Social influences on morphine conditioned place preference.

Social defeat in dominant male rats has previously been shown to result in a loss of rank, together with decreased consumption of a palatable sucrose solution, suggestive of a decrease in hedonic tone. The present study further investigated the effect of defeat on hedonic capacity using the conditioned place preference paradigm. Stable dominance hierarchies were determined in pairs of male PVG rats by repeated observation of agonistic behaviour at the onset of the dark phase of the dark-light cycle. Dominant animals and singly housed animals were then subjected to defeat by a male of the aggressive Tryon Maze Dull (TMD) strain, either 1 (singly housed animals only) 3 or 7 days prior to the first conditioning session. Each animal was tested for place conditioning with morphine (1 mg/kg i.p.) using one rewarded and one non-rewarded conditioning trial. In this paradigm, dominant animals as well as singly housed animals showed an increase in time spent in the drug-associated side but their submissive partners did not. Three days following the defeat of a dominant animal by a TMD, place conditioning to morphine was absent in the defeated animals, but was now present in their submissive partners. Seven days following defeat, conditioning was absent in defeated animals that were now submissive, but present in defeated animals that had maintained their dominant status. In isolated animals, place conditioning was absent 1 day following defeat, but was present 3 and 7 days post-defeat. These findings suggest that morphine-induced place conditioning is influenced by both the experience of defeat and the effects of defeat on social status.