Aged but not young rats develop metabolic, memory deficits after chronic brain ischaemia

Post-acute delivery of α5-GABAA antagonist, S 44819, improves functional recovery in juvenile rats following stroke

Hypo-excitability was reported in the peri-infarct tissue following stroke, an effect counteracted by a blockage of α5-GABAA receptors in adult rodents. Our present study aims to evaluate the effect of a selective α5-GABAA receptor antagonist, S 44819, in stroke in juvenile animals. We have set up and characterized an original model of transient ischemic stroke in 28 day-old Sprague-Dawley rats (45-min occlusion of the middle cerebral artery by intraluminal suture). In this model, S 44819 (1, 3 and 10 mg/kg, b.i.d) was orally administered from day 3 to day 16 after stroke onset. Sensorimotor recovery was assessed on day 1, day 9 and day 16 after stroke onset. Results show that rats treated with S 44819 at the doses of 3 and 10 mg/kg displayed a significant improvement of the neurological deficits (neuroscore) on day 9 and day 16, when compared with animals treated with vehicle. Grip-test data analysis reveals that rats treated with S 44819 at the dose of 3 mg/kg displayed a better recovery on day 9 and day 16. These results are in agreement with those previously observed in adult rats, demonstrating that targeting α5-GABAA receptors improves neurological recovery after stroke in juvenile rats.

Deciphering Alzheimer's Disease Pathogenic Pathway: Role of Chronic Brain Hypoperfusion on p-Tau and mTOR

This review examines new biomolecular findings that lend support to the hemodynamic role played by chronic brain hypoperfusion (CBH) in driving a pathway to Alzheimer's disease (AD). CBH is a common clinical feature of AD and the current topic of intense investigation in AD models. CBH is also the basis for the vascular hypothesis of AD which we originally proposed in 1993. New biomolecular findings reveal the interplay of CBH in increasing tau phosphorylation (p-Tau) in the hippocampus and cortex of AD mice, damaging fast axonal transport, increasing signaling of mammalian target of rapamycin (mTOR), impairing learning-memory function, and promoting the formation of neurofibrillary tangles, a neuropathologic hallmark of AD. These pathologic elements have been singularly linked with neurodegeneration and AD but their abnormal, collective participation during brain aging have not been fully examined. The format for this review will provide a consolidated analysis of each pathologic phase contributing to cognitive decline and AD onset, summarized in nine chronological steps. These steps galvanize each factor's active participation and contribution in constructing a biomolecular pathway to AD onset generated by CBH.

The Vascular Hypothesis of Alzheimer's Disease: A Key to Preclinical Prediction of Dementia Using Neuroimaging

The vascular hypothesis of Alzheimer's disease (VHAD) was proposed 24 years ago from observations made in our laboratory using aging rats subjected to chronic brain hypoperfusion. In recent years, VHAD has become a mother-lode to numerous neuroimaging studies targeting cerebral hemodynamic changes, particularly brain hypoperfusion in elderly patients at risk of developing Alzheimer's disease (AD). There is a growing consensus among neuroradiologists that brain hypoperfusion is likely involved in the pathogenesis of AD and that disturbed cerebral blood flow (CBF) can serve as a key biomarker for predicting conversion of mild cognitive impairment to AD. The use of cerebral hypoperfusion as a preclinical predictor of AD is becoming decisive in stratifying low and high risk patients that may develop cognitive decline and for assessing the effectiveness of therapeutic interventions. There is currently an international research drive from neuroimaging groups to seek new perspectives that can broaden our understanding of AD and improve lifestyle. Diverse neuroimaging methods are currently being used to monitor normal and dyscognitive brain activity. Some techniques are very powerful and can detect, diagnose, quantify, prognose, and predict cognitive decline before AD onset, even from a healthy cognitive state. Multimodal imaging offers new insights in the treatment and prevention of cognitive decline during advanced aging and better understanding of the functional and structural organization of the human brain. This review discusses the impact the VHAD and CBF are having on the neuroimaging technology that can usher practical strategies to help prevent AD.

