Agouti-related protein-like immunoreactivity: characterization of release from hypothalamic tissue and presence in serum

A gene-diet interaction controlling relative intake of dietary carbohydrates and fats

OBJECTIVE

Preference for dietary fat vs. carbohydrate varies markedly across free-living individuals. It is recognized that food choice is under genetic and physiological regulation, and that the central melanocortin system is involved. However, how genetic and dietary factors interact to regulate relative macronutrient intake is not well understood.

METHODS

We investigated how the choice for food rich in carbohydrate vs. fat is influenced by dietary cholesterol availability and agouti-related protein (AGRP), the orexigenic component of the central melanocortin system. We assessed how macronutrient intake and different metabolic parameters correlate with plasma AGRP in a cohort of obese humans. We also examined how both dietary cholesterol levels and inhibiting de novo cholesterol synthesis affect carbohydrate and fat intake in mice, and how dietary cholesterol deficiency during the postnatal period impacts macronutrient intake patterns in adulthood.

RESULTS

In obese human subjects, plasma levels of AGRP correlated inversely with consumption of carbohydrates over fats. Moreover, AgRP-deficient mice preferred to consume more calories from carbohydrates than fats, more so when each diet lacked cholesterol. Intriguingly, inhibiting cholesterol biosynthesis (simvastatin) promoted carbohydrate intake at the expense of fat without altering total caloric consumption, an effect that was remarkably absent in AgRP-deficient mice. Finally, feeding lactating C57BL/6 dams and pups a cholesterol-free diet prior to weaning led the offspring to prefer fats over carbohydrates as adults, indicating that altered cholesterol metabolism early in life programs adaptive changes to macronutrient intake.

CONCLUSIONS

Together, our study illustrates a specific gene-diet interaction in modulating food choice.

Pregnancy-specific Adaptations in Leptin and Melanocortin Neuropeptides in Early Human Gestation

INTRODUCTION

Pregnancy is characterized by increased appetitive drive beginning early in gestation, yet the central mechanisms underlying this adaptation are poorly understood in humans. To elucidate central mechanisms underlying appetite regulation in early pregnancy, we examine plasma and cerebrospinal fluid (CSF) leptin and Agouti-related peptide (AgRP) as well as CSF proopiomelanocortin (POMC) as surrogates for brain melanocortin activity.

METHODS

Plasma leptin, soluble leptin receptor, AgRP, and CSF leptin, POMC, and AgRP were collected from pregnant women before cerclage placement (16.6 ± 1.1 weeks; N = 24), scheduled cesarean section (39.2 ± 0.2 weeks; N = 24), and from nonpregnant controls (N = 24), matched for age and body mass index.

RESULTS

Plasma leptin was 1.5 times higher in pregnancy vs controls (P = 0.01), but CSF leptin did not differ. CSF/plasma leptin percentage was lower in early pregnancy vs controls (0.8 ± 0.1 vs 1.7 ± 0.2; P < 0.0001) and remained unchanged at term (0.9 ± 0.1), supporting a decrease in leptin transport into CSF in pregnancy. Plasma AgRP, a peripheral biomarker of the orexigenic hypothalamic neuropeptide, was higher in early pregnancy vs controls (95.0 ± 7.8 vs 67.5 ± 5.3; P = 0.005). In early gestation, CSF AgRP did not differ from controls, but CSF POMC was 25% lower (P = 0.006). In contrast, at term, CSF AgRP was 42% higher vs controls (P = 0.0001), but CSF POMC no longer differed. Overall, the CSF AgRP/POMC ratio was 1.5-fold higher in early pregnancy vs controls, reflecting a decrease in melanocortin tone favoring appetitive drive.

CONCLUSIONS

Pregnancy-specific adaptions in the central regulation of energy balance occur early in human gestation and are consistent with decreased leptin transport into brain and resistance to the effects of leptin on target melanocortin neuropeptides.

Diurnal Patterns for Cortisol, Cortisone and Agouti-Related Protein in Human Cerebrospinal Fluid and Blood

CONTEXT

Cortisol in blood has a robust circadian rhythm and exerts potent effects on energy balance that are mediated in part by central mechanisms. These interactions involve orexigenic agouti-related protein (AgRP) neurons that are stimulated by glucocorticoids. However, diurnal changes in brain or cerebrospinal fluid (CSF) cortisol and cortisone, which are interconverted by 11ß-HSD1, have not been characterized in humans.

OBJECTIVE

To conduct a secondary analysis of existing samples to characterize diurnal changes in cortisol and cortisone in CSF and examine their relationships to changes in AgRP.

METHODS

Stored CSF and plasma samples were obtained from 8 healthy subjects who served as controls for a sleep study. CSF was collected every 2h for 36h via indwelling lumbar catheter; plasma was collected every 2h.

RESULTS

There was a diurnal rhythm for cortisol and cortisone in CSF that closely followed the plasma rhythm by 2 h with peak and nadir levels at 0900h and 0100h. The ratio of cortisol (active) to cortisone (inactive) in CSF was 48% higher at the peak versus nadir. There was a diurnal rhythm for AgRP in plasma that was out of phase with the cortisol rhythm. There was a less distinct diurnal rhythm for AgRP in CSF that oscillated with a similar phase as cortisol.

CONCLUSIONS

There is a robust diurnal rhythm for cortisol and cortisone in CSF. Diurnal changes were noted for AgRP that are related to the cortisol changes. It remains to be determined if AgRP mediates adverse metabolic effects associated with disruption of the cortisol circadian rhythm.

Plasma Agouti-Related Protein Levels in Acromegaly and Effects of Surgical or Pegvisomant Therapy

CONTEXT

GH activates agouti-related protein (AgRP) neurons, leading to orexigenic responses in mice. The relationship between serum GH and plasma AgRP, which has been shown to reflect hypothalamic AgRP, has not been evaluated in humans.

OBJECTIVE

To test the hypothesis that central stimulatory actions of GH on hypothalamic AgRP could be reflected in plasma AgRP in acromegaly.

METHODS

We studied 23 patients with active acromegaly before and for ≤2 years after surgical (n = 13) or GH receptor antagonist therapy with pegvisomant (n = 10), and 100 healthy subjects with morning fasting blood samples for AgRP, leptin, GH, and IGF-1 and anthropometric measurements.

RESULTS

The plasma AgRP levels were higher in those with active acromegaly than in the matched healthy subjects [median, 100 pg/mL; interquartile range (IQR), 78 to 139 pg/mL vs median, 63 pg/mL; IQR, 58 to 67 pg/mL; P < 0.0001]. Plasma AgRP decreased from before to after surgery (median, 102 pg/mL; IQR, 82 to 124 pg/mL vs median, 63 pg/mL; IQR, 55.6 to 83 pg/mL; P = 0.0024) and from before to during pegvisomant therapy (median, 97 pg/mL; IQR, 77 to 175 pg/mL vs median, 63; IQR, 61 to 109 pg/mL; P = 0.006). The plasma AgRP level correlated with GH (r = 0.319; P = 0.011) and IGF-1 (r = 0.292; P = 0.002). In repeated measure analysis, AgRP was significantly associated with IGF-1.

