Introduction

Neurotransmitters play a key role in brain activity. Antioxidants are essential for brain protection against oxidative stress. MRS is a promising tool to evaluate these brain metabolites simultaneously in vivo. However, their distribution patterns in human brain is not fully investigated by MRS. This is probably attributed to low signal-to-noise ratios and low signal separations of these metabolites. This study aimed at revealing the distribution patterns of some neurotransmitters (N–acetylaspartylglutamate (NAAG), glutamate (Glu), and γ-aminobutyric acid (GABA)) and some antioxidants (glutathione (GSH), ascorbic acid (Asc), and taurine (Tau)) in human brain by using MRS at 7 T, where signal-to-noise ratio and signal separation are expected to be improved.

Methods

All experiments were performed on a 7-T whole-body MRI scanner (Magnetom 7T; Siemens Healthcare, Erlangen, Germany) with a 32-channel receiver head coil and a single-channel transmitter coil. Ten healthy volunteer (8 males and 2 females; mean age, 40 years; age range, 23-64 years) were examined. Proton MR spectra were acquired with a stimulated echo acquisition mode (STEAM) sequence (TE=10 ms, TR=7000 ms, 64 averages). 2.0-cm cubic regions of interest (ROIs) were positioned on the posterior cingulate cortex and the white matter (Figure 1). The number of ¹H-MRS measurements was 24 and 30 for posterior cingulate cortex and white matter, respectively. Shimming in the ROIs was performed using the FASTMAP technique. Water-unsuppressed spectra in the ROIs were also acquired. The acquired spectra were analyzed using LCModel version 6.3-1L (LA Systems Inc., Tokyo, Japan). The basis set including Asc was supplied by the software vendor. Eddy-current correction and water-scaling were carried out using the water-unsuppressed spectra. The concentrations of the metabolites were scaled by that of total creatine (creatine (Cr) and phosphocreatine (PCr)), in order to correct for the underestimation of the concentration due to CSF contamination in the ROIs.

Results

Figure 2 shows the scatter plots of the neurotransmitter concentrations and the antioxidant concentrations. The distributions of NAAG, Glu, and GABA between the gray and white matter were completely different from each other. The concentration of NAAG was significantly higher in white matters than in gray matters, that of Glu was significantly higher in gray matters than in white matters, and there was no significant difference in the concentration of GABA between the gray and white matter (Figure 3). On the other hand, the distributions of the antioxidants between the gray and white matter had a relatively similar tendency. The concentrations of Asc and Tau were significantly higher in gray matters than in white matters, although there was no significant difference in the concentration of GSH between the gray and white matter (Figure 3).

Discussion

The concentrations of GABA, GSH, and Asc have been separately measured by using spectral editing techniques, such as MEGA-PRESS. In the present study, the concentrations of these metabolites were simultaneously measured by using STEAM at 7 T. Furthermore, both glutamine (Gln) and Glu concentrations were simultaneously determined with reliable accuracy. The same is true for the N-acetylaspartate (NAA) and NAAG concentration. At 3 T and lower static magnetic field, the separation of Glu from Gln and that of NAAG from NAA are not straightforward. The concentrations of many metabolites can be simultaneously measured by using STEAM at 7 T. Our results showed that the concentration of NAAG was higher in white matters than in gray matters. This is consistent with the results of previous human studies^1,2 and a previous animal study³. However, the difference in the concentration of NAAG between the gray and white matter is still unexplained. The distribution patterns of three neurotransmitters (NAAG, Glu, and GABA) were completely different from each other. The distribution patterns of neurotransmitters may depend on the roles of the neurotransmitters (excitatory or inhibitory neurotransmitter), but the reason of these different distributions is still unclear. The concentrations of two antioxidants (Asc and Tau) were higher in gray matters where there are more cell bodies and the metabolism is more active than in white matters. But GSH was distributed equally between the gray and white matter. Further studies are needed to elucidate the reason why the neurotransmitters and the antioxidants have such distribution patterns.

Conclusion

The concentration of some neurotransmitters and some antioxidants were measured simultaneously in human brain gray and white matter by using ¹H-MRS at 7 T. The distribution patterns of the metabolites were revealed, but the reason of the distributions is still unclear. More detailed measurements of the distributions may leads to better understanding of the reason why the metabolites are distributed in human brain with such patterns.

Acknowledgements

We acknowledge Dr. Hirokazu Kawaguchi (Siemens Healthcare K. K.) for his technical support. This study was performed using the pulse sequence programs and the FASTMAP program provided by University of Minnesota.