Reduction in local cerebral blood flow induced by endothelin-1 applied topically to the middle cerebral artery in the rat

Animal models of stroke

Stroke is a devastating disease with high morbidity and mortality. Animal models are indispensable tools that can mimic stroke processes and can be used for investigating mechanisms and developing novel therapeutic regimens. As a heterogeneous disease with complex pathophysiology, mimicking all aspects of human stroke in one animal model is impossible. Each model has unique strengths and weaknesses. Models such as transient or permanent intraluminal thread occlusion middle cerebral artery occlusion (MCAo) models and thromboembolic models are the most commonly used in simulating human ischemic stroke. The endovascular filament occlusion model is characterized by easy manipulation and accurately controllable reperfusion and is suitable for studying the pathogenesis of focal ischemic stroke and reperfusion injury. Although the reproducibility of the embolic model is poor, it is more convenient for investigating thrombolysis. Rats are the most frequently used animal model for stroke. This review mainly outlines the stroke models of rats and discusses their strengths and shortcomings in detail.

Prospects of modeling poststroke epileptogenesis

This Review describes the current status of poststroke epilepsy (PSE) with an emphasis on poststroke epileptogenesis modeling for testing new therapeutic agents. Stroke is a leading cause of epilepsy in an aging population. Late-onset "epileptic" seizures have been reported in up to 30% cases after stroke. Nevertheless, the overall prevalence of PSE is 2-4%. Rodent models of stroke have contributed to our understanding of the relationship between seizures and the underlying ischemic damage to neurons. To understand whether acutely generated stroke events lead to a chronic phenotype more closely resembling PSE with recurrent seizures, a limited variety of approaches emerged in early 2000s. These limited methods of causing an occlusion in mice and rats show different infarct size and neurological deficits. The most often employed procedure for inducing focal ischemia is the middle cerebral artery occlusion. This mimics the pathophysiology seen in humans in terms of extent of damage to cortex and striatum. Photothrombosis and endothelin-1 models can similarly evoke episodes of ischemic stroke. These models are well suited to studying mechanisms and biomarkers of epileptogenesis or optimizing novel drug discoveries. However, modeling of PSE is tedious, is highly variable, and lacks validity; therefore, it is not widely implemented in epilepsy research. Moreover, the relevance of ischemic models to specific forms of human stroke remains unclear. Stroke modeling in young male rodents lacks clinical relevance to elderly populations and especially to women, likely as a result of sex differences. Nevertheless, because of the neuronal damage and epileptogenic insult that these models trigger, they are helpful tools in studying acquired epilepsy and prophylactic drug therapy. © 2016 Wiley Periodicals, Inc.

Present status and future challenges of electroencephalography- and magnetic resonance imaging-based monitoring in preclinical models of focal cerebral ischemia

Animal models are useful tools for better understanding the mechanisms underlying neurological deterioration after an ischemic insult as well as subsequent evolution of changes and recovery of functions. In response to the updated requirements for preclinical investigations of stroke to include relevant functional measurement techniques and biomarker endpoints, we here review the state of knowledge on application of some translational electrophysiological and neuroimaging methods, and in particular, electroencephalography monitoring and magnetic resonance imaging in rodent models of ischemic stroke. This may lead to improvement of diagnostic methods and identification of new therapeutic targets, which would considerably advance the translational value of preclinical stroke research.

Epileptogenesis after experimental focal cerebral ischemia

Cerebrovascular diseases are one of the most common causes of epilepsy in adults, and the incidence of stroke-induced epileptogenesis is increasing as the population ages. The mechanisms that lead to stroke-induced epileptogenesis in a subpopulation of patients, however, are still poorly understood. Recent advances in inducing epileptogenesis in rodent focal ischemia models have provided tools that can be used to identify the risk factors and neurobiologic changes leading to development of epilepsy after stroke. Here we summarize data from models in which epileptogenesis has been studied after focal ischemia; photothrombosis, middle cerebral artery (MCA) occlusion with filament, and endothelin-1-induced MCA occlusion. Analysis of the data indicates that neurobiologic changes occurring during stroke-induced epileptogenesis share some similarities to those induced by status epilepticus or traumatic brain injury.

