Age-related changes in barrier function in mouse brain I. Accelerated age-related increase of brain transfer of serum albumin in accelerated senescence prone SAM-P/8 mice with deficits in learning and memory

Blood-brain barrier dysfunction and Alzheimer's disease: associations, pathogenic mechanisms, and therapeutic potential

Alzheimer's disease (AD) is a common neurodegenerative disorder characterized by the accumulation of amyloid-beta (Aβ), hyperphosphorylation of tau, and neuroinflammation in the brain. The blood-brain barrier (BBB) limits solutes from circulating blood from entering the brain, which is essential for neuronal functioning. Focusing on BBB function is important for the early detection of AD and in-depth study of AD pathogenic mechanisms. However, the mechanism of BBB alteration in AD is still unclear, which hinders further research on therapeutics that target the BBB to delay the progression of AD. The exact timing of the vascular abnormalities in AD and the complex cause-and-effect relationships remain uncertain. Thus, it is necessary to summarize and emphasize this process. First, in this review, the current evidence for BBB dysfunction in AD is summarized. Then, the interrelationships and pathogenic mechanisms between BBB dysfunction and the risk factors for AD, such as Aβ, tau, neuroinflammation, apolipoprotein E (ApoE) genotype and aging, were analyzed. Finally, we discuss the current status and future directions of therapeutic AD strategies targeting the BBB. We hope that these summaries or reviews will allow readers to better understand the relationship between the BBB and AD.

Blood-brain barrier leakage in Alzheimer's disease: From discovery to clinical relevance

Alzheimer's disease (AD) is the most common form of dementia. AD brain pathology starts decades before the onset of clinical symptoms. One early pathological hallmark is blood-brain barrier dysfunction characterized by barrier leakage and associated with cognitive decline. In this review, we summarize the existing literature on the extent and clinical relevance of barrier leakage in AD. First, we focus on AD animal models and their susceptibility to barrier leakage based on age and genetic background. Second, we re-examine barrier dysfunction in clinical and postmortem studies, summarize changes that lead to barrier leakage in patients and highlight the clinical relevance of barrier leakage in AD. Third, we summarize signaling mechanisms that link barrier leakage to neurodegeneration and cognitive decline in AD. Finally, we discuss clinical relevance and potential therapeutic strategies and provide future perspectives on investigating barrier leakage in AD. Identifying mechanistic steps underlying barrier leakage has the potential to unravel new targets that can be used to develop novel therapeutic strategies to repair barrier leakage and slow cognitive decline in AD and AD-related dementias.

Age-Related Functional and Expressional Changes in Efflux Pathways at the Blood-Brain Barrier

During the last decade, several articles have reported a relationship between advanced age and changes in the integrity of the blood-brain barrier (BBB). These changes were manifested not only in the morphology and structure of the cerebral microvessels but also in the expression and function of the transporter proteins in the luminal and basolateral surfaces of the capillary endothelial cells. Age-associated downregulation of the efflux pumps ATP-binding cassette transporters (ABC transporters) resulted in increased permeability and greater brain exposure to different xenobiotics and their possible toxicity. In age-related neurodegenerative pathologies like Alzheimer's disease (AD), the amyloid-β (Aβ) clearance decreased due to P-glycoprotein (P-gp) dysfunction, leading to higher brain exposure. In stroke, however, an enhanced P-gp function was reported in the cerebral capillaries, making it even more difficult to perform effective neuroprotective therapy in the infarcted brain area. This mini-review article focuses on the efflux functions of the transporters and receptors of the BBB in age-related brain pathologies and also in healthy aging.

Impact of aging, Alzheimer's disease and Parkinson's disease on the blood-brain barrier transport of therapeutics

Older people are at a greater risk of medicine-induced toxicity resulting from either increased drug sensitivity or age-related pharmacokinetic changes. The scenario is further complicated with the two most prevalent age-related neurodegenerative diseases, Alzheimer's disease (AD) and Parkinson's disease (PD). With aging, AD and PD, there is growing evidence of altered structure and function of the blood-brain barrier (BBB), including modifications to tight junctions and efflux transporters, such as P-glycoprotein. The subsequent impact on CNS drug exposure and risk of neurotoxicity from systemically-acting medicines is less well characterized. The purpose of this review, therefore, is to provide an overview of the multiple changes that occur to the BBB as a result of aging, AD and PD, and the impact that such changes have on CNS exposure of drugs, based on studies conducted in aged rodents or rodent models of disease, and in elderly people with and without AD or PD.

