Tolerance to morphine "anorexia" is not contingent on experience with food while in the drugged state

To determine whether tolerance to morphine-induced anorexia requires access to milk while intoxicated, rats were given chronic injections of morphine (10 or 20 mg/kg) either before (before subgroups) or after (after subgroups) access to milk on alternate days. There were marked individual differences in initial sensitivity to the drug. After chronic treatment, there was little difference in the level of tolerance in subjects given morphine either before or after access to milk. On the intervening nondrug days, rats in the before subgroups consistently drank less milk than the other subgroups. This effect was not the result of withdrawal distress. Substitution of saline for morphine (20 mg/kg) on a scheduled drug day resulted in enhanced milk intakes in both the before and after subgroups. The results suggest that tolerance to morphine anorexia does not involve instrumental learning.