Modulation Of Bone Cancer Pain By MAPKs In Rats

Cancer pain is one of the most severe types of chronic pain, and the most common type of cancer pain is bone cancer pain, which is caused by tumors that metastasize from distant sites such as breast, prostate, and lung to the bone. Compared with neuropathic and inflammatory pain, cancer pain might share similar physiological and pathological changes. At its earlier phase, an inflammatory response is inevitable, while in its later phase, nerve infiltration and compression by tumour cells, cancer pain may also be characterized as neuropathic pain. Thus, cancer pain, neuropathic pain and inflammatory pain may share some similar molecular and cellular mechanisms.Excellular signal-regulated kinase (ERK) and p38MAPK are main family members of mitogen-activated protein kinases (MAPKs), a reserved serine/ threonine kinase family. The activation of MAPKs in spinal neuronal and glial cells has been demonstrated to be involved in the mechanisms of inflammatory and neuropathic pain. However, as to the relationship of the two MAPKs and spinal glial activation during the development of bone cancer pain, our knowledge is very limited. Therefore, in a rat model induced by intra-tibia inoculation of Walker 256 mammary gland carcinoma cells, we inverstigated the temporal change of ERK and p38 activation in different cell types in the spinal dorsal horn following the development of bone cancer pain.Syngeneic Walker 256 mammary gland carcinoma cells (4×104) were injected into the tibia medullary cavity via intercondylar eminence. Series of tests were carried out including bone radiology, bone histology and behavioral tests. The rats inoculated with carcinoma cells showed significant spontaneous pain, mechanical allodynia and thermal hyperalgesia. The pain hypersensitive behaviors were aggravated with time and destruction of bone. Interestingly, mechanical allodynia was also observed in the contralateral limb, indicating the involvement of'mirror image'pain in bone cancer pain. Following the inoculation with Walker 256 cells, microglia and astrocytes were robustly activated in the bilateral spinal dorsal horn at days 14, and 21. Intrathecal injection of propentofylline (PPF), a glial inhibitor, and minocycline, a microglial inhibitor, significantly suppressed glial activation and bone cancer-induced mechanical allodynia.At days 14, and 21 after inoculation of tumour cells, the expressions of phosph-p38 (p-p38) and phosph-ERK (pERK) in both side of the spinal dorsal horn were remarkably increased. Double immunofluorescence showed that pERK was expressed in neurons and microglia of the spinal cord at days 3 and 7 after inoculation; while at days 14 and 21, pERK was detected in all the neurons, microglia and astrocytes of the spinal cord. Differentlly, p-p38 colocalized with NeuN (a neuronal marker) and OX-42 (a microglial marker) in the spinal dorsal horn at all days we observed. Preferentially, p-p38 expressed in neurons in early phase and in microglia in late phase respectively. To address the effects of inactivating p38/MAPK on the development of behavioral hypersensitivity in rats inoculated with carcinoma cells, repeated intrathecal injection of p-p38 inhibitor SB203580 (1,5, and 10μg) were carried out once daily for 2 weeks, with the first application 2 days before inoculation. At a dose of 10μg, SB203580 significantly suppressed the development of bone cancer pain. To address the effects of inactivating p38/MAPK and ERK/MAPK on the existed behavioral hypersensitivity evoked by bone cancer, intrathecal administrations of p-p38 inhibitor SB203580 (10μg), SB239063 (100μg) or MEK inhibitor U0126 (3μg) were performed at 14 days after inoculation. The bone cancer-induced mechanical allodynia was reversed by either p38 inhibitor or MEK inhibitor. These results suggest that MAPKs in spinal cord may be involved in both development and maintenance of bone cancer pain.Taken together, the present study indicates that activation of ERK/MAPK and p38/MAPK in spinal dorsal horn contributes to the development and maintenance of bone cancer pain. And the modulation of MAPKs to bone cancer pain may be involved in the activation of spinal glial cells. Thus, MAPK might become a novel target for treating bone cancer pain.