Mechanisms of dysregulation of 11 beta-hydroxysteroid dehydrogenase type 1 in obese Zucker rats

Obesity has been associated with alterations in glucocorticoid metabolism in both man and rodents, but the underlying mechanisms remain undefined. We have previously reported tissue-specific alterations in 11 beta-hydroxysteroid dehydrogenase type 1 (11 beta-HSD1) in obese Zucker rats predicting that reactivation of corticosterone is decreased in liver but increased in omental fat. The mechanisms of dysregulation of 11 beta-HSD1 in obesity are not known, and in this study we have investigated the potential role of glucocorticoids and insulin. In one experiment lean and obese Zucker rats were adrenalectomised, and in a second experiment they were sensitised to insulin by treatment with either metformin or rosiglitazone. Adrenalectomy (ADX) of obese animals attenuated weight gain, normalised hepatic 11 beta-HSD1 kinetics by an effect on V(max) (V(max)in sham-operated animals, 6.6+/-1.1 nmol/min per mg in lean vs 3.4+/-0.6 in obese, P<0.01; in ADX animals 5.9+/-1.1 in lean vs 6.9+/-1.8 in obese, NS), and reversed the difference in omental fat 11 beta-HSD1 activity (18.9+/-4.2% in lean ADX vs 8.2+/-2.3 in obese ADX, P=0.03). Both metformin and rosiglitazone improved insulin sensitivity in obese, but not lean animals, and had no effect on 11 beta-HSD1 activity in either liver or fat. However, both treatments normalised adrenal hypertrophy in obese animals (48+/-29 mg in obese vehicle vs 37+/-1.2 in metformin and 38+/-1.8 in rosiglitazone treated, both P<0.01), and rosiglitazone tended to attenuate hypercorticosteronaemia in obese rats. Neither treatment attenuated weight gain; in fact, weight gain was enhanced by rosiglitazone in obese rats. In summary, altered 11 beta-HSD1 activity in obese Zucker rats is reversible following adrenalectomy, but the mechanism is unclear since adrenalectomy also normalises many other metabolic abnormalities. The current study suggests that hyperinsulinaemia is not responsible for tissue-specific dysregulation of 11 beta-HSD1. However, insulin sensitisation did reverse adrenal hypertrophy, suggesting that hyperinsulinaemia may be a key factor contributing to activation of the hypothalamic- pituitary-adrenal (HPA) axis in obesity independently of tissue-specific changes in 11 beta-HSD1.     

Attenuation of hyperinsulinemia by NN414, a SUR1/Kir6.2 selective K-adenosine triphosphate channel opener, improves glucose tolerance and lipid profile in obese Zucker rats

Chronic attenuation of hyperinsulinemia by diazoxide (DZ), a K-adenosine triphosphate (ATP) channel opener and an inhibitor of glucose-mediated insulin secretion, improved glucose tolerance and lipid profile and decreased the rate of weight gain in obese Zucker rats. To determine whether suppression of hyperinsulinemia alters daily food consumption, rate of weight gain, glucose tolerance, and lipid profile, we compared the effects of NN414, a potent and SUR1/Kir6.2 selective K(atp)() channel opener, with DZ in obese and lean Zucker rats. DZ (150 mg/kg/d), low-dose (LDNN414: 10 mg/kg/d), high-dose (HDNN414: 30 mg/kg/d), and vehicle (C) were administered to 7-week-old obese and lean female Zucker rats for a period of 6 weeks. Each animal underwent an intraperitoneal glucose tolerance test (IPGTT) at the end of study period. While NN414 treatment did not affect food intake and rate of weight gain in any of the strains, DZ treatment reduced food intake (P <.001) and rate of weight gain (P <.001) in obese rats. The fasting plasma insulin levels and area under the curve (AUC) insulin response to IPGTT were significantly attenuated in LDNN414 (P <.05), HDNN414 (P <.01), and DZ (P <.01) obese and lean rats compared with their controls. This was accompanied by a significant reduction in AUC glucose only in LDNN414 (P <.05), HDNN414 (P <.01), and DZ (P <.01) obese rats compared with controls. While hemoglobin A(1c) (HbA(1c)) was not affected in LDNN414 obese rats, it was higher in HDNN414 obese animals (P <.001), LD-, HDNN414 (P <.001), and DZ (P <.005) lean rats compared with their respective controls. DZ obese rats showed lower HbA(1c) levels than C obese rats (P <.02). The plasma free fatty acid (FFA) levels were only decreased in HDNN414 (P <.05) and DZ (P <.002) obese rats, whereas plasma triglyceride (TG) levels were decreased in LDNN414 (P <.05), HDNN414 (P <.001), and DZ (P <.001) obese rats compared with controls. Finally, plasma leptin level was only decreased in DZ obese rats compared with controls (P <.001). The new SUR1/Kir6.2 selective K(atp)() channel opener, NN414, reduced hyperinsulinemia in a dose-dependent manner without a significant effect on food consumption and rate of weight gain. NN414-induced beta-cell rest in obese rats was associated with a significant improvement in glucose responsiveness, suggesting an increase in insulin sensitivity after its withdrawal. There was an overall deterioration in glycemic control at the high dose as measured by HbA(1c). There was a dose-dependent improvement in lipid profiles of obese Zucker rats. These results suggest that pharmacologic attenuation of hyperinsulinemic state by low-dose NN414 may be therapeutically beneficial in insulin-resistant states without any deterioration in overall glycemic control.     

