity in hippocampal slices from mice overexpressing mutated 695-amino-acid human amyloid precursor protein (APP695SWE), which show behavioral and histopathological abnormalities simulating Alzheimer's disease. Although aged APP transgenic mice exhibit normal fast synaptic transmission and short term plasticity, they are severely impaired in in-vitro and in-vivo long-term potentiation (LTP) in both the CA1 and dentate gyrus regions of the hippocampus. The LTP deficit was correlated with impaired performance in a spatial working memory task in aged transgenics. These deficits are accompanied by minimal or no loss of presynaptic or postsynaptic elementary structural elements in the hippocampus, suggesting that impairments in functional synaptic plasticity may underlie some of the cognitive deficits in these mice and, possibly, in Alzheimer's patients.     

No evidence for cholinergic problems in apolipoprotein E knockout and apolipoprotein E4 transgenic mice

The varepsilon4 allele of the apolipoprotein E gene constitutes the major genetic risk factor to develop Alzheimer's disease. If and how this protein contributes to the pathological cascade of Alzheimer's disease is not known. The varepsilon4 allele particularly affects the cholinergic defect, which is one of the most consistent neurotransmitter problems in an Alzheimer's disease brain.We have analysed several parameters of the cholinergic system in brain of apolipoprotein E knockout mice as well as in transgenic mice overexpressing human apolipoprotein E4. We analysed the distribution of cholinergic fibers, the number and morphology of cholinergic neurons and the enzymatic activity of acetylcholinesterase and choline acetyltransferase in different brain regions. Finally, we analysed the distribution and the binding parameters of [3H]hemicholinium-3, a specific marker for the high affinity choline transporter in different brain sections and regions.This extensive effort failed to show any consistent difference in the cholinergic parameters studied, in either the apolipoprotein E4 transgenic mice or in the apolipoprotein E knockout mice, compared to age-matched non-transgenic mice. We conclude that the apolipoprotein E4 is not deleterious per se for the cholinergic system in mouse brain.     

Progressive loss of synaptic integrity in human apolipoprotein E4 targeted replacement mice and attenuation by apolipoprotein E2

Inheritance of the APOE4 allele is a well established genetic risk factor linked to the development of late onset Alzheimer's disease. As the major lipid transport protein in the central nervous system, apolipoprotein (apo) E plays an important role in the assembly and maintenance of synaptic connections. Our previous work showed that 7 month old human apoE4 targeted replacement (TR) mice displayed significant synaptic deficits in the principal neurons of the lateral amygdala, a region that is critical for memory formation and also one of the primary regions affected in Alzheimer's disease, compared to apoE3 TR mice. In the current study, we determined how age and varying APOE genotype affect synaptic integrity of amygdala neurons by comparing electrophysiological and morphometric properties in C57BL6, apoE knockout, and human apoE3, E4 and E2/4 TR mice at 1 month and 7 months. The apoE4 TR mice exhibited the lowest level of excitatory synaptic activity and dendritic arbor compared to other cohorts at both ages, and became progressively worse by 7 months. In contrast, the apoE3 TR mice exhibited the highest synaptic activity and dendritic arbor of all cohorts at both ages. C57BL6 mice displayed virtually identical synaptic activity to apoE3 TR mice at 1 month; however this activity decreased by 7 months. ApoE knockout mice exhibited a similar synaptic activity profile with apoE4 TR mice at 7 months. Consistent with previous reports that APOE2 confers protection, the apoE4-dependent deficits in excitatory activity were significantly attenuated in apoE2/4 TR mice at both ages. These findings demonstrate that expression of human apoE4 contributes to functional deficits in the amygdala very early in development and may be responsible for altering neuronal circuitry that eventually leads to cognitive and affective disorders later in life.     

