Age-related GABA Concentration Changes In Normal Human Brain:a Proton Magnetic Resonance Spectroscopy Study

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the human brain and plays an important role during brain development. GABA is primarily localized in GABAergic neurons, which synthesize GABA from glutamate by glutamic acid decarboxylase (GAD). The GABA/glutamate-glutamine cycle is a critical metabolite shuttle in the human brain:The released GABA is taken up into astrocytes, where it is further metabolized to glutamine. Glutamine released can be taken up by GABAergic neurons, where it is synthesized to glutamate and then new GABA. Numerous reports have been published indicating that the GABA/glutamate-glutamine cycle is affected in a variety of neurological and psychiatric disorders, such as depression, schizophrenia, Alzheimer's disease and multiple sclerosis.Magnetic resonance spectroscopy (MRS) of the human brain is a non-invasive and powerful technique that permits detection and quantification of key metabolites of the pathological and healthy brain tissue in vivo. Depending on the nucleus, there are many forms in MRS, including phosphorus (31P), carbon (13C), and fluorine (19F), but proton MRS (1H MRS) became more prevalent in the clinical fields because of its higher signal sensitivity and better spatial resolution. The important metabolites detectable with1H MRS include N-acetylaspartate (NAA), creatine (Cr), choline (Cho) and mI.Recently, MEGA-PRESS (MEscher-GArwood Point RESolved Spectroscopy) of J-difference edited method is the most widely used technique for measuring GABA. However, to the best of our knowledge, no previous1H-MRS study has explored the normal age-related variations in regional GABA levels using MEGA-PRESS sequence. Therefore, the purpose of this study was to determine age-related effect of GABA concentrations, and clarify if there are regional and gender differences of GABA concentrations in two brain regions:(a) frontal lobe;(b) parietal lobe in normal human using the J-difference edited MRS at3T.There are three parts in the study:Part1. Repeatability of vivo quantification of GABA by MEGA-PRESS sequence of MRSObjective:To apply edited MRS to measure GABA concentrations in the frontal region at3T, and investigate the intra-individual and inter-individual repeatability of GABA simple and relative quantification.Material and methods:Ten healthy volunteers (5men and5women) aged between20and30years were recruited from the local community (mean=25.20±2.66years). All subjects were scanned on a3T scanner (Philips 'Achieva' TX, Best, The Netherlands) and T1-weighted three-dimensional TFE images were used as a localizer. The GABA concentration in the frontal region was measured using a MEGA-PRESS sequence.All subjects were scanned at least twice within a period of24hours. In three subjects, four continuous scans were conducted within a period of3weeks. All the metabolite quantification was performed with jMRUI v.4.0software. For simple quantification, no concentration reference was chosen (GABA+), For relative quantification, the internal concentration reference was chosen Cr or NAA (GABA+/Cr or GABA+/NAA).Results:Edited spectra from the frontal region were successfully collected in all10participants. For both the intra-individual and inter-individual repeatability, coefficient of variation of GABA+/Cr was lower than GABA+/NAA and GABA+Conclusions:The MEGA-PRESS technique of MRS is stable to measure GABA in the frontal region at3T. For both the intra-individual and inter-individual repeatability, quantification with GABA+/Cr was much more reliable than GABA+/NAA and GABA+alone.Part2. Segmentation and re-creation of VOIs in the brainObjective:To segment3D T1-weighted brain images and re-create VOIs (volume of interest) for obtaining the ratio of gray matter (GM) volume to the gray matter+white matter (GM+WM) volumes of VOIs in the brain.Material and methods:Five healthy volunteers (3men and2women) aged between24and30years were recruited from the local community (mean=26.20±2.28years). All subjects were scanned on a3T scanner and T1-weighted three-dimensional TFE images were used as a localizer. The GABA concentration in the parietal region was measured using a MEGA-PRESS sequence.Each pixel in the3D T1-weighted brain images was segmented as GM, WM, or cerebrospinal fluid (CSF) using an automatic brain segmentation program, FAST (FMRIB's automated segmentation tool) in the FSL package. VOIs were co-registered to the anatomical images using the "Re-creation of VOI" Matlab tool. Tissue GM fractions were obtained by calculating the ratio of GM volume to the GM+WM volumes in the VOIs.Results:All post-processing software stably run in the personal computer (Lenovo ThinkPad T410). The VOIs of5subjects were re-created in the brain, which is consistent with the localization of VOI before MRS scanning. The mean of three tissue fractions in the5VOIs are GM (51.6%), WM (39.7%) and CSF (8.7%)。Conclusions:It is stable to segment3D T1-weighted brain images and re-create VOIs using FSL package and the "Re-creation of VOI" Matlab tool.Part3. Age-related GABA concentration changes in normal human brainObjective:To determine age-related effect of GABA concentrations, and clarify if there are regional and gender differences of GABA concentrations in normal human using the MEGA-PRESS sequence of MRS at3T. Material and methods:One hundred healthy volunteers (49men and51women) aged between20and76years were recruited from the local community (men:range24-76, mean=46.1±14.5years; women:range20-70, mean=45.0±14.7years). All subjects were scanned on a3T scanner using T1-weighted three-dimensional TFE sequence. The GABA concentrations in the frontal and parietal region were measured using a MEGA-PRESS sequence.All the metabolite quantification was performed with jMRUI v.4.0software. The brain images was segmented as gray matter (GM), white matter (WM), or cerebrospinal fluid (CSF) using the FSL package. VOIs were co-registered to the anatomical images using the "Re-creation of VOI" Matlab tool.Results:There were significant negative correlations between age and GABA+/Cr in both regions studied (frontal region:r=-0.68, p<0.001; parietal region:r=-0.54, p<0.001) from all the subjects. An analysis of variance showed a significant gender difference in the age-related reduction in GABA+/Cr ratio in the frontal region (Gender×age, p=0.025). The GABA+/Cr ratio is significantly higher in the frontal region than in the parietal region (p=0.036). There was no significant difference between men and women for the GABA+/Cr ratio in the frontal region (p=0.797) and parietal region (p=0.482).The mean gray matter tissue fraction GM/(GM+WM) was57.85%and57.95%in the frontal region and parietal region, and did not change significantly with age (p=0.14, p=0.11), and did not correlate with GABA+/Cr ratios (p=0.30, p=0.32) There were no significant gender-related and region-related difference in the fraction of GM in the VOI (p=0.37, p=0.82, respectively).Conclusions:Brain GABA+/Cr ratios decrease with age in the frontal region and parietal region, with the frontal decline is more rapid in women. GABA+/Cr ratio is significantly higher in our frontal region than our parietal region. Evidence of a measureable decline in GABA is important in considering the neurochemical basis of the cognitive decline that is associated with normal aging.