Within-meal gut feedback signaling

Nutrition and Aging: Changes in the Regulation of Energy Metabolism With Aging

Changes in energy regulation occur during normal aging and contribute to the common phenomenon of weight and fat losses late in life. This review synthesizes data on aging-related changes in energy intake and energy expenditure and on the regulation of energy intake and expenditure. The ability of older adults to accurately regulate energy intake is impaired, with a number of possible explanations including delayed rate of absorption of macronutrients secondary to reductions in taste and smell acuity and numerous hormonal and metabolic mediators of energy regulation that change with aging. There are also changes in patterns of dietary intake and a reduction in the variety of foods consumed in old age that are thought to further reduce energy intake. Additionally, all components of energy expenditure decrease with aging, in particular energy expenditure for physical activity and basal metabolic rate, and the ability of energy expenditure to increase or decrease to attenuate energy imbalance during overeating or undereating also decreases. Combined, these changes result in an increased susceptibility to energy imbalance (both positive and negative) in old age that is associated with deteriorations in health. Practical interventions for prevention of weight and fat fluctuations in old age are anticipated here based on emerging knowledge of the role of such factors as dietary variety, taste, and palatability in late-life energy regulation.

Proopiomelanocortin neurons in nucleus tractus solitarius are activated by visceral afferents: regulation by cholecystokinin and opioids

The nucleus tractus solitarius (NTS) receives dense terminations from cranial visceral afferents, including those from the gastrointestinal (GI) system. Although the NTS integrates peripheral satiety signals and relays this signal to central feeding centers, little is known about which NTS neurons are involved or what mechanisms are responsible. Proopiomelanocortin (POMC) neurons are good candidates for GI integration, because disruption of the POMC gene leads to severe obesity and hyperphagia. Here, we used POMC-enhanced green fluorescent protein (EGFP) transgenic mice to identify NTS POMC neurons. Intraperitoneal administration of cholecystokinin (CCK) induced c-fos gene expression in NTS POMC-EGFP neurons, suggesting that they are activated by afferents stimulated by the satiety hormone. We tested the synaptic relationship of these neurons to visceral afferents and their modulation by CCK and opioids using patch recordings in horizontal brain slices. Electrical activation of the solitary tract (ST) evoked EPSCs in NTS POMC-EGFP neurons. The invariant latencies, low failure rates, and substantial paired-pulse depression of the ST-evoked EPSCs indicate that NTS POMC-EGFP neurons are second-order neurons directly contacted by afferent terminals. The EPSCs were blocked by the glutamate antagonist 2,3-dihydroxy-6-nitro-7-sulfonyl-benzo[f]quinoxaline. CCK increased the amplitude of the ST-stimulated EPSCs and the frequency of miniature EPSCs, effects attenuated by the CCK1 receptor antagonist lorglumide. In contrast, the orexigenic opioid agonists [D-Ala(2), N-Me-Phe(4), Gly-ol(5)]-enkephalin and met-enkephalin inhibited both ST-stimulated EPSCs and the frequency of miniature EPSCs. These findings identify a potential satiety pathway in which visceral afferents directly activate NTS POMC-EGFP neurons with excitatory inputs that are appropriately modulated by appetite regulators.

Cholecystokinin octapeptide increases spontaneous glutamatergic synaptic transmission to neurons of the nucleus tractus solitarius centralis

Cholecystokinin (CCK) is released from enteroendocrine cells after ingestion of nutrients and induces multiple effects along the gastrointestinal tract, including gastric relaxation and short-term satiety. We used whole cell patch-clamp and immunohistochemical techniques in rat brain stem slices to characterize the effects of CCK. In 45% of the neurons of nucleus tractus solitarius subnucleus centralis (cNTS), perfusion with the sulfated form of CCK (CCK-8s) increased the frequency of spontaneous excitatory currents (sEPSCs) in a concentration-dependent manner (1-300 nM). The threshold for the CCK-8s excitatory effect was 1 nM, the EC(50) was 20 nM, and E(max) was 100 nM. The excitatory effects of CCK-8s were still present when the slices were preincubated with tetrodotoxin or bicuculline or when the recordings were conducted with Cs(+) electrodes. Pretreatment with the CCK-A receptor antagonist, lorglumide (1 microM), antagonized the effects of CCK-8s, whereas perfusion with the CCK-B preferring agonist CCK-8 nonsulfated (CCK-ns, 1 microM) did not affect the frequency of sEPSCs. Similarly, pretreatment with the CCK-B receptor antagonist, triglumide (1 microM), did not prevent the actions of CCK-8s. Although the majority (i.e., 76%) of CCK-8s unresponsive cNTS neurons had a bipolar somata shape and were TH-IR negative, no differences were found in either the morphological or the neurochemical phenotype of cNTS neurons responsive to CCK-8s. Our results suggest that the excitatory effects of CCK-8s on terminals impinging on a subpopulation of cNTS neurons are mediated by CCK-A receptors; these responsive neurons, however, do not have morphological or neurochemical characteristics that automatically distinguish them from nonresponsive neurons.

