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

Reduced Liver Mitochondrial Energy Metabolism Impairs Food Intake Regulation Following Gastric Preloads and Fasting

The capacity of the liver to serve as a peripheral sensor in the regulation of food intake has been debated for over half a century. The anatomical position and physiological roles of the liver suggest it is a prime candidate to serve as an interoceptive sensor of peripheral tissue and systemic energy state. Importantly, maintenance of liver ATP levels and within-meal food intake inhibition is impaired in human subjects with obesity and obese pre-clinical models. We demonstrate that decreased hepatic mitochondrial energy metabolism in liver-specific, heterozygous PGC1a mice results in reduced mitochondrial response to changes in ΔGATP and tissue ATP following fasting. These impairments in liver energy state are associated with larger and longer meals during chow feeding, impaired dose-dependent food intake inhibition in response to mixed and individual nutrient oral pre-loads, and greater acute fasting-induced food intake. These data support previous work proposing liver-mediated food intake regulation through modulation of peripheral satiation signals.

Acts of appetite: neural circuits governing the appetitive, consummatory, and terminating phases of feeding

The overconsumption of highly caloric and palatable foods has caused a surge in obesity rates in the past half century, thereby posing a healthcare challenge due to the array of comorbidities linked to heightened body fat accrual. Developing treatments to manage body weight requires a grasp of the neurobiological basis of appetite. In this Review, we discuss advances in neuroscience that have identified brain regions and neural circuits that coordinate distinct phases of eating: food procurement, food consumption, and meal termination. While pioneering work identified several hypothalamic nuclei to be involved in feeding, more recent studies have explored how neuronal populations beyond the hypothalamus, such as the mesolimbic pathway and nodes in the hindbrain, interconnect to modulate appetite. We also examine how long-term exposure to a calorically dense diet rewires feeding circuits and alters the response of motivational systems to food. Understanding how the nervous system regulates eating behaviour will bolster the development of medical strategies that will help individuals to maintain a healthy body weight.

Hepatic overexpression of protein targeting to glycogen attenuates obesity and improves hyperglycemia in db/db mice

Increased liver glycogen content has been shown to reduce food intake, attenuate obesity, and improve glucose tolerance in a mouse model of high-fat diet (HFD)-induced obesity. Here we studied the contribution of liver glycogen to the regulation of obesity and glucose metabolism in a model of type 2 diabetes and obesity, namely the db/db mouse. To this end, we crossed db/db mice with animals overexpressing protein targeting to glycogen (PTG) in the liver to generate db/db mice with increased liver glycogen content (db/db-PTG). Hepatic PTG overexpression reduced food intake and fat weight and attenuated obesity and hyperglycemia in db/db mice. Db/db-PTG mice showed similar energy expenditure and physical activity to db/db mice. PTG overexpression reduced liver phosphoenolpyruvate carboxykinase (PEPCK) protein levels and repressed hepatic glucose production in db/db mice. Moreover, increased liver glycogen elevated hepatic ATP content in these animals. However, lipid metabolism was not modified by PTG overexpression. In conclusion, increased liver glycogen content ameliorates the diabetic and obesity phenotype in db/db mice.

Supplementation of a propionate-producing consortium improves markers of insulin resistance in an in vitro model of gut-liver axis

Gut-liver cross talk is an important determinant of human health with profound effects on energy homeostasis. While gut microbes produce a huge range of metabolites, specific compounds such as short-chain fatty acids (SCFAs) can enter the portal circulation and reach the liver (Brandl K, Schnabl B. Curr Opin Gastroenterol 33: 128-133, 2017), a central organ involved in glucose homeostasis and diabetes control. Propionate is a major SCFA involved in activation of intestinal gluconeogenesis (IGN), thereby regulating food intake, enhancing insulin sensitivity, and leading to metabolic homeostasis. Although microbiome-modulating strategies may target the increased microbial production of propionate, it is not clear whether such an effect spreads through to the hepatic cellular level. Here, we designed a propionate-producing consortium using a selection of commensal gut bacteria, and we investigated how their delivered metabolites impact an in vitro enterohepatic model of insulin resistance. Glycogen storage on hepatocyte-like cells and inflammatory markers associated with insulin resistance were evaluated to understand the role of gut metabolites on gut-liver cross talk in a simulated scenario of insulin resistance. The metabolites produced by our consortium increased glycogen synthesis by ~57% and decreased proinflammatory markers such as IL-8 by 12%, thus elucidating the positive effect of our consortium on metabolic function and low-grade inflammation. Our results suggest that microbiota-derived products can be a promising multipurpose strategy to modulate energy homeostasis, with the potential ability to assist in managing metabolic diseases due to their adaptability.

Effects of hepatic glycogen on food intake and glucose homeostasis are mediated by the vagus nerve in mice

AIMS/HYPOTHESIS

Liver glycogen plays a key role in regulating food intake and blood glucose. Mice that accumulate large amounts of this polysaccharide in the liver are protected from high-fat diet (HFD)-induced obesity by reduced food intake. Furthermore, these animals show reversal of the glucose intolerance and hyperinsulinaemia caused by the HFD. The aim of this study was to examine the involvement of the hepatic branch of the vagus nerve in regulating food intake and glucose homeostasis in this model.

METHODS

We performed hepatic branch vagotomy (HBV) or a sham operation on mice overexpressing protein targeting to glycogen (Ptg ^OE). Starting 1 week after surgery, mice were fed an HFD for 10 weeks.

RESULTS

HBV did not alter liver glycogen or ATP levels, thereby indicating that this procedure does not interfere with hepatic energy balance. However, HBV reversed the effect of glycogen accumulation on food intake. In wild-type mice, HBV led to a significant reduction in body weight without a change in food intake. Consistent with their body weight reduction, these animals had decreased fat deposition, adipocyte size, and insulin and leptin levels, together with increased energy expenditure. Ptg ^OE mice showed an increase in energy expenditure and glucose oxidation, and these differences were abolished by HBV. Moreover, Ptg ^OE mice showed an improvement in HFD-induced glucose intolerance, which was suppressed by HBV.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that the regulation of food intake and glucose homeostasis by liver glycogen is dependent on the hepatic branch of the vagus nerve.

Disruption of re-intake after partial withdrawal of gastric food contents in rats lesioned in the gelatinous part of the nucleus of the solitary tract

Sensory information from the upper gastrointestinal tract is critical in food intake regulation. Signals from different levels of the digestive system are processed to the brain, among other systems, via the vagus nerve, which mainly projects towards the nucleus of the solitary tract (NST). The objective of this study was to analyze the participation of the gelatinous part (SolG) of the NST in short-term food intake. One-third of the stomach food content was withdrawn at 5 min after the end of a meal, and food was then available ad libitum for different time periods. SolG-lesioned and control animals ingested a similar amount of the initial liquid meal, but the former consumed significantly smaller amounts and failed to compensate for the food deficit, whereas the controls re-ingested virtually the same amount as extracted. These data suggest that the SolG, as in the case of related anatomical structures such as the vagus nerve or external lateral parabrachial subnucleus, may be relevant in particular circumstances that require the rapid processing of vagal-related food intake adjustment associated to the upper gastrointestinal tract.

