The dopaminergic mediation of a sweet reward in normal and VMH hyperphagic rats.

The role of dopamine in mediating the rewarding quality of a sweet saccharin-glucose (SG) solution was investigated by comparing the effects of the dopamine receptor blocker pimozide, the bitter adulterant quinine, and solution dilution on the consummatory response to the solution in normal and VMH rats. Experiments 1 and 2 demonstrated that pimozide and quinine caused a dose/concentration dependent dependent reduction in the intake of and the licking response to a SG solution. Pimozide treatment caused an equivalent suppression in the intake of the normal and VMH rats, in both the dynamic and static phases, whereas quinine adulteration caused a greater suppression in the intake of the VMH rats. The effects of pimozide and quinine on initial lick rate were also different. Experiment 3 demonstrated that dilution of a SG solution produced a concentration related decrease in intake and licking response. Dilution of the SG solution, like pimozide treatment, affected the intake of the normal and VMH rats in an equivalent manner. The effects of solution dilution and pimozide treatment on the licking response were also similar. The results suggest that the mechanisms by which pimozide and quinine reduce the hedonic quality of natural rewards are functionally dissimilar. The similarity between pimozide treatment and solution dilution suggests that pimozide reduces the positive affective quality of natural reinforcers. The results are discussed in terms of the dopamine theory of reward, the role of dopamine in hypothalamic hyperphagia, and VMH finickiness.