Mouse models of pain from bone cancer.
The first animal model of cancer pain was developed in 1998. In this mouse model, 105 (20 μl) fibrosarcoma cells (NCTC 2472) were implanted directly into the femur. A crucial component of this model is that the tumor cells are confined to the marrow space of the injected femur and do not invade adjacent soft tissues.148 After injection, both ongoing and movement-evoked pain-related behaviors increase as the cancer cells proliferate and the tumor develops. These behaviors are correlated with the progressive tumor-induced bone destruction that ensues, and they seem to mimic those of patients with primary or metastatic bone cancer.103 This model was used to obtain new information about the mechanisms that generate bone cancer pain.148,182,183 This femur bone cancer model has been used to examine tumor-induced bone destruction, pain behaviors, and spinal cord neurochemical changes in the mouse (B6C3-Fe-a/a and C3H/HeJ).72,148 The animals with intramedullary femur tumors showed nocifensive behaviors (vocalization and guarding of the affected limb) and mechanical allodynia (a response to nonnoxious mechanical stimuli, such as light touch or palpation). This model also revealed important neurochemical changes in the spinal cord including 1) an increase in dynorphin (a prohyperalgesic neuropeptide) expression in deep laminae of the spinal cord dorsal horn; 2) an increase in c-fos expression (a marker of neuronal activation) in spinal cord lamina I; and 3) an internalization of substance P (an important neurotransmitter in nociception) receptors in the ipsilateral tumor-injected side of the spinal cord.72,148 These spinal cord changes are normal after application of a noxious stimulus, but they also are present in cancer-affected animals after nonnoxious stimuli (in this case, palpation).