Longitudinal 1H MRS of rat forebrain from infancy to adulthood reveals adolescence as a distinctive phase of neurometabolite development

Abstract

This study represents the first longitudinal, within-subject (1) H MRS investigation of the developing rat brain spanning infancy, adolescence and early adulthood. We obtained neurometabolite profiles from a voxel located in a central location of the forebrain, centered on the striatum, with smaller contributions for the cortex, thalamus and hypothalamus, on postnatal days 7, 35 and 60. Water-scaled metabolite signals were corrected for T1 effects and quantified using the automated processing software LCModel, yielding molal concentrations. Our findings indicate age-related concentration changes in N-acetylaspartate + N-acetylaspartylglutamate, myo-inositol, glutamate + glutamine, taurine, creatine + phosphocreatine and glycerophosphocholine + phosphocholine. Using a repeated measures design and analysis, we identified significant neurodevelopment changes across all three developmental ages and identified adolescence as a distinctive phase in normative neurometabolic brain development. Between postnatal days 35 and 60, changes were observed in the concentrations of N-acetylaspartate + N-acetylaspartylglutamate, glutamate + glutamine and glycerophosphocholine + phosphocholine. Our data replicate past studies of early neurometabolite development and, for the first time, link maturational profiles in the same subjects across infancy, adolescence and adulthood.

Figure 1. Representative voxel placement for each age for scanning

A 27, 55, or 64 mm³ voxel was placed in the forebrain of 7, 35, and 60 day old rats, respectively, centered on the left striatum, below the cortex and encompassing surrounding tissues using T2 weighted images in the axial and sagittal planes.

Figure 2. Representative metabolite spectra at three measured ages

(A) Single voxel MR spectra obtained from rats on postnatal day (P) 7, 35, and 60. The source spectrum is overlaid with the fitted LCModel result (red) fit from 4.2–0.2 ppm. (B) The contribution of individual metabolites, as well as lipids and macromolecules, to the fitted LCModel result in a representative P60 spectrum, shown with baseline subtraction.

Figure 3. Neurochemical concentrations on postnatal days 7, 35, and 60

Plots show the individual values from each subject for each measured metabolite at each age. Bars show the mean at each age. Significance determined by one-way repeated measures ANOVA, followed by post hoc Student Neuman-Keuls t-tests (p < 0.05) as follows, * statistically different from P7, † statistically different from P35. Measured metabolites were myo-inositol (Ins), taurine (Tau), glycerophosphocholine + phosphocholine (GPC+PCh), N-acetylaspartate + N-acetylaspartylglutamate (NAA+NAAG), creatine + phosphocreatine (Cr+PCr), and glutamate + glutamine (Glu+Gln). N=9 for all metabolites except Ins, N=8.

Figure 4. Principal Components Analysis (PCA) of developmental metabolite profiles in P7, P35, and P60 rat brain

(A) Score plot demonstrating age-based clustering. Dashed lines indicate 95% confidence intervals. (B) Loading values of measured metabolites.