Spinal CX3CL1/CX3CR1 May Not Directly Participate in the Development of Morphine Tolerance in Rats.

CX3CL1 (fractalkine), the sole member of chemokine CX3C family, is implicated in inflammatory and neuropathic pain via activating its receptor CX3CR1 on neural cells in spinal cord. However, it has not been fully elucidated whether CX3CL1 or CX3CR1 contributes to the development of morphine tolerance. In this study, we found that chronic morphine exposure did not alter the expressions of CX3CL1 and CX3CR1 in spinal cord. And neither exogenous CX3CL1 nor CX3CR1 inhibitor could affect the development of morphine tolerance. The cellular localizations of spinal CX3CL1 and CX3CR1 changed from neuron and microglia, respectively, to all the neural cells during the development of morphine tolerance. A microarray profiling revealed that 15 members of chemokine family excluding CX3CL1 and CX3CR1 were up-regulated in morphine-treated rats. Our study provides evidence that spinal CX3CL1 and CX3CR1 may not be involved in the development of morphine tolerance directly.

Potential role of CXCL10/CXCR3 signaling in the development of morphine tolerance in periaqueductal gray.

Tolerance to morphine antinociception hinders its long-term use in clinical practice. Interaction between neuron and microglia has been proved to play critical role in the mechanism of morphine tolerance, while CXCL10/CXCR3 signaling has been implicated in neuron-glia signaling and morphine analgesia. This study aims to investigate whether CXCL10/CXCR3 signaling in periaqueductal gray (PAG) contributes to the development of morphine tolerance by modulating neuron-microglia interaction. The results showed that the expressions of CXCR3 and CXCL10 were gradually increased in parallel with repeated morphine administration and activation of microglia. CXCR3 was co-localized with neuronal marker NeuN, while CXCL10 was derived from microglia. Microglia inhibitor minocycline significantly attenuated the expression of CXCL10, besides, both minocycline and CXCR3 inhibitor alleviated the development of morphine tolerance. Taken together, our study provided the evidence that CXCL10/CXCR3 signaling in PAG is involved in the development of morphine analgesic tolerance via neuron-microglia interaction.

Involvement of chemokine CXCL11 in the development of morphine tolerance in rats with cancer-induced bone pain.

Morphine is viewed as one of the classical treatments for intractable pain, but its role is limited by side effects, including analgesic tolerance. A few chemokines have been reported to be engaged in the mechanisms of morphine tolerance. However, the exact roles of CXC chemokine 11 (CXCL11) in chronic morphine tolerance remain unknown. In this study, Walker 256 mammary gland carcinoma cells were inoculated into the tibia of rats to provoke cancer-induced bone pain. Then, morphine was intrathecally administered twice daily for seven consecutive days to induce drug tolerance. We found that the level of CXCL11 in lumbar spinal cord was increased during the development of morphine tolerance in cancer-induced bone pain rats. Meanwhile, CXCL11 was co-localized with markers of astrocytes and neurons in the spinal cord. Inhibition of CXCL11 by neutralizing antibodies could remarkably attenuate the degree of morphine tolerance and decrease the activation of astrocytes. Moreover, blocking astrocyte activation by d, l-Fluorocitric acid could distinctly alleviate morphine tolerance and reduce the expression of CXCL11. Finally, morphine stimulation could induce the release of CXCL11 by cultured astrocytes and neurons in vitro. In summary, our results provide evidence that spinal CXCL11 plays a powerful modulatory role in the development of morphine tolerance through cross-talking between astrocytes and neurons. Read the Review series "Pain".

CXCR3: latest evidence for the involvement of chemokine signaling in bone cancer pain.

Growing evidence indicates that chemokines participate in the generation and maintenance of bone cancer pain (BCP). Recent work in Exp Neurol by Guan et al. (2015) demonstrated the involvement of spinal chemokine receptor CXCR3 and its downstream PI3K/Akt and Raf/MEK/ERK signaling pathways in BCP. This work provides new evidence to support that chemokines participate in central sensitization in BCP condition. Reviewed evidence suggests that few chemokines have been proved to be related to cancer pain. The underlying relationship between CXCR3 signaling and BCP condition requires further study.

Activation of CXCL10/CXCR3 signaling attenuates morphine analgesia: involvement of Gi protein.

Morphine is a potent agonist of µ-opioid receptor and is widely used to relieve severe pain, including cancer pain. Some chemokines, for example, CX3CL1 and CCL2, participate in the regulation of opioid santinociception. In our previous study, we found overexpression of chemokine CXCL10/CXCR3 in spinal cord participated in the development of cancer-induced bone pain, so we supposed that CXCL10 may have influence in morphine analgesia in cancer pain relief. In this study, we found that a single dose of morphine could transiently increase the expression of CXCL10 in spinal cord. Blocking the function of CXCL10 enhanced morphine antinociception in cancer-induced bone pain rats. However, overexpression of CXCL10 induced acute algesia and decreased the analgesic effect of morphine in normal mice. The algesic effect of CXCL10 was blocked by inhibition of CXCR3 and Gi protein. These results suggested that CXCL10 in spinal cord serves as a novel negative regulator of morphine analgesia and provided evidence that activation of CXCL10/CXCR3 in spinal cord may attenuate antinociceptive potency of morphine in cancer pain relief.