Apolipoprotein E isoform-specific regulation of dendritic spine morphology in apolipoprotein E transgenic mice and Alzheimer's disease patients

Dendritic spines are postsynaptic sites of excitatory input in the mammalian nervous system. Apolipoprotein (apo) E participates in the transport of plasma lipids and in the redistribution of lipids among cells. A role for apoE is implicated in regeneration of synaptic circuitry after neural injury. The apoE4 allele is a major risk factor for late-onset familial and sporadic Alzheimer's disease (AD) and is associated with a poor outcome after brain injury. ApoE isoforms are suggested to have differential effects on neuronal repair mechanisms. In vitro studies have demonstrated the neurotrophic properties of apoE3 on neurite outgrowth. We have investigated the influence of apoE genotype on neuronal cell dendritic spine density in mice and in human postmortem tissue. In order to compare the morphology of neurons developing under different apoE conditions, gene gun labeling studies of dendritic spines of dentate gyrus (DG) granule cells of the hippocampus were carried out in wild-type (WT), human apoE3, human apoE4 expressing transgenic mice and apoE knockout (KO) mice; the same dendritic spine parameters were also assessed in human postmortem DG from individuals with and without the apoE4 gene. Quantitative analysis of dendritic spine length, morphology, and number was carried out on these mice at 3 weeks, 1 and 2 years of age. Human apoE3 and WT mice had a higher density of dendritic spines than human E4 and apoE KO mice in the 1 and 2 year age groups (P<0.0001), while at 3 weeks there were no differences between the groups. These age dependent differences in the effects of apoE isoforms on neuronal integrity may relate to the increased risk of dementia in aged individuals with the apoE4 allele. Significantly in human brain, apoE4 dose correlated inversely with dendritic spine density of DG neurons cell in the hippocampus of both AD (P=0.0008) and aged normal controls (P=0.0015). Our findings provide one potential explanation for the increased cognitive decline seen in aged and AD patients expressing apoE4.     

Effects of age and insulin-like growth factor-1 on neuron and synapse numbers in area CA3 of hippocampus.

Age-related effects associated with the hippocampus include declines in numbers of neurons and synapses in the dentate gyrus and area CA1, and decreased cognitive ability as assessed with the Morris water maze. The present study quantified both neuron and synapse number in the same tissue block of area CA3 of the hippocampus. No investigations of both density of neurons and synapses together in area CA3 of hippocampus have been performed previously, despite its importance as the terminal field of dentate gyrus mossy fibers, the second synapse in the trisynaptic circuit in the hippocampus. Numerical density of neurons and synapses were assessed in 4-, 18-, and 29-month-old rats receiving infusions of saline into the lateral ventricle and in 29-month-old rats receiving infusions of insulin-like growth factor-1 (IGF-1). Numerical density of neurons of the stratum pyramidale of CA3 of hippocampus remained constant across the life span as did the numerical density of synapses in stratum lucidum of area CA3. Despite the reported role of IGF-1 in synaptogenesis and improvements in behavior with age, ventricular infusion of this growth factor did not affect the numerical density of neurons or synapses in 29-month-old rats when compared to saline-infused old rats. Further, reported effects of IGF-1 on adult neurogenesis in the dentate gyrus are not reflected in an IGF-1-related increase in synapse density in this region.     

Increased synaptic inhibition in dentate gyrus of mice with reduced levels of endogenous brain-derived neurotrophic factor

