Stress-induced hypoalgesia and opioid inhibition of pigs' responses to restraint.

Pigs' responses to physical restraint were examined in order to detect a stress-induced increase in endogenous opioid activity. Tail-flick latencies in response to a source of thermal energy were used to assess the sensitivity of pigs to pain. Restraining pigs for 15 min with a nose snare resulted in a temporary increase in tail-flick latencies that was apparent after 5 min, absent after 30 min and was blocked by naloxone. Tail-flick latencies were unaffected by IV ACTH injections and were not related to plasma cortisol concentrations. Naloxone increased the pigs' vocalization during the restraint and prolonged the elevation of plasma cortisol levels that followed the restraint. The cortisol response to naloxone was larger than when the animals were not restrained. The results indicate a transient, opioid-based hypoalgesia following restraint stress that is not a result of ACTH or cortisol secretion. Endogenous opioids inhibit the pigs' behavioral and pituitary-adrenocortical responses to the restraint stress.