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

Abstract

Noninvasive 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 (ie, 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 functional magnetic resonance imaging 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 (aIns) (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 aIns and putamen was associated with both pain interference (whole brain analysis, pcorrected < 0.01) and elevated aIns choline (r = -0.37, P = 0.03). In fact, aIns/putamen connectivity statistically mediated the link between aIns choline and pain interference (P < 0.01), highlighting the pathway by which neuroinflammation can impact clinical pain dysfunction. 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.

Fig. 1.

Metabolite levels in anterior insula (aIns). (A) ¹H-MRS voxel placement for aIns (red mask represents overlapping voxels across all subjects). (B) Choline ratio to creatine was increased in FM compared to HC (p=0.02, red circle denotes mean). * indicates significant level of p < 0.05, age corrected (mean + residual), error bars denote SD.

Fig 2.

Seed-based functional connectivity for the ¹H-MRS voxel placed in right anterior insula (aIns) in FM patients. (A) A group map of aIns connectivity in FM patients (corrected p<0.01) demonstrated expected connectivity to other Salience Network regions such as mid-cingulate cortex, as well as posterior insula, visual cortex, basal ganglia, and cerebellum. (B) Correlation between aIns connectivity and pain interference score yielded a cluster in left putamen which was negatively correlated with pain interference (age and pregabalin status corrected).

Fig. 3.

Mediation modeling found that aIns functional connectivity to left putamen mediates the association between aIns choline and pain interference. For mediation modeling, (A) the correlation between aIns choline/tCr and pain interference (total effect) and (B) the correlation between aIns choline/tCr and aIns connectivity to left putamen (path a) were both significant (p<0.05). (C) A significant mediation effect of aIns connectivity to putamen was noted (CME: β=15.78, CI: 2.64 to 35.72, p=0.02; proportions mediated: β=0.55, CI: 2.11 to 1.37, p=0.02). The model corrected for the effects of age and pregabalin status (mean + residual).

Fig. 4.

In the non-human primate neuroinflammation model, post-infection choline level change in precuneus was correlated with (A) GFAP (Spearman r=0.49, p=0.03), and (B) trending for MCP-1 (Spearman r=0.32, p=0.07). (C) Medial parietal cortex (precuneus) voxel placement for the non-human primate model.

Fig. 5.

Fractional amplitude of low frequency fluctuations (fALFF) show4 correlations with choline and HADS depression score in FM patients. fALFF slow4 in aIns was inversely correlated with (A) aIns choline ratio to total creatine and (B) HADS depression scores. (C) fALFF show4 in left putamen was also inversely correlated with HADS depression scores. N.b. data are adjusted for age and pregabalin status.