Mark I. Friedman

Integrated metabolic control of food intake

Inhibition of glycolysis and fatty acid oxidation by combined treatment with 2-deoxyglucose (2DG) and methyl palmoxirate, or inhibition of glycolysis and lipolysis by combined treatment with 2DG and nicotonic acid synergistically increased food intake in rats. Methyl palmoxirate treatment alone increased food intake in rats fed a high-fat, but not low-fat diet. These results provide direct evidence for a mechanism in the control of food intake that integrates signals generated by the metabolism of glucose and fatty acids. In addition, they strongly indicate a role for fatty acid oxidation in the control of eating and raise the possibility that an interaction between glucose and fat metabolism underlies the link between regulation of body fat stores and short-term food intake.

Fuel partitioning and food intake.

Fuel metabolism generates signals that the brain uses to control food intake. Because the influence of metabolism on eating behavior depends on where and in what way metabolic fuels are utilized, the partitioning of fuels among different tissues and between metabolic pathways has significant effects on food intake. There is substantial evidence that shifts in fat fuel partitioning between oxidation and storage influence food intake, and data indicate that this may also be the case for carbohydrates. Carbohydrate and fat fuel interactions appear to affect eating behavior, and may play a role in the overconsumption of high-fat diets. A mechanism for the control of eating behavior that is sensitive to a stimulus generated at the level of oxidative phosphorylation and ATP production may underlie changes in food intake associated with shifts in fuel partitioning. A model based on such a mechanism provides a framework for understanding changes in food intake under a variety of conditions associated with alterations in energy storage and expenditure, including obesity and cachexia.

Leptin indirectly affects estrous cycles by increasing metabolic fuel oxidation.

In previous experiments, lean Syrian hamsters fasted on days 1 and 2 of the estrous cycle failed to show sex behavior and ovulation normally expected to occur on the evening of day 4. The first goal of the present experiment was to determine whether systemic treatment with the ob (obese) protein leptin could reverse the effects of fasting on estrous cyclicity, social behaviors, and ovulation rate. Fasting-induced anestrus was reversed and normal sex and social behavior and ovulation rate were restored in hamsters injected intraperitoneally with 5 mg/kg leptin every 12 h during fasting on days 1 and 2 of the estrous cycle. A second goal was to test whether the effects of leptin could be prevented by treatment with pharmacological agents that block the oxidation of metabolic fuels. Glucose oxidation was blocked by treatment with 2-deoxy-d-glucose (2DG) and fatty acid oxidation was blocked by treatment with methyl palmoxirate (MP). 2DG (1000 mg/kg) or MP (20 mg/kg) was administered at doses that did not induce anestrus in hamsters fed ad libitum. As in the first experiment, fasting-induced anestrus was reversed by leptin treatment. However, when each injection of leptin was preceded by an injection of 2DG or MP, leptin treatment did not reverse fasting-induced anestrus. In summary, estrous cyclicity was not restored when oxidation of metabolic fuels was blocked, despite high endogenous levels of leptin. These results are consistent with the hypothesis that leptin acts indirectly on the reproductive system by increasing fuel oxidation.

Control of energy intake by energy metabolism.

Eating behavior is controlled by signals that are generated in the postabsorptive metabolism of energy-yielding substrates. Recent work indicates that an event common to the metabolism of glucose and fat provides such a signal. Evidence suggests that eating behavior is triggered by a signal that is tied to hepatic ATP concentrations and is carried from the liver to brain via afferents in the vagus nerve. This metabolic control of eating behavior may link mechanisms of energy storage and expenditure to energy intake. Changes in energy intake associated with alterations in energy expenditure can be viewed as a response to a shift in the partitioning of fuels, which affects the oxidative metabolic pathways that generate the signal controlling eating behavior.

Fatty acid oxidation affects food intake by altering hepatic energy status

Inhibition of fatty acid oxidation stimulates feeding behavior in rats. To determine whether a decrease in hepatic fatty acid oxidation triggers this behavioral response, we compared the effects of different doses of methyl palmoxirate (MP), an inhibitor of fatty acid oxidation, on food intake with those on in vivo and in vitro liver and muscle metabolism. Administration of 1 mg/kg MP selectively decreased hepatic fatty acid oxidation but did not stimulate food intake. In contrast, feeding behavior increased in rats given 5 or 10 mg/kg MP, which inhibited hepatic fatty acid oxidation to the same extent as did the low dose but in addition suppressed fatty acid oxidation in muscle and produced a marked depletion of liver glycogen. Dose-related increases in food intake tracked dose-related reductions in liver ATP content, ATP-to-ADP ratio, and phosphorylation potential. The findings suggest that a decrease in hepatic fatty acid oxidation can stimulate feeding behavior by reducing hepatic energy production.Inhibition of fatty acid oxidation stimulates feeding behavior in rats. To determine whether a decrease in hepatic fatty acid oxidation triggers this behavioral response, we compared the effects of different doses of methyl palmoxirate (MP), an inhibitor of fatty acid oxidation, on food intake with those on in vivo and in vitro liver and muscle metabolism. Administration of 1 mg/kg MP selectively decreased hepatic fatty acid oxidation but did not stimulate food intake. In contrast, feeding behavior increased in rats given 5 or 10 mg/kg MP, which inhibited hepatic fatty acid oxidation to the same extent as did the low dose but in addition suppressed fatty acid oxidation in muscle and produced a marked depletion of liver glycogen. Dose-related increases in food intake tracked dose-related reductions in liver ATP content, ATP-to-ADP ratio, and phosphorylation potential. The findings suggest that a decrease in hepatic fatty acid oxidation can stimulate feeding behavior by reducing hepatic energy production.

