Role of sex differences in development of obesity in the OLETF rats
Date: November 8, 2007
Speaker’s Name, Affiliation: Aron Weller, Bar-Ilan University, Israel
Seminar Title: “Sex differences in coping with energetic challenges during development in the OLETF rat”
Summary: (Prepared by the Rapporteur)
Because of the growing health concern from the obesity epidemic and its related health complications, there is much public health interest in determining the causes of energy imbalance. Because many cases of obesity start during childhood, Dr. Weller has been investigating the development of obesity in animal models by using obesity-prone phenotypes, the Otsuka Long Evans Tokushima Fatty rats, or in short, OLETF. His research objectives include the following: 1) To examine the ability of manipulations during childhood to prevent the risk of developing obesity later in life, 2) To further study the underlying mechanisms of body weight set-points and resistance & susceptibility to environmental and dietary influences, and 3) To examine sex-differences and effects on the estrous cycle, in development of obesity in animal models.
The primary dysfunction in OLETF rats is their lack of CCK-1 receptors. Adult OLETF males exhibit the following phenotypes: hyperphagia, obesity, hyperleptinemia, complete resistance to CCK administration, hyperglycemia and NIDDM, up-regulation of NPY mRNA in the DMH, and an altered dopamine reward system. In contrast, young males and females exhibit the following phenotype: higher body weights from birth; increased intake and different feeding patterns during independent ingestion; increased nursing frequency, initiative, suckling efficiency, and weight gain as result of these nursing bouts; up-regulation of NPY mRNA in DMH at two weeks post-natal, increased body fat, and decreased intake reduction after gastric preloads seen at post-natal day 18.
To examine the mechanisms behind the feeding differences between OLETF and controls, Dr. Weller presented data on three experiments: 1) cross-fostering, 2) diet-restriction = pair feeding, and 3) voluntary exercise. In the cross-fostering experiment, the aim of the study was to explore the importance of the postnatal environment on obesity in the offspring. Animals were cross- or In-fostered with the same or opposite strains at post-natal day 1, and were sacrificed and assessed at weaning and at post-natal day 90. The primary hypotheses made were that: 1) the maternal environment of the LETO (wild-type) dam may be able to attenuate the obesity-prone future of the OLETF pups, 2) the maternal environment of the OLETF dam may be able to increase the weight-gain of lean LETO pups, and finally 3) the short-term effects (at weaning) may be more effective than the long-term effect assessed at post-natal day 90. Results revealed that OLETF males weighed and ate significantly less when they were raised by a LETO dam, but LETO males were not affected by adoption by another strain.
For females, OLETF pups who were raised by a foster OLETF animal ate and weighed significantly more than when raised by their own mother, or by a LETO mother. In summary to experiment 1, results suggest gender differences in the impact of early postnatal factors in determining later development. OLETF rats appear to be influenced by maternal strain strongly during the lactation period itself, and less in the long-term. LETO males appear not to be influenced by this manipulation in the long-term, but LETO females showed increased intake until adulthood. The genetic pre-obese tendency in the OLETF strain is extremely strong.
The purpose of experiment 2 on food restriction was to examine the long term influence of dieting (chronic, short & long-term) during sensitive periods of development in which the number of fat cells is rapidly increasing. It was hypothesized that short-term post-weaning diet-restriction may be able to attenuate the obesity-prone future of OLETF pups, and that OLETF males may exhibit more long-term effects than females. Results to this study suggested that males and females may exhibit differences in their long-term responses to early dieting and food restriction. While OLETF males can be successfully biased towards maintaining a leaner body mass in adulthood after a 3-week post-weaning manipulation, females respond more modestly. Despite the fact that females reduced body weight and intake in the long-term, they successfully regained all of their fat levels. A longer manipulation that included both childhood and the sexual maturation period was more successful in reducing obesity among female OLETFs, suggesting a different critical time period for males and females across which restriction can overcome these phenotypes.
