Development of in vivo NMR techniques for studying the impact of Ocean acidification and warming on the neurophysiology of Antartic fishes

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The most dramatic effects of ocean acidification and warming are expected in Polar regions, i.e., the impact of elevated temperatures and CO2 concentrations will be highest on Polar organisms (Antarctic and Arctic species). It was demonstrated that tropical fishes show neurological impairments under elevated CO2 concentrations expected from climate change scenarios for 2100 already, although fishes have a decent acid-base regulation. The impairments involved different neurological dysfunctions from sensory to behavioural changes and were supposed to be due to the influence of CO2 on the neurotransmitter function of GABA. Global warming might amplify these disturbances. Thus, in Antarctic eelpout it was shown that acid-base regulation of intracellular pH became impaired at temperatures above 4°C.Polar fishes can show similar or more severe neurological changes under conditions of ocean acidification and warming. However, the mechanisms responsible for the observed disturbances of marine fishes are still not understood. In vivo NMR techniques, in particular localised 31P and 1H NMR spectroscopy were extensively used to characterise the metabolic status or metabolic changes in the brain of animals and humans, including observations on changes of neurotransmitters as well as acid-base regulation. However, these measurements are limited by the inherently low sensitivity, restricting the spatial and temporal resolution.In the current research project, the prolongation of which we are applying for, we exploit the novel approach of chemical shift saturation transfer (CEST) for pH mapping and indirect metabolic imaging. CEST magnetic resonance imaging (MRI) allows measurements with high spatial and temporal resolution because the saturated longitudinal magnetisation of exchangeable protons of metabolites is transferred to water and accumulated. We have shown that CEST MRI is feasible down to polar temperatures and that the specificity of CEST MRI can be adjusted by appropriate radiofrequency (RF) pre-saturation, allowing both inverse imaging of important metabolites and pH mapping. Besides CEST MRI based on glutamate (GluCEST), we plan to develop CEST MRI based on taurine (TauCEST). TauCEST will be a novel CEST option, which seems to be applicable at low temperatures. Using the new 9.4T NMR imaging system, which is expected to be operational in January, 2016, at the AWI, will allow in vivo measurements with improved quality. Both CEST MRI and localized 1H and 31P MRS will be used for studying metabolic changes and acid-base regulation in the brain of Antarctic fishes under expected ocean acidification and warming scenarios. These studies should improve our understanding of the mechanisms underlying the observed neurological changes in Polar fishes.