Role of ERK in gender difference of fibromyalgia pain

Abstract

This study investigated the ERK pathway of the peripheral nervous system and discovered a gender-specific pattern of ERK activation in the dorsal root ganglion of an acid-induced chronic widespread muscular pain model. We employed a twice acid-induced chronic musculoskeletal pain model in rats to evaluate mechanical pain behavior in both male and female groups. We further conducted protein analysis of dissected dorsal root ganglions from both genders. Both male and female rats exhibited a similar pain behavior trend, with females demonstrating a lower pain threshold. Protein analysis of the dorsal root ganglion (DRG) showed a significant increase in phosphorylated ERK after the second acid injection in all groups. However, phosphorylation of ERK was observed in the dorsal root ganglion, with higher levels in the male ipsilateral group compared to the female group. Moreover, there was a no difference between the left and right sides in males, whereas the significant difference was observed in females. In conclusions, the administration of acid injections induced painful behavior in rats, and concurrent with this, a significant upregulation of pERK was observed in the dorsal root ganglia, with a greater magnitude of increase in males than females, and in the contralateral side compared to the ipsilateral side. Our findings shed light on the peripheral mechanisms underlying chronic pain disorders and offer potential avenues for therapeutic intervention.

Keywords: ERK pathway; Fibromyalgia syndrome; chronic pain; dorsal root ganglion; gender-specific activation.

Figure 1.

Diagram of the experimental design. The study utilized female and male rats which were divided into three groups: Naïve, Saline, and acid treatment (AI/MES). Chronic pain was induced by administering acid treatment twice, on day 0 and day 5. Pain behavior was observed for up to 19 days, and the rats were sacrificed at day 11 for further analysis.

Figure 2.

Mechanosensory response by Von Frey filaments after acid treatment. Mechanosensitivity was evaluated for up to 19 days, revealing a decrease in pain threshold after the twice acid injections (indicated by the red arrow) in both ipsilateral and contralateral sides of female and male rats. No changes were observed in the saline and Naïve groups. These results are presented in panels (a) and (b) for the female ipsilateral and contralateral sides, and panels (c) and (d) for the male ipsilateral and contralateral sides. #p < .05; ##p < .01; ###p < .001 versus BL. *p < .05; **p < .01; ***p < .001 versus acid groups. Rat number = 6-12 for each group at individual time points.

Figure 3.

ERK involved in acid injection rat model. Phosphorylated ERK activation was visualized by immunofluorescence following 11 days of acid injection in bilateral L4-5 dorsal root ganglia. The study groups included Naïve, Saline, and acid-treated groups. The male groups are represented in (A-C), while the female groups are shown in (D-F). Red indicated phosphorylated ERK.

Figure 4.

Phosphorylation of ERK expressed in satellite glia. Immunofluorescence was utilized to visualize the activation of phosphorylated ERK in satellite glia (indicated by an arrow) of bilateral L4-5 dorsal root ganglia after 11 days of acid injection. Red fluorescence represented phosphorylated ERK (A and D); Green fluorescence represented GFAP (B and E); The merged images were presented in C and F. The male groups were shown in (A-C), while the female groups were represented in (D-F).

Figure 5.

Expression of phosphorylated ERK in different group. Western blot analysis showed the levels of phosphorylated ERK (pERK1 and 2) in the ipsilateral and contralateral L4-5 dorsal root ganglia following 11 days of acid injection in female and male rats. The analyzed proteins included Naïve, saline, and acid treatment groups. The quantitative data presented the protein levels of pERK1 and 2 in the dorsal root ganglia of both female and male rats. (a) Female; (b) Male. **, p < .01; ***, <0.001.

Figure 6.

Effects of acid treatment on ERK phosphorylation in the ipsilateral and contralateral dorsal root ganglia. Western blot analysis revealed a significant increase in phosphorylated ERK in both ipsilateral (a) and contralateral (b) sides of the dorsal root ganglia. The quantitative data show the levels of phosphorylated ERK1 and 2 normalized to α-tubulin. *, p < .05; **, p < .01.

Figure 7.

Effects of acid treatment on ERK phosphorylation in left and right side of female and male rat. The western blot analysis showed a statistically significant increase in phosphorylated ERK1 and 2 in the right side of female rats compared to the left side (A). In male rats, ERK phosphorylation was elevated on both sides, but no significant difference was observed between the left and right sides. ***, p < .001.