The role of signal transduction in the delayed necrosis of the hippocampal CA1 pyramidal cells following transient ischemia.

Abstract

A short period of cerebral ischemia leads to necrosis of the hippocampal CA1 pyramidal cells. Until recently no mechanisms contributing to this selective vulnerability were known. During the last decade an increasing amount of research has been concentrated on identifying signs of disturbed signal transduction in these neurons after ischemia. The present thesis is a review of these studies with some emphasis on my own contributions to the field. Gerbil and rat models of transient global ischemia are the most frequently employed. In order to produce the selective necrosis the main arteries to the brain are occluded for 5-20 minutes. In the rat it is often also necessary to lower the blood pressure. It takes 2-7 days of recirculation before the CA1 pyramidal cells become necrotic. The studies show that the necrosis can be attenuated or aggravated by drugs acting as inhibitors or enhancers of signal transduction--also if administered shortly after ischemia. The necrosis can be similarly influenced by lesions of excitatory or inhibitory afferent neurons. The protective effect of the lesion however, can be due to the lesion-induced decrease in metabolism. During ischemia there is an increase in the extracellular concentration of several excitatory and inhibitory neurotransmitters as well as in intracellular second messengers. Some of the latter also show an increase during recirculation. In vitro autoradiographic studies of receptor proteins show either unchanged or diffusely distributed downregulation of the ligand binding to the various extra- and intracellular receptor proteins following ischemia and early recirculation. A second decrease is seen in the CA1 at the time of and probably secondary to the necrosis. The IP3 receptor decrease appears during the first minutes of recirculation and lasts for up to 14 days. The protective lesion of the excitatory afferents from CA3 also leads to a decrease in IP3 binding. The changes in receptor regulation are not accompanied by increased postischemic electrophysiological activity in the CA1. In vivo autoradiographic mapping of the regional cerebral metabolic rate of glucose show increased metabolism in the CA1 during the first hour of recirculation compared to the rest of the brain were it is depressed. This relative hypermetabolism is not seen if the CA1 has been deprived of its primary source of excitatory afferents. A later secondary increase seen in the more or less necrotic CA1 pyramidal cell layer is probably due to macrophage activity. In situ hybridization and immunohistochemical studies on the expression of c-fos mRNA and protein respectively has been used to depict neurons with increased activity.(ABSTRACT TRUNCATED AT 400 WORDS)