Leptin uptake by serotonergic neurones of the dorsal raphe

Leptin activation of dorsal raphe neurons inhibits feeding behavior

Leptin is a homeostatic regulatory element that signals the presence of energy stores -in the form of adipocytes-which ultimately reduces food intake and increases energy expenditure. Similarly, serotonin (5-HT), a signaling molecule found in both the central and peripheral nervous systems, also regulates food intake. Here we use a combination of pharmacological manipulations, optogenetics, retrograde tracing, and in situ hybridization, combined with behavioral endpoints to physiologically and anatomically identify a novel leptin-mediated pathway between 5-HT neurons in the dorsal raphe nucleus (DRN) and hypothalamic arcuate nucleus (ARC) that controls food intake. In this study, we show that microinjecting leptin directly into the DRN reduces food intake in male Sprague-Dawley rats. This effect is mediated by leptin-receptor expressing neurons in the DRN as selective optogenetic activation of these neurons at either their ARC terminals or DRN cell bodies also reduces food intake. Anatomically, we identified a unique population of serotonergic raphe neurons expressing leptin receptors that send projections to the ARC. Finally, by utilizing in vivo microdialysis and high-performance liquid chromatography, we show that leptin administration to the DRN increases 5-HT efflux into the ARC. Overall, this study identifies a novel circuit for leptin-mediated control of food intake through a DRN-ARC pathway, utilizing 5-HT as a mechanism to control feeding behavior. Characterization of this new pathway creates opportunities for understanding how the brain controls eating behavior, as well as opens alternative routes for the treatment of eating disorders.

Significance

Leptin and serotonin both play a vital role in the regulation of food intake, yet there is still uncertainty in how these two molecules interact to control appetite. The purpose of this study is to further understand the anatomical and functional connections between leptin receptor expressing neurons in the dorsal raphe nucleus, the main source of serotonin, and the arcuate nucleus of the hypothalamus, and how serotonin plays a role in this pathway to reduce food intake. Insight gained from this study will contribute to a more thorough understanding of the networks that regulate food intake, and open alternative avenues for the development of treatments for obesity and eating disorders.

Hypothalamic control of bone metabolism

Bones are structures in vertebrates that provide support to organs, protect soft organs, and give them shape and defined features, functions that are essential for their survival. To perform these functions, bones are constantly renewed throughout life. The process through which bones are renewed is known as bone remodeling, an energy demanding process sensitive to changes in energy homeostasis of the organism. A close interplay takes place between the diversity of nutritional cues and metabolic signals with different elements of the hypothalamic circuits to co-ordinate energy metabolism with the regulation of bone mass. In this review, we focus on how mouse and human genetics have elucidated the roles of hormonal signals and neural circuits that originate in, or impinge on, the hypothalamus in the regulation of bone mass. This will help to understand the mechanisms whereby regulation of bone is gated and dynamically regulated by the hypothalamus.

Clusterin/ApoJ enhances central leptin signaling through Lrp2-mediated endocytosis

Hypothalamic leptin signaling plays a central role in maintaining body weight homeostasis. Here, we show that clusterin/ApoJ, recently identified as an anorexigenic neuropeptide, is an important regulator in the hypothalamic leptin signaling pathway. Coadministration of clusterin potentiates the anorexigenic effect of leptin and boosts leptin-induced hypothalamic Stat3 activation. In cultured neurons, clusterin enhances receptor binding and subsequent endocytosis of leptin. These effects are mainly mediated through the LDL receptor-related protein-2 (Lrp2). Notably, inhibition of hypothalamic clusterin, Lrp2 or endocytosis abrogates anorexia and hypothalamic Stat3 activation caused by leptin. These findings propose a novel regulatory mechanism in central leptin signaling pathways.

Analysis of brain nuclei accessible to ghrelin present in the cerebrospinal fluid

Ghrelin is a stomach-derived peptide hormone that acts in the brain to regulate many important physiological functions. Ghrelin receptor, named the growth hormone secretagogue receptor (GHSR), is present in many brain areas with or without obvious direct access to ghrelin circulating in the bloodstream. Ghrelin is also present in the cerebrospinal fluid (CSF) but the brain targets of CSF ghrelin are unclear. Here, we studied which brain areas are accessible to ghrelin present in the CSF. For this purpose, we centrally injected mice with fluorescein-labeled ghrelin (F-ghrelin) peptide tracer and then systematically mapped the distribution of F-ghrelin signal through the brain. Our results indicated that centrally injected F-ghrelin labels neurons in most of the brain areas where GHSR is present. Also, we detected F-ghrelin uptake in the ependymal cells of both wild-type and GHSR-null mice. We conclude that CSF ghrelin is able to reach most of brain areas expressing GHSR. Also, we propose that the accessibility of CSF ghrelin to the brain parenchyma occurs through the ependymal cells in a GHSR-independent manner.

