No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Localization of Leptin Receptor (Ob-R) Messenger Ribonucleic Acid in the Rodent Hindbrain
Abstract
The behavioral and neuroendocrine effects of the adipose tissue-derived circulating protein, leptin, appear to be mediated by the hypothalamus. We have investigated whether the leptin receptor gene is expressed in hindbrain regions known to be involved in the processing of satiety and energetic signals of peripheral origin. In the mouse, gene expression was detected in the nucleus of the solitary tract, lateral parabrachial nucleus, and medullary reticular nucleus and diffusely elsewhere by in situ hybridization. Receptor messenger RNA in these neuronal areas consisted largely, if not exclusively, of the long splice variant, Ob-Rb. Presumed short receptor splice variants were abundantly expressed in the leptomeninges and the choroid plexus of the fourth ventricle. Similar levels of leptin receptor gene expression were present in the hindbrain of lean and obese (ob/ob) mice. The leptin receptor gene was expressed comparatively weakly in the nucleus of the solitary tract of the rat and was not detectable in the lateral parabrachial nucleus. However, by contrast with the mouse, a high level of receptor gene expression was observed in the cerebellum of the rat. A number of rodent hindbrain sites expressing the leptin receptor gene are activated by circulating leptin and may form a monitoring/signaling pathway to complement more direct hypothalamic interactions.
... Short forms of leptin receptors could function as specific transport systems for leptin, as they are present in high amounts in the choroid plexus and brain microvessels (Tartaglia et al., 1995; Lee et al., 1996; Guan et al., 1997; Bjorbaek et al., 1998). The long form, Ob-Rb, is the functional receptor through which leptin mediates its effects and is widely distributed in the brain including the ARC, VMH, PVN, DMH, SON, lateral POA, mPOA and SCN (Hakansson et al., 1996; Mercer et al., 1996; Fei et al., 1997; Zamorano et al., 1997; Bjorbaek et al., 1998; Elmquist et al., 1998; Mercer et al., 1998; Shioda et al., 1998; Yarnell et al., 1998). The secretory form, Ob-Re, which is spliced in ahead of the transmembrane domain, might be a soluble binding protein for leptin (Lee et al., 1996). ...
... Leptin receptor mRNA has been localized in rat brainstem (Elmquist et al., 1998; Mercer et al., 1998). Leptin receptor immunoreactivity colocalizes with TH (Tyrosine hydroxylase) and serotonin in the NTS and raphe nuclei (Hay-Schmidt et al., 2001). ...
... These findings indicate that circulating leptin may directly act in the brainstem to elicit physiological responses, or that leptin administration might activate a neurotransmitter system that subsequently activates STAT3 signaling system in the brainstem (Hosoi et al., 2002b). On the contrary, although Ob-Rb leptin receptor expression was found in the AP region (Mercer et al., 1998), leptin-induced pSTAT3 immunoreactivity was not detected in that nucleus (Hosoi et al., 2002b). Hence the brain regions where Ob-Rb receptors exist do not always respond to physiologically circulating leptin (Hosoi et al., 2002a). ...
... Deletion was only detected in hypothalamus tissue from LepRb-PTP1B –/– (Fig. 2.1). Although LepRb is also expressed outside of the hypothalamus (Elmquist et al., 1998; Mercer et al., 1998; Leshan et al., 2006), no deletion was detected in whole brain without To assess whether energy balance is affected by PTP1B-deficiency in LepRb-expressing neurons, we examined body weight in animals placed on normal chow or high-fat diet (HFD) at weaning. Both LepRb-PTP1B –/– and LepRb-PTP1B +/– mice show significantly decreased body weight on chow compared to Cre only wild type controls (Fig. 2.2A). ...
Protein tyrosine phosphatase 1B (PTP1B) is a ubiquitously expressed tyrosine phosphatase implicated in the central control of energy homeostasis via negative regulation of leptin signaling. Mice with central nervous system (CNS)-specific PTP1B deficiency demonstrate clear metabolic improvements, including decreased body weight and adiposity presumably due to enhanced leptin sensitivity. Interestingly, compound PTP1B:leptin double mutants show attenuated weight gain compared to leptin single mutants, suggesting that the metabolic effects of PTP1B deficiency may also involve non-leptin signaling pathways. Indeed, insulin and other non-leptin cytokines, such as interleukin-6, have been implicated in the CNS control of energy balance, and these pathways are also candidates of PTP1B regulation. Thus, whether or not the metabolic effects of PTP1B deficiency are due exclusively to enhanced leptin signaling remains unclear. This dissertation examines the role of central PTP1B in the control of leptin receptor-dependent energy homeostasis. A leptin receptor-expressing cell specific PTP1B-deficient (LepRb PTP1B / ) mouse model was generated and its metabolic phenotype was analyzed in comparison to wildtype controls and to whole body PTP1B knockouts. Though subtle phenotypic differences between LepRb-PTP1B-/- and PTP1B-/- mice exist, LepRb-PTP1B-/- mice demonstrate a majority of the phenotype observed in whole body PTP1B knockouts, including decreased body weight and adiposity and improved leptin sensitivity compared to wildtype controls. To further elucidate whether or not central PTP1B regulates non-leptin signaling pathways, a compound hypothalamic PTP1B:LepRb double mutant (Nkx2.1-PTP1B-/-:LepRb-/-) was generated and metabolically phenotyped in comparison to hypothalamic leptin receptor (Nkx2.1-LepRb-/-) and PTP1B (Nkx2.1-PTP1B-/-) mutant mice. While Nkx2.1-PTP1B-/- mice show decreased body weight and adiposity compared to wildtypes, both Nkx2.1-PTP1B-/-:LepRb /- and Nkx2.1-LepRb-/- show a severe obese phenotype marked by similarly increased weight gain, total fat mass, food intake, and glucose intolerance, indicating that the metabolic benefits of hypothalamic PTP1B deficiency are dependent upon functional leptin receptor signaling. Finally, whether PTP1B can regulate interleukin-6 signaling was explored using an immortalized mouse hypothalamic cell line. In summary, these data show that PTP1B is a critical regulator of energy balance within leptin receptor-expressing cells and within the hypothalamus specifically, and further begin to unravel the signaling pathways mediating the beneficial metabolic effects of central PTP1B deficiency.
