than adults (Varlinskaya & Spear, 2002). Moreover, drug exposure in a social context has been shown to enhance the rewarding value of cocaine (Thiel et al., 2008) and nicotine (Thiel et al., 2009) in adolescent rats when tested in the CPP paradigm, although no age comparisons have been done in these studies. This social enhancement of nicotine and cocaine reward seen in adolescent animals may be related to activation of the endogenous mu opioid system by social stimuli, since this system is implicated in both social behavior and drug reward (Van Ree et al., 2000; Gianoulakis, 2004).
