Bradykinin inhibits M current via phospholipase C and Ca²⁺ release from IP₃-sensitive Ca²⁺ stores in rat sympathetic neurons

Abstract

A variety of intracellular signaling pathways can modulate the properties of voltage-gated ion channels. Some of them are well characterized. However, the diffusible second messenger mediating suppression of M current via G protein-coupled receptors has not been identified. In superior cervical ganglion neurons, we find that the signaling pathways underlying M current inhibition by B₂ bradykinin and M₁ muscarinic receptors respond very differently to inhibitors. The bradykinin pathway was suppressed by the phospholipase C inhibitor U-73122, by blocking the IP₃ receptor with pentosan polysulfate or heparin, and by buffering intracellular calcium, and it was occluded by allowing IP₃ to diffuse into the cytoplasm via a patch pipette. By contrast, the muscarinic pathway was not disrupted by any of these treatments. The addition of bradykinin was accompanied by a [Ca²⁺]_i rise with a similar onset and time to peak as the inhibition of M current. The M current inhibition and the rise of [Ca²⁺]_i were blocked by depletion of Ca²⁺ internal stores by thapsigargin. We conclude that bradykinin receptors inhibit M current of sympathetic neurons by activating phospholipase C and releasing Ca²⁺ from IP₃-sensitive Ca²⁺ stores, whereas muscarinic receptors do not use the phospholipase C pathway to inhibit M current channels.