Cocaine and morphine-induced place conditioning in adolescent and adult rats

The periadolescent period in the rat is characterized by alterations in novelty seeking and exploratory behavior, as well as changes in the behavioral responsiveness to many drugs of abuse. These alterations may be predictive of alterations in the reward value of drugs of abuse. The present experiments examined whether adolescent rats (34–37 days old) differ from their adult counterparts in the expression of drug-induced place conditioning for morphine (0, 2.5, or 5 mg/kg; Experiment I) and cocaine (0, 5, or 10 mg/kg; Experiments II and III). Animals received multiple conditioning days, followed 24 h later by a drug-free CPP test. All drugs were given intraperitoneally immediately prior to confinement in the CS+ compartment, while vehicle injections were given prior to exposure to the CS− chamber. For both drugs, there were no significant differences between adolescents and adults in amount of place conditioning seen when drug exposure was paired with the nonpreferred chamber. When cocaine was paired with either the preferred or nonpreferred compartment (Experiment III), again, the magnitude of the place conditioning observed did not differ between adolescents and adults. The lack of age differences in expression of drug-induced place conditioning in the present experiments is not likely a result of ceiling effects, because the data suggest that the doses used included near-threshold doses. Although these findings need to be confirmed using other approaches for assessing drug reward before concluding that adolescent and adult rats exhibit similar sensitivity to the rewarding effects of morphine and cocaine, the current data revealed no differences between adolescents and adults in the magnitude of place conditioning expressed for morphine or cocaine.