The results from the present study demonstrate that CRP treatment is sufficient for activation of the k-opioid receptor in the presence of PGE2. CRP, however, may not directly activate opioid receptors. Another study showed that the anti-nociceptive effect of CRP can be reversed by dynorphin A anti-serum, which indicates that endogenous opioids are involved in this phenomenon. We still cannot dismiss, however, that CRP may have a direct effect on opioid receptors. Nevertheless, by the results obtained in this study and in previous studies. Based on our in vivo findings, we next wanted to determine the effect of CRP on opioid receptor activation in PGE2-sensitized DRG cells isolated from naı ¨ve animals. PGE2 increased the CRPmediated k-opioid receptor activation. This result is consistent with our in vivo result showing that the CRP anti-nociceptive effect in the PGE2-induced hyperalgesia model is mediated by k- opioid receptor activation. Because inflammation increases the frequency of interactions between opioid receptors and G proteins and activates specific intracellular signaling pathways, we decided to investigate the molecular pathways activated by CRP in the presence or absence of PGE2 sensitization. It has been shown that the opioid receptor-mediated anti-nociceptive effect is regulated by the cGMP, AKT, and MAPK Tocilizumab signaling pathways. Furthermore, we recently showed that cGMP is involved in the CRP anti-nociceptive effect. In the present study, we demonstrated that CRP increases the cGMP level and that this effect is enhanced by PGE2. AKT activation is also involved in the peripheral anti-nociception induced by opioids, such as morphine and U 50,488. Our cell culture results indicate that the kopioid receptor agonist, used as a positive control, and not CRP, phosphorylates AKT. Interestingly, recent findings obtained by our group indicate that the PI3-AKT signaling pathway is not involved in the CRP anti-nociceptive effect in the PGE2-induced hyperalgesia model. CRP and the k-opioid receptor agonist activated ERK1/2 and JNK. It is important to mention that in all of the experiments, PGE2 increased the phosphorylation levels of MAPK caused by CRP. The role that MAPKs play in the CRP-mediated anti-nociceptive effect is currently unknown. It is possible that the activation of these kinases support the expression of the receptors/ channels and/or transcriptional factors involved in anti-nociception. For example, the phosphorylation of MAPK activates transcription factors,Etanercept such as CREB, which regulates dynorphin gene expression. Our results are consistent with the previously reported results showing that the k-opioid receptor agonist U 50, 488 activates MAPKs, such as ERK1/2 and p38 MAPK. Furthermore, we propose that the effect of CRP on ERK phosphorylation is mediated by activation of the k-opioid receptor because the selective opioid receptor antagonist Nor-BNI blocked this effect. We also found that Nor-BNI activates the JNK pathway. These results are consistent with previous results showing that Nor-BNI increases the level of phospho-JNK in the HEK293 cell line. It has been well established that the pharmacological effects of Nor-BNI, such as the antagonistic effect on agonist anti-nociception and intracellular signaling, are long lasting and involve an interaction between k-opioid receptors and JNK phosphorylation.