Korean J Pain.  2024 Oct;37(4):310-319. 10.3344/kjp.24144.

Low level laser therapy alleviates mechanical allodynia in a postoperative and neuropathic pain model and alters the levels of inflammatory factors in rats

• Han X
• Jang Kc
• Kim WM
• Lee HG

• KMID: 2559693
• DOI: http://doi.org/10.3344/kjp.24144

Abstract

Background
This study aimed to investigate the analgesic and preventive effect of low-level laser therapy (LLLT) on the incisional pain model and spinal nerve ligation (SNL) model in rats and identify the possible mechanisms of action.
Methods
Male Sprague-Dawley rats were used, divided into different treatment groups. The single application group received LLLT before or after skin incision or SNL. The consecutive application group received LLLT for six consecutive days post-incision, three days pre-incision, or three consecutive days pre-SNL. The control group underwent skin incision or SNL without LLLT. The von Frey test was used to quantify the pain associated with mechanical allodynia. Pro-inflammatory cytokine level and alterations in nerve growth factor (NGF) expression were measured by using ELISA and immunohistochemistry, respectively in the skin, muscle of the paw, and spinal cord dorsal horn (SCDH).
Results
In the incisional pain model, LLLT showed significant analgesic and preventive effect. LLLT ameliorated SNL-induced mechanical allodynia but LLLT had no preventive effect. LLLT decreased interleukin-1β (IL-1β) expression levels in the skin, muscle, and SCDH and reduced the optical density of skin and spinal cord NGF in the incisional pain model.
Conclusions
LLLT alleviated incisional pain and neuropathic pain caused by SNL in rats, and reduced the levels of IL-1β and NGF in the peripheral tissue and SCDH in the incisional pain model. LLLT might be effective in patients with post-operative pain and peripheral neuropathic pain.

Keyword

Cytokines; Hyperalgesia; Interleukin-1; Low-Level Light Therapy; Pain; Postoperative; Spinal Cord Dorsal Horn; Spinal Nerves

Figure

• 

  Fig. 1 The influence of low-level laser treatment (LLLT) on analgesic recovery in a rat incision model over the course of 7 days. (A) Paw withdrawal thresholds (PWTs) to von Frey stimulation; (B) the percentage of the maximum possible effect (%MPE). PWT was measured and LLLT was subsequently irradiated for either 30 or 60 minutes (indicated by upward arrows in the figure). IM: incision model. Tx: treatment. The results are reported as the mean and SEM, with significant improvements in PWTs and %MPE compared to control group: *P < 0.05, **P < 0.01, ***P < 0.001, n = 6 rats/group (one-way ANOVA and Bonferroni post-hoc test).

• 

  Fig. 2 Preventative effect of low-level laser treatment (LLLT) in an incision model. (A) Paw withdrawal thresholds (PWTs) to von Frey stimulation; (B) the percentage of the maximum possible effect (%MPE). In the pre-treated incision model, Pre-1Tx group received LLLT for 1 hour on the day before the incision, and Pre-3Tx group received LLLT for 1 hour each day for three days before the incision. IM: incision model, Pre Tx: pretreatment. The results are reported as the mean and SEM, with significant differences observed compared to control group: *P < 0.05, **P < 0.01, n = 6 rats/group (one-way ANOVA and Bonferroni post-hoc test).

• 

  Fig. 3 Effect of low-level laser treatment (LLLT) on analgesic recovery in spinal nerve ligation (SNL) rats over 3 hours. (A) Paw withdrawal threshold (PWT) to von Frey stimulation; (B) the percentage of the maximum possible effect (%MPE). Behavioral tests were performed after 30, 60 and 120 minutes of irradiation. The results are reported as the mean and SEM, with statistically significant improvements in PWTs and %MPE compared to control group: *P < 0.05, **P < 0.01, ***P < 0.001, n = 6 rats/group (one-way ANOVA and Bonferroni post-hoc test).

• 

  Fig. 4 Preventative effect of low-level laser treatment (LLLT) in the spinal nerve ligation (SNL) model. In the pre-treated SNL model, Pre-3Tx group received LLLT for 1 hour each day for three days before the SNL was made. Pre Tx: pretreatment. The results showed no significant difference between the two groups, are presented with the P values where applicable, n = 6 rats/group (independent sample t-test). The results are reported as the mean and SEM.

• 

  Fig. 5 Cytokine levels in naive groups, control groups vs. low-level laser treatment (LLLT)-treated groups. Interleukin (IL)-1β, IL-6, and IL-10 (pg/mL) in (A, D, G) skin, (B, E, H) muscle, and (C, F, I) spinal cord dorsal horn (SCDH) tissues. Showing significant reductions in IL-1β with LLLT treatment. This indicates LLLT's role in reducing inflammatory responses associated with pain. ***P < 0.001 compared with naive group (A, C, D, F, G, I) n = 6 rats/group; ##P < 0.01, ###P < 0.001 compared with control group (B, E, H) n = 5 rats/group (one-way ANOVA and Bonferroni post-hoc test).

• 

  Fig. 6 Immunohistochemical outcomes and optical density (OD) of nerve growth factor (NGF) expression in each group. (A) Representative microphotographs depicting NGF expression in the skin. (B) Relative OD of NGF in the skin of rats in the different groups. (C) Representative microphotographs depicting NGF expression in the spinal cord dorsal horn (SCDH) in each group. (D) Comparison of the relative OD of NGF in the SCDH of the different groups. Immunohistochemical examination revealed significantly higher NGF expression in the control group than in the naive group and significant differences between the control group and LLLT group. Scale bar = 400 μm (A) and 1,000 μm (C). *P < 0.05, **P < 0.01 compared with naive group, #P < 0.05, ##P < 0.01 compared with control group, n = 5 rats/group (one-way ANOVA and Bonferroni post-hoc test).

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