ABSTRACT
Doses of amphetamine or cocaine that fail to induce psychomotor sensitization when given to a rat in its home cage can produce robust sensitization if given immediately following placement into a relatively novel, distinct environment. A drug-associated context can serve as a conditioned stimulus, and therefore may promote robust sensitization by facilitating associative learning processes. We examined this hypothesis by habituating rats to the test environment for 1 or 6--8 hr prior to each drug injection, which degrades the ability of environmental context to serve as an effective conditioned stimulus. When 0.5 mg/kg of amphetamine was administered intravenously immediately after placement into a distinct environment there was a large acute psychomotor response (rotational behavior) on the first test day, and robust sensitization developed with repeated daily injections. When the same treatment was administered in the home cage, there was a small acute response and no sensitization developed. The enhanced acute response seen in the distinct environment was significantly attenuated by 1 hr of habituation to the test environment, and completely abolished by 6--8 hr of habituation. Also, as little as 1 hr of habituation completely prevented the development of a conditioned rotational response. In contrast, neither 1 nor 6--8 hr of habituation had any effect on the ability of amphetamine to induce robust behavioral sensitization. It is concluded that the ability of a distinct environment to facilitate sensitization to amphetamine can be dissociated from its effect on acute drug responsiveness and from the ability of drug-associated environmental stimuli to elicit a conditioned response. Possible mechanisms by which a distinct environment facilitates sensitization are discussed.
