Long-term effects of exogenous leptin on body weight and fat in post-obese female rats

This study was designed to test the hypothesis that short-term leptin infusion during the post-obese refeeding phase of weight-reduced rats would reduce the rate of weight regain and, as a result, reduce the final body weight and fat content in weight-reduced rats. Ninety-six female Wistar rats were divided into four groups: (1) LFCON (low-fat control) group: Rats in this group were fed the control low-fat (LF) diet ad lib for the entire study period. (2) HFCON (high-fat control) group: Rats in this group were fed the high-fat (HF, 40% fat) diet ad lib for the study period. (3) HFRLP (high-fat fed, weight-reduced, leptin treatment) group: Obese rats in this group were weight-reduced and received leptin infusion for 2 weeks (miniosmotic pumps, 0.5 microg/kg/day) during the post-obese refeeding period. (4) HFRSM (high-fat fed, weight-reduced, sham control) group: Rats in this sham-control group were treated the same as the rats in the HFRLP group with the exception that no leptin was actually infused during the first 2 weeks of refeeding period. The results demonstrated that 2 weeks of leptin treatment during the early refeeding phase did not prevent weight regain in weight-reduced rats, but it significantly reduced body fat content in these rats as compared to ad lib fed obese control rats. One cycle of weight reduction and regain did not alter the body weight and body fat content in HFRSM rats when compared to obese control rats. Therefore, leptin treatment was effective in reducing body fat content in post-obese rats for up to 7 weeks, but the long-term effect of short-term leptin treatment needs to be further examined.

Leptin resistance in obesity is characterized by decreased sensitivity to proopiomelanocortin products

Obesity in normal animals has been demonstrated to be associated with a decrease in sensitivity to leptin especially as it relates to leptin's capacity to increase sympathetic nerve activity and enhance cardiovascular dynamics. In normal animals leptin has been demonstrated to exert significant regulatory responses by its capacity to increase proopiomelanocortin (POMC) expression and especially the increase in alpha melanocyte stimulating hormone (alphaMSH). These responses to leptin are blocked by a melanocortin-4 (MC-4) receptor antagonist. In this study we investigated the responsiveness of the sympathetic nervous system and cardiovascular system of high fat fed obese animals to the intracerebroventricular (ICV) administration of the POMC products alphaMSH and beta-endorphin (beta-END). We further investigated these responses in obese animals following leptin administration in the presence of MC-4 receptor and opioid receptor blockade. The ICV administration of leptin resulted in an increase in lumbar sympathetic nerve activity (LSNA) and mean arterial pressure (MAP) in normals but decreased it in the obese. The ICV administration of alphaMSH increased the LSNA and MAP in normal animals but to a lesser degree in obese animals. On the other hand beta-endorphin decreased the LSNA and MAP in normal animals but increased it in obese animals. Additionally ICV leptin administration in obese animals in the presence of MC-4 or opioid receptor blockade resulted in an increase in sympathetic activity and a pressor response. From these studies we conclude that obesity in high fat fed animals is characterized by a decreased sensitivity to alphaMSH and a paradoxical response to beta-endorphin and this altered responsiveness may be a factor in the altered leptin resistance characteristic of obese animals.

Obesity due to high fat diet decreases the sympathetic nervous and cardiovascular responses to intracerebroventricular leptin in rats

Obesity is associated with an increase in plasma leptin levels primarily derived from enhanced expression of the leptin gene in the adipose tissue. Leptin levels and expression are higher in females than males. The main functions of leptin are to decrease food intake and increase sympathetic nerve activity, especially in the brown adipose tissue. The high levels of leptin in obese, female rats suggest leptin resistance. In this article we describe experiments designed to investigate the effect of the intracerebroventricular (i.c.v.) administration of leptin on lumbar sympathetic nerve activity (LSNA) and cardiovascular parameters in female rats fed a low fat diet (control), a high fat diet (obese), or high fat diet followed by a period of food restrictions (reduced). The i.c.v. leptin administration increased LSNA in control rats, but decreased it in obese rats. In weight reduced animals the LSNA response to leptin returned to control levels. The i.c.v. leptin increased the mean arterial pressure in control and wt. reduced rats, but not in obese animals. The heart rate did not respond to leptin in any animal group. These results suggest that obesity decreases the central nervous system (CNS)-mediated lumbar sympathetic nervous and cardiovascular responses to leptin and that these responses recover following food restriction and wt. reduction. We conclude that obesity is associated with a decreased CNS response to leptin leading to a decrease in leptin effects to increase the activities of the autonomic nervous and cardiovascular systems.