In vitro analysis of the effects of cholecystokinin on rat brain stem motoneurons

Using whole cell patch clamp in thin brain stem slices, we tested the effects of cholecystokinin (CCK) on identified gastric-projecting neurons of the rat dorsal motor nucleus of the vagus (DMV). Perfusion with the sulfated form of CCK octapeptide (CCK8s, 30 pM-300 nM, EC50 approximately 4 nM) induced a concentration-dependent inward current in 35 and 41% of corpus- and antrum/pylorus-projecting DMV neurons, respectively. Conversely, none of the fundus-projecting DMV neurons responded to perfusion with CCK8s. The CCK8s-induced inward current was accompanied by a 65 +/- 17% increase in membrane input resistance and reversed at 90 +/- 4 mV, indicating that the excitatory effects of CCK8s were mediated by the closure of a potassium conductance. Pretreatment with the synaptic blocker TTX (0.3-1 microM) reduced the CCK8s-induced current, suggesting that a portion of the CCK8s-induced current was mediated indirectly via an action on presynaptic neurons apposing the DMV membrane. Pretreatment with the selective CCK-A receptor antagonist lorglumide (0.3-3 microM) attenuated the CCK8s-induced inward current in a concentration-dependent manner, with a maximum inhibition of 69 +/- 12% obtained with 3 microM lorglumide. Conversely, pretreatment with the selective CCK-B antagonist triglumide did not attenuate the CCK8s-induced inward current; pretreatment with triglumide (3 microM) and lorglumide (1 microM) attenuated the CCK8s-induced current to the same extent as pretreatment with lorglumide alone. Immunohistochemical experiments showed that CCK-A receptors were localized on the membrane of 34, 65, and 60% of fundus-, corpus-, and antrum/pylorus-projecting DMV neurons, respectively. Our data indicate that CCK-A receptors are present on a subpopulation of gastric-projecting neurons and that their activation leads to excitation of the DMV membrane.

A brain-liver circuit regulates glucose homeostasis

Increased glucose production (GP) is the major determinant of fasting hyperglycemia in diabetes mellitus. Previous studies suggested that lipid metabolism within specific hypothalamic nuclei is a biochemical sensor for nutrient availability that exerts negative feedback on GP. Here we show that central inhibition of fat oxidation leads to selective activation of brainstem neurons within the nucleus of the solitary tract and the dorsal motor nucleus of the vagus and markedly decreases liver gluconeogenesis, expression of gluconeogenic enzymes, and GP. These effects require central activation of ATP-dependent potassium channels (K(ATP)) and descending fibers within the hepatic branch of the vagus nerve. Thus, hypothalamic lipid sensing potently modulates glucose metabolism via neural circuitry that requires the activation of K(ATP) and selective brainstem neurons and intact vagal input to the liver. This crosstalk between brain and liver couples central nutrient sensing to peripheral nutrient production and its disruption may lead to hyperglycemia.

Gastrointestinal hormones and food intake

Despite dramatic fluctuations in calorie intake, animals maintain a very stable body weight. The reason is that energy intake and expenditure are precisely matched. Long-term regulation of energy balance is dependent on the coordination and interpretation of signals such as those given by insulin and leptin indicating sufficient long-term energy stores as well as short-term, meal-related signals such as those given by cholecystokinin (CCK). Within the last 30 years, our knowledge of short-term signals has increased dramatically. Throughout the cephalo-caudal axis of the gastrointestinal system, discrete enteroendocrine cells respond to both mechanical and chemical stimulation. Meal-associated hormone release is dependent on the concentration and composition of the nutrients ingested. Released signals are transmitted neurally through vagal afferents or humorally as circulating ligands for specific receptor populations in the periphery and central nervous system. These signals are interpreted by the CNS and manifested as a behavioral modification of feeding. This review will present past and recent literature in support of gut hormones and their roles as mediators of satiety. Evidence from pharmacologic and physiologic studies involving both humans and rodents will be presented, along with a short section outlining the knowledge gained through the use of murine knockout models. Last, the contribution of satiety hormones as likely mediators of the effectiveness seen following obesity surgery will be reviewed. Although traditionally thought of as short-term, meal-related signals, enhanced, chronic hormone secretion and signaling resulting from gut reconstruction as seen with gastric bypass surgery most likely contributes to the superior efficacy of surgery as a treatment for obesity.