... Increased leptin is associated with disrupted cerebellar activity [52] and the cerebellum plays an important role in regulating leptin-mediated processes related to food intake. The cerebellum shows strong leptin receptor gene expression [91] and is activated by circulating leptin and diet-induced obesity, which forms a monitoring/signaling pathway to complement more direct hypothalamic interactions [92]. ...
... The hypothalamic arcuate nucleus (ARC) was initially considered the main site of leptin actions in modulating energy balance and sympathetic activity; however, increasing evidence suggests that leptin may also act on a more extensive brain network that includes hindbrain areas such as the NTS and the ventral surface of the medulla (Grill et al. 2002; Ciriello & Moreau, 2013; Barnes & McDougal, 2014; Arnold & Diz, 2014; Ciriello & Caverson, 2014; Bassi et al. 2014a). The presence of functional leptin receptors (LRs) was demonstrated on cell bodies within the NTS and systemic or focal injections of leptin into the NTS enhanced c-fos expression in the caudal NTS subnuclei, suggesting that the NTS is an area activated by leptin (Mercer et al. 1998; Elias et al. 2000; Hosoi et al. 2002; Ciriello & Moreau, 2013). Injections of leptin into the NTS of anaesthetized rats acutely increased basal renal sympathetic nerve activity (RSNA) and the pressor response to peripheral chemoreflex activation (Mark et al. 2009; Ciriello & Moreau, 2012 Ciriello & Caverson, 2014). ...
With the global epidemic of obesity, breathing disorders associated with excess body weight have markedly increased. Respiratory dysfunctions caused by obesity were originally attributed to mechanical factors; however, recent studies have suggested a pathophysiological component that involves the central nervous system (CNS) and hormones such as leptin produced by adipocytes as well as other cells. Leptin is suggested to stimulate breathing and leptin deficiency causes an impairment of the chemoreflex, which can be reverted by leptin therapy. This facilitation of the chemoreflex may depend on the action of leptin in the hindbrain areas involved in the respiratory control such as the nucleus of the solitary tract (NTS), a site that receives chemosensory afferents, and the ventral surface of the medulla that includes the retrotrapezoid nucleus (RTN), a central chemosensitive area, and the rostral ventrolateral medulla (RVLM). Although the mechanisms and pathways activated by leptin to facilitate breathing are still not completely clear, evidence suggests that the facilitatory effects of leptin on breathing require the brain melanocortin system, including the POMC-MC4R pathway, a mechanism also activated by leptin to modulate blood pressure. The results of all the studies that have investigated the effect of leptin on breathing suggest that disruption of leptin signalling as caused by obesity-induced reduction of central leptin function (leptin resistance) is a relevant mechanism that may contribute to respiratory dysfunctions associated with obesity.
© 2015 The Authors. The Journal of Physiology © 2015 The Physiological Society.
... The hypothalamic arcuate nucleus (ARC) was initially considered the main site of leptin actions; however, increasing evidences suggest that leptin acts on a more extensive brain network [36]. For example, functional LRs are present in the nucleus of the solitary tract (NTS) [42,62], an important center involved in cardiorespiratory function. Thus, the focus of this mini-review is on the brain circuits and potential mechanisms that mediate the effects of leptin on respiratory function and cardiovascular regulation. ...
... Increased leptin is associated with disrupted cerebellar activity [52] and the cerebellum plays an important role in regulating leptin-mediated processes related to food intake. The cerebellum shows strong leptin receptor gene expression [91] and is activated by circulating leptin and diet-induced obesity, which forms a monitoring/signaling pathway to complement more direct hypothalamic interactions [92]. ...
Roux-en-Y gastric bypass (RYGB) surgery is a very effective bariatric procedure to achieve significant and sustained weight loss, yet little is known about the procedure's impact on the brain. This study examined the effects of RYGB on the brain's response to the anticipation of highly palatable versus regular food.
High fat diet-induced obese rats underwent RYGB or sham operation and were then tested for conditioned place preference (CPP) for the bacon-paired chamber, relative to the chow-paired chamber. After CPP, animals were placed in either chamber without the food stimulus, and brain-glucose metabolism (BGluM) was measured using positron emission tomography (μPET).
Bacon CPP was only observed in RYGB rats that had stable weight loss following surgery. BGluM assessment revealed that RYGB selectively activated regions of the right and midline cerebellum (Lob 8) involved in subjective processes related to reward or expectation. Also, bacon anticipation led to significant activation in the medial parabrachial nuclei (important in gustatory processing) and dorsomedial tegmental area (key to reward, motivation, cognition and addiction) in RYGB rats; and activation in the retrosplenial cortex (default mode network), and the primary visual cortex in control rats.
RYGB alters brain activity in areas involved in reward expectation and sensory (taste) processing when anticipating a palatable fatty food. Thus, RYGB may lead to changes in brain activity in regions that process reward and taste-related behaviors. Specific cerebellar regions with altered metabolism following RYGB may help identify novel therapeutic targets for treatment of obesity.
... The anorexigenic activity of GLP-1 could be mediated by NPY transmission; indeed, NPY-induced food intake is inhibited by GLP-1 and stimulated by Exenedin (9-39) [180, 182]. Some studies have shown that Leptin receptors and GLP-1 mRNA are co-expressed in the brainstem neurons and this observation suggests that GLP-1 may mediate the anorexigenic effects of Leptin [183, 184]. Histamine partially mediates the appetite suppression of GLP-1, because the anorexigenic effect is reduced by pharmacological or genetic loss of H1- receptor function [185]. ...
