EPAC1 and EPAC2 promote nociceptor hyperactivity associated with chronic pain after spinal cord injury.

Chronic pain following spinal cord injury (SCI) is associated with electrical hyperactivity (spontaneous and evoked) in primary nociceptors. Cyclic adenosine monophosphate (cAMP) signaling is an important contributor to nociceptor excitability, and knockdown of the cAMP effector, exchange protein activated by cAMP (EPAC), has been shown to relieve pain-like responses in several chronic pain models. To examine potentially distinct roles of each EPAC isoform (EPAC1 and 2) in maintaining chronic pain, we used rat and mouse models of contusive spinal cord injury (SCI). Pharmacological inhibition of EPAC1 or 2 in a rat SCI model was sufficient to reverse SCI-induced nociceptor hyperactivity, indicating that EPAC1 and 2 signaling activity are complementary, with both required to maintain hyperactivity. However, EPAC activation was not sufficient to induce similar hyperactivity in nociceptors from naïve rats, and we observed no change in EPAC protein expression after SCI. In the mouse SCI model, inhibition of both EPAC isoforms through a combination of pharmacological inhibition and genetic deletion was required to reverse SCI-induced nociceptor hyperactivity. This was consistent with our finding that neither EPAC1 nor EPAC2 mice were protected against SCI-induced chronic pain as assessed with an operant mechanical conflict test. Thus, EPAC1 and 2 activity may play a redundant role in mouse nociceptors, although no corresponding change in EPAC protein expression levels was detected after SCI. Despite some differences between these species, our data demonstrate a fundamental role for both EPAC1 and EPAC2 in mechanisms maintaining nociceptor hyperactivity and chronic pain after SCI.