The purpose of experiment three was to examine the long-term influence of exercise during the post-weaning period. The following hypotheses were made: 1) Short-term post-weaning exercise may be able to attenuate the obesity-prone future of the OLETF pup, 2) OLETF males may exhibit more long-term effects than females, and 3) LETO females may exhibit more long-term effects than males. In summarizing these findings, it was suggested that there are sex-differences in the long-term response to early exercising. OLETF males can be successfully biased toward a “leaner” adulthood after a 3-week manipulation involving exercise, while females responded to an identical manipulation more modestly. LETO females responded as expected, by compensating for the increased energetic demands of exercise by increasing intake. Even long after the running wheels were locked, they maintained the increased body weight (not fat) and intake levels.
In sum, OLETF pups seem to be programmed from an early point in development to eat more, gain more, weigh more, and accumulate more fat mass. Elevated NPY gene expression in the DMH during this early period in development may play a role in the above characteristics of the OLETF phenotype. For males and females, there are differences in the sensitive windows for the development of long-term obesity. In males, three-weeks post weaning (until PND45) seem to be a susceptible time. Voluntary exercise during this time was the most efficient way of reducing body fat and long-term propensity for obesity. OLETF females responded to all manipulations more modestly, suggesting that this window of sensitivity may be different in males and females. Despite modest changes, exercise also seemed to be more effective than food restriction at altering long-term prognosis of obesity.
[The contribution of main collaborators Mariana Schroeder and Tim Moran to this work was acknowledged].
Q. Why is it hard to get animal models of alcoholism?
A. Alcohol is aversive to most rodents. It is difficult to get randomly bred rats to drink it. Moderate consumption can be trained; however, it takes time and rats will not drink to intoxication.
Q. If you give OLETF rats leptin, will that correct their disorder in energy balance?
A. No, their leptin is high already (in accordance with their obese state).
Q. Is it correct that this is the first report of a genetic mutation showing an increase in the amount of intake while the animals are nursing?
A. I never realized it, but it’s possible this is the first report.
Q. Do the animals differ at birth for birth weight?
A. Yes, the OLETF animals are heavier from day 1 (Schroeder et al. Physiology and Behavior, 2006, 89, 402-409).
Q. Has anyone looked at energy expenditure in brown adipose tissue in these animals? You could also look at uncoupling proteins to find this out.
A. We have been removing brown adipose tissue, but we are not finding many differences in the weight of that specific fat pad. We have not looked at UCPs.
Q. Is leptin equally effective in reducing food intake in these animals?
A. I’m not sure how to answer that. I’m not sure what the best control group would be. Probably it will not be as effective as in LETO rats, because OLETF are hyperleptinemic. We should check this.
Q. Do the pups have a lot of control over food intake and body weight?
A. We are trying to figure that out. The feeding of the pup (as well as children) is bidirectional and symbiotic. We will be testing intake and macronutrient content of the milk in the future.
Q. Females have more sub-Q fat than males do, and you aren’t measuring that.
A. That’s true. That’ s a major critique of the study.
Q. Is there really strong evidence that there isn’t a defect in energy expenditure in OLETF rats that is causing these differences in body weight?
A. Yes, I think so.
Q. Did you measure how much the animals in your experiment ran?
A. Yes, it is in the last slide. They run a lot. And the 2 groups that were most influenced by the running included the group that ran the least – male OLETF rats.
Q. How long after exercise were these measurements taken?
A. 45 days after the running wheels were locked..
Q. What happened to food intake while the animals were running?
A. I presented that. OLETF males ate less, LETO males and OLETF females ate the same, and LETO females ate more.
Q. Comment: When the Japanese research groups did exercise studies, they started earlier, and the older animals are when they started, the less time it took them to regain lost body weight.
A. Yes, we were happy about those results.
Q. Can you hypothesize on what the endogenous levels of CCK in OLETF animals would be?
A. They should have been normal, or perhaps higher. I do not quite remember the results.
Q. How did you pair-feed the animals?
A. We measured how much the LETO animals ate on a daily basis, and then fed this amount to the OLETF rats. We fed them all in one bunch, so that they ate in a manner similar to a binge pattern of feeding.
Q. Did you measure oxytocin levels in the pups?
A. We found oxytocin to be highly correlated with increased body weight in OLETF dams, but not in the pups.