Hippocampal 11beta-hydroxysteroid dehydrogenase type 1 messenger ribonucleic acid expression has a diurnal variability that is lost in the obese Zucker rat

Circulating levels of glucocorticoids show a circadian rhythm. Obesity is associated with a flattening of the diurnal rhythm; plasma cortisol levels are slightly increased during the trough, although they are normal or low in the morning. Studies in humans and in leptin-resistant Zucker rats suggest that tissue-specific alterations in glucocorticoid exposure might play a key role for development of obesity and obesity-associated dysregulation of the hypothalamic-pituitary-adrenal axis. We hypothesized that there is a circadian rhythm in prereceptor metabolism of glucocorticoids exerted by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) in brain and/or peripheral tissues (liver, fat, and muscle) that might be abrogated in obesity. The present study demonstrates a circadian rhythm in 11beta-HSD1 mRNA expression (35-45% increase at morning vs. evening, P < 0.05) in dentate gyrus granular layer and CA1 subregions of the hippocampus in lean Zucker rats that was lost in the obese rats. Sprague Dawley rats also revealed a diurnal rhythm in hippocampal 11beta-HSD1 mRNA expression. There was no circadian variation in 11beta-HSD enzyme activity in peripheral tissues, although obese Zucker rats had a decreased enzyme activity in liver and epididymal fat (by approximately 40%, P < 0.05) compared with lean rats. In Sprague Dawley rats, 11beta-HSD activity in adipose tissue was higher in retroperitoneal and epididymal vs. sc fat (P < 0.001). In summary, obese Zucker rats lack a circadian rhythm of 11beta-HSD1 gene expression in the hippocampus, which may contribute to increased activity of the hypothalamic-pituitary-adrenal axis and altered diurnal variation of circulating corticosterone levels.     

Changes in hypothalamic expression levels of galanin-like peptide in rat and mouse models support that it is a leptin-target peptide

Galanin-like peptide (GALP) is a novel peptide that has been isolated from the porcine hypothalamus. The expression of GALP mRNA is localized to the hypothalamic arcuate nucleus and is thought to be under the regulation of leptin. First, we confirmed by real-time PCR analysis that sc administration of leptin to Wistar rats under food-deprived conditions resulted in a 1.5-fold increase in hypothalamic GALP mRNA levels. Next, GALP mRNA levels were found to be reduced by 50% in 11-wk-old male Zucker obese rats compared with age-matched Zucker lean rats, whereas neuropeptide Y mRNA levels were increased by 55% and proopiomelanocortin mRNA levels were reduced by 53% in Zucker obese rats. Analysis using a two-site enzyme immunoassay revealed a lower level of hypothalamic GALP immunoreactivity in 11-wk-old Zucker obese rats (5.9 fmol/mg protein) than in age-matched Zucker lean rats (19.6 fmol/mg protein). Immunohistochemical studies demonstrated that Zucker obese rats (11 wk old) had a reduced number of GALP immunoreactivity-positive cells (29.4 cells/3 slices) in the arcuate nucleus compared with age-matched Zucker lean rats (115 cells/3 slices). Furthermore, Zucker obese rats showed increased sensitivity to intracerebroventricularly administered GALP compared with Zucker lean rats, in that a lower dose of GALP increased plasma LH levels in male Zucker obese rats, but not in male Zucker lean rats. In addition, a reduction in the level of hypothalamic GALP mRNA was found in db/db and ob/ob mice. The result supports the hypothesis that the hypothalamic GALP gene expression is controlled by leptin signals and suggests possible involvement of GALP in the reproductive abnormalities of the Zucker obese rat.     

