Environmental modulation of the interoceptive effects of amphetamine in the rat

The aim of the present study was to investigate the effect of environmental novelty on amphetamine discrimination. Two groups of rats (home group and novelty group) were trained to perform water-reinforced operant behavior in cages equipped with two levers and a retractable liquid dipper (0.1 cc cup). The experimental procedures for the two groups were identical except for the fact that home rats were housed and tested in the operant cages whereas novelty rats were transferred daily to these cages for the testing sessions (25 min). The rats were trained to discriminate one of two doses of amphetamine (0.25 and 0.5 mg/kg) from saline. Saline and amphetamine were administered intraperitoneally immediately before each daily session according to a semi-random schedule for a total of 50 sessions. Reinforcements were delivered according to a fixed ratio that was increased daily from 2 to 30. Successful drug discrimination was achieved when activity on the appropriate lever before the first reinforcement was greater than 80% of total activity, on at least seven out of eight consecutive sessions. When training was conducted with 0.25 mg/kg of amphetamine, no home rat acquired drug discrimination whereas 56% of novelty rats reached criterion. When training was conducted with 0.5 mg/kg of amphetamine, drug discrimination was achieved by 50% of home rat versus 86% of novelty rats. These findings indicate that environmental novelty can alter the ability of amphetamine to produce interoceptive cues.