Glucose antimetabolites and hunger (theoretical article)

Effects of carbohydrate sugars and artificial sweeteners on appetite and the secretion of gastrointestinal satiety peptides

In vitro,both carbohydrate sugars and artificial sweeteners (AS) stimulate the secretion of glucagon-like peptide-1 (GLP-1). It has been suggested that the gut tastes sugars and AS through the same mechanisms as the tongue, with potential effects on gut hormone release. We investigated whether the human gut responds in the same way to AS and carbohydrate sugars, which are perceived by lingual taste as equisweet. We focused on the secretion of gastrointestinal (GI) satiety peptides in relation to appetite perception. We performed a placebo-controlled, double-blind, six-way, cross-over trial including twelve healthy subjects. On separate days, each subject received an intragastric infusion of glucose, fructose or an AS (aspartame, acesulfame K and sucralose) dissolved in 250 ml of water or water only (control). In a second part, four subjects received an intragastric infusion of the non-sweet, non-metabolisable sugar analogue 2-deoxy-d-glucose. Glucose stimulated GLP-1 (P = 0·002) and peptide tyrosine tyrosine (PYY;P = 0·046) secretion and reduced fasting plasma ghrelin (P = 0·046), whereas fructose was less effective. Both carbohydrate sugars increased satiety and fullness (albeit not significantly) compared with water. In contrast, equisweet loads of AS did not affect gastrointestinal peptide secretion with minimal effects on appetite. 2-Deoxy-d-glucose increased hunger ratings, however, with no effects on GLP-1, PYY or ghrelin. Our data demonstrate that the secretion of GLP-1, PYY and ghrelin depends on more than the detection of (1) sweetness or (2) the structural analogy to glucose.

Altered feeding responses in mice with targeted disruption of the dopamine-3 receptor gene

Dopamine signaling has been implicated in the control of food intake and body weight. In particular, dopamine is important in the control of meal size and number and is thought to mediate the response to metabolic deprivation states. In the present experiments, the authors assessed the role of the dopamine-3 receptor (D3R) in the feeding responses to 2-deoxy-D-glucose, mercaptoacetate, and peripheral insulin. All 3 compounds increased food intake in wild-type mice, but the hyperphagic responses were blunted in D3R-/- mice. In other experiments, D3R-/- mice were hyperresponsive to the administration of amylin and leptin relative to wild-type mice. These results support the hypothesis that D3Rs chronically inhibit the effects of adiposity hormones, thereby contributing to a net anabolic state.

Nutritional and physiological consequences of tumour glycolysis

A frequent characteristic of many malignant tumours is an increase in anaerobic glycolysis, that is the conversion of glucose to lactate, when compared to normal tissues. The causes of this intensification involve changes in enzyme and glucose transporter levels, shifts of the isoenzyme patterns in the cancer cells to those similar to foetal tissues and a breakdown in the normal control mechanisms, most notably the Pasteur effect. The host must adapt, with a corresponding increase in gluconeogenesis. This change, along with other adaptations made by the host, eventually results in the syndrome known as cancer cachexia, which is characterized by anorexia and depletion and redistribution of the host energy stores. In some ways many malignant tumours behave much like parasites, drawing upon the host for nutrients such as glucose and returning waste products such as lactate to the host for recycling or disposal. This cycling of glucose and lactate between host and tumour has been the target for a number of proposed and tested treatments, with regard to the possible inhibition of tumour growth and/or possible prevention of some or all of the cachectic effects. Some of these suggested treatments have reached the point of clinical testing and show promise for continued research.

Effect of glucoprivation on self-stimulation rate-frequency functions

In a series of two experiments using rats, the effect of glucoprivation, induced by 2-deoxy-D-glucose (2-DG) or insulin injections, on self-stimulation rate-frequency functions, was evaluated at two levels of current intensity. At the higher current intensity, neither insulin nor 2-DG produced a significant change in rate-frequency function parameters. At the lower current intensity, insulin suppressed asymptotic responding while 2-DG produced a lateral curve shift. Results of this study would argue that glucoprivation produces changes in self-stimulation at lateral hypothalamic electrodes that are: a) unrelated to the involvement of the neurons in stimulation-induced eating, b) are most notable when a smaller number of reward relevant neurons is stimulated, and c) can be differentially attributed to changes in motoric performance capacity during insulin tests and to changes in the reward value of stimulating current during 2-DG tests.