Abstract: Obesity is a chronic multifactorial disease caused by imbalance between caloric intake and energy
expenditure. The Neuroendocrine system is one of the main factors regulating energy intake in humans. The
Neuroendocrine system is made up of cells able to synthesize and secrete amines, peptides, growth factors and
biological mediators, known as neurohormones, which modulate various biological functions by interacting with the
nervous and immune system. In the central nervous system, neurosecretory elements are mainly located in the
hypothalamus which is the anatomical site of the hunger (lateral nucleus) and satiety (ventromedial nucleus) centers;
thus it plays a key role in chemical coding of food intake. Dopamine, Noradrenaline and Serotonin are historically
considered key points in the regulation of feeding behavior. However, other neurohormones have been identified; these
substances, also synthesized in peripheral tissues (especially adipose tissue and digestive tract), influence food intake.
Some of these hormones have orexigenic activity; conversely, other substances have anorexigenic activity. A constant
balance between orexigenic and anorexigenic neurohormones is essential to ensure a smooth feeding behavior,
whereas a subtle and progressive disruption of neurochemical transmission is sufficient to induce hyperphagia or
anorexia. Several factors affect the synthesis and release of neuropeptides: genetic, hormonal, psychological,
environmental, receptorial, type of feeding and meal frequency. In the recent past some drugs, as Sibutramine and
Rimonabant, modulating the activity of several neuroendocrine mediators (Serotonin, Noradrenaline,
Endocannabinoids), have proven to be effective in reducing weight excess, even if they were withdrawn because of
serious side effects. Recently, promising results in this way have been obtained with Glucagon like Peptide-1 analogs,
showing significant efficacy in counteracting weight excess without side effects. Further knowledge developments on
these complex neuroendocrine circuits and their hypothalamic interactions in food intake regulation could open new
frontiers for effective pharmacological therapeutic approach to Obesity and other nutritional disorders.
... The anatomical and behavioral results reported here support the hypothesis that CBS contains neurons that express Ob-Rb and are targets for the intake inhibitory effects of leptin. Previous reports of leptin receptors in the CBS left some uncertainty about the distribution of Ob-Rb (as opposed to other receptor isoforms) across structures of relevance to intake control (e.g. 2, 24, 26, 27, 29). In the present study, we used a modified FISH method with improved sensitivity for detecting neuronal mRNA species of relatively low abundance (such as those encoding most hormone receptors ) (31). ...
Three experiments were performed to investigate the hypothesis that leptin action within the caudal brain stem (CBS) contributes to its intake inhibitory effects. The first experiment evaluated the anatomical distribution of leptin receptor mRNA in rat CBS using a sensitive fluorescence in situ hybridization method with a riboprobe specific for the long form of the leptin receptor (Ob-Rb). An Ob-Rb mRNA hybridization signal was detected in neurons of several CBS nuclei involved in the control of food intake, including the dorsal vagal complex and parabrachial nucleus. A strong hybridization signal was also obtained from neuronal cell bodies of a number of other structures including the hypoglossal, trigeminal, lateral reticular, and cochlear nuclei; locus ceruleus; and inferior olive. The anatomical profile revealed by fluorescence in situ hybridization was in good agreement with immunocytochemical analysis with an antibody specific to Ob-Rb. In a second experiment, exploring the relevance of CBS Ob-Rb to feeding behavior, rats were given a fourth intracerebroventricular (i.c.v.) injection of leptin (0.1, 0.83, or 5.0μ g; n = 9–11/group) or vehicle 30 min before lights-out on three consecutive days The two higher doses reduced food intake significantly at 2, 4, and 24 h after injection and caused significant reductions of body weight. The dose-response profiles for fourth i.c.v. administration were indistinguishable from those obtained from separate groups of rats that received leptin via a lateral i.c.v. cannula. In the last experiment, a ventricle-subthreshold dose of leptin (0.1 μg) microinjected unilaterally into the dorsal vagal complex suppressed food intake at 2, 4, and 24 h. The results indicate that the CBS contains neurons that are potentially direct targets for the action of leptin in the control of energy homeostasis.
... The hypothalamic arcuate nucleus (ARC) was initially considered the main site of leptin actions; however, increasing evidences suggest that leptin acts on a more extensive brain network [36]. For example, functional LRs are present in the nucleus of the solitary tract (NTS) [42,62], an important center involved in cardiorespiratory function. Thus, the focus of this mini-review is on the brain circuits and potential mechanisms that mediate the effects of leptin on respiratory function and cardiovascular regulation. ...
... Leptin receptors are present in several hindbrain regions including the nucleus of the solitary tract (NTS), locus coeruleus (LC), rostral ventrolateral medulla (RVLM) and Bötzinger complex (BötC; Mercer et al. 1998, Elias et al. 2000, Hosoi et al. 2002, Grill & Hayes 2009, Bassi et al. 2012. The administration of leptin into the caudal portion of the NTS increases sympathetic nerve activity (Mark et al. 2009), and leptin-injected intracerebroventricularly activates pre-sympathetic neurones in the RVLM (Zhang & Felder 2004). ...
Leptin, an adipocyte-derived hormone, is suggested to participate in the central control of breathing. We hypothesized that leptin may facilitate ventilatory responses to chemoreflex activation by acting on respiratory nuclei of the ventrolateral medulla. The baseline ventilation and the ventilatory responses to CO2 were evaluated before and after daily injections of leptin into the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) for 3 days in obese leptin-deficient (ob/ob) mice.
Male ob/ob mice (40-45 g, n = 7/group) received daily microinjections of vehicle or leptin (1 μg/100 nl) for 3 days into the RTN/pFRG. Respiratory responses to CO2 were measured by whole body plethysmography. Results: Unilateral microinjection of leptin into the RTN/pFRG in ob/ob mice increased baseline ventilation (VE ) from 1447 ± 96 ml.min(-1) .kg(-1) to 2405 ± 174 ml.min(-1) .kg(-1) by increasing tidal volume (VT ) from 6.4 ± 0.4 ml.kg(-1) to 9.1 ± 0.8 ml.kg(-1) (p < 0.05). Leptin also enhanced ventilatory responses to 7% CO2 (Δ = 2172 ± 218 ml.min(-1) .kg(-1) , vs. control: Δ = 1255 ± 105 ml.min(-1) .kg(-1) ), which was also due to increased VT (Δ = 4.71 ± 0.51 ml.kg(-1) , vs. control: Δ = 2.27 ± 0.20 ml.kg(-1) ), without changes on respiratory frequency. Leptin treatment into the RTN/pFRG or into the surrounding areas decreased food intake (83% and 70%, respectively), without significantly changing body weight.
