Stress can exert modulatory effects on pain perception in animals, as exposure to a stressor can result in either the reduction or amplification of the perceived severity of pain. These phenomena are widely described as stress-induced analgesia (SIA) and stress-induced hyperalgesia (SIH). The two are mediated by the same underlying mechanisms, but occur due to different stressors and different responses from the pain pathway. SIA and SIH have been demonstrated with a variety of stress and pain stimuli in rodents, humans and other mammals. There is some evidence that SIA occurs in birds and that they have the neurological systems and brain regions necessary for SIH. Tonic immobility (TI) is related to SIA in mammals, and there is evidence the avian brain is compatible with TI having analgesic effect, but it could have a hyperalgesic effect. This review looks at the mechanisms and evidence of SIA, SIH and TI in mammals and discusses evidence relating to the occurrence of these phenomena in birds.