Dietary hyperphagia in rats : role of fat, carbohydrate, and energy content

Dietary energy, fat and carbohydrate content were varied to determine the nutritional factors responsible for hyperphagia induced by feeding rats high-fat diets. In the first experiment, rats were fed isoenergetic high-fat or high-carbohydrate diets for 2 weeks. Weight gain and energy intake were lower in rats given the high-fat diet. When some of the rats were switched to a diet that was high in fat, carbohydrate and energy, gram food intake was initially unchanged, resulting in a substantial increase in energy intake and weight gain. Energy intake gradually declined over the 4 weeks following the switch to the high-energy diet. In the second experiment, rats were fed high-fat diets that were either high or low in carbohydrate content and either high or low in energy content (kcal/g). Rats fed a high-fat diet that was high in energy and carbohydrate ate the most energy and gained the most body weight and carcass fat. In the third experiment, rats were fed high-carbohydrate diets varying in fat and cellulose content. Energy intake and body weight gain varied directly as a function of caloric density regardless of the fat or cellulose content of the diets. It is concluded that hyperphagia induced by feeding high-fat diets is not due to the high dietary fat content alone. Rather, high levels of fat, carbohydrate, and energy interact to produce overeating and obesity in rats fed high-fat diets.

Induction of Fos-like immunoreactivity (Fos-li) and stimulation of feeding by 2, 5-anhydro-d-mannitol (2, 5-AM) require the vagus nerve

The antimetabolic fructose analogue, 2,5-anhydro-D-mannitol (2,5-AM), stimulates feeding. Selective hepatic branch vagotomy has been shown to block feeding induced by low 2,5-AM doses. However, hepatic vagal fibers are not the sole mediators of 2,5-AM-induced feeding, since hepatic branch vagotomy does not impair feeding induced by higher doses of 2,5-AM. To further evaluate the role of the vagus in the response to 2,5-AM, we examined the effect of total subdiaphragmatic vagotomy on feeding induced by a high 2,5-AM dose (500 mg/kg). In addition, we assessed the ability of 2,5-AM (300 and 500 mg/kg) to induce Fos-like immunoreactivity (Fos-li) in the brain in sham-operated (SHAM), hepatic branch vagotomized (HBV) and total subdiaphragmatic vagotomized (TSDV) rats. Both doses of 2,5-AM, but not control solutions, induced Fos-li in the area postrema (AP), nucleus of the solitary tract (NTS) and lateral parabrachial nucleus (1PBN). Very weak immunoreactivity was present in the central nucleus of the amygdala and none was observed in the locus coeruleus or paraventricular nucleus of the hypothalamus. The effect of the lower 2,5-AM dose on Fos-li was blocked by HBV. The high dose effect was blocked by TSDV but not by HBV. Feeding induced by the high dose of 2,5-AM was also blocked by TSDV. Results are consistent with the hypothesis that stimulation of feeding by 2,5-AM is dependent on the vagus nerve. Hepatic branch fibers may have the lowest threshold for activation, but fibers in other vagal branches independently mediate induction of c-fos and stimulate food intake at higher doses of the analogue.

Effect of a high-fat diet on food intake and hypothalamic neuropeptide gene expression in streptozotocin diabetes

Insulin-deficient diabetic rats are markedly hyperphagic when fed a high-carbohydrate (HC) diet, but normophagic when fed a high-fat (HF) diet. When maintained on a HC diet, diabetic rats also exhibit increased gene expression of the orexigenic peptide neuropeptide Y (NPY) in the hypothalamic arcuate nucleus, and reduced expression of the anorectic peptide corticotropin-releasing hormone (CRH) in the paraventricular nucleus, and these changes are hypothesized to contribute to diabetic hyperphagia. In this experiment we assessed whether the normophagia displayed by HF-fed diabetic rats is associated with the opposite profile of NPY and CRH expression. Our results show that relative to diabetic rats on the HC diet, the diabetic rats on the HF diet exhibited significantly reduced caloric intake (-40%), NPY expression in the arcuate nucleus (-27%), and elevated CRH expression in the paraventricular nucleus (+37%). Insulin and corticosterone, which are known to affect hypothalamic NPY and CRH expression, were not different between these two groups, making it unlikely that they can account for the differences in either feeding behavior or hypothalamic peptide expression. There was a small but significant increase in plasma leptin levels in the diabetic animals maintained on the HF, and large differences in parameters associated with elevated fat oxidation. These observations support the hypothesis that the normalization of food intake observed in diabetic rats consuming a HF diet may in part be mediated by reductions in NPY expression and elevations in CRH expression.

