Leptin regulates energy balance and motivation through action at distinct neural circuits

Neurodevelopmental Programming of Adiposity: Contributions to Obesity Risk

This review analyzes the published evidence regarding maternal factors that influence the developmental programming of long-term adiposity in humans and animals via the central nervous system (CNS). We describe the physiological outcomes of perinatal underfeeding and overfeeding and explore potential mechanisms that may mediate the impact of such exposures on the development of feeding circuits within the CNS-including the influences of metabolic hormones and epigenetic changes. The perinatal environment, reflective of maternal nutritional status, contributes to the programming of offspring adiposity. The in utero and early postnatal periods represent critically sensitive developmental windows during which the hormonal and metabolic milieu affects the maturation of the hypothalamus. Maternal hyperglycemia is associated with increased transfer of glucose to the fetus driving fetal hyperinsulinemia. Elevated fetal insulin causes increased adiposity and consequently higher fetal circulating leptin concentration. Mechanistic studies in animal models indicate important roles of leptin and insulin in central and peripheral programming of adiposity, and suggest that optimal concentrations of these hormones are critical during early life. Additionally, the environmental milieu during development may be conveyed to progeny through epigenetic marks and these can potentially be vertically transmitted to subsequent generations. Thus, nutritional and metabolic/endocrine signals during perinatal development can have lifelong (and possibly multigenerational) impacts on offspring body weight regulation.

Leptin signaling and its central role in energy homeostasis

Leptin plays a critical role in regulating appetite, energy expenditure and body weight, making it a key factor in maintaining a healthy balance. Despite numerous efforts to develop therapeutic interventions targeting leptin signaling, their effectiveness has been limited, underscoring the importance of gaining a better understanding of the mechanisms through which leptin exerts its functions. While the hypothalamus is widely recognized as the primary site responsible for the appetite-suppressing and weight-reducing effects of leptin, other brain regions have also been increasingly investigated for their involvement in mediating leptin's action. In this review, we summarize leptin signaling pathways and the neural networks that mediate the effects of leptin, with a specific emphasis on energy homeostasis.

Control of energy homeostasis by the lateral hypothalamic area

The lateral hypothalamic area (LHA) is a subcortical brain region that exerts control over motivated behavior, feeding, and energy balance across species. Recent single-cell sequencing studies have defined at least 30 distinct LHA neuron types. Some of these influence specific aspects of energy homeostasis; however, the functions of many LHA cell types remain unclear. This review addresses the rapidly emerging evidence from cell-type-specific investigations that the LHA leverages distinct neuron populations to regulate energy balance through complex connections with other brain regions. It will highlight recent findings demonstrating that LHA control of energy balance extends beyond mere food intake and propose outstanding questions to be addressed by future research.

Obese-associated gut microbes and derived phenolic metabolite as mediators of excessive motivation for food reward

BACKGROUND

Excessive hedonic consumption is one of the main drivers for weight gain. Identifying contributors of this dysregulation would help to tackle obesity. The gut microbiome is altered during obesity and regulates host metabolism including food intake.

RESULTS

By using fecal material transplantation (FMT) from lean or obese mice into recipient mice, we demonstrated that gut microbes play a role in the regulation of food reward (i.e., wanting and learning processes associated with hedonic food intake) and could be responsible for excessive motivation to obtain sucrose pellets and alterations in dopaminergic and opioid markers in reward-related brain areas. Through untargeted metabolomic approach, we identified the 3-(3'-hydroxyphenyl)propanoic acid (33HPP) as highly positively correlated with the motivation. By administrating 33HPP in mice, we revealed its effects on food reward.

CONCLUSIONS

Our data suggest that targeting the gut microbiota and its metabolites would be an interesting therapeutic strategy for compulsive eating, preventing inappropriate hedonic food intake. Video Abstract.

Complementary lateral hypothalamic populations resist hunger pressure to balance nutritional and social needs

Animals continuously weigh hunger and thirst against competing needs, such as social contact and mating, according to state and opportunity. Yet neuronal mechanisms of sensing and ranking nutritional needs remain poorly understood. Here, combining calcium imaging in freely behaving mice, optogenetics, and chemogenetics, we show that two neuronal populations of the lateral hypothalamus (LH) guide increasingly hungry animals through behavioral choices between nutritional and social rewards. While increased food consumption was marked by increasing inhibition of a leptin receptor-expressing (LepR^LH) subpopulation at a fast timescale, LepR^LH neurons limited feeding or drinking and promoted social interaction despite hunger or thirst. Conversely, neurotensin-expressing LH neurons preferentially encoded water despite hunger pressure and promoted water seeking, while relegating social needs. Thus, hunger and thirst gate both LH populations in a complementary manner to enable the flexible fulfillment of multiple essential needs.

Leptin's Immune Action: A Review Beyond Satiety

The adipose tissue is an endocrine organ that secretes adipokines such as leptin, which is one of the most important hormones for controlling satiety, metabolism, and energy homeostasis. This hormone acts in the regulation of innate and adaptive immune responses since immune cells have leptin receptors from which this hormone initiates its biological action. These receptors have been identified in hematopoietic stem cells in the bone marrow and mature immune cells, inducing signaling pathways mediated by JAK/STAT, PI3K, and ERK 1/2. It is known that the bone marrow also contains leptin-producing adipocytes, which are crucial for regulating hematopoiesis through largely unknown mechanisms. Therefore, we have reviewed the roles of leptin inside and outside the bone marrow, going beyond its action in the control of satiety.

Associations of leptin and corticostriatal connectivity in bipolar disorder

Bipolar disorder (BD) and metabolic disturbance represent a chronic state of low-grade inflammation and corticostriatal circuitry alterations. Herein, we aimed to investigate whether plasma leptin, an adipokine that plays a key role in the interplay of metabolism and inflammation, is associated with corticostriatal connectivity in patients with BD. Twenty-eight BD I patients, 36 BD II patients and 66 healthy controls were enrolled and completed the Hamilton Depression Rating Scale, the Young Mania Rating Scale, and the Recent Life Change Questionnaire. Fasting plasma leptin and C-reactive protein (CRP) levels were measured, and corticostriatal connectivity was examined using functional magnetic resonance imaging (fMRI). The relationships between leptin, CRP and body mass index (BMI) identified in the controls and BD II patients were absent in the BD I patients. We did not find a significant group difference in the leptin level; nevertheless, the negative correlation between leptin level and corticostriatal connectivity (ventrolateral prefrontal cortex and inferior temporal gyrus) observed in the healthy controls was absent in the BD patients. The disproportionate increase in leptin level with increasing BMI in BD indicated a potential inflammatory role of white adipose tissue in BD. Furthermore, higher CRP levels in BD I patients might induce leptin resistance. Collectively, our results implied vulnerability to inflammatory and metabolic diseases in patients with BD, especially BD I.

