Animals and humans share similar mechanisms of pain detection and similar brain areas involved in pain processing.Also,they show similar pain behaviors,such as reflexed sensation to nociceptive stimuli.Pain is often described in sensory discrimination （algosity） and affective motivation （unpleasantness） dimensions.Both basic and clinical findings indicate that individuals with chronic pain usually suffer more from pain-associated affective disturbances than from the actual pain sensations per se.Although the neural systems responsible for the sensory component of pain have been studied extensively,the neural mechanisms underlying negative affective component are not well understood.This is partly due to the relative paucity of animal paradigms for reliable examination of each component of pain.In humans,the experience of pain and suffering can be reported by language,while in animals,pain can only be inferred through physical and behavioral reactions.Animal behaviors,cognitive psychology and functional brain imaging have made it possible to assess pain affection and pain memory in animals.Animals subjected to either neuropathic injury or inflammatory insult display significant conditioned place aversion to a pain-paired environment in behaviors.The present review aims to summarize the common methods of affective unpleasantness assessment in rats.
