title: Melanocortin-3 receptors in the limbic system mediate feeding-related motivational responses during weight loss.
abstract: OBJECTIVE
  Appetitive responses to weight loss are mediated by a nutrient-sensing neural network comprised of melanocortin neurons. The role of neural melanocortin-3 receptors (MC3R) in mediating these responses is enigmatic. Mc3r knockout mice exhibit a paradoxical phenotype of obesity and reduced feeding-related behaviors in situations of nutrient scarcity. Here we examined whether MC3Rs expressed in mesolimbic neurons regulate feeding-related motivational responses.
  
  
  METHODS
  Interactions between Mc3r genotype, cognitive function and energy balance on food self-administration were assessed using operant conditioning with fixed- and progressive ratio (FR1/PR1) settings. Inhibition of Mc3r transcription by a loxP-flanked transcriptional blocker (TB) in C57BL/6JN mice (Mc3r (TB/TB) ) was reversed in mesolimbic neurons using DAT-Cre (DAT-MC3R).
  
  
  RESULTS
  Caloric restriction (CR) caused 10-15% weight loss and increased motivation to acquire food rewards during training sessions. c-Fos-expression in the nucleus accumbens was increased 1 h following food presentation. While exhibiting weight loss, total food self-administration, enhanced motivation to self-administer food rewards in training sessions held during CR and c-Fos-activation in the nucleus accumbens following re-feeding were all markedly attenuated in Mc3r (TB/TB) mice. In contrast, cognitive abilities were normal in Mc3r (TB/TB) mice. Total food self-administration during FR1 sessions was not rescued in DAT-MC3R mice, however enhanced motivational responses to self-administer food rewards in PR1 conditions were restored. The nutrient-partitioning phenotype observed with Mc3r-deficiency was not rescued in DAT-MC3R mice.
  
  
  CONCLUSIONS
  Mesolimbic MC3Rs mediate enhanced motivational responses during CR. However, they are insufficient to restore normal caloric loading when food is presented during CR and do not affect metabolic conditions altering nutrient partitioning.
authors.affiliation: Department of Metabolism and Aging, The Scripps Research Institute, Jupiter, FL 33458, USA; Department of Pharmacology & Physiology, Saint Louis University School of Medicine, St. Louis, MO 63104, USA.
authors.forename: Maria
authors.initials: M
authors.lastname: Mavrikaki
authors.affiliation: Department of Metabolism and Aging, The Scripps Research Institute, Jupiter, FL 33458, USA; Department of Pharmacology & Physiology, Saint Louis University School of Medicine, St. Louis, MO 63104, USA.
authors.forename: Clemence
authors.initials: C
authors.lastname: Girardet
authors.affiliation: Department of Metabolism and Aging, The Scripps Research Institute, Jupiter, FL 33458, USA.
authors.forename: Andras
authors.initials: A
authors.lastname: Kern
authors.affiliation: Department of Neuroscience, The Scripps Research Institute, Jupiter, FL 33458, USA; Behavioral Core, The Scripps Research Institute, Jupiter, FL 33458, USA.
authors.forename: Alicia
authors.initials: A
authors.lastname: Faruzzi Brantley
authors.affiliation: Department of Metabolism and Aging, The Scripps Research Institute, Jupiter, FL 33458, USA; Department of Neuroscience, The Scripps Research Institute, Jupiter, FL 33458, USA.
authors.forename: Courtney A
authors.initials: CA
authors.lastname: Miller
authors.affiliation: Department of Pharmacology & Physiology, Saint Louis University School of Medicine, St. Louis, MO 63104, USA.
authors.forename: Heather
authors.initials: H
authors.lastname: Macarthur
authors.affiliation: Papé Family Pediatric Research Institute, Oregon Health & Science University, Portland, OR 97239, USA.
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Marks
authors.affiliation: Department of Metabolism and Aging, The Scripps Research Institute, Jupiter, FL 33458, USA; Department of Pharmacology & Physiology, Saint Louis University School of Medicine, St. Louis, MO 63104, USA. Electronic address: Butleraa@slu.edu.
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Butler
keywords: Appetite; Caloric restriction; Dopamine; Melanocortin-3 receptors; Melanocortins; Motivation