Clinical endocrinology and metabolism. Regulation of energy homeostasis by peripheral signals

The increased incidence of obesity makes it imperative to understand the regulation of food intake and body weight. We review the signals that interact with the brain to control energy homeostasis, i.e. energy intake and expenditure. Three broad categories can be distinguished. Signals generated in the gastrointestinal tract during meals ('satiety' signals, e.g. cholecystokinin) elicit satiation and contribute to stopping the meal. The potency of these acutely acting signals must be increased if they are to be used therapeutically. Hormonal signals whose secretion is proportional to body fat (adiposity signals, leptin and insulin) robustly reduce food intake and body weight by directly stimulating receptors locally in the brain. Therapeutic applications will have to find ways to circumvent the systemic actions of these hormones, targeting only the brain. Satiety and adiposity signals interact with neuronal circuits in the brain that utilize myriad neurotransmitters to cause net catabolic or anabolic responses. Considerable effort is being directed towards finding ways to intervene in specific circuits to help accomplish weight loss.

Gastrointestinal satiety signals II. Cholecystokinin

During a meal, ingested nutrients accumulate in the stomach, with a significant portion passing on to the small intestine. The gastrointestinal presence of ingested nutrients initiates a range of physiological responses that serve to facilitate the overall digestive process. Thus peptides and transmitters are released, and various neural elements are activated that coordinate gastrointestinal secretion and motility and can eventually lead to meal termination or satiety. Among the range of gastrointestinal peptides released by ingested nutrients is the brain/gut peptide CCK. CCK plays a variety of roles in coordinating gastrointestinal activity and has been demonstrated to be an important mediator for the control of meal size.

NMDA receptor blockade attenuates CCK-induced reduction of real feeding but not sham feeding

Systemic injection of MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor ion channels, increases meal size and delays satiation. We examined whether MK-801 increases food intake by directly interfering with actions of cholecystokinin (CCK). Prior administration of MK-801 (100 microg/kg ip) reversed the inhibitory effects of CCK-8 (2 and 4 microg/kg ip) on real feeding of both liquid and solid foods. MK-801 alone did not alter 30-min sham intake of 15% sucrose compared with intake after saline. Furthermore, while CCK-8 (2 or 4 microg/kg ip) reduced sham intake, this reduction was not attenuated by MK-801 pretreatment. To ascertain whether MK-801 attenuation of CCK-induced reduction of real feeding was associated with attenuated inhibition of gastric emptying, we tested the effect of MK-801 pretreatment on CCK-induced inhibition of gastric emptying of 5-ml saline loads. Ten-minute gastric emptying was accelerated after MK-801 (3.9 +/- 0.2 ml) compared with saline vehicle (2.72 +/- 0.2 ml). CCK-8 (0.5 microg/kg ip) reduced 10-min emptying to 1.36 +/- 0.3 ml. Pretreatment with MK-801 did not significantly attenuate CCK-8-induced reduction of gastric emptying (0.9 +/- 0.4 ml). This series of experiments demonstrates that blockade of NMDA ion channels reverses inhibition of real feeding by CCK. However, neither inhibition of sham feeding nor inhibition of gastric emptying by CCK is attenuated by MK-801. Therefore, increased food intake after NMDA receptor blockade is not caused by a direct interference with CCK-induced satiation. Rather, increased real feeding, either in the presence or absence of CCK, depends on blockade of NMDA receptor participation in other post-oral feedback signals such as gastric sensation or gastric tone.

Gastrointestinal satiety signals I. An overview of gastrointestinal signals that influence food intake

An overview is presented of those signals generated by the gastrointestinal (GI) tract during meals that interact with the central nervous system to create a sensation of fullness and satiety. Although dozens of enzymes, hormones, and other factors are secreted by the GI tract in response to food in the lumen, only a handful are able to influence food intake directly. Most of these cause meals to terminate and hence are called satiety signals, with CCK being the most investigated. Only one GI signal, ghrelin, that increases meal size has been identified. The administration of exogenous CCK or other satiety signals causes smaller meals to be consumed, whereas blocking the action of endogenous CCK or other satiety signals causes larger meals to be consumed. Satiety signals are relayed to the hindbrain, either indirectly via nerves such as the vagus from the GI tract or else directly via the blood. Most factors that influence how much food is eaten during individual meals act by changing the sensitivity to satiety signals. This includes adiposity signals as well as habits and learning, the social situation, and stressors.

Central nervous determination of food storage—a daily switch from conservation to expenditure: implications for the metabolic syndrome

Here, we present a neuroendocrine concept to review the circularly interacting energy homeostasis system between brain and body. Body-brain interaction is circular because the brain immediately integrates an input to an output, and because part of this response may be that the brain modulates the sensitivity of this perception. First, we describe how the brain senses the body through neurons and blood-borne factors. Direct neuronal connections report the state of various organs. In addition, humoral factors are perceived by the blood-brain barrier and circumventricular organs. We describe how circulating energy carriers are sensed and what signals reach the brain during food intake, exercise and an immune response. We describe that the brain regulates the homeostatic process at two fundamentally different levels during the active and inactive states. The unbalanced output of the brain in the metabolic syndrome is discussed in relation with such circadian rhythms and with regional activity of the autonomic nervous system. In line with the above, we suggest a new approach for the diagnosis and therapy of the metabolic syndrome.