Effects of 2-deoxy-D-glucose on food intake of rats are affected by diet composition

The effect of various doses of 2-deoxy-D-glucose (2-DG) on food intake in rats fed either a medium fat diet with a moderate carbohydrate content (MF-rats) or a carbohydrate-free high fat diet (HF-rats) was tested. Injections were given intraperitoneally either in the middle of the bright phase or 1 h after onset of the dark phase. During the light phase 2-DG induced a transient hyperphagia in both HF- and MF-rats, but the hyperphagia was somewhat less pronounced in HF-rats. During the dark phase 2-DG produced a hyperphagia in the MF-rats and a long-term hypophagia in the HF-rats. Since 2-DG elicited feeding in HF-rats during the light phase, the feeding response to 2-DG not only reflects hunger for carbohydrate as previously suggested but also seems to produce hunger for energy.

Alterations in regulatory behaviors induced by medial septal lesions and superior cervical ganglionectomy

Following medial septal (MS) lesions peripheral sympathetic fibers, originating from the superior cervical ganglia (SCG), grow into the hippocampus and habenula. To assess their effect on regulatory behaviors, body weight, and food and water consumption were studied under ad libitum and pharmacological stress conditions, after MS lesions, superior cervical ganglionectomy (Gx) or MS lesion + ganglionectomy (MSGx). Twenty-two animals completed the study: control (n = 7), MS lesion (n = 5), Gx (n = 6), MSGx (n = 4). No differences were observed preoperatively. Postoperatively, body weight fell but over time all groups gained weight. However, animals with MSGx were lighter than MS or Gx animals (which were equivalent), which in turn were lighter than controls (P less than 0.0001). Hypophagia was observed in the Gx and MSGx animals when compared to the MS and control groups (P less than 0.05), while hyperdipsia was seen in the MS and Gx groups (P less than 0.001). Administration of both 1 M NaCl and isoproterenol (25 micrograms/kg) increased drinking in all animals (P less than 0.001), with the MSGx group consuming significantly less than all others (P less than 0.025). Food intake increased following 2-deoxy-D-glucose (500 mg/kg) (P less than 0.0001), while epinephrine (120 micrograms/kg) treatment produced anorexia only in the MS group (P less than 0.05). Hyperthermia was found in the Gx and MSGx groups. The results of this study suggest that both the MS region and SCG contribute to the maintenance of normal regulatory behaviors, with combined loss of these neural systems resulting in severe disturbances, both qualitatively and quantitatively different from either MS lesion or Gx. Although the MS lesion group clearly regulated better than the MSGx group, it is unclear whether this is due to ingrowth or just the presence of the SCG.

Metabolic and neurochemical correlates of glucoprivic feeding

Various hypotheses are reviewed concerning the mechanisms of feeding induced by insulin or 2DG. New data are presented to show that elevated plasma ketone levels are not sufficient to suppress 2DG feeding, suggesting that nourishment of the brain either does not occur or is not sufficient to stop 2DG feeding. We find that both acetoacetate and hydroxybutyrate suppress spontaneous feeding. Another series of studies investigated the effects of 2DG and insulin on catecholamine turnover in several brain regions of animals that do (rat, mouse) or do not (hamster) eat in response to these stimuli. The effects of glucoprivic stimuli on NE turnover were minimal; however, 2DG did appear to inhibit DA turnover, especially in nucleus accumbens. Thus, brain NE does not seem specifically involved in glucoprivic feeding, data which are supported by a lack of additivity of feeding induced by 2DG and by clonidine. Finally, to resolve some of the disparate data concerning the effects of glucose infusion on insulin-induced feeding, we examined the time course for effects on feeding and for glucose tolerance. It appears that glucose strongly inhibits feeding only when it is utilized.

