Inflammatory mediators of pain.

While sensory fibres normally respond to a range of physical and chemical stimuli their activity and metabolism are profoundly altered by a variety of mediators generated by tissue injury and inflammation. These include substances produced by damaged tissue, substances of vascular origin as well as substances released by afferent fibres themselves, sympathetic fibres and various immune cells. The effects of inflammatory mediators, to activate or sensitize afferent fibres, are produced by changing membrane ion channels which are coupled directly via receptors or more commonly are regulated through receptor-coupled second messenger cascades. These latter processes also have the potential to alter gene transcription and thereby induce long-term alterations in the biochemistry of sensory neurones. This can have far-reaching consequences as the expression of novel proteins for ion channels (Na channels) and receptors (capsaicin, NPY) as well as the induction of novel enzymes (i-NOS) can profoundly affect the properties of nociceptors and their ability to transmit pain signals. However, such changes may be targeted successfully for the development of new analgesic and anti-inflammatory agents.