Abstract. The phosphorylation changes of nociceptive signaling proteins in the spinal cord dorsal horn (SCDH) are important in creating exaggerated pain following peripheral inflammation. Electroacupuncture (EA) has been widely used to relieve acute and chronic inflammatory pain in human and experimental pain models. In the present study, we performed a phosphoproteomic analysis to investigate whether EA alters protein phosphorylation in SCDH to attenuate pain development. Inflammatory hyperalgesia was induced by intraplantar injection of complete Freund's adjuvant (CFA) into the rat hind paw. EA treatment at ST36 and SP6 acupoints alleviated thermal hyperalgesia of the CFA-induced inflammatory pain model rats. The SCDH proteins from the control, inflammatory pain model and EA treatment rats were separated by 2-dimensional gel electrophoresis and the alterations in phosphoproteins were detected by Pro-Q Diamond staining. Eight proteins were differentially phosphorylated following EA treatment in the inflammatory pain model. Aldolase C, nascent polypeptide-associated complex α, stress-induced phosphoprotein 1 and heat shock protein 90 were identified as phosphoproteins whose expression was increased, whereas GDP dissociation inhibitor 1, thiamine triphosphatase, phosphoglycerate kinase 1 and 14-3-3 γ were phosphoproteins whose expression was decreased. This is the first phosphoproteomic screening study to elucidate the working mechanisms of EA analgesia. The results suggest that the regulation of cellular pathways in which the identified proteins are involved may be associated with an EA analgesic mechanism.
IntroductionPeripheral inflammation frequently leads to the development of chronic pain, which is a leading cause of disability globally. In inflammatory pain, peripheral stimuli activate primary afferent neurons and are transmitted to secondary sensory neurons in the spinal cord dorsal horn (SCDH). Persistent noxious stimuli increase the excitability of secondary neurons in the SCDH and decrease the pain threshold. Early studies attributed this central sensitization to the facilitated synaptic transmission of nociceptive signals in the SCDH (1,2). Various signaling molecules such as glutamate receptors, protein kinase A (PKA), protein kinase C (PKC), phosphatidylinositol 3-kinase (PI3K), Ca 2+ -/calmodulin-dependent kinase II (CaMKII) and mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), p38 and c-jun N-terminal kinase (JNK), have been shown to be involved in altered nociceptive processing in the SCDH (3-5). Activation of these molecules is capable of inducing signaling cascades that lead to the subsequent expression of pain-associated genes such as prodynorphin, neurokinin-1 (NK-1) and cyclooxygenase-2 (COX-2) (6,7).Acupuncture, a traditional therapeutic method in oriental medicine, has long been clinically used to relieve pain. In particular, electroacupuncture (EA), where electrical stimulation is applied to acupuncture needles, is widely used in clinical an...