CONCLUSIONS

Our data have provided evidence of a stimulatory effect of GH on plasma AgRP in humans. The levels were greater in active acromegaly and decreased in parallel with GH and IGF-1 decreases with acromegaly treatment. Data from mice suggest that AgRP may mediate some of the known effects of GH on energy metabolism. This warrants further study in patients with acromegaly and other populations.

Agouti-Related Protein 2 Is a New Player in the Teleost Stress Response System

Agouti-related protein (AgRP) is a hypothalamic regulator of food consumption in mammals. However, AgRP has also been detected in circulation, but a possible endocrine role has not been examined. Zebrafish possess two agrp genes: hypothalamically expressed agrp1, considered functionally equivalent to the single mammalian agrp, and agrp2, which is expressed in pre-optic neurons and uncharacterized pineal gland cells and whose function is not well understood. By ablation of AgRP1-expressing neurons and knockout of the agrp1 gene, we show that AgRP1 stimulates food consumption in the zebrafish larvae. Single-cell sequencing of pineal agrp2-expressing cells revealed molecular resemblance to retinal-pigment epithelium cells, and anatomic analysis shows that these cells secrete peptides, possibly into the cerebrospinal fluid. Additionally, based on AgRP2 peptide localization and gene knockout analysis, we demonstrate that pre-optic AgRP2 is a neuroendocrine regulator of the stress axis that reduces cortisol secretion. We therefore suggest that the ancestral role of AgRP was functionally partitioned in zebrafish by the two AgRPs, with AgRP1 centrally regulating food consumption and AgRP2 acting as a neuroendocrine factor regulating the stress axis.

Plasma Agouti-Related Protein and Cortisol Levels in Cushing Disease: Evidence for the Regulation of Agouti-Related Protein by Glucocorticoids in Humans

CONTEXT

Glucocorticoids regulate energy balance, in part by stimulating the orexigenic neuropeptide agouti-related protein (AgRP). AgRP neurons express glucocorticoid receptors, and glucocorticoids have been shown to stimulate AgRP gene expression in rodents.

OBJECTIVE

We sought to determine whether there is a relationship between plasma AgRP and hypothalamic AgRP in rats and to evaluate the relationship between cortisol and plasma AgRP in humans.

METHODS

We retrospectively evaluated plasma AgRP levels prior to transsphenoidal surgery in 31 patients with Cushing disease (CD) vs 31 sex- and body mass index-matched controls from a separate study. We then prospectively measured plasma AgRP, before and 6 to 12 months after surgery, in a subgroup of 13 patients with CD. Plasma and hypothalamic AgRP were measured in adrenalectomized rats with and without corticosterone replacement.

RESULTS

Plasma AgRP was stimulated by corticosterone in rats and correlated with hypothalamic AgRP expression. Plasma AgRP levels were higher in patients with CD than in controls (139 ± 12.3 vs 54.2 ± 3.1 pg/mL; P < 0.0001). Among patients with CD, mean 24-hour urine free cortisol (UFC) levels were 257 ± 39 μg/24 hours. Strong positive correlations were observed between plasma AgRP and UFC (r = 0.76; P < 0.0001). In 11 of 13 patients demonstrating surgical cure, AgRP decreased from 126 ± 20.6 to 62.5 ± 8.0 pg/mL (P < 0.05) postoperatively, in parallel with a decline in UFC.

CONCLUSIONS

Plasma AgRP levels are elevated in CD, are tightly correlated with cortisol concentrations, and decline with surgical cure. These data support the regulation of AgRP by glucocorticoids in humans. AgRP's role as a potential biomarker and as a mediator of the adverse metabolic consequences of CD deserves further study.

AgRP-Expressing Adrenal Chromaffin Cells Are Involved in the Sympathetic Response to Fasting

Fasting evokes a homeostatic response that maintains circulating levels of energy-rich metabolites and increases the drive to eat. Centrally, this reflex activates a small population of hypothalamic neurons that are characterized by the expression of AgRP, a neuropeptide with an extremely restricted distribution. Apart from the hypothalamus, the only other site with substantial expression is the adrenal gland, but there is disagreement about which cells synthesize AgRP. Using immunohistochemistry, flow cytometry, and reverse transcription-polymerase chain reaction, we show AgRP is present in the mouse adrenal medulla and is expressed by neuroendocrine chromaffin cells that also synthesize the catecholamines and neuropeptide Y. Short-term fasting led to an increase in adrenal AgRP expression. Because AgRP can act as an antagonist at MC3/4 receptors, we tested whether melanotan II, an MC3/4 receptor agonist, could regulate pre- and postsynaptic signaling within the adrenal medulla. Melanotan II decreased the paired-pulse ratio of evoked synaptic currents recorded in chromaffin cells; this effect was blocked by exogenous AgRP. In contrast, neither melanotan II nor AgRP altered the optogenetically evoked release of catecholamines from isolated chromaffin cells. These results are consistent with the idea that AgRP regulates the strength of the sympathetic input by modulation of presynaptic MC3/4 receptors located on preganglionic neurons. We conclude that a small population of neuroendocrine cells in the adrenal medulla, and the arcuate nucleus of the hypothalamus, express AgRP and neuropeptide Y and are functionally involved in the systemic response to fasting.

Regulation of Agouti-Related Protein and Pro-Opiomelanocortin Gene Expression in the Avian Arcuate Nucleus

The arcuate nucleus is generally conserved across vertebrate taxa in its neuroanatomy and neuropeptide expression. Gene expression of agouti-related protein (AGRP), neuropeptide Y (NPY), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART) has been established in the arcuate nucleus of several bird species and co-localization demonstrated for AGRP and NPY. The proteins encoded by these genes exert comparable effects on food intake in birds after central administration to those seen in other vertebrates, with AGRP and NPY being orexigenic and CART and α-melanocyte-stimulating hormone anorexigenic. We have focused on the measurement of arcuate nucleus AGRP and POMC expression in several avian models in relation to the regulation of energy balance, incubation, stress, and growth. AGRP mRNA and POMC mRNA are, respectively, up- and downregulated after energy deprivation and restriction. This suggests that coordinated changes in the activity of AGRP and POMC neurons help to drive the homeostatic response to replace depleted energy stores in birds as in other vertebrates. While AGRP and POMC expression are generally positively and negatively correlated with food intake, respectively, we review here situations in some avian models in which AGRP gene expression is dissociated from the level of food intake and may have an influence on growth independent of changes in appetite. This suggests the possibility that the central melanocortin system exerts more pleiotropic functions in birds. While the neuroanatomical arrangement of AGRP and POMC neurons and the sensitivity of their activity to nutritional state appear generally conserved with other vertebrates, detailed knowledge is lacking of the key nutritional feedback signals acting on the avian arcuate nucleus and there appear to be significant differences between birds and mammals. In particular, recently identified avian leptin genes show differences between bird species in their tissue expression patterns and appear less closely linked in their expression to nutritional state. It is presently uncertain how the regulation of the central melanocortin system in birds is brought about in the situation of the apparently reduced importance of leptin and ghrelin compared to mammals.

