Neurochemical characterization of body weight-regulating leptin receptor neurons in the nucleus of the solitary tract

5-HT obesity medication efficacy via POMC activation is maintained during aging

The phenomenon commonly described as the middle-age spread is the result of elevated adiposity accumulation throughout adulthood until late middle-age. It is a clinical imperative to gain a greater understanding of the underpinnings of age-dependent obesity and, in turn, how these mechanisms may impact the efficacy of obesity treatments. In particular, both obesity and aging are associated with rewiring of a principal brain pathway modulating energy homeostasis, promoting reduced activity of satiety pro-opiomelanocortin (POMC) neurons within the arcuate nucleus of the hypothalamus (ARC). Using a selective ARC-deficient POMC mouse line, here we report that former obesity medications augmenting endogenous 5-hydroxytryptamine (5-HT) activity d-fenfluramine and sibutramine require ARC POMC neurons to elicit therapeutic appetite-suppressive effects. We next investigated whether age-related diminished ARC POMC activity therefore impacts the potency of 5-HT obesity pharmacotherapies, lorcaserin, d-fenfluramine, and sibutramine and report that all compounds reduced food intake to a comparable extent in both chow-fed young lean (3-5 months old) and middle-aged obese (12-14 months old) male and female mice. We provide a mechanism through which 5-HT anorectic potency is maintained with age, via preserved 5-HT-POMC appetitive anatomical machinery. Specifically, the abundance and signaling of the primary 5-HT receptor influencing appetite via POMC activation, the 5-HT2CR, is not perturbed with age. These data reveal that although 5-HT obesity medications require ARC POMC neurons to achieve appetitive effects, the anorectic efficacy is maintained with aging, findings of clinical significance to the global aging obese population.

Nesfatin-1 increases energy expenditure and reduces food intake in rats

OBJECTIVE

Energy homeostasis results from a balance of food intake and energy expenditure, accomplished by the interaction of peripheral and central nervous signals. The recently discovered adipokine nesfatin-1 is involved in the central control of food intake, but whether it also participates in the regulation of thermogenesis is unknown.

METHODS

Nesfatin-1 was administered intracerebroventricularly to freely moving, male Wistar rats and direct calorimetry was performed to assess its effects on thermogenesis. Furthermore, food intake was measured and hypothalamic and N. tractus solitarius (NTS) neuropeptide expression was determined by quantitative real-time polymerace chain reaction. Leptin, which is involved in both the regulation of food intake and thermogenesis, was used as positive control.

RESULTS

For the first time it was shown that central nervous administration of nesfatin-1 profoundly increases thermogenesis in rats to a similar extent as leptin and the role of both peptides in the control of food intake was confirmed. Nesfatin-1 significantly downregulated neuropeptide Y (NPY) mRNA expression in both hypothalamus and NTS.

CONCLUSIONS

The results strongly support the prominent role of nesfatin-1 for both energy expenditure and food intake and NPY neurons appear to be involved in this effect.

Melatonin in the regulation of liver steatosis following prenatal glucocorticoid exposure

Nonalcoholic fatty liver disease patients are characterized by hepatic steatosis. Prenatal glucocorticoid overexposure can result in steatosis. In this study, we aimed to determine the mechanism and cellular apoptosis of prenatal glucocorticoid overexposure in rats and whether melatonin can rescue the prenatal glucocorticoid-induced steatosis and apoptosis in neonatal rats. Pregnant Sprague-Dawley rats at gestational days 14 to 21 were administered dexamethasone. Acute effects of prenatal programming liver were assessed at postnatal day 7. The expression of proteins involved in the apoptotic and methylation pathways was analyzed by RT-PCR and Western blotting. Apoptosis and steatosis were examined by histology staining. The liver steatosis and apoptosis were increased in prenatal glucocorticoid group more than in control group and decreased in melatonin group. The expression of leptin decreased in prenatal glucocorticoid and increased in melatonin group by liver RT-PCR and Western blot study. Caspase 3, TNF-α proteins expression, and TUNEL stains increased in prenatal glucocorticoid compared with control and decreased in melatonin group. The liver histone deacetylase, DNA methyltransferase activity, and DNA methylation were increased in prenatal glucocorticoid and decreased in melatonin group. The present study showed that the prenatal glucocorticoid induced programming liver steatosis at day 7 after delivery, possibly via altered leptin expression. Melatonin can reverse the methylation of leptin and decreased liver steatosis.

