Spinal protein kinase C/extracellular signal–regulated kinase signal pathway mediates hyperalgesia priming
Chronic pain can be initiated by one or more acute stimulations to sensitize neurons into the primed state. In the primed state, the basal nociceptive thresholds of the animal are normal, but, in response to another hyperalgesic stimulus, the animal develops enhanced and prolonged hyperalgesia. The exact mechanism of how primed state is formed is not completely understood. Here, we showed that spinal protein kinase C (PKC)/extracellular signal–regulated kinase (ERK) signal pathway is required for neuronal plasticity change, hyperalgesic priming formation, and the development of chronic hyperalgesia using acid-induced muscle pain model in mice. We discovered that phosphorylated extracellular signal–regulated kinase–positive neurons in the amygdala, spinal cord, and dorsal root ganglion were significantly increased after first acid injection. Inhibition of the phosphorylated extracellular signal–regulated kinase activity intrathecally, but not intracerebroventricularly or intramuscularly before first acid injection, prevented the development of chronic pain induced by second acid injection, which suggests that hyperalgesic priming signal is stored at spinal cord level. Furthermore, intrathecal injection of PKC but not protein kinase A blocker prevented the development of chronic pain, and PKC agonist was sufficient to induce prolonged hyperalgesia response after acid injection. We also found that mammalian target of rapamycin–dependent protein synthesis was required for the priming establishment. To test whether hyperalgesic priming leads to synaptic plasticity change, we recorded field excitatory postsynaptic potentials from spinal cord slices and found enhanced long-term potentiation in mice that received one acid injection. This long-term potentiation enhancement was prevented by inhibition of extracellular signal–regulated kinase. These findings show that the activation of PKC/ERK signal pathway and downstream protein synthesis is required for hyperalgesic priming and the consolidation of pain singling.