Evaluation of CSF and plasma biomarkers of brain melanocortin activity in response to caloric restriction in humans

The melanocortin neuronal system, which consists of hypothalamic proopiomelanocortin (POMC) and agouti-related protein (AgRP) neurons, is a leptin target that regulates energy balance and metabolism, but studies in humans are limited by a lack of reliable biomarkers to assess brain melanocortin activity. The objective of this study was to measure the POMC prohormone and its processed peptide, β-endorphin (β-EP), in cerebrospinal fluid (CSF) and AgRP in CSF and plasma after calorie restriction to validate their utility as biomarkers of brain melanocortin activity. CSF and plasma were obtained from 10 lean and obese subjects after fasting (40 h) and refeeding (24 h), and from 8 obese subjects before and after 6 wk of dieting (800 kcal/day) to assess changes in neuropeptide and hormone levels. After fasting, plasma leptin decreased to 35%, and AgRP increased to 153% of baseline. During refeeding, AgRP declined as leptin increased; CSF β-EP increased, but POMC did not change. Relative changes in plasma and CSF leptin were blunted in obese subjects. After dieting, plasma and CSF leptin decreased to 46% and 70% of baseline, CSF POMC and β-EP decreased, and plasma AgRP increased. At baseline, AgRP correlated negatively with insulin and homeostasis model assessment (HOMA-IR), and positively with the Matsuda index. Thus, following chronic calorie restriction, POMC and β-EP declined in CSF, whereas acutely, only β-EP changed. Plasma AgRP, however, increased after both acute and chronic calorie restriction. These results support the use of CSF POMC and plasma AgRP as biomarkers of hypothalamic melanocortin activity and provide evidence linking AgRP to insulin sensitivity.

Proopiomelanocortin, agouti-related protein, and leptin in human cerebrospinal fluid: correlations with body weight and adiposity

Leptin and its neuronal targets, which produce proopiomelanocortin (POMC) and agouti-related protein (AgRP), regulate energy balance. This study characterized leptin, POMC, and AgRP in the cerebrospinal fluid (CSF) of 47 healthy human subjects, 23 lean and 24 overweight/obese (OW/OB), as related to BMI, adiposity, plasma leptin, soluble leptin receptor (s-OB-R), and insulin. POMC was measured since the POMC prohormone is the predominant POMC peptide in CSF and correlates with hypothalamic POMC in rodents. Plasma AgRP was similarly characterized. CSF leptin was 83-fold lower than in plasma and correlated strongly with BMI, body fat, and insulin. The relative amount of leptin transported into CSF declined with increasing BMI, ranging from 4.5 to 0.52%, consistent with a saturable transport mechanism. CSF sOB-R was 78-fold lower than in plasma and correlated negatively with plasma and CSF leptin. CSF POMC was higher in lean vs. OW/OB subjects (P < 0.001) and correlated negatively with CSF leptin (r = -0.60, P < 0.001) and with plasma leptin, insulin, BMI, and adiposity. CSF AgRP was not different in lean vs. OW/OB; however, plasma AgRP was higher in lean subjects (P = 0.001) and correlated negatively with BMI, adiposity, leptin, insulin, and HOMA (P < 0.005). Thus, CSF measurements may provide useful biomarkers for brain leptin and POMC activity. The striking negative correlation between CSF leptin and POMC could be secondary to leptin resistance and/or neuronal changes associated with obesity but may also indicate that POMC plays a primary role in regulating body weight and adiposity. The role of plasma AgRP as a neuroendocrine biomarker deserves further study.

Peroxisome proliferator-activated receptor γ controls ingestive behavior, agouti-related protein, and neuropeptide Y mRNA in the arcuate hypothalamus

Peroxisome proliferator-activated receptor γ (PPARγ) is clinically targeted for type II diabetes treatment; however, rosiglitazone (ROSI), a PPARγ agonist, increases food intake and body/fat mass as side-effects. Mechanisms for these effects and the role of PPARγ in feeding are not understood. Therefore, we tested this role in Siberian hamsters, a model of human energy balance, and C57BL/6 mice. We tested the following: (1) how ROSI and/or GW9662 (2-chloro-5-nitro-N-phenylbenzamide; PPARγ antagonist) injected intraperitoneally or into the third ventricle (3V) affected Siberian hamster feeding behaviors; (2) whether food deprivation (FD) co-increases agouti-related protein (AgRP) and PPARγ mRNA expression in Siberian hamsters and mice; (3) whether intraperitoneally administered ROSI increases AgRP and NPY in ad libitum-fed animals; (4) whether intraperitoneally administered PPARγ antagonism blocks FD-induced increases in AgRP and NPY; and finally, (5) whether intraperitoneally administered PPARγ modulation affects plasma ghrelin. Third ventricular and intraperitoneally administered ROSI increased food hoarding and intake for 7 d, an effect attenuated by 3V GW9662, and also prevented (intraperitoneal) FD-induced feeding. FD hamsters and mice increased AgRP within the arcuate hypothalamic nucleus with concomitant increases in PPARγ exclusively within AgRP/NPY neurons. ROSI increased AgRP and NPY similarly to FD, and GW9662 prevented FD-induced increases in AgRP and NPY in both species. Neither ROSI nor GW9662 affected plasma ghrelin. Thus, we demonstrated that PPARγ activation is sufficient to trigger food hoarding/intake, increase AgRP/NPY, and possibly is necessary for FD-induced increases in feeding and AgRP/NPY. These findings provide initial evidence that FD-induced increases in AgRP/NPY may be a direct PPARγ-dependent process that controls ingestive behaviors.

