Stress induced analgesia: its opioid nature depends on the strain of rat but not on the mode of induction.

Reports by several investigators have shown that both opioid and non-opioid analgesia can be induced by non-pharmacological manipulations such as the administration of electric shock, and that such analgesia depends on shock parameters, the affective state of the animal and the region of the body shocked. We tested several manipulations which have been reported to induce opioid analgesia using a local strain of rats (CR). Such manipulations included the used of 30 min of intermittent footshock (3 mA, 1 s on, 5 s off), brief shock to the forepaws, transpinal electroconvulsive shock (ECS) and tail shock induced helplessness. Administration of either naloxone or naltrexone to rats of the CR strain failed to attenuate the analgesic effect of these manipulations and in some cases even enhanced analgesia. The existence of functional opioid analgesia systems in CR rats was evident from the fact that electrical stimulation of the periaqueductal gray area produced naloxone sensitive analgesia. In additional experiments we compared the analgesic effect of brief continuous (3 min) footshock, prolonged intermittent footshock (30 min) and ECS in young (less than 75 days of age) and old (greater than 75 days of age) rats of the Sabra strain. Young Sabra rats showed naloxone sensitive analgesia following all 3 manipulations while adult rats displayed analgesia which was naloxone insensitive. Furthermore, no decrement in learning, indicative of helplessness, could be demonstrated in young Sabras following 3 min of shock which induced naloxone sensitive analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)