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

The receptive function of hypothalamic and brainstem centres to hormonal and nutrient signals affecting energy balance

The hypothalamic arcuate nucleus (ARC) and the area postrema (AP) represent targets for hormonal and metabolic signals involved in energy homoeostasis, e.g. glucose, amylin, insulin, leptin, peptide YY (PYY), glucagon-like peptide 1 (GLP-1) and ghrelin. Orexigenic neuropeptide Y expressing ARC neurons are activated by food deprivation and inhibited by feeding in a nutrient-dependent manner. PYY and leptin also reverse or prevent fasting-induced activation of the ARC. Interestingly, hypothalamic responses to fasting are blunted in different models of obesity (e.g. diet-induced obesity (DIO) or late-onset obesity). The AP also responds to feeding-related signals. The pancreatic hormone amylin acts via the AP to control energy intake. Amylin-sensitive AP neurons are also glucose-responsive. Furthermore, diet-derived protein attenuates amylin responsiveness suggesting a modulation of AP sensitivity by macronutrient supply. This review gives an overview of the receptive function of the ARC and the AP to hormonal and nutritional stimuli involved in the control of energy balance and the possible implications in the context of obesity. Collectively, there is consistency between the neurophysiological actions of these stimuli and their effects on energy homoeostasis under experimental conditions. However, surprisingly little progress has been made in the development of effective pharmacological approaches against obesity. A promising way to improve effectiveness involves combination treatments (e.g. amylin/leptin agonists). Hormonal alterations (e.g. GLP-1 and PYY) are also considered to mediate body weight loss observed in obese patients receiving bariatric surgery. The effects of hormonal and nutritional signals and their interactions might hold the potential to develop poly-mechanistic therapeutic strategies against obesity.

Third ventricular coinjection of subthreshold doses of NPY and AgRP stimulate food hoarding and intake and neural activation

We previously demonstrated that 3rd ventricular (3V) neuropeptide Y (NPY) or agouti-related protein (AgRP) injection potently stimulates food foraging/hoarding/intake in Siberian hamsters. Because NPY and AgRP are highly colocalized in arcuate nucleus neurons in this and other species, we tested whether subthreshold doses of NPY and AgRP coinjected into the 3V stimulates food foraging, hoarding, and intake, and/or neural activation [c-Fos immunoreactivity (c-Fos-ir)] in hamsters housed in a foraging/hoarding apparatus. In the behavioral experiment, each hamster received four 3V treatments by using subthreshold doses of NPY and AgRP for all behaviors: 1) NPY, 2) AgRP, 3) NPY+AgRP, and 4) saline with a 7-day washout period between treatments. Food foraging, intake, and hoarding were measured 1, 2, 4, and 24 h and 2 and 3 days postinjection. Only when NPY and AgRP were coinjected was food intake and hoarding increased. After identical treatment in separate animals, c-Fos-ir was assessed at 90 min and 14 h postinjection, times when food intake (0-1 h) and hoarding (4-24 h) were uniquely stimulated. c-Fos-ir was increased in several hypothalamic nuclei previously shown to be involved in ingestive behaviors and the central nucleus of the amygdala (CeA), but only in NPY+AgRP-treated animals (90 min and 14 h: magno- and parvocellular regions of the hypothalamic paraventricular nucleus and perifornical area; 14 h only: CeA and sub-zona incerta). These results suggest that NPY and AgRP interact to stimulate food hoarding and intake at distinct times, perhaps released as a cocktail naturally with food deprivation to stimulate these behaviors.

Inhibitory effects of lipopolysaccharide on hypothalamic nuclei implicated in the control of food intake

The arcuate nucleus (Arc) and the lateral hypothalamic area (LHA), two key hypothalamic nuclei regulating feeding behavior, express c-Fos, a marker of neuronal activation in fasted animals. This is reversed by refeeding. In the present study we tested whether an anorectic dose of lipopolysaccharide (LPS), the cell wall component of Gram-negative bacteria, also inhibits fasting-induced c-Fos expression in these hypothalamic nuclei. This would suggest that they are involved in anorexia during bacterial infections as well. We also studied whether LPS modulates the activity of orexin-A positive (OX+) LHA neurons. Food deprived BALB/c mice were injected with LPS or saline and were sacrificed 4 or 6h later. Four hours after injection, LPS reduced the number of c-Fos positive cells in the Arc and in the LHA, but had no effect on c-Fos in OX+ neurons. Six hours after injection, LPS reduced c-Fos expression in the LHA, both in the OX- and OX+ neurons, but not in the Arc. These results show that LPS modulates neuronal activity in the Arc and LHA similar to feeding-related stimuli, suggesting that the observed effects might contribute to the anorectic effect of LPS. Thus, physiological satiety signals released during refeeding and anorexia during bacterial infection seem to engage similar neuronal substrates.

