Animal models of pain for drug discovery.

One of the greatest challenges to discovering more efficacious medications for pain control has been the heterogeneity of the chronic pain condition in humans. It is now appreciated that distinct mechanisms contribute to normal physiological pain, pain arising from tissue damage and pain arising from injury to the nervous system. To study pain transmission, identify new pain targets and characterise the potential analgesic profile of novel compounds, an array of experimental animal pain models has been developed (mainly in rodents) attempting to replicate the many human pain conditions, including inflammatory, neuropathic, visceral and cancer pain states. The authors review commonly used rodent models of acute and chronic pain that have been used in an attempt to identify novel analgesic drugs. Although these animal models have helped to better understand pain physio-pharmacology mechanisms, one should remember that even for patients labelled under the same 'pain condition', the pain experience is unique, increasing the difficulty of modelling such painful states in animals. Looking back at decades of pain research, it is clear that the transition from preclinical findings to clinical applications in the treatment of pain has been difficult and that more predictive models need to be developed to facilitate the discovery and development of novel pain medications. For these reasons, particular attention has been given in this review to the more recently developed models of visceral, osteoarthritic and bone cancer pain.