index of preference for that stimulus (i.e., the stimulus was rewarding), with more time spent on the alternative side indexing an aversion for the stimulus (i.e., a conditioned place aversion). Using this procedure, socially reared adolescent male rats were shown to exhibit CPP for novel stimuli, conditioning that was not evident in their adult counterparts (Douglas et al, 2003). Similarly, when expression of social CPP was assessed within our laboratory using a same-sex, unfamiliar partner as the social stimulus, both male and female adolescents in general exhibited more robust CPP than adults (Douglas et al., 2004). These rewarding properties of social interactions were enhanced by prior social deprivation in both adolescents and adults, although social stimuli were still rewarding to adolescents even without prior deprivation of social contact (Douglas et al., 2004). Relatively short-term social deprivation (5–7 days of isolate housing) of adolescent rats has likewise been reported to increase social behavior, particularly in terms of play fighting (Holloway & Suter, 2004; Panksepp, 1981; Takahashi & Lore, 1983; Varlinskaya et al., 1999), effects that are particularly marked early in adolescence (Varlinskaya & Spear, 2008).
