Bradykinin (BK) is an inflammatory mediator and one of the most potent endogenous pain-inducing substances. When released at sites of tissue damage or inflammation, or applied exogenously, BK produces acute spontaneous pain and causes hyperalgesia (increased sensitivity to potentially painful stimuli). The mechanisms underlying spontaneous pain induced by BK are poorly understood. Here we report that in small nociceptive neurons from rat dorsal root ganglia, BK, acting through its B2 receptors, PLC, and release of calcium from intracellular stores, robustly inhibits M-type K+ channels and opens Ca2+-activated Cl– channels (CaCCs) encoded by Tmem16a (also known as Ano1). Summation of these two effects accounted for the depolarization and increase in AP firing induced by BK in DRG neurons. Local injection of inhibitors of CaCC and specific M-channel openers both strongly attenuated the nociceptive effect of local injections of BK in rats. These results provide a framework for understanding spontaneous inflammatory pain and may suggest new drug targets for treatment of such pain.
Boyi Liu, John E. Linley, Xiaona Du, Xuan Zhang, Lezanne Ooi, Hailin Zhang, Nikita Gamper
M channel inhibition and CaCC activation account for the BK-induced excitability in small DRG neurons.