Identification of brain metabolites by magnetic resonance spectroscopy in MND/ALS

Magnetic resonance spectroscopy (MRS) has provided a novel means of studying the brain biochemistry of motor neurone disease/amyotrophic lateral sclerosis (MND/ALS) patients in vivo in situ. Previous studies have demonstrated changes in the ratios of areas under specific spectral peaks in MND/ALS patients (Jones et al., 1995). However, the significance of such findings cannot be fully elucidated without first ascertaining the biochemical identity of each peak. Each peak in a MRS spectrum corresponds to the resonance of specific protons in a particular chemical environment. Many biochemicals contain similar protons in similar environments so it is possible that a single spectral peak could represent protons from more than one biochemical. In this study of major brain MRS peaks we have demonstrated that peaks are potentially composed of a number of protons from different chemicals. For example, the peak at chemical shift 2.01 ppm, conventionally recognised as the neurotransmitter N-acetyl aspartate, may actually be a result of the protons of the N-acetyl moiety (Frahm et al., 1991). We have consequently shown that other N-acetylated compounds such as N-acetyl glutamate are also capable of producing a peak here, whereas their non-acetylated derivatives are not. We have also shown GABA is capable of producing a peak at chemical shift 3.00 ppm, a peak which is generally assigned to creatine/phosphocreatine. These findings have important implications in the identification of spectral peaks in MRS studies and in the interpretation of spectral differences between MND patients and controls.