Model for predicting and phenotyping at normal weight the long-term propensity for obesity in Sprague–Dawley rats

Maternal high-fat diet and fetal programming: increased proliferation of hypothalamic peptide-producing neurons that increase risk for overeating and obesity

Recent studies in adult and weanling rats show that dietary fat, in close association with circulating lipids, can stimulate expression of hypothalamic peptides involved in controlling food intake and body weight. In the present study, we examined the possibility that a fat-rich diet during pregnancy alters the development of these peptide systems in utero, producing neuronal changes in the offspring that persist postnatally in the absence of the diet and have long-term consequences. The offspring of dams on a high-fat diet (HFD) versus balanced diet (BD), from embryonic day 6 to postnatal day 15 (P15), showed increased expression of orexigenic peptides, galanin, enkephalin, and dynorphin, in the paraventricular nucleus and orexin and melanin-concentrating hormone in the perifornical lateral hypothalamus. The increased density of these peptide-expressing neurons, evident in newborn offspring as well as P15 offspring cross-fostered at birth to dams on the BD, led us to examine events that might be occurring in utero. During gestation, the HFD stimulated the proliferation of neuroepithelial and neuronal precursor cells of the embryonic hypothalamic third ventricle. It also stimulated the proliferation and differentiation of neurons and their migration toward hypothalamic areas where ultimately a greater proportion of the new neurons expressed the orexigenic peptides. This increase in neurogenesis, closely associated with a marked increase in lipids in the blood, may have a role in producing the long-term behavioral and physiological changes observed in offspring after weaning, including an increase in food intake, preference for fat, hyperlipidemia, and higher body weight.

Developmental changes of gut microflora and enzyme activity in rat pups exposed to fat-rich diet

The aim of this study was to investigate the effect of a high-fat (HF)/energy diet on the intestinal microbiota, the alkaline phosphatase (AP) activity, and related parameters of growth and obesity during the suckling and weaning periods in male Sprague-Dawley rats. From birth, nutrition in suckling pups was manipulated by feeding rat dams either HF or a standard diet, and then after weaning, by exposure of experimental pups to the HF, and control rats to normal diet. On days 15, 20, 40 the numbers of 2 microbial groups, i.e., Bacteroides/Prevotella (BAC) and the Lactobacillus/Enterococcus (LAB) in the jejunum, were determined by fluorescent in situ hybridization technique, and the AP activity was assayed histochemically. During all investigated periods HF pups gained body fat more rapidly than control animals, but from weaning they displayed significantly stunted growth resulting in final body weight loss. Obesity in HF rats was also accompanied by higher LAB and lower numbers of BAC and with permanently higher AP activity. Correlation of these data showed significant negative correlation between LAB, AP, and weight gain and energy efficiency, and significant positive correlation of BAC and AP activity with body fat. These data support the concept that postnatal nutritional experience represents an important factor affecting the ontogeny of intestinal microbial communities and intestinal function. These acquired changes could be a component of regulatory mechanisms involved in adverse and/or positive consequences of HF diet for adiposity, body weight, and energy-balance control in later life.

Reduced capacity for fatty acid oxidation in rats with inherited susceptibility to diet-induced obesity

High-fat, energy-dense diets promote weight gain and obesity in humans and other animals, but the mechanisms underlying such diet-induced obesity remain elusive. To determine whether a reduced capacity to oxidize fat is involved in the etiology of diet-induced obesity, we examined different measures of fatty acid oxidation in rats selectively bred for susceptibility (DIO) or resistance (DR) to dietary obesity before and after they were fed a high-fat diet and became obese. DIO rats eating a low-fat diet oxidized less dietary fatty acid in vivo and had lower levels of plasma ketone bodies during fasting compared with DR rats. Lean DIO rats fed a low-fat diet showed reduced liver messenger RNA expression of CD36, which transports fatty acids across cell membranes, and long-chain acyl-coenzyme A dehydrogenase (ACADL), which catalyzes the first step in the mitochondrial beta-oxidation of fatty acids. The deficit in CD36 and ACADL messenger RNA expression was also seen in obese DIO rats that had been eating a high-fat diet and, in addition, was accompanied by reduced expression of liver carnitine palmitoyl transferase I, the enzyme that mediates transport of long-chain fatty acids into mitochondria. No differences were found in the expression of liver enzymes involved in fat synthesis; however, in muscle, DIO rats fed the low-fat, but not high-fat, diet showed greater expression of diacylglycerol O-acyltransferase 1 and lipoprotein lipase than did DR rats. Expression of muscle enzymes involved in fatty acid oxidation was similar in the 2 groups. These findings provide a metabolic mechanism for the development of diet-induced obesity and thus suggest potential targets for intervention strategies to treat or prevent it.