Treating cognitive impairment with transcranial low level laser therapy

This report examines the potential of low level laser therapy (LLLT) to alter brain cell function and neurometabolic pathways using red or near infrared (NIR) wavelengths transcranially for the prevention and treatment of cognitive impairment. Although laser therapy on human tissue has been used for a number of medical conditions since the late 1960s, it is only recently that several clinical studies have shown its value in raising neurometabolic energy levels that can improve cerebral hemodynamics and cognitive abilities in humans. The rationale for this approach, as indicated in this report, is supported by growing evidence that neurodegenerative damage and cognitive impairment during advanced aging is accelerated or triggered by a neuronal energy crisis generated by brain hypoperfusion. We have previously proposed that chronic brain hypoperfusion in the elderly can worsen in the presence of one or more vascular risk factors, including hypertension, cardiac disease, atherosclerosis and diabetes type 2. Although many unanswered questions remain, boosting neurometabolic activity through non-invasive transcranial laser biostimulation of neuronal mitochondria may be a valuable tool in preventing or delaying age-related cognitive decline that can lead to dementia, including its two major subtypes, Alzheimer's and vascular dementia. The technology to achieve significant improvement of cognitive dysfunction using LLLT or variations of this technique is moving fast and may signal a new chapter in the treatment and prevention of neurocognitive disorders.

Vascular dysfunction associated with type 2 diabetes and Alzheimer's disease: a potential etiological linkage

The endothelium performs a crucial role in maintaining vascular integrity leading to whole organ metabolic homeostasis. Endothelial dysfunction represents a key etiological factor leading to moderate to severe vasculopathies observed in both Type 2 diabetic and Alzheimer’s Disease (AD) patients. Accordingly, evidence-based epidemiological factors support a compelling hypothesis stating that metabolic rundown encountered in Type 2 diabetes engenders severe cerebral vascular insufficiencies that are causally linked to long term neural degenerative processes in AD. Of mechanistic importance, Type 2 diabetes engenders an immunologically mediated chronic pro-inflammatory state involving interactive deleterious effects of leukocyte-derived cytokines and endothelial-derived chemotactic agents leading to vascular and whole organ dysfunction. The long term negative consequences of vascular pro-inflammatory processes on the integrity of CNS basal forebrain neuronal populations mediating complex cognitive functions establish a striking temporal comorbidity of AD with Type 2 diabetes. Extensive biomedical evidence supports the pivotal multi-functional role of constitutive nitric oxide (NO) production and release as a critical vasodilatory, anti-inflammatory, and anti-oxidant, mechanism within the vascular endothelium. Within this context, we currently review the functional contributions of dysregulated endothelial NO expression to the etiology and persistence of Type 2 diabetes-related and co morbid AD-related vasculopathies. Additionally, we provide up-to-date perspectives on critical areas of AD research with special reference to common NO-related etiological factors linking Type 2 diabetes to the pathogenesis of AD.

Vascular risk factors: a ticking time bomb to Alzheimer's disease

Evidence is growing that vascular risk factors (VRFs) for Alzheimer's disease (AD) affect cerebral hemodynamics to launch a cascade of cellular and molecular changes that initiate cognitive deficits and eventual progression of AD. Neuroimaging studies have reported VRFs for AD to be accurate predictors of cognitive decline and dementia. In regions that participate in higher cognitive function, middle temporal, posterior cingulate, inferior parietal and precuneus regions, and neuroimaging studies indicate an association involving VRFs, cerebral hypoperfusion, and cognitive decline in elderly individuals who develop AD. The VRF can be present in cognitively intact individuals for decades before mild cognitive deficits or neuropathological signs are manifested. In that sense, they may be "ticking time bombs" before cognitive function is demolished. Preventive intervention of modifiable VRF may delay or block progression of AD. Intervention could target cerebral blood flow (CBF), since most VRFs act to lower CBF in aging individuals by promoting cerebrovascular dysfunction.

Effects of hypoperfusion in Alzheimer's disease

The role of hypoperfusion in Alzheimer's disease (AD) is a vital component to understanding the pathogenesis of this disease. Disrupted perfusion is not only evident throughout disease manifestation, it is also demonstrated during the pre-clinical phase of AD (i.e., mild cognitive impairment) as well as in cognitively healthy persons at high-risk for developing AD due to family history or genetic factors. Studies have used a variety of imaging modalities (e.g., SPECT, MRI, PET) to investigate AD, but with its recent technological advancements and non-invasive use of blood water as an endogenous tracer, arterial spin labeling (ASL) MRI has become an imaging technique of growing popularity. Through numerous ASL studies, it is now known that AD is associated with both global and regional cerebral hypoperfusion and that there is considerable overlap between the regions implicated in the disease state (consistently reported in precuneus/posterior cingulate and lateral parietal cortex) and those implicated in disease risk. Debate exists as to whether decreased blood flow in AD is a cause or consequence of the disease. Nonetheless, hypoperfusion in AD is associated with both structural and functional changes in the brain and offers a promising putative biomarker that could potentially identify AD in its pre-clinical state and be used to explore treatments to prevent, or at least slow, the progression of the disease. Finally, given that perfusion is a vascular phenomenon, we provide insights from a vascular lesion model (i.e., stroke) and illustrate the influence of disrupted perfusion on brain structure and function and, ultimately, cognition in AD.