Influence of placental 11beta-hydroxysteroid dehydrogenase (11beta-HSD) inhibition on glucose metabolism and 11beta-HSD regulation in adult offspring of rats

Placental 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) converts glucocorticoids to 11-keto-products and is believed to play an important role in protecting fetuses from higher maternal glucocorticoid levels. Recent reports have speculated that prenatal glucocorticoid exposure leads to fetal growth retardation and adult offspring hypertension and hyperglycemia. To investigate the effects of placental 11beta-HSD2 inhibition on glucose metabolism and the 11beta-HSD system in adult offspring, pregnant rats were treated with daily injections of carbenoxolone (CBX), an inhibitor of 11beta-HSD. The offspring of the maternal CBX treatment group showed reduced birth weight (treated v control, 5.6 +/- 0.5 v 6.4 +/- 0.4 g, P < .0001). In adult offspring of the maternal CBX treatment group, plasma hemoglobin A1c was significantly increased (7.3% +/- 1.8% v 4.8% +/- 0.3%, P < .01) and glucose intolerance was shown on the oral glucose tolerance test. The gene expression of hepatic 11beta-HSD1 and renal 11beta-HSD2 was decreased 87.6% (P < .05) and 52.3% (P < .01) in adult offspring of the maternal CBX treatment group, whereas renal 11beta-HSD1 was not significantly altered. The change in 11beta-HSD activity corresponded to the change in the gene expression. These results suggest that inhibition of placental 11beta-HSD2 causes growth retardation, glucose intolerance, and partial suppression of the 11beta-HSD system in the offspring.     

Novel effects of the cannabinoid inverse agonist AM 251 on parameters related to metabolic syndrome in obese Zucker rats

Background and ObjectiveRecent research suggests that cannabinoid receptor CB1 antagonists can affect appetite and body weight gain, although their influence on other parameters related to metabolic syndrome is not well documented. The present study was designed to assess the effects of chronic treatment with the CB1 receptor inverse agonist AM 251 (3 mg/kg for 3 weeks) in obese and lean Zucker rats on parameters related to metabolic syndrome.Materials and MethodsFour groups of rats were used: lean Zucker rats, untreated obese Zucker rats, AM 251-treated obese Zucker rats and a pair-fed obese Zucker rat experimental group which received the same amount of food as that consumed by the animals treated with AM251. Food intake, body weight gain, energy expenditure, plasma biochemical parameters, leptin, insulin and hepatic status markers were analysed.ResultsDaily injection of AM 251 in obese Zucker rats produced a marked and sustained decrease in daily food intake and body weight and a considerable increase in energy expenditure in comparison with untreated obese Zucker rats. AM 251 administration to obese rats significantly reduced plasma levels of glucose, leptin, AST, ALT, Gamma GT, total bilirubin and LDL cholesterol whereas HDL cholesterol plasma levels increased. The results also showed a decrease in liver/weight body ratio and total fat content in the liver. The main effects of AM251 (3 mg/kg) found in this study were not observed in pair-fed obese animals, highlighting the additional beneficial effects of treatment with AM 251. The results obtained in obese rats can be interpreted as a decrease in leptin and insulin resistance, thereby improving glucose and lipid metabolism, alleviating the steatosis present in the metabolic syndrome and thus favourably modifying plasma levels of hepatic biomarkers.ConclusionOur results indicate that the cannabinoid CB1 inverse agonist AM 251 represents a promising therapeutic strategy for the treatment of obesity and metabolic syndrome.     

