Stereotypic behavior, endogenous opioids, and postfeeding hypoalgesia in pigs.

Tethered sows, some of which performed marked behavioral stereotypies after feeding, were injected IM with 1 mg/kg of naloxone 30 min before feeding (with 2 saline control days). Tail-flick latencies on a pain-sensitivity test were recorded before and after feeding. On control days, tail-flick latencies after feeding were longer than those before feeding, and this effect was abolished by naloxone pretreatment. Thus, there is an opioid-based hypoalgesia after feeding. However, sows with marked behavioral stereotypies had shorter tail-flick latencies after feeding. Thus, we have no evidence that performance of behavioral stereotypies results in increased opioid activity. Naloxone reduced the time spent in behavioral stereotypies by approximately 30% but this may be due to a reduction in time spent active. Naloxone increased the frequency and reduced the mean duration of bouts of chain manipulating, operating the drinker and rooting. We suggest that endogenous opioids are involved in the positive feedback that maintains the persistence of behavior and inhibits switching between different activities.