Activation of spinal neurons : synaptic events in physiological pain

Among different forms of persistent pain, neuropathic pain presents as a most difficult task for basic researchers and clinicians. Despite recent rapid development of neuroscience and modern techniques related to drug discovery, effective drugs based on clear basic mechanisms are still lacking. Here, I will review the basic neuronal mechanisms that maybe involved in neuropathic pain. I will present the problem of neuropathic pain as a rather difficult task for neuroscientists, and we may have to wait for a long time before we fully understand how brain encode, store, and retrieve painful information after the injury. I propose that neuropathic pain as a major brain disease, rather being a clinic problem due to peripheral injury. Background Pleasure and pain are two major emotions of animals and humans. While the pleasure from different activities is what animals and human are seeking daily, both animals and humans avoid pain. Pain is divided into two major groups: physiological pain and pathological pain. Physiological pain is an important physiological function for survival. Depending on pain experience, animals and humans gain knowledge of potential dangerous stimuli in the environment, and pain-related unpleasantness help to form long-term avoidance memory in order to protect themselves. Although animals have the capability to enhance their sensitivity as well as motor responses to subsequent noxious stimuli, animals' ability to distinguish pain from other sensation is intact, at least not permanently altered. Unlike physiological pain, pathological pain only happen after injury (e.g., tissue or nerve injury), and is not the result of repetitive application of physiological pain. Long-term changes are likely to occur after injury, both peripherally and centrally. Consequently, the injury and injury-related areas undergo long-term plastic changes, and pain sensation is significantly enhanced (hyperalgesia) or non-noxious stimuli cause pain (allodynia). It should be pointed out that allodynia is one of the major problems in pathological pain. Because it is induced by non-noxious stimuli, it is mostly likely that central plastic changes play important roles. Thus, pathological pain is likely a result of long-term plastic changes along somatosensory pathways, from the periphery to cortex. Due to long-term plastic changes in central regions, pain specificity is lost at the first synapses of the somatosensory pathway, at least from areas where allodynia was reported. Here I will review recent progress related to neuronal mechanism for neuropathic pain, a form of persistent pain resistant to conventional treatment. I will focus on central plasticity, in particular the forebrain areas that are critical for the processing pain and pain-related emotional responses. Readers are referred to Published: 6 June 2007 Molecular Pain 2007, 3:14 doi:10.1186/1744-8069-3-14 Received: 28 May 2007 Accepted: 6 June 2007 This article is available from: http://www.molecularpain. com/content/3/1/14 © 2007 Zhuo; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.