The present results suggest that leptin acting in the respiratory nuclei of the ventrolateral medulla improves baseline VE and VT and facilitates respiratory responses to hypercapnia in ob/ob mice. This article is protected by copyright. All rights reserved.
... Leptin mRNA, and protein are found with particularly high levels in the thalamus and the Purkinje and granular cell layers of the cerebellum (Morash et al. 1999;Ur et al. 2002) indicating that leptin may be released locally in the brain, acting on specific sensory and motor systems. In human, LepR mRNA is expressed at significantly higher levels in the cerebellum than in the hypothalamus (Elmquist et al. 1998;Mercer et al. 1998). In rodents, LepR expression is also evident in cerebellar neurons (Elmquist et al. 1998;Udagawa et al. 2000) during both embryonic and postnatal stages of development (Elmquist et al. 1998;Udagawa et al. 2006). ...
The hormone leptin, by binding to hypothalamic receptors, suppresses food intake and decreases body adiposity. Leptin receptors are also widely expressed in extra-hypothalamic areas such as hippocampus, amygdala and cerebellum, where leptin modulates synaptic transmission. Here we show that a defective leptin receptor affects the electrophysiological properties of cerebellar Purkinje neurons (PNs). PNs from (db/db) mice recorded in cerebellar slices display a higher firing rate of spontaneous action potentials than PNs from wild type (WT) mice. Blockade of GABAergic tonic inhibition with bicuculline in WT mice changes the firing pattern from continuous, uninterrupted spiking into bursting firing, but bicuculline does not produce these alterations in db/db neurons, suggesting that they receive a weaker GABAergic inhibitory input. Our results also show that the intrinsic firing properties (auto-rhythmicity) of WT and db/db PNs are different. Tonic firing of PNs, the only efferent output from the cerebellar cortex, is a persistent signal to downstream cerebellar targets. The significance of leptin modulation of PNs spontaneous firing is not known. Also, it is not clear if the increased excitability of cerebellar PNs in db/db mice results from hyperglycemia or from the lack of leptin signaling, since both conditions coexist in the db/db strain.
... One study examining the impact of combined Htr2c null and ob/ob leptin null mutations revealed a synergistic effect on glucose regulation, indicated by a marked exacerbation of the diabetes phenotype characteristic of the ob/ob genotype (Wade et al., 2008 ). Early indications that leptin might have a direct effect on serotonin function arose from several histological studies demonstrating that long-form leptin receptor (LEPRb) is expressed in neurons of the raphe nuclei (Elmquist et al., 1998; Mercer et al., 1998; Shioda et al., 1998) where it co-localizes with SERT, a marker of serotonin-producing neurons in the raphe (Collin et al., 2000; Finn et al., 2001). Furthermore, serotonergic neurons of the dorsal raphe have been shown to take up a labeled leptin analog infused into the lateral ventricle, a phenomenon indicative of LEPRb binding and internalization (Fernandez-Galaz et al., 2002 ). ...
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.
... More specifically, that study found that leptin injected into the 4V activated the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathway in forebrain tissues. Because STAT3 phosphorylation is widely considered a marker of leptin receptor binding and activation (Mercer et al., 1998;Myers, 2004;Peters et al., 2007;Tartaglia et al., 1995), this finding could be explained by leptin reaching structures near the forebrain ventricles. As discussed in more depth by Ruiter et al (2010), a more likely possibility is that leptin was able to access forebrain sites by entering the subarachnoid space with the flow of CSF from the 4V. ...
In spite of evidence to the contrary, concern that substances injected into the fourth ventricle (4V) reach forebrain structures challenges the validity of using these injections to evaluate the role of hindbrain structures. Injection of AngII into the lateral ventricle (LV) increases water intake, but a similar response is not observed after injection into the 4V. This alone suggests the requirement of forebrain structures, but the potential for a counteracting, anti-dipsogenic pressor response to hindbrain AngII allows for lingering concern that this competing effect of AngII, rather than lack of forebrain access, underlies the negative result. Here, we used a double cannulation approach (LV and 4V) to evaluate the effect of the AngII receptor antagonist, losartan, on the drinking response to AngII injected into the LV. Injections of losartan into the LV blocked the dipsogenic response to AngII given 5min later into the LV. There was no effect, however, when losartan was injected into 4V, even when we used a dose of losartan that was 25 times greater than needed when injected into the LV. Collectively, these experiments suggest that concerns about diffusion from hindbrain ventricles to forebrain structures are overstated and can be circumvented using proper dose and timing of injections. Moreover, these data provide additional support to the existing literature showing that forebrain structures are key sites in the stimulation of drinking behavior by AngII.
... Systemic injections of leptin have been shown not only to increase c-fos expression (27, 30) but also increase STAT3 expression, which is indicative of activation of functional leptin receptors within NTS (42). Consistent with this latter interpretation of direct actions of leptin within NTS, leptin receptors have been identified within the nucleus, primarily within the cardiovascular responsive aspects of the caudal NTS complex (29, 30, 36, 37, 42, 53, 68). In addition, both in vivo and in vitro studies recording the electrical activity of neurons within NTS have shown that leptin can either directly activate or inhibit the discharge frequency of these neurons (28, 76, 77). ...
