Analgesic Effect of Crotalphine in a New Model of Rat Bone Cancer Pain

Abstract

Background: Crotalphine (CRP), a peptide first identified and isolated from the South American rattlesnake Crotalus durissus terrificus venom, induces analgesic effect mediated by opioid receptors. The aim of this work is to characterize the analgesic effect of crotalphine in a new model of bone cancer pain induced by inoculation of Walker 256 tumor cells into the rat femoral cavity. Methods: Bone tumor implantation and metastasis were determined by histopathological analysis. Bone metabolic alterations were determined by scintigraphy, using 99mTc-MDP. Femoral images were obtained before and 7, 14 and 21 days after tumor cell injection. Bone cancer pain was characterized by the presence of hyperalgesia (rat paw pressure test) and allodynia (von Frey filaments). Results and Discussion: Photomicrographs analyzed 21 days after injection of tumor cells, demonstrated the presence of tumor cells in the femur of the animals. Incorporation of 99mTc-MDP was significant 7, 14 and 21 days, suggesting the development of tumor on the femoral cavity. Histopathological analysis demonstrated the presence of tumor cells in the lung and spleen, but not in the liver and kidneys of the rats. The results indicate that cells inoculated into femoral bone marrow can spread to some organs, including lymphoid organs. Hyperalgesia and allodynia were detected on days 1, 3, 7, 14 and 21 after cell inoculation. Interestingly, the paw withdrawal threshold in the von Frey test was reduced not only in the ipsilateral hind paw but also in the contralateral one, demonstrating the existence of bilateral allodynia (mirror-image pain). To evaluate the involvement of prostanoids in these nociceptive phenomena, Indomethacin, a cyclooxygenase inhibitor, was administered 3,7,14 and 21 days after tumor cell injection. Indomethacin only partially inhibited hyperalgesia and allodynia induced by bone cancer, indicating the involvement of prostanoids in bone cancer pain. The contribution of prostanoids is more significant within the first 3 days after cell injection. CRP (8μg/kg) administered on day 21, blocked hyperalgesia, allodynia and mirror image pain. The analgesic effect was detected up to 2 days after peptide administration and was blocked by κ-opioid receptor antagonist and partially inhibited by δ-opioid antagonists, indicating the involvement of opioid receptors. Morphine only partially inhibited allodynia and hyperalgesia. Conclusions: Results indicate that injection of tumor cells causes bone cancer and pain. CRP induces a potent and long-lasting antinociception in this model, with higher efficacy as compared to standard analgesic drugs.