The GRK2 Overexpression Is a Primary Hallmark of Mitochondrial Lesions during Early Alzheimer Disease

Increasing evidence points to vascular damage as an early contributor to the development of two leading causes of age-associated dementia, namely Alzheimer disease (AD) and AD-like pathology such as stroke. This review focuses on the role of G protein-coupled receptor kinases (GRKs) as they relate to dementia and how the cardio and cerebrovasculature is involved in AD pathogenesis. The exploration of GRKs in AD pathogenesis may help bridge gaps in our understanding of the heart-brain connection in relation to neurovisceral damage and vascular complications of AD. The a priori basis for this inquiry stems from the fact that kinases of this family regulate numerous receptor functions in the brain, myocardium and elsewhere. The aim of this review is to discuss the finding of GRK2 overexpression in the context of early AD pathogenesis. Also, we consider the consequences for this overexpression as a loss of G-protein coupled receptor (GPCR) regulation, as well as suggest a potential role for GPCRs and GRKs in a unifying theory of AD pathogenesis through the cerebrovasculature. Finally, we synthesize this newer information in an attempt to put it into context with GRKs as regulators of cellular function, which makes these proteins potential diagnostic and therapeutic targets for future pharmacological intervention.

Insights into cerebrovascular complications and Alzheimer disease through the selective loss of GRK2 regulation

Alzheimer disease (AD) and stroke are two leading causes of age-associated dementia. Increasing evidence points to vascular damage as an early contributor to the development of AD and AD-like pathology. In this review, we discuss the role of G protein-coupled receptor kinase 2 (GRK2) as it relates to individuals affected by AD and how the cardiovasculature plays a role in AD pathogenesis. The possible involvement of GRKs in AD pathogenesis is an interesting notion, which may help bridge the gap in our understanding of the heart–brain connection in relation to neurovisceral damage and vascular complications in AD, since kinases of this family are known to regulate numerous receptor functions both in the brain, myocardium, and elsewhere. The aim of this review is to discuss our findings of overexpression of GRK2 in the context of the early pathogenesis of AD, because increased levels of GRK2 immunoreactivity were found in vulnerable neurons of AD patients as well as in a two-vessel occlusion (2-VO) mammalian model of ischaemia. Also, we consider the consequences for this overexpression as a loss of G-protein coupled receptor (GPCR) regulation, as well as suggest a potential role for GPCRs and GRKs in a unifying theory of AD pathogenesis, particularly in the context of cerebrovascular disease. We synthesize this newer information and attempt to put it into context with GRKs as regulators of diverse physiological cellular functions that could be appropriate targets for future pharmacological intervention.

Chronic ischemia and neurocognition

Cognitive impairment from a major stroke as a consequence of carotid disease is an acknowledged clinical outcome; however, cognitive impairment without major stroke is open to discussion. The three recognized mechanisms for cognitive dysfunction from internal carotid artery are microembolization, white-matter disease, and hypoperfusion. The last has been most difficult to characterize physiologically. In this article, the authors review evidence supporting the existence of chronic ischemia in the brain and its direct impact on cognitive functions. By incorporating the pathophysiology of chronic ischemia into the algorithm of the management of carotid artery disease, we may be able to extend the goals of carotid artery revascularization beyond merely preventing stroke to include preventing or reversing cognitive decline.