Decline in Sirtuin-1 expression and activity plays a critical role in blood-brain barrier permeability in aging

Accumulating evidence suggest that cerebral microvascular disease increases with advancing age and is associated with lacunar stroke, leukoaraiosis, vascular dementia and Alzheimer disease. Increased blood brain barrier (BBB) permeability/leakage takes "center stage" in ongoing age-related vascular/brain parenchymal injury. Although significant effort has been made in defining the gene mutations and risk factors involved in microvascular alterations in vascular dementia and Alzheimer disease, the intra- and intercellular pathogenic mechanisms responsible for vascular hyperpermeability are still largely unknown. The present study aimed to reveal the ongoing senescence process in brain endothelial cells and its effect on BBB integrity in healthy/non-disease conditions. An analysis of BBB integrity during the life span of C56Bl6 mice (young, 2-6 months; middle-aged, 6-12, months; old, 16-22 months) showed increased BBB permeability for different molecular sized tracers (sodium fluorescein, inulin and 20 kDa dextran) in aged mice which was accompanied by modifications in tight junction (TJ) complex organization, manifested as altered TJ protein expression (particularly claudin-5). A gene screening analysis of aging associated markers in brain microvessels isolated from "aged" mice (C56Bl6, 18-20 months) and human brain samples showed a significant decline in sirtuin-1 expression (Sirt1; ~2.8-fold) confirmed at mRNA and protein levels and by activation assay. Experiments in Sirt1 transgenic mice and brain endothelial cell-specific Sirt1 knockout mice indicated that Sirt1 affects BBB integrity, with loss increasing permeability. Similarly, in vitro, overexpressing Sirt1 or increasing Sirt1 activity with an agonist (Sirt1720) protected against senescence-induced brain endothelial barrier hyperpermeability, stabilized claudin-5/ZO-1 interactions and rescued claudin-5 expression. These findings reveal a novel role of Sirt1 in modulating aging-associated BBB persistent leakage.

SAMP8 mice as a neuropathological model of accelerated brain aging and dementia: Toshio Takeda's legacy and future directions

Senescence accelerated mice P8 (SAMP8) show significant age-related deteriorations in memory and learning ability in accordance with early onset and rapid advancement of senescence. Brains of SAMP8 mice reveal an age-associated increase of PAS-positive granular structures in the hippocampal formation and astrogliosis in the brain stem and hippocampus. A spongy degeneration in the brain stem appears at 1 month of age and reaches a maximum at 4-8 months. In addition, clusters of activated microglia also appear around the vacuoles in the brain stem. β/A4(Aβ) protein-like immunoreactive granular structures are observed in various regions and increase in number markedly with age. Other age-associated histological changes include cortical atrophy, neuronal cell loss in locus coeruleus and lateral tegmental nuclei, intraneuronal accumulation of lipopigments in Purkinje cells and eosinophilic inclusion bodies in thalamic neurons. A blood-brain barrier dysfunction and astrogliosis are also prominent with advancing age in the hippocampus. These changes are generally similar to the pathomorphology of aging human brains and characterized by their association with some specific glioneuronal reactions. As for the hallmarks of Alzheimer brains, tau morphology has not yet been confirmed regardless of the age-related increase in phosphorylated tau in SAMP8 mice brains, but early age-related Aβ deposition in the hippocampus has recently been published. SAMP8 mice are, therefore, not only a senescence-accelerated model but also a promising model for Alzheimer's disease and other cognitive disorders.

