Drug-induced stimulus control and the concept of breaking point: LSD and quipazine.

Discriminative stimulus control was established in rats (N = 12) with LSD (100 microgram/kg) and saline using a two-lever response choice task and an FR10 schedule of water reinforcement. Subjects were then tested once per week with either LSD or quipazine (3 mg/kg) and every other week the test ratio was doubled, i.e., each drug was tested at ratios of 10, 20, 40, and 80. In contrast with LSD, which maintained stimulus control at all ratios, LSD-appropriate responding following quipazine declined significantly at FR80. In addition, five of eight subjects tested with quipazine failed to complete the FR80 in 15 min. In subsequent experiments, the breaking point, here defined as the number of LSD-appropriate responses prior to emission of ten responses on the saline-appropriate lever, was determined for LSD and for quipazine. Mean values (N = 12) for LSD and quipazine were 161 +/- 28 and 65 +/- 19, respectively.