The role of the small bowel in the regulation of circulating ghrelin levels and food intake in the obese Zucker rat

Circulating levels of ghrelin, a stomach peptide that promotes food intake, rise before and fall after meal. We aimed to investigate whether there is an independent contribution of the small bowel to the regulation of ghrelin and appetite. A duodenal-jejunal bypass (DJB) with preservation of normal gastric volume and exposure to nutrients was performed in 12-wk-old obese Zucker ZDF fa/fa rat. Food intake, weight gain, 48-h fasting, and 24-h refeeding levels of total and acylated ghrelin were measured. The DJB was challenged against gastric banding (GB), diet, and a sham operation in matched animals. Normal controls were age-matched Wistar rats, which underwent either DJB or a sham operation. The Zucker obese animals showed a paradoxical increase of acylated ghrelin levels after refeeding (+30% with respect to fasting levels; P = 0.001), an abnormality that was completely reversed only by the DJB (-30%; P = 0.01) but not after GB, diet, or sham operation. In obese rats, the DJB resulted in significantly less food intake and weight gain compared with both GB (P < 0.05) and sham operation (P < 0.01). In sharp contrast, the DJB did not alter food intake and weight gain in normal rats. The DJB does not physically restrict the flow of food but restores meal-induced suppression of acylated ghrelin and significantly reduces food intake in Zucker obese rats. These findings suggest an independent intestinal contribution to the regulation of the dynamic ghrelin response to eating and the possibility that defective signaling from the proximal bowel could be involved in the pathogenesis of obesity/hyperphagia.     

Obese Zucker rats have reduced mineralocorticoid receptor and 11beta-hydroxysteroid dehydrogenase type 1 expression in hippocampus-implications for dysregulation of the hypothalamic-pituitary-adrenal axis in obesity

Obese Zucker rats have elevated basal corticosterone levels and an increased stress response suggestive of an increased activity of the hypothalamic-pituitary-adrenal (HPA) axis. We hypothesized that altered central expression of glucocorticoid receptors (GR), mineralocorticoid receptors (MR), and/or 11beta-hydroxysteroid dehydrogenase type 1 (11betaHSD1) contribute to these changes. In brains from young adult male rats, in situ hybridization and Western blotting showed that obese rats had normal hippocampal GR mRNA and protein levels. In contrast, in obese rats, 11betaHSD1 mRNA levels were reduced in a subpopulation of hippocampal cells in the main neuronal layers (by 37-47%, P < 0.05), whereas 11betaHSD1 levels in sparse high-expressing cells did not differ. MR mRNA was decreased in all regions of the hippocampus (by 37-49%, P < 0.05 for CA1-2 and P < 0.01 for dentate gyrus) and in frontal cortex (by 16%, P < 0.05) in obese rats. In whole hippocampal homogenates, however, neither the protein concentration of MR by Western blot nor activity of 11betaHSD1 was measurably different between the phenotypes. To test the functional importance of lower central MR expression, groups of lean and obese rats were given spironolactone before restraint stress. In vehicle-treated animals, obese rats had higher plasma corticosterone levels than lean rats after stress (by ANOVA, P < 0.05). Spironolactone markedly increased the corticosterone response in both groups, but the incremental rise was smaller in the obese rats, so that spironolactone abolished the differences between groups. We conclude that lower levels of MR, but not GR, contribute to the increased HPA activity in the obese Zucker rats and that this seems more influential during stress than in the basal state. This may be exacerbated by impaired local regeneration of corticosterone by 11betaHSD1. These abnormalities could contribute to the subtle changes in the HPA axis in rodent and human obesity.     

Anti-obesity effect of two different levels of dehydroepiandrosterone in lean and obese middle-aged female Zucker rats

Dehydroepiandrosterone has previously been shown to prevent weight gain in growing lean and obese mice and rats. In the present study, lean and obese female Zucker rats were treated with either 0.6 or 1.0 percent DHEA in the diet from 8 until 14 months of age. In lean rats, 0.6 percent DHEA prevented weight gain and 1.0 percent DHEA resulted in significant weight loss compared to initial body weight. Control lean rats had a significant weight gain. Both 0.6 and 1.0 percent DHEA obese rats lost weight over the experimental period while control obese rats gained weight. Food intake of DHEA-treated obese rats was lowered compared to control obese rats but was similar to that of all lean groups. DHEA lowered serum insulin levels in both lean and obese rats relative to control groups. Both 0.6 and 1.0 percent DHEA lean rats had elevated hepatic G6PD activity compared to control lean rats. DHEA obese rats had lowered G6PD activity compared to the control obese rats. Hepatic malic enzyme was elevated by DHEA treatment in both lean and obese Zucker rats. Adipose tissue weights were lowered substantially in DHEA treated lean and obese rats versus their control groups. These data indicate that DHEA treatment in adult rats has an anti-obesity effect.     

