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

Relatively little is known about the neurochemical and pharmacological mechanisms involved in flavor preference learning. The present study examined the ability of the opioid antagonist, naltrexone to alter the acquisition and expression of flavor preferences conditioned by the sweet taste of sucrose. This was accomplished by adding a novel flavor (the CS+) to a sucrose solution, and a different flavor (the CS-) to a less-preferred saccharin solution. Rats were trained to drink these solutions with an open gastric fistula (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 CS- solutions during one-bottle training. Preferences were assessed in two-bottle tests (with the CS+ and CS- flavors presented in mixed sucrose-saccharin solutions) following vehicle or naltrexone (0.1-10 mg/kg, SC) treatment. The rats displayed significant CS+ preferences following vehicle, particularly after unlimited access training. In four of five experiments, naltrexone significantly reduced total intakes during the two-bottle, sham-feeding tests. Except for one instance, however, the drug failed to block the preference 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-feeding rats under limited (Experiment 3A) and unlimited (Experiment 3B) training access conditions. Rats treated with naltrexone during training displayed similar CS+ preferences as did saline-treated rats, even though they consumed less CS+ during training. The naltrexone-trained rats also displayed smaller reductions in total or CS+ intakes than did saline-trained rats when 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 reduces 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.