Evidence that hypothalamic RFamide related peptide-3 neurones are not leptin-responsive in mice and rats

Leptin, a permissive hormonal regulator of fertility, provides information about the body's energy reserves to the hypothalamic gonadotrophin-releasing hormone (GnRH) neuronal system that drives reproduction. Leptin does not directly act on GnRH neurones, and the neuronal pathways that it uses remain unclear. RFamide-related peptide-3 (RFRP-3) neurones project to GnRH neurones and primarily inhibit their activity. We tested whether leptin could act via RFRP-3 neurones to potentially modulate GnRH activity. First, the effects of leptin deficiency or high-fat diet-induced obesity on RFRP-3 cell numbers and gene expression were assessed in male and female mice. There was no significant difference in Rfrp mRNA levels or RFRP-3-immunoreactive cell counts in wild-type versus leptin-deficient ob/ob animals, or in low-fat versus high-fat diet fed wild-type mice. Second, the presence of leptin-induced signalling in RFRP-3 neurones was examined in male and female wild-type mice and rats. Dual label immunohistochemistry revealed leptin-induced phosphorylated signal transducer and activator of transcription-3 in close proximity to RFRP-3 neurones, although there was very little (2-13%) colocalisation and no significant differences between vehicle and leptin-treated animals. Furthermore, we were unable to detect leptin receptor mRNA in a semi-purified RFRP-3 cell preparation. Because GABA neurones form critical leptin-responsive GnRH inputs, we also determined whether RFRP-3 and GABA cells were colocalised. No such colocalisation was detected. These results support the concept that leptin has little or no effects on RFRP-3 neurones, and that these neurones are unlikely to be an important neuronal pathway for the metabolic regulation of fertility by leptin.

Leptin signaling in the medial nucleus tractus solitarius reduces food seeking and willingness to work for food

The adipose-derived hormone leptin signals in the medial nucleus tractus solitarius (mNTS) to suppress food intake, in part, by amplifying within-meal gastrointestinal (GI) satiation signals. Here we show that mNTS leptin receptor (LepRb) signaling also reduces appetitive and motivational aspects of feeding, and that these effects can depend on energy status. Using the lowest dose that significantly suppressed 3-h cumulative food intake, unilateral leptin (0.3 μg) administration to the mNTS (3 h before testing) reduced operant lever pressing for sucrose under increasing work demands (progressive ratio reinforcement schedule) regardless of whether animals were energy deplete (food restricted) or replete (ad libitum fed). However, in a separate test of food-motivated responding in which there was no opportunity to consume food (conditioned place preference (CPP) for an environment previously associated with a palatable food reward), mNTS leptin administration suppressed food-seeking behavior only in chronically food-restricted rats. On the other hand, mNTS LepRb signaling did not reduce CPP expression for morphine reinforcement regardless of energy status, suggesting that mNTS leptin signaling differentially influences motivated responding for food vs opioid reward. Overall results show that mNTS LepRb signaling reduces food intake and appetitive food-motivated responding independent of energy status in situations involving orosensory and postingestive contact with food, whereas food-seeking behavior independent of food consumption is only reduced by mNTS LepRb activation in a state of energy deficit. These findings reveal a novel appetitive role for LepRb signaling in the mNTS, a brain region traditionally linked with processing of meal-related GI satiation signals.

Effects of metformin in children and adolescents with Prader-Willi syndrome and early-onset morbid obesity: a pilot study

Prader-Willi syndrome (PWS) is one of the most commonly recognized causes of early-onset childhood obesity. Individuals with PWS have significant hyperphagia and decreased recognition of satiety. The exact etiology of the hyperphagia remains unknown and, therefore, untreatable. We conducted a pilot, open-label study of response to metformin in 21 children with PWS and six with early morbid obesity (EMO). Participants had significant insulin resistance and glucose intolerance on oral glucose tolerance testing (OGTT) and were started on metformin for these biochemical findings. We administered the Hyperphagia Questionnaire to parents of patients before and after starting metformin treatment. Both the PWS and EMO groups showed significant improvements in food-related distress, anxiety, and ability to be redirected away from food on the Hyperphagia Questionnaire. In the PWS group, improvements were predominantly seen in females. Within the PWS group, responders to metformin had higher 2-h glucose levels on OGTT (7.48 mmol/L vs. 4.235 mmol/L; p=0.003) and higher fasting insulin levels (116 pmol/L vs. 53.5 pmol/L; p=0.04). Additionally, parents of 5/13 individuals with PWS and 5/6 with EMO reported that their child was able to feel full while on metformin (for many this was the first time they had ever described a feeling of fullness). Metformin may improve sense of satiety and decrease anxiety about food in some individuals with PWS and EMO. Positive response to metformin may depend on the degree of hyperinsulinism and glucose intolerance. Nonetheless, the results of this pilot study bear further investigation.