Liver glycogen reduces food intake and attenuates obesity in a high-fat diet-fed mouse model

We generated mice that overexpress protein targeting to glycogen (PTG) in the liver (PTG(OE)), which results in an increase in liver glycogen. When fed a high-fat diet (HFD), these animals reduced their food intake. The resulting effect was a lower body weight, decreased fat mass, and reduced leptin levels. Furthermore, PTG overexpression reversed the glucose intolerance and hyperinsulinemia caused by the HFD and protected against HFD-induced hepatic steatosis. Of note, when fed an HFD, PTG(OE) mice did not show the decrease in hepatic ATP content observed in control animals and had lower expression of neuropeptide Y and higher expression of proopiomelanocortin in the hypothalamus. Additionally, after an overnight fast, PTG(OE) animals presented high liver glycogen content, lower liver triacylglycerol content, and lower serum concentrations of fatty acids and β-hydroxybutyrate than control mice, regardless of whether they were fed an HFD or a standard diet. In conclusion, liver glycogen accumulation caused a reduced food intake, protected against the deleterious effects of an HFD, and diminished the metabolic impact of fasting. Therefore, we propose that hepatic glycogen content be considered a potential target for the pharmacological manipulation of diabetes and obesity.

Ghrelin as a starvation signal

SUMMARY

Ghrelin, a gastrointestinal hormone, functions as an endogenous ligand for the growth hormone secretagogue receptor (GHS-R). It comprises 28 amino acids, of which the Ser-3 residue is post-translationally modified by the addition of octanoyl acid. Ghrelin stimulates feeding and the secretion of growth hormone; it is also thought to function in energy conservation. Here, we provide an overview of the discovery of ghrelin, its pathophysiological significance, and the mechanism of ghrelin-induced feeding.:

Dehydroepiandrosterone (DHEA) blocks the increase in food intake caused by neuropeptide Y (NPY) in the Zucker rat

Recent studies have demonstrated that neuropeptide Y (NPY) reduced the neural production of dehydroepiandrosterone (DHEA) in frog hypothalamic explants. The objective of this study was to assess if DHEA can block the NPY induced increase in food intake in lean and obese Zucker rats. Rats were given one of the following four treatments: sterile water/dimethylsulfoxide (DMSO), NPY/DMSO, water/DHEA, and NPY/DHEA. Immediately after administration of their respective treatment, rats were exposed to macronutrients for 4 h and food intake was monitored. NPY caused a significant increase in total calories consumed compared to control. Co-administration of DHEA along with NPY blocked this NPY dependent effect. These results suggest that DHEA blocks the over-eating in satiated rats induced by NPY. Measurement of changes in regional hypothalamic and raphe monoamine neurotransmitters known to affect food intake suggested a possible role of serotonin fluctuations in the ventromedial hypothalamus (VMH) guiding this behaviour.

Effects of isoflurane and alpha-chloralose anesthesia on BOLD fMRI responses to ingested L-glutamate in rats

It is important to investigate the effect of anesthesia on blood oxygenation level-dependent (BOLD) signals in an animal model. Many researchers have investigated the BOLD response to visual, sensory, and chemical stimuli in anesthetized rats. There are no reports, however, comparing the differences in the BOLD signal change between anesthetized and conscious rats when a visceral nutrient signal arises. Here, using functional magnetic resonance imaging (fMRI), we investigated the differences in the BOLD signal changes after intragastric administration of l-glutamate (Glu) under three anesthesia conditions: conscious, alpha-chloralose-anesthetized, and isoflurane-anesthetized condition. Under the conscious and alpha-chloralose condition, we observed the significant BOLD signal increase in the medial prefrontal cortex (mPFC), insular cortex (IC), hippocampus, and several hypothalamic regions including the lateral and ventromedial nucleus. In chloralose group, however, gut Glu stimulation induced BOLD signal increase in the prelimbic cortex and orbital cortex, which did not activate in conscious condition. Meanwhile, under isoflurane-anesthetized condition, we did not observe the BOLD signal increase in these areas. BOLD signal intensity in the nucleus of the solitary tract (NTS), to which vagus nerve transmits the visceral information from the gastrointestinal tract, increased in all conditions. Importantly, under conscious condition, we observed increased BOLD signal intensity in several regions related to the metabolic state (i.e. hunger or satiety), such as the mPFC, ventromedial and lateral hypothalamus (LH). Our results suggest that alpha-chloralose and isoflurane anesthesia caused distinct effects on BOLD response to the gut l-Glu stimulation in several brain regions.

Pathological satiety caused by brainstem hemangioblastoma

Because of the multiplicity and saltatory growth pattern associated with central nervous system hemangioblastomas in von Hippel-Lindau (VHL) disease, resection of individual tumors is usually reserved until symptoms occur, to avoid unnecessary surgery over the lifetime of a patient. Brainstem hemangioblastomas in VHL typically cause lower cranial nerve dysfunction, long-tract signs, sensory impairment, and gait abnormalities. The authors report on a 16-year-old girl with VHL who presented with abnormal early satiety resulting in growth and developmental arrest associated with a growing obex hemangioblastoma. Tumor resection resulted in restoration of appetite, with rapid weight gain, growth in stature, and onset of menses. These findings indicate that caudal brainstem-mediated mechanisms have a profound effect on satiety. Moreover, brainstem hemangioblastomas may present with abnormalities in satiety and feeding that can be effectively reversed with resection.

Visceral afferents directly activate catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 neurons are catecholamine (CA) neurons within the solitary tract nucleus (NTS) that influence many homeostatic functions, including cardiovascular reflexes, food intake, and stress. Because NTS is a major interface between sensory visceral afferents and the CNS, NTS CA neurons are ideally suited to coordinate complex responses by their projections to multiple brain regions. To test how NTS CA neurons process visceral afferent information carried by solitary tract (ST) afferents, we identified CA neurons using transgenic mice expressing TH-EGFP (enhanced green fluorescent protein under the control of the tyrosine hydroxylase promoter) and recorded synaptic responses to ST activation in horizontal slices. ST shocks evoked large-amplitude, short-latency, glutamatergic EPSCs (ST-EPSCs) in 90% of NTS CA neurons. Within neurons, ST-EPSCs had constant latency, rarely failed, and depressed substantially at high ST frequencies, indicating that NTS CA neurons receive direct monosynaptic connections from afferent terminals. NTS CA neurons received direct ST inputs from only one or two afferent fibers, with one-half also receiving smaller amplitude indirect inputs. Up to 90% of ST shocks evoked action potentials in NTS CA neurons. However, transmission of sensory afferent information through NTS CA neurons critically depended on the expression of an A-type potassium current (I(KA)), which when active attenuated ST-activated action potentials to a 37% success rate. The satiety peptide, cholecystokinin, presynaptically facilitated glutamate transmission in one-half of NTS CA neurons. Thus, NTS CA neurons are directly driven by visceral afferents with output being modulated by presynaptic peptide receptors and postsynaptic potassium channels.

