Conditioned suppression with cocaine as the unconditioned stimulus

A conditioned-suppression procedure was used to study drug conditioning using cocaine as the unconditioned stimulus (UCS). Rats were first trained to nose poke for food-reinforcement during daily 60-min sessions. At least 1 week following jugular vein catheterization, a 5-min tone-light compound stimulus was presented 30 min into the food-reinforcement session. Two minutes after the onset of the stimulus, either 0 (saline), 1.0, 3.0 or 5.6 mg/kg cocaine, was administered i.v. to separate groups of rats. For another group, the stimulus was presented, and the 5.6 mg/kg dose of cocaine was injected in an unpaired fashion (i.e., at different times). After 5 days of training a test was given with the tone-light stimulus presented alone. No disruption of responding during the tone-light stimulus was observed in the saline and 1.0 mg/kg cocaine groups or for the unpaired group. When the tone-light stimulus was paired with 5.6 mg/kg cocaine; however, it produced nearly a 50% reduction in responding, which then gradually extinguished when the stimulus was presented alone for 5 days. The 3.0 mg/kg cocaine group produced intermediate suppression. When the tone-light compound stimulus was shortened to 70 s and the interstimulus interval (ISI) was 0, 30, or 60 s in three separate groups of rats, the most robust conditioned suppression was observed at the 60 s ISI. Therefore, the conditioned suppression procedure, using 3.0 or 5.6 mg/kg i.v. cocaine doses as the UCS, produced robust conditioning effects comparable to other drugs and more conventional reinforcers. The conditioned suppression procedure may be a useful model for studying the classically conditioned effects of cocaine.