The impact of anxiety on chronic musculoskeletal pain and the role of astrocyte activation

Abstract

Anxiety and depression are associated with increased pain responses in chronic pain states. The extent to which anxiety drives chronic pain, or vice versa, remains an important question that has implications for analgesic treatment strategies. Here, the effect of existing anxiety on future osteoarthritis (OA) pain was investigated, and potential mechanisms were studied in an animal model. Pressure pain detection thresholds, anxiety, and depression were assessed in people with (n = 130) or without (n = 100) painful knee OA. Separately, knee pain and anxiety scores were also measured twice over 12 months in 4730 individuals recruited from the general population. A preclinical investigation of a model of OA pain in normo-anxiety Sprague-Dawley (SD) and high-anxiety Wistar Kyoto (WKY) rats assessed underlying neurobiological mechanisms. Higher anxiety, independently from depression, was associated with significantly lower pressure pain detection thresholds at sites local to (P < 0.01) and distant from (P < 0.05) the painful knee in patients with OA. Separately, high anxiety scores predicted increased risk of knee pain onset in 3274 originally pain-free people over the 1-year period (odds ratio = 1.71; 95% confidence interval = 1.25-2.34, P < 0.00083). Similarly, WKY rats developed significantly lower ipsilateral and contralateral hind paw withdrawal thresholds in the monosodium iodoacetate model of OA pain, compared with SD rats (P = 0.0005). Linear regressions revealed that baseline anxiety-like behaviour was predictive of lowered paw withdrawal thresholds in WKY rats, mirroring the human data. This augmented pain phenotype was significantly associated with increased glial fibrillary acidic protein immunofluorescence in pain-associated brain regions, identifying supraspinal astrocyte activation as a significant mechanism underlying anxiety-augmented pain behaviour.

Figure 1.

Association between anxiety status (low = HADS score ≤10; high = HADS score >10) and pain measures in patients with knee OA (n = 130) vs healthy controls (n = 100). Association between anxiety status and self-reported pain (A) in patients with knee OA, and pressure pain detection thresholds (PPTs) at the anterior tibia (B), and the lateral (C) and medial (D) aspects of the index knee, and at the sternum (E) in knee OA vs healthy controls. Data are mean log10 PPT ± SEM. P-values are derived from linear regression where pain scores or PPTs are the outcome and anxiety status (low or high) is the predictor variable, adjusting for age, sex, BMI, and depression status as per the analysis in Table 1. †P < 0.10, *P < 0.05, **P < 0.01, ***P < 0.0001. BMI, body mass index; HADS, Hospital Anxiety and Depression Scale; ICOAP, Intermittent and Constant Osteoarthritis Pain scale; OA, osteoarthritis.

Figure 2.

Augmented MIA-induced pain behaviour, but not knee pathology, in the WKY rat. (A) Intra-articular injection of MIA produced similar alterations in weight-bearing asymmetry in SD and WKY rats, compared with their respective saline-treated controls. However, a significantly greater decrease in ipsilateral and contralateral hind paw withdrawal thresholds (PWTs) was observed in WKY-MIA rats, demonstrating augmented pain behaviour in this strain. Data are mean ± SEM, 2-way analysis of variance with Bonferroni post hoc testing. *P < 0.05, **P < 0.01 ***P < 0.001 vs saline; #P < 0.05, ##P < 0.01 vs SD-MIA. (B) Enhanced MIA-induced pain behaviour in the WKY strain was not accompanied by altered joint pathology, with similar cartilage damage and joint inflammation scores compared to SD-MIA rats (n = 8). Data are medians, Kruskal–Wallis test with Dunn post hoc testing *P < 0.05. MIA, monosodium iodoacetate; SD, Sprague-Dawley.

Figure 3.

Enhanced MIA-induced pain behaviour in the WKY rat is accompanied by activation of spinal astrocytes, but not microglial cells. Representative images of astrocyte activation visualised through GFAP immunofluorescence in the ipsilateral spinal cord dorsal horn in the WKY strain 21 days after intra-articular injection of saline (A) or MIA (B). Quantification revealed MIA-induced astrocyte activation in the ipsilateral spinal cord of both SD and WKY rats when compared with saline-treated controls (C), but changes in GFAP expression were more pronounced in WKY rats, and spinal GFAP expression levels were significantly higher bilaterally in both saline and MIA-treated WKY rats compared with their SD counterparts. GFAP expression (defined as GFAP+ area) was significantly correlated with decreased contralateral PWTs in the WKY strain (D), suggesting a clear association between astrocyte activation and augmented pain behaviour. GFAP+ area was calculated as the number of pixels with intensity >55 in each area. Scale bars = 50 µm. Data are mean ± SEM. ***P < 0.001, ****P < 0.0001 vs saline control, #P < 0.05, ####P < 0.0001 vs SD-MIA, ^^^^P < 0.0001 WKY-MIA vs SD-MIA. By contrast, no strain differences were observed in spinal microglial activation assessed through P-p38 expression (E and F) or morphological analyses (G). Representative spinal cord image demonstrating P-p38 (red) expressed almost exclusively in IBA-1–positive (green) activated microglial cells (E). Quantification revealed a similar bilateral activation in P-p38–expressing spinal microglial cells after MIA injection in both SD and WKY rat strains (F). Morphological analysis also revealed a similar MIA-induced increase in ipsilateral microglial activation in both strains (G). Data are mean ± SEM, analysed using 1-way (microglia) or 2-way (P-p38) analysis of variance with Bonferroni post hoc testing. *P < 0.05, **P < 0.01, ***P < 0.001 vs respective saline-treated controls. (n = 5 rats per group, 6-7 sections per rat). GFAP, glial fibrillary acidic protein; MIA, monosodium iodoacetate; PWT, paw withdrawal threshold; SD, Sprague-Dawley.

Figure 4.

Monosodium iodoacetate–induced pain behaviour in the WKY rat is accompanied by activation of supraspinal astrocytes not seen in normo-anxiety SD rats. Representative images demonstrate increased GFAP immunofluorescence in the right PAG (B) and ACC (F) in the WKY strain compared with saline-treated controls (A and E, n = 5/group). Quantification revealed MIA-induced astrocyte activation bilaterally in the PAG (C), and unilaterally in the ACC (G), of WKY, but not SD rats (see Figure S3, available at http://links.lww.com/PAIN/A696). GFAP expression (defined as GFAP+ area) in the right PAG and right ACC correlated with contralateral PWTs in the WKY strain (D and H), suggesting a clear association between supraspinal astrocyte activation and augmented pain behaviour in the WKY strain. GFAP+ area was calculated as the number of pixels with intensity >55 in each area. Scale bars = 50 µm. Data are mean ± SEM. **P < 0.01, ****P < 0.0001 vs saline control, ##P < 0.01, ####P < 0.0001 vs SD-MIA. ACC, anterior cingulate cortex; GFAP, glial fibrillary acidic protein; MIA, monosodium iodoacetate; PAG, periaqueductal gray; PWT, paw withdrawal threshold; SD, Sprague-Dawley.