Erk1/2 mediates leptin receptor signaling in the ventral tegmental area

Metabolic Hormone Action in the VTA: Reward-Directed Behavior and Mechanistic Insights

Dysfunctional signaling in midbrain reward circuits perpetuates diseases characterized by compulsive overconsumption of rewarding substances such as substance abuse, binge eating disorder, and obesity. Ventral tegmental area (VTA) dopaminergic activity serves as an index for how rewarding stimuli are perceived and triggers behaviors necessary to obtain future rewards. The evolutionary linking of reward with seeking and consuming palatable foods ensured an organism's survival, and hormone systems that regulate appetite concomitantly developed to regulate motivated behaviors. Today, these same mechanisms serve to regulate reward-directed behavior around food, drugs, alcohol, and social interactions. Understanding how hormonal regulation of VTA dopaminergic output alters motivated behaviors is essential to leveraging therapeutics that target these hormone systems to treat addiction and disordered eating. This review will outline our current understanding of the mechanisms underlying VTA action of the metabolic hormones ghrelin, glucagon-like peptide-1, amylin, leptin, and insulin to regulate behavior around food and drugs of abuse, highlighting commonalities and differences in how these five hormones ultimately modulate VTA dopamine signaling.

Leptin Promotes Striatal Dopamine Release via Cholinergic Interneurons and Regionally Distinct Signaling Pathways

Dopamine (DA) is a critical regulator of striatal network activity and is essential for motor activation and reward-associated behaviors. Previous work has shown that DA is influenced by the reward value of food, as well as by hormonal factors that reguate food intake and energy expenditure. Changes in striatal DA signaling also have been linked to aberrant eating patterns. Here we test the effect of leptin, an adipocyte-derived hormone involved in feeding and energy homeostasis regulation, on striatal DA release and uptake. Immunohistochemical evaluation identified leptin receptor (LepR) expression throughout mouse striatum, including on striatal cholinergic interneurons (ChIs) and their extensive processes. Using fast-scan cyclic voltammetry (FSCV), we found that leptin causes a concentration-dependent increase in evoked extra-cellular DA concentration ([DA]o) in dorsal striatum (dStr) and nucleus accumbens (NAc) core and shell in male mouse striatal slices, and also an increase in the rate of DA uptake. Further, we found that leptin increases ChI excitability, and that the enhancing effect of leptin on evoked [DA]o is lost when nicotinic acetylcholine (ACh) receptors are antagonized or when examined in striatal slices from mice lacking ACh synthesis. Evaluation of signaling pathways underlying leptin's action revealed a requirement for intracellular Ca2+, and the involvement of different downstream pathways in dStr and NAc core versus NAc shell. These results provide the first evidence for dynamic regulation of DA release and uptake by leptin within brain motor and reward pathways, and highlight the involvement of ChIs in this process.SIGNIFICANCE STATEMENT Given the importance of striatal dopamine (DA) in reward, motivation, motor behavior and food intake, identifying the actions of metabolic hormones on DA release in striatal subregions should provide new insight into factors that influence DA-dependent motivated behaviors. We find that one of these hormones, leptin, boosts striatal DA release through a process involving striatal cholinergic interneurons (ChIs) and nicotinic acetylcholine (ACh) receptors. Moreover, we find that the intracellular cascades downstream from leptin receptor (LepR) activation that lead to enhanced DA release differ among striatal subregions. Thus, we not only show that leptin regulates DA release, but also identify characteristics of this process that could be harnessed to alter pathologic eating behaviors.

The role of hypoleptinemia in the psychological and behavioral adaptation to starvation: implications for anorexia nervosa

This narrative review aims to pinpoint mental and behavioral effects of starvation, which may be triggered by hypoleptinemia and as such may be amenable to treatment with leptin receptor agonists. The reduced leptin secretion results from the continuous loss of fat mass, thus initiating a graded triggering of diverse starvation related adaptive functions. In light of leptin receptors located in several peripheral tissues and many brain regions adaptations may extend beyond those of the hypothalamus-pituitary-end organ-axes. We focus on gastrointestinal tract and reward system as relevant examples of peripheral and central effects of leptin. Despite its association with extreme obesity, congenital leptin deficiency with its many parallels to a state of starvation allows the elucidation of mental symptoms amenable to treatment with exogenous leptin in both ob/ob mice and humans with this autosomal recessive disorder. For starvation induced behavioral changes with an intact leptin signaling we particularly focus on rodent models for which proof of concept has been provided for the causative role of hypoleptinemia. For humans, we highlight the major cognitive, emotional and behavioral findings of the Minnesota Starvation Experiment to contrast them with results obtained upon a lesser degree of caloric restriction. Evidence for hypoleptinemia induced mental changes also stems from findings obtained in lipodystrophies. In light of the recently reported beneficial cognitive, emotional and behavioral effects of metreleptin-administration in anorexia nervosa we discuss potential implications for the treatment of this eating disorder. We postulate that leptin has profound psychopharmacological effects in the state of starvation.

