Making Sense Out of Calories

Blood glucose dynamics and control of meal initiation: a pattern detection and recognition theory

A new framework for understanding the control of feeding behavior, with special emphasis on the evolution of hunger, the initiation of feeding, and its dependence on patterns of blood glucose, is the subject of this review. A perspective on the current status and future directions of this search for a more complete understanding of the regulation of feeding behavior in laboratory rats and humans is presented including theoretical and experimental components. First, a historical perspective on the role of blood glucose in the control of feeding is presented. Next, the theoretical approaches that have been applied to the control of feeding and had a strong influence on experimental feeding research are summarized. This is followed by a statement and overview of a current theory that has emerged from studies of the role of transient declines in blood glucose in the control of meal initiation. The current working hypothesis that transient declines in blood glucose are endogenous metabolic patterns that are detected and recognized by the central nervous system and are mapped into meal initiation in rats and are correlated with meal requests in humans are then presented. Then, the experimental studies on meal initiation and its dependence on patterns of blood glucose, first in rats and then in humans, are reviewed in detail. Finally, the future directions of the work, limitations, and the implications for the understanding of the control of feeding behavior and the regulation of energy balance are discussed.

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.

Effects of mu and kappa opioid receptor antagonists on glucoprivic induction of Fos immunoreactivity in the rat preoptic area and hypothalamus

Interoreceptors in the central nervous system elicit compensatory behavioral and physiological responses to cellular glucopenia. Antagonism of mu and kappa opioid receptors attenuates glucoprivic hyperphagia, findings that implicate these peptidergic receptors in the central processing of metabolic regulatory signals. Several hypothalamic structures of critical importance for the regulation of energy balance exhibit one or both of these receptors. The following studies investigated the role of these opioid receptors in glucoprivic induction of immediate-early gene expression in these brain sites. Male rats were pretreated with beta-funaltrexamine (mu antagonist), Mr-1452 MS (kappa antagonist), or vehicle prior to intraperitoneal injection of the glucose antimetabolite, 2-deoxy-D-glucose (2DG), then sacrificed by transcardial perfusion 2 h later. Nuclear immunolabeling for the transcription factor, Fos, was observed in several preoptic and hypothalamic sites following 2DG administration. Rats pretreated with the mu antagonist exhibited significantly fewer Fos-positive neurons in the medial preoptic area and dorsomedial hypothalamic nucleus in response to 2DG, compared to vehicle-pretreated controls. Blockade of kappa receptors diminished 2DG and induced Fos staining in the paraventricular and supraoptic nuclei. Numbers of Fos-positive cells in the arcuate nucleus and ventrolateral hypothalamic area were not altered by either antagonist. The present data implicate mu and kappa opioid receptors in neural mechanisms underlying glucoprivic induction of the Fos stimulus-transcription pathway by local neurons in discrete hypothalamic sites.

Metabolic and hormonal controls of food intake: highlights of the last 25 years--1972-1997

The six major research advances in metabolic and hormonal controls of food intake that have altered the direction or have broadened the scope of the field in the last 25 years are discussed. The advances selected are: (1) GI processes and meal termination-the CCK pathway; (2) Brain insulin hypothesis; (3) Glucose-dependent processes in periphery, plasma, and brain including the transient declines in blood glucose signaling meal initiation; (4) Fatty acid oxidation in the liver; (5) Behavioral and metabolic patterns; and (6) New pathways from molecular genetics and molecular biology-the OB protein pathway.

An energy sensor for control of energy intake

Control of energy intake, either in response to changes in the energy content of food or in energy expenditures and storage, is based on the detection of a feedback signal generated in the processing of metabolic fuels for energy. Evidence from studies of the fructose analogue, 2,5-AM, indicates a sensor in liver responds to changes in intracellular ATP or some closely associated event and communicates this information to the brain via vagal afferent neurons. Such a mechanism could serve as the energy sensor which controls energy intake.

Why do we eat?

The urge to eat is the main apparent issue underlying obesity. Although vast information regarding the physiology and psychology of eating behavior has been accumulated, a comprehensive concept is still missing. The model presented suggests that feeding behavior is ultimately controlled by the rate of work performed in the muscle or in an as yet unidentified compartment. It suggests a novel approach of a dynamic set-point weight and explains why diets usually fail to resolve the disorder while physical activity is beneficial in losing excess weight. Obesity is presented as a syndrome of high efficiency of energy conversion resulting in a variety of symptoms of which over-weight is only the more apparent. Other symptoms manifested in the predisposition to a variety of illnesses constitute the main health problem and can prevail in the obese subject even without the excess weight. Therefore, resolution of the disorder requires developing approaches which directly affect the efficiency of energy conversion.

Cholecystokinin, but not bombesin, has interoceptive sensory consequences like 1-h food deprivation

A comparison of the interoceptive sensory consequences of cholecystokinin, bombesin, and 1-h food deprivation was conducted in rats. In two experiments, rats were first trained to use stimuli arising from different degrees of food deprivation (1 and 24 h or 1 and 4 h) as discriminative signals for shock, and were then tested for generalized control of discriminative responding (i.e., conditioned freezing) by cues arising from injection of 4 micrograms/kg (Experiment 1 only) and 8 micrograms/kg cholecystokinin (Experiment 1A only), 12 micrograms/kg bombesin (BBS), and isotonic saline, respectively. The effects of these peptides on food intake were also assessed. In both experiments, BBS and cholecystokinin (CCK) each suppressed feeding relative to saline; however, generalization of discriminative responding was obtained only following CCK. Generalization depended on prior learning about food deprivation cues--CCK had little effect on freezing in rats that showed weak discrimination performance (i.e., rats trained with 1- and 4-h deprivation cues). The results indicated that CCK but not BBS produces interoceptive cues like those occurring under 1-h food deprivation.