Early-life low-calorie sweetener consumption disrupts glucose regulation, sugar-motivated behavior, and memory function in rats

Low-calorie sweetener (LCS) consumption in children has increased dramatically due to its widespread presence in the food environment and efforts to mitigate obesity through sugar replacement. However, mechanistic studies on the long-term impact of early-life LCS consumption on cognitive function and physiological processes are lacking. Here, we developed a rodent model to evaluate the effects of daily LCS consumption (acesulfame potassium, saccharin, or stevia) during adolescence on adult metabolic, behavioral, gut microbiome, and brain transcriptomic outcomes. Results reveal that habitual early-life LCS consumption impacts normal postoral glucose handling and impairs hippocampal-dependent memory in the absence of weight gain. Furthermore, adolescent LCS consumption yielded long-term reductions in lingual sweet taste receptor expression and brought about alterations in sugar-motivated appetitive and consummatory responses. While early-life LCS consumption did not produce robust changes in the gut microbiome, brain region-specific RNA-Seq analyses reveal LCS-induced changes in collagen- and synaptic signaling-related gene pathways in the hippocampus and nucleus accumbens, respectively, in a sex-dependent manner. Collectively, these results reveal that habitual early-life LCS consumption has long-lasting implications for glucoregulation, sugar-motivated behavior, and hippocampal-dependent memory in rats, which may be based in part on changes in nutrient transporter, sweet taste receptor, and central gene pathway expression.

Molecular profile and response to energy deficit of leptin-receptor neurons in the lateral hypothalamus

Leptin exerts its effects on energy balance by inhibiting food intake and increasing energy expenditure via leptin receptors in the hypothalamus. While LepR neurons in the arcuate nucleus of the hypothalamus, the primary target of leptin, have been extensively studied, LepR neurons in other hypothalamic nuclei remain understudied. LepR neurons in the lateral hypothalamus contribute to leptin's effects on food intake and reward, but due to the low abundance of this population it has been difficult to study their molecular profile and responses to energy deficit. We here explore the transcriptome of LepR neurons in the LH and their response to energy deficit. Male LepR-Cre mice were injected in the LH with an AAV carrying Cre-dependent L10:GFP. Few weeks later the hypothalami from fed and food-restricted (24-h) mice were dissected and the TRAP protocol was performed, for the isolation of translating mRNAs from LepR cells in the LH, followed by RNA sequencing. After mapping and normalization, differential expression analysis was performed with DESeq2. We confirm that the isolated mRNA is enriched in LepR transcripts and other known neuropeptide markers of LepR^LH neurons, of which we investigate the localization patterns in the LH. We identified novel markers of LepR^LH neurons with association to energy balance and metabolic disease, such as Acvr1c, Npy1r, Itgb1, and genes that are differentially regulated by food deprivation, such as Fam46a and Rrad. Our dataset provides a reliable and extensive resource of the molecular makeup of LH LepR neurons and their response to food deprivation.

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.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Identification of Novel Neurocircuitry Through Which Leptin Targets Multiple Inputs to the Dopamine System to Reduce Food Reward Seeking

BACKGROUND

Leptin reduces the motivation to obtain food by modulating activity of the mesolimbic dopamine (DA) system upon presentation of cues that predict a food reward. Although leptin directly reduces the activity of ventral tegmental area (VTA) DA neurons, the majority of leptin receptor (LepR)-expressing DA neurons do not project to the nucleus accumbens, the projection implicated in driving food reward seeking. Therefore, the precise locus of leptin action to modulate motivation for a food reward is unresolved.

METHODS

We used transgenic mice expressing Cre recombinase under the control of the LepR promoter, anatomical tracing, optogenetics-assisted patch-clamp electrophysiology, in vivo optogenetics with fiber photometric calcium measurements, and chemogenetics to unravel how leptin-targeted neurocircuitry inhibits food reward seeking.

RESULTS

A large number of DA neurons projecting to the nucleus accumbens are innervated by local VTA LepR-expressing GABA (gamma-aminobutyric acid) neurons. Leptin enhances the activity of these GABA neurons and thereby inhibits nucleus accumbens-projecting DA neurons. In addition, we find that lateral hypothalamic LepR-expressing neurons projecting to the VTA are inhibited by leptin and that these neurons modulate DA neurons indirectly via inhibition of VTA GABA neurons. In accordance with such a disinhibitory function, optogenetically stimulating lateral hypothalamic LepR projections to the VTA potently activates DA neurons in vivo. Moreover, we found that chemogenetic activation of lateral hypothalamic LepR neurons increases the motivation to obtain a food reward only when mice are in a positive energy balance.

CONCLUSIONS

We identify neurocircuitry through which leptin targets multiple inputs to the DA system to reduce food reward seeking.

Peripheral versus central insulin and leptin resistance: Role in metabolic disorders, cognition, and neuropsychiatric diseases

Insulin and leptin are classically regarded as peptide hormones that play key roles in metabolism. In actuality, they serve several functions in both the periphery and central nervous system (CNS). Likewise, insulin and leptin resistance can occur both peripherally and centrally. Metabolic disorders such as diabetes and obesity share several key features including insulin and leptin resistance. While the peripheral effects of these disorders are well-known (i.e. cardiovascular disease, hypertension, stroke, dyslipidemia, etc.), the CNS complications of leptin and insulin resistance have come into sharper focus. Both preclinical and clinical findings have indicated that insulin and leptin resistance are associated with cognitive deficits and neuropsychiatric diseases such as depression. Importantly, these studies also suggest that these deficits in neuroplasticity can be reversed by restoration of insulin and leptin sensitivity. In view of these observations, this review will describe, in detail, the peripheral and central functions of insulin and leptin and explain the role of insulin and leptin resistance in various metabolic disorders, cognition, and neuropsychiatric diseases.