Effects of the CRF1 receptor antagonist SSR125543 on energy balance and food deprivation-induced neuronal activation in obese Zucker rats

The corticotropin-releasing factor (CRF) system is involved in numerous physiological and behavioral actions, including the regulation of energy balance. We examined the effects of the CRF(1) receptor antagonist, SSR125543, on energy balance and food deprivation-induced neuronal activation in obese rats. Lean (Fa/?) and obese (fa/fa) Zucker rats were treated orally with SSR125543 at a daily dose of 30 mg/kg for 21 days. Rats were killed either fed ad libitum or food deprived for 6 h in order to induce a mild stress response in obese rats. SSR125543 reduced plasma corticosterone levels in lean rats, prevented corticosterone response to fasting in obese rats, and increased CRF mRNA levels in the paraventricular hypothalamic nucleus (PVN) of both lean and obese rats, further confirming that the antagonist partially blocked CRF(1) receptors. SSR125543 increased protein gain in obese rats. Whole carcass analyses showed reduced energy and fat gains in lean rats. Consistent with reduced fat gain, circulating triglyceride and leptin levels were reduced in SSR125543-treated lean rats. In obese rats, circulating glucose levels and the homeostasis model assessment of insulin resistance index of insulin resistance were reduced by SSR125543 treatment. CRF(1) receptor blockade increased uncoupling protein-1 mRNA levels in interscapular brown adipose tissue of obese rats. The antagonist partly blocked the fasting-induced changes in c-fos mRNA levels in the PVN and arcuate nucleus of obese rats. Overall, these results suggest that although SSR125543 had relatively mild effects on energy balance, CRF(1) receptor blockade attenuated several metabolic effects of short-term fasting and improved plasma variables related to the metabolic syndrome and diabetes.     

Influence of clenbuterol on energy balance, thermogenesis and body composition in lean and genetically obese Zucker rats

Daily injection of lean Zucker rats with a beta 2-adrenergic agonist (clenbuterol, 1 mg/kg) for 22 day increased weight gained by 38 per cent; there were significant increases in carcass protein and water, but fat content was unaltered. Clenbuterol did not affect energy intake or expenditure, the acute thermogenic response to food, or brown adipose tissue (BAT) activity (assessed from mitochondrial purine nucleotide (GDP) binding). In obese Zucker rats, clenbuterol significantly depressed energetic efficiency and increased the thermogenic response to food and BAT activity in these mutants. Body weight gain was not significantly affected by clenbuterol in obese Zucker rats but this was because of a 19 per cent reduction in fat content accompanied by a simultaneous 13 per cent increase in protein content. The ratio of protein/fat gained during the study was increased by 50 and 173 per cent by clenbuterol in lean and obese rats, respectively. Thus, clenbuterol exhibits potent anabolic effects on lean body mass in genetically obese as well as lean rats, but also increases thermogenesis and reduces body fat content in the obese mutants.     

Some effects of adrenalectomy in the fatty rat.

Function of the pituitary-adrenal axis was examined in lean and genetically obese fatty (Zucker) rats. The diurnal rhythms of plasma corticosterone and plasma ACTH were similar in both groups. The secretion of corticosterone by adrenal glands incubated in vitro with graded doses of ACTH was also comparable in lean and fatty rats. Adrenalectomy reduced food intake and weight gain in the fatty rats to levels similar to those in the lean animals and raised plasma ACTH to the same level. The injection of corticosterone (2 and 10 mg/day) increased food intake more in adrenalectomized fatty rats than in the corresponding lean ones. Progesterone increased food intake similarly in both groups. The enhanced responsiveness to corticosterone could account for many of the metabolical defects in the fatty rat.     

PRL-releasing peptide interacts with leptin to reduce food intake and body weight.

