title: The opioid system contributes to the acquisition of reinforcement for dietary fat but is not required for its maintenance.
abstract: The opioid system plays an important role in ingestive behavior, especially with regard to palatable high-fat or sweetened foods. In the present study, we investigated the role of the opioid system in the regulation of ingestive behavior in mice with regard to dietary fat intake, reinforcement, and particularly the processes involved in development of these behavior types. Subcutaneous administration of the non-selective opioid receptor antagonist naltrexone (0.5 or 2.0mg/kg body weight [BW]) reduced the spontaneous intake of fat emulsion (Intralipid). We investigated the effect of naltrexone on reinforcement by using an operant behavioral paradigm under a progressive ratio schedule in which the number of lever presses required to obtain a test sample increased progressively. Mice showed stronger reinforcement by Intralipid as a function of concentration. However, naltrexone (0.5 or 2.0mg/kg BW) did not affect reinforcement at any concentration of Intralipid in mice that had repeatedly ingested Intralipid before testing was carried out. Intralipid ingestion also induced conditioned place preference (CPP), which is another evaluation index of reinforcement. High-dose naltrexone (2.0mg/kg BW) administration during CPP conditioning suppressed the reinforcement induced by Intralipid ingestion, although the drug administration (0.5 or 2.0mg/kg BW) during CPP testing did not affect reinforced behavior. These results suggest that the amount of fat ingestion and reinforcement for fat ingestion are separately regulated by the opioid system. Furthermore, our results indicate that the opioid system plays an important role in acquiring reinforcement for fat but is not required for maintenance of learned reinforcement.
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan. Electronic address: rohizuka@kais.kyoto-u.ac.jp.
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Sakamoto
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Shigenobu
authors.initials: S
authors.lastname: Matsumura
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Okafuji
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Ai
authors.initials: A
authors.lastname: Eguchi
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Yoneda
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Takafumi
authors.initials: T
authors.lastname: Mizushige
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Tsuzuki
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Inoue
authors.affiliation: Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Oiwakecho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Tohru
authors.initials: T
authors.lastname: Fushiki
mesh_terms: D000818:Animals; D003216:Conditioning, Operant / Q000187:drug effects / Q000502:physiology; D004041:Dietary Fats / Q000008:administration & dosage*; D018492:Dopamine Antagonists / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004435:Eating / Q000187:drug effects / Q000502:physiology; D005518:Food Preferences / Q000187:drug effects* / Q000502:physiology*; D006185:Habituation, Psychophysiologic / Q000187:drug effects / Q000502:physiology; D008297:Male; D008807:Mice, Inbred BALB C; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D011897:Random Allocation; D012054:Reinforcement, Psychology; D013028:Space Perception / Q000187:drug effects / Q000502:physiology; D013469:Sulpiride / Q000494:pharmacology
chemical_list: D004041:Dietary Fats; D018492:Dopamine Antagonists; D009292:Narcotic Antagonists; D009271:Naltrexone; D013469:Sulpiride; C055382:naltrindole
keywords: Dietary fat; Food preference; Naltrexone; Opioid