Overexpression of GRK2 in Alzheimer disease and in a chronic hypoperfusion rat model is an early marker of brain mitochondrial lesions

Heterotrimeric guanine nucleotide-binding (G) protein-coupled receptor kinases (GRKs) are cytosolic proteins that are known to contribute to the adaptation of the heptahelical G protein-coupled receptors (GPCRs) and to regulate downstream signals through these receptors. GPCRs mediate the action of messengers that are key modulators of cardiac and vascular cell function, such as growth and differentiation. GRKs are members of a multigene family, which are classified into three subfamilies and are found in cardiac, vascular and cerebral tissues. Increasing evidence strongly supports the hypothesis that vascular damage is an early contributor to the development of Alzheimer disease (AD) and/or other pathology that can mimic human AD. Based on this hypothesis, and since kinases of this family are known to regulate numerous receptor functions both in the brain, myocardium and elsewhere, we explored cellular and subcellular localization by immunoreactivity of G protein-coupled receptor kinase 2 (GRK2), also known as beta-adrenergic receptor kinase-1(betaARK1), in the early pathogenesis of AD and in ischemia reperfusion injury models of brain hypoperfusion. In the present study, we used the two-vessel carotid artery occlusion model, namely the 2-VO system that results in chronic brain hypoperfusion (CBH) and mimics mild cognitive impairment (MCI) and vascular changes in AD pathology. Our findings demonstrate the early overexpression of GRK2 member kinase in the cerebrovasculature, especially endothelial cells (EC) following CBH, as well as in select cells from human AD tissue. We found a significant increase in GRK2 immunoreactivity in the EC of AD patients and after CBH, which preceded any amyloid deposition. Since GRK2 activity is associated with certain compensatory changes in brain cellular compartments and in ischemic cardiac tissue, our findings suggest that chronic hypoperfusion initiates oxidative stress in these conditions and appears to be the main initiating injury stimulus for disruption of brain and cerebrovascular homeostasis and metabolism.

How do heart disease and stroke become risk factors for Alzheimer's disease?

BACKGROUND

Heart disease and stroke are two of the major leading causes of death and disability in the world. Mainly affecting the elderly population, heart disease and stroke are important risk factors for Alzheimer's disease (AD).

METHODS

This review examines the evidence linking chronic brain hypoperfusion (CBH) produced by several types of heart disease and stroke on the development of AD.

RESULTS

The evidence indicates a strong association between such risk factors as coronary artery bypass surgery (CABG), atrial fibrillation, aortic/mitral valve damage, hypertension, hypotension, congestive heart failure, cerebrovascular-carotid atherosclerosis, and transient ischemic attacks in producing CBH. In people whose cerebral perfusion is already diminished by their advanced age, further cerebral blood flow reductions from heart-brain vascular-related risk factors, seemingly increases the probability of AD. The evidence also suggests that a neuronal energy crisis brought on by a relentless CBH is responsible for protein synthesis defects that later result in the classic AD neurodegenerative lesions such as the formation of excess beta-amyloid plaques and neurofibrillary tangles.

CONCLUSIONS

Knowledge of how heart disease and stroke can progress to AD should provide a better understanding of the physiopathology characteristic of AD and also target more precise therapy in preventing, controlling or reversing this dementia.

Age-dependent MRI-detected lesions at early stages of transient global ischemia in rat brain

Although ischemic stroke has higher incidence and severity in aged than in young humans, the age factor is generally neglected in ischemia animal models. This study was aimed at comparing age-dependent effects at early stages of transient global cerebral ischemia (TGCI) in rats. TGCI was induced in two groups of rats (3-6 and 20-24 months old, respectively) by exposure to 15% oxygen and 15 min occlusion of the two common carotid arteries. Brains were analysed in vivo by MRI-apparent diffusion coefficient (ADC) and T2 maps--at 1-3 h post-TGCI and in vitro by histochemical examination of triphenyltetrazolium chloride (TTC)-stained slices. At 1-3 h post-TGCI, a higher incidence of lesions was found in aged than in young rats especially in the hippocampus and cortex (occipital plus parietal) but not in the thalamus. The lesioned regions showed lower ADC values in aged than in younger rats. The most substantial ADC decreases were associated with enhanced spin-spin relaxation and lower TTC staining. The different responses of the two age groups support the use of aged animals for investigations on different ischemia models. Our model of brain ischemia appears appropriate for further studies including drug effects.