Circulating levels of leptin are elevated in individuals suffering from chronic intermittent hypoxia (CIH). Systemic and central administration of leptin elicits increases in sympathetic nervous activity (SNA), arterial pressure (AP), and heart rate (HR), and it attenuates the baroreceptor reflex, cardiovascular responses that are similar to those observed during CIH as a result of activation of chemoreceptors by the systemic hypoxia. Therefore, experiments were done in anesthetized Wistar rats to investigate the effects of leptin in nucleus of the solitary tract (NTS) on AP and HR responses, and renal SNA (RSNA) responses during activation of NTS neurons and the chemoreceptor reflex. Microinjection of leptin (5-100 ng; 20 nl) into caudal NTS pressor sites (l-glutamate; l-Glu; 0.25 M; 10 nl) elicited dose-related increases in AP, HR, and RSNA. Leptin microinjections (5 ng; 20 nl) into these sites potentiated the increase in AP and HR elicited by l-Glu. Additionally, bilateral injections of leptin (5 ng; 100 nl) into NTS potentiated the increase in AP and attenuated the bradycardia to systemic activation of the chemoreflex. In the Zucker obese rat, leptin injections into NTS neither elicited cardiovascular responses nor altered the cardiovascular responses to activation of the chemoreflex. Taken together, these data indicate that leptin exerts a modulatory effect on neuronal circuits within NTS that control cardiovascular responses elicited during the reflex activation of arterial chemoreceptors and suggest that increased AP and SNA observed in individuals with CIH may be due, in part, by leptin's effects on the chemoreflex at the level of NTS.
... The search for these specific neuronal populations has become a critical issue to the field. LepR is highly expressed in numerous brainstem and hypothalamic sites [59, 113, 121, 231, 232, 242, 294]. In the brainstem, the nucleus of the solitary tract (NTS) might possibly mediate leptin's effects as it functions as a sensory relay of the internal milieu via vagal afferents or the area postrema [132, 172, 175]. ...
Leptin is an adipocyte-derived hormone involved in a myriad of physiological process, including the control of energy balance and several neuroendocrine axes. Leptin-deficient mice and humans are obese, diabetic, and display a series of neuroendocrine and autonomic abnormalities. These individuals are infertile due to a lack of appropriate pubertal development and inadequate synthesis and secretion of gonadotropins and gonadal steroids. Leptin receptors are expressed in many organs and tissues, including those related to the control of reproductive physiology (e.g., the hypothalamus, pituitary gland, and gonads). In the last decade, it has become clear that leptin receptors located in the brain are major players in most leptin actions, including reproduction. Moreover, the recent development of molecular techniques for brain mapping and the use of genetically modified mouse models have generated crucial new findings for understanding leptin physiology and the metabolic influences on reproductive health. In the present review, we will highlight the new advances in the field, discuss the apparent contradictions, and underline the relevance of this complex physiological system to human health. We will focus our review on the hypothalamic circuitry and potential signaling pathways relevant to leptin's effects in reproductive control, which have been identified with the use of cutting-edge technologies of molecular mapping and conditional knockouts.
... These broadly distributed effects of hormones and neurotransmitters provide opportunities for them to modulate several aspects of food intake regulation in a coordinated fashion, including those mediated by viscerosensory, motivational, affective and motor systems [34, 115]. For example, leptin receptors are expressed in peripheral tissues, peripheral nerves, and numerous CNS nuclei116117118119120. Moreover, leptin has been shown to have effects on food intake and body weight at many of these sites121122123124125126127128129130131132. ...
Abnormal perinatal nutrition (APN) results in a predisposition to develop obesity and the metabolic syndrome and thus may contribute to the prevalence of these disorders. Obesity, including that which develops in organisms exposed to APN, has been associated with increased meal size. Vagal afferents of the gastrointestinal (GI) tract contribute to regulation of meal size by transmitting satiation signals from gut-to-brain. Consequently, APN could increase meal size by altering this signaling, possibly through changes in expression of factors that control vagal afferent development or function. Here two studies that addressed these possibilities are reviewed. First, meal patterns, meal microstructure, and the structure and density of vagal afferents that innervate the intestine were examined in mice that experienced early postnatal overnutrition (EPO). These studies provided little evidence for EPO effects on vagal afferents as it did not alter meal size or vagal afferent density or structure. However, these mice exhibited modest hyperphagia due to a satiety deficit. In parallel, the possibility that brain-derived neurotrophic factor (BDNF) could mediate APN effects on vagal afferent development was investigated. Brain-derived neurotrophic factor was a strong candidate because APN alters BDNF levels in some tissues and BDNF knockout disrupts development of vagal sensory innervation of the GI tract. Surprisingly, smooth muscle-specific BDNF knockout resulted in early-onset obesity and hyperphagia due to increases in meal size and frequency. Microstructure analysis revealed decreased decay of intake rate during a meal in knockouts, suggesting that the loss of vagal negative feedback contributed to their increase in meal size. However, meal-induced c-Fos activation within the dorsal vagal complex suggested this effect could be due to augmentation of vago-vagal reflexes. A model is proposed to explain how high-fat diet consumption produces increased obesity in organisms exposed to APN, and may be required to reveal effects of EPO on vagal function.
The hypothalamus and dorsal vagal complex (DVC) are both important for integration of signals that regulate energy balance. Increased leptin transgene expression in either the hypothalamus or DVC of female rats was shown to decrease white adipose tissue and circulating levels of leptin and adiponectin. However, in contrast to hypothalamus, leptin transgene expression in the DVC had no effect on food intake, circulating insulin, ghrelin and glucose, nor on thermogenic energy expenditure. These findings imply different roles for hypothalamus and DVC in leptin signaling. Leptin signaling is required for normal bone accrual and turnover. Leptin transgene expression in the hypothalamus normalized the skeletal phenotype of leptin-deficient ob/ob mice but had no long-duration (≥10 weeks) effects on the skeleton of leptin-replete rats. The goal of this investigation was to determine the long-duration effects of leptin transgene expression in the DVC on the skeleton of leptin-replete rats. To accomplish this goal, we analyzed bone from three-month-old female rats that were microinjected with recombinant adeno-associated virus encoding either rat leptin (rAAV-Leptin, n = 6) or green fluorescent protein (rAAV-GFP, control, n = 5) gene. Representative bones from the appendicular (femur) and axial (3rd lumbar vertebra) skeleton were evaluated following 10 weeks of treatment. Selectively increasing leptin transgene expression in the DVC had no effect on femur cortical or cancellous bone microarchitecture. Additionally, increasing leptin transgene expression had no effect on vertebral osteoblast-lined or osteoclast-lined bone perimeter or marrow adiposity. Taken together, the findings suggest that activation of leptin receptors in the DVC has minimal specific effects on the skeleton of leptin-replete female rats.
