The role of endorphins in stress-induced analgesia.

We have seen that exposure of an organism to any of a wide range of stressful situations can induce alterations in sensitivity to pain that outlast the exposure. Not all stressors induce analgesia; among those that do not are some that produce maximal elevations in plasma beta-endorphin, ACTH, and adrenal corticosteroids. Some examples of SIA are sensitive to opiate receptor blockade by naloxone, but others are not. Hypophysectomy produces a similarly uneven profile of effects across different stressors. This diversity has often been interpreted as evidence for the existence of an array of pain inhibitory systems, with differing physiological properties and activated by different stressors. However, it might also suggest that stressors can prompt a variety of behavioral changes, many of which can be interpreted as analgesia if a pain reflex test is employed as the dependent measure.