The analgesic effect and mechanism of celastrol in formalin-induced inflammatory pain mouse model

Celastrol (CEL), a quinonic methyl triterpenoid extracted from the rootstock of the plant Tripterygium wilfordii, which shows therapeutic potential in improving inflammatory diseases, tumor, metabolic diseases and neurodegenerative diseases. However, the role of CEL in acute inflammatory pain is still unclear. In our experiment, the inflammatory pain model induced by formalin was used to explore the mechanism and effect of CEL on pain regulation. The results showed that intraperitoneal injection (i.p.) or intragastric gavage (i.g.) of CEL (0.5 mg/kg and 1.0 mg/kg) obviously reduced the time of licking/biting the right hind paw in the formalin test, without affecting normal motor function or voluntary activity. Moreover, both naloxone (NLX, 1.0 mg/kg, co-injected with CEL) and nor-binaltorphimine (nor-BNI, 10 mg/kg, administrated 24 h prior to CEL) significantly antagonized the antinociceptive effect of CEL, indicating that the κ-opioid receptor was involved in the antinociceptive effect of CEL. In addition, CEL significantly up-regulated the gene expression and protein levels of dynorphin mRNA and Oprk1, while did not influence the expression of Penk, Pomc, Oprm1 and Oprd1. The protein expression levels of p-CaMKII, p-ERK and p-CREB were also significantly decreased in the CEL treated group. Moreover, the immunohistochemistry and immunofluorescence results demonstrated the numbers of positive c-Fos and GFAP in the dorsal horn (L4-6) were obviously reduced. However, all of these protein's expression change induced by CEL were blocked by nor-BNI. Thus, our results indicate that CEL may produce analgesic effect through activating KOR, which subsequently reduce the expression of p-CaMKII/ERK/CREB and inhibit the activation of neural cells in the dorsal horn (L4-6).