Nicotine treatment regulates neuropeptide S system expression in the rat brain

Nicotine has marked effects on sleep, arousal and body weight. However, the neuronal mechanisms underlying these actions are not fully understood. Neuropeptide S (NPS) is a recently discovered neuropeptide regulating sleep, anxiety and feeding. Here, we examined the effect of acute and chronic nicotine treatment on the expression of NPS and its receptor (NPS-R) in the hypothalamus and brainstem of rats by using real-time PCR. Our results showed that chronic nicotine treatment induced significant changes in NPS and NPS-R expression whereas acute treatment exclusively induces a marked increase in the mRNA levels of NPS-R in the brainstem. Interestingly, we detected no changes in the expression levels of other set of genes present both in hypothalamus and brainstem. Overall, these data suggest that NPS system is specifically regulated by nicotine in the rat hypothalamus and brainstem.

Leptin signaling in neurotensin neurons involves STAT, MAP kinases ERK1/2, and p38 through c‐Fos and ATF1

The adipokine leptin signals energy status to the hypothalamus, which triggers a network of neuropeptide responses. Each hypothalamic cell type expresses a unique complement of neuropeptides, receptors, and second messengers; thus each likely responds specifically to peripheral hormones. We describe here the analysis of leptin signaling in a clonal population of mouse neurotensin (NT) -expressing hypothalamic neurons, N-39. Leptin induced phosphorylation of STAT3 and MAPK ERK1/2, but not the downstream effector of PI3K, Akt, and also induced c-Fos protein. We found activation of p38 MAPK by leptin, accompanied by phosphorylation of its downstream effector ATF-1. Phosphorylation of ATF-1 is blocked by the p38 MAPK inhibitor SB 203580. We linked this signaling directly to NT transcription. Protein binding analysis indicates that both ATF-1 and c-Fos are capable of binding to the mouse NT/N gene predominantly at physiological or high concentrations of leptin. The evidence indicates activation of distinct leptin signal transduction pathways that directly result in changes in NT gene expression and links these specific neurons to the control of energy homeostasis.

Monitoring energy balance: metabolites of fatty acid synthesis as hypothalamic sensors

Because energy balance is important for survival, a system is required to monitor energy status and to make appropriate adjustments in energy intake and energy expenditure. In higher animals, a centrally located system has evolved to accomplish this task. When caloric intake exceeds expenditure, the surplus is channeled into energy storage pathways, primarily the synthesis of fatty acids, which are converted into fat and stored in adipose tissue. Thus, metabolic flux through the pathway of fatty acid synthesis, located in the lipogenic tissues, reflects the "energy status" of the animal. The enzymatic machinery of this pathway is also expressed in the brain, notably the hypothalamus. In the hypothalamus, intermediates in this pathway appear to serve as energy sensors that signal higher brain centers to produce appropriate responses, e.g., altered food intake and energy expenditure.

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.

Attenuation of fasting-induced phosphorylation of mitogen-activated protein kinases (ERK/p38) in the mouse hypothalamus in response to refeeding

Nutritional status modify the expression of hypothalamic neuropeptides through various signal molecules, including mitogen-activated protein kinases (MAPKs) and cAMP/calcium-responsive element-binding protein (CREB), for the regulation of energy balance. Previously, we demonstrated fasting-induced activation of extracellular signal-regulated kinase 1/2 (ERK) and p38 mitogen-activated protein kinase (p38) in the murine hypothalamus. To study how caloric intake after food deprivation influences intracellular signal transduction, we investigated the phosphorylation of ERK and p38 in the murine hypothalamus of refed mice. In the arcuate nucleus, refeeding significantly attenuated fasting-induced phosphorylation of ERK and CREB. In the paraventricular nucleus, fasting-induced phosphorylation of ERK and p38 was also significantly decreased by refeeding. Thus, refeeding rapidly reduced the fasting-induced phosphorylation of ERK, p38, and CREB, suggesting that the activation of these signal molecules in the hypothalamus is precisely regulated with feeding status.

