ReviewPermanent, bilateral common carotid artery occlusion in the rat: A model for chronic cerebral hypoperfusion-related neurodegenerative diseases
Introduction
Disorders of the cerebral circulation are the causes of or factors contributing to numerous neurological and psychiatric illnesses. A sudden disruption of the blood supply to distinct brain regions leads to stroke, while a moderate but persistent reduction in regional cerebral blood flow (CBF) compromises memory processes and contributes to the development and progression of dementia. The association of decreased CBF, particularly in the temporal and parietal cortices, with Alzheimer's disease (AD) has been firmly established (de la Torre, 2002, Farkas and Luiten, 2001, Matsuda, 2001). Additionally, the degree or pattern of cerebral hypoperfusion in mild cognitive impairment has been suggested as a predictive marker for the progression to AD (Borroni et al., 2006, Hirao et al., 2005). Furthermore, global cerebral hypoperfusion can occur in patients who have suffered a cardiac arrest or those who undergo complex cardiac surgery, and this condition can lead to a poor neurologic outcome (Hypothermia after Cardiac Arrest Study Group, 2002, Nussmeier, 2002). For an understanding of the role of the cerebrovascular pathology in the development of a cognitive dysfunction and dementia, it is important to explore the cerebral hypoperfusion-related metabolic changes, the distinct neurodegenerative and cognitive correlates of hypoperfusion, and the causal relationships between these factors. Furthermore, recognition of particular mechanisms in the chain of events from chronic cerebral hypoperfusion to a cognitive decline may identify potential targets for effective therapies. For these purposes, a number of animal models have been introduced.
Vessel occlusion studies aim at creating ischemic or oligemic injuries with various degrees of severity in the brains of experimental animals. The rat is a frequently used species in consequence of the good survival rate, the satisfactory recovery from surgery, the easy and reproducible behavioral testing, the relatively low costs, and ethical acceptance. A great variety of rat vessel occlusion models have emerged, most of which are applied in stroke research. Since stroke is an acute pathophysiological condition, the experimental stroke models are generally employed to investigate the short-term effects of vessel occlusion, and the possibilities of rapid interventions designed to limit neurodegenerative processes. To this end, two experimental approaches have evolved in the rat: transient occlusion of the middle cerebral artery and permanent occlusion of extracranial vessels (e.g. vertebral and carotid arteries) in combination with experimental hypotension or hypoxia. In the frequently used transient ischemia models, there is an obvious component of reperfusion, the brain damage is focal and severe, the ischemic site can typically be divided into a core and a penumbra region, motor dysfunction evidently appears as a functional correlate of the brain damage, and even seizures may develop.
For the reproduction of chronic cerebral hypoperfusion as it occurs in human aging and AD, permanent, bilateral occlusion of the common carotid arteries of rats (2-vessel occlusion, 2VO) has been introduced. The rat is a suitable rodent species for this purpose because the complete circle of Willis affords incessant (but reduced) blood flow after the onset of 2VO. In contrast, the lack or the underdevelopment of the posterior communicating arteries of the circle of Willis causes severe ischemia during 2VO in gerbils and most of the mouse strains. In the rat 2VO model, the surgical methods, the permanent nature of the ligation of the common carotid arteries, the lack of exacerbating factors (hypotension or hypoxia), and the experimental animal species (the rat) are standard, and therefore the 2VO model can be regarded as a single defined entity from a technical point of view. As opposed to stroke research, 2VO studies have the aim of investigation of the long-term effects of chronic cerebral hypoperfusion. In 2VO rats, the vessel occlusion is permanent and long-lasting, reperfusion injury (as a result of an instant recovery of perfusion) does not occur, the cerebral hypoperfusion is global, and thus a distinct ischemic core and penumbra region cannot be outlined, the damage to the nervous tissue is less dramatic, and there are no obvious signs of motor dysfunction or seizures.
In aging and AD, the sequence of the events of cerebral hypoperfusion and neurodegeneration has been a subject of debate. A chronic reduction in CBF was earlier believed to induce the neurodegenerative processes; however, the view was also raised that an earlier neuronal cell loss would require a consequently lower supply of energy substrates, and therefore a lower perfusion rate. Following the introduction of the concept of the neurovascular unit (the functionally integrated network of neural and vascular cells), this controversy can be explained: damage to any of the components of the unit results in consequences to the functioning of the entire unit.
With the help of the 2VO model, the causal and sequential interactions of cerebral hypoperfusion, neuronal injury and memory deficits have been elucidated. Experimental evidence has demonstrated the initiating role of chronic cerebral hypoperfusion in neural damage to the hippocampus, the cerebral cortex, the white matter (WM) areas and the visual system. Because of the vulnerability of the WM, the retina and the visual pathways, the 2VO model has recently been applied with success in other research fields, such as ischemic WM injury and ischemic eye diseases.
The aims of the current review are to provide a comprehensive survey of the experimental evidence that has accumulated from use of the 2VO model in the rat, to relate the results to human neurological conditions, and to consider the particular subtleties and pitfalls of the method.