Weight gain model in prepubertal rats: prediction and phenotyping of obesity-prone animals at normal body weight

OBJECTIVE

Male Sprague-Dawley rats maintained from birth on a high-fat diet were examined to determine whether a specific measure before puberty can identify and allow one to characterize prepubertal rats at normal weight with high vs low risk for adult obesity.

MATERIALS AND METHODS

Measures from weaning (day 21) to around puberty (day 45) were taken of weight gain, absolute body weight and daily energy intake on a high-fat diet and related to the amount of body fat accumulated at maturity (80-100 days of age). Rats identified by a specific prepubertal measure as obesity-prone (OP) vs obesity-resistant (OR) were then characterized before and after puberty.

RESULTS

Prepubertal weight gain from days 30 to 35 of age was the strongest and earliest positive correlate of ultimate body fat accrual in adult rats. The highest (8-10 g/day) compared to lowest (5-7 g/day) weight-gain scores identified accurately and reproducibly distinct OP and OR subgroups at day 35 that became obese or remained lean, respectively, as adults. The OP rats with rapid prepubertal weight gain and 50% greater adiposity at maturity (day 100) exhibited the expected phenotype of already-obese rats. These included elevated levels of leptin, insulin, triglycerides and glucose, increased galanin (GAL) peptide levels in the paraventricular nucleus (PVN) and reduced neuropeptide Y (NPY) levels in the arcuate nucleus (ARC). Before puberty (day 35), the OP rats with normal fat pad weights, energy intake and endocrine profile similar to OR rats exhibited these disturbances characteristic of obese rats. They had decreased capacity for fat oxidation in muscle, increased GAL expression in PVN and reduced expression of NPY and agouti-related protein in ARC.

CONCLUSION

Prepubertal weight gain can identify OP rats on day 35 when they have minimal body fat but exhibit specific metabolic and neurochemical disturbances expected to promote obesity and characteristics of already-obese adult rats.

Leptin secretion after a high-fat meal in normal-weight rats: strong predictor of long-term body fat accrual on a high-fat diet

The objective of this study was to investigate meal-related endocrine changes that permit one to identify Sprague-Dawley rats at normal weight that are prone (OP) vs. resistant (OR) to obesity. In blood collected via chronic cardiac catheters, a 2-h high-fat meal (HFM, 50% fat, 40 kcal) at dark onset caused a significant increase in leptin, insulin, and triglycerides compared with premeal levels. Similar to patterns in already obese compared with lean rats on a high-fat diet, these meal-induced endocrine changes in normal-weight rats on lab chow were almost twofold larger in OP rats that, compared with OR rats, subsequently accumulated 100% more fat mass on a chronic high-fat diet. These exaggerated endocrine changes were similarly observed in blood collected using a simpler tail vein puncture procedure. In three separate experiments, the HFM-induced rise in leptin was found to be the strongest, positive correlate (r = +0.58, +0.62 and +0.64) of long-term body fat accrual. The lowest (2-5 ng/ml) vs. highest (6-9 ng/ml) scores for this post-HFM leptin measurement identified distinct OR and OP subgroups, respectively, when they were similar in body weight (340-350 g), premeal leptin (2.6-3.4 ng/ml), and meal size (40 kcal). Subsequent tests in these normal-weight OP rats revealed a distinct characteristic compared with OR rats, namely, exaggerated HFM-induced rise in expression of the orexigenic peptide galanin in the paraventricular nucleus. Thus, with this HFM-induced leptin measurement, OP rats can be identified while still at normal weight and then investigated for mechanisms that contribute to their excessive body fat accrual on a high-fat diet.