A mouse model of neuropathic cancer pain

We developed a mouse model of neuropathic cancer pain by inoculating Meth A sarcoma cells to the immediate proximity of the sciatic nerve in BALB/c mice. The tumor grows predictably with time and gradually compresses the nerve, thereby causing nerve injury. Time courses of thermal hyperalgesia and mechanical sensitivity to von Frey hairs were determined and signs of spontaneous pain were evaluated. We compared this model with the chronic constriction injury (CCI) model, which is a neuropathic pain model widely utilized in the rat. Furthermore, to characterize the difference in nerve injury between the two models, we performed histological examination of the nerve of the two models by light and electron microscopy. Progressive compression of the sciatic nerve by growth of a tumor mass resulted in a gradual development of thermal hyperalgesia and mechanical allodynia in the ipsilateral hind paw. Signs of spontaneous pain, such as lifting of the paw, were also observed. However, further growth of the tumor reversed the mechanical hypersensitivity and produced mechanical hyposensitivity, while thermal hyperalgesia and signs of spontaneous pain still persisted. Histologically, gradual compression by the tumor resulted in a progressive damage to both myelinated and unmyelinated fibers. However, the severity of damage to the myelinated fibers was considerably less compared to that of the CCI mice. In the CCI mice, severe damage to myelinated fibers, especially large fibers, was observed and unmyelinated fibers were damaged to a lesser degree. These results suggest that gradual compression of a nerve by a malignant tumor results in nerve damage with a profile considerably different from that of chronic constriction injury produced by loose ligation of the nerve. Our new tumor model may be useful in studies of neuropathic cancer pain due to nerve compression by malignant tumors.