Bang-bang control of feeding: role of hypothalamic and satiety signals

Rats, people, and many other omnivores eat in meals rather than continuously. We show by experimental test that eating in meals is regulated by a simple bang-bang control system, an idea foreshadowed by Le Magnen and many others, shown by us to account for a wide range of behavioral data, but never explicitly tested or tied to neurophysiological facts. The hypothesis is simply that the tendency to eat rises with time at a rate determined by satiety signals. When these signals fall below a set point, eating begins, in on-off fashion. The delayed sequelae of eating increment the satiety signals, which eventually turn eating off. Thus, under free conditions, the organism eats in bouts separated by noneating activities. We report an experiment with rats to test novel predictions about meal patterns that are not explained by existing homeostatic approaches. Access to food was systematically but unpredictably interrupted just as the animal tried to start a new meal. A simple bang-bang model fits the resulting meal-pattern data well, and its elements can be identified with neurophysiological processes. Hypothalamic inputs can provide the set point for longer-term regulation carried out by a comparator in the hindbrain. Delayed gustatory and gastrointestinal aftereffects of eating act via the nucleus of the solitary tract and other hindbrain regions as neural feedback governing short-term regulation. In this way, the model forges real links between a functioning feedback mechanism, neuro-hormonal data, and both short-term (meals) and long-term (eating-rate regulation) behavioral data.

Learned preferences induced by electrical stimulation of a food-related area of the parabrachial complex: Effects of naloxone

Electrical stimulation of the External Lateral Parabrachial Subnucleus (LPBe), a food-related area, induced behavioral preferences for associated stimuli in a taste discrimination learning task. Although this stimulation appeared to be ineffective to elicit standard lever press self-stimulation, it induced place preference for one of two training compartments of a rectangular maze in which animals (adult male Wistar rats) received concurrent electrical brain stimulation. In subjects that consistently showed a preference behavior in different trials, administration of the opioid antagonist naloxone (4 mg/ml/kg) blocked concurrent learning when the test was made in a new maze but not in the same maze in which animals had learned the task. These results are discussed in terms of the possible participation of the LPBe subnucleus in different natural and artificial brain reward systems.

Neuroendocrinology of nutritional infertility

Natural selection has linked the physiological controls of energy balance and fertility such that reproduction is deferred during lean times, particularly in female mammals. In this way, an energetically costly process is confined to periods when sufficient food is available to support pregnancy and lactation. Even in the face of abundance, nutritional infertility ensues if energy intake fails to keep pace with expenditure. A working hypothesis is proposed in which any activity or condition that limits the availability of oxidizable fuels (e.g., undereating, excessive energy expenditure, diabetes mellitus) can inhibit both gonadotropin-releasing hormone (GnRH)/luteinizing hormone secretion and female copulatory behaviors. Decreases in metabolic fuel availability appear to be detected by cells in the caudal hindbrain. Hindbrain neurons producing neuropeptide Y (NPY) and catecholamines (CA) then project to the forebrain where they contact GnRH neurons both directly and also indirectly via corticotropin-releasing hormone (CRH) neurons to inhibit GnRH secretion. In the case of estrous behavior, the best available evidence suggests that the inhibitory NPY/CA system acts primarily via CRH or urocortin projections to various forebrain loci that control sexual receptivity. Disruption of these signaling processes allows normal reproduction to proceed in the face of energetic deficits, indicating that the circuitry responds to energy deficits and that no signal is necessary to indicate that there is an adequate energy supply. While there is a large body of evidence to support this hypothesis, the data do not exclude nutritional inhibition of reproduction by other pathways and processes, and the full story will undoubtedly be more complex than this.

2,5-Anhydro-D-mannitol increases hepatocyte calcium: implications for a hepatic hunger stimulus

The fructose analogue, 2,5-anhydro-D-mannitol (2,5-AM), triggers feeding in rats via a mechanism linked to its ability to trap phosphate and deplete hepatic ATP. This metabolic inhibitor is particularly useful in the study of the role of the liver in initiation of feeding as its effects are preferentially localized to the liver, and its metabolic consequences have been extensively characterized. To determine whether changes in intracellular calcium may participate in a mechanism conveying information about hepatic energy status to the nervous system, we studied the effects of 2,5-AM on intracellular calcium in isolated hepatocytes using the ratiometric indicator, fura-2. 2,5-AM elicited a marked elevation of intracellular calcium within 2-3 min of exposure that returned to baseline upon removal of the agent. Removal of external calcium failed to prevent this response, while emptying intracellular stores prevented it. These data are consistent with the hypothesis that hepatic energy status may be conveyed to the nervous system via a calcium-mediated secretion event.

The hepatic vagus mediates fat-induced inhibition of diabetic hyperphagia

Diabetic rats both overeat high-carbohydrate diet and have altered hypothalamic neuropeptide Y (NPY) and corticotropin-releasing factor (CRF). In contrast, a high-fat diet reduces caloric intake of diabetics to normal, reflected by normal hypothalamic NPY and CRF content. How the brain senses these changes in diet is unknown. To date, no hormonal changes explain these diet-induced changes in caloric intake. We tested whether the common branch of the hepatic vagus mediates the fat signal. We presented fat in two ways. First, diabetic and vehicle-treated rats were offered a cup of lard in addition to their normal high-carbohydrate diet. Second, we switched diabetic rats from high-carbohydrate diet to high-fat diet, without choice. In streptozotocin-treated rats, both methods resulted in fat-induced inhibition of caloric intake and normalization of hypothalamic neuropeptides to nondiabetic levels. Strikingly, common branch hepatic vagotomy (unlike gastroduodenal vagotomy) entirely blocked these fat-induced changes. Although a shift in hepatic energy status did not explain the lard-induced changes in diabetic rats, the data suggested that common hepatic branch vagotomy does not interfere with hepatic energy status. Furthermore, common branch hepatic vagotomy without diabetes induced indexes of obesity. Abnormal function of the hepatic vagus, as occurs in diabetic neuropathy, may contribute to diabetic obesity.

Lessons from experimental disruption of estrous cycles and behaviors

In female mammals reproduction is highly sensitive to the food supply. During lean times, females suspend reproductive attempts in favor of maintaining processes necessary for survival; fertility is restored once the food supply is again abundant. Nearly all aspects of reproduction are affected, including puberty, adult ovulatory cycles, and reproductive behaviors. Work with experimental animals reveals that caloric restriction inhibits release of luteinizing hormone (LH) and female sexual behavior via similar, although separate, processes. The primary metabolic event affecting LH release as well as female sexual behavior is the short-term (minute-to-minute, hour-to-hour) availability of oxidizable metabolic fuels, rather than any aspect of body size or composition (e.g., body fat content). Metabolic fuel availability is detected in the hindbrain and perhaps in peripheral tissues. Metabolic information is then transmitted synaptically from the visceral hindbrain to the forebrain effector circuits. In the forebrain, signaling via corticotropin-releasing hormone receptors appears to be crucial for inhibition of both LH secretion and female sexual behavior.

The role of the gastric afferent vagal nerve in ghrelin-induced feeding and growth hormone secretion in rats

BACKGROUND & AIMS

Visceral sensory information is transmitted to the brain through the afferent vagus nerve. Ghrelin, a peptide primarily produced in the stomach, stimulates both feeding and growth hormone (GH) secretion. How stomach-derived ghrelin exerts these central actions is still unknown. Here we determined the role of the gastric afferent vagal nerve in ghrelin's functions.

METHODS

Food intake and GH secretion were examined after an administration of ghrelin intravenously (IV) to rats with vagotomy or perivagal application of capsaicin, a specific afferent neurotoxin. We investigated Fos expression in neuropeptide Y (NPY)-producing and growth hormone-releasing hormone (GHRH)-producing neurons by immunohistochemistry after administration IV of ghrelin to these rats. The presence of the ghrelin receptor in vagal afferent neurons was assessed by using reverse-transcription polymerase chain reaction and in situ hybridization histochemistry. A binding study on the vagus nerve by (125)I-ghrelin was performed to determine the transport of the ghrelin receptor from vagus afferent neurons to the periphery. We recorded the electric discharge of gastric vagal afferent induced by ghrelin and compared it with that by cholecystokinin (CCK), an anorectic gut peptide.

