Animal models of ischemic stroke
Encyclopedia
Animal model
s of ischemic stroke are procedures inducing cerebral ischemia
. The aim is the study of basic processes or potential therapeutic interventions in this disease, and the extension of the pathophysiological knowledge on and/or the improvement of medical treatment of human ischemic stroke.
Ischemic stroke has a complex pathophysiology
involving the interplay of many different cells and tissues such as neuron
s, glia, endothelium
, and the immune system
. These events cannot be mimicked satisfactorily in vitro
yet. Thus a large portion of stroke research is conducted on animals.
and stroke can be induced by common carotid artery occlusion alone.
. It is often used as a model of focal stroke to evaluate candidate pro-regenerative therapies. One advantage of this model of stroke is that it causes highly reproducible infarcts. Another benefit is that it can be used in elderly rats with only very low resulting mortality.
(MCA) occlusion is achieved in this model by injecting particles like blood clots (thrombembolic MCAO) or artificial spheres into the carotid artery of animals as an animal model of ischemic stroke. Thromembolic MCAO is achieved either by injecting clots that were formed in vitro or by endovascular instillation of thrombin
for in situ clotting . The thrombembolic model is closest to the pathophysiology of human cardioembolic stroke. When injecting spheres into the cerebral circulation, their size determines the pattern of brain infarction: Macrospheres (300 – 400 µm) induce infarcts similar to those achieved by occlusion of the proximal MCA , whereas microsphere (~ 50 µm) injection results in distal, diffuse embolism . However, the quality of MCAO – and thus the volume of brain infarcts – is very variable, a fact which is further aggravated by a certain rate of spontaneous lysis
of injected blood clots.
and forwarded until the tip occludes the origin of the middle cerebral artery
, resulting in a cessation of blood flow and subsequent brain infarction
in its area of supply. If the suture is removed after a certain interval, reperfusion is achieved (transient MCAO); if the filament is left in place the procedure is suitable as model of permanent MCAO, too. The most common modification is based on Longa (1989) who described filament introduction via the external carotid artery
, allowing closure of the access point with preserved blood supply via the common
and internal carotid artery to the brain after the removal of the filament. Known pitfalls of this method are insufficient occlusion, subarachnoid hemorrhage
, hyperthermia
, and necrosis
of the ipsilateral extracranial tissue . Filament MCAO is not applicable to all rat strains
.
(MCA) is surgically dissected and subsequently permanently occluded, e.g. by electrocautery or ligation
. Occlusion can be performed on the proximal or distal part of the MCA. In the latter, ischemic damage is restricted to the cerebral cortex
. MCAO can be combined with temporal or permanent common carotid artery
occlusion. These models require a small craniotomy
.
being reperfused after a defined period of focal cerebral ischemia . Like permanent MCAO, craniotomy is required and common carotid artery
(CCA) occlusion can be combined. Occluding one MCA and both CCAs is referred to as the three vessel occlusion model of focal cerebral ischemia.
Animal model
An animal model is a living, non-human animal used during the research and investigation of human disease, for the purpose of better understanding the disease without the added risk of causing harm to an actual human being during the process...
s of ischemic stroke are procedures inducing cerebral ischemia
Cerebral ischemia
Brain ischemia, also known as cerebral ischemia, is a condition in which there is insufficient blood flow to the brain to meet metabolic demand. This leads to poor oxygen supply or cerebral hypoxia and thus to the death of brain tissue or cerebral infarction / ischemic stroke...
. The aim is the study of basic processes or potential therapeutic interventions in this disease, and the extension of the pathophysiological knowledge on and/or the improvement of medical treatment of human ischemic stroke.
Ischemic stroke has a complex pathophysiology
Pathophysiology
Pathophysiology is the study of the changes of normal mechanical, physical, and biochemical functions, either caused by a disease, or resulting from an abnormal syndrome...
involving the interplay of many different cells and tissues such as neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s, glia, endothelium
Endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...
, and the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
. These events cannot be mimicked satisfactorily in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
yet. Thus a large portion of stroke research is conducted on animals.
