Long-term exposure to PGE(2) causes homologous desensitization of receptor-mediated activation of protein kinase A

BACKGROUND: Acute exposure to prostaglandin E(2) (PGE(2)) activates EP receptors in sensory neurons which triggers the cAMP-dependent protein kinase A (PKA) signaling cascade resulting in enhanced excitability of the neurons. With long-term exposure to PGE(2), however, the activation of PKA does not appear to mediate persistent PGE(2)-induced sensitization. Consequently, we examined whether homologous desensitization of PGE(2)-mediated PKA activation occurs after long-term exposure of isolated sensory neurons to the eicosanoid. METHODS: Sensory neuronal cultures were harvested from the dorsal root ganglia of adult male Sprague-Dawley rats. The cultures were pretreated with vehicle or PGE(2) and used to examine signaling mechanisms mediating acute versus persistent sensitization by exposure to the eicosanoid using enhanced capsaicin-evoked release of immunoreactive calcitonin gene-related peptide (iCGRP) as an endpoint. Neuronal cultures chronically exposed to vehicle or PGE(2) also were used to study the ability of the eicosanoid and other agonists to activate PKA and whether long-term exposure to the prostanoid alters expression of EP receptor subtypes. RESULTS: Acute exposure to 1 μM PGE(2) augments the capsaicin-evoked release of iCGRP, and this effect is blocked by the PKA inhibitor H-89. After 5 days of exposure to 1 μM PGE(2), administration of the eicosanoid still augments evoked release of iCGRP, but the effect is not attenuated by inhibition of PKA or by inhibition of PI3 kinases. The sensitizing actions of PGE(2) after acute and long-term exposure were attenuated by EP2, EP3, and EP4 receptor antagonists, but not by an EP1 antagonist. Exposing neuronal cultures to 1 μM PGE(2) for 12 h to 5 days blocks the ability of PGE(2) to activate PKA. The offset of the desensitization occurs within 24 h of removal of PGE(2) from the cultures. Long-term exposure to PGE(2) also results in desensitization of the ability of a selective EP4 receptor agonist, L902688 to activate PKA, but does not alter the ability of cholera toxin, forskolin, or a stable analog of prostacyclin to activate PKA. CONCLUSIONS: Long-term exposure to PGE(2) results in homologous desensitization of EP4 receptor activation of PKA, but not to neuronal sensitization suggesting that activation of PKA does not mediate PGE(2)-induced sensitization after chronic exposure to the eicosanoid.