AMPHETAMINE-INDUCED CONDITIONED ACTIVITY DOES NOT RESULT FROM A FAILURE OF RATS TO HABITUATE TO NOVELTY

Psychostimulant-induced conditioned activity is characterized by the p resence of a hyperactivity in drug-free rats exposed to an environment previously paired with the effects of a psychostimulant. According to the habituation hypothesis, conditioned activity arises not through a Pavlovian conditioning process but rather because rats under the effe cts of the psychostimulant would be unable to habituate normally to th e environment paired with these effects. This hypothesis predicts that conditioned activity should not develop in a previously habituated en vironment, This prediction was tested using a within-subject design, I n this design, conditioned activity is evidenced when a group of rats, following a vehicle injection, was more active in a previously amphet amine-paired environment than in a previously vehicle-paired environme nt. The drug-environment pairing involved administering rats with d-am phetamine (1.25 mg/kg; SC) immediately prior to their placement in one of two distinctive environments. On alternate days, the rats received the vehicle and were placed in the other environment, With this desig n, it was found that: a) conditioned activity developed in a previousl y habituated environment; b) its magnitude was independent of the numb er of amphetamine-environment pairings (two, four or eight pairings); c) this development of conditioned activity did not result from a for- getting of the habituated environment due to a state-dependent retenti on of the habituation. Taken together, these results do not support th e habituation hypothesis of psychostimulant-induced conditioned activi ty.