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Abstract
The ability to directly monitor in vivo brain metabolism in real time in a matter of seconds using the dissolution dynamic nuclear polarization technology holds promise to aid the understanding of brain physiology in health and disease. However, translating the hyperpolarized signal observed in the brain to cerebral metabolic rates is not straightforward, as the observed in vivo signals reflect also the influx of metabolites produced in the body, the cerebral blood volume, and the rate of transport across the blood brain barrier. We introduce a method to study rapid metabolism of hyperpolarized substrates in the viable rat brain slices preparation, an established ex vivo model of the brain. By retrospective evaluation of tissue motion and settling from analysis of the signal of the hyperpolarized [1-13C]pyruvate precursor, the T1s of the metabolites and their rates of production can be determined. The enzymatic rates determined here are in the range of those determined previously with classical biochemical assays and are in agreement with hyperpolarized metabolite relative signal intensities observed in the rodent brain in vivo.
Highlights
Magnetic resonance spectroscopy (MRS) of 13C-labeled substrates is an attractive approach to study brain metabolism as it can non-invasively quantify the flux of isotopic label in living tissues
The development of the dissolution dynamic nuclear polarization methodology by Ardenkjaer-Larsen et al.[10] that can enhance the liquid state 13C NMR signal by four orders of magnitude, reduces the amount of labeled material and measurement times allowing a re-examination of cerebral MRS
Ex vivo brain slices are an ideal model system for studying the metabolism of brain tissue, as these slices preserve the in vivo architecture of the brain but the metabolic rates observed will not be affected by the influx of metabolites from the periphery or the blood brain barrier (BBB) transport[21,22,23,24,25,26,27,28]
Summary
Magnetic resonance spectroscopy (MRS) of 13C-labeled substrates is an attractive approach to study brain metabolism as it can non-invasively quantify the flux of isotopic label in living tissues. In this preparation, the microenvironment of the brain tissue can be controlled and finely tuned, promising a new window to study the factors influencing brain metabolism and the potential effects of exogenous compounds To this end, we modified previously described systems for preparing rat brain slices and maintaining them in the NMR magnet[29,30] to allow rapid injections of hyperpolarized metabolite solutions to the perfused brain slices. In a couple of previous reports of hyperpolarized metabolic studies in perfused tumor tissue slices[27,31], the authors do not report fitting the dynamic spectra to a kinetic model, but instead sum all spectra and consider the ratio of the total product to precursor signal and it is not reported whether this problem was experienced in that preparation as well. Using the experimental system and analytical approach described we have characterized the cerebral metabolic rates of [1-13C]pyruvate in real time
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