RESULTS

Blockade of the gastric vagal afferent abolished ghrelin-induced feeding, GH secretion, and activation of NPY-producing and GHRH-producing neurons. Ghrelin receptors were synthesized in vagal afferent neurons and transported to the afferent terminals. Ghrelin suppressed firing of the vagal afferent, whereas CCK stimulated it.

CONCLUSIONS

This study indicated that the gastric vagal afferent is the major pathway conveying ghrelin's signals for starvation and GH secretion to the brain.

The role of the external lateral parabrachial subnucleus in flavor preferences induced by predigested food administered intragastrically

A study was undertaken of the role of the external lateral parabrachial subnucleus (PBNLe) in flavor preferences induced by the intragastric administration of predigested/cephalic food. These preferences were developed using two different learning procedures, concurrent and sequential. In the concurrent procedure, two different-flavored stimuli were presented at the same time: one stimulus was paired with the simultaneous intragastric administration of partially digested food and the other with physiological saline. In the sequential learning procedure, the two stimuli were presented at alternate sessions. The results showed that PBNLe lesions blocked acquisition of concurrent learning but had no effect on the sequential procedure. In the latter case, both lesioned and control animals showed a strong preference for the gustatory stimulus paired with partially digested food. These results are interpreted in terms of a dual neurobiological system involved in the rewarding effects of visceral signals.

Interactions of dietary fat and 2,5-anhydro-D-mannitol on energy metabolism in isolated rat hepatocytes

The fructose analog 2,5-anhydro-D-mannitol (2,5-AM) stimulates feeding in rats by reducing ATP content in the liver. These behavioral and metabolic effects occur with rats fed a high-carbohydrate/low-fat (HC/LF) diet, but they are prevented or attenuated when the animals eat high-fat/low-carbohydrate (HF/LC) food. To examine the metabolic bases for this effect of diet, we assessed the actions of 2,5-AM on ATP content, oxygen consumption, and substrate oxidation in isolated hepatocytes from rats fed one of the two diets. Compared with cells from rats fed the HC/LF diet ("HC/LF" cells), cells from rats fed the HF/LC diet ("HF/LC" cells) had similar ATP contents but lower oxygen consumption, decreased fructose, and increased palmitate oxidation. 2,5-AM did not decrease ATP content or oxygen consumption in HF/LC cells as much as it did in HC/LF hepatocytes, and it only affected fructose and palmitate oxidation in HC/LF cells. 31P-NMR spectroscopy indicated that differences in phosphate trapping accounted for differences in depletion of ATP by 2,5-AM. These results suggest that intake of the HF/LC diet prevents the eating response and attenuates the decline in liver ATP by shifting hepatocyte metabolism to favor fat over carbohydrate as an energy-yielding substrate.

Neuronal activation and corticotropin-releasing hormone expression in the brain of obese (fa/fa) and lean (fa/?) Zucker rats in response to refeeding

The present study was conducted to investigate the pattern of neuronal activation and corticotropin-releasing hormone (CRH) expression in fed, food deprived and refed lean (Fa/?) and obese (fa/fa) Zucker rats. The pattern of neuronal activation was studied by measuring the expression of the immediate-early gene c-fos. Expression of c-fos and CRH mRNA was determined by in situ hybridization histochemistry. In both lean and obese rats, one hour of refeeding led to a transient increase in c-fos mRNA levels which was detected in the paraventricular hypothalamic nucleus (PVH), the dorsomedial hypothalamic nucleus, the supraoptic nucleus, the paraventricular thalamic nucleus, the central nucleus of amygdala (CeA), the lateral and medial parabrachial nuclei, the nucleus of the solitary tract, and the area postrema. In addition, refeeding led to strong activation of the arginine-vasopressin neurons located in the magnocellular part of the PVH. Following 24 h of food deprivation, CRH expression in the parvocellular division of the PVH was significantly higher in obese rats compared to lean animals. During refeeding, PVH CRH mRNA levels in obese rats decreased to reach control values. The decrease in CRH expression in obese rats was accompanied by the alleviation of the hypercorticosteronemia that characterized obese Zucker rats. CRH mRNA levels in the central nucleus of the amygdala were significantly higher in lean rats than in obese animals, when the rats were fed ad libitum During food deprivation, CeA CRH mRNA levels decreased in lean rats and gradually returned to predeprivation values during refeeding. In refed obese rats, CeA levels of CRH mRNA were higher than those of ad libitum fed or food-deprived obese mutants. In the perifornical region of the lateral hypothalamic area (LHA), the expression of CRH mRNA rose significantly in response to refeeding in lean rats, but not in obese animals. Following the first hour of refeeding, the number of neurons expressing CRH mRNA in the LHA in lean rats almost doubled. The present results demonstrate that refeeding has a stimulating effect in obese Zucker rats in a pattern of activation similar to that seen in lean Fa/? rats. They also demonstrate differences in CRH expression between Fa/? and fa/fa rats after refeeding. The most apparent of these differences was seen in the lateral hypothalamus in which refeeding failed to up-regulate CRH expression in obese rats.

High-fat diet prevents eating response and attenuates liver ATP decline in rats given 2,5-anhydro-D-mannitol

Administration of the fructose analog 2,5-anhydro-D-mannitol (2,5-AM) stimulates eating in rats fed a low-fat diet but not in those fed a high-fat diet that enhances fatty acid oxidation. The eating response to 2,5-AM treatment is apparently triggered by a decrease in liver ATP content. To assess whether feeding a high-fat diet prevents the eating response to 2,5-AM by attenuating the decrease in liver ATP, we examined the effects of the analog on food intake, liver ATP content, and hepatic phosphate metabolism [using in vivo 31P-NMR spectroscopy (NMRS)]. Injection (intraperitoneal) of 300 mg/kg 2,5-AM increased food intake in rats fed a high-carbohydrate/low-fat diet, but not in those fed high-fat/low-carbohydrate (HF/LC) food. Liver ATP content decreased in all rats given 2,5-AM compared with saline, but it decreased about half as much in rats fed the HF/LC diet. NMRS on livers of anesthetized rats indicated that feeding the HF/LC diet attenuates the effects of 2,5-AM on liver ATP by reducing phosphate trapping. These results suggest that rats consuming a high-fat diet do not increase food intake after injection of 2,5-AM, because the analog is not sufficiently phosphorylated and therefore fails to decrease liver energy status below a level that generates a signal to eat.