PRL-releasing peptide (PrRP) is a novel anorexigen that reduces food intake and body weight gain in rats. In common with other anorexigens, PrRP mRNA expression is reduced during states of negative energy balance, i.e. lactation and fasting in female rats. In this study, we examined the interaction between PrRP and the adiposity signal, leptin, which interacts with a number of peptidergic systems in the brain to regulate energy homeostasis. Intracerebroventricular coadministration of 4 nmol PrRP and 1 microg leptin in rats resulted in additive reductions in nocturnal food intake and body weight gain and an increase in core body temperature compared with each peptide alone. We show also, by quantitative in situ hybridization, that PrRP mRNA is reduced in fasted male rats and obese Zucker rats, indicating that PrRP mRNA expression, like that of other anorexigens, may be regulated by leptin. Finally we show, using immunohistochemistry, that greater than 90% of PrRP neurons in all regions where PrRP is expressed contain leptin receptors. Thus, we provide evidence for PrRP neurons forming part of the leptin-sensitive brain circuitry involved in the regulation of food intake and energy homeostasis.     

Increased hypothalamic content of preproneuropeptide Y messenger ribonucleic acid in genetically obese Zucker rats and its regulation by food deprivation

Neuropeptide Y (NPY) is a potent orexigenic agent capable of producing hyperphagia and obesity. NPY-containing neurons project from the hypothalmic arcuate nucleus to the paraventricular nucleus, an area known to be sensitive to the orexigenic effects of NPY. In this study we investigated the possibility that preproNPY messenger RNA (mRNA) content may be altered in obese Zucker rats compared to that of their lean littermates. Total RNA was isolated from hypothalamic dissections from male and female, obese and lean Zucker rats. RNA was also isolated from dissections of: olfactory bulb, entorhinal cortex, hippocampus, and striatum of female obese and lean rats. PreproNPY mRNA content was determined by solution hybridization-RNase protection analysis. The results revealed a 2- to 3-fold increase in preproNPY mRNA levels in the hypothalamus of obese animals compared to lean. The increase was observed in both sexes and was specific to the hypothalamus. In situ hybridization localized this increase to the arcuate nucleus. An additional RNase protection study was pursued to investigate the effects of 72 h food deprivation on hypothalamic preproNPY mRNA levels in lean and obese animals. Lean animals displayed an approximate 2-fold increase in preproNPY mRNA content, whereas obese animals showed no significant increase after food deprivation. These data are consistent with the hypothesis that NPY projections within the hypothalamus are involved in regulating feeding behavior and weight gain, and that disturbed regulation of hypothalamic NPY expression may play a role in the etiology of obesity in the genetically obese Zucker rat.     

Diazoxide enhances basal metabolic rate and fat oxidation in obese Zucker rats

Persistent suppression of hyperinsulinemia in genetically obese (fa/fa) Zucker rats by diazoxide (DZ) reduces food intake and weight gain; improves insulin sensitivity, glycemic control, and lipid profile; and enhances β₃-adrenergic function and lipolysis in adipose tissue. The aim of this study was to elucidate the effects of DZ on basal metabolic rate (BMR), fat oxidation, and adrenergic function of lean and obese Zucker rats. Diazoxide (150 mg/kg/d) or vehicle (control) was administered for 4 weeks in 7-week-old obese and lean Zucker rats (n = 8-9 per subgroup). Animals underwent indirect calorimetry, body composition analysis, and determination of uncoupling proteins (UCPs) messenger RNA (mRNA) in brown and white adipose tissues (BAT and WAT) and skeletal muscle (SM), β₃-adrenergic receptor (AR) mRNA in BAT and WAT, β₂-AR in SM as well as WAT, and SM adenylate cyclase (AC) activity at the completion of study. Diazoxide treatment decreased food intake, weight gain, and body fat in obese rats (P < .01). Although DZ treatment lowered fasting plasma glucose, insulin, leptin, adiponectin, and lipids in obese rats (P < .01), it increased adiponectin-leptin ratio (P < .01). Plasma adiponectin-leptin ratio was inversely correlated with fat mass in obese and lean rats (r = −0.86, P < .0001). Diazoxide treatment resulted in higher BMR and fat oxidation rate in obese compared with control animals (P < .01), without any effect in lean animals. Furthermore, plasma adiponectin was inversely correlated with BMR (−0.56, P < .001) and lipid oxidation rate (−0.61, P < .0005) and was positively correlated with nonprotein respiratory quotient (r = 0.41, P < .01) in obese and lean rats. This was associated with increased β₃-AR mRNA expression in BAT and WAT (P < .01), UCP-1 and UCP-3 in BAT and AC activity in WAT (P < .02), and AC activity in SM of DZ obese rats compared with controls (P < .01), without significant change in SM β₂-AR mRNA expressions. Diazoxide attenuation of hyperinsulinemia decreased the rate of weight gain but enhanced insulin sensitivity, BMR, and fat oxidation in obese rats. This was associated with increased receptor- and non-receptor-mediated adrenergic function in adipose and muscle tissues in obese rats, respectively. These metabolic changes in obese Zucker rats suggest that antiobesity effects of DZ appear to be not only through its anorectic effect, modification of disturbed insulin metabolism, and inhibition of lipogenesis, but also due to augmentation of adrenergic function, energy expenditure, and fat utilization.     