Leptin Receptor Expressing Neurons in the Substantia Nigra Regulate Locomotion, and in The Ventral Tegmental Area Motivation and Feeding

Leptin is an anorexigenic hormone, important in the regulation of body weight. Leptin plays a role in food reward, feeding, locomotion and anxiety. Leptin receptors (LepR) are expressed in many brain areas, including the midbrain. In most studies that target the midbrain, either all LepR neurons of the midbrain or those of the ventral tegmental area (VTA) were targeted, but the role of substantia nigra (SN) LepR neurons has not been investigated. These studies have reported contradicting results regarding motivational behavior for food reward, feeding and locomotion. Since not all midbrain LepR mediated behaviors can be explained by LepR neurons in the VTA alone, we hypothesized that SN LepR neurons may provide further insight. We first characterized SN LepR and VTA LepR expression, which revealed LepR expression mainly on DA neurons. To further understand the role of midbrain LepR neurons in body weight regulation, we chemogenetically activated VTA LepR or SN LepR neurons in LepR-cre mice and tested for motivational behavior, feeding and locomotion. Activation of VTA LepR neurons in food restricted mice decreased motivation for food reward (p=0.032) and food intake (p=0.020), but not locomotion. In contrast, activation of SN LepR neurons in food restricted mice decreased locomotion (p=0.025), but not motivation for food reward or food intake. Our results provide evidence that VTA LepR and SN LepR neurons serve different functions, i.e. activation of VTA LepR neurons modulated motivation for food reward and feeding, while SN LepR neurons modulated locomotor activity.

A role for leptin and ghrelin in the augmentation of heroin seeking induced by chronic food restriction

RATIONAL

Caloric restriction increases the risk of relapse in abstinent drug users. Hormones involved in the regulation of energy balance and food intake, such as leptin and ghrelin, are implicated in drug-related behaviors.

OBJECTIVES

We investigated the role of leptin and ghrelin in the augmentation of heroin seeking induced by chronic food restriction.

METHODS

Rats self-administered heroin (0.1 mg/kg/infusion) for 10 days followed by 14 days of drug withdrawal. During withdrawal, rats were food restricted to 90% of their original body weight or were given free access to food. In experiment 1, we measured the plasma concentrations of leptin and ghrelin following heroin self-administration and withdrawal. In experiment 2, leptin was administered centrally (2.0 or 4.0 μg; i.c.v.) prior to a heroin-seeking test under extinction conditions. High density of both leptin and ghrelin receptors was previously identified in the ventral tegmental area (VTA), suggesting a direct effect on reward and motivation. Hence, we administered leptin (experiment 3; 0.125 or 0.250 μg/side), or ghrelin receptor antagonist JMV 2959 (experiment 4; 2.0 or 10.0 μg/side) directly into the VTA prior to the heroin-seeking test.

RESULTS

Chronic food restriction significantly decreased plasma levels of leptin and elevated plasma levels of ghrelin. Central administration of leptin had no statistically significant effect on heroin seeking. Intra-VTA administration of either leptin or JMV 2959 dose-dependently and selectively decreased heroin seeking in the food-restricted rats.

CONCLUSIONS

Leptin and ghrelin transmission in the VTA can modulate the augmentation of heroin seeking induced by chronic food restriction.

Ventral tegmental area inactivation alters hormonal, metabolic, and locomotor responses to inescapable stress

Context: The involvement of unilateral and bilateral inhibition of the ventral tegmental area (VTA) in response to stress is not well understood. Objective: In the present study, the effect of unilateral and bilateral inhibition of the VTA on hormonal, metabolic, and locomotor responses to stress was assessed. Material and methods: Male rats seven days after cannulation into the VTA received electro foot-shock stress for seven consecutive days. Twenty minutes before induction of stress, 2% lidocaine hydrochloride or sterile saline (control) was injected either uni- or bi-laterally into the VTA. Results: Results showed that stress significantly increased serum corticosterone level, adrenal gland weight and anorexia, reduced weight gain, food-intake, and locomotor activity. However, bilateral inactivation of VTA prevented stress-induced these parameters changes. Conclusion: The present study demonstrated that the bilateral VTA blockade effectively relieves the symptoms of stress, while the unilateral VTA blockade does not significantly improve the changes caused by stress.