Overview
Several models in different species are currently known to produce cerebral ischemia. Global ischemia models, both complete and incomplete, tend to be easier to perform. However, they are less immediately relevant to human stroke than the focal stroke models, because global ischemia is not a common feature of human stroke. However, in various settings global ischemia is also relevant, e.g. in global anoxic brain damage due to cardiac arrest. Different species also vary in their susceptibility to the various types of ischemic insults. An example is gerbils. They do not have a Circle of WillisCircle of Willis
The Circle of Willis is a circle of arteries that supply blood to the brain...
and stroke can be induced by common carotid artery occlusion alone.
Mechanisms of inducing ischemic stroke
Some of the mechanisms which have been used are:- Complete global ischemia
- Decapitation
- Aorta/vena cava occlusion
- External neck torniquet or cuff
- Cardiac arrest
- Incomplete global ischemia
- Hemorrhage or hypotension
- Hypoxic ischemia
- Intracranial hypertension and common carotid artery occlusion
- Two-vessel occlusion and hypotension
- Four-vessel occlusion
- Unilateral common carotid artery occlusion (in some species only)
- Focal cerebral ischemia
- Endothelin-1-induced constriction of arteries and veins
- Middle cerebral artery occlusion
- Spontaneous brain infarction (in spontaneously hypertensive rats)
- Macrosphere embolization
- Multifocal cerebral ischemia
- Blood clot embolization
- Microsphere embolization
- Photothrombosis
Hypoxic Ischemia models
One of the most commonly used animal models of hypoxic ischemia was originally described by Levine in 1960 and later refined by Rice et al., in 1981. This approach is useful to study hypoxic ischemia in the developing brain, since newborn rat pups are utilized in this model. Briefly, 7 day old rat pups undergo a permanent unilateral carotid artery ligation with a subsequent 3 hour exposure to a hypoxic environment (8% oxygen). This model creates a unilateral infarct in the hemisphere ipsilateral to the ligation, since the hypoxia alone is subthreshold for injury at this age. The area of injury is typically concentrated in periventricular regions of the brain, especially cortical and hippocampal areas.Focal ischemia models
They are divided into techniques including reperfusion of the ischemic tissue (transient focal cerebral ischemia) and those without reperfusion (permanent focal cerebral ischemia). The following models are established :- Endothelin-1 -induced constriction of arteries and veins
- Middle cerebral arteryMiddle cerebral artery-External links:*...
occlusion (MCAO)- MCAO avoiding craniotomyCraniotomyA craniotomy is a surgical operation in which a bone flap is temporarily removed from the skull to access the brain. Craniotomies are often a critical operation performed on patients recording, brain imaging, and for neurological manipulations such as electrical stimulation and chemical...
- Embolic middle cerebral artery occlusion
- Endovascular filament middle cerebral artery occlusion (transient or permanent)
- MCAO involving craniotomy
- Permanent transcranial middle cerebral artery occlusion
- Transient transcranial middle cerebral artery occlusion
- MCAO avoiding craniotomy
- Direct tissue damage
- Cerebrocortical photothrombosis
Endothelin-1 -induced constriction of arteries and veins
Endothelin-1 is a potent vasoconstrictor which is produced endogenously during ischemic stroke and which contributes to overall loss of cells and disability. Exogenous endothelin-1 can also be used to induce stroke and cell death after sustained vasoconstriction with reperfusion. It can be microinjected to induce focal stroke in small tissue volumes (e.g., cortical grey matter, white matter or subcortical tissue) or after injection near the Middle cerebral arteryMiddle cerebral artery
-External links:*...
. It is often used as a model of focal stroke to evaluate candidate pro-regenerative therapies. One advantage of this model of stroke is that it causes highly reproducible infarcts. Another benefit is that it can be used in elderly rats with only very low resulting mortality.
Embolic middle cerebral artery occlusion
Middle cerebral arteryMiddle cerebral artery
-External links:*...
(MCA) occlusion is achieved in this model by injecting particles like blood clots (thrombembolic MCAO) or artificial spheres into the carotid artery of animals as an animal model of ischemic stroke. Thromembolic MCAO is achieved either by injecting clots that were formed in vitro or by endovascular instillation of thrombin
Thrombin
Thrombin is a "trypsin-like" serine protease protein that in humans is encoded by the F2 gene. Prothrombin is proteolytically cleaved to form thrombin in the first step of the coagulation cascade, which ultimately results in the stemming of blood loss...
for in situ clotting . The thrombembolic model is closest to the pathophysiology of human cardioembolic stroke. When injecting spheres into the cerebral circulation, their size determines the pattern of brain infarction: Macrospheres (300 – 400 µm) induce infarcts similar to those achieved by occlusion of the proximal MCA , whereas microsphere (~ 50 µm) injection results in distal, diffuse embolism . However, the quality of MCAO – and thus the volume of brain infarcts – is very variable, a fact which is further aggravated by a certain rate of spontaneous lysis
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
of injected blood clots.
