Substance P and Bradykinin Activate Alternative Gq/11-Coupled Signalling Cascades and Impose Opposite Effects on M Current in DRG Neurons

We investigated signalling cascades and coupling to M channel modulation of two types of Gq/11-coupled receptors in rat nociceptive DRG neurons: bradykinin (BK) B2 and substance P (SP) neurokinin (NK) receptors. In patch clamp experiments, BK induced a rapid and reversible inhibition of M current which was prevented by blocking phospholipase C or buffering cytosolic Ca2+. In contrast, SP (1uM) failed to inhibit M current in 35 neurons tested, instead producing slow augmentation (162 ± 18%) in 19/35 predominantly TRPV1-positive neurons. The augmentation was not reversible by washout but was completely reversed (to 80 ± 19% of initial value, n = 19) by 1 mM DTT suggesting that the effect may be mediated by oxidative modification of M channels. DTT applied alone did not inhibit M current. In current clamp, BK induced an increase in action potential firing whereas SP induced a hyperpolarization in 3/8 neurons tested. In Ca2+ imaging experiments, SP elicited small rises in [Ca2+]i in only 9% of neurons while BK induced robust [Ca2+]i rises in 61 % of neurons, indicating that the NK receptors couple poorly to cytosolic Ca2+ signals. In ∼50% of DRG neurones SP did induce sensitisation of TRPV1 suggesting abundant expression of NK receptors. When expressed in CHO cells all three NK receptor isoforms (NK1-3) induced robust Ca2+ rises, hydrolysis of PIP2 and inhibition of KCNQ2/3 currents. Injection of BK into the hind paw of rats induced prominent nocifensive behaviour (65 ± 5 s/20 min) whereas SP evoked only small (but significant) responses (9 ± 2 s/20 min). Our data indicate that BK and SP couple to different subroutine of Gq/11 signalling resulting in opposite effects on M current and excitability of nociceptors.