Abstract
Background: Endothelin (ET), a peptide best known for its vascular effects, also evokes pain and hyperalgesia, independently of its vascular actions. Data suggest that ET can have nociceptive effects, acting directly on receptors expressed in sensory neurons. As such, the aim this study was to investigate the direct effect of ET on hyperalgesia and edema, induced by carrageenan, on the temporomandibular joint (TMJ) of capsaicin-treated rats. Methods: Capsaicin was administered by subcutaneous injection to newborn, male Wistar rats. Inflammation was induced 60 days later by a single intra-articular injection of carrageenan into the left TMJ (control group received sterile saline). Inflammatory parameters, such as plasma extravasation, leukocyte influx and mechanical allodynia (measured as the head-withdrawal force threshold) were evaluated 4 h after edematogenic stimulus. ET-1 and ET-3, and the ET-B receptor (ETBR) antagonist were administered 3 min before edematogenic stimulus. ET and transient receptor potential vanilloid (TRPV1) mRNA expression was assessed by reverse-transcription polymerase chain reaction (RT-PCR). Edema formation was evaluated by measurement of the extravascular accumulation of injected 125I-human serum albumin into the TMJ soft tissues of anesthetized rats. Results: Capsaicin neonatal treatment significantly reduced edema formation, leukocyte influx and mechanical allodynia in TMJ, when compared to the control group, while the ETBR antagonist increased plasma extravasation and hyperalgesia in the capsaicin-treated group. ET-1 treatment reduced both plasma extravasation and myeloperoxidase activity. Capsular mRNA for ET-1 was significantly augmented in the TMJ of capsaicin-treated rats, when compared to controls. Conclusions: Our results suggest, for the first time, that ET-1, via ETBR activation, reduces plasma extravasation, leukocyte influx and inflammatory pain in the temporomandibular joint of capsaicin-treated rats.
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