Serotonin and the regulation of mammalian energy balance

Maintenance of energy balance requires regulation of the amount and timing of food intake. Decades of experiments utilizing pharmacological and later genetic manipulations have demonstrated the importance of serotonin signaling in this regulation. Much progress has been made in recent years in understanding how central nervous system (CNS) serotonin systems acting through a diverse array of serotonin receptors impact feeding behavior and metabolism. Particular attention has been paid to mechanisms through which serotonin impacts energy balance pathways within the hypothalamus. How upstream factors relevant to energy balance regulate the release of hypothalamic serotonin is less clear, but work addressing this issue is underway. Generally, investigation into the central serotonergic regulation of energy balance has had a predominantly “hypothalamocentric” focus, yet non-hypothalamic structures that have been implicated in energy balance regulation also receive serotonergic innervation and express multiple subtypes of serotonin receptors. Moreover, there is a growing appreciation of the diverse mechanisms through which peripheral serotonin impacts energy balance regulation. Clearly, the serotonergic regulation of energy balance is a field characterized by both rapid advances and by an extensive and diverse set of central and peripheral mechanisms yet to be delineated.

The two faces of serotonin in bone biology

The serotonin molecule has some remarkable properties. It is synthesized by two different genes at two different sites, and, surprisingly, plays antagonistic functions on bone mass accrual at these two sites. When produced peripherally, serotonin acts as a hormone to inhibit bone formation. In contrast, when produced in the brain, serotonin acts as a neurotransmitter to exert a positive and dominant effect on bone mass accrual by enhancing bone formation and limiting bone resorption. The effect of serotonin on bone biology could be harnessed pharmacologically to treat diseases such as osteoporosis.

Regional and temporal differences in leptin signaling in rat brain

Leptin regulates energy homeostasis through activation of different hypothalamic pathways. Evidence indicates that leptin is a pleiotropic hormone that acts on many brain areas, altering food intake, metabolism, and locomotion, among other functions. Because short-term effects of leptin infusion and intracellular pathways in other brain areas involved in food regulation have not been thoroughly analysed, we have studied the acute effect of intracerebroventricular leptin administration on the levels of the long form of leptin receptor (Ob-Rb), as well as on activation of Janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3), protein kinase B (Akt), extracellular regulated kinases (ERKs) and levels of suppressor of cytokine signaling-3 (SOCS3) in the hypothalamus, hippocampus, frontal cortex and cerebellum of adult male Wistar rats at 15min, 1 and 6h. The levels of Ob-Rb increased at 6h in hypothalamus only. Leptin activated the JAK2/STAT3 pathway in all areas, although in a temporally specific pattern. In contrast, this hormone decreased Akt activation in hypothalamus, hippocampus and cerebellum and ERK activation in frontal cortex, while it increased ERK activation in hypothalamus and hippocampus. These differences in modulation of Ob-Rb levels and signaling indicate that the rapid effects of leptin in non-hypothalamic areas are mediated, at least in part, through the intracellular pathways involved in hypothalamic energy balance, but in a temporally specific manner.

