Abstract

This study investigated whether the duration of electric foot shock is a critical factor to induce analgesia mediated by opiate and/or non-opiate system, whether analgesia by the opiate system is at the same time mediated by the anterior pituitary secreting ACTH and fl-endorphin and whether pain reactivity returns normal when the subjects experience the foot shock repeatedly. In Exp. I, 40 male rats were randomly assigned to one of four groups and intraperitoneally injected with naloxone, dexamethasone or saline, or pricked by the needle only respectively. All the animals were then subjected to footshock for 17.5 min. delivered under a 2-sec-on/5-sec-off paradigm, after which paw-licking latency on a hot plate was measured as an index of pain reactivity. 10 minutes later, the subjects again received the same patterns of the footshock, after which the latency was also measured. In Exp. II, new 40 male rats were employed and subjected to the same treatment as in Exp.I except that this time the footshock was delivered for 5 min. continuously. Results indicate that analgesia is mediated by the opiate system when the duration of footshock is relatively long (17.5 min. intermittent) and by the non-opiate system when it is relatively short (5 min. continuous), that the opiate system-mediated analgesia is simultaneously mediated by the anterior pituitary which is indicated by the fact that only under the same conditions antagonizing effects on analgesia of naloxone and dexamethasone took place, and that when the animal is repeatedly exposed to the footshock pain reactivity returns normal presumably because of the development of tolerance to analgesia.