8-OHDPAT effects upon cocaine unconditioned and conditioned behaviors: a role for drug stimulus effects

The effects of the 5-HT1A agonist, (+/-)-8-hydroxy-dipropylaminotetralin (8-OHDPAT) upon the unconditioned and conditioned behavior induced by cocaine were assessed in rats. Separate groups (n=7) received saline, cocaine (10 mg/kg), 8-OHDPAT (0.2 mg/kg), or 8-OHDPAT (0.2 mg/kg) plus cocaine (10 mg/kg) for eight treatment sessions (two per week) in which the rats were tested for 20 min in an open-field. On the eighth treatment session, cocaine enhanced locomotion and rearing but decreased grooming. 8-OHDPAT also decreased grooming and, when given in combination with cocaine, enhanced locomotion but attenuated cocaine-induced rearing. The two 8-OHDPAT groups differed substantially from each other and from the cocaine group in terms of locomotion during the drug treatment phase. Subsequently, all groups received a series of conditioning tests in which they received saline, 0.1, 0.2, or 0.4 mg/kg 8-OHDPAT prior to testing. Groups which had received either 8-OHDPAT or cocaine prior to the conditioning tests exhibited equivalent conditioned effects on the saline conditioning test. When conditioning tests were conducted with 8-OHDPAT, however, only the group which had previously received the combined 0.2 mg/kg 8-OHDPAT plus cocaine treatment exhibited a conditioned response and this effect only occurred at the 0.2 8-OHDPAT dose level. These observations indicate the important influence of the stimulus properties of drugs for the study of drug conditioning and for understanding drug interactions with cocaine.