Chronic spontaneous neuropathic pain arising from peripheral nerve injury is driven by ectopic activity of injured axons from a genetically-defined C-fiber nociceptor subtype (SSTR2+ neurons) within neuromas, establishing a cellular and mechanistic separation from stimulus-evoked neuropathic pain mediated by intact larger-diameter sensory neurons in spared nerves.

Spontaneous pain is a common but poorly understood consequence of peripheral nerve injury, including injuries that lead to the formation of neuromas. We developed a chronic neuroma model for measuring spontaneous pain-related behaviours in mice, which revealed that limb flicks - emerging predominantly 2 months post-injury - reflect spontaneous paroxysmal pain. Ectopic activity of injured dorsal root ganglia (DRG) sensory neurons whose axonal endings terminate within the neuroma drives this spontaneous pain. In vivo imaging showed that a subset of small-diameter DRG sensory neurons are the source of spontaneous neural signals emanating from the neuroma, and these spontaneously active neurons are distinct from the intact larger diameter sensory neurons that mediate stimulus-evoked mechanical allodynia from spared nerves. Cell-type–specific gain- and loss-of-function studies identified a genetically- and functionally-defined subtype of small-diameter C-fibre nociceptors whose injured axons in neuromas drive spontaneous limb flicks/neuropathic pain. These findings establish the neurobiological basis of spontaneous pain enabling targeted pain management strategies and define a cellular and mechanistic separation between spontaneous and evoked neuropathic pain.