Effects of fixed-ratio length on the development of tolerance to decreased responding by l-nantradol.

Key pecking of pigeons was maintained under either a 100-response or a 300-response fixed-ratio schedule of food presentation, and animals received 0.03 mg/kg/day l-nantradol prior to experimental sessions. Tolerance developed for initial rate decreases under fixed ratio 100 in 10-12 sessions, but tolerance did not develop under fixed ratio 300 for up to 30 sessions. When the fixed ratio was changed from 300 back to 100, tolerance developed in three to four sessions, and when the fixed ratio was changed from 100 back to 300, tolerance diminished in two to three sessions. The importance of fixed-ratio parameter for the observation of tolerance extends the generality of the influence of reinforcement processes on tolerance.