Kinin B1 receptors contributes to acute pain following minor surgery in humans

Abstract

Background: Kinins play an important role in regulation of pain and hyperalgesia after tissue injury and inflammation by activating two types of G-protein-coupled receptors, the kinin B1 and B2 receptors. It is generally accepted that the B2 receptor is constitutively expressed, whereas the B1 receptor is induced in response to inflammation. However, little is known about the regulatory effects of kinin receptors on the onset of acute inflammation and inflammatory pain in humans. The present study investigated the changes in gene expression of kinin receptors and the levels of their endogenous ligands at an early time point following tissue injury and their relation to clinical pain, as well as the effect of COX-inhibition on their expression levels.

Results: Tissue injury resulted in a significant up-regulation in the gene expression of B1 and B2 receptors at 3 hours post-surgery, the onset of acute inflammatory pain. Interestingly, the up-regulation in the gene expression of B1 and B2 receptors was positively correlated to pain intensity only after ketorolac treatment, signifying an interaction between prostaglandins and kinins in the inflammatory pain process. Further, the gene expression of both B1 and B2 receptors were correlated. Following tissue injury, B1 ligands des-Arg9-BK and des-Arg10-KD were significantly lower at the third hour compared to the first 2 hours in both the placebo and the ketorolac treatment groups but did not differ significantly between groups. Tissue injury also resulted in the down-regulation of TRPV1 gene expression at 3 hours post-surgery with no significant effect by ketorolac treatment. Interestingly, the change in gene expression of TRPV1 was correlated to the change in gene expression of B1 receptor but not B2 receptor.

Conclusions: These results provide evidence at the transcriptional level in a clinical model of tissue injury that up-regulation of kinin receptors are involved in the development of the early phase of inflammation and inflammatory pain. The up-regulation of B1 receptors may contribute to acute inflammatory pain through TRPV1 activation.

Figure 1

The levels of immunoreactive kinins were significantly lower at the third hour compared to the first 2 hours. There was no significant difference between the placebo and ketorolac treatment groups. Fig. (1a) shows the levels of B₂ receptor ligands (BK and KD) and Fig. (1b) shows the levels of B₁ receptor ligands (des-Arg⁹-BK and des-Arg¹⁰-KD). Concentrations of immunoreactive kinins (pg/ml) were transformed into Ln (X+1), which is shown here. For details, please refer to the methods section. Figures show the levels measured at the left side. The right side showed similar results. Data are presented as mean ± SEM; *indicated p < 0.05, 3 way ANOVA.

Figure 2

(a) Pain intensity assessed by 100 mm VAS was lower in the keterolac treatment group (p = 0.023, two way ANOVA); Data are presented as mean ± SEM. (b-e) The change in gene expression level (RQ) of both B1 and B2 receptors was correlated to pain intensity (VAS) in the keterolac treatment group but not in the placebo treatment group. The association between the gene expression and pain scale was examined using Pearson correlation. The y-axis represents the sum of the pain intensity over the first 3 hours post-surgery. The x-axis represents the relative changes in gene expression (RQ) from qRT-PCR.

Figure 3

(a) The relative changes in gene expression (RQ) from qRT-PCR of B1 and B2 receptors were correlated. (b) Similarly, the relative change in gene expression of TRPV1 was correlated to that of B1 receptor. The relative change in gene expression is calculated as 2^-ΔΔCt to show the fold increase. In case of TRPV1, the downregulation is shown as the negative fold change. The association was examined using Pearson correlation at 3 hours post-surgery.

Figure 4

Tissue injury results in the production of kinins and cytokines. Kinins activate both B₁ and B₂ receptors on nerve ending and on other cells (possibly, gingival fibroblasts, epithelial cells or inflammatory cells). The activation of both B₁ and B₂ receptors leads via different signaling pathways to the production of IL-6, IL-8 and CLL2, all of which can activate their corresponding receptors on the sensory nerve endings. Activation of TRPV1 is thought to mediate kinins effects on sensory neurons and might possibly mediate their effect on other cells to produce cytokines.