Focal Lesions in the Rat Central Nervous System Induced by Endothelin-1

Axon injury following cerebral ischemia has received little scientific attention compared to the abundance of information dealing with the pathophysiology of grey matter ischemia. There are differences in the initial response of grey and white matter to ischemia in vitro. In this study we investigate whether the vasoactive peptide, endothelin-1, can generate a focal ischemic lesion in the white matter and compare the findings with endothelin-1-induced lesions in the grey matter. Using a minimally invasive technique to microinject endothelin-1 into selected brain regions, we observed an acute reduction in local MRI perfusion in the injected hemisphere after 1 hour. Twenty-four hours after microinjection of 10 pmoles of endothelin-1, we observed a loss of neurons in the grey matter. At 72 hours, neutrophils were absent and a macrophage/microglia response and astrocyte gliosis were detected. No breakdown in the blood-brain barrier was detected. After injection of 10 pmoles endothelin-1 into the cortical white matter, we observed prolific amyloid precursor protein-positive immunostaining (indicative of axonal disruption) and an increase in tau-1 immunostaining in oligodendrocytes at 6 hours. Similar to the grey matter lesions, no neutrophils were present, a macrophage/microglia response did not occur until 72 hours and there was no disruption in the blood-brain barrier. Focal injections of endothelin-1 into specific areas of the rat CNS represent a model to investigate therapeutic approaches to white matter ischemia.

Hypoxic modulation of striatal lesions induced by administration of endothelin-1

Levels of endothelin-1 (ET-1), a potent endogenous vasoconstrictor, are elevated in plasma and cerebrospinal fluid (CSF) following cerebral ischemia and reperfusion injury. The present study sought insight into the potential differential vasoactive effects on the cerebral vasculature and resultant neural damage of ET-1 during normoxic vs. ischemic conditions and upon reperfusion. Under normoxic conditions, intrastriatal stereotaxic injection of exogenous ET-1 (40 pmol) induced a significant (P<0.05) reduction (</=29+/-12%) in the regional (striatal) cerebral blood flow measured by Laser Doppler flowmetry (CBF(LDF)) for up to 40 min in halothane-anesthetized male Long-Evans rats. Intrastriatal injection of ET-1 10 min after the onset of hypoxia (12% O(2), balance N(2)) tended to blunt, but not significantly, the striatal CBF(LDF) responses to the 35 min period of hypoxia. ET-1 given during reoxygenation significantly (P<0.05) reduced striatal CBF(LDF), which was similar to the effect of ET-1 during normoxia. ET-1-induced infarction when administered prior to hypoxia, but not during or post-hypoxia, was significantly (P<0.05) exacerbated compared to infarction of ET-1 without hypoxia. These results suggest that exogenous ET-1 administered into the brain parenchyma can induce an infarction associated with modulation of CBF(LDF) during the normoxic or reoxygenation period, but not during the hypoxic period and that the increased release of ET-1 in any pathological phase of cerebral ischemia contributes to irreversible neural damage with associated hemodynamic disturbances.

Endothelin type A-antagonist improves long-term neurological recovery after cardiac arrest in rats

OBJECTIVE

Antagonists of endothelin (ET(A)) receptors improve postischemic hypoperfusion. In this study we investigated whether the selective ET(A)-antagonist BQ123 also improves postischemic functional recovery.

STUDY DESIGN

Cardiac arrest of 12 mins duration was induced in rats by electrical fibrillation of the heart, followed by advanced cardiopulmonary resuscitation. BQ123 (0.8 mg/kg; n = 9) or its vehicle (saline; n = 9) was injected intravenously at 15 mins after the return of spontaneous circulation. The neurologic deficit was scored daily for 7 days after resuscitation by rating consciousness, various sensory and motor functions, and coordination tests. On day 7, we measured functional coupling of cerebral blood flow under halothane anesthesia by recording laser-Doppler flow during electrical forepaw stimulation, and we measured vascular reactivity to CO2 by measuring the laser-Doppler flow change during ventilation with 6% CO2. The brains were perfusion-fixated with 4% paraformaldehyde, and the histopathologic damage was evaluated in the CA1 sector of hippocampus, in the motor cortex, and in the cerebellum.

RESULTS

Treatment with BQ123 had no effect on histopathologic damage, but it significantly improved neurologic recovery. In all nine treated rats, neurologic performance returned to near normal within 2 days whereas four of nine untreated animals developed spastic paralysis of the hind limbs and severe coordination deficits. BQ123 also normalized CO2 reactivity and improved the functional cerebral blood flow response to somatosensory stimulation.

