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 opi oid receptor antagonists. These behavioral responses in stress are acc ompanied by changes in the contents of opioid peptides, the mRNAs enco ding their precursors and opioid receptor binding in the brain. In the present article, experimental data concerning stress-induced analgesi a and motor suppression is reviewed and discussed in relation to a pos sible involvement of different opioid systems in the various observed behavioral responses in stress. Pharmacological studies with subtype-s elective antagonists have demonstrated that not only mu- but also delt a- and/or kappa-opioid receptors are involved in opioid-mediated stres s-induced analgesia. There are two types of stress-induced analgesia r eferred to as opioid-mediated and non-opioid mediated forms. It has be en 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 inhibito ry systems interact each other according to a collateral inhibition mo del. Recent studies show that parallel activation of multiple opioid r eceptors mediates non-opioid froms of stress-induced analgesia. Dynorp hins, by acting at kappa-opioid receptors, may play a pivotal role in the expression of stress-induced motor suppression, whereas enkephalin s may act to attenuate this response.