Key role for hypothalamic interleukin-6 in food-motivated behavior and body weight regulation

The pro-inflammatory role of interleukin-6 (IL-6) is well-characterized. Blockade of IL-6, by Tocilizumab, is used in patients with rheumatoid arthritis and those diagnosed with cytokine storm. However, brain-produced IL-6 has recently emerged as a critical mediator of gut/adipose communication with the brain. Central nervous system (CNS) IL-6 is engaged by peripheral and central signals regulating energy homeostasis. IL-6 is critical for mediating hypophagia and weight loss effects of a GLP-1 analog, exendin-4, a clinically utilized drug. However, neuroanatomical substrates and behavioral mechanisms of brain IL-6 energy balance control remain poorly understood. We propose that the lateral hypothalamus (LH) is an IL-6-harboring brain region, key to food intake and food reward control. Microinjections of IL-6 into the LH reduced chow and palatable food intake in male rats. In contrast, female rats responded with reduced motivated behavior for sucrose, measured by the progressive ratio operant conditioning test, a behavioral mechanism previously not linked to IL-6. To test whether IL-6, produced in the LH, is necessary for ingestive and motivated behaviors, and body weight homeostasis, virogenetic knockdown by infusion of AAV-siRNA-IL6 into the LH was utilized. Attenuation of LH IL-6 resulted in a potent increase in sucrose-motivated behavior, without any effect on ingestive behavior or body weight in female rats. In contrast, the treatment did not affect any parameters measured (chow intake, sucrose-motivated behavior, locomotion, and body weight) in chow-fed males. However, when challenged with a high-fat/high-sugar diet, the male LH IL-6 knockdown rats displayed rapid weight gain and hyperphagia. Together, our data suggest that LH-produced IL-6 is necessary and sufficient for ingestive behavior and weight homeostasis in male rats. In females, IL-6 in the LH plays a critical role in food-motivated, but not ingestive behavior control or weight regulation. Thus, collectively these data support the idea that brain-produced IL-6 engages the hypothalamus to control feeding behavior.

Anxiety-like behavior in female mice is modulated by STAT3 signaling in midbrain dopamine neurons

The central signaling actions of cytokines are mediated by signal transducer and activator of transcription (STAT3). STAT3 activation plays a pivotal role in the behavioral responses to the adiposity hormone leptin, including in midbrain dopamine (DA) neurons where it mediates the influence of leptin to diminish physical activity and running reward in male mice. Leptin also has anxiolytic effects which have been tied to the mesolimbic DA system. To assess the contribution of STAT3 signaling in mesolimbic DA neurons on feeding, mesolimbic DA tone and anxiodepressive behaviors in female mice, we generated DA-specific STAT3 knockout mice by crossing mice expressing Cre under the control of the dopamine transporter with STAT3-LoxP mice. Feeding, locomotion, wheel running, conditioned place preference for palatable food and amphetamine locomotor sensitization were unaffected by DA-specific STAT3 deletion. Conversely, knockout mice exhibited heightened anxiety-like behavior (open field test and elevated plus maze) along with increased basal and stress-induced plasma corticosterone, whereas indices of behavioral despair (forced swim and tail-suspension tasks) were unchanged. In accordance with biochemical evidence of increased D1 receptor signaling (phospho-DARPP32_Thr34) in the central nucleus of the amygdala (CeA) of knockout mice, local microinjections of a D1 receptor antagonist reversed the anxiogenic phenotype of knockout mice. In addition to alluding to sex differences in the signaling mechanisms mediating anxiety-like behavior, our findings suggest that activation of STAT3 in midbrain dopamine neurons projecting to the CeA dampens anxiety in a D1R-dependent manner in female mice.

Eating behavior as a new frontier in memory research

The study of memory is commonly associated with neuroscience, aging, education, and eyewitness testimony. Here we discuss how eating behavior is also heavily intertwined-and yet considerably understudied in its relation to memory processes. Both are influenced by similar neuroendocrine signals (e.g., leptin and ghrelin) and are dependent on hippocampal functions. While learning processes have long been implicated in influencing eating behavior, recent research has shown how memory of recent eating modulates future consumption. In humans, obesity is associated with impaired memory performance, and in rodents, dietary-induced obesity causes rapid decrements to memory. Lesions to the hippocampus disrupt memory but also induce obesity, highlighting a cyclic relationship between obesity and memory impairment. Enhancing memory of eating has been shown to reduce future eating and yet, little is known about what influences memory of eating or how memory of eating differs from memory for other behaviors. We discuss recent advancements in these areas and highlight fruitful research pursuits afforded by combining the study of memory with the study of eating behavior.

Optimization of whole-brain rabies virus tracing technology for small cell populations

The lateral hypothalamus (LH) is critically involved in the regulation of homeostatic energy balance. Some neurons in the LH express receptors for leptin (LepRb), a hormone known to increase energy expenditure and decrease energy intake. However, the neuroanatomical inputs to LepRb-expressing LH neurons remain unknown. We used rabies virus tracing technology to map these inputs, but encountered non-specific tracing. To optimize this technology for a minor cell population (LepRb is not ubiquitously expressed in LH), we used LepRb-Cre mice and assessed how different titers of the avian tumor virus receptor A (TVA) helper virus affected rabies tracing efficiency and specificity. We found that rabies expression is dependent on TVA receptor expression, and that leakiness of TVA receptors is dependent on the titer of TVA virus used. We concluded that a titer of 1.0–3.0 × 10⁷ genomic copies per µl of the TVA virus is optimal for rabies tracing. Next, we successfully applied modified rabies virus tracing technology to map inputs to LepRb-expressing LH neurons. We discovered that other neurons in the LH itself, the periventricular hypothalamic nucleus (Pe), the posterior hypothalamic nucleus (PH), the bed nucleus of the stria terminalis (BNST), and the paraventricular hypothalamic nucleus (PVN) are the most prominent input areas to LepRb-expressing LH neurons.

Bariatric surgery improves clinical outcomes and adiposity biomarkers but not inflammatory cytokines SAA and MCP-1 after a six-month follow-up

Obesity is a global health problem and is associated with a chronic low-grade inflammatory state. Surgical obesity treatment is being increasingly common due to its efficacy. From this, we evaluate the metabolic state improvement and inflammation remission in patients with obesity undergoing bariatric surgery. Methods: The clinical data and serum levels of leptin and adiponectin were assessed in patients with obesity before and one, three and six months after bariatric surgery. Also, serum amyloid A (SAA), monocyte chemoattractant protein-1 (MCP-1) levels were measured during the follow-up surgery and compared with a lean group of individuals. Results: Weight loss decreased body mass index (BMI), comorbidities percentage, drugs use and leptin levels. Adiponectin levels increased after surgery. SAA and MCP-1 showed no difference after surgery, but a trend decrease for MCP-1 and a significant decrease was observed when the patients with obesity were compared to the lean participants. Conclusion: Bariatric surgery alters metabolic status improving obesity-related comorbidities and the adiposity biomarkers leptin and adiponectin, but not inflammatory cytokines SAA and MCP-1.