Elucidation of mechanism of blood-brain barrier damage for prevention and treatment of vascular dementia

It is well-known that the blood-brain barrier (BBB) plays significant roles in transporting intravascular substances into the brain. The BBB in cerebral capillaries essentially impedes the influx of intravascular compounds from the blood to the brain, while nutritive substances, such as glucose, can be selectively transported through several types of influx transporters in endothelial cells. In the choroid plexus, intravascular substances can invade the parenchyma as fenestrations exist in endothelial cells of capillaries. However, the substances cannot invade the ventricles easily as there are tight junctions between epithelial cells in the choroid plexus. This restricted movement of the substances across the cytoplasm of the epithelial cells constitutes a blood-cerebrospinal fluid barrier (BCSFB). In the brain, there are circumventricular organs, in which the barrier function is imperfect in capillaries. Accordingly, it is reasonable to consider that intravascular substances can move in and around the parenchyma of the organs. Actually, it was reported in mice that intravascular substances moved in the corpus callosum, medial portions of the hippocampus, and periventricular areas via the subfornical organs or the choroid plexus. Regarding pathways of intracerebral interstitial and cerebrospinal fluids to the outside of the brain, two representative drainage pathways, or perivascular drainage and glymphatic pathways, are being established. The first is the pathway in a retrograde direction to the blood flow through the basement membrane in walls of cerebral capillaries, the tunica media of arteries, and the vessels walls of the internal carotid artery. The second is in an anterograde direction to blood flow through the para-arterial routes, aquaporin 4-dependent transport through the astroglial cytoplasm, and para-venous routes, and then the fluids drain into the subarachnoid CSF. These fluids are finally considered to drain into the cervical lymph nodes or veins. These clearance pathways may play a role in maintenance of the barrier in the entire brain. Obstruction of the passage of fluids through the perivascular drainage and glymphatic pathways as well as damage of the BBB and BCSFB may induce several kinds of brain disorders, such as vascular dementia. In this review, we focus on the relationship between damage of the barriers and the pathogenesis of vascular dementia and introduce recent findings including our experimental data using animal models.

Blood-brain barrier and blood-cerebrospinal fluid barrier in normal and pathological conditions

Blood-borne substances can invade into the extracellular spaces of the brain via endothelial cells in sites without the blood-brain barrier (BBB), and can travel through the interstitial fluid (ISF) of the brain parenchyma adjacent to non-BBB sites. It has been shown that cerebrospinal fluid (CSF) drains directly into the blood via the arachnoid villi and also into lymph nodes via the subarachnoid spaces of the brain, while ISF drains into the cervical lymph nodes through perivascular drainage pathways. In addition, the glymphatic pathway of fluids, characterized by para-arterial pathways, aquaporin4-dependent passage through astroglial cytoplasm, interstitial spaces, and paravenous routes, has been established. Meningeal lymphatic vessels along the superior sagittal sinus were very recently discovered. It is known that, in mice, blood-borne substances can be transferred to areas with intact BBB function, such as the medial regions of the hippocampus, presumably through leaky vessels in non-BBB sites. In the present paper, we review the clearance mechanisms of interstitial substances, such as amyloid-β peptides, as well as summarize models of BBB deterioration in response to different types of insults, including acute ischemia followed by reperfusion, hypertension, and chronic hypoperfusion. Lastly, we discuss the relationship between perivascular clearance and brain disorders.

Astrocytic response to cerebral ischemia is influenced by sex differences and impaired by aging

Ischemic stroke occurs more often among the elderly, and within this demographic, women are at an increased risk for stroke and have poorer functional recovery than men. This is also well replicated in animal studies where aging females are shown to have more extensive brain tissue loss as compared to adult females. Astrocytes provide nutrients for neurons, regulate glutamate levels, and release neurotrophins and thus play a key role in the events that occur following ischemia. In addition, astrocytes express receptors for gonadal hormones and synthesize several neurosteroids suggesting that the sex differences in stroke outcome may be mediated through astrocytes. This review discusses key astrocytic responses to ischemia including, reactive gliosis, excitotoxicity, and neuroinflammation. In light of the age and sex differences in stroke outcomes, this review highlights how aging and gonadal hormones influence these responses. Lastly, astrocyte specific changes in gene expression and epigenetic modifications during aging and following ischemia are discussed as possible molecular mechanisms for impaired astrocytic functioning.

