Historically, much of the classic research on stress in rodents has examined how aversive stimulation, such as painful electric shock, affects unlearned defensive responses, stress-induced analgesia, drug reactivity, learned responses, and more complex cognitive behaviors.
Williams, Worland, and Smith (1990) found that this paradigm of social defeat not only elicited freezing and interfered with prod burying, but the odors of the alpha conspecifics also produced stress-induced analgesia during a post-defeat formalin test for pain sensitivity.
Research has demonstrated that both of these responses are motivated by fear (Fanselow, 1980) and stress-induced analgesia can be regulated by either opioid or nonopioid processes (Maier, 1989).
The significant reduction in recuperative paw licking in Group SAL/TMT, relative to Group SAL/CIT, indicates that the predator odor of TMT produced a stress-induced analgesia that was not simply the result of the novelty of the odors.
This type of transituational analgesic reaction, following repeated defeat, appears to be similar to the stress-induced analgesia that has been reported in "learned helplessness" studies in which rats were exposed to a series of inescapable shocks (Maier, 1989).
Long-term stress-induced analgesia and activation of the opiate system.