[Anorexia nervosa - from a neuroscience perspective]

Anorexia nervosa is a frequent disorder especially among adolescent girls and young women, with high morbidity, mortality, and relapse rates. To date, no single therapeutic approach has proved to be superior to others (Herpertz et al., 2011). It remains unclear how its etiology and pathology are encoded within cognitive, neural, and endocrinological processes that modulate important mechanisms in appetitive processing and weight regulation. Yet, several trait characteristics have been identified in AN which might reflect predisposing factors. Further, altered levels of neuropeptides and hormones that regulate appetite and feeding behavior have been found during both the acute and the recovered state, pointing to dysfunctional mechanisms in AN that persist even after malnutrition has ceased. Researchers are also hoping that brain imaging techniques will allow for a more detailed investigation of the neural basis of reward and punishment sensitivity that appears to be altered in AN. The integration and extension of recent findings in these areas will hopefully provide a more comprehensive understanding of the disorder and hence enable the development of more effective treatments.

Coordinated regulation of hepatic energy stores by leptin and hypothalamic agouti-related protein

Like obesity, prolonged food deprivation induces severe hepatic steatosis; however, the functional significance of this phenomenon is not well understood. In this study, we show that the fall in plasma leptin concentration during fasting is required for the development of hepatic steatosis in mice. Removal of leptin receptors from AGRP neurons diminishes fasting-induced hepatic steatosis. Furthermore, the suppressive effects of leptin on fasting-induced hepatic steatosis are absent in mice lacking the gene encoding agouti-related protein (Agrp), suggesting that this function of leptin is mediated by AGRP. Prolonged fasting leads to suppression of hepatic sympathetic activity, increased expression of acyl CoA:diacylglycerol acyltransferase-2 in the liver, and elevation of hepatic triglyceride content and all of these effects are blunted in the absence of AGRP. AGRP deficiency, despite having no effects on feeding or body adiposity in the free-fed state, impairs triglyceride and ketone body release from the liver during prolonged fasting. Furthermore, reducing CNS Agrp expression in wild-type mice by RNAi protected against the development of hepatic steatosis not only during starvation, but also in response to consumption of a high-fat diet. These findings identify the leptin-AGRP circuit as a critical modulator of hepatic triglyceride stores in starvation and suggest a vital role for this circuit in sustaining the supply of energy from the liver to extrahepatic tissues during periods of prolonged food deprivation.

Trajectories of agouti-related protein and leptin levels during antipsychotic-associated weight gain in patients with schizophrenia

OBJECTIVE

Some but not all second-generation antipsychotics can induce considerable weight gain and metabolic syndrome. Although the exact biochemical mechanisms for these adverse effects are unclear, appetite-regulating neuropeptides of the central nervous system are thought to be implicated in this process. The hypothalamic mediator Agouti-related protein (AGRP) is inhibited by leptin and was shown to increase food intake. The aim of the present study was to investigate the trajectory of AGRP levels during antipsychotic-induced weight gain.

METHODS

As part of a controlled prospective clinical study, we determined indicators of body fat mass, plasma AGRP, and leptin levels in 16 patients with schizophrenia treated with ziprasidone and 21 patients with schizophrenia treated with olanzapine. Measurements by enzyme-linked immunosorbent assay were obtained before treatment (T0), after 4 weeks (T1), and after 3 months (T2) of treatment.

RESULTS

Whereas body mass index and leptin levels increased in patients treated with olanzapine compared to patients treated with ziprasidone, plasma AGRP levels did not differ among the treatment groups and did not change over time. Associations between AGRP and fat mass as well as appetite were disrupted in the olanzapine-treated patients but not in the ziprasidone group.

CONCLUSION

Future studies are needed to test whether the lack of a decrease in AGRP levels during weight gain in patients treated with olanzapine could perpetuate adverse metabolic long-term effects.

Agouti-related protein in patients with acute and weight-restored anorexia nervosa

BACKGROUND

An imbalance in appetite-regulating neuropeptides of the central nervous system has been associated with anorexia nervosa (AN), but the mechanisms of action are poorly understood. Agouti-related protein (AGRP), an orexigenic mediator of the hypothalamus, increases food intake and decreases energy expenditure in times of negative energy balance. The aim of the present study was to investigate AGRP in acute and fully weight-restored patients with AN, as well as during weight gain.

METHOD

Plasma AGRP and leptin levels were assessed using an enzyme-linked immunosorbent assay kit in a total of 175 female participants, including 75 patients with acute AN, 37 weight-restored AN patients and 63 healthy controls. Of the patients with acute AN, 33 were reassessed after partial weight gain.

RESULTS

In weight-restored AN patients plasma AGRP levels were similar to those in healthy controls, whereas in patients with acute AN, AGRP was elevated. AGRP was inversely correlated with indicators of undernutrition such as body mass index and plasma leptin. In addition, AGRP levels normalized during weight gain of longitudinally assessed AN patients.

CONCLUSIONS

Our results underline the significance of undernutrition and hypoleptinemia for the interpretation of peripheral AGRP concentrations. This provides support for the hypothesis that abnormal AGRP plasma levels in AN patients reflect undernutrition, rather than disease-specific traits.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Hypothalamic proopiomelanocortin processing and the regulation of energy balance

Hypothalamic proopiomelanocortin (POMC) neurons play a key role in regulating energy balance and neuroendocrine function. Much attention has been focused on the regulation of POMC gene expression with less emphasis on regulated peptide processing. This is particularly important given the complexity of posttranslational POMC processing which is essential for the generation of biologically active MSH peptides. Mutations that impair POMC sorting and processing are associated with obesity in humans and in animals. Specifically, mutations in the POMC processing enzymes prohormone convertase 1/3 (PC1/3) and in carboxypeptidase E (CPE) and in the α-MSH degrading enzyme, PRCP, are associated with changes in energy balance. There is increasing evidence that POMC processing is regulated with respect to energy balance. Studies have implicated both the leptin and insulin signaling pathways in the regulation of POMC at various steps in the processing pathway. This article will review the role of hypothalamic POMC in regulating energy balance with a focus on POMC processing.

Treadmill training enhances rat agouti-related protein in plasma and reduces ghrelin levels in plasma and soleus muscle

Ghrelin and agouti-related protein (AgRP) are orexigenic peptides secreted from stomach mucosa and the arcuate nucleus of the hypothalamus, respectively. Both peptides affect feeding behavior and play a role in energy balance, glucose homeostasis, and adiposity. The purpose of the current study was to determine the effects of moderate-term (6 weeks) running regimen on resting levels of ghrelin, AgRP, adenosine triphosphate, and glycogen in soleus muscle as well as plasma concentrations of the orexigenic hormones. Eighteen adult Wistar male rats (12 weeks old, 235-255 g) were randomly assigned to training (n = 10) and control (n = 8) groups. The training group ran for 60 min/d, 5d/wk at 25 m/min and 0% grade for 6 weeks. Forty-eight hours after the last exercise session, rats were killed; and soleus muscle and plasma were collected and frozen in liquid nitrogen for later analysis. Results demonstrated that 6 weeks of treadmill exercise reduced ghrelin and increased AgRP levels in plasma. Trained rat soleus muscle had higher levels of glycogen but not adenosine triphosphate or AgRP compared with untrained controls. Data indicate that training lowers ghrelin levels in rat soleus and plasma, which is accompanied by higher plasma AgRP and soleus glycogen content.

