Factors which might modify analgesic effect of morphine in differentially housed rats.

The analgesic effect of morphine was studied by tail compression method in grouped and chronically isolated Wistar male rats developing muricide behavior showed increased pain threshold whilst indifferent (non-killer) showed reduced responses to the analgesic action of morphine. "Natural" killer (housed in groups) showed, on the other hand, increased sensitivity to morphine. The role of factors such as isolation period, behavioral pattern and contact with the victim was discussed.