| Chronic inflammatory pain is a widespread medical problem projected to afflict nearly 60 million adults by the year 2020. If left untreated, the condition takes a devastating toll on the physical and mental health of the individual. A major component of this syndrome is persistent hypersensitivity of sensory neurons that conduct pain signals. However, the intracellular signaling events in sensory neurons that mediate the transition from acute to chronic pain are poorly understood. Recently, a class of guanine nucleotide exchange factors (GEFs) known as exchange factors directly activated by cyclic AMP (Epacs) have emerged as critical signaling molecules mediating this transition. Therefore, the fundamental goal of this thesis was to determine signaling pathways downstream of Epacs which mediate the ability of these molecules to elicit hypersensitivity in sensory neurons.;Treatment with the widely-used Epac agonist, 8-(4-chlorophenylthio)-3'5'-cAMP-AM (8CPT-AM), elicits sensitization of sensory neurons as evidenced by an increase in the number of action potentials (APs) fired in response to a ramp of depolarizing current and an increase in the chemically-evoked release of immunoreactive calcitonin gene-related peptide (iCGRP). This sensitization is attenuated by reducing expression of Epac2, but not Epac1. In addition, treatment with 8CPT-AM activates both Ras and Rap1 GTPases; however, inhibition of Ras, but not Rap1, attenuates the sensitization elicited by 8CPT-AM. Furthermore, inhibition of the downstream Ras effectors, phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI-3K), attenuates the ability of 8CPT-AM to trigger a sensitized response. Overall, this work reveals a novel mechanism whereby the Ras signaling pathway mediates the Epac-induced sensitization of sensory neurons. Therefore, the Epac signaling pathway and it associated components could present novel therapeutic targets for treating chronic pathological pain, without altering acute pain. |
