Proton MRS of early post-natal mouse brain modifications in vivo

NMR provides a non-invasive tool for the phenotypic characterisation of mouse models. The aim of the present study was to apply reliable in vivo MRS techniques for non-invasive investigations of brain development in normal and transgenic mice, by monitoring metabolite concentrations in different brain regions. The conditions of anaesthesia, immobilisation and respiratory monitoring were optimized to carry out in vivo MRS studies in young mice. All the experiments were performed in normal mice, at 9.4 T, applying a point-resolved spectroscopy (PRESS) sequence (TR = 2,000 ms; TE = 130 ms). We obtained reproducible in vivo (1)H NMR spectra of wild-type mouse brains as early as post-natal day 5, which allowed us to follow brain maturation variations from post-natal days 5 to 21. The survival rate of animals was between 66 and 90% at post-natal days 5 and 21, respectively. Developmental changes of metabolite concentrations were measured in three brain regions: the thalamus, a region rich in cell bodies, the olfactory bulb, rich in fibre tracts actively myelinated during brain maturation, and the cerebellum. The voxel size varied from 2 to 8 microL according to the size of the brain structure analysed. The absolute concentrations of the total creatine, taurine, total choline, N-acetylaspartate and of the glutamate/glutamine pool were determined from (1)H NMR spectra obtained in the different brain regions at post-natal day 5, 10, 15 and 21. Variations observed during brain development were in accordance with those previously reported in mice using ex vivo MRS studies, and also in rats and humans in vivo. Possibilities of longitudinal MRS analysis in maturing mice brains provide new perspectives to characterise better the tremendous number of transgenic mutant mice generated with the aim of decrypting the complexity of brain development and neurodegenerative diseases but also to follow the impact of environmental and therapeutic factors.