Insulin and adipokine signaling and their cross-regulation in postmortem human brain

Aberrant insulin and adipokine signaling has been implicated in cognitive decline associated with both type 2 diabetes mellitus and neurodegenerative diseases. We established methods that reliably measure insulin, adiponectin and leptin signaling, and their crosstalk, in thawed postmortem mid-frontal cortical tissue from cognitively normal older subjects with a short postmortem interval. Insulin-evoked insulin receptor (IR) activation increases activated, tyrosine-phosphorylated IRβ on tyrosine residues 960, 1150, and 1151, insulin receptor substrate-1 recruitment to IRβ and phosphorylated RAC-α-serine/threonine-protein kinase. Adiponectin augments, but leptin inhibits, insulin signaling. Adiponectin activates adiponectin receptors to induce APPL1 binding to adiponectin receptor 1 and 2 and T-cadherin and downstream adenosine monophosphate-dependent protein kinase phosphorylation. Insulin inhibited adiponectin-induced signaling. In addition, leptin-induced leptin receptor (OB-R) signaling promotes Janus kinase 2 recruitment to OB-R and Janus kinase 2 and downstream signal transducer and activator of transcription 3 phosphorylation. Insulin enhanced leptin signaling. These data demonstrate insulin and adipokine signaling interactions in human brain. Future studies can use these methods to examine insulin, adiponectin, and leptin metabolic dysregulation in aging and disease states, such as type 2 diabetes and Alzheimer's disease-related dementias.

Leptin attenuates D2 receptor-mediated inhibition of putative ventral tegmental area dopaminergic neurons

Obesity causes hyperleptinemia. We have previously shown that D₂ receptor‐mediated inhibition of ventral tegmental area (VTA) dopaminergic neurons is attenuated in diet‐induced mice with obesity. Consequently, we hypothesized that high concentrations of serum leptin during obesity might modulate D₂ receptor‐mediated effects on VTA dopaminergic neurons. To investigate our hypothesis, we examined leptin effects on D₂ receptor‐mediated inhibition of putative VTA dopaminergic neurons from lean mice using electrophysiological techniques. Leptin (100 nmol/L) directly inhibited spontaneous firing in 71% of putative VTA dopaminergic neurons (leptin‐responsive), whereas the remaining 29% of neurons were leptin‐nonresponsive. In 41% of leptin‐responsive neurons, leptin attenuated the reduced firing rate produced by quinpirole (100 nmol/L), whereas the remaining 59% of neurons exhibited no effect of leptin. In leptin‐nonresponsive neurons, no significant leptin‐induced effect was observed on reduced firing rate produced by quinpirole. In leptin‐responsive neurons with positive leptin‐induced attenuation of quinpirole effects, leptin‐induced attenuation persisted for >20 min, whereas no such persistent attenuation was observed in other types of neurons. In conclusion, leptin attenuates D₂ receptor‐mediated inhibition in a subpopulation of putative VTA dopaminergic neurons. We suggest that leptin directly decreases, and indirectly increases, excitability of VTA dopaminergic neurons. In turn, this may contribute to a change in feeding behavior through the mesolimbic dopaminergic system during the development of obesity.

Development of leptin resistance in sucrose drinking rats is associated with consuming carbohydrate-containing solutions and not calorie-free sweet solution

Rats offered 30% sucrose solution plus chow or a sucrose-free diet develop leptin resistance within 4 weeks. This experiment tested whether leptin resistance was associated with the reward of sweet taste or the pre- or post-absorptive effects of consumption of simple carbohydrate. Male Sprague Dawley rats were offered a sucrose-free diet (NS), a diet containing 67% calories as sucrose (HS) or NS diet plus 30% sucrose (LS), 0.03% saccharin (Sacc) or 20% SolCarb^® solution for 38 days. SolCarb^® is a maltodextrin powder. Sacc rats initially drank more than LS rats, but intakes were the same after Day 20. SolCarb^® and LS rats drank the same number of calories from Day 15 to the end of the experiment. SolCarb^® and LS rats ate less dry food than other groups, but total energy intake was greater than that of NS, HS and Sacc groups and over 80% of their energy intake was from carbohydrate. Leptin responsiveness was tested on Days 27 and 32 with each rat acting as its own control. An i.p. injection of 2 mg/kg leptin inhibited food intake of NS, HS and Sacc rats, but had no effect on energy intake of LS or SolCarb^® rats or on consumption of Sacc, sucrose or SolCarb^® solution. At the end of the experiment all of the rats were insulin sensitive, had the same body composition and serum leptin concentrations. These data indicate that consumption of a calorie containing carbohydrate solution and not sweet taste drives the development of leptin resistance and suggest that there is lower threshold for inhibition of hunger than for inhibition of reward by leptin.

