Pain is a subjective experience comprised of physiological and affective components. Previous decades of research have placed an emphasis on pain “sensation″, which involves assessing location and intensity of noxious stimuli. However, somatosensory localization and intensity coding are not necessarily linked with emotional responses, as indicated by the IASP (International Association for the Study of Pain) definition, “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage″. Therefore, it is important to consider that not only the sensory, but also the emotional, perspective of pain plays an important role in pain suffering.
　Our group previously demonstrated, for the first time, that long-term persistent pain in an animal model of neuropathic pain, resulted in anxiety and depression related behavior. The many human emotions are all capable of altering pain sensitivity, as demonstrated by experimental and clinical studies focused on the associations between pain sensation and various emotions in human imaging studies. Negative emotions, such as anger, sadness, and anxiety, result in increased pain intensity. In contrast, positive emotion can regulate various pain sensations.
　Patients with certain psychiatric disorders, such as schizophrenia and borderline personality disorder, are less sensitive to pain. However, one of the main symptoms of depressive patients is “pain″. Although many of the neurobiological mechanisms of these diseases remain unclear, psychiatric disorders could reflect brain mechanisms of pain processing, because patients with psychiatric disorders exhibit varying reactions to experimental and clinical pain. Certain psychiatric disorders, in particular schizophrenia, could be considered to be human diseases that exhibit symptoms completely opposite to chronic pain.
　As stated by Prof. Loeser, “It is not pain, but suffering, that brings patients into doctor's offices in hopes of finding relief". Doctors of modern medicine tend to believe that it is more important to remove the cause of pain through methods such as nerve blockade. However, injuries, and the diseases that cause pain, might only be the trigger. The most important problem for these patients could be the change in social environment triggered by the injuries and diseases. In other words, affective components of pain are the main problems for these patients. Although it is difficult to distinguish whether patient pain is influenced by the affective components of pain, it is recommended to attempt to simultaneously treat patients according to sensory and emotional perspectives. It is expected that studies focused on the affective components of pain will make great advancements, and drug discovery will likely aim at specifically reducing suffering from pain, with an eventual paradigm shift in pain treatment.

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　Objective. Acupuncture is considered to be useful for clinical management of muscle pain. However, the mechanisms underlying the effect of acupuncture on muscle pain are still unknown. The purpose of our investigation was to determine the effect of electroacupuncture on muscle pain induced by local injection of 3% λ-carrageenan.
　Methods. A total of 28 male rats were allocated into 4 groups: control (CONT, n=7), electro-acupuncture (4 Hz, 20 min, 2 weeks) (EA, n=7), electroacupuncture under administration of naloxone (10 mg/kg, ip) (NAL, n=7) and under restriction (REST, n=7). All rats were injected with 3% λ-carrageenan (0.1 ml, im) in the left gastrocnemius (GS) muscle. Pain thresholds were measured with the Randall-Selitto test (RST) and von Frey filament test (VFT) over the GS muscle at scheduled intervals for 8 weeks after the injection.
　Results. In the CONT and REST groups, the RST thresholds on the injection side were decreased within 4 h after injection, which was sustained for 6 weeks. The influence was also observed on the contralateral side at two weeks after the injection. In the EA group, however, the thresholds recovered to the baseline within 4 days, although the decrease in the thresholds at 4 h after injection was comparable to that observed in the other groups, while in the NAL group, the recovery of the RST thresholds was similar as the REST group. The VFT thresholds on the injection and the contralateral side were not substantially changed in the time course in all groups.
　Conclusion. In the present study, GS muscle pain was not detected in the EA group. This result suggests that electroacupuncture reversed mechanical hyperalgesia in a muscle pain model through activation of the endogenous opioid system.

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