Poly-phosphoinositide-mediated messengers in focal cerebral ischemia and reperfusion

Abstract

The receptor-mediated poly-phosphoinositide (PI) signalling pathway is known to play an important role in maintaining intracellular calcium homeostasis, which in turn, is critical for mediating neuronal function. In this study, we examined the effects of focal cerebral ischemia induced in rats by temporary occlusion of the middle cerebral artery (MCA) and both common carotid arteries (CCAs) on this signal transduction pathway. Results indicate that several parts of the pathway are altered, both during the early phase of focal cerebral ischemic insult and after recirculation. Cerebral ischemia induced a decrease in levels of phosphatidylinositol 4,5-biphosphate (PIP₂) in the ischemic MCA cortex, due partly to stimulated poly-PI hydrolysis and partly to the depletion of ATP required for resynthesis of this substrate. ATP depletion during ischemia was also attributed to a sustained decrease in inositol 1,4,5-triphosphate (IP₃) levels. On the other hand, the decline in IP₃ 3-kinase activity after 30 min of ischemic insult was not related to ATP depletion. During reperfusion upon prolonged ischemic insult, neither IP₃ level nor IP₃ 3-kinase activity were able to show recovery after reperfusion, despite that ATP levels recovered by 80%. In situ hybridization studies indicated a decrease in mRNA expression of IP₃ receptor but not IP₃ 3-kinase during the initial 4 h of reperfusion after a 45 min ischemic insult. Under this same condition, insulted cortical neurons started to show morphological changes between 4 and 8 h after reperfusion and extensive cell death could be observed by 16 h. Taken together, these results demonstrated early and delayed changes in the poly-PI signalling pathway due to focal cerebral ischemia. These effects are likely to cause impairment in neuronal function and underline the process of cerebral ischemic damage.