Cerebral amyloid angiopathy, blood-brain barrier disruption and amyloid accumulation in SAMP8 mice

Cerebrovascular dysfunction and β-amyloid peptide deposition on the walls of cerebral blood vessels might be an early event in the development of Alzheimer's disease. Here we studied the time course of amyloid deposition in blood vessels and blood-brain barrier (BBB) disruption in the CA1 subzone of the hippocampus of SAMP8 mice and the association between these two variables. We also studied the association between the amyloid deposition in blood vessels and the recently described amyloid clusters in the parenchyma, as well as the association of these clusters with vessels in which the BBB is disrupted. SAMP8 mice showed greater amyloid deposition in blood vessels than age-matched ICR-CD1 control mice. Moreover, at 12 months of age the number of vessels with a disrupted BBB had increased in both strains, especially SAMP8 animals. At this age, all the vessels with amyloid deposition showed BBB disruption, but several capillaries with an altered BBB showed no amyloid on their walls. Moreover, amyloid clusters showed no spatial association with vessels with amyloid deposition, nor with vessels in which the BBB had been disrupted. Finally, we can conclude that vascular amyloid deposition seems to induce BBB alterations, but BBB disruption may also be due to other factors.

Ginkgo biloba extract EGb761 protects against aging-associated mitochondrial dysfunction in platelets and hippocampi of SAMP8 mice

Standardized Ginkgo biloba extract, EGb761, has been shown to possess polyvalent properties, such as anti-oxidation, anti-apoptosis and anti-inflammation. Recently, it has also been proposed to have direct protective effects on mitochondria. The effects of EGb761 make it a potential anti-aging drug. Despite that, the 'anti-aging' effect of EGb761, particularly its effect on the central nervous system, is still inconclusive. Using two age groups (3-week-old and 40-week-old) of SAMP8 mice (a senescence-accelerated strain of mice), the effects of EGb761 on mitochondrial function in platelets and hippocampi were investigated in this study. It was found that mitochondrial functions, evaluated as cytochrome c oxidase (COX) activity, mitochondrial ATP (adenosine-5'-triphosphate) content and mitochondrial glutathione (GSH) content, decreased with age. EGb761 protected against mitochondrial dysfunction in platelets of young and old mice, suggesting a peripheral effect of this herb in the prevention and treatment of age-associated degeneration. In contrast, in hippocampi, protective effects of EGb761 were observed only in the old mice, probably due to an age-associated increase in the permeability of the blood brain barrier (BBB). Therefore, while EGb761 has a potential anti-aging effect, its central effect can be affected by in vivo factors such as the BBB permeability. A better understanding of the in vivo pharmacological actions of EGb761 may contribute to a better understanding of the effectiveness and complexity of this drug.

Transport of drugs across the blood–brain barrier in Alzheimer’s disease

Alzheimer’s disease, a neurodegenerative disorder, is associated with various pathological alterations to the blood–brain barrier, including disruption to the inter-endothelial tight junction proteins, altered expression of transport proteins involved in drug efflux, a reduction in cerebral blood flow and a thickening of the brain capillary basement membrane. There are many conflicting reports on whether such changes alter the ability of endogenous proteins to extravasate into the brain parenchyma, and there are even fewer reports focusing on the potential impact of these changes on drug transport into the CNS. The purpose of this review is to critically evaluate how the reported changes to the blood–brain barrier in Alzheimer’s disease have (or have not) resulted in altered CNS drug delivery, and to highlight the requirement for more rigorous and systematic studies in this field for the benefit of drug discovery and delivery scientists.

Senescence accelerated mouse strain is sensitive to neurodegeneration induced by mild impairment of oxidative metabolism

Neuronal loss and impairment of oxidative metabolism are frequently observed in aging associated neurodegenerative diseases. Thiamine deficiency (TD) induces the region selective neuronal loss in the brain, which has been used to model neurodegeneration, accompanied by mild impairment of oxidative metabolism. C57BL/6 mice were commonly used animals for TD experiments; however, the individual variations among C57BL/6 mice in response to TD limited the consistence of brain pathology. The senescence accelerated prone 8 (SAMP8) mouse strain exhibits age-related morphological changes in the brain and deficits in learning and memory. In this study, we compared the effects of TD on SAMP8 mice, senescence accelerated resistant 1 (SAMR1) mice and C57BL/6 mice. TD-induced body weight loss in SAMP8 mice was much greater than in SAMR1 and C57BL/6 mice. In addition, earlier and more severe loss of neurons in the submedial thalamic nucleus (SmTN) of the thalamus was detected in the SAMP8 mice. After 8 days of TD (TD8), the loss of NeuN-positive neurons in the SmTN of SAMP8, SAMR1 and C57BL/6 mice was 65%, 50%, and 36%, respectively. TD also caused accumulation of amyloid precursor protein (APP) in the thalamus. After TD10, APP immunoreactivity in the thalamus of SAMP8 was much more intense than that of SAMR1 and C57BL/6 mice. These results suggest that SAMP8 mice are sensitive to TD and therefore offer a useful model for studying aging related neurodegeneration caused by the impairment of oxidative metabolism.

