title: Naltrexone does not prevent acquisition or expression of flavor preferences conditioned by fructose in rats.
abstract: The effects of the general opioid antagonist, naltrexone, on the acquisition and expression of flavor preferences conditioned by the sweet taste of fructose were examined. Food-restricted rats were trained over eight daily alternating one-bottle sessions (2 h) to drink an 8% fructose solution containing one novel flavor (CS+/F) and a less preferred 0.2% saccharin solution containing a different flavor (CS-/S). Four groups of rats were treated daily with either saline (control group) or naltrexone doses of 0.1, 1.0, or 5.0 mg/kg during training. Preferences were assessed in two-bottle tests with the CS+/S and CS-/S flavors presented in 0.2% saccharin solutions following saline injections. Naltrexone dose-dependently reduced fructose and saccharin intakes during training, confirming the drug's well-known suppressive effect on the intake of sweet solutions. Despite their reduced training intakes, the naltrexone groups displayed preferences for the CS+/S over the CS-/S (72-86%) that were similar to that of the control group (78%). The effect of naltrexone on the expression of the CS+/S flavor preference was evaluated by treating control rats with naltrexone (0.1-5 mg/kg) prior to CS+/S vs. CS-/S choice tests. The drug doses produced a dose-dependent reduction in CS+/S intake but did not significantly attenuate the CS+/S preference. These data are consistent with the relative inability of naltrexone to reduce flavor-flavor conditioning by sucrose in sham-feeding rats and flavor-nutrient conditioning in rats receiving intragastric sucrose infusions. In contrast, dopamine antagonists reduce both sucrose- and fructose-conditioned flavor preferences, which indicates the sensitivity of these conditioning paradigms to neuropharmacological manipulations. These data indicate that the endogenous opioid system, unlike the dopamine system, does not play a major role in either the acquisition or expression of flavor preference learning as measured in two-bottle choice tests.
authors.affiliation: Department of Psychology, Queens College, City University of New York, CUNY, 65-30 Kissena Boulevard, Flushing, NY 11367, USA.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Baker
authors.forename: Yin
authors.initials: Y
authors.lastname: Li
authors.forename: Mariel G
authors.initials: MG
authors.lastname: Lee
authors.forename: Anthony
authors.initials: A
authors.lastname: Sclafani
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Bodnar
mesh_terms: D000818:Animals; D003213:Conditioning, Psychological / Q000187:drug effects* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D005632:Fructose / Q000494:pharmacology*; D008297:Male; D009271:Naltrexone / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013649:Taste / Q000187:drug effects* / Q000502:physiology
chemical_list: D005632:Fructose; D009271:Naltrexone