Obesity hypoventilation syndrome (OHS) is defined as daytime hypercapnia in obese individuals in the absence of other underlying causes. In the United States OHS is present in 10-20% of obese patients with obstructive sleep apnea and is linked to hypoventilation during sleep. OHS leads to high cardiorespiratory morbidity and mortality, and there is no effective pharmacotherapy. The depressed hypercapnic ventilatory response plays a key role in OHS. The pathogenesis of OHS has been linked to resistance to an adipocyte-produced hormone, leptin, a major regulator of metabolism and control of breathing. Mechanisms by which leptin modulates the control of breathing are potential targets for novel therapeutic strategies in OHS. Recent advances shed light on the molecular pathways related to the central chemoreceptor function in health and disease. Leptin signaling in the nucleus of the solitary tract, retrotrapezoid nucleus, hypoglossal nucleus, and dorsomedial hypothalamus, and anatomical projections from these nuclei to the respiratory control centers, may contribute to OHS. In this review, we describe current views on leptin-mediated mechanisms that regulate breathing and CO2 homeostasis with a focus on potential therapeutics for the treatment of OHS.
The lateral parabrachial nucleus (LPBN) has been shown to be involved in the suppression of appetite at the pharmacological, optogenetic and chemogenetic levels. However, the signalling that mediates activation of these neurons in physiological conditions has been hindered by difficulties in segregating different cell populations in this region. Using reporter mice, we identify at the electrophysiological level the effects of an anorexic hormone, leptin, on leptin receptor (ObR)-expressing neurons in the LPBN (LPBNObR neurons). Application of leptin caused inhibition in a subpopulation of LPBNObR neurons. This effect was mediated by an increased potassium conductance and was also accompanied by a decrease in excitatory synaptic input onto these neurons. However, mimicking the inhibitory effects of leptin on LPBNObR neurons through chemogenetics led to no changes in feeding or glucose levels, which suggests that leptin action on LPBNObR neurons may not be sufficient to regulate these metabolic aspects.
Obstructive sleep apnea (OSA) is recurrent obstruction of the upper airway due to the loss of upper airway muscle tone during sleep. OSA is highly prevalent, especially in obesity. There is no pharmacotherapy for OSA. Previous studies have demonstrated the role of leptin, an adipose-tissue-produced hormone, as a potent respiratory stimulant. Leptin signaling via a long functional isoform of leptin receptor, LEPRb, in the nucleus of the solitary tract (NTS), has been implicated in control of breathing. We hypothesized that leptin acts on LEPRb positive neurons in the NTS to increase ventilation and maintain upper airway patency during sleep in obese mice. We expressed designer receptors exclusively activated by designer drugs (DREADD) selectively in the LEPRb positive neurons of the NTS of Leprb-Cre-GFP mice with diet-induced obesity (DIO) and examined the effect of DREADD ligand, J60, on tongue muscle activity and breathing during sleep. J60 was a potent activator of LEPRb positive NTS neurons, but did not stimulate breathing or upper airway muscles during NREM and REM sleep. We conclude that, in DIO mice, the stimulating effects of leptin on breathing during sleep are independent of LEPRb signaling in the NTS.
Past studies have suggested that non-neuronal brain cells express the leptin receptor. However, the identity and distribution of these leptin receptor-expressing non-neuronal brain cells remain debated. This study assessed the distribution of the long form of the leptin receptor (LepRb) in non-neuronal brain cells using a reporter mouse model in which LepRb-expressing cells are permanently marked by tdTomato fluorescent protein (LepRb-CretdTomato). Double immunohistochemistry revealed that, in agreement with the literature, the vast majority of tdTomato-tagged cells across the mouse brain were neurons (i.e., based on immunoreactivity for NeuN). Non-neuronal structures also contained tdTomato-positive cells, including the choroid plexus and the perivascular space of the meninges and, to a lesser extent, the brain. Based on morphological criteria and immunohistochemistry, perivascular cells were deduced to be mainly pericytes. Notably, tdTomato-positive cells were immunoreactive for vitronectin and platelet derived growth factor receptor beta (PDGFBR). In situ hybridization studies confirmed that most tdTomato-tagged perivascular cells were enriched in leptin receptor mRNA (all isoforms). Using qPCR studies, we confirmed that the mouse meninges were enriched in Leprb and, to a greater extent, the short isoforms of the leptin receptor. Interestingly, qPCR studies further demonstrated significantly altered expression for Vtn and Pdgfrb in the meninges and hypothalamus of LepRb-deficient mice. Collectively, our data demonstrate that the only intracranial non-neuronal cells that express LepRb in the adult mouse are cells that form the blood-brain barrier, including, most notably, meningeal perivascular cells. Our data suggest that pericytic leptin signaling plays a role in the integrity of the intracranial perivascular space and, consequently, may provide a link between obesity and numerous brain diseases.
Plaguing humans for more than two millennia, manifest on every continent studied, and with more than one billion patients having an attack in any year, migraine stands as the sixth most common cause of disability on the planet. The pathophysiology of migraine has emerged from a historical consideration of the “humors” through mid-20th century distraction of the now defunct Vascular Theory to a clear place as a neurological disorder. It could be said there are three questions: why, how, and when? Why: migraine is largely accepted to be an inherited tendency for the brain to lose control of its inputs. How: the now classical trigeminal durovascular afferent pathway has been explored in laboratory and clinic; interrogated with immunohistochemistry to functional brain imaging to offer a roadmap of the attack. When: migraine attacks emerge due to a disorder of brain sensory processing that itself likely cycles, influenced by genetics and the environment. In the first, premonitory, phase that precedes headache, brain stem and diencephalic systems modulating afferent signals, light-photophobia or sound-phonophobia, begin to dysfunction and eventually to evolve to the pain phase and with time the resolution or postdromal phase. Understanding the biology of migraine through careful bench-based research has led to major classes of therapeutics being identified: triptans, serotonin 5-HT1B/1D receptor agonists; gepants, calcitonin gene-related peptide (CGRP) receptor antagonists; ditans, 5-HT1F receptor agonists, CGRP mechanisms monoclonal antibodies; and glurants, mGlu5 modulators; with the promise of more to come. Investment in understanding migraine has been very successful and leaves us at a new dawn, able to transform its impact on a global scale, as well as understand fundamental aspects of human biology.
