Role of prostaglandin receptor EP1 in the spinal dorsal horn in carrageenan-induced inflammatory pain

Background: Prostaglandin E-2 (PGE(2)) and the receptor for PGE(2) (EP receptor) are key factors contributing to the generation of hyperalgesia caused by inflammation. The current study was designed to investigate the roles of PGE(2) and EP, receptors in the spinal cord in the development and maintenance of inflammatory pain, using behavioral, microdialysis, and intracellular calcium ion concentration ([Ca2+](i)) assays. Methods: inflammation was induced by an injection of carrageenan into the plantar surface of the rat hind paw. The effects of inflammation were evaluated at the time points of 3 h (early phase) and 15 h (late phase) after carrageenan injection. in behavioral assays, withdrawal thresholds to mechanical stimuli were evaluated. The effect of an intrathecally administered selective EP1 antagonist, ONO-8711, on the carrageenan-induced hyperalgesia was examined. Using a spinal microdialysis method, PGE(2) concentration in the spinal dorsal horn was measured. In [Ca2+](i) assays, we measured [Ca2+](i) in the spinal dorsal horn in transverse spinal slices and examined the effects of pretreatment with ONO-8711. Sensitivities of the changes in [Ca2+](i) to PGE(2) perfusion were also assessed. Results. Mechanical hyperalgesia and paw edema were observed in both the early and late phases. The hyperalgesia was inhibited by intrathecal ONO-8711 in the late, but not early, phase. The concentration of PGE(2) in the spinal dorsal horn increased in the late phase. The [Ca2+](i) in the dorsal horn increased on the ipsilateral side to the inflammation in the late, but not early phase. This increase was suppressed by the pretreatment with ONO-8711. Magnitude of the increase in [Ca2+](i) on the ipsilateral side in response to PGE(2) perfusion was greater in the late phase than in the early phase. Conclusion: The results suggested that activation of spinal EP1 receptors was crucial in the carrageenan-induccd mechanical hyperalgesia in the late phase. it seems that some of the mechanisms underlying inflammation-induced plastic changes are mediated by time-dependent increase in PGE(2) concentration, activation of EP1 receptors, and increase in [Ca2+](i) in the spinal dorsal horn.