Dietary obesity: effects of drugs on food intake in S 5B/P1 and Osborne-Mendel rats

Extracellular Hypothalamic Monoamines Measured by In Vivo Microdialysis in a Rat Model of Dietary Fat-Induced Obesity

We tested two hypotheses about monoamine neurotransmitters in two strains of rats that differ in their sensitivity to obesity when eating a high-fat diet; 1) that the concentrations of norepinephrine and serotonin and of their metabolites differ in the extracellular fluid of the ventromedial hypothalamus of conscious, unrestrained Osborne-Mendel and S 5B/Pl rats, and 2) that these monoamines are altered differently between strains by a high-fat diet. The monoamines were measured by HPLC in dialysate collected by in vivo microdialysis in rats eating a semisynthetic low-fat diet (10% of kcal as fat) and again after either two or seven days of eating a high-fat diet (56% of kcal as fat). Norepinephrine, serotonin (5-HT), and 5-hydroxyindole-3-acetic acid (5-HIAA) were lower in Osborne-Mendel rats than in S 5B/Pl rats eating the low-fat diet. Norepinephrine and serotonin both increased in Osborne-Mendel rats with the onset of the high-fat diet so that Osborne-Mendel and S 5B/Pl rats no longer differed in these neurotransmitters. By day 7 of high-fat feeding, the concentrations of 3-methoxy-4-hydroxyphenylglycol (MHPG), 5-HIAA and the 5-HIAA/5-HT ratio rose in both strains. Ambient extracellular monoamines in the medial hypothalamus are lower in Osborne-Mendel rats than in S 5B/Pl rats and the response of these catecholamines to dietary fat was greater in Osborne-Mendel rats than in S 5B/Pl rats.

Differential response to kappa-opioidergic agents in dietary fat selection between Osborne-Mendel and S5B/P1 rats

We have investigated the central effect of a kappa-opioid agonist and an antagonist on the macronutrient preference in two strains of rat, the Osborne Mendel (OM) and S5B/P1 rats, that have different susceptibility to obesity and differential preference for dietary fat intake. OM rats prefer diets high in fat and are sensitive to diet-induced obesity, whereas S5B/P1 prefer a low fat diet and are resistant to high-fat diet-induced obesity. Rats adapted to a two-choice high fat (HF)/low fat (LF) diet were food deprived (20 h) and then infused into the third cerebroventricle with 10 micrograms nor-binaltorphimine (nor-BNI), a selective kappa-antagonist. Nor-BNI preferentially suppressed HF intake, but not LF intake in OM rats, whereas it affected neither diet in S5B rats. Infusion of U50488, a selective kappa-agonist (33 nmol), into the third cerebroventricle in sated rats, potently stimulated the intake of HF only in the OM rats, whereas it induced a significant but moderate stimulation of intake of both HF and LF diets in the S5B/P1 rats. Total energy intake following U50488 was not significantly different between the two strains. These findings suggest that the enhanced sensitivity of the OM rats to kappa-opioid stimulation for dietary fat may contribute to their preference for dietary fat and possibly their increased susceptibility for obesity.

Feeding response to mercaptoacetate in Osborne-Mendel and S5B/PL rats

The purpose of this experiment was to determine if Osborne-Mendel (OM) rats, which are susceptible to dietary-induced obesity, and S5B/PL (S5B) rats, which are resistant to dietary-induced obesity, differ in their feeding responses to mercaptoacetate (MA), which blocks fatty acid oxidation, or 2-deoxy-D-glucose (2DG), which blocks glucose utilization. 2DG (100 mg/kg or 200 mg/kg) increased food intake in both strains of rats on a high-fat diet (56% energy from fat). Mercaptoacetate (600 mumol/kg) increased food intake in OM but not S5B rats on a high-fat diet. When maintained on a low-fat diet (10% energy from fat), MA (400 mumol/kg or 600 mumol/kg) stimulated food intake in OM rats, whereas S5B rats increased food intake only after the highest dose of MA (600 mumol/kg). MA stimulated carbohydrate and protein intake in OM rats maintained on a macronutrient selection diet, whereas S5B rats maintained on this diet did not significantly increase intake of any macronutrient after MA. These results demonstrate that OM and S5B rats have a similar food intake response to 2DG but a dissimilar response to MA. The variable response to MA in these strains may be due to a difference in peripheral or central signaling systems related to fatty acid oxidation or a difference in metabolic environments between the strains, which in turn affects the feeding response to MA. These studies suggest that a difference in control of fatty acid oxidation may account for the difference in susceptibility to obesity when eating a high-fat diet.

Comparison of Osborne-Mendel and S5B/PL strains of rat: central effects of galanin, NPY, beta-casomorphin and CRH on intake of high-fat and low-fat diets

The effects of central administration of galanin, neuropeptide Y (NPY), beta-casomorphin(1-7) and corticotropin releasing hormone (CRH) on intake of either a high-fat or low-fat diet have been compared in two strains of rat, the dietary fat-sensitive Osborne-Mendel (OM) rat and the dietary fat-resistant S5B/Pl rat. Injection of galanin (0.1, 0.3 nmoles) into the 3rd cerebral ventricle stimulated the intake of both a high-fat and a low-fat diet in OM rats in a dose dependent manner but the response was significantly smaller in rats fed the low-fat diet. In S5B/Pl rats, galanin had a small stimulatory effect on food intake but only at a high dose (2 nmole). Beta-casomorphin(1-7) (5 nmoles), an opioid-like peptide, increased the intake of the high-fat but not the low-fat diet in OM rats, whereas S5B/Pl rats fed either a high-fat or low-fat diet did not respond to beta-casomorphin(1-7). Both strains showed a similar stimulatory response to NPY (0.1, 0.5 nmoles) on the intake of the high-fat or low-fat diet, but the magnitude of the response was attenuated in S5B/Pl rats. In contrast, the anorectic effects of CRH (0.26 nmoles) on food deprived animals was similar in both strains for both diets. We speculate that the regulatory system controlling the intake of fat activated by galanin and beta-casomorphin(1-7) may be defective in S5B/Pl rats.