Senescence-accelerated mouse (SAM) with special references to neurodegeneration models, SAMP8 and SAMP10 mice

The SAM strains, a group of related inbred strains consisting of senescence-prone inbred strains (SAMP) and senescence-resistant inbred strains (SAMR), have been successfully developed by selective inbreeding of the AKR/J strain of mice donated by the Jackson laboratory in 1968. The characteristic feature of aging common to the SAMP and SAMR is accelerated senescence and normal aging, respectively. Furthermore, SAMP and SAMR strains of mice manifest various pathobiological phenotypes spontaneously. Among SAMP strains, SAMP8 and SAMP10 mice show age-related behavioral deterioration such as deficits in learning and memory, emotional disorders (reduced anxiety-like behavior and depressive behavior) and altered circadian rhythm associated with certain pathological, biochemical and pharmacological changes. Here, the previous and recent literature on SAM mice are reviewed with an emphasis on SAMP8 and SAMP10 mice. A spontaneous model like SAM with distinct advantages over the gene-modified model is hoped by investigators to be used more widely as a biogerontological resource to explore the etiopathogenesis of accelerated senescence and neurodegenerative disorders.

Time-course of blood-brain barrier disruption in senescence-accelerated mouse prone 8 (SAMP8) mice

Senescence of the cerebrovascular system and an abnormal function of the blood-brain barrier have been related with Alzheimer's disease. We studied here the time-course of blood-brain barrier disruption in senescence-accelerated mouse prone 8 (SAMP8) mice, which is a murine model of senescence and is also considered a model of Alzheimer's disease. We used a previously described method that allows evaluating blood-brain barrier integrity by observing Evans blue extravasation from brain blood vessels. Three brain regions (cortex, hippocampus and hippocampal fissure) of SAMP8 brains were analyzed at 3, 6, 9, 12 and 15 months of age. Moreover, genetically related senescence-accelerated mouse resistant 1 (SAMR1) and ICR-CD1 mice were studied. Results indicate that Evans blue permeability in SAMP8 and SAMR1 increases from 6 to 15 months in the three studied regions. At 15 months of age, SAMP8 and SAMR1 mice showed higher Evans blue extravasation in CA1 and Fissure than ICR-CD1 mice. Further studies are required to understand the senescence process in SAMR1 mice, as blood-brain barrier alterations in old age have unexpectedly been observed. On the other hand, as blood-brain barrier permeability in SAMP8 mice increases with age, blood-brain barrier alterations may contribute to the cerebral pathology observed in this strain.

Increased permeability of blood-brain barrier on the hippocampus of a murine model of senescence

SAMP8 mice show several indicative characteristics of accelerated aging and have been used to study the physiological and physiopathological processes that take place during senescence. There is some controversy about the presence of a functional blood-brain barrier (BBB) disturbance on these animals, which could be related to the oxidative stress or the amyloidosis present in their brain. In order to elucidate BBB status in the hippocampus of SAMP8 mice, in this study we have determined the extravasation from brain microvessels of endogenous IgG in SAMP8 mice aged 3, 7 and 12 months and in age-matched control SAMR1 mice. Immunohistochemistry, confocal microscopy and an imaging methodology specially designed to quantify IgG extravasation have been used. The choroid plexus was analyzed as a control for positive extravasation in SAMP8 and SAMR1 mice and, as expected, in all studied ages high IgG immunoreactivity was observed in both strains. We have found significantly higher levels of IgG extravasation in the hippocampus of 12-month-old SAMP8 mice compared to SAMR1 mice, indicating an increased permeability of BBB in aged senescence-accelerated mice.