The aim of this study was to explore the role of endogenous neurotrophins for inhibitory synaptic transmission in the dentate gyrus of adult mice. Heterozygous knockout (+/-) mice or neurotrophin scavenging proteins were used to reduce the levels of endogenous brain-derived neurotrophic factor and neurotrophin-3. Patch-clamp recordings from dentate granule cells in brain slices showed that the frequency, but not the kinetics or amplitude, of miniature inhibitory postsynaptic currents was modulated in brain-derived neurotrophic factor +/- compared to wild-type (+/+) mice. Furthermore, paired-pulse depression of evoked inhibitory synaptic responses was increased in brain-derived neurotrophic factor +/- mice. Similar results were obtained in brain slices from brain-derived neurotrophic factor +/+ mice incubated with tyrosine receptor kinase B-immunoglobulin G, which scavenges endogenous brain-derived neurotrophic factor. The increased inhibitory synaptic activity in brain-derived neurotrophic factor +/- mice was accompanied by decreased excitability of the granule cells. No differences in the frequency, amplitude or kinetics of miniature inhibitory postsynaptic currents were seen between neurotrophin-3 +/- and +/+ mice.From these results we suggest that endogenous brain-derived neurotrophic factor, but not neurotrophin-3, has acute modulatory effects on synaptic inhibition onto dentate granule cells. The site of action seems to be located presynaptically, i.e. brain-derived neurotrophic factor regulates the properties of inhibitory interneurons, leading to increased excitability of dentate granule cells. We propose that through this mechanism, brain-derived neurotrophic factor can change the gating/filtering properties of the dentate gyrus for incoming information from the entorhinal cortex to hippocampus. This will have consequences for the recruitment of hippocampal neural circuitries both under physiological and pathological conditions, such as epileptogenesis.     

Synaptic correlates of memory and menopause in the hippocampal dentate gyrus in rhesus monkeys

Aged rhesus monkeys exhibit deficits in hippocampus-dependent memory, similar to aging humans. Here we explored the basis of cognitive decline by first testing young adult and aged monkeys on a standard recognition memory test (delayed nonmatching-to-sample test; DNMS). Next we quantified synaptic density and morphology in the hippocampal dentate gyrus (DG) outer (OML) and inner molecular layer (IML). Consistent with previous findings, aged monkeys were slow to learn DNMS initially, and they performed significantly worse than young subjects when challenged with longer retention intervals. Although OML and IML synaptic parameters failed to differ across the young and aged groups, the density of perforated synapses in the OML was coupled with recognition memory accuracy. Independent of chronological age, monkeys classified on the basis of menses data as peri- or post-menopausal scored worse on DNMS, and displayed lower OML perforated synapse density, than premenopausal monkeys. These results suggest that naturally occurring reproductive senescence potently influences synaptic connectivity in the DG OML, contributing to individual differences in the course of normal cognitive aging.     

Breaches of the pial basement membrane are associated with defective dentate gyrus development in mouse models of congenital muscular dystrophies

A subset of congenital muscular dystrophies (CMDs) has central nervous system manifestations. There are good mouse models for these CMDs that include POMGnT1 knockout, POMT2 knockout and Large^myd mice with all exhibiting defects in dentate gyrus. It is not known how the abnormal dentate gyrus is formed during the development. In this study, we conducted a detailed morphological examination of the dentate gyrus in adult and newborn POMGnT1 knockout, POMT2 knockout, and Large^myd mice by immunofluorescence staining and electron microscopic analyses. We observed that the pial basement membrane overlying the dentate gyrus was disrupted and there was ectopia of granule cell precursors through the breached pial basement membrane. Besides these, the knockout dentate gyrus exhibited reactive gliosis in these mouse models. Thus, breaches in the pial basement membrane are associated with defective dentate gyrus development in mouse models of congenital muscular dystrophies.     

Dominant negative effects of apolipoprotein E4 revealed in transgenic models of neurodegenerative disease

