Stress and the endogenous opioid system. II. Stress, stress models and endogenous opioid peptides

Biochemical and pharmacological findings indicate an activation of the opioid system under certain stress influences. In this paper findings are discussed, according to which the activation of the opioid system depends on the temporary distribution pattern and the kind of the stressor. In response to stress opioid peptides are involved in excitatory and inhibitory processes. The participation of different types of opioids in central and peripheral stress-induced effects is discussed. Some findings suggest that beta-endorphin acts mainly centrally rather than peripherally, whereas the enkephalins take part in peripheral regulatory processes (gastrointestinal system, adrenal glands). Now there is evidence that the endogenous opioids modulate stress-induced effects, but there has been much controversy on how this fact should be established. These controversies mainly resulted from the many factors that influence activation.