Peptide YY directly inhibits ghrelin-activated neurons of the arcuate nucleus and reverses fasting-induced c-Fos expression

The hypothalamic arcuate nucleus (Arc) monitors and integrates hormonal and metabolic signals involved in the maintenance of energy homeostasis. The orexigenic peptide ghrelin is secreted from the stomach during negative status of energy intake and directly activates neurons of the medial arcuate nucleus (ArcM) in rats. In contrast to ghrelin, peptide YY (PYY) is released postprandially from the gut and reduces food intake when applied peripherally. Neurons in the ArcM express ghrelin receptors and neuropeptide Y receptors. Thus, PYY may inhibit feeding by acting on ghrelin-sensitive Arc neurons. Using extracellular recordings, we (1) characterized the effects of PYY on the electrical activity of ghrelin-sensitive neurons in the ArcM of rats. In order to correlate the effect of PYY on neuronal activity with the energy status, we (2) investigated the ability of PYY to reverse fasting-induced c-Fos expression in Arc neurons of mice. In addition, we (3) sought to confirm that PYY reduces food intake under our experimental conditions. Superfusion of PYY reversibly inhibited 94% of all ArcM neurons by a direct postsynaptic mechanism. The PYY-induced inhibition was dose-dependent and occurred at a threshold concentration of 10(-8)M. Consistent with the opposite effects of ghrelin and PYY on food intake, a high percentage (50%) of Arc neurons was activated by ghrelin and inhibited by PYY. In line with this inhibitory action, peripherally injected PYY partly reversed the fasting-induced c-Fos expression in Arc neurons of mice. Similarly, refeeding of food-deprived mice reversed the fasting-induced activation in the Arc. Furthermore, peripherally injected PYY reduced food intake in 12-hour fasted mice. Thus the activity of Arc neurons correlated with the feeding status and was not only reduced by feeding but also by administration of PYY in non-refed mice. In conclusion, our current observations suggest that PYY may contribute to signaling a positive status of energy intake by inhibiting Arc neurons, which are activated under a negative status of energy intake by signals such as ghrelin.

Effect of the anorectic fatty acid synthase inhibitor C75 on neuronal activity in the hypothalamus and brainstem

Intraperitoneal (i.p.) injection of C75, a fatty acid synthase inhibitor, causes a rapid (<or=2-h) and persistent (to at least 24-h) approximately 95% decrease in food intake. The persistent effect seems to be due to inhibition of the fasting-induced up-regulation of expression of hypothalamic orexigenic neuropeptides neuropeptide Y and agouti-related protein and down-regulation of expression of anorexigenic neuropeptides pro-opiomelanocortinalpha-melanocyte-stimulating hormone and cocaine-amphetamine-related transcript. The effect of C75 on neuronal activity in the hypothalamus and brainstem was assessed by c-Fos expression. Consistent with its effect on neuropeptide expression, C75 blocked fasting-induced c-Fos expression in the arcuate nucleus (Arc), lateral hypothalamic area (LHA), and paraventricular nucleus (PVN) 10-24 h after i.p. injection. However, i.p. C75 induced a rapid (<or=2-h) c-Fos expression in the nucleus of the solitary tract (NTS) and area postrema of the brainstem but not in the Arc or LHA. Intracerebroventricular administration of C75 rapidly induced c-Fos expression in the Arc, PVN, and NTS, supporting a central role of C75 in the regulation of food intake. Thus, suppression of food intake by C75 administered i.p. seems to be mediated in two phases, a rapid initial phase via the NTSarea postrema of the brainstem and a delayed phase via the Arc, LHA, and PVN of the hypothalamus. The delayed effect of C75 on the Arc, LHA, and PVN correlates well with its ability to interfere with the fasting-induced effects on the expression of key orexigenic (neuropeptide Y and agouti-related protein) and anorexigenic (pro-opiomelanocortinalpha-melanocyte-stimulating hormone and cocaine-amphetamine-related transcript) messages in the hypothalamus.

Orexins/hypocretins in the ob/ob mouse: hypothalamic gene expression, peptide content and metabolic effects

Orexins (forms A and B) belong to a new family of peptides that, as neuropeptide Y (NPY), stimulate food intake when centrally injected. The ob/ob mouse is a well-characterized model of hyperphagia and obesity associated with strong metabolic disturbances and a central dysregulation of peptides involved in the control of feeding. In the present report, we investigated the hypocretin (Hcrt)/orexin (OX) peptide pathway in lean and ob/ob mice. Prepro-Hcrt/OX mRNA expression, measured by in situ hybridization was restricted to the lateral hypothalamus area. It was significantly decreased in ob/ob mice (-18%; p<0.01). When estimated by real time RT-PCR in the whole hypothalamus, this decrease amounted to 65% (p<0.001). Hcrt-1/OX-A peptide concentrations, measured by RIA in microdissected hypothalamic nuclei were high in the lateral hypothalamus (LH) and lower in the arcuate (ARC) and paraventricular nuclei (PVN). In ob/ob mice, OX-A levels were significantly lower than in lean mice in the LH (-34%; p<0.02) and in the PVN (-72%; p<0.005). Acute intracerebroventricular injection of Hcrt-1/OX-A (1-10 nmol) stimulated feeding in lean, but not in ob/ob mice, whereas Hcrt-2/OX-B (1-10 nmol) had the opposite effect. Acute third ventricle (i3vt) injections of Hcrt/OX peptides in ob/ob mice transiently increased their metabolic rate and stimulated lipid substrate utilization. These findings provide direct evidence that Hcrt/OX peptides are down-regulated in the hypothalamus of ob/ob mice, contrary to the NPY system. The present data argues that Hcrt/OX peptides are not primarily responsible for the metabolic syndrome of the ob/ob mice. The diminution in the OX tone might participate in a counterregulatory system necessary to limit the adverse effects of NPY on food intake and body weight.

