Stress-induced behavioral responses and multiple opioid systems in the brain

Various stressor produce a wide range of behavioral responses such as analgesia, catalepsy and motor suppression, which are sensitive to opioid receptor antagonists. These behavioral responses in stress are accompanied by changes in the contents of opioid peptides, the mRNAs encoding their precursors and opioid receptor binding in the brain. In the present article, experimental data concerning stress-induced analgesia and motor suppression is reviewed and discussed in relation to a possible involvement of different opioid systems in the various observed behavioral responses in stress. Pharmacological studies with subtype-selective antagonists have demonstrated that not only μ- but also δ- and/or κ-opioid receptors are involved in opioid-mediated stress-induced analgesia. There are two types of stress-induced analgesia referred to as opioid-mediated and non-opioid mediated forms. It has been proposed that the intensity and temporal pattern of stressor may be a critical factor determining the nature of stress-induced analgesia. Accumulated evidence demonstrate that these two foms of pain inhibitory systems interact each other according to a collateral inhibition model. Recent studies show that parallel activation of multiple opioid receptors mediates non-opioid forms of stress-induced analgesia. Dynorphins, by acting at κ-opioid receptors, may play a pivotal role in the expression of stress-induced motor suppression, whereas enkephalins may act to attenuate this response.