Utilizing the GLP-1 signaling system to treat diabetes: sorting through the pharmacologic approaches

Hypothalamic control of energy and glucose metabolism

The central nervous system (CNS), generally accepted to regulate energy homeostasis, has been implicated in the metabolic perturbations that either cause or are associated with obesity. Normally, the CNS receives hormonal, metabolic, and neuronal input to assure adequate energy levels and maintain stable energy homeostasis. Recent evidence also supports that the CNS uses these same inputs to regulate glucose homeostasis and this aspect of CNS regulation also becomes impaired in the face of dietary-induced obesity. This review focuses on the literature surrounding hypothalamic regulation of energy and glucose homeostasis and discusses how dysregulation of this system may contribute to obesity and T2DM.

Meal-anticipatory glucagon-like peptide-1 secretion in rats

Animals anticipating a meal initiate a series of responses enabling them to better cope with the meal's metabolic impact. These responses, such as cephalic insulin, occur prior to the onset of ingestion and are especially evident in animals maintained on a meal-feeding schedule with limited but predictable access to food each day. We tested the hypothesis that meal-fed rats secrete the incretin hormone glucagon-like peptide-1 (GLP-1) cephalically when anticipating a large meal. Male Long-Evans rats were fed ad libitum (controls) or adapted to a schedule on which food was available for the same 4-h period each day (meal fed animals). Plasma GLP-1 increased in meal-fed rats over an interval from 75 to 60 min prior to feeding time, from a baseline of 10 to around 40 pm, and then returned to baseline prior to food presentation. Controls had steady plasma GLP-1 levels (10-15 pm) over the same span. Meal-fed rats also secreted cephalic insulin starting around 15 min prior to food presentation. Administration of the selective GLP-1 receptor antagonist exendin-4[desHis-1,Glu-9] prior to the premeal spike of GLP-1 caused meal-fed rats to eat significantly less food than normal, whereas administration of the antagonist after the GLP-1 spike but prior to food presentation resulted in a significant increase in food consumption. These findings document for the first time a cephalic increase of plasma GLP-1 and suggest that it functions to facilitate consumption of a large meal.

The control of food intake: behavioral versus molecular perspectives

To meet the continuous demand for energy, organisms use diverse signals to match food intake with energy needs. This paper reviews the effect of satiation signals and adiposity signals on food intake, including how they interact in the brain and how their influence changes with experience. Whereas meal initiation is influenced by external environmental factors, meal size is influenced by an array of signals that can be partitioned according to their reliability in indicating caloric content of food. It is argued that the malleability of satiation signals renders them poor candidates as pharmacological targets to control body weight.

Central control of body weight and appetite

CONTEXT

Energy balance is critical for survival and health, and control of food intake is an integral part of this process. This report reviews hormonal signals that influence food intake and their clinical applications.

EVIDENCE ACQUISITION

A relatively novel insight is that satiation signals that control meal size and adiposity signals that signify the amount of body fat are distinct and interact in the hypothalamus and elsewhere to control energy homeostasis. This review focuses upon recent literature addressing the integration of satiation and adiposity signals and therapeutic implications for treatment of obesity.

EVIDENCE SYNTHESIS

During meals, signals such as cholecystokinin arise primarily from the GI tract to cause satiation and meal termination; signals secreted in proportion to body fat such as insulin and leptin interact with satiation signals and provide effective regulation by dictating meal size to amounts that are appropriate for body fatness, or stored energy. Although satiation and adiposity signals are myriad and redundant and reduce food intake, there are few known orexigenic signals; thus, initiation of meals is not subject to the degree of homeostatic regulation that cessation of eating is. There are now drugs available that act through receptors for satiation factors and which cause weight loss, demonstrating that this system is amenable to manipulation for therapeutic goals.

CONCLUSIONS

Although progress on effective medical therapies for obesity has been relatively slow in coming, advances in understanding the central regulation of food intake may ultimately be turned into useful treatment options.