Keratinocytes acting on injured afferents induce extreme neuronal hyperexcitability and chronic pain

Keratinocytes play an important role in the dialog between skin and cutaneous sensory neurons. They are an essential source of cutaneous nerve growth factor (NGF), a neurotrophin that contributes to persistent pain in inflammation and neuropathy. We studied the interaction of human keratinocytes (hKTs) and regenerating afferent nerve fibers by transplanting hKTs into a ligated and transected peripheral nerve. The hKTs self-assembled into a multi-laminar spheroid cellular structure resembling the stratum spinosum of epidermis. Axonal sprouts surrounded the structure although they were excluded from entry. Levels of NGF were elevated at the transplant site. Whole cell patch-clamp recordings from primary afferent neurons whose cut axons were present near the transplanted hKTs displayed extreme hyperexcitability. These neurons generated high frequency trains of action potentials during step depolarization stimuli, and they sometimes showed afterdischarge and fired spontaneously at resting membrane potential. This spontaneous firing originated from subthreshold membrane potential oscillations. The animals with the hKT transplants exhibited spontaneous pain behavior manifest as autotomy. The results demonstrate that an interaction between injured/regenerating nerve fibers and keratinocytes such as may occur during wound healing, results in afferent hyperexcitability and pain. These results have implications for persistent pain associated with burn and traumatic skin injuries.