Impact of Gut and Metabolic Hormones on Feeding Reward

Ingestion of food activates a cascade of endocrine responses (thereby reflecting a contemporaneous feeding status) that include the release of hormones from the gastrointestinal (GI) tract, such as cholecystokinin (CCK), glucagonlike peptide YY (PYY), peptide PP, and oleoylethanolamide, as well as suppression of ghrelin secretion. The pancreas and adipose tissue, on the other hand, release hormones that serve as a measure of the current metabolic state or the long-term energy stores, that is, insulin, leptin, and adiponectin. It is well known and intuitively understandable that these hormones target either directly (by crossing the blood-brain barrier) or indirectly (e.g., via vagal input) the "homeostatic" brainstem-hypothalamic pathways involved in the regulation of appetite. The current article focuses on yet another target of the metabolic and GI hormones that is critical in inducing changes in food intake, namely, the reward system. We discuss the physiological basis of this functional interaction, its importance in the control of appetite, and the impact that disruption of this crosstalk has on energy intake in select physiological and pathophysiological states. We conclude that metabolic and GI hormones have a capacity to strengthen or weaken a response of the reward system to a given food, and thus, they are fundamental in ensuring that feeding reward is plastic and dependent on the energy status of the organism. © 2021 American Physiological Society. Compr Physiol 11:1425-1447, 2021.

Amyloid Beta Peptide Is an Endogenous Negative Allosteric Modulator of Leptin Receptor

INTRODUCTION

Metabolic dysfunction is now recognized as a pivotal component of Alzheimer's disease (AD), the most common dementia worldwide. However, the precise molecular mechanisms linking metabolic dysfunction to AD remain elusive.

OBJECTIVE

Here, we investigated the direct impact of soluble oligomeric amyloid beta (Aβ) peptides, the main molecular hallmark of AD, on the leptin system, a major component of central energy metabolism regulation.

METHODS

We developed a new time-resolved fluorescence resonance energy transfer-based Aβ binding assay for the leptin receptor (LepR) and studied the effect of Aβ on LepR function in several in vitro assays. The in vivo effect of Aβ on LepR function was studied in an Aβ-specific AD mouse model and in pro-opiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus.

RESULTS

We revealed specific and high-affinity (Ki = 0.1 nM) binding of Aβ to LepR. Pharmacological characterization of this interaction showed that Aβ binds allosterically to the extracellular domain of LepR and negatively affects receptor function. Negative allosteric modulation of LepR by Aβ was detected at the level of signaling pathways (STAT-3, AKT, and ERK) in vitroand in vivo. Importantly, the leptin-induced response of POMC neurons, key players in the regulation of metabolic function, was completely abolished in the presence of Aβ.

CONCLUSION

Our data indicate that Aβ is a negative allosteric modulator of LepR, resulting in impaired leptin action, and qualify LepR as a new and direct target of Aβ oligomers. Preventing the interaction of Aβ with LepR might improve both the metabolic and cognitive dysfunctions in AD condition.

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.

NTS and VTA oxytocin reduces food motivation and food seeking

Oxytocin (OT) is a neuropeptide whose central receptor-mediated actions include reducing food intake. One mechanism of its behavioral action is the amplification of the feeding inhibitory effects of gastrointestinal (GI) satiation signals processed by hindbrain neurons. OT treatment also reduces carbohydrate intake in humans and rodents, and correspondingly, deficits in central OT receptor (OT-R) signaling increase sucrose self-administration. This suggests that additional processes contribute to central OT effects on feeding. This study investigated the hypothesis that central OT reduces food intake by decreasing food seeking and food motivation. As central OT-Rs are expressed widely, a related focus was to assess the role of one or more OT-R-expressing nuclei in food motivation and food-seeking behavior. OT was delivered to the lateral ventricle (LV), nucleus tractus solitarius (NTS), or ventral tegmental area (VTA), and a progressive ratio (PR) schedule of operant reinforcement and an operant reinstatement paradigm were used to measure motivated feeding behavior and food-seeking behavior, respectively. OT delivered to the LV, NTS, or VTA reduced 1) motivation to work for food and 2) reinstatement of food-seeking behavior. Results provide a novel and additional interpretation for central OT-driven food intake inhibition to include the reduction of food motivation and food seeking.

Leptin level as a biomarker of uncontrolled eating in obesity and overweight

BACKGROUND

Uncontrolled eating (UE) showed important relationships with the development of obesity. Homeostatic regulations of feeding and energy balance, as well as hedonic eating, are regulated by leptin.

AIMS

The aims of this study were (1) to assess eating behaviors of Algerian adults as measured by the 51-item eating inventory; we also evaluate changes in the Three-Factor Eating Questionnaire (TFEQ) scores according to the body mass index (BMI) category; (2) to examine the association between the scores of the three TFEQ factors and the BMI values of the participants; and (3) to examine whether leptin concentrations are associated with eating behavior. Our hypothesis is that participants with obesity and high concentrations of leptin might display uncontrolled eating behavior.

METHODS

The subjects were 190 participants (60 obese, 60 overweight, and 70 lean subjects). The eating behavior was measured by using the 51-item eating inventory. Serum insulin concentrations were assessed by radioimmunoassay and were used to calculate homeostasis model assessment (HOMA). Serum leptin was quantified by the enzyme-linked immunosorbent assay (ELISA).

RESULTS

Obese and overweight subjects showed hyperphagic behavior, i.e., uncontrolled eating. The logistic regression analysis showed an effect of leptin, HOMA, uncontrolled eating, and emotional eating on BMI. Leptin levels were associated with the uncontrolled eating and influenced by insulin sensitivity.

CONCLUSIONS

The uncontrolled eating reflects hyperphagic eating behavior in obese and overweight subjects. Coexistence of uncontrolled eating and high level of leptin demonstrates a state of leptin resistance resulting in an inability to detect satiety. High circulating leptin can be considered a potential biomarker of uncontrolled eating.

Endothelial-specific deficiency of megalin in the brain protects mice against high-fat diet challenge

BACKGROUND

The increasing risk of obesity and diabetes among other metabolic disorders are the consequence of shifts in dietary patterns with high caloric-content food intake. We previously reported that megalin regulates energy homeostasis using blood-brain barrier (BBB) endothelial megalin-deficient (EMD) mice, since these animals developed obesity and metabolic syndrome upon normal chow diet administration. Obesity in mid-life appears to be related to greater dementia risk and represents an increasing global health issue. We demonstrated that EMD phenotype induced impaired learning ability and recognition memory, neurodegeneration, neuroinflammation, reduced neurogenesis, and mitochondrial deregulation associated with higher mitochondrial mass in cortical tissues.

METHODS

EMD mice were subjected to normal chow and high-fat diet (HFD) for 14 weeks and metabolic changes were evaluated.

RESULTS

Surprisingly, BBB megalin deficiency protected against HFD-induced obesity improving glucose tolerance and preventing hepatic steatosis. Compared to wild type (wt), the brain cortex in EMD mice showed increased levels of the mitochondrial biogenesis regulator, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), and uncoupling protein 2 (UCP2), a thermogenic protein involved in the regulation of energy metabolism. This agreed with the previously found increased mitochondrial mass in the transgenic mice. Upon HFD challenge, we demonstrated these two proteins were found elevated in wt mice but reported no changes over the already increased levels in EMD animals.

CONCLUSION

We propose a protective role for megalin on diet-induce obesity, suggesting this could be related to metabolic disturbances found in dementia through brain endocrine system communications.