The neuroanatomical axis for control of energy balance

The hypothalamic feeding-center model, articulated in the 1950s, held that the hypothalamus contains the interoceptors sensitive to blood-borne correlates of available or stored fuels as well as the integrative substrates that process metabolic and visceral afferent signals and issue commands to brainstem mechanisms for the production of ingestive behavior. A number of findings reviewed here, however, indicate that sensory and integrative functions are distributed across a central control axis that includes critical substrates in the basal forebrain as well as in the caudal brainstem. First, the interoceptors relevant to energy balance are distributed more widely than had been previously thought, with a prominent brainstem complement of leptin and insulin receptors, glucose-sensing mechanisms, and neuropeptide mediators. The physiological relevance of this multiple representation is suggested by the demonstration that similar behavioral effects can be obtained independently by stimulation of respective forebrain and brainstem subpopulations of the same receptor types (e.g., leptin, CRH, and melanocortin). The classical hypothalamic model is also challenged by the integrative achievements of the chronically maintained, supracollicular decerebrate rat. Decerebrate and neurologically intact rats show similar discriminative responses to taste stimuli and are similarly sensitive to intake-inhibitory feedback from the gut. Thus, the caudal brainstem, in neural isolation from forebrain influence, is sufficient to mediate ingestive responses to a range of visceral afferent signals. The decerebrate rat, however, does not show a hyperphagic response to food deprivation, suggesting that interactions between forebrain and brainstem are necessary for the behavioral response to systemic/ metabolic correlates of deprivation in the neurologically intact rat. At the same time, however, there is evidence suggesting that hypothalamic-neuroendocrine responses to fasting depend on pathways ascending from brainstem. Results reviewed are consistent with a distributionist (as opposed to hierarchical) model for the control of energy balance that emphasizes: (i) control mechanisms endemic to hypothalamus and brainstem that drive their unique effector systems on the basis of local interoceptive, and in the brainstem case, visceral, afferent inputs and (ii) a set of uni- and bidirectional interactions that coordinate adaptive neuroendocrine, autonomic, and behavioral responses to changes in metabolic status.

Role of vagal afferent innervation in feeding and brain Fos expression produced by metabolic inhibitors

Hepatic vagal afferent fibers have been implicated in the feeding responses initiated by administration of 2,5-anhydro-D-mannitol (2,5-AM; an inhibitor of hepatic metabolism) and methyl palmoxirate (MP; an inhibitor of fat metabolism). 2,5-AM and MP also increase brain Fos expression, an indicator of neural activity, which suggests that Fos expression can reveal the central neural pathways involved in the stimulation of feeding by these agents. To more closely test the hypothesis that brain Fos expression is related to the effects of 2,5-AM and MP on feeding, the vagus was lesioned by application of capsaicin, which destroys afferent fibers, directly to the cervical vagi. Perivagal capsaicin treatment blocked 2,5-AM-induced eating and attenuated MP-induced eating. Although perivagal capsaicin treatment attenuated MP-induced Fos expression, capsaicin treatment did not affect brain Fos expression produced by 2,5-AM. It is concluded that (1) brain Fos expression is not always related to the effects of 2,5-AM on feeding, (2) capsaicin-sensitive hepatic vagal afferent fibers carry the signal that stimulates feeding following 2,5-AM treatment, and (3) MP-induced feeding and brain Fos expression is mediated in part by capsaicin-sensitive fibers.

The effect of cholecystokinin in controlling appetite and food intake in humans

The present review of the satiating effect of cholecystokinin in humans has revealed that cholecystokinin is a physiological satiety factor in humans. The results demonstrate the efficacy of the satiating actions of exogenous and endogenous CCK in humans. The therapeutic potential of CCK analogues cannot be estimated until further studies are performed that demonstrate the efficacy of CCK analogues for decreasing body weight, and the safety of CCK when administered repetitively for prolonged periods.

Differential actions of dopamine receptor antagonism in rats upon food intake elicited by either mercaptoacetate or exposure to a palatable high-fat diet

Selective dopamine receptor antagonists have been shown to reduce food intake of rats under such regulatory challenge conditions as food deprivation and 2-deoxy-D-glucose-induced glucoprivation, and under such palatable conditions as acute exposure to sucrose solutions. Food intake is increased following either pretreatment with the free fatty acid oxidation inhibitor, mercaptoacetate (MA), or acute exposure to a palatable high-fat source. The present study examined whether equimolar doses (50-800 nmol/kg, s.c.) of either the selective D(1) receptor antagonist, SCH23390, or the selective D(2) receptor antagonist, raclopride, would alter food intake elicited by either MA (70 mg/kg, i.p.) or acute exposure to a high-fat diet (67% ground rat chow, 33% vegetable shortening). SCH23390 significantly and dose-dependently reduced MA-induced feeding with the two higher (400 and 800 nmol/kg) doses eliminating this response after the first 2 h and the two lower (50 and 200 nmol/kg) doses preventing the occurrence of significant MA-induced feeding. Raclopride eliminated MA-induced feeding at the highest dose, and produced dose-dependent reductions at lower doses. A different pattern of dopamine antagonist effects emerged for high-fat intake. The identical dose range of SCH23390 failed to alter high-fat intake. In contrast, whereas the highest (800 nmol/kg) dose of raclopride significantly reduced high-fat intake after 1 h, the middle (200 and 400 nmol/kg) doses of raclopride significantly increased high-fat intake after 2 h. These data are discussed in terms of the modulatory actions of dopamine upon food intake, of the differential actions of dopamine receptor subtypes upon intake under challenge and palatable conditions, and of the potential participation of presynaptic and postsynaptic receptor populations in these responses.

Central nervous system control of food intake

New information regarding neuronal circuits that control food intake and their hormonal regulation has extended our understanding of energy homeostasis, the process whereby energy intake is matched to energy expenditure over time. The profound obesity that results in rodents (and in the rare human case as well) from mutation of key signalling molecules involved in this regulatory system highlights its importance to human health. Although each new signalling pathway discovered in the hypothalamus is a potential target for drug development in the treatment of obesity, the growing number of such signalling molecules indicates that food intake is controlled by a highly complex process. To better understand how energy homeostasis can be achieved, we describe a model that delineates the roles of individual hormonal and neuropeptide signalling pathways in the control of food intake and the means by which obesity can arise from inherited or acquired defects in their function.

Intraventricular 2-deoxy-D-glucose induces Fos expression by hypothalamic vasopressin, but not oxytocin neurons

The glucostatic theory supports the role of central and peripheral substrate "sensors" in monitoring cellular glucose metabolism. Induction of hyperphagia and hyperglycemia by intracerebroventricular (i.c.v.) delivery of drugs inhibiting glucose uptake or oxidation suggests that glucose "sensors" are accessible from the cerebroventricular system. Although glucopenia elevates neurohypophyseal vasopressin (VP) and oxytocin (OXY) secretion and induces c-fos expression by hypothalamic paraventricular (PVN) and supraoptic (SON) neurons, the origin of glucoprivic regulatory signals impinging upon these cell populations is unclear. The following study evaluated immunolabeling of hypothalamic VP and OXY neurons for the nuclear transcription factor, Fos, following systemic vs. i.c.v. delivery of the glucose antimetabolite, 2-deoxy-D-glucose (2DG). Intraperitoneal drug treatment resulted in Fos expression by a high proportion of AVP- and OXY-ir neurons in the PVN and SON, whereas i.c.v. antimetabolite administration resulted in immunostaining of a smaller proportion of AVP neurons and a lack of colabeling of OXY neurons in both sites. These results suggest that decreased glucose metabolism within the periventricular CNS is a stimulus for central mechanisms that activate the Fos stimulus-transcription cascade in a discrete subpopulation of VP neurons in the PVN and SON. Alternatively, the absence of demonstrable Fos expression by OXY neurons in the same structures suggests that the functional status of these cells is regulated by glucoprivic stimuli of peripheral and/or nonperiventricular central origin.