Homeostatic and non-homeostatic controls of feeding behavior: Distinct vs. common neural systems

Understanding the neurobiological controls of feeding behavior is critical in light of the growing obesity pandemic, a phenomenon largely based on excessive caloric consumption. Feeding behavior and its underlying biological substrates are frequently divided in the literature into two separate categories: [1] homeostatic processes involving energy intake based on caloric and other metabolic deficits, and [2] non-homeostatic processes that involve feeding driven by environmental and cognitive factors. The present review summarizes both historic and recent research examining the homeostatic regulation of feeding with specific emphasis on hypothalamic and hindbrain circuitry that monitor and regulate various metabolic signals. Regarding non-homeostatic controls, we highlight higher-order brain structures that integrate feeding-relevant external, interoceptive, and cognitive factors, including sensory cortical processing, learned associations in the hippocampus, and reward-based processing in the nucleus accumbens and interconnected mesolimbic circuitry. Finally, the current review focuses on recent evidence that challenges the traditional view that distinct neural systems regulate homeostatic vs. non-homeostatic controls of feeding behavior. Specifically, we highlight several feeding-related endocrine systems that act on both lower- and higher-order substrates, present evidence for the modulation of learned and cognitive feeding-relevant behaviors by lower-order brain regions, and highlight data showing that apparent homeostatic-based feeding behavior is modulated by higher-order brain regions. Our concluding perspective is that the classic dissociation between homeostatic and non-homeostatic constructs in relation to feeding behavior is limited with regards to understanding the complex integrated neurobiological systems that control energy balance.

Neuroendocrine integration of nutritional signals on reproduction

Reproductive function in mammals is energetically costly and therefore tightly regulated by nutritional status. To enable this integration of metabolic and reproductive function, information regarding peripheral nutritional status must be relayed centrally to the gonadotropin-releasing hormone (GNRH) neurons that drive reproductive function. The metabolically relevant hormones leptin, insulin and ghrelin have been identified as key mediators of this 'metabolic control of fertility'. However, the neural circuitry through which they act to exert their control over GNRH drive remains incompletely understood. With the advent of Cre-LoxP technology, it has become possible to perform targeted gene-deletion and gene-rescue experiments and thus test the functional requirement and sufficiency, respectively, of discrete hormone-neuron signaling pathways in the metabolic control of reproductive function. This review discusses the findings from these investigations, and attempts to put them in context with what is known from clinical situations and wild-type animal models. What emerges from this discussion is clear evidence that the integration of nutritional signals on reproduction is complex and highly redundant, and therefore, surprisingly difficult to perturb. Consequently, the deletion of individual hormone-neuron signaling pathways often fails to cause reproductive phenotypes, despite strong evidence that the targeted pathway plays a role under normal physiological conditions. Although transgenic studies rarely reveal a critical role for discrete signaling pathways, they nevertheless prove to be a good strategy for identifying whether a targeted pathway is absolutely required, critically involved, sufficient or dispensable in the metabolic control of fertility.

Reciprocal Inhibitory Interactions Between the Reward-Related Effects of Leptin and Cocaine

Cocaine is habit-forming because of its ability to enhance dopaminergic neurotransmission in the forebrain. In addition to neuronal inputs, forebrain dopamine circuits are modulated by hormonal influences; one of these is leptin, an adipose-derived hormone that attenuates the rewarding effects of food- and hunger-associated brain stimulation reward. Here we report reciprocal inhibition between the reward-related effects of leptin and the reward-related effects of cocaine in rats. First, we report that cocaine and the expectancy of cocaine each depresses plasma leptin levels. Second, we report that exogenous leptin, given systemically or directly into the ventral tegmental area, attenuates the ability of cocaine to elevate dopamine levels in the nucleus accumbens, the ability of cocaine to establish a conditioned place preference, and the ability of cocaine-predictive stimuli to prolong responding in extinction of cocaine-seeking. Thus, whereas leptin represents an endogenous antagonist of the habit-forming and habit-sustaining effects of cocaine, this antagonism is attenuated by cocaine and comes to be attenuated by the expectancy of cocaine.

