Magnetic resonance imaging of neuroinflammation in chronic pain: a role for astrogliosis?

Non-invasive measures of neuroinflammatory processes in humans could substantially aid diagnosis and therapeutic development for many disorders, including chronic pain. Several proton Magnetic Resonance Spectroscopy (H-MRS) metabolites have been linked with glial activity (i.e. choline and myo-inositol) and found to be altered in chronic pain patients, but their role in the neuroinflammatory cascade is not well known. Our multimodal study evaluated resting fMRI connectivity and H-MRS metabolite concentration in insula cortex in 43 patients suffering from fibromyalgia, a chronic centralized pain disorder previously demonstrated to include a neuroinflammatory component, and 16 healthy controls. Patients demonstrated elevated choline (but not myo-inositol) in anterior insula (p=0.03), with greater choline levels linked with worse pain interference (r=0.41, p=0.01). In addition, reduced resting functional connectivity between anterior insula and putamen was associated with both pain interference (whole brain analysis, pcorrected<0.01) and elevated anterior insula choline (r=-0.37, p=0.03). In fact, anterior insula/putamen connectivity statistically mediated the link between anterior insula choline and pain interference (p<0.01), highlighting the pathway by which neuroinflammation can impact clinical pain dysfunction. In order to further elucidate the molecular substrates of the effects observed, we investigated how putative neuroinflammatory H-MRS metabolites are linked with ex-vivo tissue inflammatory markers in a nonhuman primate model of neuroinflammation. Results demonstrated that cortical choline levels were correlated with glial fibrillary acidic protein, a known marker for astrogliosis (Spearman r=0.49, p=0.03). Choline, a putative neuroinflammatory H-MRS assessed metabolite elevated in fibromyalgia and associated with pain interference, may be linked with astrogliosis in these patients.