Ultrastructural and permeability features of microvessels in the hippocampus, cerebellum and pons of senescence-accelerated mice (SAM)

We previously reported that the accumulation of blood-borne radiolabelled serum albumin in brain parenchyma increased with aging, especially in senescence-accelerated mice (SAMP8), which showed age-related deficits in learning and memory. In this study, in order to examine morphological events related to the age-related increase of the brain accumulation of serum albumin, the transvascular passage of blood-borne horseradish peroxidase (HRP) and ultrastructural features of microvessels were examined in the hippocampus, cerebellum and pons of SAMP8 and SAMR1 (control) mice. Ultrastructural examination of the hippocampus showed that the staining for HRP was occasionally spreading throughout the parajunctional cytoplasm of the endothelial cell of aged SAMP8 mice, but not in young SAMP8 mice nor in SAMR1 mice. The number of vessels showing the staining reaction for HRP in the parajunctional cytoplasm of the endothelial cells in aged SAMP8 mice increased significantly compared with that in the others. Electron microscopic morphometry showed that there were no significant differences among the number of HRP-positive vesicles per unit area of the endothelial cell cytoplasm in young and old mice of both strains. The staining reaction for HRP was not seen in the basal lamina of microvessels and the perivascular neuropil in all mice examined. Perivascular lipofuscin-like granules and collagen deposits, swelling of astroglial perivascular endfeet and perivascular cells containing foamy, lipid-like droplets were frequently found in several brain regions of aged SAMP8 mice. The perivascular cells with a few lipid-like droplets and more electron-homogeneous lysosomes were occasionally seen in SAMR1 and young SAMP8, while the other findings were scarcely observed in SAMR1 and young SAMP8 mice. These findings suggest that the blood-brain barrier to HRP was preserved in microvessels in three brain regions of SAM mice but the blood microvessels showed some age-related ultrastructural alterations in SAMP8 brains. Uncontrolled passage of HRP through the parajunctional cytoplasm of the endothelial cells may partly contribute to the age-related increase of accumulation of serum albumin in SAMP8 brains.

Ultrastructural and permeability features of microvessels in the periventricular area of senescence-accelerated mice (SAM)

Brain transfer of intravenously injected horseradish peroxidase (HRP) and the ultrastructural features of the vessels were examined in periventricular areas in senescence-accelerated mice (SAMP8), which show age-related deficits in learning and memory, and senescence-accelerated resistant mice (SAMR1), which do not show age-related deficits. In all mice examined with light microscopy, staining reaction for HRP was seen in the periventricular area adjacent to the medial side of the lateral ventricle. Electron microscopic examination in the periventricular area of young and old mice of both strains showed that the staining reaction for HRP appeared in the vesicular profiles of the endothelial cytoplasm, the cytoplasm of the perivascular cells, the basal lamina, and the adjoining extracellular spaces of the white matter, suggesting an incomplete blood-brain barrier (BBB) in the periventricular white matter. In addition, irregularly thickened endothelial cell cytoplasm, membranous inclusions within the basal lamina, and electron-dense endothelial cell cytoplasm were occasionally seen in aged SAMP8 mice. These findings were not observed in 3-month-old SAMP8 mice and 3- and 13-month-old SAMR1 mice. Perivascular collagen deposits were also frequently seen in aged SAMP8 mice. These findings indicate that the endothelial cells and pericytes in the periventricular white matter in aged SAMP8 mice have an ultrastructure with damaged BBB function. Intravascular substances can easily penetrate the periventricular white matter and the BBB of the vessels in the area can be deteriorated with aging in SAMP8 mice.

Aging of blood-brain barrier and neuronal cells of eye and ear in SAM mice

The SAMP, Senescence-Accelerated Mouse strains show senescence acceleration and age-associated pathological phenotypes similar to geriatric disorders seen in humans. Among these strains, SAMP8 mice show age-associated deficits in learning and memory. Histopathological studies revealed various neurodegenerative changes in the brain, including age-associated appearance of spongiform degeneration in the brain stem and of PAS-positive granular structures in the hippocampus. The blood-brain barrier (BBB) function of SAMP8 mice was also impaired with advancing age. The compromised BBB function in the olfactory bulb, the hippocampus and the pons of SAMP8 mice coincided with and might have been the cause of some morphological changes. Age-associated degeneration of receptor cells and ganglion neurons in the retina and cochlea also occurred in the SAM mice. Oxidative stress partly caused by mitochondrial dysfunction was detected and may be a cause of the neuronal cell degeneration. The SAM strains are useful tool in the attempt to understand the mechanisms of age-dependent neurodegeneration and to develop clinical interventions.

