Biomedical Sciences ETDs

Publication Date

5-1-2011

Abstract

Spreading depression (SD) is a severe depolarization of both neurons and astrocytes that can propagate throughout CNS tissue. SD has long been associated with migraine, and recent work has strongly suggested that closely related events are likely to be involved in pathophysiological conditions where they provide a large additional metabolic burden to injured tissues. The major goal of this dissertation was to test the overall hypothesis that manipulation of astrocyte metabolic capacity can modify the initiation and propagation of deleterious spreading depression (SD)-like events. If this were the case, then it may support targeting these cells in a range of pathological conditions where SD-like events are known to occur. Work in this dissertation utilized acutely prepared hippocampal brain slice preparations from adult mice to study mechanisms regulating SD initiation and propagation. For the majority of studies, slices were prepared from Naive mice to study functions of normal astrocytes. One set of studies utilized preparations in which astrocytes were activated by bilateral lenti-viral injections of ciliary neurotrophic factor (CNTF) into the hippocampus. SD was generated under either normoxic conditions (using localized microinjections of KCl or block of Na+/K+-ATPase activity) or in a model of ischemia (using deprivation of oxygen and glucose). Initiation and propagation of SD and related events was monitored using a combination of extracellular voltage recordings and optical imaging methods. Simultaneous recordings of NAD(P)H autofluorescence and evoked excitatory potentials (fEPSPs) were also used to assess changes in mitochondrial redox potential and neuronal viability. Initial studies sought to determine the contribution of astrocyte metabolism on the initiation and propagation of SD and related events. Previous work had shown that selective inhibition of astrocyte oxidative metabolism increased SD propagation rate, but little was known about roles of astrocyte glycogen. Pharmacologically preventing glycogen availability significantly increased SD propagation rate and increasing astrocyte glycogen stores resulted in either compete block of SD initiation or, in preparations where SD still occurred, a significant decrease in propagation rate (Chapter 2). Other studies confirmed the effectiveness of inhibition of astrocyte oxidative metabolism, but showed that targeting glycogen stores may prove more efficacious therapeutically. Previous work has shown that localized astrocyte activation can be achieved in vivo by lenti-viral injections of ciliary neurotrophic factor (CNTF) in rat striatum and lead to neuroprotection. My studies sought to establish a similar model of astrocyte activation suitable for subsequent electrophysiological studies in the well-characterized hippocampal slice preparation. I then used this model to assess the effects of astrocyte activation on SD propagation and initiation. CNTF-infection resulted in sustained astrocyte activation and preparations from these tissues showed profound resistance to SD initiation and propagation under both normoxic and ischemic conditions (Chapter 3). This model may be suitable for many studies of astrocyte activation and these results suggest that targeting astrocyte activation may be useful therapeutically to limit SD. A final set of studies was conducted to test the hypothesis that an additional metabolic burden in the context of SD, even under normoxic conditions, can lead to irrecoverable neuronal damage. Nitric oxide (NO) has been proposed as a possible treatment to reduce the number of SD-like events following subarachnoid hemorrhage. My studies investigated the effects of manipulation of NO concentrations on the progression and consequences of SD under normoxic conditions. Exogenous NO led to inhibition of mitochondrial respiration, which has been well described previously, and converted recoverable SD events into irreversible depolarizations leading to neuronal damage. These studies suggest that if NO is to be used therapeutically to limit SD, care must be taken to ensure concentrations do not inhibit oxidative metabolism (Chapter 4). Overall the studies in this dissertation suggest that strategies which target the maintenance of brain metabolism and normal astrocyte function, including the clearance of extracellular glutamate and K+, may reduce the incidence of SD and may also limit neuronal injury in pathologic conditions where SD-like events still occur. This work may provide a useful starting point for future studies to determine specific therapeutic interventions including in tissue where astrocyte phenotype may be modified by pathologic processes.

Keywords

Hippocampal slice, Spreading depression, Astrocytes, Metabolism, Ischemia

Sponsors

Supported by National Institute of Health grants: NS 051288 and NHLBI T32 HL007736

Document Type

Dissertation

Language

English

Degree Name

Biomedical Sciences

Level of Degree

Doctoral

Department Name

Biomedical Sciences Graduate Program

First Committee Member (Chair)

Partridge, L. Donald

Second Committee Member

Cunningham, Lee Anna

Third Committee Member

Bizzozero, Oscar

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