Peripheral versus central effects of glucagon-like peptide-1 receptor agonists on satiety and body weight loss in Zucker obese rats

The present study explores the potential utility of peripheral versus central administration of glucagon-like peptide-1 (GLP-1) receptor agonists in the regulation of feeding behavior in Wistar and Zucker obese rats. Acute central (intracerebroventricular [i.c.v.]) and peripheral (subcutaneous [s.c.]) administration of both GLP-1 (7-36) amide and exendin-4 resulted in a reduction in food intake for at least 4 hours, exendin-4 being much more potent than GLP-1 (7-36) amide, especially after peripheral administration. Both Zucker obese rats (fa/fa) and their lean littermates (Fa/-) responded to acute central and peripheral administration of exendin-4. Moreover, in situ hybridization revealed specific labeling for the mRNA for GLP-1 receptors in several brain areas of both the obese and lean rats. The presence of this receptor was also detected by affinity cross-linking assays. Long-term s.c. administration of exendin-4 (1 single injection per day, 1 hour prior to the onset of the dark phase of the cycle) decreased daily food intake and practically blocked weight gain in obese rats. In contrast to previous studies, these findings show that peripheral (s.c.) administration of both GLP-1 receptor agonists also induces satiety and weight loss in rats, and suggest the potential usefulness of exendin-4 as a therapeutic tool for the treatment of diabetes and/or obesity.     

Impact of obesity and leptin treatment on adipocyte gene expression in Psammomys obesus

We examined the effects of leptin treatment on the expression of key genes in adipocyte metabolism in Psammomys obesus (P. obesus), a polygenic rodent model of obesity. Lean and obese P. obesus were given three daily intraperitoneal injections of either saline or leptin (total of 45 mg/kg per day) for 7 days. In lean animals, leptin treatment led to reductions in food intake, body weight and fat mass. Pair-fed animals matched for the reduction in food intake of the lean leptin-treated animals demonstrated similar reductions in body weight and fat mass. In obese P. obesus, leptin treatment failed to have any effect on body weight or body fat mass, indicating leptin resistance. Lipoprotein lipase, hormone-sensitive lipase and peroxisome proliferator activated receptor gamma 2 mRNA levels were significantly reduced in lean leptin-treated animals, whereas pair-fed animals were similar to lean controls. Uncoupling protein 2 and glycerol phosphate acyltransferase were also reduced in the lean leptin-treated animals, but not significantly so. Obese animals did not show any gene expression changes after leptin treatment. In conclusion, high circulating concentrations of leptin in lean P. obesus resulted in decreased gene expression of a number of key lipid enzymes, independent of changes in food intake, body weight and fat mass. These effects of leptin were not found in obese P. obesus.     

Effect of diazoxide on brain capillary insulin receptor binding and food intake in hyperphagic obese Zucker rats.

