3.1.3 EP4 but not EP3 signaling mediates bradykinin-enhancing capsaicin responses
The observation that bradykinin-mediated sensitization of capsaicin-induced calcium responses in cultured DRG neurons is inhibited by COX enzyme inhibitors and thus involves prostaglandins, led to speculation that prostaglandins, of unknown type, could be released from DRG neurons to act in an autocrine manner. Since it is already known that the EP4 receptors for PGE2 are important in the sensitisation of capsaicin responses by PKA modulation of TRPV1 ion channels (Lin et al. 2006), we examined the effect of EP receptor antagonists on the bradykinin-mediated sensitisation of capsaicin responses in cultured DRG neurons.
Figure 4A shows traces of calcium responses that resulted from the repeated capsaicin application (80nM; for 10 sec via U tube) in the presence of bradykinin (100nM; 3 min), after treatment with an antagonist cocktail of an EP4 inhibitor (GW627368, 1μM) and an EP3 inhibitor (L-798106; 1μM) for 10 min. In the presence of an antagonist cocktail, bradykinin failed to sensitise the second capsaicin-induced calcium response. The histogram in figure 4C shows that the mean response ratio for bradykinin + AC was 58.7 ±7 % which was significantly reduced (* P<0.05, one-way ANOVA and Bonferroni’s multiple comparison test) from the mean response ratio observed with bradykinin alone.
Figure 4B shows representative traces of the effect of the EP4 antagonist or an EP3 antagonist on capsaicin-induced calcium responses in the presence of bradykinin (100nM for 3 min). The EP4 inhibitor, GW627368 (1μM), but not the EP3 antagonist, L-798106 (1μM), significantly inhibited bradykinin-induced sensitisation of capsaicin sensitive neurons. The response ratios of first and second capsaicin responses in the presence of bradykinin + GW627368 and for bradykinin + L-798106 were 48.4 ± 7.6 % (*P < 0.05 one-way ANOVA followed by Bonferroni’s multiple comparison test) and 90.7 ± 10.6 % (P >0.05), respectively.