While it remains impossible to directly measure neurotransmitters and metabolites in the living human brain, radiologists can come close by using magnetic resonance spectroscopy. MRS uses the same equipment as magnetic resonance imaging but allows scientists to detect specific chemicals. In the June 24 JAMA Neurology, researchers led by first author Bradley Foerster and senior author Eva Feldman of the University of Michigan in Ann Arbor report that MRS reveals lower-than-normal levels of the neurotransmitter GABA in the motor cortex of people with amyotrophic lateral sclerosis. Those patients have elevated brain glutamate as well. This represents the first time GABA and glutamate in the brain have been measured simultaneously by MRS. The data support the theory that loss of inhibitory GABAergic activity overstimulates excitatory neurons, leading to glutamate toxicity in ALS.
Magnetic resonance spectroscopy uses the same principle as nuclear magnetic resonance spectroscopy of small molecules. With the help of a much stronger magnetic field it obtains chemical spectra from whole tissues and organs. In an image akin to an MRI, MRS quantifies the most abundant metabolites in the brain. Researchers have used MRS to detect N-acetylaspartate, choline, creatine, myoinositol, and glutamate/glutamine (Glx) in the brain. GABA levels are harder to infer because their spectral peaks appear smaller than, but in the same position as, those of creatine. To measure GABA, the researchers used a fairly standard subtraction method. They altered the magnetic field provided by the MR machine to specifically change GABA’s output signal. By subtracting the GABA-altered spectrogram from the regular one, they could quantify creatine signals and, by removing them, measure GABA levels (Mescher et al., 1998; Stagg et al., 2009). “The technique they used … is very solid,” said Erik Pioro of the Cleveland Clinic Neurological Institute in Ohio, who was not involved with the study. The researchers used a three-Tesla magnet; a seven-Tesla would work even better, Pioro said, but is only available at specialized centers.
Overall, the GABA loss and glutamate gain appeared in 29 people with ALS compared to 30 healthy controls. This confirms the group’s earlier finding in a subset of these patients (Foerster et al., 2012). Researchers believe glutamate excitotoxicity is one mechanism underlying ALS (Rao and Weiss, 2004). “Perhaps GABA [reduction] could be a predisposing factor to glutamate excitotoxicity,” Foerster said. However, he cautioned that a direct link between low GABA and high Glx remains unproven. “This is the first demonstration in patients with ALS that substantiates the notion that there is a relative imbalance between an inhibitory system and an excitatory one,” Pioro said.
In fact, not all participants with ALS showed a rise in Glx. The 15 people taking riluzole daily showed Glx levels more like those of controls. Riluzole is the only drug approved for ALS, and it is believed to attenuate glutamate signaling (Cifra et al., 2011). Foerster suggested it would be interesting to follow patients longitudinally to observe if the treatment reduced the glutamate levels.
Could neurotransmitter MRS help identify people with ALS or predict their disease course? “I do think that the time has come to re-evaluate MRS, given the technical refinements and higher field strengths than we had available 10 years ago,” commented Nigel Leigh of King’s College London, U.K., in an e-mail to Alzforum. “There may still be significant sensitivity and specificity issues relating to the MRS peaks for GABA and glutamate, so it is difficult to see how far this approach can get us in monitoring therapy (for example) but is worth trying again.”
The fact that most clinics lack access to a three- or seven-Tesla magnet also weakens the potential for MRS at this time, Pioro said. But in research settings, the technique might be applicable to study neuron inhibition and excitation in other neurodegenerative diseases, as well as stroke, he suggested. For example, MRS has already been applied to GABA and glutamate in an Alzheimer’s rat model (Nilsen et al., 2012).
Bill Klunk of the University of Pittsburgh Medical Center in Pennsylvania, who was not involved with the study, noted that there was too much overlap between the GABA levels of patients and controls for the parameter to be of much clinical use. He added that MRS only identifies neurotransmitters and metabolites but does not tell radiologists whether those compounds exist in neurons or glia, or whether they are acting as neurotransmitters or merely participating in cellular metabolism, as glutamate does. “The [MRS] field has not really produced that big an impact,” said Klunk, who worked with MRS for AD in the past but said he was disinclined to continue.
Foerster acknowledged these limitations. “I do not think spectroscopy has the key for diagnosis,” he said. “I do not think any individual imaging metric is going to be a sufficient biomarker in and of itself.” Perhaps combined with diffusion tensor imaging and other standard MRI, and even other measures such as serum or cerebrospinal fluid metabolites, MRS could be one among a panel of biomarkers that assist in diagnosis, prognosis, or stratifying clinical trial participants, he said.
Foerster BR, Pomper MG, Callaghan BC, Petrou M, Edden RA, Mohamed MA, Welsh RC, Carlos RC, Barker PB, Feldman EL. An imbalance between excitatory and inhibitory neurotransmitters in amyotrophic lateral sclerosis revealed by use of 3-T proton magnetic resonance spectroscopy. JAMA Neurol. 2013 Jun 24:1-8. Abstract
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