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Abstract

Innervation of the joint with thinly myelinated and unmyelinated sensory nerve fibres is crucial for the occurrence of joint pain. During inflammation in the joint, sensory fibres show changes in the expression of receptors that are important for the activation and sensitization of the neurones and the generation of joint pain. We recently reported that both neurokinin 1 receptors and bradykinin 2 receptors are upregulated in dorsal root ganglion (DRG) neurones (the cell bodies of sensory fibres) in the course of acute and chronic antigen-induced arthritis in the rat. In this study, we begin to address mechanisms of the interaction between fibroblast-like synovial (FLS) cells and sensory neurones by establishing a co-culture system of FLS cells and DRG neurones. The proportion of DRG neurones expressing neurokinin 1 receptor-like immunoreactivity was not altered in the co-culture with FLS cells from normal joints but was significantly upregulated using FLS cells from knee joints of rats with antigen-induced arthritis. The proportion of DRG neurones expressing bradykinin 2 receptors was slightly upregulated in the presence of FLS cells from normal joints but upregulation was more pronounced in DRG neurones co-cultured with FLS cells from acutely inflamed joints. In addition, the expression of the transient receptor potential V1 (TRPV1) receptor, which is involved in inflammation-evoked thermal hyperalgesia, was mainly upregulated by co-culturing DRG neurones with FLS cells from chronically inflamed joints. Upregulation of neurokinin 1 receptors but not of bradykinin 2 and TRPV1 receptors was also observed when only the supernatant of FLS cells from acutely inflamed joint was added to DRG neurones. Addition of indomethacin to co-cultures inhibited the effect of FLS cells from acutely inflamed joints on neurokinin 1 receptor expression, suggesting an important role for prostaglandins. Collectively, these data show that FLS cells are able to induce an upregulation of pain-related receptors in sensory neurones and, thus, they could contribute to the generation of joint pain. Importantly, the influence of FLS cells on DRG neurones is dependent on their state of activity, and soluble factors as well as direct cellular contacts are crucial for their interaction with neurones.

Sensitization of knee-innervating sensory neurons by tumor necrosis factor-α-activated fibroblast-like synoviocytes: an in vitro, coculture model of inflammatory pain

“… 29 Finally, supernatants from cultured FLS derived from rats 3 days into AIA model show increased IL-6 and PGE 2 . 4 Here, we observed that neither the expression level of Il-6 , Il-1r1, and Cox-2 , nor the level of secreted cytokines was upregulated in FLS derived from CFA-injected knee (our local ethics review body did not permit investigation of > 24 hours after CFA injection). This is possibly because the model used here is too brief to influence FLS gene expression, which is supported by a previous study where FLS derived from rats with longer (3-28 days) AIA-induced knee inflammation did have higher PGE 2 and IL-6 concentrations in culture supernatants compared to control.…”

“…Dorsal root ganglion neurons in coculture with inflamed FLS derived from AIA rats reportedly have increased expression of TRPV1. 4 Here, we sought to investigate whether FLS can modulate knee neuron responses to TRP channel agonists using whole-cell patch-clamp recordings. Knee neurons in monoculture and when cocultured with control FLS displayed very similar responses, i.e., a mean capsaicin peak current density response of 3.5 ± 1.0 pA/pF (n = 7/19) and 3.2 ± 0.9 pA/pF (n = 7/18), respectively.…”

“…However, a genome-wide study on DNA methylation has shown that important differences exist between knee and hip FLS, including of genes involved in IL-6 signalling. 1 By using a cell-outgrowth method to culture mouse FLS, as previously described in humans 34 and rats, 4 we have avoided the biological ambiguity introduced by joint-to-joint variability. Using these FLS, we investigated the expression of inflammatory genes Il-6 , Il-1r1 , and Cox-2 , all of which have been linked to the inflammatory phenotype of FLS.…”

“… 31 Fibroblast-like synoviocytes derived from chronic antigen-induced arthritic (AIA) rats were observed to increase the expression of receptors associated with nociception, namely neurokinin 1, bradykinin 2, and transient receptor potential vanilloid 1 (TRPV1) in DRG neurons when cocultured. 4 However, this study did not functionally assess modulation of DRG neuron excitability, which is a key mechanism of peripheral sensitization and hence pain. Indeed, we recently showed that human osteoarthritic (OA) synovial fluid (a lubricating fluid largely secreted by FLS 5 ) causes hyperexcitability of murine sensory neurons and increased TRPV1 function.…”

Knee osteoarthritis: a role for bradykinin?

“…Other mechanisms have been accounted for BK noxious activity which can be further emphasized in inflamed joints. This has been proven by co-culturing dorsal root ganglia with synoviocytes isolated from rat inflamed knee which causes a significant up-regulation of bradykinin B 2 receptors [45] and increases the c-fos expression both at the spinal and supraspinal level depending on the time of exposure of the noxius stimulus [46]. B 2 receptor stimulation is in turn responsible for the activation of transcription factors (nuclear factor of activated T-cells) and for the induction of cyclooxygenase-2 (COX-2) expression [47].…”