P2X7-induced nociception in the temporomandibular joint of rats depends on inflammatory mechanisms and C-fibres sensitization.

BACKGROUND: P2X7 receptors are responsible for triggering inflammatory responses contributing to processes of pain in articular tissues. This study aimed to investigate whether the activation of the P2X7 receptor located in the temporomandibular joint (TMJ) tissues induces nociception through an inflammatory mechanisms and/or the activation of C-fibres (small-diameter primary afferents) of rats' TMJ. METHODS: The TMJ hypernociception induced by the activation of P2X7 receptor was assessed by measuring the behavioural nociceptive responses. After behavioural experiments, the animals were terminally anaesthetized and periarticular tissues were removed and homogenate for enzyme-linked immunosorbent assay, leukocyte infiltration and western blotting analysis. RESULTS: The nonselective P2X7 receptor agonist BzATP induced a dose-dependent TMJ nociception, which was blocked by the selective P2X7 receptor antagonist A-438079. The co-administration of the selective ß2-adrenoceptor antagonist (ICI-118,551) and the pre-treatment with cyclooxygenase inhibitor indomethacin or with the nonspecific selectin inhibitor Fucoidan significantly reduced BzATP-induced TMJ nociception. BzATP also induced an increase of pro-inflammatory cytokines TNFα, IL-1ß and CINC-1 levels, as well as leukocyte recruitment in TMJ tissue, effects that were reduced by A-438079. Moreover BzATP-induced TMJ nociception was inhibited in rats neonatal-treated with Capsaicin (depleting C-fibers). Finally, BzATP-induced an increase in TRPV1 expression in TMJ tissue. CONCLUSIONS: These findings suggest that P2X7 receptor activation in TMJ of rats induces nociceptive responses mediated by sympathomimetic amines, prostaglandins, leukocyte migration and increased levels of pro-inflammatory cytokines. Furthermore, the P2X7 receptor activation induces nociceptive responses dependent on the activation of the primary afferent nociceptors of rats' TMJ. SIGNIFICANCE: The activation of P2X7 receptors has an essential role in TMJ nociception and could be an interesting target to control the inflammatory pain in temporomandibular disorders.