Meal initiation controlled by learned cues: basic behavioral properties

The behavioral properties of meals initiated in response to the presentation of food-associated external stimuli are examined. A Pavlovian conditioning procedure was used to teach animals an association between an arbitrary external cue and food. Subsequent presentations of the conditioned cue reliably led to the initiation of feeding even though rats were tested under conditions of satiety. Several properties of learned external control of feeding were identified. First, the size of meals motivated by exposure to signals for food resembled the level of ingestion characteristic of spontaneous feeding. Second, the potency of externally-controlled intake was influenced by satiety signals arising from previous meals. Third, although presentation of the conditioned cue could be used to influence meal pattern, rats regulated the amount of calories consumed in a 24-h period. These results establish that learning contributes to meal initiation and that mechanisms based on learning do not require (but may interact with) internal energy depletion signals in the control of feeding. The implications of these findings to models of feeding behavior are discussed.

Meal initiation controlled by learned cues: effects of peripheral cholinergic blockade and cholecystokinin

Sated animals can be induced to initiate meals by exposing them to external stimuli which they have learned, via Pavlovian conditioning, to associate with food. This study examined physiological properties of this control of feeding. The initial hypothesis examined was that conditioned feeding depended on the elaboration of cholinergic cephalic phase responses (e.g., anticipatory insulin secretion). This idea was evaluated by comparing feeding responses to presentation of conditioned cues following an injection of either a peripheral cholinergic blocker, atropine methyl nitrate, or a control substance, physiological saline. Peripheral cholinergic blockade had no effect on the meal initiated by presentation of conditioned cues even though the dose of atropine methyl nitrate used was demonstrated to be sufficient to completely suppress cholinergic cephalic phase responses. These results indicate that cholinergic anticipatory digestive secretions do not contribute to feeding in this preparation. The effects of exogenously administered cholecystokinin on feeding controlled by learned cues were also studied. Cholecystokinin suppressed the size of the meal induced by presentation of conditioned stimuli but did not influence the latency, or initial rate of eating. The implications of these results to a conclusion that cholecystokinin is a satiety factor are discussed.

Increased carbohydrate consumption by rats as a function of 2-deoxy-D-glucose administration

Dietary self-selection was examined following the administration of the glucoprivic agent, 2-deoxy-D-glucose (2-DG), in adult male rats given access to separate sources of the three macronutrients, protein, fat and carbohydrate. All animals received injections (IP) of saline, 250, 500 and 750 mg/kg 2-DG with nutrient intakes measured at 2, 4, 6 and 24 hrs following injections. Animals consumed significantly more carbohydrate at 4, 6 and 24 hrs after injections of 500 and 750 mg/kg 2-DG than after saline injections. In contrast, fat intake was significantly suppressed by all three doses of 2-DG at 2 hr, by 250 and 750 mg/kg 2-DG at 4 and 6 hrs, and by 750 mg/kg 2-DG at 24 hr after injections. Protein intake was significantly decreased by all three doses of 2-DG at 2 hr after injections. As a result of the increase in carbohydrate intake and complimentary decrease in fat intake following 2-DG injections, total caloric intake of animals given the self-selection regime was not modified as a function of drug administration. In comparison, rats given a single nutritionally complete diet (ground Purina Laboratory Chow) consumed significantly more calories following 2-DG administration than following saline injections. The ability of animals to make appropriate modifications in nutrient selection following regulatory challenges is discussed.

Food and water intake following injection of glucose into the lateral ventricle of the brain of broiler type chicks

The effects of glucose and 2-deoxy-D-glucose (2-DG) on food intake and water consumption were investigated in broiler chicks. Artificial cerebrospinal fluid (aCSF) containing either glucose or the glucose antimetabolite 2-DG was injected into the lateral brain ventricles of fasted and satiated chicks. The satiated chicks received 10 microliters of CSF containing either 5 or 10% 2-DG, whereas those fasted for 24 hr received CSF with 10, 20, or 30% glucose (dextrose) in 10 microliters volumes. Chicks were maintained in a thermoneutral environment with continuous lighting. Intraventricular injections of 5 or 10% 2-DG had no significant effect on food intake or water consumption in the fully-fed chicks. In the 24-hr fasted chicks, 10% glucose had no effect on food or water intake. The decrease in food intake observed following the injection of 20 and 30% glucose appeared to be an osmotic effect. Based on results obtained, it was suggested that selection for increased growth rate, such as in broilers, may have resulted in a decrease in glucoreceptor sensitivity. Such a decrease in sensitivity could lead to an increased food intake as occurs in broilers.

