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.