Pharmacology of flavor preference conditioning in sham-feeding rats: Effects of naltrexone

Relatively little is known about the neurochemical and pharmacological mech anisms involved in flavor preference learning. The present study examined t he ability of the opioid antagonist, naltrexone to alter the acquisition an d expression of flavor preferences conditioned by the sweet taste of sucros e. This was accomplished by adding a novel flavor (the CS+) to a sucrose so lution, and a different flavor (the CS-) to a less-preferred saccharin solu tion. Rats were trained to drink these solutions with an open gastric fistu la (sham-feeding), which minimized postingestive actions. Food-restricted ( Experiments 1 and 2A) and ad lib-fed (Experiment 2B) rats were given either limited (Experiment 1) or unlimited (Experiment 2) access to the CS+ and C S- solutions during one-bottle training. Preferences were assessed in two-b ottle tests (with the CS+ and CS- flavors presented in mixed sucrose-saccha rin solutions) following vehicle or naltrexone (0.1-10 mg/kg, SC) treatment . The rats displayed significant CS+ preferences following vehicle, particu larly after unlimited access training. In four of five experiments. naltrex one significantly reduced total intakes during the two-bottle, sham feeding tests. Except for one instance, however, the drug failed to block the pref erence for the CS+ flavor over the CS- flavor. The effects of naltrexone (0 .1 mg/kg) on the acquisition of flavor preferences were studied in sham fee ding rats under limited (Experiment 3A) and unlimited (Experiment 3B) train ing access conditions. Rats treated with naltrexone during training display ed similar CS+ preferences as did saline-treated rats, even though they con sumed less CS+ during training. The naltrexone-trained rats also displayed smaller reductions in total or CS+ intakes than did saline-trained rats whe n all rats were treated with a 2.5 mg/kg dose of naltrexone during testing. As in previous studies, these results show that naltrexone significantly r educes the intake of sweet solutions, yet it has little or no effect on the acquisition or expression of flavor preferences conditioned by sucrose in sham-feeding rats. (C) 1999 Elsevier Science Inc.