Evidence that Alzheimer's disease is a microvascular disorder: the role of constitutive nitric oxide

Evidence is fast accumulating which indicates that Alzheimer's disease is a vascular disorder with neurodegenerative consequences rather than a neurodegenerative disorder with vascular consequences. It is proposed that two factors need to be present for AD to develop: (1) advanced ageing, (2) presence of a condition that lowers cerebral perfusion, such as a vascular-risk factor. The first factor introduces a normal but potentially insidious process that lowers cerebral blood flow in inverse relation to increased ageing; the second factor adds a crucial burden which further lowers brain perfusion and places vulnerable neurons in a state of high energy compromise leading to a cascade of neuronal metabolic turmoil. Convergence of the two factors above will culminate in a critically attained threshold of cerebral hypoperfusion (CATCH). CATCH is a hemodynamic microcirculatory insufficiency that will destabilize neurons, synapses, neurotransmission and cognitive function, creating in its wake a neurodegenerative state characterized by the formation of senile plaques, neurofibrillary tangles, amyloid angiopathy and in some cases, Lewy bodies. Since any of a considerable number of vascular-related conditions must be present in the ageing individual for cognition to be disturbed, CATCH identifies an important aspect of the heterogeneic disease profile assumed to be present in the AD syndrome. It is proposed that CATCH initiates AD by distorting regional brain capillary structure involving endothelial cell shape changes and impairment of nitric oxide (NO) release which affect signaling between the immune, cardiovascular and nervous systems. Evidence is presented that in many tissues there is a basal level of NO being produced and that the actions of several signaling molecules may initiate increases in basal NO levels. Moreover, these temporary increases in basal NO levels exert inhibitory cellular actions, via cellular conformational changes. Findings indicate that (a) constitutive NO is responsible for a basal or 'tonal' level of NO; (b) this NO keeps particular types of cells in a state of inhibition and (c) activation of these cells occurs through disinhibition. Consequently, tissues not maintaining a basal NO level are more prone to excitatory, immune, vascular and neural influences. Under such circumstances, these tissues cannot be down-regulated to normal basal levels, thus prolonging their excitatory state. Thus, the clinical convergence of advanced ageing in the presence of a chronic, pre-morbid vascular risk factor, can, in time, contribute to an endotheliopathy involving basal NO deficit, to the degree where regional metabolic dysfunction leads to cognitive meltdown and to progressive neurodegeneration characteristic of Alzheimer's disease.

Chronic cerebrovascular ischemia in aged rats: effects on brain metabolic capacity and behavior

The objective of this study was to model one of the risk factors for the development of late-onset Alzheimer's disease, decreased cerebral blood flow. Aging rats were tested for visuospatial behavioral deficits after permanent surgical occlusion of both carotid arteries. This was followed after 4 weeks by quantitative cytochrome oxidase histochemical mapping of metabolic capacity throughout the brain. The brain regions affected were related to observed deficits in spatial memory (CA1 and posterior parietal cortex), visually guided movements (superior colliculus and secondary visual cortex), motor coordination (red nucleus), and escape behavior (central gray). The results suggest that deficits in visuospatial learning are not exclusively the result of hippocampal dysfunction, but may be directly correlated with altered oxidative energy metabolism in other integrative visuomotor regions identified in this study. It was concluded that chronic cerebrovascular ischemia in this aged rat model produces neurometabolic and behavioral alterations that may be relevant for an increased risk for the development of Alzheimer's disease.

Critically attained threshold of cerebral hypoperfusion: the CATCH hypothesis of Alzheimer's pathogenesis

This review discusses the experimental and clinical data which indicate that chronic cerebral hypoperfusion can affect metabolic, anatomic, and cognitive function adversely. In aged but not young animals, chronic brain hypoperfusion results in regional pre- and post-synaptic changes, protein synthesis abnormalities, energy metabolic dysregulation, reduced glucose utilization, cholinergic receptor loss, and visuo-spatial memory deficits. Additionally, aging animals that are kept for prolonged periods of time after chronic brain hypoperfusion, also develop brain capillary degeneration in CA1 hippocampus and neuronal damage extending from the hippocampal region to the temporo-parietal cortex where neurodegenerative tissue atrophy eventually forms. All these pathologic events occur in rodents in the absence of senile plaques and neurofibrillary tangles. Alzheimer brains reveal similar biochemical and structural changes as those experimentally induced in aging animals. Moreover, regional cerebral hypoperfusion is one of the earlier (if not the earliest) clinical manifestations in both the sporadic and familial forms of Alzheimer's disease. In addition, therapy that improves or increases cerebral perfusion is generally of some benefit to Alzheimer patients. Conversely, a variety of disorders with different etiologies that impair or diminish cerebral perfusion are reported to be risk factors for this dementia. These findings have prompted us to propose the concept that advanced aging in the presence of a vascular risk factor can converge to create a critically attained threshold of cerebral hypoperfusion (CATCH) that triggers regional brain microcirculatory disturbances and impairs optimal delivery of energy substrates needed for normal brain cell function. The outcome of this defect generates a chain of events leading to the progressive evolution of brain metabolic, cognitive and tissue pathology that characterize Alzheimer's disease. The possible role of CATCH in familial and early onset Alzheimer's disease is briefly discussed from a theoretical vantagepoint. The growing and most recent evidence in support of the CATCH concept is the focus of this review.