Section snippets
The spatial and temporal patterns of blood flow in experimental cerebral hypoperfusion
It has been well established that aging and dementia are accompanied by a reduced CBF (Farkas and Luiten, 2001). The degree of cerebral hypoperfusion observed in clinical studies ranges from ∼ 94% in the parietal and temporal cortices of elderly individuals as compared with younger volunteers (Claus et al., 1998) to ∼ 73% in the same regions and the hippocampus of AD patients as compared with age-matched, healthy controls (Farkas and Luiten, 2001). In order to tackle the neurological and
The effects of chronic cerebral hypoperfusion on the blood–brain barrier
The blood–brain barrier (BBB), the interface between blood-borne molecules and the central nervous system, is quite vulnerable to ischemia. For example, the BBB becomes readily permeable to large molecules in a severely ischemic environment (Lenzsér et al., 2005, Preston and Foster, 1997). However, there is little evidence of disruption of the BBB during healthy aging (Mooradian, 1988, Stewart et al., 1987), and only a few studies have indicated actual leakage of the BBB in AD (Schlageter et
Alterations in the electrophysiological activity of the brain during chronic cerebral hypoperfusion
Relatively few studies have recorded hypoperfusion-induced changes in the electrophysiological activity of the rat brain. The retina of 2VO rats has been flash-stimulated, and visual evoked potentials recorded from the occipital cortex. The latency of the positive peaks (P2) was increased, while the amplitude (the difference between the negative and positive peaks, N2–P2) was diminished 10 days after 2VO induction (Aytac et al., 2006). However, these results may not reflect the failing activity
Impairment of learning and memory in chronic cerebral hypoperfusion
The hypothesis that chronic cerebral hypoperfusion contributes to the progression of dementia was proposed long ago (de la Torre, 2000, Farkas and Luiten, 2001). Traditionally, the two most frequently used tests to measure the hippocampus-related spatial learning capacity in chronic cerebral hypoperfusion in rats are the Morris water maze and the 8-arm radial maze. In these learning paradigms, substantial evidence has been compiled in support of learning being impaired by 2VO (Farkas and
Neuronal damage
For several reasons, the favored brain region for the study of 2VO-induced neurodegeneration is the hippocampus. First of all, the hippocampus is the area that displays the most characteristic neuropathological damage in AD. Second, the hippocampus is highly implicated in spatial learning and memory as assessed by the Morris water maze and the 8-arm radial maze. For this reason, neuronal injury in the hippocampus and impaired spatial learning can be related. Third, the hippocampus (and
Chronic cerebral hypoperfusion-related white matter injury
Cerebral WM lesions that accompany human aging and dementia have received increasing attention as WM injury visualized with clinical imaging techniques has been found to coincide with cognitive and psychiatric disorders in the elderly and AD patients (Barber et al., 1999, de Groot et al., 2000, de Leeuw et al., 2001). Cerebral ischemia has been hypothesized as the most probable cause of WM lesions, but direct confirmation of the assumption requires experimental models. The 2VO model has emerged
The effects of chronic cerebral hypoperfusion on the visual system
The 2VO model in rats has also been applied for ischemic eye research. In investigations of the ischemic component of diabetic retinopathy, chronic glaucoma, or ocular ischemic syndrome, a number of 2VO-induced functional and morphological pathologies of the optic system have been identified (Lavinsky et al., 2006, Yamamoto et al., 2006). Approximately 50% of 2VO animals lose their pupillary reflex (Davidson et al., 2000, Lavinsky et al., 2006, Stevens et al., 2002), which makes the retina
Body weight
Like larger surgical interventions in general, 2VO is followed by a decrease in body weight. On postoperative days 1 and 2, a loss in body weight can be observed in both the sham-operated and 2VO groups. Since certain muscles in the ventral cervical region (e.g. the sternohyoid and the sternomastoid muscles) are slightly damaged during the preparation of the carotid arteries, discomfort during movement of the head, mastication, and swallowing may contribute to this initial weight loss. The
Species and strains
It is widely known that 2VO induces neuronal damage with varying degrees of severity in different mammal species. Two species frequently employed for carotid occlusion studies are the rat and the gerbil. These two models create distinct ischemic conditions in the brain. Because of the lack of communicating arteries between the carotid and vertebral systems, carotid occlusion in the gerbil leads to severe forebrain ischemia. In contrast, the complete circle of Willis in the rat affords
Summary and conclusions
The cerebral hemodynamic, metabolic, functional, and neuropathological data necessitated a survey of the achievements with the 2VO model. The nature and gravity of the insult inflicted by 2VO, and the severity of the consequent neuronal damage demand a review because they have not been comprehensively and conclusively determined. Most investigators agree that the level of cerebral hypoperfusion in the model is moderate, and the resulting neuronal injury is relatively mild. However, the terms
Acknowledgments
This work was supported by the Hungarian Scientific Research Fund (OTKA) grants F042803 (E.F.) and K63401 (F.B.), the Hungarian National Office for Research and Technology (NKTH), a Visiting Scholar grant from the Royal Netherlands Academy of Arts and Sciences to E.F., and a Bolyai János Research Scholarship of the Hungarian Academy of Sciences to E.F.
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