Processing nociceptive information is a consistent feature of the nervous system in all vertebrate species. This chapter reviews the nonmammalian models developed for pain and analgesia research, with a special emphasis on models that were used for detecting opioid antinociception. Nociceptive pathways in nonmammalian vertebrates are reviewed and endogenous opioid systems are described. Compared to mammalian models for pain research, there are relatively few models in each vertebrate class. Details are provided for the methods and the results of using an amphibian model for the testing of opioid analgesics. The chapter ends with a case study of the comparative approach in pain and analgesia research applied to the molecular evolution of opioid receptor proteins.
- Animal models