Imbalanced insulin action in chronic over nutrition: Clinical harm, molecular mechanisms, and a way forward

The growing worldwide prevalence of overnutrition and underexertion threatens the gains that we have made against atherosclerotic cardiovascular disease and other maladies. Chronic overnutrition causes the atherometabolic syndrome, which is a cluster of seemingly unrelated health problems characterized by increased abdominal girth and body-mass index, high fasting and postprandial concentrations of cholesterol- and triglyceride-rich apoB-lipoproteins (C-TRLs), low plasma HDL levels, impaired regulation of plasma glucose concentrations, hypertension, and a significant risk of developing overt type 2 diabetes mellitus (T2DM). In addition, individuals with this syndrome exhibit fatty liver, hypercoagulability, sympathetic overactivity, a gradually rising set-point for body adiposity, a substantially increased risk of atherosclerotic cardiovascular morbidity and mortality, and--crucially--hyperinsulinemia. Many lines of evidence indicate that each component of the atherometabolic syndrome arises, or is worsened by, pathway-selective insulin resistance and responsiveness (SEIRR). Individuals with SEIRR require compensatory hyperinsulinemia to control plasma glucose levels. The result is overdrive of those pathways that remain insulin-responsive, particularly ERK activation and hepatic de-novo lipogenesis (DNL), while carbohydrate regulation deteriorates. The effects are easily summarized: if hyperinsulinemia does something bad in a tissue or organ, that effect remains responsive in the atherometabolic syndrome and T2DM; and if hyperinsulinemia might do something good, that effect becomes resistant. It is a deadly imbalance in insulin action. From the standpoint of human health, it is the worst possible combination of effects. In this review, we discuss the origins of the atherometabolic syndrome in our historically unprecedented environment that only recently has become full of poorly satiating calories and incessant enticements to sit. Data are examined that indicate the magnitude of daily caloric imbalance that causes obesity. We also cover key aspects of healthy, balanced insulin action in liver, endothelium, brain, and elsewhere. Recent insights into the molecular basis and pathophysiologic harm from SEIRR in these organs are discussed. Importantly, a newly discovered oxide transport chain functions as the master regulator of the balance amongst different limbs of the insulin signaling cascade. This oxide transport chain--abbreviated 'NSAPP' after its five major proteins--fails to function properly during chronic overnutrition, resulting in this harmful pattern of SEIRR. We also review the origins of widespread, chronic overnutrition. Despite its apparent complexity, one factor stands out. A sophisticated junk food industry, aided by subsidies from willing governments, has devoted years of careful effort to promote overeating through the creation of a new class of food and drink that is low- or no-cost to the consumer, convenient, savory, calorically dense, yet weakly satiating. It is past time for the rest of us to overcome these foes of good health and solve this man-made epidemic.

Mesolimbic dopamine and its neuromodulators in obesity and binge eating

Obesity has reached epidemic prevalence, and much research has focused on homeostatic and nonhomeostatic mechanisms underlying overconsumption of food. Mesocorticolimbic circuitry, including dopamine neurons of the ventral tegmental area (VTA), is a key substrate for nonhomeostatic feeding. The goal of the present review is to compare changes in mesolimbic dopamine function in human obesity with diet-induced obesity in rodents. Additionally, we will review the literature to determine if dopamine signaling is altered with binge eating disorder in humans or binge eating modeled in rodents. Finally, we assess modulation of dopamine neurons by neuropeptides and peripheral peptidergic signals that occur with obesity or binge eating. We find that while decreased dopamine concentration is observed with obesity, there is inconsistency outside the human literature on the relationship between striatal D2 receptor expression and obesity. Finally, few studies have explored how orexigenic or anorexigenic peptides modulate dopamine neuronal activity or striatal dopamine in obese models. However, ghrelin modulation of dopamine neurons may be an important factor for driving binge feeding in rodents.

Integration of reward signalling and appetite regulating peptide systems in the control of food-cue responses

Understanding the neurobiological substrates that encode learning about food-associated cues and how those signals are modulated is of great clinical importance especially in light of the worldwide obesity problem. Inappropriate or maladaptive responses to food-associated cues can promote over-consumption, leading to excessive energy intake and weight gain. Chronic exposure to foods rich in fat and sugar alters the reinforcing value of foods and weakens inhibitory neural control, triggering learned, but maladaptive, associations between environmental cues and food rewards. Thus, responses to food-associated cues can promote cravings and food-seeking by activating mesocorticolimbic dopamine neurocircuitry, and exert physiological effects including salivation. These responses may be analogous to the cravings experienced by abstaining drug addicts that can trigger relapse into drug self-administration. Preventing cue-triggered eating may therefore reduce the over-consumption seen in obesity and binge-eating disorder. In this review we discuss recent research examining how cues associated with palatable foods can promote reward-based feeding behaviours and the potential involvement of appetite-regulating peptides including leptin, ghrelin, orexin and melanin concentrating hormone. These peptide signals interface with mesolimbic dopaminergic regions including the ventral tegmental area to modulate reactivity to cues associated with palatable foods. Thus, a novel target for anti-obesity therapeutics is to reduce non-homeostatic, reward driven eating behaviour, which can be triggered by environmental cues associated with highly palatable, fat and sugar rich foods.

