Ca²⁺ transient evoked by chemical stimulation is enhanced by PGE₂ in vagal sensory neurons: Role of cAMP/PKA signaling pathway

The effect of prostaglandin E₂ (PGE₂) on chemical stimulation-evoked calcium (Ca²⁺) transient was investigated in isolated vagal sensory neurons of the rat using fura-2-based ratiometric Ca²⁺ imaging. Application of capsaicin (3 × 10^-8 to 10^-7 M; 15 s) caused a rapid surge of intracellular Ca²⁺ concentration in small- and medium-size neurons; the response was reproducible when >10 min elapsed between two challenges and was absent in nominally Ca²⁺-free solution. After pretreatment with PGE₂ (3 × 10^-7 M; 5 min), the peak of this capsaicin-evoked Ca²⁺ transient was increased by almost fourfold, and its duration was also prolonged. This augmented response to capsaicin induced by PGE₂ gradually declined but remained higher than control after 15-min washout. Similarly, PGE₂ pretreatment also markedly enhanced the Ca²⁺ transients induced by other chemical stimulants to C neurons, such as phenylbiguanide (PBG), adenosine 5′-triphosphate (ATP), and KCl. The Ca²⁺ transients evoked by PBG, ATP, and KCl were potentiated after the pretreatment with PGE₂ to 242, 204, and 163% of their control, respectively. This potentiating effect of PGE₂ could be mimicked by forskolin (10^-6 M; 5 min), an activator of adenylyl cyclase, and 8-(4-chlorophenylthio)adenosine-3′-5′-cyclic monophosphate (CPT-cAMP; 3 × 10^-6 M, 10 min), a membrane-permeable cAMP analogue. Furthermore, the potentiating effects of PGE₂, forskolin, and CPT-cAMP were abolished by N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H89; 10^-5 M; 15-20 min), a protein kinase A (PKA) inhibitor. In summary, these results show that PGE₂ reversibly potentiates the chemical stimuli-evoked Ca²⁺ transients in cultured rat vagal sensory neurons, and this potentiating effect is mediated through the cyclic AMP/PKA transduction cascade.