Leptin targets in the mouse brain

The central actions of leptin are essential for homeostatic control of adipose tissue mass, glucose metabolism, and many autonomic and neuroendocrine systems. In the brain, leptin acts on numerous different cell types via the long-form leptin receptor (LepRb) to elicit its effects. The precise identification of leptin's cellular targets is fundamental to understanding the mechanism of its pleiotropic central actions. We have systematically characterized LepRb distribution in the mouse brain using in situ hybridization in wildtype mice as well as by EYFP immunoreactivity in a novel LepRb-IRES-Cre EYFP reporter mouse line showing high levels of LepRb mRNA/EYFP coexpression. We found substantial LepRb mRNA and EYFP expression in hypothalamic and extrahypothalamic sites described before, including the dorsomedial nucleus of the hypothalamus, ventral premammillary nucleus, ventral tegmental area, parabrachial nucleus, and the dorsal vagal complex. Expression in insular cortex, lateral septal nucleus, medial preoptic area, rostral linear nucleus, and in the Edinger-Westphal nucleus was also observed and had been previously unreported. The LepRb-IRES-Cre reporter line was used to chemically characterize a population of leptin receptor-expressing neurons in the midbrain. Tyrosine hydroxylase and Cre reporter were found to be coexpressed in the ventral tegmental area and in other midbrain dopaminergic neurons. Lastly, the LepRb-IRES-Cre reporter line was used to map the extent of peripheral leptin sensing by central nervous system (CNS) LepRb neurons. Thus, we provide data supporting the use of the LepRb-IRES-Cre line for the assessment of the anatomic and functional characteristics of neurons expressing leptin receptor.

Melanin-concentrating hormone (MCH) immunoreactivity in non-neuronal cells within the raphe nuclei and subventricular region of the brainstem of the cat

Neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator are localized within the postero-lateral hypothalamus and zona incerta. These neurons project diffusely throughout the central nervous system and have been implicated in critical physiological processes such as energy homeostasis and sleep. In the present report, we examined the distribution of MCH immunoreactivity in the brainstem of the cat. In addition to MCH+ axons, we found MCH-immunoreactive cells that have not been previously described either in the midbrain raphe nuclei or in the periaqueductal and periventricular areas. These MCH+ cells constituted: 1. ependymal cells that lined the fourth ventricle and aqueduct, 2. ependymal cells with long basal processes that projected deeply into the subventricular (subaqueductal) parenchyma, and, 3. cells in subventricular regions and the midbrain raphe nuclei. The MCH+ cells in the midbrain raphe nuclei were closely related to neuronal processes of serotonergic neurons. Utilizing Neu-N and GFAP immunohistochemistry we determined that the preceding MCH+ cells were neither neurons nor astrocytes. However, we found that vimentin, an intermediate-filament protein that is used as a marker for tanycytes, was specifically co-localized with MCH in these cells. We conclude that MCH is present in tanycytes whose processes innervate the midbrain raphe nuclei and adjacent subependymal regions. Because tanycytes are specialized cells that transport substances from the cerebrospinal fluid (CSF) to neural parenchyma, we suggest that MCH is absorbed from the CSF by tanycytes and subsequently liberate to act upon neurons of brainstem nuclei.

Synergistic impairment of glucose homeostasis in ob/ob mice lacking functional serotonin 2C receptors

To investigate how serotonin and leptin interact in the regulation of energy balance and glucose homeostasis, we generated a genetic mouse model, the OB2C mouse, which lacks functional serotonin 2C receptors and the adipocyte hormone leptin. The OB2C mice exhibited a dramatic diabetes phenotype, evidenced by a synergistic increase in serum glucose levels and water intake. The severity of the animals' diabetes phenotype would not have been predicted from the phenotypic characterization of mice bearing mutations of either the leptin (OB mutant mice) or the serotonin 2C receptor gene (2C mutant mice). The synergistic impairment in glucose homeostasis developed at an age when OB2C mice did not differ in body weight from OB mice, suggesting that this impairment was not an indirect consequence of increased adiposity. We also demonstrated that the improvement in glucose tolerance in wild-type mice treated with the serotonin releaser and reuptake inhibitor fenfluramine was blunted in 2C mutant mice. These pharmacological and genetic findings provide evidence that the serotonin 2C receptor has direct effects on glucose homeostasis.

Polymorphisms in the serotonin reuptake transporter gene modify the consequences of social status on metabolic health in female rhesus monkeys