Leptin Suppresses the Rewarding Effects of Running via STAT3 Signaling in Dopamine Neurons

The adipose hormone leptin potently influences physical activity. Leptin can decrease locomotion and running, yet the mechanisms involved and the influence of leptin on the rewarding effects of running ("runner's high") are unknown. Leptin receptor (LepR) signaling involves activation of signal transducer and activator of transcription-3 (STAT3), including in dopamine neurons of the ventral tegmental area (VTA) that are essential for reward-relevant behavior. We found that mice lacking STAT3 in dopamine neurons exhibit greater voluntary running, an effect reversed by viral-mediated STAT3 restoration. STAT3 deletion increased the rewarding effects of running whereas intra-VTA leptin blocked it in a STAT3-dependent manner. Finally, STAT3 loss-of-function reduced mesolimbic dopamine overflow and function. Findings suggest that leptin influences the motivational effects of running via LepR-STAT3 modulation of dopamine tone. Falling leptin is hypothesized to increase stamina and the rewarding effects of running as an adaptive means to enhance the pursuit and procurement of food.

Effects of temporary low-dose galactose supplements in children aged 5-12 y with classical galactosemia: a pilot study

BACKGROUND

Classical galactosemia is caused by severe galactose-1-phosphate uridyltransferase deficiency. Despite life-long galactose-restriction, many patients experience long-term complications. Intoxication by galactose and its metabolites as well as over-restriction of galactose may contribute to the pathophysiology. We provided temporary low-dose galactose supplements to patients. We assessed tolerance and potential beneficial effects with clinical monitoring and measurement of biochemical, endocrine, and IgG N-glycosylation profiles.

METHODS

We enrolled 26 patients (8.6 ± 1.9 y). Thirteen were provided with 300 mg of galactose/day followed by 500 mg for 2 wk each (13 patient controls).

RESULTS

We observed no clinical changes with the intervention. Temporary mild increase in galactose-1-phosphate occurred, but renal, liver, and bone biochemistry remained normal. Patients in the supplementation group had slightly higher leptin levels at the end of the study than controls. We identified six individuals as "responders" with an improved glycosylation pattern (decreased G0/G2 ratio, P < 0.05). There was a negative relationship between G0/G2 ratio and leptin receptor sOb-R in the supplementation group (P < 0.05).

CONCLUSION

Temporary low-dose galactose supplementation in children over 5 y is well tolerated in the clinical setting. It leads to changes in glycosylation in "responders". We consider IgG N-glycan monitoring to be useful for determining individual optimum galactose intake.

Cooperative interaction between leptin and amylin signaling in the ventral tegmental area for the control of food intake

Peripheral coadministration of amylin and leptin produces enhanced suppression of food intake and body weight, but the central nuclei mediating these effects remain unclear. Because each of these peptides controls feeding via actions at the ventral tegmental area (VTA), we tested the hypothesis that the VTA is a site of action for the cooperative effects of leptin and amylin on energy balance control. First, we show that intra-VTA injection of amylin and leptin at doses of each peptide that are effective in reducing food intake and body weight when administered separately produces an enhanced suppression of feeding when administered in combination. We also demonstrate that subthreshold doses of both amylin and leptin cause significant hypophagia and body weight loss when coadministered into the VTA. Additionally, we provide evidence that VTA amylin receptor blockade significantly attenuates the ability of intra-VTA leptin to reduce feeding and body weight gain. Together, these data provide the first evidence that the VTA mediates the interaction of amylin and leptin to cooperatively promote negative energy balance.

Neonatal onset of leptin signalling in dopamine neurones of the ventral tegmental area in the rat

Leptin inhibits feeding by acting on hypothalamic and mesolimbic dopamine (DA) pathways involved in the homeostatic and hedonic control of energy balance. In the rodent, the neonatal period is characterised by high circulating leptin concentrations and an insensitivity to the anorectic effects of this hormone, suggesting that the modulation of these circuits by leptin is reduced during this period. The present study aimed to examine the onset of the functional ventral tegmental area (VTA) response to leptin during the neonatal period and to characterise the phenotype of leptin-responsive VTA neurones. On postnatal day (PND) 10 in pups insensitive to the anorectic effects of leptin and exclusively dependent on their mother for feeding, leptin administration failed to increase phosphorylated signal transducer of activation and transcription 3 (pSTAT3) and phosphorylated extracellular signal-regulated kinase (pERK)1/2 immunoreactivity in the midbrain. At the onset of independent feeding on PND16, leptin stimulated pSTAT3 production in the lateral parabrachial pigmented area of the midbrain, with a subset of these pSTAT3-positive neurones co-localising with tyrosine hydroxylase, a marker of DA neurones. Leptin did not increase pERK1/2 immunoreactivity in DA neurones on PND16. These results suggest that the insensitivity of PND10 pups to the anorectic effects of leptin might be mediated, at least in part, by a lack of signalling through the Janus kinase/STAT signalling pathway in VTA DA neurones in response to leptin before the onset of independent feeding.

