In vivo somatosensory stimuli evoked the release of substance P from primary afferent neurons that terminate in the spinal cord and stimulated endocytosis of substance P receptors in rat spinal cord neurons. The distal dendrites that showed substance P receptor internalization underwent morphological reorganization, changing from a tubular structure to one characterized by swollen varicosities connected by thin segments. This internalization and dendritic structural reorganization provided a specific image of neurons activated by substance P. Thus receptor internalization can drive reversible structural changes in central nervous system neurons in vivo. Both of these processes may be involved in neuronal plasticity.

It has been suggested that neuroimmunologic mechanisms may be involved in the development and maintenance of neuropathic pain. To further address this concept, the immunoreactive spinal expression of the pro-inflammatory cytokine, interleukin-6 (IL-6), was determined in the mononeuropathy model in the rat, sciatic cryoneurolysis (SCN). This well-established animal model expresses behaviors suggestive of neuropathic pain in humans. Immunohistochemical localization in the spinal cord was determined at 3, 7, 14, 21, 35, and 120 days after SCN (n = 6 per time point). Immunoreactive IL-6 increased incrementally in the substantia gelatinosa and motoneurons over time following SCN as compared with normal rats. In an additional study, recombinant human IL-6 was administered intrathecally to normal and previously SCN-lesioned rats. Intrathecal IL-6 produced touch-evoked allodynia (increased sensitivity to a nonnoxious stimulus) in normal rats and thermal hyperalgesia (increased sensitivity to a noxious stimulus) in previously lesioned SCN rats. These results provide evidence that IL-6 may be involved in the cascade of events leading to the development and maintenance of behaviors suggestive of neuropathic pain following peripheral nerve injury.

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Recently, investigators began using radiofrequency to manage knee osteoarthritis pain in patients at high risk who cannot undergo surgical intervention. To our knowledge, no study has investigated the use of radiofrequency ablation of the genicular nerves to alleviate chronic knee pain after total knee replacement. A single case is presented here in which genicular nerve ablation successfully improved pain and restored function. We believe that these preliminary results could be used in the development of future prospective cohort studies and randomized controlled trials that focus on the use of radiofrequency ablation to treat persistent knee pain after total knee replacement.

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