Role of D₁ and D₂ dopamine receptors in the acquisition and expression of flavor-preference conditioning in sham-feeding rats

The present study examined the effects of D₁ and D₂ antagonists on flavor-preference conditioning by the sweet taste of sucrose. All sessions were conducted under sham-feeding conditions to minimize post-ingestive influences. The rats were trained in alternating, one-bottle sessions to sham-feed a 16% sucrose solution containing one novel flavor (CS+) and a less-preferred 0.2% saccharin solution containing a different flavor (CS-). Three groups of food-restricted rats were treated with either vehicle (control group), the D₁ antagonist, SCH23390 (200 nmol/kg), or the D₂ antagonist, raclopride (200 nmol/kg) during one-bottle training. A fourth group (yoked group) was vehicle-treated and its training intakes were matched to that of the D₁ and D₂ drug groups. Preferences were assessed in two-bottle tests with the CS+ and CS- flavors presented in mixed 8% sucrose+0.1% saccharin solutions following systemic doses of 0, 200, or 800 nmol/kg of either the D₁ or D₂ antagonists. All groups significantly preferred the CS+ flavor in vehicle tests, although the preferences were weaker in the D₁, D₂, and yoked groups compared to the control group. All groups selectively reduced their CS+ intakes when treated with either D₁ or D₂ antagonists during two-bottle testing, and the CS+ preference was blocked at the higher doses. These data show that D₁ and D₂ receptor antagonists block the expression of a sucrose-conditioned preference, but produces substantially lesser effects upon the acquisition of this form of flavor conditioning.