Dopamine D1 and opioid receptor antagonists differentially reduce the acquisition and expression of fructose-conditioned flavor preferences in BALB/c and SWR mice

Sugar appetite is influenced by unlearned and learned preferences in rodents. The present study examined whether dopamine (DA) D1 (SCH23390: SCH) and opioid (naltrexone: NTX) receptor antagonists differentially altered the expression and acquisition of fructose-conditioned flavor preferences (CFPs) in BALB/c and SWR mice. In expression experiments, food-restricted mice alternately (10 sessions, 1 h) consumed a flavored (e.g., cherry) 8% fructose + 0.2% saccharin solution (CS +) and a differently-flavored (e.g., grape) 0.2% saccharin solution (CS −). Two-bottle CS choice tests (1 h) occurred 0.5 h following vehicle: SCH (200 or 800 nmol/kg) or NTX (1 or 5 mg/kg). SCH, but not NTX significantly reduced CS + preference in both strains. In acquisition experiments, 0.5 h prior to 10 acquisition training sessions, vehicle, SCH (50 nmol/kg), NTX (1 mg/kg) or Limited Control vehicle treatments were administered, followed by two-bottle CS choice tests without injections. SCH and NTX reduced training intakes in both strains. BALB/c mice displayed hastened extinction of the fructose-CFP following training with SCH, but not NTX. SCH eliminated fructose-CFP acquisition in SWR mice, whereas NTX hastened extinction of the CFP. These results are compared to previous drug findings obtained with sucrose-CFPs in SWR and BALB/c mice, and are discussed in terms of differential effects of these sugars on oral and post-oral conditioning.