Individual differences in sucrose consumption in the rat: motivational andneurochemical correlates of hedonia

Rationale: Rats exhibit marked individual differences in consumption of fre ely available sucrose; however, the underlying mechanism(s) contributing to such interindividual differences remain unclear. Objectives: The current s tudy examined whether: 1) motivational differences (as reflected by the deg ree of operant output to procure sucrose reward) underlie variability in su gar consumption, and 2) whether potential differences in dopaminergic and/o r opioidergic systems contribute to such differences. Methods: In the initi al experiment, Sprague-Dawley rats were divided into those that spontaneous ly consumed High or Low amounts of sucrose, based on the consumption of fre ely available sugar (+/-2 SD from group median) over 7 consecutive test day s. The potential differences in their motivation to seek and "earn" sucrose solution rewards were measured using the progressive ratio (PR) schedule o f reinforcement. Results: Performance of both groups on the PR schedule was sensitive to the concentration of sucrose (i.e. the higher the concentrati on, the greater the behavioral output). Furthermore, the High sucrose consu mers earned a greater number of reinforcements (20% sucrose solution) than the Low group. Parenthetically, the degree of behavioral output by the High (but not the Low) group was comparable to that emitted for water under wat er-deprived condition. Treatment with the opioid antagonist, naloxone, atte nuated PR performance for sucrose, whereas d-amphetamine (0.5 mg/kg) enhanc ed it. Furthermore, naloxone attenuated amphetamine-enhanced responding for sucrose reward, suggesting an interaction between the dopaminergic and opi oidergic systems in the mediation of sucrose reward. Conclusions: These res ults support the contention that motivational differences may partially acc ount for individual variability in sucrose consumption, and that dopaminerg ic and/or opioidergic agents differentially affect the "wanting" and/or "li king" of sucrose in the High and Low sucrose consumers.