Increased feeding and hyperglycemia elicited by intracerebroventricular 5-thioglucose

5-Thioglucose (5-TG) is a glucose analogue in which sulfur is substituted for the pyranose ring oxygen. Intracerebroventricular infusion of 5-TG caused prompt initiation of feeding and a marked rise in plasma glucose concentration at doses as low as 0.54 mumol (106 microgram). 5-TG is, therefore, more potent for eliciting feeding than any other previously tested glucose antimetabolite. The potency of 5-TG for eliciting feeding should make 5-TG a useful tool for localizing brain glucoreceptors which participate in the control of food intake.

Fall in blood glucose level precedes meal onset in free-feeding rats

It has been suggested for a long time that the metabolic stimulation to eat or the hunger arousal of eating, originated from a fall in the blood glucose level induced by the periodic failure of hepatic glucose production to match the peripheral glucose uptake. However, this suggestion has not been substantiated directly by the results of periodic blood glucose evaluations performed during intermeal intervals in free-fed rats. In this experiment, a technique involving a continuous blood glucose determination over several hours was used in free-feeding, undisturbed rats. It was shown that all nocturnal and diurnal meals were preceded by a 6 to 8% fall of blood level, starting 5 to 6 min prior to meal onset. The overall consequences of these findings are discussed.

Differential effects of 2-deoxy-D-glucose on plasma glucose, free fatty acids and feeding during the light and dark cycle in rats

The effects of 2-deoxy-D-glucose (2-DG) combined with 0 to 6 hr of food deprivation on plasma glucose (PG), plasma free fatty acid (PFFA) levels and feeding were compared during the 2 parts of the diurnal cycle. In one group, PG and PFFA were measured prior to the injection of either 2-DG or saline and 90 min after injections without restoration of access to food. In a second group, food was reintroduced following injection and intake was measured. At night, PG and PFFA measured prior to injections decreased and increased with respect to the length of food deprivation. Ninety minutes after 2-DG, PG and PFFA were elevated above control levels as a function of previous deprivation. During the day, fasting time did not affect PG and PFFA 90 min after a saline injection; but, after 2-DG, PG increased and PFFA decreased irrespective of the previous deprivation period. At night, food intake increased after saline and 2-DG as a function of fast duration, but increases were significantly lower after 2-DG. During the day, the duration of food deprivation did not affect food intake after 2-DG and saline; but irrespective of previous food deprivation, the cumulative intake of the first 2 hr was higher after 2-DG. Results are discussed in relation to the dual activation by 2-DG of facilitatory and inhibitory mechanisms of food intake.

Hunger in humans induced by 2-deoxy-D-glucose: glucoprivic control of taste preference and food intake

Intracellular glucopenia induced by 2-deoxy-D-glucose (2DG) administration in man produces increased hunger ratings and magnitude estimates of pleasantness for sucrose solutions. Augmented food intake substantiates these changes in affective behavior and relieves experimentally induced hunger. Intracellular glucopenia activates counterregulatory mechanisms to raise plasma glucose concentrations. Inducing hunger experimentally with 2DG provides a useful method for studying appetitive behavior in humans. The neurohumoral control of pituitary hormone release and other hypothalamic functions may be examined after 2DG infusion.

Hepatic-portal nutrient infustion: effect on feeding in intact and vagotomized rabbits

Hepatic-portal infusions of isotonic glucose did not influence food intake in free-feeding rabbits. In contrast, several other macronutrients and some of their metabolites, infused via the same route, cause anorexia followed by prolonged hypophagia. Its duration was generally decreased when the same infusions were performed during the nocturnal period. Vagotomy did not significantly alter the food intake responses to hepatic-portal infusions of either glucose or the other experimental substances. The results of related experiments have demonstrated that prolonged anorexia was not a consequence of the fast infusion rate or the generalized behavioral mallaise. However, when the infusions of most anorexigenic substances were followed by intraperitoneal administration of insulin, the duration of anorexia was shortened and food consumption was elevated. The results provided indirect evidence for the vagally mediated regulatory contribution of the previously studied neural and humoral intestinal mechanisms which are bypassed by infusing nutrient loads directly into the hepatic-portal circulation.