The administration of morphine causes a short-term increase in food intake, and repeated administration of morphine has been shown to cause progressively larger increases in intake and/or the relative intake of dietary fat. In this experiment, we measured the effects of continuous morphine infusions on diet choice and total intake. Male rats were given ad lib access to two diets: a high-carbohydrate diet (80% carbohydrate, 20% protein) and a high-fat diet (80% fat, 20% protein). Diet intakes were measured daily for 21 days. Via the implantation of osmotic minipumps, one group received continuous infusions of morphine sulfate (approx. 2.8 mg/kg/h) for days 1-7 and of saline for days 8-14. A second group was infused with saline for days 1-7 and with morphine for days 8-14. A third group received sham surgery but no minipumps. Total caloric intake was significantly decreased on the final 6 days of morphine infusions. The percentage of total caloric intake consumed from the high-fat diet was significantly increased for the first 2-3 days of morphine treatment; this effect was due to an initial reduction in carbohydrate intake and an increase in fat intake. Over the course of the infusion period, fat intake gradually decreased and carbohydrate intake increased. The effects of morphine when infused on days 1-7 were similar to those observed when the drug was infused during days 8-14. Termination of morphine infusions caused significant decreases in body weight gain and total daily intake and tended to decrease relative preference for the high-fat diet; daily intake generally returned to control (sham) levels within 4-5 days. These effects (a sustained decrease in total intake with only an initial increase in fat preference) differ from those reported after repeated intermittent administration of morphine in short-term trials.
- Diet selection
- Food intake