Obesity and the chronic diseases associated with obesity are rapidly becoming a public health crisis in the world. The existence of a circulating factor with effects on body weight has long been anticipated. The discovery of the hormone leptin in 1994 dramatically accelerated the pace of research in obesity and the neurobiology of feeding. Following the initial characterization of leptin, the concept of a peripheral satiety signal gained new credibility. Early research typically focused on the weight-reducing effects of leptin, consequently leading to the idea of leptin as an antiobesity hormone. Subsequent studies, however, have indicated that leptin plays a role in many functions related to energy balance Many insights into leptin function have been gained from rodent models of obesity, which either lack functional leptin or do not respond to its effects due to the mutation in the leptin receptor gene. An obvious and important search has begun to see whether a significant proportion of human obesity can be due to mutations in the ob gene. The coding region of the leptin gene has been sequenced from hundreds of people, but mutations have not been found. Although mutations other than in the coding region may affect leptin messenger RNA, the demonstration that most obese individuals have high circulating leptin levels did exclude the possibility that the problem lied in the ob gene itself. Indeed, human obesity does not appear to be due to impairment in leptin or its receptor. Overweight and obese humans are hyperleptinemic but they seem to be resistant to leptin, which means they do not respond to high levels of leptin in blood. Biological processes potentially involved in leptin resistance include transport of leptin into the brain, alterations of leptin hypothalamic receptor expression, and downstream signaling pathways. This chapter focuses on possible mechanisms that may underlie leptin resistance.
Obesity is associated with significant health risks including stroke and heart disease. The prevalence of obesity has dramatically increased over the past 20 years. Although the development of obesity is clearly related to changing lifestyles, the central nervous system plays a key role in regulation of energy balance. To develop effective strategies for treating obesity, we must gain a clearer understanding of the neurocircuitry and signaling mechanisms involved. Toward this end, recent progress has been made in the understanding of the roles played by the sensory circumventricular organs (CVOs) of the brain. These areas lack the normal blood-brain barrier and thus act as transducers of signals between the blood, other centers in the brain, and the cerebrospinal fluid. This review focuses on the roles played by the sensory CVOs in detecting and responding to a number of signals that carry information regarding nutritional status, including cholecystokinin, amylin, ghrelin, peptide YY, pancreatic polypeptide, leptin, adiponectin, and glucose.
Background: In a murine myotube cell line (C2C12 myotubes), leptin at low physiological concentrations (1 ng/ml) has been shown to stimulate glucose transport and glycogen synthesis. The aim of the present study was to test whether an analogous leptin effect on glucose transport is detectable in the heart. Methods and Results: We used the isolated Langendorff rat heart preparation with hemodynamic function control. Using polymerase chain reaction (RT-PCR), a 346- and 375-base fragment indicative for the short and long leptin receptor isoform was detected in the rat heart. Glucose transport rates were calculated using equimolar double tracer perfusion with the non-metabolizable glucose analog 3-O-methylglucose (3-O-MG) and the non-transportable tracer mannitol and two-compartimental modeling. 3-O-MG uptake at a perfusate glucose concentration of 11 mM was measured over 15 minutes in control hearts, hearts perfused with insulin (10 mU/ml), leptin (1 ng/ml) or insulin (10 mU/ml) plus leptin (1 ng/ml; n = 8 in each group). The basal 3-O-MG transport rate of 0,7351 +/- 0,051 mumol/min/g wet weight was increased 4.18fold with insulin, 2.69fold with leptin, and 4.2fold with leptin plus insulin. Simultaneous monitoring of hemodynamic function revealed a minor and transient effect of leptin on left ventricular pressure, which was strongly augmented in coperfusion with insulin. Conclusion: The data suggest that leptin at low physiological concentrations is able to exert a partial insulin like effect on glucose uptake. We speculate that the effect might be mediated by both leptin receptor isoforms. This leptin effect is additive to the effect of insulin and might therefore contribute to the insulin independent basal glucose supply of the heart. It can not be completely excluded that the observed leptin effect on glucose transport is secondary to altered myocardial function.
Study objectives:
Obesity hypoventilation and obstructive sleep apnea are common complications of obesity linked to defects in respiratory pump and upper airway neural control. Leptin-deficient ob/ob mice have impaired ventilatory control and inspiratory flow limitation during sleep, which are both reversed with leptin. We aimed to localize central nervous system (CNS) site(s) of leptin action on respiratory and upper airway neuroventilatory control.
Methods:
We localized the effect of leptin to medulla versus hypothalamus by administering intracerbroventricular leptin (10 μg/2 μL) versus vehicle to the lateral (n = 14) versus fourth ventricle (n = 11) of ob/ob mice followed by polysomnographic recording. Analyses were stratified for effects on respiratory (nonflow-limited breaths) and upper airway (inspiratory flow limitation) functions. CNS loci were identified by (1) leptin-induced signal transducer and activator of transcription 3 (STAT3) phosphorylation and (2) projections of respiratory and upper airway motoneurons with a retrograde transsynaptic tracer (pseudorabies virus).
Results:
Both routes of leptin administration increased minute ventilation during nonflow-limited breathing in sleep. Phrenic motoneurons were synaptically coupled to the nucleus of the solitary tract, which also showed STAT3 phosphorylation, but not to the hypothalamus. Inspiratory flow limitation and obstructive hypopneas were attenuated by leptin administration to the lateral but not to the fourth cerebral ventricle. Upper airway motoneurons were synaptically coupled with the dorsomedial hypothalamus, which exhibited STAT3 phosphorylation.
Conclusions:
Leptin relieves upper airway obstruction in sleep apnea by activating the forebrain, possibly in the dorsomedial hypothalamus. In contrast, leptin upregulates ventilatory control through hindbrain sites of action, possibly in the nucleus of the solitary tract.