Control of food intake by fatty acid oxidation and ketogenesis

Fatty acid oxidation seems to provide an important stimulus for metabolic control of food intake, because various inhibitors of fatty acid oxidation (mercaptoacetate, methyl palmoxirate, R-3-amino-4-trimethylaminobutyric acid) stimulated feeding in rats and/or mice, in particular when fed a fat-enriched diet, and long-term intravascular infusion of lipids reduced voluntary food intake in various species, including humans. The feeding response to decreased fatty acid oxidation was due to a shortening of the intermeal interval with meal size remaining unaffected. Thus, energy derived from fatty acid oxidation seems to contribute to control of the duration of postmeal satiety and meal onset. Since inhibition of glucose metabolism by 2-deoxy-D-glucose affects feeding pattern similarly, and spontaneous meals were shown to be preceded by a transient decline in blood glucose in rats and humans, a decrease in energy availability from glucose and fatty acid oxidation seems to be instrumental in eliciting eating. Since the feeding response of rats to inhibition of fatty acid oxidation was abolished by total abdominal vagotomy and pretreatment with capsaicin destroying non-myelinated afferents and attenuated by hepatic branch vagotomy, fatty acid oxidation in abdominal tissues, especially in the liver, apparently is signalled to the brain by vagal afferents to affect eating. Brain lesions and Fos immunohistochemistry were employed to identify pathways within the brain mediating eating in response to decreased fatty acid oxidation. According to these studies, the nucleus tractus solitarii (NTS) of the medulla oblongata represents the gate for central processing of vagally mediated afferent information related to fatty acid oxidation. The lateral parabrachial nucleus of the pons seems to be a major relay for pertinent ascending input from the NTS. In particular the central nucleus of the amygdala, a projection area of the parabrachial nucleus, appears to be crucial for eating in response to decreased fatty acid oxidation. As ketones are products of hepatic fatty acid oxidation that are released into the circulation and peripheral (and central) administration of 3-hydroxybutyrate reduced voluntary food intake in rats, ketones being utilized as fuels by the peripheral and central nervous system might contribute to control of eating by fatty acid oxidation, especially when high levels of circulating ketones occur. Whether a modulation of the hepatic membrane potential resulting from changes in the rate of fatty acid oxidation and/or ketogenesis represent a signal for control of eating transmitted to the brain by vagal afferents remains to be established. Recent in vivo studies investigating the effects of mercaptoacetate on the hepatic membrane potential and on afferent activity of the hepatic vagus branch are consistent with this notion. Further investigations are necessary to delineate the coding mechanisms by which fatty acid oxidation and/or ketogenesis modulate vagal afferent activity.

2-Deoxy-D-glucose and mercaptoacetate induce different patterns of macronutrient ingestion

2-Deoxy-D-glucose (2DG) and mercaptoacetate (MA) are antimetabolic agents that reduce the metabolism of glucose and fatty acids, respectively, and stimulate feeding. The present study compared the effects of MA and 2DG on macronutrient self-selection. Because 2DG and MA have different metabolic actions and appear to activate different neural pathways, our hypothesis was that 2DG and MA would elicit different patterns of macronutrient selection. The first experiment examined macronutrient selection in response to 2DG, MA, and 0.9% saline in rats maintained on a three-macronutrient self-selection diet consisting of cornstarch, casein, and vegetable oil. Subsequently, one macronutrient source was replaced in each of three similar experiments with Polycose, albumin, or solid vegetable shortening. Finally, 2DG and MA tests were conducted in which only one macronutrient (cornstarch, casein, or oil) was available during the test. Results show that MA and 2DG elicit different macronutrient preferences. 2DG elicits intake of all three macronutrients in the same relative proportion consumed during spontaneous feeding across a number of dietary conditions, suggesting that glucoprivation activates interoceptive signals and neural pathways similar to those involved in normal hunger. MA elicits a selective intake of protein. Conditions in which carbohydrate palatability is enhanced or protein palatability is diminished lead to a relative increase in carbohydrate intake in response to MA. However, MA did not increase the intake of fat. Results suggest that intake of each macronutrient is subject to separate neural or endocrine control, and that these controls are linked to metabolic cues.

Neural substrate for an integrated metabolic control of feeding behavior

Evidence indicates that feeding behavior in rats is controlled by a mechanism that integrates information about different aspects of fuel metabolism. We investigated the neural substrate for this integrated control by measuring the effect of metabolic inhibitors given alone and in combination on food intake and neuronal activity as reflected by the expression of c-Fos protein. Combined administration of methyl palmoxirate (5 mg/kg po), an inhibitor of fatty acid oxidation, and 2,5-anhydro-D-mannitol (150 mg/kg ip), which decreases liver ATP content, increased feeding in rats more than expected on the basis of eating responses after treatment with either inhibitor given alone. Combined treatment also produced a synergistic increase in Fos-like immunoreactivity in several brain areas, including the nucleus of the solitary tract, area postrema, and parvocellular portion of the hypothalamic paraventricular nucleus. These findings provide strong evidence for the involvement of selected brain regions in the metabolic control of food intake and suggest that metabolic information used to control feeding behavior is integrated in the periphery or at the level of the brain stem.

Neurons containing hypocretin (orexin) project to multiple neuronal systems

The novel neuropeptides called hypocretins (orexins) have recently been identified as being localized exclusively in cell bodies in a subregion of the tuberal part of the hypothalamus. The structure of the hypocretins, their accumulation in vesicles of axon terminals, and their excitatory effect on cultured hypothalamic neurons suggest that the hypocretins function in intercellular communication. To characterize these peptides further and to help understand what physiological functions they may serve, we undertook an immunohistochemical study to examine the distribution of preprohypocretin-immunoreactive neurons and fibers in the rat brain. Preprohypocretin-positive neurons were found in the perifornical nucleus and in the dorsal and lateral hypothalamic areas. These cells were distinct from those that express melanin-concentrating hormone. Although they represent a restricted group of cells, their projections were widely distributed in the brain. We observed labeled fibers throughout the hypothalamus. The densest extrahypothalamic projection was found in the locus coeruleus. Fibers were also seen in the septal nuclei, the bed nucleus of the stria terminalis, the paraventricular and reuniens nuclei of the thalamus, the zona incerta, the subthalamic nucleus, the central gray, the substantia nigra, the raphe nuclei, the parabrachial area, the medullary reticular formation, and the nucleus of the solitary tract. Less prominent projections were found in cortical regions, central and anterior amygdaloid nuclei, and the olfactory bulb. These results suggest that hypocretins are likely to have a role in physiological functions in addition to food intake such as regulation of blood pressure, the neuroendocrine system, body temperature, and the sleep-waking cycle.

