Sensitization of
purinergic P2X3 receptors (P2X3Rs) is a major mechanism contributing to injury-induced exaggerated
pain responses. We showed in a previous study that cyclic
adenosine monophosphate (cAMP)-dependent
guanine nucleotide exchange factor 1 (Epac1) in rat sensory dorsal root ganglia (DRGs) is upregulated after inflammatory injury, and it plays a critical role in P2X3R sensitization by activating
protein kinase C epsilon (PKCε) inside the cells.
protein kinase C epsilon has been established as the major PKC
isoform mediating injury-induced hyperalgesic responses. On the other hand, the role of PKCα in receptor sensitization was seldom considered. Here, we studied the participation of PKCα in
Epac signaling in P2X3R-mediated
hyperalgesia. The expression of both Epac1 and Epac2 and the level of cAMP in DRGs are greatly enhanced after complete
Freund adjuvant (CFA)-induced
inflammation. The expression of phosphorylated PKCα is also upregulated. Complete
Freund adjuvant (CFA)-induced P2X3R-mediated
hyperalgesia is not only blocked by
Epac antagonists but also by the classical PKC
isoform inhibitors,
Go6976, and PKCα-
siRNA. These CFA effects are mimicked by the application of the
Epac agonist, 8-(4-chlorophenylthio)-2 -O-methyl-cAMP (
CPT), in control rats, further confirming the involvement of Epacs. Because the application of
Go6976 prior to
CPT still reduces
CPT-induced
hyperalgesia, PKCα is downstream of Epacs to mediate the enhancement of P2X3R responses in DRGs. The pattern of translocation of PKCα inside DRG neurons in response to
CPT or CFA stimulation is distinct from that of PKCε. Thus, in contrast to prevalent view, PKCα also plays an essential role in producing complex
inflammation-induced receptor-mediated
hyperalgesia.