CONCLUSIONS

The ET(A)-antagonist BQ123 significantly improves neurologic outcome after 12 mins of cardiac arrest. The apparent restoration of vascular reactivity demonstrates a correlation between hemodynamic factors and functional recovery.

Cerebral vasoconstriction in comatose patients resuscitated from a cardiac arrest?

OBJECTIVE

To determine the role of cerebral vasoconstriction in the delayed hypoperfusion phase in comatose patients after cardiac arrest.

DESIGN

Prospective study.

SETTING

Medical intensive care unit in a university hospital.

PATIENTS

10 comatose patients (Glasgow Coma Score +/- 6)successfully resuscitated from a cardiac arrest occurring outside the hospital.

MEASUREMENTS

We measured the pulsatility index (PI) and mean blood flow velocity (MFV) of the middle cerebral artery, the cerebral oxygen extraction ratio and jugular bulb levels of endothelin, nitrate, and cGMP during the first 24 h after cardiac arrest.

RESULTS

The PI decreased significantly from 1.86 +/- 1.02 to 1.05 +/- 0.22 (p = 0.03). The MFV increased significantly from 29 +/- 10 to 62 +/- 25 cm/s (p = 0.003). Cerebral oxygen extraction ratio decreased also from 0.39 +/- 0.13 to 0.24 +/- 0.11 (p = 0.015). Endothelin levels were high but did not change during the study period. Nitrate levels varied widely and showed a slight but significant decrease from 37.1 mumol/l (median; 25th-75th percentiles: 26.8-61.6) to 31.3 mumol/l (22.1-39.6) (p = 0.04). Cyclic guanosine monophosphate levels increased significantly from 2.95 mumol/l (median; 25th-75th percentiles: 2.48-5.43) to 7.5 mumol/l (6.20-14.0) (p = 0.02).

CONCLUSIONS

We found evidence of increased cerebrovascular resistance during the first 24 h after cardiac arrest with persistent high endothelin levels, gradually decreasing nitrate levels, and gradually increasing cGMP levels, This suggests that active cerebral vasoconstriction due to an imbalance between local vasodilators and vasoconstrictors plays a role in the delayed hypoperfusion phase.

Endothelin peptides in brain diseases

Recently, scientists interested in diseases of the human brain have paid much attention to the endothelin group of peptides. Under normal conditions they are found in some types of neurons and in endothelial cells of microvessels but not in glial cells. This review focuses on the endothelin peptides and their involvement in various brain diseases. Particular attention is paid to their expression in reactive astrocytes seen in many pathological conditions of the human brain. Endothelin-1 is a very potent vasoconstrictor which may be involved in the vasospasm occurring in subarachnoid haemorrhage. Intracerebral injection or application to cerebral arteries in animals will cause a focal necrosis, apparently due to severe vasoconstriction. Reactive astrocytes occurring in cases with infarcts, lacunae, Alzheimer's disease, progressive multifocal leukoencephalopathy (PML) and subacute sclerosing panencephalitis (SSPE) express endothelin-like immunoreactivity. Astrocytes in vitro may produce, store and release endothelins. To some extent astrocytes grown in vitro mimic reactive astrocytes in vivo since in cultures astrocytes are removed from their natural environment which may trigger reactive responses. Therefore, in vivo reactive astrocytes may produce, store and release endothelins just as in vitro. If endothelins are released from reactive astrocytes they may act as mitogens and may influence microcirculation by inducing vasoconstriction of intracerebral arterioles. In such ways endothelins may contribute to the final lesions seen in cases with infarcts, lacunae, traumatic conditions, Alzheimer's disease and inflammatory diseases of the brain.

Potentiation by endothelin-1 of Ca2+ sensitivity of contractile elements depends on Ca2+ influx through L-type Ca2+ channels in the canine cerebral artery

1. Endothelin-1 (ET-1) contracted canine cerebral artery in a concentration-dependent manner with an increase in intracellular Ca2+ concentration ([Ca2+]i), and at higher concentrations it produced a greater contraction with a smaller increase in [Ca2+]i. 2. Ca2+ channel antagonist such as d-cis-diltiazem inhibited the tension more effectively than the [Ca2+]i increased by ET-1. 3. In Ca(2+)-free solution containing 0.2 mM EGTA, ET-1 elicited a transient increase in [Ca2+]i and tension. 4. In the Staphylococcus aureus alpha-toxin-permeabilized artery, ET-1 shifted the pCa-tension relationship leftwards in the presence of GTP. 5. These findings suggest that ET-1 contracts the canine cerebral artery by increasing not only the Ca2+ influx through L-type Ca2+ channels, but also Ca2+ release from the intracellular storage sites, and also Ca2+ sensitivity of contractile elements. The degree of Ca2+ sensitivity is strongly affected by [Ca2+]i which is increased by the Ca2+ influx through L-type Ca2+ channels.

