Opiates and homing.

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Abstract
Beginning at 15 days of age, Long-Evans rat pups were trained to run toward their home cage in a T-maze task. Morphine (0.5-1.0 mg/kg s.c.) slowed initial acquisition running times but did not change the number of trials required to learn the position habit. Morphine markedly impeded extinction of the homing behavior. Opiate-treated animals ran as accurately and as quickly toward home on the 12th day of extinction as on the 1st (10 trials given per day). Naloxone (1 mg/kg s.c.) reduced resistance to extinction. The morphine effect was not state-dependent since the drug also impeded extinction in animals that had acquired the task under saline. The morphine effect was blocked by naloxone, which indicates that the increased resistance to extinction was due to an opiate receptor effect. Morphine has a strong capacity to sustain a social habit in the absence of reinforcement.