Roles of dopamine D1 and D2 receptors in the acquisition and expression of fat-conditioned flavor preferences in rats

Abstract

Sugars and fats elicit innate and learned flavor preferences with the latter mediated by flavor–flavor (orosensory) and flavor–nutrient (post-ingestive) processes. Systemic dopamine (DA) D1 (SCH23390: SCH) and D2 (raclopride: RAC), but not opioid antagonists blocked the acquisition and expression of flavor–flavor preferences conditioned by sugars. In addition, systemic D1, but not D2 or opioid antagonists blocked the acquisition of flavor–nutrient preferences conditioned by intragastric (IG) sugar infusions. Given that DA antagonists reduce fat intake, the present study examined whether systemic D1 or D2 antagonists altered the acquisition and/or expression of conditioned flavor preferences (CFP) produced by pairing one novel flavor (CS+, e.g., cherry) with a 3.5% corn oil (CO: fat) solution relative to another flavor (CS−, e.g., grape) paired with a 0.9% CO solution. In an expression study, food-restricted rats were trained to drink either flavored 3.5% or 0.9% CO solutions on alternate days. Subsequent two-bottle tests with the CS+ and CS− flavors mixed in 0.9% CO solutions occurred 0.5h after systemic administration of vehicle (VEH), SCH (50–800nmol/kg) or RAC (50–800nmol/kg). The rats displayed a robust CS+ preference following VEH treatment (87–88%) the expression of which was attenuated by treatment with moderate doses of RAC, and to a lesser degree, SCH. In an acquisition study, six groups of rats received VEH, SCH (25, 50, 200nmol/kg) or RAC (50, 200nmol/kg) 0.5h prior to 1-bottle training trials with CS+ flavored 3.5% and CS− flavored 0.9% (CS−) CO solutions. A seventh Limited VEH group was trained with its training intakes limited to that of the SCH and RAC groups. Subsequent two-bottle tests were conducted with the CS+ and CS− flavors presented in 0.9% CO without injections. Significant and persistent CS+ preferences were observed in VEH (75–82%), Limited VEH (70–88%), SCH25 (75–84%), SCH50 (64–87%), SCH200 (78–91%) and RAC200 (74–91%) groups. In contrast, the group trained with RAC50 displayed a significant initial CS+ preference (76%) which declined over testing to 61%. These data indicate limited DA D1 and D2 receptor signaling involvement in the expression and acquisition of a fat-CFP relative to previous robust effects for sugar-CFP.