Insulin directly regulates NPY and AgRP gene expression via the MAPK MEK/ERK signal transduction pathway in mHypoE-46 hypothalamic neurons

Insulin plays a key role in the maintenance of nutrient homeostasis through central regulation of neuropeptides. Neuropeptide Y (NPY) and agouti-related peptide (AgRP) are vital orexigenic peptides that are regulated by insulin, although the processes utilized are unknown. Using a hypothalamic, clonal cell line, mHypoE-46, which endogenously expresses NPY, AgRP and the insulin receptor, we studied the mechanisms involved in the regulation of the NPY/AgRP neuron by insulin. We determined that insulin has direct actions on the neurons and acts to repress NPY/AgRP gene expression through a MAPK MEK/ERK-dependent pathway. Transient transfection analysis determined that human NPY and AgRP 5' flanking gene regions were not regulated by insulin in the mouse cell line, while sequence comparison analysis indicated only a 50% sequence similarity between human and mouse NPY and AgRP 5' flanking regions. These experiments indicate that insulin acts directly on specific hypothalamic neurons to regulate neuropeptide transcription.

Fasting enhances the response of arcuate neuropeptide Y-glucose-inhibited neurons to decreased extracellular glucose

Fasting increases neuropeptide Y (NPY) expression, peptide levels, and the excitability of NPY-expressing neurons in the hypothalamic arcuate (ARC) nucleus. A subpopulation of ARC-NPY neurons ( approximately 40%) are glucose-inhibited (GI)-type glucose-sensing neurons. Hence, they depolarize in response to decreased glucose. Because fasting enhances NPY neurotransmission, we propose that during fasting, GI neurons depolarize in response to smaller decreases in glucose. This increased excitation in response to glucose decreases would increase NPY-GI neuronal excitability and enhance NPY neurotransmission. Using an in vitro hypothalamic explant system, we show that fasting enhances NPY release in response to decreased glucose concentration. By measuring relative changes in membrane potential using a membrane potential-sensitive dye, we demonstrate that during fasting, a smaller decrease in glucose depolarizes NPY-GI neurons. Furthermore, incubation in low (0.7 mM) glucose enhanced while leptin (10 nM) blocked depolarization of GI neurons in response to decreased glucose. Fasting, leptin, and glucose-induced changes in NPY-GI neuron glucose sensing were mediated by 5'-AMP-activated protein kinase (AMPK). We conclude that during energy sufficiency, leptin reduces the ability of NPY-GI neurons to sense decreased glucose. However, after a fast, decreased leptin and glucose activate AMPK in NPY-GI neurons. As a result, NPY-GI neurons become depolarized in response to smaller glucose fluctuations. Increased excitation of NPY-GI neurons enhances NPY release. NPY, in turn, shifts energy homeostasis toward increased food intake and decreased energy expenditure to restore energy balance.

Neuropeptide y receptor selective ligands in the treatment of obesity

Obesity is a serious public health problem throughout the world, affecting both developed societies and developing countries. The central nervous system has developed a meticulously interconnected circuitry in order to keep us fed and in an adequate nutritional state. One of these consequences is that an energy-dense environment favors the development of obesity. Neuropeptide Y (NPY) is one of the most abundant and widely distributed peptides in the central nervous system of both rodents and humans and has been implicated in a variety of physiological actions. Within the hypothalamus, NPY plays an essential role in the control of food intake and body weight. Centrally administered NPY causes robust increases in food intake and body weight and, with chronic administration, can eventually produce obesity. NPY activates a population of at least six G protein-coupled Y receptors. NPY analogs exhibit varying degrees of affinity and specificity for these Y receptors. There has been renewed speculation that ligands for Y receptors may be of benefit for the treatment of obesity. This review highlights the therapeutic potential of Y(1), Y(2), Y(4), and Y(5) receptor agonists and antagonists as additional intervention to treat human obesity.

Diet-induced obesity causes severe but reversible leptin resistance in arcuate melanocortin neurons

Despite high leptin levels, most obese humans and rodents lack responsiveness to its appetite-suppressing effects. We demonstrate that leptin modulates NPY/AgRP and alpha-MSH secretion from the ARH of lean mice. High-fat diet-induced obese (DIO) mice have normal ObRb levels and increased SOCS-3 levels, but leptin fails to modulate peptide secretion and any element of the leptin signaling cascade. Despite this leptin resistance, the melanocortin system downstream of the ARH in DIO mice is over-responsive to melanocortin agonists, probably due to upregulation of MC4R. Lastly, we show that by decreasing the fat content of the mouse's diet, leptin responsiveness of NPY/AgRP and POMC neurons recovered simultaneously, with mice regaining normal leptin sensitivity and glycemic control. These results highlight the physiological importance of leptin sensing in the melanocortin circuits and show that their loss of leptin sensing likely contributes to the pathology of leptin resistance.

Enterostatin inhibition of dietary fat intake is modulated through the melanocortin system

Enterostatin injected into the amygdala selectively reduces dietary fat intake by an action that involves a serotonergic component in the paraventricular nucleus. We have investigated the role of melanocortin signaling in the response to enterostatin by studies in melanocortin 4 receptor (MC4R) knock out mice and by the use of the MC4R and MC3R antagonist SHU9119, and by neurochemical phenotyping of enterostatin activated cells. We also determined the effect of enterostatin in vivo on the expression of AgRP in the hypothalamus and amygdala of rats and in culture on a GT1-7 neuronal cell line. Enterostatin had no effect on food intake in MC4R knock out mice. SHU9119 i.c.v. blocked the feeding response to amygdala enterostatin in rats. Amygdala enterostatin induced fos activation in alpha-melanocyte stimulating hormone (alpha-MSH) neurons in the arcuate nucleus. Enterostatin also reduced the expression of AgRP in the hypothalamus and amygdala and in GT1-7 cells. These data suggest enterostatin inhibits dietary fat intake through a melanocortin signaling pathway.

Role of Agouti-related protein in adrenal steroidogenesis

The levels of Agouti-related protein (AgRP) mRNA in the adrenal are second only to those in the hypothalamus, raising questions regarding its target binding sites and its specific role in adrenal steroidogenesis. We and others demonstrated the presence of a population of melanocortin receptor-4 (MC4R) positively coupled to steroidogenesis in adrenal cells. Moreover, AgRP inhibited both the acute and long-term steroidogenic responses of these cells to NDP-alphaMSH through its antagonistic properties towards MC4R. Although AgRP had no antagonistic properties towards the MC2R and did not modify the acute steroidogenic response to ACTH, it exerted a biphasic sustained inhibitory effect on the long-term response to ACTH through an undefined alternate mechanism. Since adrenal cells release a relatively large amount of AgRP, this protein likely exerts a local paracrine/autocrine control on adrenal steroidogenesis.