Differential effects of enterostatin, galanin and opioids on high-fat diet consumption

Enterostatin, the activation peptide of pancreatic procolipase, suppresses high-fat diet consumption both centrally and peripherally. kappa-opioid agonists are also known to stimulate fat intake. These experiments were conducted to determine if an opioidergic central pathway might mediate the effects of enterostatin and galanin on fat intake. Male Sprague-Dawley rats were adapted to a high-fat diet (56% energy) and were implanted with cannulae aimed at the lateral cerebral ventricle (LV) or third cerebral ventricle (3V). Injection of enterostatin (1 nmol, LV) suppressed high-fat diet consumption in fasted (20 h) rats. This inhibition of high-fat intake by enterostatin was attenuated by central injection of the specific kappa-agonist U50488 (2.15, 21.5 and 215 nmol, LV) in a dose-dependent manner in fasted rats while only the highest dose of U50488 (215 nmol, LV) independently produced stimulation of high-fat diet consumption in sated rats. Galanin (0.1 nmol, 3V) induced consumption of high-fat diet in sated rats similar to that seen with U50488 and this stimulation was attenuated by peripheral injection of naloxone (1.0 mg/kg i.p.). We present a model which integrates the present data, as well as previous findings, in explaining a potential common opioid pathway modulating fat consumption.

Brain 3-hydroxybutyrate, glutamate, and GABA in a rat model of dietary obesity

Whole brain concentrations of 3-hydroxybutyrate, glutamate and gamma-aminobutyric acid (GABA) have been measured in two strains of rats with differing susceptibility to obesity. S 5B/Pl rats are resistant to developing obesity when eating a high-fat diet, whereas Osborne-Mendel rats readily develop obesity when eating the same diet. We tested the hypotheses that brain 3-hydroxybutyrate, glutamate and GABA differ between S 5B/Pl rats and Osborne-Mendel rats, and that these substrates/neuroregulators are altered when eating a high-fat diet primarily in S 5B/Pl (resistant) rats. Blood and brain 3-hydroxybutyrate concentrations were higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.05) but diet effects were not significant. Brain glutamate concentration, like 3-hydroxybutyrate, was higher in S 5B/Pl rats than in Osborne-Mendel rats (p less than 0.01) and was not affected by adding fat to the diet. Brain GABA differed only slightly between strains but increased after adding fat to the diet (p less than 0.05) in both strains with a greater increase occurring in S 5B/Pl rats. The brains of S 5B/Pl rats are chronically exposed to higher levels of 3-hydroxybutyrate and glutamate than are those of Osborne-Mendel rats. Thus, 3-hydroxybutyrate is a potential signal in the regulation of body weight. Brain GABA increases with fat feeding, especially in S 5B/Pl rats, suggesting that the ability to adjust to an energy dense diet may be through suppression of food intake by elevated brain GABA.

Nutrient balance and obesity: an approach to control of food intake in humans

This article has examined the regulated systems that control nutrient balance. From this analysis, the following conclusions may be suggested: 1. Each nutrient is regulated separately in a feedback system. 2. The control of glucose is regulated by the size of the glycogen stores; the size of the fat depots, by the rate of hepatic fatty acid oxidation; and protein, by the size of the protein depots. 3. Obesity can occur as a result of hyperphagia or from repartitioning the deposition of nutrients. In either case, there is a relative or absolute reduction in the activity of the sympathetic nervous system, requiring adequate levels of circulating corticosteroids.

Experimental obesity: a homeostatic failure due to defective nutrient stimulation of the sympathetic nervous system

The basic hypothesis of this review is that studies on models of experimental obesity can provide insight into the control systems regulating body nutrient stores in humans. In this homeostatic or feedback approach to analysis of the nutrient control system, we have examined the afferent feedback signals, the central controller, and the efferent control elements regulating the controlled system of nutrient intake, storage, and oxidation. The mechanisms involved in the beginning and ending of single meals must clearly be related to the long-term changes in fat stores, although this relationship is far from clear. Changes in total nutrient storage in adipose tissue can arise as a consequence of changes in the quantity of nutrients ingested in one form or another or a decrease in the utilization of the ingested nutrients. A change in energy intake can be effected by increased size of individual meals, increased number of meals in a 24-hour period, or a combination of these events. Similarly, a decrease in utilization of these nutrients can develop through changes in resting metabolic energy expenditure which are associated with one of more of the biological cycles such as protein metabolism, triglyceride for glycogen synthesis and breakdown, or maintenance of ionic gradients for Na+ + K+ across cell walls. In addition, differences in energy expenditure related to the thermogenesis of eating or to the level of physical activity may account for differences in nutrient utilization.

Experimental manipulations of eating: advances in animal models for studying anorectic agents

The material set out in this text has been designed to show the wide range of procedures which have the capacity to modify eating behavior--to produce hyper- or hypophagia, to alter the profile of eating patterns, or to adjust dietary preferences and selection. Accordingly, in investigating anorectic drugs it seems necessary to observe the effects of drug actions in a variety of experimental models. This strategy will provide a more complete description of the effect of a drug, will throw light on the mechanism of action, and will provide a more realistic base for predicting the effects of drugs in man.