Leptin signaling in GABA neurons, but not glutamate neurons, is required for reproductive function

The adipocyte-derived hormone leptin acts in the brain to modulate the central driver of fertility: the gonadotropin releasing hormone (GnRH) neuronal system. This effect is indirect, as GnRH neurons do not express leptin receptors (LEPRs). Here we test whether GABAergic or glutamatergic neurons provide the intermediate pathway between the site of leptin action and the GnRH neurons. Leptin receptors were deleted from GABA and glutamate neurons using Cre-Lox transgenics, and the downstream effects on puberty onset and reproduction were examined. Both mouse lines displayed the expected increase in body weight and region-specific loss of leptin signaling in the hypothalamus. The GABA neuron-specific LEPR knock-out females and males showed significantly delayed puberty onset. Adult fertility observations revealed that these knock-out animals have decreased fecundity. In contrast, glutamate neuron-specific LEPR knock-out mice displayed normal fertility. Assessment of the estrogenic hypothalamic-pituitary-gonadal axis regulation in females showed that leptin action on GABA neurons is not necessary for estradiol-mediated suppression of tonic luteinizing hormone secretion (an indirect measure of GnRH neuron activity) but is required for regulation of a full preovulatory-like luteinizing hormone surge. In conclusion, leptin signaling in GABAergic (but not glutamatergic neurons) plays a critical role in the timing of puberty onset and is involved in fertility regulation throughout adulthood in both sexes. These results form an important step in explaining the role of central leptin signaling in the reproductive system. Limiting the leptin-to-GnRH mediators to GABAergic cells will enable future research to focus on a few specific types of neurons.

Role of oxytocin signaling in the regulation of body weight

Obesity and its associated metabolic disorders are growing health concerns in the US and worldwide. In the US alone, more than two-thirds of the adult population is classified as either overweight or obese [1], highlighting the need to develop new, effective treatments for these conditions. Whereas the hormone oxytocin is well known for its peripheral effects on uterine contraction during parturition and milk ejection during lactation, release of oxytocin from somatodendrites and axonal terminals within the central nervous system (CNS) is implicated in both the formation of prosocial behaviors and in the control of energy balance. Recent findings demonstrate that chronic administration of oxytocin reduces food intake and body weight in diet-induced obese (DIO) and genetically obese rodents with impaired or defective leptin signaling. Importantly, chronic systemic administration of oxytocin out to 6 weeks recapitulates the effects of central administration on body weight loss in DIO rodents at doses that do not result in the development of tolerance. Furthermore, these effects are coupled with induction of Fos (a marker of neuronal activation) in hindbrain areas (e.g. dorsal vagal complex (DVC)) linked to the control of meal size and forebrain areas (e.g. hypothalamus, amygdala) linked to the regulation of food intake and body weight. This review assesses the potential central and peripheral targets by which oxytocin may inhibit body weight gain, its regulation by anorexigenic and orexigenic signals, and its potential use as a therapy that can circumvent leptin resistance and reverse the behavioral and metabolic abnormalities associated with DIO and genetically obese models.

Selective leptin resistance revisited

In addition to effects on appetite and metabolism, leptin influences many neuroendocrine and physiological systems, including the sympathetic nervous system. Building on my Carl Ludwig Lecture of the American Physiological Society, I review the sympathetic and cardiovascular actions of leptin. The review focuses on a critical analysis of the concept of selective leptin resistance (SLR) and the role of leptin in the pathogenesis of obesity-induced hypertension in both experimental animals and humans. We introduced the concept of SLR in 2002 to explain how leptin might increase blood pressure (BP) in obese states, such as diet-induced obesity (DIO), that are accompanied by partial leptin resistance. This concept, analogous to selective insulin resistance in the metabolic syndrome, holds that in several genetic and acquired models of obesity, there is preservation of the renal sympathetic and pressor actions of leptin despite attenuation of the appetite and weight-reducing actions. Two potential overlapping mechanisms of SLR are reviewed: 1) differential leptin molecular signaling pathways that mediate selective as opposed to universal leptin action and 2) brain site-specific leptin action and resistance. Although the phenomenon of SLR in DIO has so far focused on preservation of sympathetic and BP actions of leptin, consideration should be given to the possibility that this concept may extend to preservation of other actions of leptin. Finally, I review perplexing data on the effects of leptin on sympathetic activity and BP in humans and its role in human obesity-induced hypertension.

Leptin at the intersection of neuroendocrinology and metabolism: current evidence and therapeutic perspectives

Since its discovery as an adipocyte-secreted hormone, leptin has been found to impact food intake, energy homeostasis, and metabolism through its effects on the central nervous system and peripheral organs. Recent research indicates that leptin may also be involved in cognition, immune function, and bone metabolism. These findings place leptin at the intersection of neuroendocrinology and metabolism, and possibly immune function, and render it an appealing therapeutic target for several niche areas of unmet clinical need. Current evidence regarding classic and emerging roles of leptin as well as the pros and cons of its potential clinical use are summarized herein.

