SUMMARYMany studies have investigated the consequences of individual variation in resting metabolic rate at thermoneutrality (RMRt) on reproductive performance. Despite strong theoretical reasons for expecting such an association, results have generally been disappointing. A fundamental assumption of these studies is that RMRt is a repeatable trait. We examined repeatability of RMRt in female MF1 mice over short (15days apart; N238) and long intervals (110days apart; N33). In the long-term experiment, after the first RMRt measurement, females were separated in two groups: the first was kept virgin (N16); the second was allowed to breed (N17) and measured 15days after they had weaned their pups. We also examined the association between RMRt and reproduction. We used Pearson's correlation (r) and intraclass correlation coefficients () to estimate repeatability. There was a strong effect of body mass on RMRt for all measurements. Over the short interval, repeatability was significant for body mass (r0.86; 0.86), RMRt (r0.68; 0.68,) and residual-RMRt (r0.58; 0.58). Over long intervals, repeatability of residual-RMRt was high in virgin females (r0.59; 0.60), but not in the breeders (r0.38; 0.39); body mass was repeatable only for non-breeders measured by r (r0.55). There was no significant correlation between RMRt or residual-RMRt and litter size or litter mass. In conclusion, RMRt and residual-RMRt are highly repeatable traits in virgin MF1 female mice. The lack of association between nonreproductive RMRt and reproductive performance in MF1 mice does not come about because of its poor repeatability.Key words: Mus musculus, repetability, resting metabolic rate, mouse, reproductive performance. 1989;Weiner, 1992;Peterson et al., 1990;Hammond and Diamond, 1992;Hammond and Diamond, 1997;Lovegrove, 2000). Although the first ideas were that SusMR and BMR might be linked via the energy acquisition system, it was later recognized that links between the two might also come about because of commonality in the demands of the tissues where energy is ultimately utilized. If the tissues involved in energy utilisation have high rates of energy demand in the basal state, then a similar functional linkage between tissues, BMR (RMRt), SusMR and SusEI would emerge. THE JOURNAL OF EXPERIMENTAL BIOLOGY279 Repeatability of RMRLactation is one of the most energetically demanding periods in the lives of small rodents (reviewed by Speakman, 2008;Naya et al., 2008). Many studies have been performed comparing RMRt of lactating animals to non-breeding individuals (Hammond and Diamond, 1992;Hammond et al., 1996;Konarzewski and Diamond, 1995;Rogowitz and McClure, 1995;Rogowitz, 1998;Speakman and McQueenie, 1996;Johnson and Speakman, 2001;Johnson et al., 2001a;Johnson et al., 2001b;Johnson et al., 2001c;Król and Speakman, 2003a;Król and Speakman, 2003b;Król et al., 2003;Król et al., 2007). These studies are consistent with the idea that BMR (RMRt) and SusMR (SusEI) should be closely associated. Moreover, this association i...

SUMMARYLimits to sustained energy intake (SusEI) are important because they constrain many aspects of animal performance. Individual variability in SusEI may be imposed by genetic factors that are inherited from parents to offspring. Here, we investigated heritability of reproductive performance in MF1 mice. Food intake, milk energy output (MEO) and litter mass were measured in mothers (F 0 ) and daughters (F 1 ) that were raising litters of 10 pups. Cross-fostering was designed so that half of each litter consisted of biological offspring and the rest came from one unrelated female (i.e. fostered pups). Food intake increased linearly during early lactation and reached a plateau during late lactation (day 9-13, called the asymptotic food intake, FI AS , equivalent to SusEI). Parent-offspring regression showed that FI AS , MEO and litter mass were all positively and significantly related between mothers and their biological daughters, but no significant relationships were found between the same traits for mothers and fostered daughters. FI AS at peak lactation was significantly correlated to adult food intake and body mass when the mice were 6months old and not lactating. In conclusion, a large part of the variation in FI AS could be explained by genetic variation or maternal effects in pregnancy whereas non-genetic maternal effects in lactation were negligible. As a consequence, biological daughters of mothers with high reproductive performance (i.e. high milk production and hence higher litter mass at weaning) had a better reproductive performance themselves, independent of the mother that raised them during lactation.

Objective: The melanin-concentrating hormone (MCH) is a centrally acting peptide implicated in the regulation of energy homeostasis and body weight, although its role in glucose homeostasis is uncertain. Our objective was to determine effects of MCHR1 antagonism on energy budgets and glucose homeostasis in mice. Methods: Effects of chronic oral administration of a specific MCHR1 antagonist (GW803430) on energy budgets and glucose homeostasis in diet-induced obese (DIO) C57BL/6J mice were examined. Results: Oral administration of GW803430 for 30 days reduced food intake, body weight, and body fat. Circulating leptin and triglycerides were reduced but insulin and nonesterified fatty acids were unaffected. Despite weight loss there was no improvement in glucose homeostasis (insulin levels and intraperitoneal glucose tolerance tests). On day 4-6, mice receiving MCHR1 antagonist exhibited decreased metabolisable energy intake and increased daily energy expenditure. However these effects had disappeared by day 22-24. Physical activity during the dark phase was increased by MCHR1 antagonist treatment throughout the 30-day treatment. Conclusions: GW803430 produced a persistent anti-obesity effect due to both a decrease in energy intake and an increase in energy expenditure via physical activity but did not improve glucose homeostasis.

The endocannabinoids have been recognized as an important system involved in the regulation of energy balance. Rimonabant (SR141716), a selective inverse agonist of cannabinoid receptor 1 (CB1), has been shown to cause weight loss. However, its suppressive impact on food intake is transient, indicating a likely additional effect on energy expenditure. To examine the effects of rimonabant on components of energy balance, we administered rimonabant or its vehicle to diet‐induced obese (DIO) C57BL/6 mice once daily for 30 days, by oral gavage. Rimonabant induced a persistent weight reduction and a significant decrease in body fatness across all depots. In addition to transiently reduced food intake, rimonabant‐treated mice exhibited decreased apparent energy absorption efficiency (AEAE), reduced metabolizable energy intake (MEI), and increased daily energy expenditure (DEE) on days 4–6 of treatment. However, these effects on the energy budget had disappeared by days 22–24 of treatment. No chronic group differences in resting metabolic rate (RMR) or respiratory quotient (RQ) (P > 0.05) were detected. Rimonabant treatment significantly increased daily physical activity (PA) levels both acutely and chronically. The increase in PA was attributed to elevated activity during the light phase but not during the dark phase. Taken together, these data suggested that rimonabant caused a negative energy balance by acting on both energy intake and expenditure. In the short term, the effect included both reduced intake and elevated PA but the chronic effect was only on increased PA expenditure.

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