Fig. 3.

Effects of the opioid receptor agonist morphine (MOR; 1 mg/kg, s.c., 1 h before test) on social play behavior. When social play behavior was assessed per pair of animals (a–c), there was an increase in pinning only when both rats in a pair were treated with morphine (a); pouncing, however, was increased when either one or both rats in a pair were treated with morphine (b). Social exploration was not affected by morphine treatment (c). When behavior of the members of a pair was scored separately (d–g), pinning (d) and pouncing (e) were increased in all morphine-treated rats, irrespective of whether they were playing with vehicle- or morphine-treated partners. Social exploration (f) and percentage of responses to play solicitation (g) were unaffected. Data represent mean ± SEM frequency of pinning and pouncing, mean ± SEM duration of social exploration and mean ± SEM percentage of responses to play solicitation. *p<0.05, **p<0.01 vs. couples in which both rats were treated with vehicle (white bar); ^$p<0.05 vs. couples in which one animal was treated with morphine (Tukey’s post hoc test, n = 8–16 per treatment group).