Apolipoprotein E fulfills fundamental functions in lipid transport and neural tissue repair after injury.(6,8) Its three most common isoforms (E2, E3, and E4) are critical determinants of diverse human diseases, including major cardiovascular and neurodegenerative disorders.(8,14) Apolipoprotein E4 is associated with an increased risk for Alzheimer's disease(3,5) and poor clinical outcome after head injury or stroke.(11,16) The precise role of apolipoprotein E4 in these conditions remains unknown. To characterize the effects of human apolipoprotein E isoforms in vivo, we analysed transgenic Apoe knockout mice that express apolipoprotein E3 or E4 or both in the brain. Hemizygous and homozygous apolipoprotein E3 mice were protected against age-related and excitotoxin-induced neurodegeneration, whereas apolipoprotein E4 mice were not. Apolipoprotein E3/E4 bigenic mice were as susceptible to neurodegeneration as apolipoprotein E4 singly-transgenic mice. At eight months of age neurodegeneration was more severe in homozygous than in hemizygous apolipoprotein E4 mice consistent with a dose effect. Thus, apolipoprotein E4 is not only less neuroprotective than apolipoprotein E3 but also acts as a dominant negative factor that interferes with the beneficial function of apolipoprotein E3. The inhibition of this apolipoprotein E4 activity may be critical for the prevention and treatment of neurodegeneration in APOE varepsilon4 carriers.     

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.     

Age-related loss of synaptophysin immunoreactive presynaptic boutons within the hippocampus of APP751SL, PS1M146L, and APP751SL/PS1M146L transgenic mice

Neuron and synapse loss are important features of the neuropathology of Alzheimer's disease (AD). Recently, we observed substantial age-related hippocampal neuron loss in APP751SL/PS1M146L transgenic mice but not in PS1M146L mice. Here, we investigated APP751SL mice, PS1M146L mice, and APP751SL/PS1M146L mice for age-related alterations in synaptic integrity within hippocampal stratum moleculare of the dentate gyrus (SM), stratum lucidum of area CA3 (SL), and stratum radiatum of area CA1-2 (SR) by analyzing densities and numbers of synaptophysin-immunoreactive presynaptic boutons (SIPBs). Wild-type mice, APP751SL mice and PS1M146L mice showed similar amounts of age-related SIPB loss within SM, and no SIPB loss within SL. Both APP751SL mice and PS1M146L mice showed age-related SIPB loss within SR. Importantly, APP751SL/PS1M146L) mice displayed the severest age-related SIPB loss within SM, SL, and SR, even in regions free of extracellular Abeta deposits. Together, these mouse models offer a unique framework to study the impact of several molecular and cellular events caused by mutant APP and/or mutant PS1 on age-related alterations in synaptic integrity. The observation of age-related SIPB loss within SR of PS1M146L mice supports a role of mutant PS1 in neurodegeneration apart from its contribution to alterations in Abeta generation.     

No hippocampal neuron or synaptic bouton loss in learning-impaired aged β-Amyloid precursor protein-null mice

Aged β-amyloid precursor protein-null mice were used to investigate the relationship between β-amyloid precursor protein, hippocampal neuron and synaptic bouton number, and cognitive function. Learning and memory performance of aged β-amyloid precursor protein-null mice and age-matched controls were assessed in the Morris water maze. β-Amyloid precursor protein-null mice demonstrated impaired task acquisition as measured by significantly longer swim path lengths, a higher percentage of failed trials, and more frequent thigmotaxis behavior than controls. In a subsequent probe trial, β-amyloid precursor protein-null mice spent significantly less time in the old goal quadrant, and made fewer crossings over the old platform location than did controls. No differences in motor or visual skills were observed which could account for the performance differences. In light of these findings and previous evidence for a role of β-amyloid precursor protein in neuronal maintenance and synaptogenesis, we pursued the hypothesis that the learning impairment of β-amyloid precursor protein-null mice may be a reflection of differences in neuron or synaptophysin-positive presynaptic bouton number. Thus, unbiased stereological analysis was used to estimate neuron and synaptic bouton number in dentate gyrus and hippocampal CA1 of the behaviorally characterized mice. No difference in neuron or synaptophysin-positive presynaptic bouton number was found between the β-amyloid precursor protein-null mice and age-matched controls.

Our results suggest that the learning impairment of β-amyloid precursor protein-null mice is not mediated by a loss of hippocampal neurons or synaptic boutons.