Species-specific danger signals, endogenous opioid analgesia, and defensive behavior

The effects of handling stimuli and stress odors on species-specific defensive behavior and pain sensitivity were examined in rats. Animals not adapted to handling had longer jump latencies on the hot plate test of pain sensitivity than those with extensive handling experience. In a postshock freezing test, naltrexone enhanced defensive freezing relative to saline controls in nonadapted animals. However, naltrexone produced no such effect in rats that were adapted to handling. These two studies indicate that the handling procedure triggered an endogenous opioid analgesic response in rats not adapted to handling. Experiment 3 showed that a similar naltrexone-reversible opioid analgesia can be triggered by stress odors. Naltrexone, when compared to saline, enhanced postshock freezing in the presence of conspecific stress odors, but not in their absence. In Experiment 4, stress odors and nonadapted handling were able to activate defensive freezing directly, when tested in compound but not in isolation. The studies are consistent with the view that stress odors and handling stimuli are danger signals that activate endogenous opioid analgesia as well as defensive behavior, suggesting that analgesia is a component of the rat's defensive behavior system.