Age-related changes in the brains of senescence-accelerated mice (SAM): association with glial and endothelial reactions

Twelve substrains of inbred senescence-accelerated mice (SAM) have been developed, among which the SAMP8 and SAMP10 strains show a significant age-related deterioration in learning and memory for passive and active avoidance tasks. These strains have, respectively, a low and high incidence of systemic senile amyloidosis. Although we found no amyloid deposits in their brain parenchyma, a variety of age-related alterations were identified, involving neurons, glia, and vessels in the brain tissues. Here we review the degenerative changes in aged SAMP8 and SAMP10 brains. These changes are generally similar to the pathology of aging human brain and may be characterized by their association with some specific glial reactions.

Learning and memory in the SAMP8 mouse

The SAMP8 (P8) mouse strain develops deficits in learning and memory relatively early in its lifespan. This review provides an overview of the age-related changes that occur in P8 mice. Behavioral studies with P8 mice show impaired acquisition and retention as early as 4 months of age. Deficits in acquisition and retention occur with both aversive and appetitive training tasks. Anatomical studies have detected a number of age-related changes that occur in the central nervous system of P8 mice. The age-related increase in amyloid beta protein is well correlated with the age-related decline in learning and memory. Antibody to amyloid beta protein injected prior to training alleviated impaired acquisition and retention, whereas post-training injections alleviated retention deficits in older P8 mice. Biochemical studies have detected numerous age-related changes with reduced NMDA receptor activity most closely related to impaired learning and memory in P8 mice. Pharmacological studies have found age-related functional changes in the ability of drugs to improve memory processing in P8 mice in the septum and the hippocampus. The specific pattern of pharmacological changes and the inferred change in neurotransmitter activity suggest that age-related impairment in memory processing may be due to impaired septohippocampal interactions. The proposal that P8 mice may be a useful model for studying the early phases of age-related dementia of the Alzheimer type, while still requiring considerable study, seems reasonable.

Immunocytochemical evaluation of the blood-brain barrier to endogenous albumin in the olfactory bulb and pons of senescence-accelerated mice (SAM)

The blood-brain barrier (BBB) to endogenous albumin was studied in the olfactory bulb and pons of the senescence-accelerated prone (SAMP8) mouse and senescence-accelerated resistant (SAMR1) mouse strains by using a quantitative immunocytochemical procedure. Ultrathin sections of Lowicryl K4M-embedded samples were exposed to anti-mouse albumin antiserum followed by protein A-gold. Morphometric analysis of the electron micrographs revealed that in the olfactory bulb of both groups of animals, especially in the internal granular layer, some percentage of capillaries and slightly larger microvessels showed leakage of albumin. However, this percentage was larger in SAMP8 than in SAMR1 mice. In the pons, no significant differences in the permeability of blood microvessels were observed in both groups of mice, although a small fraction of capillaries in SAMP8 mice showed limited extravasation of blood plasma albumin. These observations indicate that the BBB in the olfactory bulb of control and SAMP8 mice is not as tight as it is in the pons or in the previously examined cerebral cortex. The labelling density of the neuropil was slightly higher than in the cerebral cortex, suggesting that albumin may have extravasated locally, in addition to having acces to the parenchyma of the olfactory bulb and pons from neighbouring areas supplied with the non-BBB-type of microvasculature. Furthermore, the data obtained suggest that there is limited (segmental), premature age-related impairment of the BBB function in SAMP8 mice.

Accumulation of blood-borne horseradish peroxidase in medial portions of the mouse hippocampus

The intracerebral distribution of intravenously injected horseradish peroxidase (HRP) in young adult DDD mice was examined. HRP-tetramethylbenzidine reaction products were observed in the medial portions of the hippocampus, particularly the medial CA1 region and medial dentate gyrus. Reaction products were observed in the subfornical organ in mice decapitated 5 min after HRP injection, and then also progressively more caudally in the medial portions of the hippocampus as postinjection survival time increased. These findings suggest that blood-borne macromolecules have ready access to the medial portions of the hippocampus, particularly the medial CA1 region and medial dentate gyrus.