Substance P (SP) is a prominent neuromodulator, which is produced and released by peripheral damage-sensing (nociceptive) neurons; these neurons also express SP receptors. However, the mechanisms of peripheral SP signaling are poorly understood. We report a signaling pathway of SP in nociceptive neurons: Acting predominantly through NK1 receptors and G i/o proteins, SP stimulates increased release of reactive oxygen species from the mitochondrial electron transport chain. Reactive oxygen species, functioning as second messengers, induce oxidative modification and augment M-type potassium channels, thereby suppressing excitability. This signaling cascade requires activation of phospholipase C but is largely uncoupled from the inositol 1,4,5-trisphosphate sensitive Ca 2+ stores. In rats SP causes sensitization of TRPV1 and produces thermal hyperalgesia. However, the lack of coupling between SP signaling and inositol 1,4,5-trisphosphate sensitive Ca 2+ stores, together with the augmenting effect on M channels, renders the SP pathway ineffective to excite nociceptors acutely and produce spontaneous pain. Our study describes a mechanism for neurokinin signaling in sensory neurons and provides evidence that spontaneous pain and hyperalgesia can have distinct underlying mechanisms within a single nociceptive neuron.G protein coupled receptors | inflammatory pain | intracellular signaling | KCNQ | M current E xcitation of peripheral terminals of sensory neurons is a primary event in somatosensation, including pain. A large proportion of sensory neurons (mostly TRPV1 + damage-sensing or "nociceptive" neurons) produce neuropeptides such as substance P (SP), which are released in response to nociceptive stimulation both at spinal cord synapses and in the periphery (1). In the spinal cord SP acts as an excitatory neurotransmitter or cotransmitter (2, 3), whereas peripheral SP release is thought to underlie the inflammatory response known as "neurogenic inflammation" (4). Many peripheral nociceptors also express receptors for SP [neurokinin receptors (NKR) 1-3 (NK1-3)]; this expression may suggest paracrine or autocrine actions of this peptide. However, the nature of such actions is controversial, and the molecular events triggered by NKR in peripheral nociceptors are not well established. The NKR traditionally are classed as G q/11 -coupled G protein-coupled receptors (GPCR) that activate phospholipase C (PLC), with subsequent hydrolysis of membrane phosphatidylinositol 4,5-bisphosphate (PIP 2 ) and release of inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG) (5, 6). Common downstream steps of G q/11 signaling include IP 3 -mediated release of Ca 2+ from intracellular stores and DAG-mediated activation of PKC. However, it was hitherto unknown whether NKR in peripheral sensory neurons couple fully to this signaling cascade, because a lack of coupling between NKR and Ca 2+ release in dorsal root ganglia (DRG) neurons has been noted (ref. 7 and 8, but cf. ref. 9). Nevertheless, it is accepted that peripherally act...