Morphine enhances hedonic taste palatability in rats.

The question of whether opiates stimulate feeding by enhancing taste pleasure was investigated by examining the effect of morphine upon hedonic and aversive reactions to taste (tongue protrusions, gapes, etc.). Rats (n = 12) were given SC injections of morphine (4 mg/kg) or equal volumes of isotonic saline 2 h after the start of their daily light cycle. Food intake was measured in a 2-h test. On days when they were given morphine, rats ate significantly more food than when given saline. Hedonic and aversive taste reactions were elicited by an infusion of sucrose-quinine solution into the mouth and were measured subsequently in a slow-motion video analysis. The same rats that showed an increase in food intake after treatment with morphine showed a significant increase in their positive hedonic responses. Aversive reactions were unchanged by morphine. The results support the hypothesis that morphine enhances feeding by increasing the hedonic palatability of food.