Subgroups of hindbrain catecholamine neurons are selectively activated by 2-deoxy-D-glucose induced metabolic challenge

Glucose is a major fuel for body energy metabolism and an essential metabolic fuel for the brain. Consequently, glucose deficit (glucoprivation) elicits a variety of physiological and behavioral responses crucial for survival. Previous work indicates an important role for brain catecholamine neurons in mediation of responses to glucoprivation. This experiment was conducted to identify the specific catecholamine neurons that are activated by glucoprivation. Activation of hindbrain catecholamine neurons by the antimetabolic glucose analogue, 2-deoxy-D-glucose (2DG; 50, 100, 200 or 400 mg/kg, s.c.) was evaluated using double label immunohistochemistry. Fos protein was used as the marker for neuronal activation and the enzymes tyrosine hydroxylase (TH) and phenethanolamine-N-methyl transferase (PNMT) were used as the markers for norepinephrine (NE) and epinephrine (E) neurons. 2-Deoxy-D-glucose (200 and 400 mg/kg) produced selective activation of distinct hindbrain catecholamine cell groups. In the ventrolateral medulla, doubly labeled neurons were concentrated in the area of A1/C1 and were predominantly adrenergic in phenotype. In the dorsal medulla, doubly labeled neurons were limited to C2 and C3 cell groups. In the pons, some A6 neurons were Fos-positive. Neurons in rostral C1, ventral C3, A2, A5 and A7 did not express Fos-ir in response to 2DG. Our results identify specific subpopulations of catecholamine neurons that are selectively activated by 2DG. Previously demonstrated connections of these subpopulations are consistent with their participation in the feeding and hyperglycemic response to glucoprivation. Finally, the predominant and seemingly preferential activation of epinephrine neurons suggests that they may play a unique role in the brain's response to glucose deficit.

Brain fos-like immunoreactivity in chronic decerebrate and neurologically intact rats given 2,5-anhydro-D-mannitol

Injection of the fructose analogue, 2,5-anhydro-d-mannitol (2,5-AM), increases food intake and Fos-like immunoreactivity (Fos-li) in both brainstem and forebrain structures. Because of the interconnections between brainstem and forebrain areas, it has not been possible to determine whether or to what extent induction of Fos-li in a given region reflects brainstem-forebrain interactions. We addressed this issue using chronic decerebrate (CD) rats with complete transections of the neuroaxis at the meso-diencephalic juncture. CD and neurologically intact control rats were injected (i.p.) with saline or 400 mg/kg 2,5-AM and brains were examined for Fos-li. Both intact and CD rats showed increased Fos-li in the nucleus of the solitary tract (NTS) after injection of 2,5-AM as compared with saline. 2, 5-AM treatment increased Fos-li in the external lateral division of parabrachial nucleus (PBNel) in intact but not in CD rats, suggesting that descending projections from the forebrain may play a role in the activation of PBNel neurons after 2,5-AM injection. Decerebration eliminated significant 2,5-AM-induced Fos-li responses in forebrain structures, including the paraventricular nucleus, supraoptic nucleus, bed nucleus of the stria terminalis and central nucleus of the amygdala. The results are consistent with the hypothesis that the activation of forebrain structures after 2,5-AM treatment is due to stimulation by ascending projections from the brainstem.

Metabolic inhibition increases feeding and brain Fos-like immunoreactivity as a function of diet

Whether administration of 2,5-anhydro-D-mannitol (2,5-AM) or methyl palmoxirate (MP) elicits eating behavior in rats depends on the composition of the maintenance diet. To assess whether specific brain sites are involved in triggering the eating responses to these metabolic inhibitors, we measured food intake and Fos-like immunoreactivity (Fos-li) in rats maintained on either a low-fat/high-carbohydrate (LF/HC) or high-fat/low-carbohydrate (HF/LC) diet. Rats fed the LF/HC diet increased food intake after administration of 2,5-AM (200 mg/kg ip) but not after treatment with MP (10 mg/kg po), whereas rats maintained on the HF/LC diet increased food intake in response to MP administration but not after 2,5-AM injection. The effects of these inhibitors on brain Fos-li in several specific brain nuclei paralleled those on feeding behavior; that is, the number of cells showing Fos-li increased only under dietary conditions in which 2,5-AM or MP stimulated eating. These results suggest that the eating response to metabolic inhibition is tied to increased neuronal activity in brain regions that process vagal afferent signals.

Hyperpolarization of the rat hepatocyte membrane by 2,5-anhydro-D-mannitol in vivo

The fructose analogue 2,5-anhydro-D-mannitol (2,5-AM), that inhibits glucose release and ATP formation in liver cells, seems to stimulate feeding by acting on the liver, because hepatic portal injection was more effective than jugular vein injection and because hepatic branch vagotomy attenuated 2,5-AM's hyperphagic effect. Russek's "potentiostatic" hypothesis postulates a role for the hepatic membrane potential in the control of food intake with depolarization of hepatocytes signaling hunger and hyperpolarization representing a satiety signal. Therefore, the aim of the present study was to find out, whether 2,5-AM affects the hepatic membrane potential under in vivo conditions. The membrane potential was measured with microelectrodes in anesthetized rats after intraperitoneal (i.p.) or intraportal (i.p.v.) administration of 2,5-AM or control solution. 2,5-AM significantly hyperpolarized the hepatocyte membrane 50 min after i.p. injection (100 mg/kg: 3.6 mV; 300 mg/kg: 9.9 mV). In a second experiment, 2,5-AM (300 mg/kg) elicited a significant hyperpolarization of hepatocytes as soon as 5-9 min after i.p.v. infusion. These effects occurred at doses that have been shown to increase the afferent discharge rate in the common hepatic vagus branch, and to stimulate food intake. 2,5-AM's hyperphagic effect therefore is associated with an increase in the hepatic membrane potential. These findings contradict the predictions of the "potentiostatic" hypothesis and are consistent with the notion, that the feeding response to 2,5-AM might be due to ATP depletion in the terminals of vagal afferents.

The region of the pontine parabrachial nucleus is a major target of dehydration-sensitive CRH neurons in the rat lateral hypothalamic area

Neurons in a restricted part of the lateral hypothalamic area (LHA) show increased expression of corticotropin-releasing hormone (CRH) mRNA as a consequence of cellular dehydration. In the present study, we have investigated the organization of their efferent projections by using anterograde and retrograde tracing techniques. Additionally, we have compared the distribution of CRH mRNA-containing neurons after cellular dehydration and intraventricular (i.c.v.) colchicine injections. Our results show that cellular dehydration activates a more restricted neuronal population than does i.c.v. colchicine. Iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHAL) were placed in the LHA of animals drinking hypertonic saline and their proximity to activated CRH neurons determined by in situ hybridization for CRH mRNA. Although labelled fibers from these injections were seen throughout the brain, the region of the parabrachial nucleus and nucleus of the solitary tract (NTS) were most conspicuous in also having CRH immunoreactive fibers. Injections of Fluoro-Gold placed in these two structures were used to confirm these findings in dehydrated animals. Significant numbers of neurons containing both Fluoro-Gold and CRH mRNA were seen in the lateral hypothalamus after injections in the lateral and medial parts of the parabrachial nucleus; far fewer were seen after injections in the NTS. These results strongly suggest that the CRH neurons in the LHA activated by cellular dehydration provide an input to the region of the parabrachial nucleus. The altered biochemical composition of this pathway may well be able to modify sensory and motor patterns both during and after dehydration.

Methyl palmoxirate increases eating behavior and brain Fos-like immunoreactivity in rats

Administration of methyl palmoxirate (MP), an inhibitor of fatty acid oxidation, stimulates eating behavior in rats. Fos immunohistochemistry was used to determine neural pathways that may play a role in the eating response to MP. The number of cells showing Fos-like immunoreactivity (Fos-li) was quantified by computerized image analysis. MP treatment, at a dose that increased food intake (10 mg/kg, p.o.), induced Fos expression in the nucleus of the solitary tract, area postrema, lateral parabrachial nucleus, central lateral nucleus of the amygdala, dorsal lateral bed nucleus of the stria terminalis, and the paraventricular nucleus of the hypothalamus. The results suggest that MP activates an afferent pathway projecting from the hindbrain to the forebrain, which may be involved in the eating response after MP treatment.

