Evidence for brain glial activation in chronic pain patients

Abstract

Although substantial evidence has established that microglia and astrocytes play a key role in the establishment and maintenance of persistent pain in animal models, the role of glial cells in human pain disorders remains unknown. Here, using the novel technology of integrated positron emission tomography-magnetic resonance imaging and the recently developed radioligand (11)C-PBR28, we show increased brain levels of the translocator protein (TSPO), a marker of glial activation, in patients with chronic low back pain. As the Ala147Thr polymorphism in the TSPO gene affects binding affinity for (11)C-PBR28, nine patient-control pairs were identified from a larger sample of subjects screened and genotyped, and compared in a matched-pairs design, in which each patient was matched to a TSPO polymorphism-, age- and sex-matched control subject (seven Ala/Ala and two Ala/Thr, five males and four females in each group; median age difference: 1 year; age range: 29-63 for patients and 28-65 for controls). Standardized uptake values normalized to whole brain were significantly higher in patients than controls in multiple brain regions, including thalamus and the putative somatosensory representations of the lumbar spine and leg. The thalamic levels of TSPO were negatively correlated with clinical pain and circulating levels of the proinflammatory citokine interleukin-6, suggesting that TSPO expression exerts pain-protective/anti-inflammatory effects in humans, as predicted by animal studies. Given the putative role of activated glia in the establishment and or maintenance of persistent pain, the present findings offer clinical implications that may serve to guide future studies of the pathophysiology and management of a variety of persistent pain conditions.

Keywords: 11C-PBR28; TSPO; chronic pain; glia; neuroinflammation; translocator protein (18kDa).

Figure 1

Evidence for glial activation in the thalamus of chronic LBP patients. (A) Boxplots are presented for the mean ¹¹C-PBR28 SUVRs extracted for all 10 patients with chronic LBP and nine control subjects from the thalamic regions of interest (insert). The P-values refer to matched-pairs analyses (sign test) performed using nine chronic LBP-control matching pairs. The analyses were repeated twice, each time using one of the two patients matching the same control, with statistically significant results in both analyses. *P < 0.05, **P < 0.01. (B) Voxel-wise distribution of thalamic SUVRs, showing that patients with chronic LBP have a substantial number of voxels at values ≥ 1.4 (green arrows), whereas controls have a median voxel count of 0. (C) Individual thalamic SUVRs are presented as axial sections (left), and 3D rendering of values higher than the threshold of 1.4 (right). Each row displays SUVRs for each patient-control matched pair. TSPO polymorphism (Ala/Ala or Ala/Thr) is indicated.

Figure 2

Whole-brain voxel-wise analyses. (A) Median SUVR map from healthy controls (n = 9) and patients with chronic LBP (n = 10) are presented. Matched-pairs tests (nine versus nine) revealed significantly higher TSPO levels in patients, in thalamus, pre- and postcentral gyri, and paracentral lobule (P < 0.05 corrected for multiple comparisons; permutation testing, 10 000 permutations). As two patients were matching the same controls, the analyses were performed first using the patient best matching the control. A second analysis was performed using the other patient, limiting our search to significant clusters from the first analysis, with identical results. (B) Boxplots for each of the four regions demonstrating statistically higher SUVRs in patients are shown for illustrative purposes. postc. = postcentral; g. = gyrus; parac. lob. = postcentral lobule; prec. = precentral.

Figure 3

Anti-inflammatory and anti-nociceptive role of TSPO. SUVRs were negatively associated with pain outcomes (A–C) and blood levels of interleukin-6 (D). The scatterplots show the residuals adjusting for the effect of genotype. MPQ = McGill Pain Questionnaire.