Individuals vary substantially in their vulnerability to physical and psychosocial stressors. The causes of such variation in susceptibility to stress are poorly understood, but are thought to relate in part to genetic factors. The present study evaluated the extent to which polymorphisms in the gene encoding the serotonin reuptake transporter (5HTTLPR or SERT) modulated physiologic responses to the imposition of psychosocial stress (social reorganization and subordinate social status) in female rhesus monkeys. Forty females, drawn from the middle ranking genealogies of several large social groups, were reorganized into eight groups containing 5 monkeys each; four groups were comprised entirely of animals homogeneous for the long promoter variant in the SERT gene (l/l), while the other four groups had monkeys with at least one allele of the short promoter variant (l/s or s/s). Females were sequentially introduced into these new groups in random order and dominance ranks were established within several days. During the ensuing 6 weeks, dominant monkeys exhibited elevated rates of aggression while subordinates displayed high rates of submission. Notably, females with the s-variant SERT genotype, collapsed across social status positions, exhibited the highest overall rates of both aggression and submission. Although neither social status nor SERT genotype influenced morning cortisol concentrations, glucocorticoid negative feedback was reduced significantly in subordinate compared to dominant females irrespective of genotype. All animals lost weight and abdominal fat across the experiment. However, decreases were greatest in subordinates, regardless of genotype, and least in dominant females with the l/l genotype. Serum concentrations of insulin, glucose, and ghrelin decreased significantly during the group formation process, effects that were independent of genotype or social status. In contrast, social status and genotype interacted to influence changes in serum concentrations of leptin and triiodothyronine (T3), as dominant, l/l females had the highest levels while subordinate s-variant females had the lowest levels. The order in which a female was introduced to her group generally predicted her eventual social rank. However, rank was additionally predicted by pre-experimental T3 and abdominal fat values, but only in the l/l animals. While these findings must be replicated with a larger sample size, the data suggest that the s-variant SERT genotype confers increased vulnerability to the adverse effects of psychosocial stress associated with subordinate status while the l/l genotype benefits the most from the absence of stress conferred by dominant social status. These findings suggest that genetic factors modify the responses of monkeys to social subordination and perhaps other psychosocial stressors.

Antidepressant-like effect of leptin in streptozotocin-induced diabetic mice

We previously reported that streptozotocin (STZ)-induced diabetic mice showed the depressive-like behavior in the tail suspension test. It has also been reported that leptin-deficient obese mice demonstrate the depressive-like behavior. Since STZ-induced diabetes causes a marked decrease in plasma leptin levels, it is possible that decrease in leptin levels and the depressive-like behavior may somehow be related. Therefore, we examined the effect of leptin on the depressive-like behavior of STZ-induced diabetic mice in the tail suspension test. The prolonged duration of immobility in diabetic mice was dose-dependently and significantly suppressed by single treatment with leptin (0.1-1 mg/kg, i.p.) without affecting on the locomotor activity. Leptin did not affect either the duration of immobility or the locomotor activity in non-diabetic mice. The anti-immobility effect of leptin (1 mg/kg, i.p.) in diabetic mice was significantly antagonized by the selective serotonin2 (5-HT2) receptor antagonist LY53,857 (0.03 mg/kg, s.c.), but not by the selective 5-HT1A receptor antagonist WAY-100635 (0.03 mg/kg, s.c.). Antagonists administered alone did not affect either the duration of immobility or the locomotor activity in diabetic mice. In conclusion, we suggest that leptin exerts the antidepressant-like effect in diabetic mice mediated by, at least in part, 5-HT2 receptors.

Chronic subcutaneous leptin infusion diminishes the responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis in female rhesus monkeys

The fat derived protein leptin has its anorexic action through a number of neuropeptides including an upregulation of corticotropin releasing hormone (CRH) expression in the hypothalamus. However, the influence of leptin on these neuropeptides may be different during stress. The present study used ovariectomized female rhesus monkeys (n=8) to further define the effect of leptin on HPA responsivity. To accomplish this, we assessed the effects of constant leptin infusion on cortisol and ACTH secretion in both a predictable and unpredictable situation as well as in response to dexamethasone suppression-CRH stimulation test. We hypothesized that leptin would attenuate the increase in cortisol and ACTH to a novel, unpredictable situation and would enhance glucocorticoid negative feedback and diminish the response to CRH. Animals were assessed under control placebo conditions and during a 28 day infusion with recombinant human leptin (6 microg/kg/day, SC). Within each treatment condition, HPA responsivity was assessed during no estradiol replacement and acute estradiol replacement that produced serum concentrations of approximately 40 pg/ml. However, the results indicated that neither estradiol alone or in combination with leptin had any consistent effect on the outcome measures. Compared to the control condition, leptin had no effect on the cortisol diurnal rhythm; however, evening but not morning plasma ACTH concentrations were significantly lower during leptin infusion. In contrast, the response in plasma cortisol and ACTH to an unpredictable situation was significantly attenuated by chronic leptin infusion. Furthermore, leptin enhanced glucocorticoid negative feedback and blunted CRH-induced increase in both cortisol and ACTH. Taken together, these data suggest that in the female monkey, leptin has little effect on basal cortisol. However, when the HPA axis is activated, leptin attenuates the neuroendocrine response by enhancing glucocorticoid negative feedback. These data underscore the potential importance of leptin in maintaining homeostasis through its diverse interaction with the HPA axis.