Intake and preference for dried bonito dashi in male Sprague-Dawley rats and C57BL/6 N mice

Dried bonito dashi is a traditional Japanese fish broth that enhances palatability of various dishes due to its specific flavor. The present study examined influences of dietary fat levels (10% vs. 45% fat), presentation order of dried bonito dashi (ascending vs. descending concentrations), and prior experience with dashi on subsequent dashi intake and preference using two-bottle choice tests in two rodent strains, Sprague-Dawley (SD) rats and C57BL/6 N (B6N) mice. In the ascending concentration tests, SD rats on a low fat diet preferred 10-100% dashi to water, whereas B6N mice showed a blunted preference for dashi. Consumption of a high fat diet reduced dashi preference in SD rats. The B6N mice on the high fat diet never preferred dashi at any concentration. In the descending concentration tests, SD rats on the low fat diet preferred dashi over a wide range (0.03-100% dashi). The B6N mice showed a trend similar to that of SD rats. Ingestion of the high fat diet in both strains reduced dashi preference in the descending concentration tests. However, introduction of the high fat diet to dashi experienced rats maintained on the low fat diet, reduced neither dashi intake nor dashi preference. Dashi intake affected neither high fat diet intake, caloric intake, nor preference for high fat diet. These results suggest that preference for dried bonito dashi is influenced at least by 1) dietary fat levels, 2) presentation order of dashi, and 3) prior experience with dashi.

The Novel Perspectives of Adipokines on Brain Health

First seen as a fat-storage tissue, the adipose tissue is considered as a critical player in the endocrine system. Precisely, adipose tissue can produce an array of bioactive factors, including cytokines, lipids, and extracellular vesicles, which target various systemic organ systems to regulate metabolism, homeostasis, and immune response. The global effects of adipokines on metabolic events are well defined, but their impacts on brain function and pathology remain poorly defined. Receptors of adipokines are widely expressed in the brain. Mounting evidence has shown that leptin and adiponectin can cross the blood–brain barrier, while evidence for newly identified adipokines is limited. Significantly, adipocyte secretion is liable to nutritional and metabolic states, where defective circuitry, impaired neuroplasticity, and elevated neuroinflammation are symptomatic. Essentially, neurotrophic and anti-inflammatory properties of adipokines underlie their neuroprotective roles in neurodegenerative diseases. Besides, adipocyte-secreted lipids in the bloodstream can act endocrine on the distant organs. In this article, we have reviewed five adipokines (leptin, adiponectin, chemerin, apelin, visfatin) and two lipokines (palmitoleic acid and lysophosphatidic acid) on their roles involving in eating behavior, neurotrophic and neuroprotective factors in the brain. Understanding and regulating these adipokines can lead to novel therapeutic strategies to counteract metabolic associated eating disorders and neurodegenerative diseases, thus promote brain health.

Hypothalamus-hippocampus circuitry regulates impulsivity via melanin-concentrating hormone

Behavioral impulsivity is common in various psychiatric and metabolic disorders. Here we identify a hypothalamus to telencephalon neural pathway for regulating impulsivity involving communication from melanin-concentrating hormone (MCH)-expressing lateral hypothalamic neurons to the ventral hippocampus subregion (vHP). Results show that both site-specific upregulation (pharmacological or chemogenetic) and chronic downregulation (RNA interference) of MCH communication to the vHP increases impulsive responding in rats, indicating that perturbing this system in either direction elevates impulsivity. Furthermore, these effects are not secondary to either impaired timing accuracy, altered activity, or increased food motivation, consistent with a specific role for vHP MCH signaling in the regulation of impulse control. Results from additional functional connectivity and neural pathway tracing analyses implicate the nucleus accumbens as a putative downstream target of vHP MCH1 receptor-expressing neurons. Collectively, these data reveal a specific neural circuit that regulates impulsivity and provide evidence of a novel function for MCH on behavior.

Weight Loss: How Does It Fit in With Liposuction?

BACKGROUND

Weight loss is traditionally viewed as straightforward counting of calories in and calories out, with little regard to the role of the adipocytes tasked with storing said calories. However, the body executes a complex compensatory response to any intervention that depletes its energy stores. Here, the authors discuss the methods used to attain weight loss, the body's response to this weight loss, and the difficulties in maintaining weight loss. Furthermore, the authors provide an overview of the literature on the physiological effects of liposuction.

OBJECTIVE

To describe the role of adipose tissue in energy homeostasis, methods of weight loss, weight regain, and the effect of liposuction on endocrine signaling.

METHODS

The authors conducted a narrative review of representative studies.

CONCLUSION

A variety of strategies for weight loss exist, and optimizing one's weight status may in turn optimize the aesthetic outcomes of liposuction. This is most apparent in the preferential reaccumulation of fat in certain areas after liposuction and the ability to avoid this with a negative energy balance.

Central leptin signaling transmits positive valence

Hunger resulting from food deprivation is associated with negative affect. This is supported by recent evidence showing that hunger-sensitive neurons drive feeding through a negative valence teaching signal. However, the complementary hypothesis that hormonal signals of energy surfeit counteract this negative valence, or even transmit positive valence, has received less attention. The adipose-derived hormone leptin signals in proportion to fat mass, is an indicator of energy surplus, and reduces food intake. Here, we showed that centrally-delivered leptin reduced food intake and conditioned a place preference in food-restricted as well as ad libitum fed rats. In contrast, leptin did not reduce food intake nor condition a place preference in obese rats, likely due to leptin resistance. Despite a well-known role for hindbrain leptin receptor signaling in energy balance control, hindbrain leptin delivery did not condition a place preference in food-restricted rats, suggesting that leptin acting in midbrain or forebrain sites mediates place preference conditioning. Supporting the hypothesis that leptin signaling induces a positive affective state, leptin also decreased the threshold for ventral tegmental area brain stimulation reward. Together, these data suggest that leptin signaling is intrinsically preferred, and support the view that signals of energy surfeit are associated with positive affect. Harnessing the positive valence of signals such as leptin may attenuate the negative affect associated with hunger, providing a compelling new approach for weight loss maintenance.

Examining weight suppression as a transdiagnostic factor influencing illness trajectory in bulimic eating disorders

Recent research indicates that weight suppression (WS: defined as the difference between highest lifetime and current weight) prospectively predicts illness trajectory across eating disorders characterized by binge eating, including AN binge-purge subtype (ANbp), bulimia nervosa (BN), and binge eating disorder (BED), collectively referred to as bulimic eating disorders. Through a series of studies, we have developed a model to explain the link between WS and illness trajectory in bulimic eating disorders. Our model posits that WS contributes to reduced circulating leptin, which leads to reduced postprandial glucagon-like peptide 1 (GLP-1) response. Diminished leptin and GLP-1 function contribute to alterations in two reward-related constructs in the Research Domain Criteria (RDoC): reward value/effort and reward satiation. Respectively, these changes increase drive/motivation to consume food and decrease ability for food consumption to lead to a state of satiation/satisfaction. Combined, these alterations increase risk for experiencing large, out-of-control binge-eating episodes. The following review presents evidence that contributed to the development of this model as well as preliminary findings from an on-going project funded to test this model.

