Background: At excitatory synapses, glutamate released from neurons is taken up by glial cells and converted to glutamine, which is cycled back to neurons. Alterations in this system are believed to play a role in the pathophysiology of bipolar disorder, but they have not been characterized in vivo. We examined the glutamine/glutamate ratio and levels of other metabolites in acute mania and schizophrenia in this exploratory study.
Methods: Data were obtained from 2 x 2 x 2 cm voxels in the anterior cingulate cortex (ACC) and parieto-occipital cortex (POC) using two-dimensional J-resolved proton magnetic resonance spectroscopy at 4 Tesla and analyzed using LCModel. Fifteen bipolar disorder patients with acute mania and 17 schizophrenia patients with acute psychosis were recruited from an inpatient unit; 21 matched healthy control subjects were also studied. Glutamine/glutamate ratio and N-acetylaspartate, creatine, choline, and myo-inositol levels were evaluated in a repeated measures design. Medication effects and relationship to demographic and clinical variables were analyzed.
Results: Glutamine/glutamate ratio was significantly higher in ACC and POC in bipolar disorder, but not schizophrenia, compared with healthy control subjects. N-acetylaspartate was significantly lower in the ACC in schizophrenia. Patients on and off lithium, anticonvulsants, or benzodiazepines had similar glutamine/glutamate ratios.
Conclusions: The elevated glutamine/glutamate ratio is consistent with glutamatergic overactivity and/or defective neuronal-glial coupling in acute mania, although medication effects cannot be ruled out. Abnormalities in glutamatergic neurotransmission and glial cell function in bipolar disorder may represent targets for novel therapeutic interventions.