Reduced cytochrome oxidase and memory dysfunction after chronic brain ischemia in aged rats

The effects of chronic cerebrovascular ischemia on memory function and cytochrome oxidase (CO) activity were investigated. Cerebrovascular insufficiency was induced by permanent bilateral carotid artery ligation (2-VO) in 19 month old rats. Sham surgery in no-vessel occlusion (no-VO) rats were used for controls. Memory function was tested 1 week prior to surgery and then weekly for 21 days using the Morris water maze. Regional brain activity of CO was measured 4 weeks after surgery by quantitative histochemistry. Histologic examination of brain slices was used to evaluate any neuropathology present. Results showed that 2-VO rats were significantly impaired in the water maze task at each testing period with respect to no-VO controls. In addition, CO activity in 2-VO rats was markedly reduced only in the dorsal CA1 region of the hippocampus and in the posterior parietal cortex. These brain regions are involved in visuo-spatial memory mechanisms. Analysis of other brain regions in 2-VO rats did not reveal further CO activity changes. There were no damaged or loss of neurons in 2-VO or no-VO groups in any region examined, including CA1 and posterior parietal cortex. The CA1 region however, is known to undergo neuronal loss 25 weeks after chronic 2-VO suggesting that this vascular insult can induce a slowly-evolving cascade consisting of neuronal damage, atrophy and death. The present findings indicate that reduced CO activity in CA1 and posterior parietal regions can predict neural damage and atrophy prior to structural perikaryal pathology following chronic brain ischemia. In addition, the data shows that neuronal energy metabolic deficiency may initiate visuo-spatial memory impairment in this aging rat model.

Castration increases [125I]MK801 binding in the hippocampus of male rats

This study examines the effect of castration and androgen replacement on [125I]MK801 binding in the hippocampus. In castrated male rats, [125I]MK801 binding was significantly increased in both the stratum oriens and radiatum and the pyramidal cell layer of CA1. In contrast, no increase in [125I]MK801 binding was observed in the stratum oriens and radiatum of CA1 of castrated rats that were treated with dihydrotestosterone. No change in [125I]MK801 binding was observed in the CA3 region or dentate gyrus after castration. The observed increase in [125I]MK801 binding in pyramidal cell neurons within CA1 suggests that androgens may potentially affect hippocampal function by modulating pyramidal cell NMDA receptors.

Correlates between nuclear magnetic resonance spectroscopy, diffusion weighted imaging, and CA1 morphometry following chronic brain ischemia

Chronic brain ischemia (CBI) was induced in aging (13 month) rats by ligating the left subclavian artery and placing temporary occluders in each common carotid artery [three-vessel occlusion (3-VO)]. Carotid artery occluders were removed after 1, 2, or 3 weeks following brain ischemia or maintained for 9 weeks. Two rats were kept with their occluders in place for 25 weeks. On weeks 3 and 9 after CBI, 31P-/1H-nuclear magnetic resonance (NMR) spectroscopy and high resolution diffusion weighted imaging were performed in vivo, non-invasively for detection of hippocampal high energy phosphates, lactate, intracellular pH, N-acetyl-aspartate, choline, glutamate, creatine, and structural alterations of the brain following CBI. Brains were histologically processed for morphometry of glial fibrillary acidic protein (GFAP) and CA1 damaged neurons 9 weeks after CBI. 31P-/1H-NMR spectroscopy showed that high energy substrates remained normal in ischemic animals when compared to non-ischemic controls except for an elevation of phosphomonesters in the hippocampal region. Rats deoccluded 1 and 2 weeks after initiation of CBI had no NMR spectroscopic or imaging changes. Rats kept ischemic for 9 weeks showed high signal intensities in the parietal cortex detected by diffusion weighted imaging as well as CA1 damage and increased GFAP density but no cortical atrophy or neuronal damage could be detected histologically. Rats kept ischemic for 25 weeks showed extensive cortical atrophy which corresponded to the high signal intensity observed with diffusion weighted imaging in the group kept ischemic for 9 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