Effects of GABA and Leptin Receptor-Expressing Neurons in the Lateral Hypothalamus on Feeding, Locomotion, and Thermogenesis

OBJECTIVE

The lateral hypothalamus (LH) is known for its role in feeding, and it also regulates other aspects of energy homeostasis. How genetically defined LH neuronal subpopulations mediate LH effects on energy homeostasis remains poorly understood. The behavioral effects of chemogenetically activating LH gamma-aminobutyric acid (GABA) and the more selective population of LH GABA neurons that coexpress the leptin receptor (LepR) were compared.

METHODS

LepR-cre and VGAT-cre mice were injected with AAV5-hSyn-DIO-hM3DGq-mCherry in the LH. The behavioral effects of LH GABA or LH LepR neuronal activation on feeding, locomotion, thermogenesis, and body weight were assessed.

RESULTS

The activation of LH GABA neurons increased body temperature (P ≤ 0.008) and decreased body weight (P ≤ 0.01) despite decreased locomotor activity (P = 0.03) and transiently increased chow intake (P ≤ 0.009). Also, similar to other studies, this study found that activation of LH GABA neurons induced gnawing on both food and nonfood (P = 0.001) items. Activation of LH LepR neurons decreased body weight (P ≤ 0.01) and chow intake when presented on the cage floor (P ≤ 0.04) but not when presented in the cage top and increased locomotor activity (P = 0.002) and body temperature (P = 0.03).

CONCLUSIONS

LH LepR neurons are a subset of LH GABA neurons, and LH LepR activation more specifically regulates energy homeostasis to promote a negative energy balance.

Vagal Interoceptive Modulation of Motivated Behavior

In addition to regulating the ingestion and digestion of food, sensory feedback from gut to brain modifies emotional state and motivated behavior by subconsciously shaping cognitive and affective responses to events that bias behavioral choice. This focused review highlights evidence that gut-derived signals impact motivated behavior by engaging vagal afferents and central neural circuits that generally serve to limit or terminate goal-directed approach behaviors, and to initiate or maintain behavioral avoidance.

The hindbrain is a site of energy balance action for prolactin-releasing peptide: feeding and thermic effects from GPR10 stimulation of the nucleus tractus solitarius/area postrema

PURPOSE

Prolactin-releasing peptide (PrRP) is a neuropeptide that suppresses food intake and increases body temperature when delivered to the forebrain ventricularly or parenchymally. However, PrRP's receptor GPR10 is widely distributed throughout the brain with particularly high levels found in the dorsomedial hindbrain. Thus, we hypothesized that hindbrain-directed PrRP administration would affect energy balance and motivated feeding behavior.

METHODS

To address this hypothesis, a range of behavioral and physiologic variables were measured in Sprague-Dawley rats that received PrRP delivered to the fourth ventricle (4V) or the nucleus of the solitary tract (NTS) at the level of the area postrema (AP).

RESULTS

4V PrRP delivery decreased chow intake and body weight, in part, through decreasing meal size in ad libitum maintained rats tested at dark onset. PrRP inhibited feeding when delivered to the nucleus tractus solitarius (NTS), but not to more ventral hindbrain structures. In addition, 4V as well as direct NTS administration of PrRP increased core temperature. By contrast, 4V PrRP did not reduce ad libitum intake of highly palatable food or the motivation to work for or seek palatable foods.

CONCLUSIONS

The dorsomedial hindbrain and NTS/AP, in particular, are sites of action in PrRP/GPR10-mediated control of chow intake, core temperature, and body weight.

Lateral hypothalamic GLP-1 receptors are critical for the control of food reinforcement, ingestive behavior and body weight

Increased motivation for highly rewarding food is a major contributing factor to obesity. Most of the literature focuses on the mesolimbic nuclei as the core of reward behavior regulation. However, the lateral hypothalamus (LH) is also a key reward-control locus in the brain. Here we hypothesize that manipulating glucagon-like peptide-1 receptor (GLP-1R) activity selectively in the LH can profoundly affect food reward behavior, ultimately leading to obesity. Progressive ratio operant responding for sucrose was examined in male and female rats, following GLP-1R activation and pharmacological or genetic GLP-1R blockade in the LH. Ingestive behavior and metabolic parameters, as well as molecular and efferent targets, of the LH GLP-1R activation were also evaluated. Food motivation was reduced by activation of LH GLP-1R. Conversely, acute pharmacological blockade of LH GLP-1R increased food motivation but only in male rats. GLP-1R activation also induced a robust reduction in food intake and body weight. Chronic knockdown of LH GLP-1R induced by intraparenchymal delivery of an adeno-associated virus-short hairpin RNA construct was sufficient to markedly and persistently elevate ingestive behavior and body weight and ultimately resulted in a doubling of fat mass in males and females. Interestingly, increased food reinforcement was again found only in males. Our data identify the LH GLP-1R as an indispensable element of normal food reinforcement, food intake and body weight regulation. These findings also show, for we believe the first time, that brain GLP-1R manipulation can result in a robust and chronic body weight gain. The broader implications of these findings are that the LH differs between females and males in its ability to control motivated and ingestive behaviors.

Hedonic Eating and the "Delicious Circle": From Lipid-Derived Mediators to Brain Dopamine and Back

Palatable food can be seductive and hedonic eating can become irresistible beyond hunger and negative consequences. This is witnessed by the subtle equilibrium between eating to provide energy intake for homeostatic functions, and reward-induced overeating. In recent years, considerable efforts have been devoted to study neural circuits, and to identify potential factors responsible for the derangement of homeostatic eating toward hedonic eating and addiction-like feeding behavior. Here, we examined recent literature on “old” and “new” players accountable for reward-induced overeating and possible liability to eating addiction. Thus, the role of midbrain dopamine is positioned at the intersection between selected hormonal signals involved in food reward information processing (namely, leptin, ghrelin, and insulin), and lipid-derived neural mediators such as endocannabinoids. The impact of high fat palatable food and dietary lipids on endocannabinoid formation is reviewed in its pathogenetic potential for the derangement of feeding homeostasis. Next, endocannabinoid signaling that regulates synaptic plasticity is discussed as a key mechanism acting both at hypothalamic and mesolimbic circuits, and affecting both dopamine function and interplay between leptin and ghrelin signaling. Outside the canonical hypothalamic feeding circuits involved in energy homeostasis and the notion of “feeding center,” we focused on lateral hypothalamus as neural substrate able to confront food-associated homeostatic information with food salience, motivation to eat, reward-seeking, and development of compulsive eating. Thus, the lateral hypothalamus-ventral tegmental area-nucleus accumbens neural circuitry is reexamined in order to interrogate the functional interplay between ghrelin, dopamine, orexin, and endocannabinoid signaling. We suggested a pivotal role for endocannabinoids in food reward processing within the lateral hypothalamus, and for orexin neurons to integrate endocrine signals with food reinforcement and hedonic eating. In addition, the role played by different stressors in the reinstatement of preference for palatable food and food-seeking behavior is also considered in the light of endocannabinoid production, activation of orexin receptors and disinhibition of dopamine neurons. Finally, type-1 cannabinoid receptor-dependent inhibition of GABA-ergic release and relapse to reward-associated stimuli is linked to ghrelin and orexin signaling in the lateral hypothalamus-ventral tegmental area-nucleus accumbens network to highlight its pathological potential for food addiction-like behavior.