Agouti-related protein is posttranslationally cleaved by proprotein convertase 1 to generate agouti-related protein (AGRP)83-132: interaction between AGRP83-132 and melanocortin receptors cannot be influenced by syndecan-3

Agouti-related protein (AGRP) plays a key role in energy homeostasis. The carboxyl-terminal domain of AGRP acts as an endogenous antagonist of the melanocortin-4 receptor (MC4-R). It has been suggested that the amino-terminal domain of AGRP binds to syndecan-3, thereby modulating the effects of carboxyl-terminal AGRP at the MC4-R. This model assumes that AGRP is secreted as a full-length peptide. In this study we found that AGRP is processed intracellularly after Arg(79)-Glu(80)-Pro(81)-Arg(82). The processing site suggests cleavage by proprotein convertases (PCs). RNA interference and overexpression experiments showed that PC1/3 is primarily responsible for cleavage in vitro, although both PC2 and PC5/6A can also process AGRP. Dual in situ hybridization demonstrated that PC1/3 is expressed in AGRP neurons in the rat hypothalamus. Moreover, hypothalamic extracts from PC1-null mice contained 3.3-fold more unprocessed full-length AGRP, compared with wild-type mice, based on combined HPLC and RIA analysis, demonstrating that PC1/3 plays a role in AGRP cleavage in vivo. We also found that AGRP(83-132) is more potent an antagonist than full-length AGRP, based on cAMP reporter assays, suggesting that posttranslational cleavage is required to potentiate the effect of AGRP at the MC4-R. Because AGRP is cleaved into distinct amino-terminal and carboxyl-terminal peptides, we tested whether amino-terminal peptides modulate food intake. However, intracerebroventricular injection of rat AGRP(25-47) and AGRP(50-80) had no effect on body weight, food intake, or core body temperature. Because AGRP is cleaved before secretion, syndecan-3 must influence food intake independently of the MC4-R.

Selective tissue uptake of agouti-related protein(82-131) and its modulation by fasting

The blood concentration of agouti-related protein (AgRP), a protein related to hyperphagia and obesity, is increased in obese human and fasted lean subjects. Because there is no saturable transport system at the blood-brain barrier for circulating AgRP to reach its central nervous system target, uptake of AgRP by peripheral organs might be physiologically meaningful. Using the biologically active fragment AgRP(82-131), we determined the pharmacokinetics of its radioactively labeled tracer after iv bolus injection and compared it with that of the vascular marker albumin. AgRP enters peripheral organs at different influx rates, all of which were higher than into brain and spinal cord. At 10 min after iv injection, the radioactivity recovered in the liver, which had the fastest influx rate for AgRP, represented intact (125)I-AgRP. The adrenal gland had a moderately fast uptake (but the highest initial volume of distribution), followed by the heart, lungs, and skeletal muscle. By comparison, epididymal fat, testis, and pancreas had low permeability to AgRP. Saturation of influx was determined by coadministration of excess unlabeled AgRP and was shown to be present in the liver and adrenal gland. The influx rate and initial volume of distribution did not show a linear correlation with vascular permeability or regional blood flow. AgRP uptake by the liver and epididymal fat was significantly increased by overnight fasting, whereas that by the adrenal gland was significantly decreased in fasted mice. Thus, the differential uptake of AgRP by peripheral organs could be a regulated process that is modulated by food deprivation.

Two ethnic-specific polymorphisms in the human Agouti-related protein gene are associated with macronutrient intake

BACKGROUND

The Agouti-related protein (AGRP), an appetite modulator, induces hyperphagia when administered intracerebroventricularly or when overexpressed in transgenic mice. Exogenous administration of AGRP in rodents predisposes to high fat and high sugar intakes.

OBJECTIVE

The objective was to examine the potential associations of 2 ethnic-specific polymorphisms in the AGRP gene (Ala67Thr in whites and -38C>T in blacks) in the Health, Risk Factors, Exercise Training, and Genetics (HERITAGE) Family Study.

DESIGN

We examined the effect of the 2 polymorphisms in the AGRP gene on self-reported macronutrient intakes in 478 white and 272 black participants in the HERITAGE Family Study.

RESULTS

Both AGRP polymorphisms showed a significant association with energy intake. In whites, a smaller proportion of total energy was derived from fat by the Ala67Thr heterozygotes (mean +/- SEM: 29.4 +/- 0.7%) than by the Ala67Ala homozygotes (31.5 +/- 0.5%; P = 0.009), mainly because of a lower intake of saturated (P = 0.06) and monounsaturated (P = 0.01) fats by the Ala67Thr heterozygotes. The percentage of energy from carbohydrates was 2.6% greater in the Ala67Thr heterozygotes (55.1 +/- 1.1%) than in the Ala67Ala homozygotes (52.5 +/- 0.6%; P = 0.03). In blacks, protein intake was associated with the -38C>T promoter polymorphism. T/T homozygotes had a significantly lower protein intake than did the C-allele carriers (C/C: 16.8 +/- 0.4%; C/T: 17.2 +/- 0.2%; T/T: 15.4 +/- 0.7%; P = 0.04). No significant differences in total energy and alcohol intakes existed between genotype groups in blacks or whites.

CONCLUSIONS

The present study suggests that 2 ethnic-specific AGRP variants, previously shown to be associated with leanness in the HERITAGE Family Study, are also associated with macronutrient intake.

Hypothalamic cocaine- and amphetamine-regulated transcript (CART) and agouti-related protein (AgRP) neurons coexpress the NOP1 receptor and nociceptin alters CART and AgRP release

Nociceptin or orphanin FQ (N/OFQ) and its receptor NOP1 are expressed in hypothalamic nuclei involved in energy homeostasis. N/OFQ administered by intracerebroventricular or arcuate nucleus (ARC) injection increases food intake in satiated rats. The mechanisms by which N/OFQ increases food intake are unknown. We hypothesized that N/OFQ may regulate hypothalamic neurons containing peptides involved in the control of food intake such as cocaine- and amphetamine-regulated transcript (CART), alphaMSH, neuropeptide Y (NPY), and agouti-related protein (AgRP). We investigated the ability of N/OFQ to alter the release of CART, alphaMSH, NPY, and AgRP using ex vivo medial basal hypothalamic explants. Incubation of hypothalamic explants with N/OFQ (1, 10, 100 nM) resulted in significant changes in CART and AgRP release. One hundred nanomoles N/OFQ caused a 33% decrease in release of CART (55-102) immunoreactivity (IR) and increased release of AgRP-IR to 163% but produced no change in either alphaMSH-IR or NPY-IR. Double immunocytochemistry/in situ hybridization demonstrated that CART-IR and NOP1 mRNA are colocalized throughout the hypothalamus, in particular in the paraventricular nucleus, lateral hypothalamus, zona incerta, and ARC, providing an anatomical basis for N/OFQ action on CART release. Dual in situ hybridization demonstrated that AgRP neurons in the ARC also express the NOP1 receptor. Our data suggest that nociceptin via the NOP1 receptor may increase food intake by decreasing the release of the anorectic peptide CART and increasing the release of the orexigenic peptide AgRP.