Pica—A model of nausea? Species differences in response to cisplatin

Rats lack the emetic reflex but exhibit pica in response to stimuli that induce emesis in species with an emetic reflex, hence it has been proposed that pica may be analogous to emesis in species lacking the reflex. In the present study, we investigated whether pica was present in Suncus murinus (with an emetic reflex) as well as in rats and mice (without emetic reflex) to provide a further insight to the validity of pica as a model for nausea/vomiting. Cisplatin (6 mg/kg, i.p.) induced pica in rats, indicated by a significant increase in kaolin consumption at 24 h (but not 48 h) post-treatment whereas we failed to demonstrate this effect in mice (inbred or outbred strain, 6 or 20 mg/kg i.p.) and whilst cisplatin (20 mg/kg, i.p.) induced emesis in Suncus, kaolin intake was not significantly affected. Furthermore, cisplatin significantly increased the weight of gastric contents at 48 h post-injection in rats and mice indicating delayed gastric emptying whereas this effect was not present in Suncus. These results show that Suncus and two strains of mice, unlike rats, do not develop pica in response to cisplatin which suggests that the consumption of kaolin induced by cisplatin may not be associated with whether or not an emetic reflex is present. The differences in ingestive behaviour and gastric response between species with and without an emetic reflex in response to cisplatin treatment as well as the difference between mice and rats, is discussed in relation to the selection of models for the study of nausea and vomiting.

METABOLIC, GASTROINTESTINAL, AND CNS NEUROPEPTIDE EFFECTS OF BRAIN LEPTIN ADMINISTRATION IN THE RAT

To investigate whether brain leptin involves neuropeptidergic pathways influencing ingestion, metabolism, and gastrointestinal functioning, leptin (3.5 μg) was infused daily into the third cerebral ventricular of rats for 3 days. To distinguish between direct leptin effects and those secondary to leptin-induced anorexia, we studied vehicle-infused rats with food available ad libitum and those that were pair-fed to leptin-treated animals. Although body weight was comparably reduced (-8%) and plasma glycerol was comparably increased (142 and 17%, respectively) in leptin-treated and pair-fed animals relative to controls, increases in plasma fatty acids and ketones were only detected (132 and 234%, respectively) in pair-fed rats. Resting energy expenditure (-15%) and gastrointestinal fill (-50%) were reduced by pair-feeding relative to the ad libitum group, but they were not reduced by leptin treatment. Relative to controls, leptin increased hypothalamic mRNA for corticotropin-releasing hormone (CRH; 61%) and for proopiomelanocortin (POMC; 31%) but did not reduce mRNA for neuropeptide Y. These results suggest that CNS leptin prevents metabolic/gastrointestinal responses to caloric restriction by activating hypothalamic CRH- and POMC-containing pathways and raise the possibility that these peripheral responses to CNS leptin administration contribute to leptins anorexigenic action.To investigate whether brain leptin involves neuropeptidergic pathways influencing ingestion, metabolism, and gastrointestinal functioning, leptin (3.5 micrograms) was infused daily into the third cerebral ventricular of rats for 3 days. To distinguish between direct leptin effects and those secondary to leptin-induced anorexia, we studied vehicle-infused rats with food available ad libitum and those that were pair-fed to leptin-treated animals. Although body weight was comparably reduced (-8%) and plasma glycerol was comparably increased (142 and 17%, respectively) in leptin-treated and pair-fed animals relative to controls, increases in plasma fatty acids and ketones were only detected (132 and 234%, respectively) in pair-fed rats. Resting energy expenditure (-15%) and gastrointestinal fill (-50%) were reduced by pair-feeding relative to the ad libitum group, but they were not reduced by leptin treatment. Relative to controls, leptin increased hypothalamic mRNA for corticotropin-releasing hormone (CRH; 61%) and for proopiomelanocortin (POMC; 31%) but did not reduce mRNA for neuropeptide Y. These results suggest that CNS leptin prevents metabolic/gastrointestinal responses to caloric restriction by activating hypothalamic CRH- and POMC-containing pathways and raise the possibility that these peripheral responses to CNS leptin administration contribute to leptins anorexigenic action.