Willingness to work for sucrose: Impact of schedules, reinforcer alternatives, homeostatic value, and individual differences in male mice

Motivated behavior is characterized by a high degree of behavioral activation and effort. Preclinical models of operant effort-based decision-making are useful for studying motivational symptoms seen in many psychopathologies as well as observing individual vulnerability. In this study, we evaluated the impact of having alternative reward choices with different effort demands on the decision-making process that may lead to individual differences in working. Adult CD1 male mice, trained on a fixed ratio (FR) 8, were divided into two groups: the no-choice (FR8 for 10% sucrose) and choice (FR8 for 10% sucrose plus concurrent free access to 3% sucrose) conditions. After 5 weeks, a progressive ratio (PROG) schedule was introduced under the same choice conditions. Results indicate that only the no-choice group adjusted their work-output after changing from FR8 to PROG. The choice group did not increase lever pressing when PROG was introduced, but it maintained total fluid intake (10% plus 3%). Water restriction increased intake of all fluids in both choice conditions, independently of the effort required. Additionally, a median split based on PROG performance was used to compare high and low performers. Individual differences in lever pressing were seen under both choice conditions, but only high-performers significantly increased work output when PROG was introduced. Among the choice group, high-PROG performers did not increase total volume obtained compared with low-performers. Thus, the choice condition induced individual differences based mostly on willingness to work rather than to obtain the maximum amount of reinforcer. Further studies in both sexes should characterize this "high-worker" profile.