Functional analysis of the Ala67Thr polymorphism in agouti related protein associated with anorexia nervosa and leanness

AgRP is a neuropeptide that stimulates food intake through inhibition of central melanocortin receptors (MCRs). In humans, the non-conservative amino acid substitution Alanine (Ala) 67 Threonine (Thr) has been associated with Anorexia Nervosa and with leanness. In the present study, the cellular distribution, processing and in vitro and in vivo activities of Ala67 and Thr67 AgRP were investigated. Western blots of media and lysates of BHK cells stably transfected with Ala67 or Thr67 expression constructs showed identical AgRP bands. Both Ala67 and Thr67 AgRP colocalised with the Golgi apparatus, but not with the ER or lysosomes when expressed in Att20 D16V cells. Also, no differences were observed between the potencies of bacterially expressed Ala67 and Thr67 AgRP to stimulate MC4R in a reporter gene assay or inhibit food intake in rats. Taken together, no evidence was found for a functional defect of Thr67 AgRP related to MC4R interactions.

Effects of fasting, leptin, and insulin on AGRP and POMC peptide release in the hypothalamus

Agouti-related protein (AGRP) and proopiomelanocortin (POMC) have opposing effects on melanocortin receptor (MC-R) signaling and energy balance, and are important targets for leptin and insulin in the hypothalamus. While food intake and leptin have documented effects on POMC and AGRP gene expression, and insulin has effects on POMC gene expression, little is known about their effects on POMC or AGRP peptide release. Here we have examined the effects of fasting, leptin, and insulin on the release of AGRP and the POMC-derived peptide gamma(3)-MSH from the perifused rat hypothalamus in vitro. In the first experiment, fasting (48 h) resulted in a significant overall decrease in gamma(3)-MSH release measured every 20 min during a 3-h baseline perifusion period and after depolarization with 56 mM KCl (p = 0.02); there was a trend towards an overall increase in the release of AGRP but this was not significant. When the ratio of gamma(3)-MSH/AGRP release was calculated at each time point, there was an overall decrease in gamma(3)-MSH/AGRP with fasting (p < 0.01). Further examination of the ratio of gamma(3)-MSH/AGRP revealed a 34% reduction (p < 0.05) in the basal area under the curve (AUC) and a 33% reduction (p < 0.01) in the post-KCl stimulated AUC in fasted vs. fed animals. In the second experiment, perifusion of hypothalamic slices with 10(-8) or 10(-7) M leptin for 2 h resulted in a significant decrease in the release of AGRP noted primarily after depolarization with KCl (p < 0.01); no effect was seen on gamma(3)-MSH release. Similarly, in a third experiment, perifusion with 10(-7) M insulin caused a significant decrease in AGRP release (p < 0.001) without affecting gamma(3)-MSH release. Thus, there is a significant decrease in gamma(3)-MSH and the ratio of gamma(3)-MSH to AGRP released during fasting, consistent with a net inhibition of hypothalamic MC-R signaling. In contrast, short-term treatment with leptin and insulin may inhibit MC-R signaling primarily by decreasing the release of AGRP.

Expression of agouti-related protein (AgRP) in the hypothalamus and adrenal gland of the duck ( Anas platyrhynchos)

The presence and distribution of agouti-related protein (AgRP) immunoreactivity were investigated in the hypothalamus and adrenal gland of the duck using immunohistochemistry and Western blot analysis. Expression of AgRP mRNA was also studied using reverse transcriptase polymerase chain reaction (RT-PCR). A partial coding sequence (cds) of the duck AgRP gene was identified. Western blot analysis showed the presence of an AgRP-like peptide having a molecular weight consistent with the number of predicted amino acids of the avian AgRP. In the hypothalamus, AgRP immunoreactivity was found in neurons of the nucleus infundibularis and in fibers projecting to the median eminence. In the adrenals, AgRP immunoreactivity was observed in medullary cells. These findings suggest that in the duck, AgRP may play a role in regulating energy homeostasis and adrenal endocrine functions.

Agouti-related protein has an inhibitory paracrine role in the rat adrenal gland

alpha-Melanocyte-stimulating-hormone (alpha-MSH) is an agonist at the melanocortin 3 receptor (MC3-R) and melanocortin 4 receptor (MC4-R). alpha-MSH stimulates corticosterone release from rat adrenal glomerulosa cells in vitro. Agouti-related protein (AgRP) an endogenous antagonist at the MC3-R and MC4-R, is expressed in the adrenal gland. We investigated the expression of the MC3-R and MC4-R and the role of AgRP in the adrenal gland. MC3-R and MC4-R expression was detected in rat adrenal gland using RT-PCR. The effect of AgRP on alpha-MSH-induced corticosterone release was investigated using dispersed rat adrenal glomerulosa cells. AgRP administered alone did not affect corticosterone release, but co-administration of AgRP and alpha-MSH attenuated alpha-MSH-induced corticosterone release. To investigate glucocorticoid feedback, adrenal AgRP expression was compared in rats treated with dexamethasone to controls. AgRP mRNA was increased in rats treated with dexamethasone treatment compared to controls. Our findings demonstrate that adrenal AgRP mRNA is regulated by glucocorticoids. AgRP acting via the MC3-R or MC4-R may have an inhibitory paracrine role, blocking alpha-MSH-induced corticosterone secretion.

