Sensitization of
purinergic P2X receptors is one of the mechanisms responsible for exaggerated
pain responses to inflammatory
injuries.
Prostaglandin E2 (
PGE2), produced by inflamed tissues, is known to contribute to abnormal
pain states. In a previous study, we showed that
PGE2 increases fast inactivating
ATP currents that are mediated by homomeric P2X3 receptors in dorsal root ganglion (DRG) neurons isolated from normal rats.
Protein kinase A (PKA) is the signalling pathway used by
PGE2. Little is known about the action of
PGE2 on
ATP currents after
inflammation, although the information is crucial for understanding the mechanisms underlying
inflammation-induced sensitization of P2X receptors. We therefore studied the effects of
PGE2 on
P2X3 receptor-mediated
ATP currents in DRG neurons dissociated from complete
Freund's adjuvant (CFA)-induced inflamed rats. We found that
PGE2 produces a large increase in
ATP currents. PKCepsilon, in addition to PKA, becomes involved in the modulatory action of
PGE2. Thus,
PGE2 signalling switches from a solely PKA-dependent pathway under normal conditions to both PKA- and PKC-dependent pathways after
inflammation. Studying the mechanisms underlying the switch, we demonstrated that cAMP-responsive
guanine nucleotide exchange factor 1 (Epac1) is up-regulated after
inflammation. The
Epac agonist
CPT-OMe mimics the potentiating effect of
PGE2 and occludes the PKC-mediated
PGE2 action on
ATP currents. These results suggest that
Epac plays a critical role in P2X3 sensitization by activation of de novo PKC-dependent signalling of
PGE2 after
inflammation and would be a useful therapeutic target for
pain therapies.