Abstract

Chronic inflammatory pain, when not effectively treated, is a costly health problem and has a harmful effect on all aspects of health-related quality of life. Previous studies suggested that in male Sprague Dawley rats, prostaglandin E2 (PGE2)-induced short-term hyperalgesia depends on protein kinase A (PKA) activity, whereas long-lasting hyperalgesia induced by PGE2 with carrageenan pre-injection, requires protein kinase Cε (PKCε). However, the mechanism underlying the kinase switch with short- to long-term hyperalgesia remains unclear. In this study, we used the inflammatory agents carrageenan or complete Freund’s adjuvant (CFA) to induce long-term hyperalgesia, and examined PKA and PKCε dependence and switching time. Hyperalgesia induced by both agents depended on PKA/PKCε and Gs/Gi-proteins, and the switching time from PKA to PKCε and from Gs to Gi was about 3 to 4 h after inflammation induction. Among the single inflammatory mediators tested, PGE2 and 5-HT induced transient hyperalgesia, which depended on PKA and PKCε, respectively. Only acidic solution-induced hyperalgesia required Gs-PKA and Gi-PKCε, and the switch time for kinase dependency matched inflammatory hyperalgesia, in approximately 2 to 4 h. Thus, acidosis in inflamed tissues may be a decisive factor to regulate switching of PKA and PKCε dependence via proton-sensing G-protein–coupled receptors.

Highlights

  • Tissue injury, infection or tumor growth induces inflammation, which is often accompanied by persistent and chronic pain

  • Prolonged hyperalgesia depends on a switch of protein kinase A (PKA) and protein kinase Cε (PKCε) kinase activities Carrageenan and complete Freund’s adjuvant (CFA) are commonly used in models of inflammatory pain

  • To understand whether the prolonged hyperalgesia induced by these two agents depends on PKA and/or PKCε, mice received intraplantar injection of CFA or carrageenan to induce peripheral inflammation

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Summary

Introduction

Infection or tumor growth induces inflammation, which is often accompanied by persistent and chronic pain. More sustained pain effects are achieved only with high concentration (10-5 M) of a PLOS ONE | DOI:10.1371/journal.pone.0125022. Acidosis Mediates the Switching of Gs-PKA and Gi-PKCε in Hyperalgesia combination of inflammatory mediators (including BK, 5-HT, PGE2, and histamine)[7]. High local proton concentrations in inflamed tissues excite and sensitize rat skin nociceptors and cause sustained pain in human skin [5,8,9]. The combination of inflammatory mediators (BK, 5-HT, PGE2, and histamine) in acid solution (pH 6.1) excites and sensitizes rat skin nociceptors [10]. Acidosis in inflamed tissues is the decisive factor for ongoing nociceptor excitation and sustained pain [13]

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