Neuropeptide Y and corticotropin-releasing hormone concentrations within specific hypothalamic regions of lean but not ob/ob mice respond to food-deprivation and refeeding

Leptin is proposed to control food intake at least in part by regulating hypothalamic neuropeptide Y (NPY), a stimulator of food intake, and corticotropin-releasing hormone (CRH), an inhibitor of food intake. Ob/ob mice are leptin-deficient and would thus be expected to exhibit alterations in hypothalamic NPY and CRH. We therefore measured concentrations of NPY and CRH in discrete regions of the hypothalamus (i.e., ARC, arcuate nucleus; PVN, paraventricular nucleus; VMH, ventromedial nucleus; DMH, dorsomedial nucleus; and SCN, suprachiasmatic nucleus) of 6.5-7-wk-old ob/ob and lean mice with free access to stock diet, 24 h after food deprivation, and 1 h after refeeding. Fed ob/ob mice had 55-75% higher concentrations of NPY in the ARC, VMH and SCN than lean mice. Food deprivation increased NPY concentrations approximately 70% in the ARC, PVN and VMH of lean mice, and refeeding lowered NPY concentrations approximately 70% in the PVN of these mice. NPY in these hypothalamic regions of ob/ob mice was unresponsive to food deprivation or refeeding. The most pronounced change in CRH concentrations within the regions examined (i.e., ARC, PVN and VMH) occurred in the ARC of lean mice where refeeding lowered CRH concentrations by 75% without influencing ARC CRH concentrations in ob/ob mice. The hypothalamic concentrations of two neuropeptides involved in body weight regulation (i.e., NPY and CRH) in leptin-deficient ob/ob mice respond abnormally to abrupt changes in nutritional status.

Altered c-fos expression in autonomic regulatory centers of genetically obese (ob/ob) mouse brain

This study examined c-fos-like immunoreactivity (FLI) in the brain of obese (ob/ob) and lean (+/+) mice during states of satiety and hunger. The results demonstrated that there is an altered neural activity primarily in the hypothalamus of the obese mouse brain. Most interestingly, only obese mice had an increased FLI in the dorsomedial and supramammillary nuclei. Conversely, only lean mice showed increase in FLI in the medial part of perifornical nucleus following food deprivation for 24 h.

Fos expression in the brain induced by peripheral injection of CCK or leptin plus CCK in fasted lean mice

We previously reported a synergistic interaction between leptin and cholecystokinin (CCK) to reduce food intake through CCK-A receptors in lean mice fasted for 24 h. To identify the activated neuronal pathways, we investigated changes in Fos expression in brain nuclei 2 h after single or combined intraperitoneal (i.p.) injections of leptin (120 microg/kg) and sulfated CCK-8 (3.5 microg/kg) in male lean mice (C57BL/6) fasted for 24 h using immunohistochemistry for Fos, the protein product of the early gene, c-fos. Leptin did not increase Fos expression in the brain compared with vehicle-treated mice. CCK increased the numbers of Fos-positive neurons in the nucleus of the solitary tract (NTS)/area postrema (AP), central nucleus of the amygdala (CeA) and, to a smaller extent, in the paraventricular nucleus of the hypothalamus (PVN) (5.2-, 2.3- and 0. 3-fold respectively). Injections of leptin-CCK further enhanced Fos expression by 40% in the PVN compared with that induced by CCK alone, but not in the other nuclei. Devazepide (a CCK-A receptor antagonist, 1 mg/kg, i.p.) prevented the increase in Fos expression induced by leptin-CCK in the PVN and by CCK alone in the PVN, CeA and NTS/AP. These results indicate that in fasted mice, i.p. injection of CCK increases Fos expression in specific brain nuclei through CCK-A receptors while leptin alone had no effect. Leptin in conjunction with CCK selectively enhanced Fos expression in the PVN. The PVN may be an important site mediating the synergistic effect of leptin-CCK to regulate food intake.

