Relative response cost determines the sensitivity of instrumental reward seeking to dopamine receptor blockade

Dopamine is a critical mediator of instrumental reward seeking behavior and appears to have a particularly important role in motivating actions that require considerable effort. As with rewards, response costs can be evaluated in both absolute and relative terms. The current study investigated whether the extent to which instrumental performance is dependent on dopamine transmission is influenced by relative or absolute response cost. Three groups of rats were rewarded for lever pressing on different fixed ratio (FR) schedules that required 1 (FR-1), 10 (FR-10), or 20 (FR-20) presses for each food reward. Rats were then injected systemically with flupentixol, a dopamine receptor antagonist, or vehicle before testing all groups on an intermediate-cost (FR-10) schedule, such that only the relative cost of responding differed across groups. Rats experiencing an upshift in cost (group FR-1/FR-10) showed greater response suppression following flupentixol administration than rats experiencing no shift in cost (group FR-10/FR-10), whereas flupentixol treatment had no effect on rats experiencing a downshift in cost (group FR-20/FR-10). A second round of flupentixol tests was conducted using the rats' maintenance schedules, such that only absolute response costs differed across groups. Here, the pattern was reversed among the groups, in line with previous reports. Specifically, flupentixol had a stronger suppressive effect in group FR-20/FR-20 than in group FR-10/FR-10, and had no detectable effect in group FR-1/FR-1. These findings suggest that response costs are evaluated in both absolute and relative terms and that dopamine has a role in overcoming both kinds of cost.