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

The periadolescent period in the rat is characterized by alterations in nov elty 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 experime nts examined whether adolescent rats (34-37 days old) differ from their adu lt counterparts in the expression of drug-induced place conditioning for mo rphine (0, 2.5, or 5 mg/kg; Experiment I) and cocaine (0, 5, or 10 mg/kg; E xperiments II and III). Animals received multiple conditioning days, follow ed 24 h later by a drug-free CPP test. All drugs were given intraperitoneal ly immediately prior to confinement in the CSS compartment, while vehicle i njections were given prior to exposure to the CS- chamber. For both drugs, there were no significant differences between adolescents and adults in amo unt of place conditioning seen when drug exposure was paired with the nonpr eferred chamber. When cocaine was paired with either the preferred or nonpr eferred compartment (Experiment III), again, the magnitude of the place con ditioning 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 th ese findings need to be confirmed using other approaches for assessing drug reward before concluding that adolescent and adult rats exhibit similar se nsitivity to the rewarding effects of morphine and cocaine, the current dat a revealed no differences between adolescents and adults in the magnitude o f place conditioning expressed for morphine or cocaine; (C) 2000 Elsevier S cience Inc. All rights reserved.