Cholecystokinin-octapeptide effects on eating elicited by “external” versus “internal” cues in rats

Abstract

Animals were pretrained using a paradigm that involved 3 weeks of restricted daily feeding. As a result of this experience, presentation of food-associated stimuli (“external cues”) to minimally (1-h) deprived (or even undeprived) rats is followed immediately by vigorous eating. This paradigm was used in order to test whether animals that are compelled to eat large meals despite being only minimally deprived do so because they are insensitive to endogenous cholecystokinin-octapeptide (CCK-8). CCK-8 is the active fragment of a polypeptide hormone and it is thought by some to be released after a meal and to serve as a short term satiety factor. Following minimal, or 24-h, food-deprivation, the effects of 0, 2, 4, or 8 μg/kg doses of CCK-8 were examined in these animals. Latency to contact food, meal initiation, time spent eating, meal size and a hoarding-like reaction were among the measures recorded. The major finding was that meal initiation was delayed dose-dependently in the minimally-deprived condition. Thus, animals eating apparently “by force of habit” were not insensitive to circulating CCK-8. Approach to food and hoarding-like behavior were virtually unaffected. In the 24-h food-deprivation condition, the pretrained animals were relatively resistant to the inhibitory effects of CCK-8 on eating. The effectiveness of CCK-8 appeared to be greatly enhanced by (and may even depend on) its summation with other satiety factors.