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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.

In the absence of skin innervation, wound healing is delayed and chronic nonhealing wounds may occur. Keratinocytes produce neurotrophic factors, such as nerve growth factor (NGF), which has been suggested to attract primary cutaneous afferent axons and exert mitogenic effects on keratinocytes. The present study was performed to examine the interaction of primary human keratinocytes (hKTs) and rat cutaneous primary afferent dorsal root ganglion (DRG) neurons with regard to neuritic outgrowth and keratinocyte proliferation. Neuritic outgrowth was assessed with neurofilament immunostaining where cell bodies and fine neuritic processes were identified. Neuritic outgrowth of neurons alone in culture is spatially random and radial. Neurites in cocultures of DRG neurons insinuated between the hKTs and grew to "clumps" of hKTs within the cultures. Immunostaining with anti-NGF antibody indicates that hKTs expressed the neurotrophin NGF. Proliferation of keratinocytes was significantly enhanced in coculture with DRG and hKT, and NGF levels were increased as compared to DRG or hKT culture alone. These results indicate a dynamic interaction between DRG neurons and hKTs whereby the DRG neurons issue neurites in association with hKTs and the hKTs up-regulate NGF and increase their proliferation rate. These findings support the hypothesis that nerve-skin interactions play a significant role in wound healing.

Mammalian motor and sensory fibers respond differently to the potassium channel-blocking agent, 4-aminopyridine (4-AP). The action potentials of the motor fibers increase in duration after 4-AP, while the sensory fibers respond with bursts of action potentials after a single stimulus. These differences may account for the paresthesias reported by patients with multiple sclerosis following treatment with 4-AP.