The effects of naloxone and cage size on social play and activity in isolated young rats.

The effects of large and small housing environments as well as naloxone on social play (as defined by pinning behavior) in isolated postweanling male rats were investigated. Animals housed in small cages played significantly more than those housed in larger cages. This effect was not observed when cage size was reversed. Administration of 0.5, 1.0, and 5.0 mg/kg of naloxone resulted in significant decreases in play behavior as compared to saline controls, both before housing environments were switched, and after. Animals in both isolation conditions were also compared to animals that were socially housed. Isolation was found to increase social play as well as the time spent in active social interaction, but had no effect on locomotor activity. While housing in a small cage increases play behavior, it had no effect on the amount of time spent in active social interaction or on activity. While administration of 0.5, 1.0, and 5.0 mg/kg naloxone to habituated animals differentially housed did not result in an overall decrease in activity, when compared with saline controls, only those animals housed in small cages and injected with the high dose of naloxone differed significantly from controls. In animals having no prior play experience, the activity of animals housed in the large and small cages differed significantly from each other only in the saline and 5.0 mg/kg naloxone conditions. Since naloxone's effect on play behavior was strong, but its effect on activity was not profound, our data suggest that naloxone primarily affects the affective component of play rather than altering activity levels.