Behavioral tolerance to stimulating effects of pentobarbital: a within-subject determination

Squirrel monkeys were trained to press a lever under a multiple schedule of food presentation. In one stimulus condition responses that terminated interresponse times greater than 28 sec were followed by food presentation. In the other stimulus condition, an interval schedule of food presentation was presented that provided approximately the same frequency and distribution of food delivery as that observed under the interresponse-time schedule. Except when it was administered for the first time, 5.6 mg/kg sodium pentobarbital produced reliable increases in responding during the interresponse-time schedule. Behavioral tolerance to the rate-increasing effect was assessed in individual subjects by first administering the drug daily following each session, and then giving it daily before each session. Following post-session drugging, the effects of 5.6 mg/kg were not changed, but tolerance developed when the drug was administered pre-session. The way in which tolerance developed was consistent with the reinforcement-loss hypothesis.