Chronic amphetamine: effects on defensive flight in the rat

Hooded rats were injected with physiological saline or d-amphetamine sulfate for 13 days on a schedule designed to mimic patterns of abuse: one injection on days 1-11, two injections on day 12, and three injections on day 13; amphetamine dosage for the first three injections was 3.5 mg/kg and for all subsequent injections was 5.0 mg/kg. Amphetamine-treated rats (Amphet) showed a dramatic flight reaction in response to a novel stimulus (mechanical robot) that did not elicit flight from saline control animals. Tested on a slow-moving treadmill that carried them toward the stimulus, Amphet rats accumulated only 15% of the trial time at the front of the apparatus nearest the stimulus and accumulated approximately 75% of the trial time at the extreme rear of the apparatus, farthest from the stimulus. Control tests of Amphet rats in the absence of the stimulus ruled out interpretations in terms of motor behavior. In fact, a major advantage of the present procedure is that animals are able to execute the relatively simple defense response despite the occurrence of motor stereotypy. These results suggest that the defense-response paradigm is suitable for the study of chronic amphetamine and may provide a useful adjunctive to existing models of amphetamine psychosis.