Differential gender response produced by meal and ad lib feedings of a high-fat diet in Osborne-Mendel rats

We investigated if there were gender differences in metabolic consequences produced by meal feeding of a high-fat (HF) diet in male and female Osborne-Mendel rats. An HF diet was fed either ad lib (AL) or in 1 meal (MF) during the last 3 h of the dark cycle for 4 weeks (lights off from 2400 h to 1200 h). All rats were sacrificed at 1 of 3 time-points: prior to MF rats receiving their food at 0900 h, after food was taken from MF groups at 1200 h or 1500 h. Food intake, body weight, body fat weight, retroperitoneal adipose tissue lipoprotein lipase activity, plasma cholesterol, or HDL-cholesterol levels did not differ between MF- and AL-fed groups, but were higher in male than in female rats. There were no differences between the male and female groups in plasma levels of insulin and glucose or systolic blood pressures. Plasma triglyceride levels at 1200 h were significantly different between MF and AL Groups within either gender, but this was not observed at 0900 h or 1500 h. Body weights were correlated with internal fat weights and plasma cholesterol levels in both males (r = 0.57, p < 0.05) and females (r = 0.59, p < 0.05). Hence, it is the amount of HF diet ingested, rather than the pattern of meal feeding, that was the most significant factor related to gender differences in weight gain, increases in fat mass, and metabolic differences.

Long-term weight cycling in female Wistar rats: effects on metabolism

Weight cycling (WC) induced by ad-lib and restricted high fat (HF) feeding has been shown to reduce final body weight but not body fat percent in female Wistar rats. We examined the metabolic consequences of this type of WC. Five groups of female Wistar rats were fed a HF diet and the sixth group was fed a low fat diet to serve as a control group. Of the five HF groups, four groups were weight cycled by ad-lib and restricted feeding of the HF diet. One of these groups weight cycled three times (HFCYC group) while the remaining three groups weight cycled once only, corresponding to the first, second and the third cycle of the HFCYC group. HF feeding induced hyperinsulinemia, hypertriglyceridemia, insulin resistance and elevated adipose tissue lipoprotein lipase (AT-LPL) activity levels as compared to rats fed the low fat (LF) control diet. WC further increased blood insulin concentrations and insulin resistance in rats with three cycles of WC. However, blood pressure was not affected by HF feeding or WC. The magnitude of increase of AT-LPL was reduced in weight cycled, HF fed obese rats after 15 weeks refeeding. We concluded that even though WC did not enhance weight gain nor impair weight loss, it did facilitate the development of insulin resistance and may predispose animals to diabetes.

Low dose fructose ingestion during gestation and lactation affects carbohydrate metabolism in rat dams and their offspring

This study examined whether drinking fructose or glucose water with a balanced diet affects pregnant and lactating rats and their offspring. The animals were divided into three groups and drank tap water, 100 g/L fructose water or 100 g/L glucose water. The fructose-fed dams ate significantly more food but drank less water than the glucose-fed group. On d 19 of pregnancy, the fructose-fed dams had significantly heavier livers and significantly higher plasma glucose and insulin concentrations than dams consuming tap water. Five days after litters were weaned, dams fed fructose had the heaviest body weights, significantly higher plasma glucose concentration compared with the group receiving tap water and significantly higher plasma triglyceride concentration compared with the glucose-fed group. Weanlings of the fructose-fed dams had significantly lower plasma glucose concentration but a significantly higher plasma insulin concentration than the weanlings of the group receiving tap water. These findings suggest that intake of fructose during gestation can cause hyperglycemia and hyperinsulinemia in dams and, at weaning, greater weight gain in dams and hyperinsulinemia in offspring. These abnormalities in dams and weanlings could be the result of insulin resistance.