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.