rats received intragastric infusions of ethanol (the unconditioned stimulus [US]) paired with intra-oral infusions of sucrose (CS1) during phase 1, whereas unpaired control animals were exposed to the sucrose CS1 four hours prior to administration of the ethanol US (Pautassi et al., 2008). In the second conditioning phase, animals in both paired and unpaired groups were exposed to the sucrose CS1 in a discrete environment (CS2). At test, when adolescent and adult rats were then given the opportunity to explore a 3-chamber apparatus that contained the CS2 environment, adolescents in the paired condition showed greater preference for the CS2 than their unpaired controls, suggesting that the CS2 had attained positive reinforcing properties through a CS1-mediated association with the ethanol US. Such second order conditioning was not evident in adults, with adults receiving paired exposure to the CS1/US in phase 1 not differing from unpaired adults when examining CS2 preference at test.
