Food motivated behavior of melanocortin-4 receptor knockout mice under a progressive ratio schedule
Introduction
The obesity epidemic has been a topic of interest recently due to the rise in obesity-related illnesses. The sharp increase in the incidence of humans identified as overweight in the past 10–20 years is directly attributable to environmental and lifestyle changes. Physiologic and genomic research using animal models have additionally identified genes and polymorphisms that predispose carriers to obesity. The melanocortin system has received recent attention both in the human domain as well as in animal models including genetically engineered mice (see [11] for review). The two central melanocortin receptors (MCRs) that are most pertinent to feeding are the MC3R and the MC4R (see [4], [25] for review). Functionally, the MC3R is related to fat metabolism while the MC4R is related to food intake and energy expenditure [2], [6], [9], [12], [15].
There is emerging evidence that feeding in the MC4R knockout (KO) mouse model is subject to environmental modulation [21], [23]. Specifically, while hyperphagia is essential to maintain their obese state [23], MC4RKO mice did not become obese when a running wheel was present but they developed hyperphagia and obesity when housed individually without a wheel [16]. We have also reported a protocol in which MC4RKO mice were not hyperphagic, and repeatedly lost weight, when required to work for food in operant chambers [24]. However, each time they were returned to free feeding, their weight rebounded, presumably due to hyperphagia. The operant protocol that we used was one of imposed procurement costs, when to initiate a meal a fixed ratio (FR) of lever presses had to be emitted. Both wild type and MC4RKO mice showed changes in meal size and frequency as a result of the imposed FR, but the KO mice ate the same amount as the wild types. One interpretation of this finding is that MC4RKO mice are less motivated to work for food, so in the present study we apply a stringent test of this hypothesis using progressive ratio (PR) schedules that are thought to be sensitive indicators of motivation for either drugs or food [13], [14], [18], [22].
PR schedules require a subject to emit an increasing number of responses to gain access to successive reinforcers [13]. Typically, these are used in short sessions that are terminated when the subject will not emit the work needed to obtain another reinforcer, and this is termed the breakpoint: greater motivation produces higher breakpoints [10], [13]. We have adapted the PR protocol to measure meal patterns by using a closed economy in which mice live and obtain all of their food. To do this, we have employed a reset condition; if there is an elapsed criterion time without responding the response requirement reverts to the initial value (one press) and the animal gains the opportunity to initiate a new meal. Hence, the animal has complete control over the initiation and termination of meals.
Due to the fact that the first pellets in any meal are the cheapest, a strategy of many small meals will minimize the average cost of food (presses per pellet). If, as we suggested above, MC4RKO mice are less motivated than wild types, then we would expect they will eat smaller meals, and depending on their compensation in intermeal interval, in a day they may eat either less, the same, or more than wild type mice.
Section snippets
Subjects and housing environment
Untimed pregnant heterozygous mothers were from a colony originating from Millennium Pharmaceuticals and maintained by one of us (CHL) at the University of Florida. Mothers gave birth 3–19 days after being moved to a vivarium in the Psychology department. As part of another study, these offspring were reared in small (∼4 pups) or larger (∼8 pups) litters and were fed Chow or high fat diet until weaned and were maintained on these diets until 77 days of age when they were returned to Chow for at
Results
The mean intakes of food for each genotype in each phase of the experiment are shown in Table 1. In the FR3 condition, MC4RKO mice ate significantly more than the other two genotypes (p < 0.001). This pattern was also present during the two PR phases, but in these phases the heterozygotes also showed a modest hyperphagia. The KO mice initially were obese, weighing ∼50% more than wild types and because of their sustained hyperphagia, their elevated weight was maintained throughout the experiment (
Discussion
To our knowledge this is the first report using a PR schedule in a closed economy to study meal patterns. PR schedules have been used predominantly in short sessions where breakpoints end the session, but in our case they defined elective meal termination and so our interpretation of the breakpoint was most relevant to the assessment of the subjects’ ability to sense fullness and end a discrete bout of feeding. Under these conditions, the PR schedule may emulate depletion of a patch of finite
Acknowledgements
The MC4RKO mice were generously provided by Millennium Pharmaceuticals and this work was supported in part by NIHDK57080 (CHL).
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