Do peptide-induced changes in feeding occur because of changes in motivation to eat?

The usual paradigm in which peptides are tested for their effect on food intake involves measuring intake of readily available food. In the lever press apparatus, the subjects must "work" to get food. Such work has traditionally been used as a means of measuring motivation. Mice were trained to press a lever for milk reinforcement. After achieving a stable level of performance, we tested the effects of gastrin-related peptide (GRP), bombesin (BBS) and cholecystokinin octapeptide (CCK-8) on lever pressing. All three peptides suppressed lever pressing for milk reinforcement. Prefeeding mice with milk increased the suppression of lever pressing to a greater extent in peptide-treated mice than in saline-treated mice. As the duration of prefeeding increased, lever pressing decreased. When mice were required to make more lever presses to obtain milk, both saline- and CCK-8-treated mice increased their lever pressing. However, saline-treated mice pressed at a higher rate than CCK-8-treated mice. Unlike the results obtained with saline and CCK-8, administration of a known gustatory adversant, lithium chloride, suppressed lever pressing to the same degree in mice fed or not fed prior to training. The results are consistent with the hypothesis that these peptides act as satiety agents.