Thalamostriatal and cerebellothalamic pathways in a songbird, the Bengalese finch

The thalamostriatal system is a major network in the mammalian brain, originating principally from the intralaminar nuclei of thalamus. Its functions remain unclear, but a subset of these projections provides a pathway through which the cerebellum communicates with the basal ganglia. Both the cerebellum and basal ganglia play crucial roles in motor control. Although songbirds have yielded key insights into the neural basis of vocal learning, it is unknown whether a thalamostriatal system exists in the songbird brain. Thalamic nucleus DLM is an important part of the song system, the network of nuclei required for learning and producing song. DLM receives output from song system basal ganglia nucleus Area X and sits within dorsal thalamus, the proposed avian homolog of the mammalian intralaminar nuclei that also receives projections from the cerebellar nuclei. Using a viral vector that specifically labels presynaptic axon segments, we show in Bengalese finches that dorsal thalamus projects to Area X, the basal ganglia nucleus of the song system, and to surrounding medial striatum. To identify the sources of thalamic input to Area X, we map DLM and cerebellar-recipient dorsal thalamus (DT_CbN ). Surprisingly, we find both DLM and dorsal anterior DT_CbN adjacent to DLM project to Area X. In contrast, the ventral medial subregion of DT_CbN projects to medial striatum outside Area X. Our results suggest the basal ganglia in the song system, like the mammalian basal ganglia, integrate feedback from the thalamic region to which they project as well as thalamic regions that receive cerebellar output.

Association of serum leptin levels and pulmonary tuberculosis: a meta-analysis

Background

We performed a meta-analysis to investigate the association of serum leptin levels with the pathogenesis of pulmonary tuberculosis (PTB).

Methods

The retrieval of related articles was achieved through searching the electronic databases according to strict inclusion criteria. The STATA version 12.0 statistical software was employed to calculate the standardized mean difference (SMD) and 95% confidence interval (CI) during the statistical analysis.

Results

Twelve case-control studies were enrolled in this meta-analysis. Our finding showed that serum leptin levels of healthy controls were markedly higher than those of PTB patients (SMD =0.70, 95% CI =0.51-0.89, P<0.001). Stratified analysis based on ethnicity presented that lower serum leptin levels were apparently associated with the development of PTB among both Asians and Caucasians (Asians: SMD =0.38, 95% CI =0.15-0.61, P=0.001; Caucasians: SMD =1.54, 95% CI =1.17-1.91, P<0.001). Furthermore, subgroups analysis based on the detecting method also showed that there was an association between the serum leptin levels and the development of PTB in both ELISA subgroup and RIA subgroup (ELISA: SMD =1.03, 95% CI =0.81-1.26, P<0.001; RIA: SMD =-0.41, 95% CI =-0.82 to -0.01, P=0.045).

Conclusions

In conclusion, our present findings suggest that decreased serum leptin levels may be associated with the pathogenesis of PTB.

Weight suppression and bulimic syndrome maintenance: Preliminary findings for the mediating role of leptin

OBJECTIVE

Longitudinal studies support a prospective relationship between weight suppression (WS) and bulimic syndrome (BN-S) maintenance. Although biobehavioral mechanisms have been proposed to explain this link, such mechanisms have yet to be identified. Given that weight loss would reduce leptin levels which may influence eating, this study examined whether reduced leptin levels mediate the link between greater WS and longer illness duration.

METHOD

Women (N = 53), ages 18-45 years, were recruited from the community if they met criteria for a BN-S, including either DSM-5 bulimia nervosa (BN; n = 33) or purging disorder (PD: n = 20), and fell within a healthy weight range (18.5-26.5 kg/m² ). Participants completed clinical assessments and provided blood samples to measure circulating leptin.

RESULTS

Significant associations were found among greater WS, lower leptin concentrations, and longer duration of illness. Mediation analyses using bootstrapping procedures indicated all paths were significant and that leptin mediated the link between WS and illness duration. An alternative model in which longer illness duration contributed to leptin, via greater WS, was not supported.

DISCUSSION

Longitudinal research is needed to support temporal associations and explore behavioral mechanisms linking leptin to illness trajectory.

Dopamine D2 gene expression interacts with environmental enrichment to impact lifespan and behavior

Aging produces cellular, molecular, and behavioral changes affecting many areas of the brain. The dopamine (DA) system is known to be vulnerable to the effects of aging, which regulate behavioral functions such as locomotor activity, body weight, and reward and cognition. In particular, age-related DA D₂ receptor (D2R) changes have been of particular interest given its relationship with addiction and other rewarding behavioral properties. Male and female wild-type (Drd₂ +/+), heterozygous (Drd₂ +/−) and knockout (Drd₂ −/−) mice were reared post-weaning in either an enriched environment (EE) or a deprived environment (DE). Over the course of their lifespan, body weight and locomotor activity was assessed. While an EE was generally found to be correlated with longer lifespan, these increases were only found in mice with normal or decreased expression of the D₂ gene. Drd₂ +/+ EE mice lived nearly 16% longer than their DE counterparts. Drd₂ +/+ and Drd₂ +/− EE mice lived 22% and 21% longer than Drd₂ −/− EE mice, respectively. Moreover, both body weight and locomotor activity were moderated by environmental factors. In addition, EE mice show greater behavioral variability between genotypes compared to DE mice with respect to body weight and locomotor activity.