Roles for central leptin receptors in the control of meal size

The adiposity hormone leptin has been implicated in the regulation of behavioral and metabolic controls of body weight. Leptin receptors are found in multiple peripheral and central tissues, particularly within hypothalamic and brainstem neuronal populations. Central leptinergic signaling acts as an indirect control to modulate the feeding inhibitory potency of the direct controls of meal size. Mouse models of neuronal leptin loss and gain of function have helped to identify and characterize how central leptin contributes to the central control of food intake.

Satiation and stress-induced hypophagia: examining the role of hindbrain neurons expressing prolactin-releasing Peptide or glucagon-like Peptide 1

Neural circuits distributed within the brainstem, hypothalamus, and limbic forebrain interact to control food intake and energy balance under normal day-to-day conditions, and in response to stressful conditions under which homeostasis is threatened. Experimental studies using rats and mice have generated a voluminous literature regarding the functional organization of circuits that inhibit food intake in response to satiety signals, and in response to stress. Although the central neural bases of satiation and stress-induced hypophagia often are studied and discussed as if they were distinct, we propose that both behavioral states are generated, at least in part, by recruitment of two separate but intermingled groups of caudal hindbrain neurons. One group comprises a subpopulation of noradrenergic (NA) neurons within the caudal nucleus of the solitary tract (cNST; A2 cell group) that is immunopositive for prolactin-releasing peptide (PrRP). The second group comprises non-adrenergic neurons within the cNST and nearby reticular formation that synthesize glucagon-like peptide 1 (GLP-1). Axonal projections from PrRP and GLP-1 neurons target distributed brainstem and forebrain regions that shape behavioral, autonomic, and endocrine responses to actual or anticipated homeostatic challenge, including the challenge of food intake. Evidence reviewed in this article supports the view that hindbrain PrRP and GLP-1 neurons contribute importantly to satiation and stress-induced hypophagia by modulating the activity of caudal brainstem circuits that control food intake. Hindbrain PrRP and GLP-1 neurons also engage hypothalamic and limbic forebrain networks that drive parallel behavioral and endocrine functions related to food intake and homeostatic challenge, and modulate conditioned and motivational aspects of food intake.

Altered hepatic lipid metabolism contributes to nonalcoholic fatty liver disease in leptin-deficient Ob/Ob mice

Nonalcoholic fatty liver disease (NAFLD) is strongly linked to obesity, insulin resistance, and abnormal hepatic lipid metabolism; however, the precise regulation of these processes remains poorly understood. Here we examined genes and proteins involved in hepatic oxidation and lipogenesis in 14-week-old leptin-deficient Ob/Ob mice, a commonly studied model of obesity and hepatic steatosis. Obese Ob/Ob mice had increased fasting glucose, insulin, and calculated HOMA-IR as compared with lean wild-type (WT) mice. Ob/Ob mice also had greater liver weights, hepatic triglyceride (TG) content, and markers of de novo lipogenesis, including increased hepatic gene expression and protein content of acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), and stearoyl-CoA desaturase-1 (SCD-1), as well as elevated gene expression of PPARγ and SREBP-1c compared with WT mice. While hepatic mRNA levels for PGC-1α, PPARα, and TFAM were elevated in Ob/Ob mice, measures of mitochondrial function (β-HAD activity and complete (to CO(2)) and total mitochondrial palmitate oxidation) and mitochondrial OXPHOS protein subunits I, III, and V content were significantly reduced compared with WT animals. In summary, reduced hepatic mitochondrial content and function and an upregulation in de novo lipogenesis contribute to obesity-associated NAFLD in the leptin-deficient Ob/Ob mouse.

Physiological Roles of GPR10 and PrRP Signaling

Prolactin-releasing peptide (PrRP) was first isolated from bovine hypothalamus, and was found to act as an endogenous ligand at the G-protein-coupled receptor 10 (GPR10 or hGR3). Although originally named as it can affect the secretion of prolactin from anterior pituitary cells, the potential functions for this peptide have been greatly expanded over the past decade. Anatomical, pharmacological, and physiological studies indicate that PrRP, signaling via the GPR10 receptor, may have a wide range of roles in neuroendocrinology; such as in energy homeostasis, stress responses, cardiovascular regulation, and circadian function. This review will provide the current knowledge of the PrRP and GPR10 signaling system, its putative functions, implications for therapy, and future perspectives.