A chronic physiological rat model of dementia

We have developed an aging rat model that mimics specific pathology reported in dementia, particularly Alzheimer's disease (AD). The model involves subjecting rats to chronic cerebrovascular insufficiency (CVI) for 1-9 weeks. Gross and sensory-motor function remains normal but spatial memory acquisition and retention are impaired after 1 week and worsens progressively with time. In vivo [31P]NMR spectroscopy evaluation in CVI animals showed membrane phospholipid synthesis increase in the hippocampal-cortex region of affected animals which increases with time. Post-mortem examination revealed that CA1 neurons can express selective damage 1 week after CVI and the number of CA1 neurons thus affected increases in proportion with time. MOreover, there is progressive increase in GFAP hypertrophy and hyperplasia in the hippocampal region during the 9-week observation period. Reduction of microtubule-associated protein 2 and pre-terminal noradrenergic varicosities in the hippocampus-cortex is seen after 9 weeks but not 1 week of CVI. All the above changes have been reported in AD-affected brains. The present CVI model appears as a useful screen in investigating potential therapy for AD as well as increasing understanding of this disorder.

Chronic cerebral hypoperfusion and impaired neuronal function in rats

BACKGROUND AND PURPOSE

Studies in acute cerebral ischemia have shown that reductions in cerebral blood flow of up to 50% do not lead to infarction or alterations in neuronal electric activity. Little is known about the effects of chronic reductions in cerebral blood flow. The purpose of this study was to evaluate neuronal electrophysiological function in brain that had been subjected to a chronic reduction of cerebral blood flow of less than 50%. Based on existing knowledge of thresholds of cerebral ischemia, neuronal electrophysiological function should be unaffected by hypoperfusion of this magnitude.

METHODS

An arteriovenous fistula model in the rat was used to induce chronic cerebral hypoperfusion with reductions of cerebral blood flow of 25% to 50% as measured previously by 14C-labeled autoradiography. Using in vitro electrophysiological brain slice techniques, long-term potentiation in hippocampal CA1 neurons was examined extracellularly after 6 months of chronic noninfarctional cerebral hypoperfusion. Brains were also examined histologically at this time for evidence of cerebral infarction.

RESULTS

There was no evidence of cerebral infarction. Long-term potentiation was produced in 9 of 12 control animals and only 2 of 8 hypoperfused animals. This difference was significant (P < .05) and demonstrated that long-term potentiation was impaired in animals with chronic hypoperfusion.

CONCLUSIONS

Noninfarctional reductions in cerebral blood flow of up to 50% do impair neuronal function in chronic cerebral ischemia, a result quite distinct from that seen in acute ischemia. The threshold for neuronal dysfunction in chronic cerebral hypoperfusion is lower than that found in acute cerebral ischemia, suggesting that duration as well as severity of ischemic insult determines cellular viability. Chronic hypoperfusion may lead to a noninfarctional state with impaired neuronal function, a category of chronic cerebral ischemia not previously identified.

Brain blood flow restoration 'rescues' chronically damaged rat CA1 neurons

Middle aged rats (13 months) were subjected to chronic cerebrovascular insufficiency (CVI) for 9 weeks using a 3-vessel occlusion technique. This CVI injury targets CA1 neuron damage selectively. Three groups of rats had their cerebral blood flow restored after 1, 2 or 3 weeks following CVI by removal of their carotid artery occluders. Another rat group did not undergo deocclusion for the 9 week observation period. Rats were tested for memory acquisition and retention 6 and 9 weeks after CVI using a modified water maze test. At the end of the 9 weeks, cerebral blood flow was measured in the fronto-parietal cortex and rats were killed by fixation-perfusion. Hippocampal morphometry was done to assess the % of damaged CA1 neurons and the density of GFAP-positive hyperplasia and hypertrophy. Results show that restoration of cerebral blood flow 1 and 2 weeks after CVI but not after 3 weeks of CVI, reversed a significant increase in reactive astrocytosis and prevented memory impairment in these deoccluded rats when compared to the non-deoccluded group. It appears from these results that 'neuronal rescue' of CA1 neurons is possible when cerebral blood flow is restored in rats subjected to chronic CVI during a 2 week (but not 3 week) 'window of opportunity'. This chronic brain ischemia model may be useful in screening potential therapy in patients with dementia where spatial memory impairment and hippocampal damage may be manifested.