Complex response competition and dopamine blocking: choosing of high cost sucrose solution versus low cost water in rats.

The effect of DA blocking with pimozide on complex response competition involving coupling of cost and benefit was studied with a double-bottle test in lickometer: The competition occurred between two responses; licking of 2% sucrose solution requiring a high effort of standing up and licking of water requiring a low effort of crouching posture. Rats normally chose sucrose over water (76% with vehicle); however, pimozide (0, 0.25, 0.375 and 0.5 mg/kg) diminished licking of the sucrose solution dose-dependently (57% at 0.5 mg/kg) without affecting licking of water. Analysis of lick pattern suggested that this shift in choice was attributable not much to a potential effect of pimozide to hamper motor capability or to reduce rewarding impact of sucrose because (a) the difference in licking profile for sucrose and water remained intact, (b) rats remained fully capable of assuming the standing posture for sustained and efficient licking of sucrose, and (c) the integrity of licking pattern largely remained. Pimozide appeared to selectively affect the behavior that cost more but with higher reward; rats settled with behavior that cost less with lower reward. Rats became less motivated to invest effort for a better reward, a behavior pattern described as "indolent". A more complex view of incentive motivation including coupling of behavioral cost and benefit was presented as a theory to explain the DA mediation of reward-maintained behaviors.