Endovascular filament middle cerebral artery occlusion
The technique of endovascular filament (intraluminal suture) MCAO as an animal model of ischemic stroke was described first by Koizumi . It is applied to rats and mice. A piece of surgical filament is introduced into the internal carotid arteryInternal carotid artery
In human anatomy, the internal carotid arteries are two major arteries, one on each side of the head and neck. They arise from the common carotid arteries where these bifurcate into the internal and external carotid artery, and they supply the brain....
and forwarded until the tip occludes the origin of the middle cerebral artery
Middle cerebral artery
-External links:*...
, resulting in a cessation of blood flow and subsequent brain infarction
Infarction
In medicine, infarction refers to tissue death that is caused by a local lack of oxygen due to obstruction of the tissue's blood supply. The resulting lesion is referred to as an infarct.-Causes:...
in its area of supply. If the suture is removed after a certain interval, reperfusion is achieved (transient MCAO); if the filament is left in place the procedure is suitable as model of permanent MCAO, too. The most common modification is based on Longa (1989) who described filament introduction via the external carotid artery
External carotid artery
In human anatomy, the external carotid artery is a major artery of the head and neck. It arises from the common carotid artery when it bifurcates into the external and internal carotid artery.-Course:...
, allowing closure of the access point with preserved blood supply via the common
Common carotid artery
In human anatomy, the common carotid artery is an artery that supplies the head and neck with oxygenated blood; it divides in the neck to form the external and internal carotid arteries. - Structure :...
and internal carotid artery to the brain after the removal of the filament. Known pitfalls of this method are insufficient occlusion, subarachnoid hemorrhage
Subarachnoid hemorrhage
A subarachnoid hemorrhage , or subarachnoid haemorrhage in British English, is bleeding into the subarachnoid space—the area between the arachnoid membrane and the pia mater surrounding the brain...
, hyperthermia
Hyperthermia
Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate...
, and necrosis
Necrosis
Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...
of the ipsilateral extracranial tissue . Filament MCAO is not applicable to all rat strains
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...
.
Permanent transcranial middle cerebral artery occlusion
In this animal model of ischemic stroke the middle cerebral arteryMiddle cerebral artery
-External links:*...
(MCA) is surgically dissected and subsequently permanently occluded, e.g. by electrocautery or ligation
Ligation
Ligation may refer to:* In molecular biology, the covalent linking of two ends of DNA molecules using DNA ligase* In medicine, the making of a ligature * Chemical ligation, the production of peptides from amino acids...
. Occlusion can be performed on the proximal or distal part of the MCA. In the latter, ischemic damage is restricted to the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. MCAO can be combined with temporal or permanent common carotid artery
Common carotid artery
In human anatomy, the common carotid artery is an artery that supplies the head and neck with oxygenated blood; it divides in the neck to form the external and internal carotid arteries. - Structure :...
occlusion. These models require a small craniotomy
Craniotomy
A craniotomy is a surgical operation in which a bone flap is temporarily removed from the skull to access the brain. Craniotomies are often a critical operation performed on patients recording, brain imaging, and for neurological manipulations such as electrical stimulation and chemical...
.
Transient transcranial middle cerebral artery occlusion
The technique of modeling ischemic stroke by transient transcranial MCAO is similar to that of permanent transcranial MCAO, with the MCAMiddle cerebral artery
-External links:*...
being reperfused after a defined period of focal cerebral ischemia . Like permanent MCAO, craniotomy is required and common carotid artery
Common carotid artery
In human anatomy, the common carotid artery is an artery that supplies the head and neck with oxygenated blood; it divides in the neck to form the external and internal carotid arteries. - Structure :...
(CCA) occlusion can be combined. Occluding one MCA and both CCAs is referred to as the three vessel occlusion model of focal cerebral ischemia.