Long-term retention of tolerance to amphetamine hypophagia following cessation of drug injections and feeding tests.

According to the instrumental learning model, tolerance to amphetamine hypophagia involves learning to suppress stereotyped movements that interfere with feeding. If both drug injections and feeding tests are then suspended, learning should be retained and no loss of tolerance should occur. However, previous studies have only assessed the retention of tolerance for 3-4 weeks. In the present study, retention intervals of 4-31 weeks were used. Rats were given daily injections of amphetamine (2 mg/kg) and access to milk for 30 min until tolerance developed to drug-induced hypophagia. Yoked controls were injected with saline. Both before and after this phase, dose-response (DR) tests were conducted. Drug injections and feeding tests were then suspended. At 4, 10, 18, and 31 weeks, both groups were injected with 2 mg/kg amphetamine and given access to milk for 30 min to assess the retention of tolerance. A final DR determination was then conducted. Most (88%) rats retained tolerance to 2 mg/kg amphetamine for 31 weeks. However, DR tests revealed that tolerance was not retained at 4 mg/kg. The results demonstrate that learned tolerance to amphetamine can be retained over long intervals when both drug injections and feeding tests are suspended.