title: Decreased motivation to eat in mu-opioid receptor-deficient mice.
abstract: Altered motivational processes might participate to the physiopathology of eating-related disorders. The endogenous opioid system is thought to mediate the hedonic properties of food intake. To assess the role for the micro-opioid receptor (MOR) pathway in the motivational properties of food intake, in the present study we tested wild-type and MOR-deficient mice (MOR-/-) in a nose-poke operant paradigm for chow or sucrose pellets. To avoid confounding factors linked to food restriction/deprivation experience, mice were always provided with food ad libitum. Although less MOR-/- than wild-type mice initiated operant behaviour, under a fixed ratio-1 (FR-1) reinforcement schedule the two genotypes showed similar patterns of food-driven nose-poking, indicating preserved cognitive abilities in MOR-deficient mice. However, during FR-3 and progressive ratio (PR) reinforcement experiments, MOR-/- mice showed lower levels of nose-poking for either chow or sucrose pellets than wild-type mice, indicating a crucial role for the MOR pathway in the motivational properties of food intake. Moreover, under the PR reinforcement schedule mice nose-poking for sucrose pellets showed higher genotype-independent breakpoint levels than mice working for chow pellets, indicating that the MOR pathway is not essential for hedonic processing of palatable food intake. Finally, MOR-/- mice did not differ from wild-type mice in the rate of operant responding extinction, further supporting the notion of unaltered cognitive abilities in the MOR-deficient mice. The present findings strongly indicate that the MOR pathway mediates the motivational properties of food intake, but it is not essential for hedonic processing of ingestive behaviour.
authors.affiliation: Laboratoire Homéostasie-Allostasie-Pathologie, EA 3666, Université Victor Segalen Bordeaux 2, 146 Rue Léo Saignat, 33076 Bordeaux, France.
authors.forename: Francesco
authors.initials: F
authors.lastname: Papaleo
authors.forename: Brigitte L
authors.initials: BL
authors.lastname: Kieffer
authors.forename: Antoine
authors.initials: A
authors.lastname: Tabarin
authors.forename: Angelo
authors.initials: A
authors.lastname: Contarino
mesh_terms: D000818:Animals; D001522:Behavior, Animal; D003216:Conditioning, Operant / Q000187:drug effects / Q000502:physiology; D004435:Eating / Q000187:drug effects / Q000235:genetics*; D005108:Extinction, Psychological; D005260:Female; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009042:Motivation; D017450:Receptors, Opioid, mu / Q000172:deficiency*; D012055:Reinforcement Schedule; D012054:Reinforcement, Psychology; D013395:Sucrose / Q000008:administration & dosage; D013549:Sweetening Agents / Q000008:administration & dosage
chemical_list: D017450:Receptors, Opioid, mu; D013549:Sweetening Agents; D013395:Sucrose