Genetic induction of hypometabolism by ablation of MC4R does not suppress ALS-like phenotypes in the G93A mutant SOD1 mouse model

Dysfunction and death of motor neurons leads to progressive paralysis in amyotrophic lateral sclerosis (ALS). Recent studies have reported organism-level metabolic dysfunction as a prominent but poorly understood feature of the disease. ALS patients are hypermetabolic with increased resting energy expenditure, but if and how hypermetabolism contributes to disease pathology is unknown. We asked if decreasing metabolism in the mutant superoxide dismutase 1 (SOD1) mouse model of ALS (G93A SOD1) would alter motor function and survival. To address this, we generated mice with the G93A SOD1 mutation that also lacked the melanocortin-4 receptor (MC4R). MC4R is a critical regulator of energy homeostasis and food intake in the hypothalamus. Loss of MC4R is known to induce hyperphagia and hypometabolism in mice. In the MC4R null background, G93A SOD1 mice become markedly hypometabolic, overweight and less active. Decreased metabolic rate, however, did not reverse any ALS-related disease phenotypes such as motor dysfunction or decreased lifespan. While hypermetabolism remains an intriguing target for intervention in ALS patients and disease models, our data indicate that the melanocortin system is not a good target for manipulation. Investigating other pathways may reveal optimal targets for addressing metabolic dysfunction in ALS.

Leptin resistance and hippocampal behavioral deficits

The adipocyte-derived hormone leptin is an important regulator of body weight and metabolism through activation of brain leptin receptors expressed in regions such as the hypothalamus. Beyond these well described and characterized activities of leptin in the hypothalamus, it is becoming increasingly clear that the central activities of leptin extend to the hippocampus. Indeed, leptin receptors are expressed in the hippocampus where these receptors are proposed to mediate various aspects of hippocampal synaptic plasticity that ultimately impact cognitive function. This concept is supported by studies demonstrating that leptin promotes hippocampal-dependent learning and memory, as well as studies indicating that leptin resistance is associated with deficits in hippocampal-dependent behaviors and in the induction of depressive-like behaviors. The effects of leptin on cognitive/behavioral plasticity in the hippocampus may be regulated by direct activation of leptin receptors expressed in the hippocampus; additionally, leptin-mediated activation of synaptic networks that project to the hippocampus may also impact hippocampal-mediated behaviors. In view of these previous observations, the goal of this review will be to discuss the mechanisms through which leptin facilitates cognition and behavior, as well as to dissect the loci at which leptin resistance leads to impairments in hippocampal synaptic plasticity, including the development of cognitive deficits and increased risk of depressive illness in metabolic disorders such as obesity and type 2 diabetes mellitus (T2DM).

PINK1/Parkin-Dependent Mitochondrial Surveillance: From Pleiotropy to Parkinson's Disease

Parkinson's disease (PD) is one of the most frequent neurodegenerative disease caused by the preferential, progressive degeneration of the dopaminergic (DA) neurons of the substantia nigra (SN) pars compacta. PD is characterized by a multifaceted pathological process involving protein misfolding, mitochondrial dysfunction, neuroinflammation and metabolism deregulation. The molecular mechanisms governing the complex interplay between the different facets of this process are still unknown. PARK2/Parkin and PARK6/PINK1, two genes responsible for familial forms of PD, act as a ubiquitous core signaling pathway, coupling mitochondrial stress to mitochondrial surveillance, by regulating mitochondrial dynamics, the removal of damaged mitochondrial components by mitochondria-derived vesicles, mitophagy, and mitochondrial biogenesis. Over the last decade, PINK1/Parkin-dependent mitochondrial quality control emerged as a pleiotropic regulatory pathway. Loss of its function impinges on a number of physiological processes suspected to contribute to PD pathogenesis. Its role in the regulation of innate immunity and inflammatory processes stands out, providing compelling support to the contribution of non-cell-autonomous immune mechanisms in PD. In this review, we illustrate the central role of this multifunctional pathway at the crossroads between mitochondrial stress, neuroinflammation and metabolism. We discuss how its dysfunction may contribute to PD pathogenesis and pinpoint major unresolved questions in the field.

Further investigation of phenotypes and confounding factors of progressive ratio performance and feeding behavior in the BACHD rat model of Huntington disease

Huntington disease is an inherited neurodegenerative disorder characterized by motor, cognitive, psychiatric and metabolic symptoms. We recently published a study describing that the BACHD rat model of HD shows an obesity phenotype, which might affect their motivation to perform food-based behavioral tests. Further, we argued that using a food restriction protocol based on matching BACHD and wild type rats' food consumption rates might resolve these motivational differences. In the current study, we followed up on these ideas in a longitudinal study of the rats' performance in a progressive ratio test. We also investigated the phenotype of reduced food consumption rate, which is typically seen in food-restricted BACHD rats, in greater detail. In line with our previous study, the BACHD rats were less motivated to perform the progressive ratio test compared to their wild type littermates, although the phenotype was no longer present when the rats' food consumption rates had been matched. However, video analysis of food consumption tests suggested that the reduced consumption rate found in the BACHD rats was not entirely based on differences in hunger, but likely involved motoric impairments. Thus, restriction protocols based on food consumption rates are not appropriate when working with BACHD rats. As an alternative, we suggest that studies where BACHD rats are used should investigate how the readouts of interest are affected by motivational differences, and use appropriate control tests to avoid misleading results. In addition, we show that BACHD rats display distinct behavioral changes in their progressive ratio performance, which might be indicative of striatal dysfunction.

Obesity: Current and potential pharmacotherapeutics and targets

Obesity is a global epidemic that contributes to a number of health complications including cardiovascular disease, type 2 diabetes, cancer and neuropsychiatric disorders. Pharmacotherapeutic strategies to treat obesity are urgently needed. Research over the past two decades has increased substantially our knowledge of central and peripheral mechanisms underlying homeostatic energy balance. Homeostatic mechanisms involve multiple components including neuronal circuits, some originating in hypothalamus and brain stem, as well as peripherally-derived satiety, hunger and adiposity signals that modulate neural activity and regulate eating behavior. Dysregulation of one or more of these homeostatic components results in obesity. Coincident with obesity, reward mechanisms that regulate hedonic aspects of food intake override the homeostatic regulation of eating. In addition to functional interactions between homeostatic and reward systems in the regulation of food intake, homeostatic signals have the ability to alter vulnerability to drug abuse. Regarding the treatment of obesity, pharmacological monotherapies primarily focus on a single protein target. FDA-approved monotherapy options include phentermine (Adipex-P®), orlistat (Xenical®), lorcaserin (Belviq®) and liraglutide (Saxenda®). However, monotherapies have limited efficacy, in part due to the recruitment of alternate and counter-regulatory pathways. Consequently, a multi-target approach may provide greater benefit. Recently, two combination products have been approved by the FDA to treat obesity, including phentermine/topiramate (Qsymia®) and naltrexone/bupropion (Contrave®). The current review provides an overview of homeostatic and reward mechanisms that regulate energy balance, potential therapeutic targets for obesity and current treatment options, including some candidate therapeutics in clinical development. Finally, challenges in anti-obesity drug development are discussed.