The role of the melanocortin system and the melanocortin-4 receptor in ring dove (Streptopelia risoria) feeding behavior

The melanocortin-4 receptor (MC4-R) is an important mediator of the effects of two melanocortin system ligands, alpha melanocyte stimulating hormone (alpha-MSH) and agouti-related peptide (AGRP), on feeding behavior and energy balance in mammals. Although an avian homologue of the mammalian MC4-R has recently been identified, there is little information on the role of this receptor and the melanocortin system in avian feeding and body weight regulation. In these studies, we measured changes in feeding behavior in ring doves (Streptopelia risoria) following intracerebroventricular (i.c.v.) injection of various melanocortin receptor agonists and antagonists. The selective MC4-R antagonist HS014 elevated food intake within 4 h at all three doses tested (0.02, 0.2, and 2 nmol). A 1 nmol dose of the endogenous antagonist AGRP also stimulated feeding but only after a post-injection interval of 10 h. Surprisingly, the MC3-R and MC4-R antagonist SHU9119 not only failed to stimulate food intake at the same doses as HS014, but actually inhibited food intake at 8 h after injection. Whether this was due to toxicity effects or differences in the pharmacology of avian and mammalian melanocortin receptors remains to be determined. Food-deprived doves showed a fourfold increase in the number of AGRP-immunoreactive cells in the tuberal region of the hypothalamus and 5 ng of the MC3-R and MC4-R agonist MTII significantly attenuated the amount of food consumed by food-deprived birds that were allowed to re-feed. These data support a role for the melanocortin system and the melanocortin-4 receptor in the ring dove feeding behavior.

Plasma agouti-related protein level: a possible correlation with fasted and fed states in humans and rats

We measured plasma concentrations of agouti-related protein (AGRP) in humans and rats and determined whether these were affected by ingestion of a meal after fasting. In 17 healthy human subjects, the mean plasma concentration of AGRP was lower in the fed state than in the fasted state. Two hours after a breakfast meal, AGRP levels dropped by 39%. By contrast, a continued fast for 2 h increased the average AGRP concentration by 73%. In rats with diet-induced obesity, refeeding resulted in a 50% decrease in plasma AGRP concentrations following a fasting-refeeding protocol. Our results support the notion that plasma AGRP may serve as a biomarker for the transition from a fasted to the satiated state.

Hyperleptinemia in A(y)/a mice upregulates arcuate cocaine- and amphetamine-regulated transcript expression

The effects of leptin on cocaine- and amphetamine-regulated transcript (CART) and agouti-related protein (AGRP) expression in the hypothalamic arcuate nucleus of obese A(y)/a mice were investigated. CART mRNA expression was upregulated by 41% and AGRP mRNA downregulated by 78% in hyperleptinemic A(y)/a mice relative to levels in lean a/a mice. The mRNA expression of these neuropeptides in either young nonobese A(y)/a mice or rats treated with SHU-9119, a synthetic melanocortin-4 receptor (MC4R) antagonist, did not differ significantly from that in the corresponding controls. After a 72-h fast, which decreased the concentration of serum leptin, CART and AGRP mRNA expression decreased and increased, respectively, in A(y)/a mice. The expression levels of these neuropeptides in leptin-deficient A(y)/a ob/ob double mutants were comparable to those in a/a ob/ob mice. Leptin thus modulates both CART and AGRP mRNA expression in obese A(y)/a mice, whereas leptin signals are blocked at the MCR4R level. Taken together, the present findings indicate that differential expression of these neuropeptides in A(y)/a and ob/ob mice results in dissimilar progression toward obesity.

PRL-releasing peptide inhibits food intake in male rats via the dorsomedial hypothalamic nucleus and not the paraventricular hypothalamic nucleus

PRL-releasing peptide inhibits food intake after intracerebroventricular injection. PRL-releasing peptide immunoreactivity is found in several hypothalamic nuclei involved in feeding, with highest levels in the paraventricular and dorsomedial hypothalamic nuclei. The aim of this study was to examine the effect of PRL-releasing peptide on food intake after administration into these nuclei. Paraventricular nucleus injection of PRL-releasing peptide did not alter food intake. Dorsomedial hypothalamic nucleus injection of PRL-releasing peptide decreased 1 h food intake [PRL-releasing peptide (1 nmol) 83.4 +/- 6.1% saline all; P < 0.05]; and continued until 8 h postinjection [PRL-releasing peptide (1 nmol) 89.2 +/- 4.1% saline; P < 0.05]. To investigate the mechanism of this inhibition of food intake, we examined PRL-releasing peptide's effect on neuropeptide release from hypothalamic explants. alpha MSH release was increased [PRL-releasing peptide (100 nmol), 5.4 +/- 1.6 pmol/explant; change vs. basal, P < 0.01], whereas agouti-related protein release was unchanged. The release of cocaine- and amphetamine-regulated transcript was inhibited [PRL-releasing peptide (100 nmol), -33.5 +/- 12.6 pmol/explant; change vs. basal, P < 0.01]. PRL-releasing peptide dose-dependently increased neurotensin release [PRL-releasing peptide (1 nmol), 3.7 +/- 2.6 pmol/explant; change vs. basal, P = NS; PRL-releasing peptide (10 nmol), 7.2 +/- 2.7 pmol/explant; change vs. basal, P < 0.01; PRL-releasing peptide (100 nmol), 36.8 +/- 5.4 pmol/explant; change vs. basal, P < 0.001]. Our data suggest that the dorsomedial hypothalamic nucleus is important in the inhibitory effect of PRL-releasing peptide on food intake and that PRL-releasing peptide alters the release of several hypothalamic neuropeptides important in the control of food intake.

A role for the Agouti-Related Protein promoter in obesity and type 2 diabetes

The murine Agouti-Related Protein (mAGRP) is upregulated in obese and diabetic mice and can stimulate hyperphagia when overexpressed in transgenic models. Here we report upstream nucleotide sequences of the human hAGRP gene with putative recognition sites for transcription factors including a site for the STAT transactivators. A polymorphism (-38C-->T) was identified in the promoter region and the C/C genotype had significantly higher promoter activity and affinity for transcription factors as tested in periphery- and hypothalamus-derived cell lines. The polymorphic site could affect the expression levels of hAGRP and the high expressing C/C genotype was significantly associated with high BMI and type 2 diabetes in Africans.

Opioid receptor involvement in the effect of AgRP- (83-132) on food intake and food selection

Agouti-related peptide (AgRP) is a receptor antagonist of central nervous system (CNS) melanocortin receptors and appears to have an important role in the control of food intake since exogenous CNS administration in rats and overexpression in mice result in profound hyperphagia and weight gain. Given that AgRP is heavily colocalized with neuropeptide Y (NPY) and that orexigenic effects of NPY depend on activity at opioid receptors, we hypothesized that AgRP's food-intake effects are also mediated by opioid receptors. Subthreshold doses of the opioid receptor antagonist naloxone blocked AgRP-induced intake when given simultaneously but not 24 h after AgRP injection. Opioids not only influence food intake but food selection as well. Hence, we tested AgRP's effect to alter food choice between matched diets with differing dietary fat content. AgRP selectively enhanced intake of the high-fat but not the low-fat diet. Additionally, AgRP selectively increased chow intake in rats given ad libitum access to a 20% sucrose solution and standard rat chow. The current results indicate that AgRP influences not only caloric intake but food selection as well and that the early effects of AgRP depend critically on an interaction with opioid receptors.