Endocannabinoids and energy homeostasis: an update

The endocannabinoid system (ECS) is a widespread intercellular signaling system that plays a critical role in energy homeostasis, meant as the precise matching of caloric intake with energy expenditure which normally keeps body weight stable over time. Complex interactions between environmental and neurohormonal systems directly contribute to the balance of energy homeostasis. This review highlights established and more recent data on the brain circuits in which the ECS plays an important regulatory role, with focus on the hypothalamus, a region where numerous interacting systems regulating feeding, satiety, stress, and other motivational states coexist. Although not meant as an exhaustive review of the field, this article will discuss how endocannabinoid tone, in addition to reinforcing reward circuitries and modulating food intake and the salience of food, controls lipid and glucose metabolism in several peripheral organs, particularly the liver and adipose tissue. Direct actions in the skeletal muscle and pancreas are also emerging and are briefly discussed. This review provides new perspectives into endocannabinoid control of the neurochemical causes and consequences of energy homeostasis imbalance, a knowledge that might lead to new potential treatments for obesity and related morbidities.

Food for thought: hormonal, experiential, and neural influences on feeding and obesity

Obesity is a growing public health problem. Although convenient, the notion that obesity is simply a problem of will power is increasingly antiquated. It is becoming clear that complex interactions of environment, neurohormonal systems, and transgenerational effects directly contribute to obesity. This review highlights data presented at the Society for Neuroscience Annual Meeting in San Diego, California in 2013; and although not meant as an exhaustive review of the area, this reivew will explore seemingly disparate areas of research that, when taken as a whole, illuminate the complex topography of the causes and consequences of obesity. We discuss how disruption of the biological clock, a consequence of modern society, can lead to changes in the brain and periphery that lead to obesity. We explore how obesity can actually cause pathological changes within the hypothalamus of the brain (a key regulator of food intake and metabolic homeostasis). How reward circuitry, particularly the ventral tegmental area, responds to insulin and how these effects modulate feeding and the salience of feeding cues are mechanistically described. We also investigate how nutrition may cross generational boundaries to affect the development and function of offspring, underscoring the long reach of metabolic effects. Finally, the role of the endocannabinoid system is emphasized as a critical node in the transduction of many of these effects. Together, this review should provide perspective into the neural causes and consequences of obesity, and hopefully lead to new areas of interdisciplinary research to tackle this important public health epidemic.

Feelings about food: the ventral tegmental area in food reward and emotional eating

Overconsumption of high caloric food plays an important role in the etiology of obesity. Several factors drive such hedonic feeding. High caloric food is often palatable. In addition, when an individual is sated, stress and food-related cues can serve as potent feeding triggers. A better understanding of the neurobiological underpinnings of food palatability and environmentally triggered overconsumption would aid the development of new treatment strategies. In the current review we address the pivotal role of the mesolimbic dopamine reward system in the drive towards high caloric palatable food and its relation to stress- and cue-induced feeding. We also discuss how this system may be affected by both established and potential anti-obesity drug targets.

Neural circuits and motivational processes for hunger

How does an organism's internal state direct its actions? At one moment an animal forages for food with acrobatic feats such as tree climbing and jumping between branches. At another time, it travels along the ground to find water or a mate, exposing itself to predators along the way. These behaviors are costly in terms of energy or physical risk, and the likelihood of performing one set of actions relative to another is strongly modulated by internal state. For example, an animal in energy deficit searches for food and a dehydrated animal looks for water. The crosstalk between physiological state and motivational processes influences behavioral intensity and intent, but the underlying neural circuits are poorly understood. Molecular genetics along with optogenetic and pharmacogenetic tools for perturbing neuron function have enabled cell type-selective dissection of circuits that mediate behavioral responses to physiological state changes. Here, we review recent progress into neural circuit analysis of hunger in the mouse by focusing on a starvation-sensitive neuron population in the hypothalamus that is sufficient to promote voracious eating. We also consider research into the motivational processes that are thought to underlie hunger in order to outline considerations for bridging the gap between homeostatic and motivational neural circuits.

