¹³C Magnetic Resonance Spectroscopy and Glutamate Metabolism in Mood Disorders: Current Challenges, Potential Opportunities

The importance of the amino acid neurotransmitter glutamate to the normal functioning of the human brain cannot be overstated. Approximately 85% of all neocortical synapses are glutamatergic (1). As the primary excitatory neurotransmitter in the CNS, glutamate is responsible for essentially all action potentials and brain activity. In addition, glutamate receptors can detect coincident presynaptic and postsynaptic activations and strengthen synapses where they occur, thereby playing an important role in synaptic plasticity and learning (2).

Not surprisingly, then, abundant evidence implicates aberrations in glutamate activity in the pathophysiology of major psychiatric illnesses, including mood disorders. Much of these data come from proton magnetic resonance spectroscopy (¹H MRS) studies, which permit the in vivo quantification of glutamate in the brain. Meta-analyses of studies examining glutamate in persons with major depressive disorder and bipolar disorder suggest that glutamate levels in limbic brain areas are decreased in major depression and increased in bipolar disorder (3, 4). It is humbling for psychiatrists to contemplate that the medications that form the bedrock of our therapeutic armamentarium—serotonin-, norepinephrine-, and dopamine-altering agents—function by indirectly modulating the physiology of glutamatergic neurons. Emerging evidence suggests that directly altering the glutamatergic system with medications such as ketamine may produce much more rapid and robust antidepressant effects (5).

However, ¹H MRS has significant limitations. Its signal-to-noise ratio is poor, and its spatial resolution is low. At magnetic field strengths below 4 T, it cannot reliably distinguish between the related molecules glutamate and glutamine (6). Moreover, it is only able to measure static levels of metabolites, and not their rate of creation or breakdown. Finally, ¹H MRS cannot localize the source of the glutamate signal—that is, which cell type it resides in (neuron versus astrocyte) or where in the cell it is localized (cytosol, synaptic vesicles, synaptic cleft). This is problematic because glutamate functioning is complex, and abnormalities at multiple steps might contribute to mood disorders. After glutamate is released from presynaptic neurons into the synaptic cleft, it is taken up primarily into astrocytes, where it is converted into glutamine. Glutamine is transported back to the presynaptic neuron, where it is recycled into glutamate, or to GABA-ergic neurons, where it is converted into GABA. The flow of glutamate from the presynaptic neuron to astrocytes and back is referred to as the glutamate-glutamine cycle, and when it is converted to GABA, as the GABA-glutamine cycle.

In this issue, Abdallah et al. (7) report on the first study using carbon-13 (¹³C) MRS in patients with mood disorders to explore abnormalities in glutamate cycling and neuroenergetics in major depression. Carbon-12, the carbon isotope most abundant in the human body (comprising 99% of brain carbon), is invisible to spectroscopy. However, the minor isotope ¹³C (<1% of brain carbon) is spectroscopically detectable. ¹³C from infused ¹³C-glucose is incorporated into specific molecular positions in glutamate, glutamine, and GABA, creating unique spectroscopic peaks that enable them to be reliably quantified. Moreover, by measuring and mathematically modeling the rate of incorporation of ¹³C into each molecule, information about the glutamate-glutamine and GABA-glutamine cycles can be obtained. In addition, ¹³C MRS enables measurement of the neuronal tricarboxylic acid cycle, allowing information about the neuronal energy production to be acquired (8). The ¹³C MRS method is highly technically complex and requires expertise, specialized hardware, and software present at only a handful of centers in the world.

Abdallah and colleagues’ study included 23 medication-free patients with major depressive disorder and 17 healthy subjects. All underwent standard ¹H MRS on a 4-T scanner using a voxel placed midline over the occipital cortex. One week later, after an infusion of [1-¹³C]-glucose, ¹³C MRS data were collected from a voxel placed midline at the parietal-occipital junction. The main finding was a 26% reduction in neuronal mitochondrial energy production in patients with major depression relative to healthy comparison subjects. No group differences were observed in glutamate-glutamine cycling or GABA-glutamine cycling. In contrast to most previous ¹H MRS studies, no differences in either glutamate or GABA levels were found between patients and healthy comparison subjects.

In principle, ¹³C MRS can greatly expand our understanding of how aberrations in glutamate cycling and neuroenergetics contribute to mood disorders. It has already been used with some success to demonstrate abnormalities in glutamate-glutamine cycling and neuronal energetics in patients with hepatic encephalopathy (9) and epilepsy (10) as well as in healthy aging (11), and it is now beginning to be applied to psychiatric disorders. Abdallah and colleagues’ finding of reduced neuronal energy production in depressed patients is in keeping with previous positron emission tomography studies showing brain hypometabolism in major depression. However, their unexpected negative findings raise important issues about whether this method, as currently employed, is suitable to the study of depression. First, it is unclear whether the occipital lobe is a biologically relevant region in which to measure glutamate in major depression. For technical and safety reasons, the occipital cortex has been the main target of ¹³C-MRS studies in humans. However, while a number of studies have reported reduced occipital GABA levels in major depression, glutamate abnormalities in this region are less consistent (12, 13). Indeed, ¹H MRS studies suggest that glutamatergic abnormalities may be most pronounced in prefrontal limbic brain regions (11, 14). A second issue has to do with the subtypes of major depression that were studied. Sanacora et al. previously reported (12) reduced occipital GABA and increased glutamate in a major depression sample in which only 20% of patients had atypical depression. In contrast, in the Abdallah et al. study, no abnormalities in glutamate or GABA levels or cycling were found in a sample in which 65% of the patients had atypical depression. Thus, it may be that glutamate-related abnormalities are more pronounced in certain subtypes of depression, such as melancholic. Adapting ¹³C-MRS to target limbic brain areas, and determining which subgroups of depression are most fruitful to study, may provide future directions for this potentially important method of investigating neurotransmitter cycling profiles to elucidate the molecular underpinnings of depression, thereby guiding new diagnostic and treatment approaches.