Entwicklung von in vivo NMR-Techniken zur Untersuchung der Auswirkung von Ozeanversauerung und -erwärmung auf die Neurophysiologie Antarktischer Fische

Abstract

Ocean acidification and warming as a consequence of climate change affect marine organisms. Thus, for example, neurological changes could be observed in polar cod, which were induced by ocean acidification and warming. As a potential cause, a connection between the neurological impairments and changes in intracellular pH (pHi) has been postulated as it was described for animal models in preclinical research. Therefore, the non-invasive determination of pHi with high spatial and temporal resolution is of great interest. A promising tool for non-invasive pH measurements is the CEST contrast (Chemical Exchange Saturation Transfer), which enables the indirect detection of endogenous or exogenous molecules with exchangeable protons using changes in the NMR signal of the water pool. The CEST effect mainly depends on the concentration of a metabolite and the exchange rate between this metabolite and water, which in turn is influenced by physical and physiological parameters. These properties allow for CEST to be used for in vivo determination of changes in metabolite concentration and pH. The present thesis describes the development and adaption of the measurement method CEST MRI (Magnetic Resonance Imaging) and its application for determining changes in pHi with a high temporal and spatial resolution in the brain of marine fish. CEST from glutamate to water (GluCEST) was experimentally investigated in a broad temperature and pH range. The applicability of GluCEST to determine relative changes in pH even at low temperatures depends on glutamate concentration and on the parameters used in the experiments. Localized 1H NMR spectroscopy in combination with the application of quantification algorithms is an established method for the quantification of metabolite concentrations. In most cases, these algorithms use model functions based on chemical shifts at 37AdegreeC. Therefore, the temperature dependency of the chemical shifts for important metabolites has been examined in order to avoid systematic quantification errors caused by the use of incorrect prior knowledge for spectra recorded at temperatures different from 37AdegreeC. The exchange rates show an exponential behaviour as a function of temperature, thus offering a completely new picture of metabolites whose exchange regime offers the possibility to determine a CEST effect at temperatures near the freezing point. In this context, the taurine based TauCEST effect was investigated in a broad pH and temperature range. Investigations showed that the specificity of TauCEST offers the possibility to use this method as a suitable tool to detect pHi changes in the brain of polar cod. The first in vivo application of the TauCEST effect in the brain of polar cod showed an increase of about 1.5-3% under different CO2 concentrations. Localized 1H NMR spectroscopy, which was successfully adapted for the in vivo application in the brain of marine fish at 9.4 T, indicated no significant changes in concentration for those metabolites that mainly determine the CEST effect. Thus, the in vivo application of TauCEST proves to be an adequate method for determining non-invasively relative changes in pHi with high spatial and temporal resolution in the brain of polar cod during exposure to elevated CO2 concentrations.