2,5-Anhydro-D-mannitol induces Fos-like immunoreactivity in hindbrain and forebrain: relationship to eating behavior

Injection of the fructose analogue, 2,5-anhydro-D-mannitol (2,5-AM), stimulates eating behavior in rats. Previous studies have shown that administration of 2,5-AM in doses that elicit eating induces Fos-like immunoreactivity (Fos-li) primarily in hindbrain structures, including the nucleus of the solitary tract (NTS), area postrema (AP), and lateral parabrachial nucleus (PBN). To more closely assess the relationship between neural activation and the eating response to 2,5-AM treatment, we measured food intake and brain Fos-li in rats given a range of doses of 2,5-AM. The numbers of neurons showing Fos-li were quantified by computerized image analysis. Doses of 2,5-AM that reliably stimulated food intake induced Fos-li in both the hindbrain and forebrain, including in the NTS, AP, lateral PBN, central lateral nucleus of the amygdala, dorsal lateral bed nucleus of the stria terminalis (BNSTdl), anterior paraventricular nucleus of the thalamus, supraoptic nucleus, subfornical organ, and paraventricular hypothalamic nuclei. A low dose of 2,5-AM that did not elicit eating increased Fos-li marginally only in the AP, PBN, and BNSTdl. The results suggest that 2,5-AM treatment activates a vagal afferent pathway projecting from the hindbrain to forebrain that is involved in initiating the eating response to the fructose analogue.

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.

Intraportal infusion of 2,5-anhydro-D-mannitol increases afferent activity in the common hepatic vagus branch

Peripheral administration of the fructose analogue 2,5-anhydro-D-mannitol (AM), that inhibits hepatic glucose release and ATP formation, stimulates food intake in rats. This effect is partly generated in the hepato-portal area and transmitted to the central nervous system by the common hepatic vagus branch because hepatic branch vagotomy eliminated the feeding response to AM. In the present study, we investigated if the pertinent signal to increase food intake changes the discharge rate in hepatic vagal afferents. An in vivo preparation was used to record afferent hepatic vagal activity following intraportal infusion of AM in anaesthetized rats. Fine nerve filaments were isolated from the distal cut end of the hepatic vagus branch. Nerve activity was recorded by a bipolar electrode and analyzed after conversion of raw data to standard pulses. Standard pulses were integrated into spike counts of 5 s duration and the mean number of spikes in a 50 s interval at baseline was compared to spike count 10, 30 and 50 min after infusion of AM (100 or 300 mg/kg) or saline (control). Saline infusion did not influence afferent hepatic vagal activity. Intraportal infusion of AM, however, dose-dependently increased afferent activity in the hepatic vagus branch. In conclusion, AM increased the afferent discharge rate in the common hepatic vagus branch at doses that have previously been shown to increase food intake. These findings agree with the proposed role of fuel metabolism in the hepato-portal area in the control of food intake and with the suggestion that fuel availability controls food intake by influencing the hepatic afferent discharge rate.

The physiology and brain mechanisms of feeding

This article is designed as an introduction to the major theoretical models in the field of regulation of eating behavior, and a selective review of relevant neurobiological data. We first critically consider the paradigm of homeostasis as it relates to body energy content, and argue that additional theoretical constructs will be needed to account for the complexity of eating behavior in both nonhumans and humans. We then summarize some of the methods available to the neuroscientist in this area, and address some of their limitations. We review treatments and potential mechanisms that increase food intake, including deprivation, antimetabolites, norepinephrine, and several peptides including neuropeptide Y. We next review treatments that decrease food intake, including a variety of humoral, gastrointestinal, and pancreatic factors, as well as examine central pathways of satiety. This includes a discussion of leptin and other potential anorectic agents. We conclude with a discussion of human obesity and anorexias, and prospects for pharmacotherapy of eating disorders. We emphasize throughout that most regions of the human brain probably make some contribution to feeding behavior, and so a focus on any one area of transmitter/hormone is an unrealistic approach both in basic and applied areas.

Control of fertility by metabolic cues

In female mammals, reproduction is extremely sensitive to the availability of oxidizable metabolic fuels. When food intake is limited or when an inordinate fraction of the available energy is diverted to other uses such as exercise or fattening, reproductive attempts are suspended in favor of processes necessary for individual survival. Both reproductive physiology and sexual behaviors are influenced by food availability. Nutritional effects on reproductive physiology are mediated by changes in the activity of gonadotropin-releasing hormone (GnRH) neurons in the forebrain, whereas the suppression of sexual behaviors appears to be due, at least in part, to decreases in estrogen receptor in the ventromedial hypothalamus. Work using pharmacological inhibitors of glucose and fatty acid oxidation indicates that reproductive physiology and behavior respond to short-term (minute-to-minute or hour-to-hour) changes in metabolic fuel oxidation, rather than to any aspect of body size or composition (e.g., body fat content or fat-to-lean ratio). These metabolic cues seem to be detected in the viscera (most likely in the liver) and in the caudal hindbrain (probably in the area postrema). This metabolic information is then transmitted to the GnRH-secreting or estradiol-binding effector neurons in the forebrain. There is no evidence to date for direct detection of metabolic cues by these forebrain effector neurons. This metabolic fuels hypothesis is consistent with a large body of evidence and seems to account for the infertility that is seen in a number of situations, including famine, eating disorders, excessive exercise, cold exposure, lactation, some types of obesity, and poorly controlled diabetes mellitus.

Elevated neuronal c-Fos-like immunoreactivity and messenger ribonucleic acid (mRNA) in genetically obese (ob/ob) mice

Adult genetically obese (ob/ob) mice display a number of metabolic alterations, the primary cause of which may be a defect in their central nervous system (CNS). The protein encoded by the protooncogene c-fos, c-Fos, functions as a nuclear transcription factor, and also serves as a marker of neuronal activity. The specific objectives of this study were (1) to use c-Fos immunohistochemistry to identify regions with altered neuronal activity in 6-7 week old male lean and ob/ob mice; (2) to examine c-fos relative mRNA abundance by northern blot analysis in brains of these mice and compare it with that of neuropeptide Y (NPY), a peptide well known to alter feeding and (3) determine changes in c-Fos immunoreactivity and mRNA caused by food deprivation. Fos-like immunoreactivity (FLI) tended to be higher in ad libitum fed ob/ob mice than in lean controls in most brain regions examined. The most prominent and consistent differences were in the paraventricular nuclei (PVN) where the numbers of Fos-positive nuclei were approximately 3 fold higher in ob/ob mice. Food deprivation for 24 h increased FLI in the PVN in lean mice but did not further augment FLI in the PVN of ob/ob mice. Arcuate nuclei of lean and ob/ob mice showed minimal FLI staining under ad libitum fed conditions. Food deprivation however, induced FLI in arcuate nuclei of both lean and ob/ob mice. The abundance of c-fos mRNA in whole brain of ob/ob mice averaged several fold higher than in leans under both fed and fasted conditions.(ABSTRACT TRUNCATED AT 250 WORDS)