Effects of naloxone and naltrexone on meal patterns of freely-feeding rats

The effects of naloxone and naltrexone on the night-time meal patterning of freely-feeding male rats were investigated using a Kissileff-type eatometer. Naloxone (5.0 mg/kg) and naltrexone (2.5 mg/kg) reduced intake for two hours after IP injection. This effect resulted from a shortening of duration of meals and an extension of postmeal intervals. Unlike other anorexic agents neither drug affected meal frequency or the eating rate within meals. These particular opioid antagonists therefore appear to produce anorexia by advancing meal termination and extending the inhibition of feeding which follows a meal. These specific changes in the structure of the meal pattern consolidate previous findings and support the hypothesis that naloxone and naltrexone reduce food intake in rats by promoting satiation and prolonging satiety.