Intrathecal administration of endothelin-1 in the rat: impact on spinal cord blood flow and the blood-spinal cord barrier

The purpose of this study was to examine the integrity of the blood-spinal cord barrier after intrathecal administration of endothelin-1 (ET-1) in the rat spinal cord. A laser Doppler probe was positioned in the exposed subarachnoid space at the T8 vertebral level. In the first experiment, horseradish peroxidase (HRP), a marker of barrier integrity, was given intravenously prior to the intrathecal application of ET-1. Blood flow was then recorded for 3 h, after which the anesthetized animals were euthanized. In the second experiment, animals exposed to endothelin were recovered after confirming a 50% reduction in blood flow. HRP was given 10 min prior to euthanasia at 24 h post infusion. The intensity of staining for HRP was quantified by optical density in fixed sections of spinal cord. There was a significant sustained reduction in spinal cord blood flow and significant barrier breakdown to HRP at both 3 and 24 h after administration of the peptide. Based upon these results we conclude that intrathecal infusion of ET-1 reduces spinal cord blood flow and results in prolonged breakdown of the blood-spinal cord barrier.

Immunolocalization of endothelin in the traumatized spinal cord: relationship to blood-spinal cord barrier breakdown

The purpose of this study was to determine the relationship between endothelin-1 (ET-1), a prominent vasoactive agent, and the breakdown of the blood-spinal cord barrier along the axis of the cord after a moderate spinal cord injury. In the first study rats (n = 10) were euthanized 24 h after spinal cord injury and compared to sham (n = 5) and unoperated (n = 10) controls. Endothelin and immunoglobulins (IgG) were immunolocalized in adjacent sections of spinal cord using semiquantitative immunocytochemical techniques. In the second study animals were pretreated with the endothelin antagonist Bosentan (n = 6) or vehicle (n = 6) prior to spinal cord injury. Animals were euthanized at 24 h postinjury. Ten minutes prior to euthanasia animals were given horseradish peroxidase (HRP) intravenously. After perfusion fixation sections of cord were prepared for quantitative HRP histochemistry. After spinal cord injury there was enhanced staining for endothelin along the axis of the cord that correlated with the anatomical pattern of barrier breakdown to IgG. In those animals that were pretreated with Bosentan, there was a significant reduction in barrier breakdown along the axis of the injured cord as compared to those animals that received vehicle only. Taken together, this data implicate involvement of endothelin in the axial pattern of barrier breakdown after spinal cord injury.

Reactive astrocytes in viral infections of the human brain express endothelin-like immunoreactivity

In order to investigate the expression of endothelin-like immunoreactivity in astrocytes of viral infections of the human brain the avidin-biotin peroxidase complex method and a polyclonal antiserum were used. Autopsy material was obtained from 5 cases of herpes simplex encephalitis, two of progressive multifocal leukoencephalopathy (PML) and two of subacute sclerosing panencephalitis (SSPE). All the 5 herpes simplex encephalitis cases presented groups of immunoreactive astrocytes around necrotic, inflammatory lesions. The PML cases exhibited a large number of immunoreactive astrocytes in and around lesions of the white matter. The cases of SSPE disclosed numerous, markedly stained fibrillary immunoreactive astrocytes; they were most abundant in degenerated regions of the white matter. The processes and peripheral cytoplasm of giant astrocytes in the PML cases contained immunoreactive material but the perinuclear region was devoid of such material. In the herpes simplex and the SSPE cases immunoreactivity was present throughout the cytoplasm and processes of reactive fibrillary astrocytes. Many nerve cells in the cerebral cortex, hippocampus, cerebellum and pons of control cases exhibited endothelin-like immunoreactivity but this occurred in only exceptional astrocytes of control cases. Endothelin-like immunoreactivity was not present in the oligodendrocytes and vascular endothelial cells of controls and cases of virus infection. The expression of endothelin-like immunoreactivity in astrocytes in human viral diseases reflects probably an increased intracellular content of endothelin. If this peptide is released from such astrocytes, it may act as a mitogen and by inducing constriction of arterioles it may influence the microcirculation.

