Acquisition and expression of fat-conditioned flavor preferences are differentially affected by NMDA receptor antagonism in BALB/c and SWR mice
Conditioned flavor preferences are elicited by fat (Intralipid) in inbred mouse strains with BALB/c and SWR mice displaying among the most robust preferences. Dopamine D1 and opioid receptor antagonism differentially reduces the acquisition (learning) and expression (maintenance) of fat-conditioned flavor preferences in these two strains. Because noncompetitive NMDA receptor antagonism with MK-801 differentially altered sugar-conditioned flavor preferences in these strains, and because NMDA receptors are involved in fat intake, the present study examined whether MK-801 differentially altered expression and acquisition of fat (Intralipid)-conditioned flavor preferences in BALB/c and SWR mice. In expression studies, food-restricted male mice alternately consumed a flavored (CS+, e.g., cherry, 5 sessions) 5% Intralipid solution and a differently-flavored (CS−, e.g., grape, 5 sessions) 0.5% Intralipid solution. Two-bottle CS choice tests occurred following vehicle or MK-801 (100, 200 μg/kg). MK-801 blocked expression of Intralipid-CFP at both doses in BALB/c mice, but only at the 100 μg/kg dose in SWR mice. In acquisition studies, groups of BALB/c (0, 100 μg/kg) and SWR (0, 100 μg/kg) male mice were treated prior to the ten acquisition training sessions followed by six 2-bottle CS choice tests without injections. MK-801 eliminated acquisition of Intralipid-conditioned flavor preferences in BALB/c mice, and actually changed the preference to an avoidance response in SWR mice. Thus, NMDA receptor signaling appears essential especially for the learning of fat-conditioned flavor preferences in both mouse strains.