Insulin is believed to act as a central adiposity signal by binding to hypothalamic and other brain insulin receptors. Entry of circulating insulin into the brain is accomplished by a saturable receptor-mediated transendothelial transport system and is believed to be impaired in hyperinsulinemic, insulin-resistant, and hyperphagic obese Zucker rats. Theoretically, if hyperinsulinemia is decreased simultaneously while brain capillary insulin binding is increased, uptake of insulin into the brain can be enhanced leading to reduced food intake. To test this hypothesis, we administered diazoxide (DZ, 150 mg/kg/day), an inhibitor of glucose-mediated insulin secretion, or vehicle (control) to 7-week-old female obese and lean Zucker rats for 4 weeks (n = 24-28/subgroup-strain). Animals were assigned to either fasted (FD) or free-fed (FF) protocol for determination of plasma and cerebrospinal fluid (CSF) insulin and brain capillary insulin binding at the end of 4 weeks. DZ obese consumed fewer calories (P<0.01) and gained less weight than control obese (P<0.01), whereas DZ lean had similar amounts of caloric intake and weight gain compared with lean controls. DZ obese had lower fasting and random plasma glucose than control obese (P<0.05). FD and FF DZ-treated obese and lean rats had lower plasma insulin than their respective obese (P<0.01) and lean (P<0.01) controls. FD and FF DZ-treated obese rats demonstrated higher CSF insulin (P<0.05) and CSF/ plasma insulin ratio (P<0.01) than their controls, while only FF DZ lean animals showed higher CSF/plasma insulin ratio (P<0.01) than their controls (P<0.05). This was associated with enhanced brain capillary insulin binding in FD and FF DZ-treated obese (P<0.01) and lean (P<0.05) animals compared with their respective controls. It was concluded that DZ treatment in obese Zucker rats caused a decrease in insulin secretion and partially reversed impaired insulin binding to brain capillaries, leading to enhanced brain insulin uptake, and resulted in reduced food intake and weight gain observed in these animals.     

Testosterone supplementation in male obese Zucker rats reduces body weight and improves insulin sensitivity but increases blood pressure

Androgen levels are lower in obese men as compared with normal weight individuals. However, there are no safety data regarding the chronic use of androgen supplements in middle-aged men. The present study was undertaken to determine the cardiovascular and metabolic effects of chronic (10 weeks) testosterone treatment in male obese Zucker rats, starting at 22 weeks of age, when testosterone levels were significantly decreased. Testosterone supplements increased plasma levels, 10-fold in both obese Zucker rats and lean Zucker rats. In obese Zucker rats, testosterone supplements reduced body weight, plasma insulin, and cholesterol levels and improved the oral glucose tolerance test. None of these parameters were affected in lean Zucker rats. Mean arterial pressure was significantly increased in obese Zucker rats but not lean Zucker rats. Testosterone supplements increased proteinuria and accelerated renal injury in lean Zucker rats only. Thus, treatment of obese men with chronic testosterone supplements should be done with careful monitoring of blood pressure.     

Adjustments in daily energy expenditure to caloric restriction and weight loss by adult obese and lean Zucker rats

The 24 h resting (postabsorptive) energy expenditure of adult female lean and obese Zucker rats was examined both at the body weights each genotype spontaneously maintained and at body weights that were reduced 6-10 percent by restricting food intake for 2 weeks to 70 or 55 percent of normal. At their normally-maintained body weight, lean rats expended energy at a daily rate appropriate to their body mass according to the Kleiber equation (predicted kcal/day = 70.0 x BW-0.75kg observed kcal/day = 70.66 x BW-0.75kg). However, expenditure rates declined significantly in the weight-reduced lean rats restricted to 70 or 55 percent of normal intake (to 64.1 and 61.3 kcal/kg0.75). The obese rats expended energy at a rate lower than predicted by the Kleiber equation prior to weight loss (49.5 x BW-0.75kg). But, rates of daily expenditure in the obese rats was further reduced by 70 or 55 percent restriction (to 43.7 and 42.4 x BW-0.75kg), a decline proportionately as large or larger than that displayed by restricted lean rats. With this reduced metabolic rate, the total daily caloric expenditure of obese rats restricted to 55 percent of normal intake approximated that of unrestricted lean rats weighing only half as much (27.8 vs 27.5 kcal/day; 569 vs 285 g). Evidently, Zucker obese rats display the same adaptive reductions in resting metabolism as do leans in response to energy deficit and weight loss. In both, these adjustments in expenditure act to stabilize body weight at the levels that typify each genotype.