In an effort to examine gene expression changes during experimental arthritis, we found that three members of the endothelin system - namely ET-1, ET-2, and ETA - were markedly upregulated during the acute phase of AIA. This is in line with previous findings showing that patients with RA exhibit increased ET-1 serum levels as well as high ET-1 concentrations in synovial fluid [15–17]. Moreover, it is widely accepted that endothelins induce hypernociception in rodents [18–22]. So far, studies investigating the role of endothelins in the pathophysiology of arthritis are sparse [18, 23, 24]. It has been shown, however, that local administration of endothelin receptor antagonists reduces edema, neutrophil infiltration, and production of inflammatory mediators [21, 25–32].
Given the availability of potent endothelin receptor antagonists, we investigated the effects of systemic administration of the mixed ETA and ETB endothelin receptor antagonist bosentan and the ETA-selective antagonist ambrisentan on pain-related behavior, inflammation, and histopathological manifestations during the course of AIA. We found that daily oral administration of bosentan significantly attenuated knee joint swelling. In contrast, ambrisentan failed to promote any detectable anti-inflammatory activity. These findings indicate that the anti-inflammatory effects of bosentan are mediated predominantly via the ETB receptor.
Bosentan selectively inhibited mechanical hyperalgesia but not thermal hyperalgesia. Acute and chronic models of joint inflammation reliably produce mechanical hyperalgesia. In some arthritic models, thermal hyperalgesia can also be observed; however, it is not known to what extent thermal hyperalgesia is important in humans. Interestingly, intradermal injection of ET-1 induces mechanical hyperalgesia in humans, whereas thermal hyperalgesia could not be observed. Moreover, previous findings revealed different contributions of ETA and ETB receptors to thermal and mechanical hyperalgesia, respectively [2, 9, 21, 25, 28, 29, 31–34]. Whereas ETA receptors have been shown to mediate ET-1-induced thermal hyperalgesia, ETB receptors have been linked to mechanical hyperalgesia [2, 9, 21, 25, 28, 29, 31–34]. Both ambrisentan and bosentan had no effect on thermal hyperalgesia. In contrast, dexamethasone produced a significant inhibition of thermal hyperalgesia, suggesting that mechanisms in addition to an upregulation of ET-1 or ET-2 may contribute to the development of thermal hyperalgesia in our AIA model. At present, we do not know whether ETB-selective antagonists could exert therapeutic effects similar to those of mixed ETA and ETB receptor antagonists. Nevertheless, daily oral bosentan administration was well tolerated over the 42-day treatment period in our murine AIA model.
To assess gene expression changes in lumbar DRGs during the acute phase of AIA, we used transcriptional profiling by genome-wide microarray analysis. Our results indicate that an acute peripheral inflammation of the knee joint induces robust changes in gene expression patterns in DRGs, suggesting that dynamic adaptations occur in primary sensory neurons in response to peripheral inflammation. However, this approach is based on the isolation of total mRNA from DRGs and, hence, cannot differentiate between mRNAs originating from neurons, glial cells, endothelial cells, or infiltrating leukocytes. Nevertheless, we detected a total of 451 AIA-regulated genes, 436 of which were upregulated (fold change of at least 5) and only 15 of which were downregulated (fold change of not more than -5) in DRGs from the affected side in comparison with the contralateral side and control animals. Table 1 shows a selection of upregulated genes. This selection includes regulatory peptides (for example, secretin, peptide YY, and guanylin) as well as chemokines, receptors, enzymes, and carriers. Several of these genes, including phospholipase A2, kallikrein, IL-18, and CX3CL1, have been associated with arthritis or inflammatory pain.