Localization of leptin receptor immunoreactivity in the lean and obese Zucker rat brain

Leptin, a product of the obese (ob) gene, is secreted by adipocytes and appears to act as a hormone to regulate food intake, metabolism and body weight. Subcutaneous administration of leptin causes reductions in food intake and body and fat-depot weights in both lean and genetically obese (ob/ob) mice, and leptin infusion into the lateral cerebral ventricles decreases feeding with short latency, suggesting a central site of action. A gene defect in the Zucker obese rat causes an amino acid substitution in the leptin receptor and reduced leptin binding at the cell surface. An antiserum to a portion of the mouse leptin receptor (AA 877-894) located within the intracellular domain was used to label Zucker lean (Fa/?) and obese (fa/fa) rat brain sections. At optimal dilution (1:8000), only cells in the basal forebrain, preoptic area, hypothalamus and brainstem were moderately or intensely labeled. The most intensely-labeled nuclei, the anterior commissural, magnocellular paraventricular, supraoptic, circularis in the anterior hypothalamus and fornical in the lateral hypothalamus contain large neurons that synthesize and secrete vasopressin or oxytocin and their respective neurophysins. Diminished leptin transport into the central nervous system or defective signal transduction in Zucker obese rats may sufficiently compromise leptin regulation of the HPA axis, NPY-immunoreactive neurons or other hypothalamic elements to cause obesity.

Neuroanatomical patterns of Fos-like immunoreactivity induced by naltrexone in food-restricted and ad libitum fed rats

Chronic food restriction produces a variety of adaptive changes in physiology and behavior aimed at the preservation of energy homeostasis. The brain opioid system may be involved in the adaptation to food restriction since regional levels of opioid peptides, precursor mRNA, and receptor binding have previously been observed. In the present study, c-Fos immunohistochemistry was used to localize cells that are released from opioid-mediated inhibition by naltrexone under conditions of food restriction and ad libitum feeding. In the majority of hypothalamic and forebrain areas examined, Fos-like immunoreactivity (FLI) was higher in food-restricted rats regardless of injection treatment. This may reflect the persistent stress of underfeeding or the synchronizing effect of afternoon feeding on spontaneous c-fos mRNA expression in food-restricted rats. In two brain regions, bed nucleus of the stria terminalis (BNST) and central amygdala (CEA), naltrexone increased FLI in ad libitum fed rats, exclusively. This result suggests the presence of tonic opioid secretion under basal conditions that is suppressed by food restriction. Interestingly, work in other laboratories indicates that anorectic agents consistently increase FLI in BNST and CEA. In three brain regions--lateral (LH), dorsomedial (DMH) and arcuate hypothalamus (ARC)--naltrexone increased FLI in food-restricted rats, exclusively. This result suggests the presence of opioid secretion that is unique to the state of food restriction. The hypothalamic pattern of FLI is discussed in terms of NPY-opioid interactions that result from the ARC response to changes in circulating insulin, corticosterone and leptin levels during food restriction.

Synergistic interaction between leptin and cholecystokinin to reduce short-term food intake in lean mice

Leptin is a circulating protein involved in the long-term regulation of food intake and body weight. Cholecystokinin (CCK) is released postprandially and elicits satiety signals. We investigated the interaction between leptin and CCK-8 in the short-term regulation of food intake induced by 24-hr fasting in lean mice. Leptin, injected intraperitoneally (i.p.) at low doses (4-120 microg/kg), which did not influence feeding behavior for the first 3 hr postinjection, decreased food intake dose dependently by 47-83% during the first hour when coinjected with a subthreshold dose of CCK. Such an interaction was not observed between leptin and bombesin. The food-reducing effect of leptin injected with CCK was not associated with alterations in gastric emptying or locomotor behavior. Leptin-CCK action was blocked by systemic capsaicin at a dose inducing functional ablation of sensory afferent fibers and by devazepide, a CCK-A receptor antagonist but not by the CCK-B receptor antagonist, L-365,260. The decrease in food intake which occurs 5 hr after i.p. injection of leptin alone was also blunted by devazepide. Coinjection of leptin and CCK enhanced the number of Fos-positive cells in the hypothalamic paraventricular nucleus by 60%, whereas leptin or CCK alone did not modify Fos expression. These results indicate the existence of a functional synergistic interaction between leptin and CCK leading to early suppression of food intake which involves CCK-A receptors and capsaicin-sensitive afferent fibers.