Preference for Morphine and Drug-Seeking Behavior in Morphine Dependent Rats.

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Abstract
It was previously reported that morphine pretreated rats preferred morphine-admixed food during choice trials with the 2 cup method. Both the 5 and 2 cup methods were used and the preference for morphine in rats forcedly pretreated with the drug and the increasing rate of preference for the drug in rats where the feeding time was limited was observed. In morphine pretreated rats, preference rate for morphine was 61.2 .+-. 3.0% with the 5 cup method and 61.8 .+-. 3.3% with the 2 cup method during the choice trials. In rats that were treated with a limited amount of morphine, each preference rate for morphine during choice trials rapidly increased in the 5 cup method and the preference rate for morphine stabilized at approximately 60%. Findings with the 2 cup method were similar. After the preference for morphine was stabilized at the 60% level, morphine was given s.c. and the preference rate was dependent on the dose injected. When the drug-admixed concentration was changed from 1 mg/g food to 0.5 and 2 mg/g food, the preference rate changed in parallel with the concentration. When the number of food cups containing morphine-admixed food was changed from 1/5 to 2/5, 3/5 and 4/5 food cup, the preference rate was not effected. In the process of preference for morphine, morphine treatment enhanced spontaneous intake of morphine-admixed food.