A reassessment of the role of serotonergic system in the control of feeding behavior

The role of serotonergic system in the feeding behavior was appraised by electrolytic lesions in the dorsal raphe nucleus (DRN) and administration of para-chlorophenylalanine (PCPA, 3 mg/5 microl, icv). Chronic evaluations were accomplished through 120 and 360 days in PCPA-injected and DRN-lesioned rats, respectively. Acute food intake was evaluated in fasted rats and submitted to injection of PCPA and hydroxytryptophan (LHTP, 30 mg/kg, ip). DRN-lesioned rats exhibited 22-80% increase in food intake up to sixth month, whereas the obesity was evident and sustained by whole period. In PCPA-injected rats was observed an initial increase in the food intake followed by hypophagy from 25th to 30th day and a transitory increase of body weight from 5th to 60th day. In the acute study, the LHTP reverted partially the PCPA-induced increase in food intake of fasted rats suggesting a sustained capacity of decarboxylation of precursor by serotonergic neurons. Slow restoration of the levels of food intake in DRN-lesioned rats reveals a neuroplasticity in the systems that regulate feeding behavior. A plateau on the body weight curve in lesioned rats possibly represents the establishment of a new and higher set point of energetic balance.

Acute fluoxetine administration differentially affects brain C-Fos expression in fasted and refed rats

In the present study we investigated the effect of acute fluoxetine administration on the expression of c-Fos in the rat brain under two different metabolic conditions: fed and fasting states. Wistar male rats, weighing 220+/-30g, received i.p. injections of saline solution or fluoxetine (10mg/kg), and were killed 2 h later. The brains were removed after transcardiac perfusion with phosphate-buffered saline followed by paraformaldehyde, and were then processed for immunohistochemistry. Fos-like immunoreactivity was quantified by a computerized system. Fasted animals faced an 18-h suppression of food intake, while fed groups were submitted to an initial 14-h period of fast followed by a 4-h period in which food was freely available. Both in fasting and fed states, fluoxetine-treated animals presented a significant increase in c-Fos expression in hypothalamic areas, limbic structures, circumventricular areas, and in mesencephalic and rhomboencephalic regions, as compared with saline-treated controls. The quantitative comparison of data obtained from fasted and fed animals showed that fasted rats treated with fluoxetine presented a higher c-Fos expression in the ventromedial hypothalamus and the paraventricular nuclei compared with the fed group, while in fluoxetine-treated fed rats c-Fos expression was higher in the arcuate nuclei, medial amygdala, locus coeruleus and dorsal raphe nuclei, as compared with fasted, fluoxetine-treated animals. These data indicate that the metabolic condition of the animals significantly modifies fluoxetine-induced brain c-Fos expression, suggesting that visceral and behavioral fluoxetine effects may be influenced by the metabolic state of the individual.

Interaction between estradiol replacement and chronic stress on feeding behavior and on serum leptin

Exposure to stress may cause either an increase or a decrease in food intake. Behavioral and physiological responses to stress, including alterations in feeding behavior, are sexually dimorphic. This study aimed to evaluate the interaction between estradiol levels and chronic variate stress on the intake of sweet food and on serum levels of leptin, a hormone secreted by the adipose cells with a role in the regulation of body weight. Adult female Wistar rats were used. After ovariectomy, the animals received estradiol replacement (or oil) subcutaneously. Rats were then divided in controls and stressed (submitted to 30 days of variate stress). Consumption of sweet food and of serum leptin was measured. Although animals receiving estradiol replacement presented smaller weight gain, they showed an increased consumption of sweet food. Chronic variate stress decreased sweet food intake at 30, but not at 20, days of treatment. Estradiol replacement in the stressed group prevented both the reduction observed in sweet food intake and the increase in leptin levels. These results suggest that there is an interaction between chronic stress and estradiol replacement in feeding behavior concerning sweet food consumption, and this interaction may be related to altered leptin levels.