Presynaptic leptin action suppresses excitatory synaptic transmission onto ventral tegmental area dopamine neurons

BACKGROUND

Leptin is an adipocyte-derived cytokine that can act in the brain to suppress feeding and maintain energy homeostasis. Additionally, leptin activates its receptors in the ventral tegmental area (VTA), a critical site for neuroadaptations to rewarding stimuli, to modulate reward-seeking behaviors. Although leptin can decrease intrinsic excitability of dopamine neurons in the VTA, it is unknown whether leptin can modulate excitatory synaptic transmission in this brain region. Because plasticity of glutamatergic synapses onto VTA neurons can encode predictive information about reward, we hypothesized that leptin can decrease excitatory synaptic transmission onto dopamine neurons.

METHODS

Using whole-cell patch clamp electrophysiology in mouse midbrain slices, we tested the effects of leptin on evoked α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) or N-methyl-D-aspartate receptor (NMDAR)-mediated excitatory postsynaptic currents (EPSCs) onto VTA dopamine neurons.

RESULTS

Leptin depressed both AMPAR and NMDAR EPSCs in VTA dopamine neurons and reduced frequency but not amplitude of mini EPSCs. Bath application of the MEK1/2 inhibitor U0126 did not alter leptin-induced suppression of AMPAR EPSCs. However, external, but not internal, application of the phosphoinositol 3-kinase (PI3K) or Janus kinase 2 (Jak2) tyrosine kinase inhibitors abolished leptin-induced synaptic depression.

CONCLUSIONS

This study demonstrates that leptin causes a presynaptic inhibition of the probability of glutamate release onto VTA dopamine neurons. This synaptic inhibition requires Jak2 and PI3K activation. Leptin-induced weakening of synaptic strength onto dopamine cells may underlie its inhibitory effects on appetitive behavior for rewarding stimuli.

Leptin induces cyclin D1 expression and proliferation of human nucleus pulposus cells via JAK/STAT, PI3K/Akt and MEK/ERK pathways

Increasing evidence suggests that obesity and aberrant proliferation of nucleus pulposus (NP) cells are associated with intervertebral disc degeneration. Leptin, a hormone with increased circulating level in obesity, has been shown to stimulate cell proliferation in a tissue-dependent manner. Nevertheless, the effect of leptin on the proliferation of human NP cells has not yet been demonstrated. Here, we show that leptin induced the proliferation of primary cultured human NP cells, which expressed the leptin receptors OBRa and OBRb. Induction of NP cell proliferation was confirmed by CCK8 assay and immunocytochemistry and Real-time PCR for PCNA and Ki-67. Mechanistically, leptin induced the phosphorylation of STAT3, Akt and ERK1/2 accompanied by the upregulation of cyclin D1. Pharmacological inhibition of JAK/STAT3, PI3K/Akt or MEK/ERK signaling by AG490, Wortmannin or U0126, respectively, reduced leptin-induced cyclin D1 expression and NP cell proliferation. These experiments also revealed an intricate crosstalk among these signaling pathways in mediating the action of leptin. Taken together, we show that leptin induces human NP cell cyclin D1 expression and proliferation via activation of JAK/STAT3, PI3K/Akt or MEK/ERK signaling. Our findings may provide a novel molecular mechanism that explains the association between obesity and intervertebral disc degeneration.

Leptin levels in children and adults with classic galactosaemia

Among the long-term complications of Classic Galactosaemia (Gal) is premature ovarian insufficiency (POI) in female patients with subtle abnormalities of reproductive function also reported in male patients. Leptin is a circulating hormone which reflects body energy stores and which affects the neuroendocrine reproductive axis and pubertal development.We measured serum leptin in 28 children (10 girls, 18 boys; mean age 7.6 years, range 0.5-17.9 years) and in 22 adults (10 females, 12 males; mean age 23.9 years, range 18-37 years) with Gal on a strict galactose-restricted diet in comparison with control data.Leptin levels (expressed as SDS for gender and pubertal stage) were lower in Gal children than controls (mean leptin-SDS = -0.71 for girls, p < 0.05, -0.97 for boys compared with SDS = 0 for controls, p < 0.05). In an age-related analysis, leptin levels did not correlate with age in children with Gal for both sexes as it did for matched controls.As expected, females had higher leptin levels than males in either group. In adults with Gal, leptin concentrations were within normal limits for both sexes when adjusted for gender and BMI. There was a linear relationship between log-leptin and BMI in children with Gal and in controls. For Gal women, log-leptin was also associated with BMI. However, for Gal men, and hence for the entire group of adult Gal patients, this association between log-leptin and BMI was not detectable. Our findings suggest that leptin dysregulation may play a role in fertility issues in individuals with Gal from an early age.