Photobiomodulation Therapy Partially Restores Cartilage Integrity and Reduces Chronic Pain Behavior in a Rat Model of Osteoarthritis: Involvement of Spinal Glial Modulation

Abstract

Objective: Chronic pain associated with osteoarthritis (OA) often leads to reduced function and engagement in activities of daily living. Current pharmacological treatments remain relatively ineffective. This study investigated the efficacy of photobiomodulation therapy (PBMT) on cartilage integrity and central pain biomarkers in adult male Wistar rats.

Design: We evaluated the cartilage degradation and spinal cord sensitization using the monoiodoacetate (MIA) model of OA following 2 weeks of delayed PBMT treatment (i.e., 15 days post-MIA). Multiple behavioral tests and knee joint histology were used to assess deficits related to OA. Immunohistochemistry was performed to assess chronic pain sensitization in spinal cord dorsal horn regions. Furthermore, we analyzed the principal components related to pain-like behavior and cartilage integrity.

Results: MIA induced chronic pain-like behavior with respective cartilage degradation. PBMT had no effects on overall locomotor activity, but positive effects on weight support (P = 0.001; effect size [ES] = 1.01) and mechanical allodynia (P = 0.032; ES = 0.51). Greater optical densitometry of PBMT-treated cartilage was evident in superficial layers (P = 0.020; ES = 1.34), likely reflecting the increase of proteoglycan and chondrocyte contents. In addition, PBMT effects were associated to decreased contribution of spinal glial cells to pain-like behavior (P = 0.001; ES = 0.38).

Conclusion: PBMT during the chronic phase of MIA-induced OA promoted cartilage recovery and reduced the progression or maintenance of spinal cord sensitization. Our data suggest a potential role of PBMT in reducing cartilage degradation and long-term central sensitization associated with chronic OA.

Keywords: monoiodoacetate; osteoarthritis; photobiomodulation therapy; rats.

Figure 1.

Monoiodoacetate (MIA) induced pain-like behavior, while photobiomodulation therapy (PBMT) reduced most of this behavioral deficit. (A) Schematics depicting PBMT application sites. (B) Experiment timeline. (C) Open field behavioral outcomes showed a lesion effect, but absence of a treatment effect. (D, left panel) Illustration of the weight bearing apparatus used to measure unilateral weight support by the affected (MIA) and nonaffected (Saline) hindlimbs. (D, right panel) Weight bearing behavioral deficits were evident following MIA injections and persisted over time. PBMT enhanced behavioral recovery if compared to placebo at d22 and d29 (note that high values indicate less weight support by the affected hindlimb). (E) Mechanical allodynia behavioral deficits showed a delayed onset and peaked on d8-15 post-MIA injection, with moderate PBMT effects. Paw withdrawal threshold of saline injected knees showed variable response throughout the experiment. Shaded rectangles in D and E represent the PBMT treatment period. n_Control = 9, n_PBMT = 10; 2-way analysis of variance with Sidak post hoc, P < 0.05. Data are mean ± SEM (mean ± standard error of the mean).

Figure 2.

Osteoarthritis (OA) induced by monoiodoacetate (MIA) was characterized by cartilage degradation that was partially rescued following 14 days of photobiomodulation therapy (PBMT). (A) The knee cartilage displayed pronounced loss of toluidine blue staining following 30 days of MIA injection. (B) Whereas saline-injected knees displayed symmetrical toluidine blue staining throughout superficial and deep cartilage layers, MIA-injected knees showed remarkable asymmetry alongside with subcondral bone erosions and tidemark violations (arrow). (C) Optical densitometry (OD) of the joint cartilage was performed in the medial, central and lateral portions of the medial femoral condyle (MFC) and medial tibial plateau (MTP). (D, E). Reduction of the superficial cartilage OD and increase of the basal cartilage OD were evident following MIA injection. (F) The net effect of PBMT was a combination of increased superficial and decreased basal cartilage OD (G) PBMT promoted greater cartilage symmetry between superficial and deep layers, which was often accompanied by increased nuclei/cytoplasm (arrowheads). n_Control = 7, n_PBMT = 8; 2-way analysis of variance with Sidak post hoc, P < 0.05. Data are mean ± SEM (mean ± standard error of the mean).

Figure 3.

Cartilage optical densitometry (OD) was correlated with pain-like behavior, and photobiomodulation therapy (PBMT)–promoted pain relief also correlated with cartilage OD (A) The correlation map analyses including all animals showed consistent correlations between cartilage OD and weight bearing behavioral deficits from d4 to d22. (B) The correlation map analyses including only PBMT animals indicated that PBMT was potentially involved in restoring superficial cartilage OD and reducing pain-like behavioral deficits following the first treatment week. (C) A principal component (PC) explained most of the variance in the PBMT dataset, and this PC related the superficial cartilage OD to, mostly, weight bearing-associated deficits. n_Control = 7, n_PBMT = 8. Pearson correlations and principal component analysis; white hashed rectangles are P < 0.05.

Figure 4.

Photobiomodulation therapy (PBMT) was effective in reducing reactive astrogliosis in the spinal cord dorsal horn. (A, B) MIA-injections resulted in greater calcitonin gene-related peptide (CGRP) transmission and modulation of pain in the spinal cord dorsal horn, but PBMT was not effective in controlling this increase. (C, D) PBMT reduced reactive astrogliosis in the spinal cord dorsal horn. CGRP: n_Control = 6, n_PBMT = 4; glial fibrillary acidic protein (GFAP): n_Control = 4 (560 cells), n_PBMT = 3 (180 cells); 2-way analysis of variance with Sidak post hoc, P < 0.05. Data are mean ± SEM (mean ± standard error of the mean).