Biomedical Sciences ETDs

Author

Isamu Aiba

Publication Date

12-1-2012

Abstract

Spreading depolarization (SD) is a slowly propagating wave of depolarization of neurons and glia. Since its original discovery in 1944, SD has been observed in various experimental settings, both in vitro and in vivo. SD has recently been detected in brains of human patients with various injuries, and a contribution of SD to deleterious consequences of acute brain injury is increasingly accepted. iv The present dissertation work focus mainly on the characterization of SD and cellular events associated with it. Main interests of these studies are on underlying mechanisms that convert non-injurious factors into mediators of acute neuronal injury. One of the major findings is an important role of NMDA-type glutamate receptors (NMDAR) in the injurious consequences of SD. Prior to my work, contributions of NMDARs to the initiation and propagation of SD were well acknowledged, however there was little evidence regarding NMDAR contributions to injurious consequences of SD. My studies have identified a brief period of sustained glutamate release and concomitant NMDAR activation following the onset of SD, and demonstrate a significant contribution of this excitatory phase to neuronal injury in the setting of metabolic compromise. Selective block of this later excitatory phase was sufficient to prevent Ca2+ deregulation and membrane damage in metabolically compromised tissue. These glutamate-dependent injury processes suggest a novel link between SD and previous studies of acute excitotoxic neuronal injury. Another important focus of the present dissertation is Zn2+ physiology and its contribution to SD. In the mammalian brain, Zn2+ is enriched in synaptic vesicles of many glutamergic neurons and is released as a neuromodulator. Importantly Zn2+ is well known as a potent antagonist of NMDARs. Based on the significant contribution of NMDAR activation to SDs, Zn2+ could potentially play a protective role in brain injury by reducing the number of SD occurring in the injured brain. However Zn2+ is also often considered as a neurotoxic factor. In v fact, it is suggested that synaptically released Zn2+ can translocate in postsynaptic neurons and contribute to neuronal death. My studies have evaluated roles of synaptic Zn2+ release in regulation of SD as well as the contribution to potentially toxic intracellular Zn2+ accumulation as a consequence of transmembrane Zn2+ flux. My studies revealed that extracellular actions of Zn2+ indeed inhibit SD initiation and propagation in healthy tissue. However the inhibitory effects of Zn2+ were highly sensitive to oxygen concentration, such that in severe hypoxia or anoxic conditions, Zn2+ antagonisms of NMDARs and SD were profoundly reduced. Studies on potential mechanism revealed that this was partly due to redox potential shit and resultant modification of the receptors. Interestingly, such redox modulation mechanism could partly contribute to ineffectiveness of other NMDAR antagonists. Thus loss of oxygen supply in metabolic compromised tissue could be important modulator of the effects of Zn2+ on SD. Another line of study examined potential use of intracellular dialysis for detection of Zn2+ influx to postsynaptic neurons. One of the confounds for studying intracellular Zn2+ responses was how to distinguish contributions from transmembrane Zn2+ influx and Zn2+ release from intracellular sites. A Key finding of this part of studies is that intracellular dialysis significantly disrupt intracellular Zn2+ homeostasis mechanisms and enabled to selective detect transmembrane Zn2+ influx over intracellular Zn2+ release. Development of the new detection method significantly improved understanding of intracellular Zn2+ signal sources in SD and related events. vi In summary, these studies show that the metabolic state of tissue can significantly influence regulation of NMDAR activation and synaptic Zn2+ release. These findings should be of value for developing novel interventions to minimize the deleterious consequences of SD in acute brain injury.

Keywords

Neuron, Ischemia, Calcium, zinc, brain injury

Document Type

Dissertation

Language

English

Degree Name

Biomedical Sciences

Level of Degree

Doctoral

Department Name

Biomedical Sciences Graduate Program

First Advisor

Shuttleworth, C. William

First Committee Member (Chair)

Resta, Tom

Second Committee Member

Partridge, Donald

Third Committee Member

Varenzuela, Fernando

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