Development and Characterization of a Global Brain Ischemia Model in the Mouse -A Genetic and Pharmacological Approach

University dissertation from Division IV

Abstract: Brain ischemia is a major cause of death and morbidity in the industrialized countries. It is mainly caused by stroke, but may also be the consequence of successful cardiopulmonary resuscitation in victims of cardiac arrest, known as transient global ischemia. Although extensive effort in the field of brain research using experimental rodent models, no neuroprotective treatments are at present available. Advances in gene technology, to manipulate the mouse genome by deleting or overexpressing genes has opened new possibilities to reveal novel pathways in the pathophysiology of ischemic brain damage. The elucidation of the selective neuronal damage following global ischemia, using this genetic approach, has been hampered due to the lack of reproducible models in the mouse. The first aim of this thesis was to develop a global brain ischemia model in the mouse. Twelve minutes of ischemia was induced by bilateral common carotid artery occlusion under halothane anesthesia and artificial ventilation controlled by online measurement of exhaled CO2. Body and head temperature was rigorously controlled. Bilateral monitoring of cerebral blood flow by laser Doppler during the experiment was crucial to only include hemispheres with a sufficient dense ischemia. Extensive damage was found in the striatum and marked cell damage was observed in hippocampal CA1 and CA2 and thalamus, whereas the CA3 region, dentate gyrus and cortex suffered less damage. Also, hippocampal CA1 injury developed in a delayed manner. The second aim was to test the applicability of this model in earlier established views on the mechanisms of ischemic cell death. The robust protective effect of intraischemic hypothermia of 33?C was confirmed, and NMDA receptor blockade could if applied directly following ischemia provide protection. Also the endogenous protective effect of adenosine through the adenosine A1 receptor was enlightened by the aggravated effect of blockade of the A1 receptor. However, and possibly even more interesting was the result that did not support and in fact contradicted general views and also some findings in other animals species. Deletion of the gene coding for the adenosine A1 receptor did not aggravate ischemia-induced damage, contrary to pharmacological inhibition of the receptor, indicating that other compensatory neuroprotective mechanism has been activated due to the gene deletion. The notion of a neuroprotective effect of the stress protein heat shock protein 70 (Hsp70) could not be verified in our murine model of global ischemia were Hsp70 was genetically overexpressed. Using a mouse strain deficient in the protease inhibitor cystatin C, we found that the cell death following global and focal ischemia appears to develop differently, or perhaps more correctly stated, different mechanisms are more or less important. The absence of cystatin C aggravated focal ischemic damage while diminishing to the selective neuronal damage induced by global ischemia. In conclusion, this thesis describes a reliable model of global brain ischemia in the mouse and its applicability to different genetically modified mouse strains show that compensatory mechanisms can be induced in knockout animals and that different mechanisms are involved in focal compared with global ischemic brain damage.

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