Endothelin levels increase in rat focal and global ischemia

Endothelin-1, a peptide exhibiting extremely potent cerebral vasoactive properties, is elevated in the cerebrospinal fluid after hemorrhagic stroke and implicated in cerebral vasospasm. The purpose of this study was to determine changes in endothelin in ischemic rat brain by assaying endothelin tissue and extracellular levels. Immunoreactive endothelin levels in ischemic brain tissue following permanent or transient focal ischemia produced by middle cerebral artery occlusion was determined. In addition, endothelin levels were assayed in striatal extracellular fluid collected by microdialysis before, during, and after global ischemia produced by two-vessel occlusion combined with hypotension. Twenty-four hours after the onset of permanent middle cerebral artery occlusion, the ischemic cortex level (0.58 +/- 0.27 fmol/mg protein) of immunoreactive endothelin was significantly (p < 0.05) increased, by 100%, over that in the nonischemic cortex (0.29 +/- 0.13 fmol/mg protein). Transient artery occlusion for 80 min with reperfusion for 24 h also resulted in a similar significant (p < 0.05) increase, 78%, in immunoreactive endothelin in the ischemic zone. Global forebrain ischemia significantly (p < 0.05) increased the level of immunoreactive endothelin collected in striatal microdialysis perfusate, from a basal level of 14.6 +/- 6.7 to 26.5 +/- 7.7 and 26.2 +/- 7.4 amol/microliters (i.e. 82 and 79%). These changes reflect the relative picomolar extracellular concentration increases during ischemia and following reperfusion, respectively. This is the first demonstration of elevated levels of endothelin in focal ischemic tissue and in the extracellular fluid in global ischemia and suggests a role of the peptide in ischemic and postischemic derangements of cerebral vascular function and tissue injury.

Astrocytes in Alzheimer's disease express immunoreactivity to the vaso-constrictor endothelin-1

The avidin-biotin peroxidase complex method and a polyclonal antiserum were used to investigate the distribution of endothelin-1-like immunoreactivity of cerebral astrocytes in autopsy cases of Alzheimer's disease compared with controls. The cases of Alzheimer's disease presented numerous astrocytes with intense endothelin-1-like immunoreactivity of the cell body often extending into the finest ramifications of the cell processes. Absorption of the antiserum by the corresponding antigen eliminated this immunostaining. The immunoreactive astrocytes were most consistently present in the subcortical white matter of the cerebral hemispheres and the folia of the cerebellum. The immunoreactive cells were often located in small clusters close to blood vessels. Five of the seven cases showed immunoreactive astrocytes in the molecular layer of the cerebral cortex and three of the seven cases presented regions in which immunoreactive astrocytes appeared to be located in the periphery of plaques. The pons contained small groups of immunoreactive astrocytes in five of the cases. The cerebellum had such cells in six of the seven investigated patients. Immunoreactive astrocytes were very rare in control cases without cerebral disease. Many nerve cells in the cerebral neocortex, hippocampus, cerebellum and pons of Alzheimer cases and controls exhibited endothelin-1-like immunoreactivity. Oligodendrocytes and endothelial cells of blood vessels of controls and Alzheimer cases did not show such immunoreactivity. The expression of endothelin-1-like immunoreactivity in astrocytes of Alzheimer's disease probably reflects an increased content of endothelin-1. If endothelin-1 is released from such astrocytes it may reach smooth muscle cells of the intracerebral blood vessels and disturb micro-circulation since this compound is a most powerful vasoconstrictor peptide.

The structure and specificity of endothelin receptors: their importance in physiology and medicine

In addition to involvement in vascular endothelium-smooth muscle communication, the secretion of and receptors for, endothelins are widely distributed. Two cloned receptor subtypes are G-protein-coupled to several intracellular messengers, predominantly inositol phosphates. From a knowledge of structure-activity relationships and peptide conformations, details of receptor architecture and selective agents, including nonpeptides and antagonists, have been discovered. From the nature of the actions of endothelins, receptor distributions (including CNS) and plasma levels, it is concluded that they are paracrine factors normally involved in long-term cellular regulation, but which may be important in several pathologies, many of which are stress-related.