Activation of spinal Extacellular Signal-Regulated Kinases and c-jun N-terminal kinase signaling pathways contributes to morphine-induced acute and chronic hyperalgesia in mice

Background This study investigated the activation of mitogen-activated protein kinases in the spinal dorsal horn to explore the mechanisms underlying morphine-induced acute and chronic hyperalgesia in mice. Methods Male adult mice were given a single subcutaneous injection (SC) of morphine (1 mu g/kg) or twice-daily administration of morphine (10 mg/kg/day) for 8 days. Thermal hyperalgesia and mechanical allodynia were assessed using the radiant heat and von Frey filament test. Levels of phospho (p)-extracellular signal-regulated kinases (p-ERK), p-c-Jun N-terminal kinase (p-JNK), p-p38, p-PKC gamma, N-methyl-d-aspartate receptor (NMDAr), and c-Fos protein in the spinal dorsal horn were examined by Western blot assays. Results A single ultra-low dose or repeated administration of morphine induced hyperalgesia in mice and caused a significant increase in the levels of p-ERK and p-JNK, but not p-p38, in the spinal dorsal horn. The level of c-Fos protein was significantly elevated following administration of morphine. The protein levels of p-PKC gamma and NMDAr subunits (NR2B and NR2A) were also altered. Pretreatment with the NMDAr antagonist MK-801 or the protein kinase C (PKC) inhibitor calphostin C (CC) suppressed the morphine-induced increase in p-ERK, p-JNK, and c-Fos. Administration of MK-801 and CC also relieved morphine-induced hyperalgesia. Conclusion These findings suggest that activation of the spinal ERK and JNK signaling pathways contribute to morphine-induced acute and chronic hyperalgesia in mice.