Focal ischemic reperfusion stroke model in rats and the role of galanin

University dissertation from Linköping : Linköping University Electronic Press

Abstract: Stroke is the third most common cause for mortality in industrialised countries and amongst the major causes of long- time morbidity. While the mortality due to myocardial infarction has been dramatically reduced during the last 10-15 years, mortality due to stroke remains almost the same, despite the fact that the two share similar basic pathogenic mechanisms including atherosclerosis, hypertension and diabetes. Treatment modalities of reperfusion therapy for acute ischemic stroke, including the use of tissue plasminogen activator for thrombolysis and endovascular treatments, are eff ective if applied early after onset of the first symptoms. The more frequent use of reperfusion therapy, especially in the most common type of stroke aff ecting the middle cerebral artery (MCA), increase the clinical relevance and demand for experimental models of temporary and focal ischemia of the brain. The primary goal of the present work was to develop a model in rats for studying the mechanisms underlying focal and temporary ischemia in brain regions supplied by the MCA.We have modified the intracranial method of occluding the MCA originally described by Tamura et al. in the early 1980es by introducing a microclip to occlude the artery and induce reperfusion under direct visual control through an operating microscope. The goal was to create a mild ischemia model with low morbidity and mortality, optimizing conditions for the animals postoperatively and allowing longterm (weeks) observation periods of high relevance for human stroke. Morbidity and mortality in experimental stroke models are crucial confounders. Change of anesthesia from intraperitoneally administrated chloral hydrate to isoflurane inhalation anesthesia with endotracheal intubation and controlled ventilation reduced mortality markedly from 25% to ~10%. Improved overall skills in anesthesia and surgical techniques further reduced mortality to <3%.Hypothermia reduces brain lesions caused by ischemia not only when administered before and during the ischemic episode, but also afterwards. Several studies have shown that galanin concentrations are increased in response to various types of lesions to the nervous system, and galanin may be amongst the factors supporting neuronal survival and functions. We therefore investigated whether or not hypothermia-induced alterations in galanin concentrations could constitute a part of the established neuroprotective effect of hypothermia in our rat stroke model. Hypothermia induced an overall increase in the concentrations of immunoreactive galanin (p < 0.001). The elevated galanin levels were predominantly found in the non-ischemic control hemisphere. The galanin concentrations were lower in the ischemic hemisphere in both the normo- and hypothermic animals compared to the corresponding contralateral intact hemisphere (p = 0.049). The hypothermia and not the ischemic/reperfusion lesions explained the major part of the observed changes in galanin concentrations. Hypothermia-induced elevation in galanin concentration is therefore not likely to be amongst the major protective mechanisms of hypothermia. Our results support the notion that hypothermia-induced increase in tissue concentrations of galanin in the brain are the result of changes from optimal homeostatic conditions – the hypothermia-induced stress – rather than the ischemic/reperfusion lesion- induced changes in galanin concentrations.Whether the lesion-induced increase in galanin concentrations is primarily a signal that a lesion has occurred, a consequence of the lesion or a mechanism for facilitating neuronal survival is an open question. We therefore infused three different concentrations of galanin intracerebroventricularly in a direct attempt to investigate whether or not galanin has neuroprotective properties in a rat model of MCA occlusion. Furthermore, we infused the GalR2/3 agonist Gal(2-11) (AR-M1896) shown to subserve neuroprotective functions. The lesion was 98% larger seven days after a 60 min transient MCA occlusion and continuous administration of the GalR2/3 agonist Gal(2-11). No differences were found after seven days in the groups treated with galanin in three different concentrations (0.24, 2.4 and 24 nmol/day; p = 0.939, 0.715 and 0.977, respectively). There was also no difference in the size of the ischemic lesion measured after three days in the galanin-treated group (2.4 nmol/d) compared to artificial cerebrospinal fl uid (p = 0.925).The expression of the galanin, GalR1, GalR2 and GalR3 receptor genes were investigated in the female rat brain seven days after a 60-min unilateral occlusion/reperfusion of the MCA. Galanin gene expression showed a 2.5-fold increase and GalR1 a 1.5-fold increase in the locus coeruleus of the ischemic hemisphere compared to the control side, and the GalR1 mRNA levels decreased by 35% in the cortex of the ischemic hemisphere. Thus, stroke-induced forebrain lesion upregulates synthesis of galanin and GalR1 in the locus coeruleus, a noradrenergic cell group projecting to many forebrain areas, including cortex and the hippocampal formation, supporting the notion that galanin may play a role in the response of the central nervous system to injury and have trophic effects.