Abstract
BackgroundAcute exposure to prostaglandin E2 (PGE2) 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 PGE2, however, the activation of PKA does not appear to mediate persistent PGE2-induced sensitization. Consequently, we examined whether homologous desensitization of PGE2-mediated PKA activation occurs after long-term exposure of isolated sensory neurons to the eicosanoid.MethodsSensory neuronal cultures were harvested from the dorsal root ganglia of adult male Sprague-Dawley rats. The cultures were pretreated with vehicle or PGE2 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 PGE2 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.ResultsAcute exposure to 1 μM PGE2 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 PGE2, 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 PGE2 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 PGE2 for 12 h to 5 days blocks the ability of PGE2 to activate PKA. The offset of the desensitization occurs within 24 h of removal of PGE2 from the cultures. Long-term exposure to PGE2 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.ConclusionsLong-term exposure to PGE2 results in homologous desensitization of EP4 receptor activation of PKA, but not to neuronal sensitization suggesting that activation of PKA does not mediate PGE2-induced sensitization after chronic exposure to the eicosanoid.
Highlights
Acute exposure to prostaglandin E2 (PGE2) activates EP receptors in sensory neurons which triggers the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) signaling cascade resulting in enhanced excitability of the neurons
Prostaglandin E2 and agents that increase production of cAMP augment PKA activity in sensory neuronal culture Previous studies have shown that exposing sensory neurons in culture to PGE2 or prostaglandin I2 (PGI2) increases the production of cAMP [6, 7]
Because cAMP has multiple downstream effectors, we measured whether exposing sensory neuronal cultures to increasing concentrations of PGE2 would directly increase PKA activity
Summary
Acute exposure to prostaglandin E2 (PGE2) activates EP receptors in sensory neurons which triggers the cAMP-dependent protein kinase A (PKA) signaling cascade resulting in enhanced excitability of the neurons. In animal models of inflammation or in animals chronically exposed to PGE2, the ability of the eicosanoid to enhance nociception does not diminish, but subsequent administration of PGE2 results in a stronger and more prolonged hyperalgesia [18,19,20] This phenomenon, termed “hyperalgesic priming” [21], can be modeled in isolated sensory neurons where acute exposure to PGE2 sensitizes neurons to various stimuli [1, 7, 22] and, like their in vivo counterparts, the sensitizing actions of eicosanoids are not diminished by chronic exposure [23, 24]
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