Differences in temporal aspects of food acquisition between rats and two strains of mice in a closed operant economy

Male and female mice show equal variability in food intake across 4-day spans that encompass estrous cycles

The exclusion of female rodents from biomedical research is well documented and persists in large part due to perceptions that ovulatory cycles render female traits more variable than those of males, and females must be tested at each of four stages of the estrous cycle to generate reliable data. These beliefs are not empirically based. The magnitude of trait variance associated with the estrous cycle may be sufficiently low and of little impact, or trait variability of males tested on 4 consecutive days may be as great as that of females over the 4 days of the estrous cycle. Here, we analyzed food intake data from mice in 4-day blocks, corresponding to the females' 4-day estrous cycle in several schedules of food procurement or reward. Variance was compared within and across individual mice. In no instance did the overall variance differ by sex under any of the food reward schedules. This extends earlier observations of trait variability in body temperature and locomotor activity of mice and supports the claim that there is no empirical basis for excluding female rodents from biomedical research.

Effect of Food Predictability on Life Span in Male Mice

The purpose of this study is to compare the effect of unpredictable (U) or predictable (P) food delivery on health and longevity in mice. From 2 months of age until end of life, singly-housed male C57BL/6 mice were fed a semisynthetic diet either ad libitum (AL), or as imposed meals delivered as small pellets at either P or U times, frequencies, or amounts. The total daily food consumed by all groups was the same. The AL group gained body weight faster than either P or U groups, and had ~12% shorter median life span compared with either P or U groups. Bimonthly noninvasive body composition determinations showed that the differences in body weights were due to differences in fat and lean mass. Postmortem examinations revealed that the organ pathologies were similar in all groups, but a larger fraction of P and U mice were euthanized due to end-of-life suffering. There were no systematic differences in outcome measures between P and U groups suggesting that, within the range studied, the temporal pattern of food delivery did not have a significant metabolic effect.

Protocols Using Rodents to Model Eating Disorders in Humans

Dysfunctional feeding behavior has a bidirectional aspect, too little and too much. The former reflects restricted eating and, in extreme, becomes an eating disorder such as anorexia nervosa (AN). The latter reflects lack of restraint and leads to obesity and life-shortening metabolic syndrome. Both of these dysfunctions have proven extremely difficult to prevent or treat, and the use of animal models that have translational validity may be one of the most cost-effective ways of advancing. This chapter describes some of the laboratory protocols using rodents that are available to model human eating dysfunctions.

Cost-based anorexia: A novel framework to model anorexia nervosa

Anorexia nervosa (AN) is an eating disorder that is thought to emerge through biological predisposition(s) within sociocultural context(s). Practical and ethical concerns limit study of the etiology of this disorder in humans, and in particular the biological aspects. Laboratory animal models have a pivotal role in advancing our understanding of the neurobiological, physiological and behavioral aspects of this disorder, and developing new treatment strategies. One shortcoming of animal models, including activity based anorexia (ABA) in rodents, is that they cannot fully capture the contextual aspects of AN. In this article we discuss the merits of an alternate approach, cost-based anorexia (CBA). CBA is conceptually founded in behavioral economics and its magnitude is influenced by several relevant contextual aspects of feeding.

Temporal relationships between food acquisition and voluntary exercise in mice

Patterns of operant food acquisition in a closed economy and bouts of either voluntary wheel running (WR) or spontaneous locomotor activity in a standard condition (SC) with no wheel were examined in young adult male and female C57BL/6 mice across a range of nose poke prices (FUP) per food pellet. Both sexes showed vigorous WR or locomotor activity. At each FUP, WR groups had higher food intake than SC groups. Despite substantially higher mean body weight of males compared with females, intakes and activity did not differ by sex in the SC groups and males lost weight more rapidly as FUP increased. In contrast, WR males ran ∼33% further per day than females, increased their food intake (above that of SC counterparts) more than females, and lost less body weight than SC males. By parsing the night in four 3h epochs it was found that food intake declined progressively through the night in both WR and SC mice and that the hyperphagia of WR relative to SC groups was most evident early in the night, coincident with highest activity. No large or systematic sex differences were revealed in these temporal analyses. Analysis of data at 60s resolution showed that pellet acquisition occurred in many small or short bouts, the timing of which was either intercalated or concurrent with either locomotor activity or WR. The results show that increased eating due to WR occurs concurrently with maximum running, and with no evidence of delayed compensation.

Restricted temporal access to food and anorexia in mice: Microstructure of eating within feeding opportunities

Intake and body weight were recorded in a closed economy as male and female C57BL/6 mice progressed through either fixed interval (FI) or fixed unit price (FUP) schedules of cost for 20-mg food pellets. Access to food was constrained to four 40 min food opportunities (FOs) per day, spaced 4-h apart through the dark phase. Nose poke responses and pellet deliveries were collected at 10-s resolution to allow pellet-by-pellet analysis. In the FI protocol, mice maintained adequate food intake and body weight through the study, even though at the highest FI (50-s) they spent the entire 40-min FOs engaged in eating at or near the maximum rate allowed by the schedule. In the FUP protocol, mice greatly reduced their intake and lost weight at the highest FUP (50 responses/pellet). The analysis of response and pellet distributions showed these mice were not filling the FOs with responding and ate less at dusk (FO #1) and dawn (FO #4) than at FOs #2 and 3 in the middle of the night. The principal, and unexpected, sex difference was that females tended to eat more than males despite lower body weight, but behavioral changes as a function of feeding cost or schedule were qualitatively similar in both sexes. These results show that slow eating as imposed by an FI is not sufficient to produce hypophagia and, in the FUP protocol, hypophagia cannot be explained by slowed eating due to response requirements. We discuss the role of effort or time in FUP-induced anorexia, and suggest this murine model may emulate some aspects of human anorexia nervosa better than current activity-based protocols.