Involvement of Central Endothelin B Receptors in Focal Cerebral Ischemia

While it has long been known that part of the pathology of cerebral ischemia involves a disruption of the endogenous endothelin (ET) system, very little is known about the involvement of endothelin B (ETB) receptors specifically. The purpose of this study, therefore, was to determine the involvement of central ETB receptors in an experimental model of cerebral ischemia. Male Sprague-Dawley rats were subject to permanent middle cerebral artery occlusion and treated with either vehicle (isotonic saline) at a dose of 1 ml/kg or ETB receptor agonist IRL-1620 at a dose of 5 µg/kg intravenously (i.v.) at 2, 4 and 6 hours post infarct. Endothelin B antagonist BQ788 was also administered at a dose of 1 mg/kg, i.v., 15 minutes prior to treatment with either vehicle or IRL-1620 in order to further classify the role of ETB receptors in cerebral ischemia.;Animals during both the acute and sub-acute phases of cerebral ischemia demonstrated significant neurological and motor function impairment. When treated with IRL-1620, however, animals showed a significant reduction in functional impairment. This reduction in motor deficit coincided with a significantly smaller infarct volume, with animals treated with IRL-1620 presenting with infarcts of 24.47 ± 4.37 mm3 and 54.06 ± 14.12 mm3 at 24 h and 1 w, respectively, as compared to vehicle-treated animals who presented with infarcts of 153.23 ± 32.18 mm3 and 177.06 ± 13.21 mm3 at 24 h and 1 w, respectively. Pretreatment with ETB antagonist BQ788, on the other hand, blocked the effects of IRL-1620, confirming the role of ETB receptor stimulation in the reduction of motor impairment and infarct volume.;In order to determine the mechanism of action responsible for improvement observed following ETB receptor stimulation in cerebral ischemia, oxidative stress parameters were determined. Whereas cerebral ischemia increased levels of lipid peroxidation and decreased antioxidants, treatment with IRL-1620 resulted in lower levels of the lipid peroxidation parameter, malodialdehyde, and higher levels of antioxidants, reduced glutathione and superoxide dismutase, during both the acute and sub-acute phases of cerebral ischemia. These results indicate that the stimulation of ETB receptors following cerebral ischemia provides significant neuroprotection. Additionally, while no alteration in the expression of ETB receptors was noted in the first 24 hours, these receptor levels were significantly increased in the infarcted hemisphere of IRL-1620-treated animals at 1 w following induction of cerebral ischemia, indicating that an up-regulation of these receptors may play a role in neuroprotection.;In confirmation of the observed neuroprotection, we found that neuronal numbers within the cortex, striatum and subventricular zones of animals treated with IRL-1620 were preserved at 24 h post infarct. Moreover, at this time point, there was a reduction in astrocytic conversion and an increase in the number of vessels staining positive for vascular endothelial growth factor (VEGF), indicating that vascular remodeling was occurring. By 1 w following middle cerebral artery occlusion, VEGF+ vessels in the IRL-1620 treatment group numbered 11.33 ± 2.13 versus 4.19 ± 0.79 per 30 µm-thick brain slice in the vehicle-treated group. Furthermore, animals treated with IRL-1620 displayed increased numbers of proliferating cells as well as cells staining positively for nerve growth factor in the cortex, striatum, and subventricular zones of the infarcted brains. Pretreatment with BQ788 blocked these effects. The results indicate that treatment with IRL-1620, administered on the day of infarct, enhances angiogenic and neurogenic remodeling as well as neuroprotection for up to 1 w following experimental cerebral ischemia.;The results of this study clearly demonstrate that ETB receptors are involved in cerebral ischemia. This is the first report indicating that stimulation of ETB receptors with selective ETB agonist IRL-1620 following cerebral ischemia results in neuroprotection. We have also shown, for the first time, that the mechanism behind ETB receptor-induced neuroprotection involves a reduction in oxidative stress parameters and an enhancement of vascular and neuronal growth factors. We therefore speculate that selective stimulation of ETB receptors may be a novel therapeutic target for the treatment of focal ischemic stroke. (Abstract shortened by UMI.).