Amylin, a peptide hormone produced in the pancreas and in the brain, has well-established physiological roles in glycemic regulation and energy balance control. It improves postprandial blood glucose levels by suppressing gastric emptying and glucagon secretion; these beneficial effects have led to the FDA-approved use of the amylin analog pramlintide in the treatment of diabetes mellitus. Amylin also acts centrally as a satiation signal, reducing food intake and body weight. The ability of amylin to promote negative energy balance, along with its unique capacity to cooperatively facilitate or enhance the intake- and body weight-suppressive effects of other neuroendocrine signals like leptin, have made amylin a leading target for the development of novel pharmacotherapies for the treatment of obesity. In addition to these more widely studied effects, a growing body of literature suggests that amylin may play a role in processes related to cognition, including the neurodegeneration and cognitive deficits associated with Alzheimer's disease (AD). Although the function of amylin in AD is still unclear, intriguing recent reports indicate that amylin may improve cognitive ability and reduce hallmarks of neurodegeneration in the brain. The frequent comorbidity of diabetes mellitus and obesity, as well as the increased risk for and occurrence of AD associated with these metabolic diseases, suggests that amylin-based pharmaceutical strategies may provide multiple therapeutic benefits. This review will discuss the known effects of amylin on glycemic regulation, energy balance control, and cognitive/motivational processes. Particular focus will be devoted to the current and/or potential future clinical use of amylin pharmacotherapies for the treatment of diseases in each of these realms.
Leptin has profound effects on food intake, body weight, and neuroendocrine status. The lack of leptin results in hormonal and metabolic alterations and a dramatic increase in body weight. Leptin acts in the brain, especially in the hypothalamus; however, the central nervous system sites that respond to leptin have not been examined comprehensively. In this study, we explored systematically the distribution of leptin-activated neurons throughout the rat brain. Furthermore, we investigated the chemical identity of subsets of these leptin-activated cells. Fos-like immunoreactivity (Fos-IR) was investigated in the rat brain after two different doses of leptin (1.0 mg/kg and 5.0 mg/kg) at 2 hours and 6 hours after injections. The induction of Fos-IR was observed in hypothalamic nuclei, including the paraventricular nucleus (PVH), the retrochiasmatic area (RCA), the ventromedial nucleus (VMH), the dorsomedial nucleus (DMH), the arcuate nucleus (Arc), and the ventral premammillary nucleus (PMV). In addition, leptin-induced Fos-IR was found in several nuclei of the brainstem, including the superior lateral and external lateral subdivisions of the parabrachial nucleus (slPB and elPB, respectively), the supragenual nucleus, and the nucleus of the solitary tract (NTS). By using double-labeling immunohistochemistry or immunohistochemistry coupled with in situ hybridization, leptin-activated neurons were found that contained cocaine- and amphetamine-regulated transcript mRNA in several hypothalamic nuclei, including the RCA, Arc, DMH, and PMV. In the Arc and DMH, leptin-induced Fos-IR was observed in neurons that expressed neurotensin mRNA. Dynorphin neurons in the VMH and in the Arc also expressed Fos-IR. In the brainstem, we found that cholecystokinin neurons in the slPB and glucagon-like peptide-1 neurons in the NTS were activated by leptin. We also investigated the coexpression of Fos-IR and the long form of the leptin receptor (OBRb) mRNA. We found double-labeled neurons surrounding the median eminence and in the RCA, Arc, VMH, DMH, and PMV. However, in brainstem sites, very little OBRb mRNA was found; thus, there were very few double-labeled cells. These results suggest that leptin stimulates brain pathways containing neuropeptides that are involved in the regulation of energy balance, autonomic homeostasis, and neuroendocrine status. J. Comp. Neurol. 423:261–281, 2000.
The brainstem contains all the necessary circuitry that is required to control the basic mechanisms of feeding and to regulate the size of individual meals. This is achieved through reciprocal connections with the whole length of the gastrointestinal tract. Any meal is initiated by feed-forward sensory information and is terminated by feedback information. To modulate eating with respect to other facets of energy homeostasis, the brainstem detects neuronal and hormonal signals from the gut and other organs, and integrates this information by communication with higher brain centers. This chapter is focused to assess the role of neuropeptides with particular reference to the dorsal vagal complex. The term neuropeptide is treated in its widest context to include peptide transmitters and peptide hormones. Some neuropeptides that are intrinsic to the brainstem itself may have as important roles in energy homeostasis as their hypothalamic counterparts. The modulation of brainstem function by descending peptidergic pathways from higher brain centers suggests that the brainstem is a target for pharmaceutical intervention. There are peptidergic inputs from the periphery either carried neuronally or humorally. These signals are integrated by the nucleus of the tractus solitarius (NTS) and projected back to the periphery or to other brain centers. Descending peptidergic pathways further modulate the NTS. The importance of peptidergic neurons that are intrinsic to the NTS is being appreciated and they may yet prove to be as crucial in regulating appetite and body weight as some of their hypothalamic counterparts.
Obese leptin-deficient (ob/ob) mice demonstrate defects in upper airway structural and neuromuscular control. We hypothesized that these defects predispose to upper airway obstruction during sleep, and improve with leptin administration. High-fidelity polysomnographic recordings were conducted to characterize sleep and breathing patterns in conscious, unrestrained ob/ob mice (23 weeks, 67.2±4.1g, n=13). In a parallel-arm crossover study, we compared responses to subcutaneous leptin (1µg/hr) vs. vehicle on respiratory parameters during NREM and REM sleep. Upper airway obstruction was defined by the presence of inspiratory airflow limitation (IFL), as characterized by an early inspiratory plateau in airflow at a maximum level (VImax) with increasing effort. The severity of upper airway obstruction (VImax) was assessed along with minute ventilation (VE), tidal volume (VT), respiratory rate (RR), inspiratory duty cycle, and mean inspiratory flow at each time point. IFL occurred more frequently in REM sleep (37.6±0.2% vs. 1.1±0.0% in NREM sleep, p<0.001), and leptin did not alter its frequency. VImax (3.7±1.1 vs. 2.7±0.8mL/s, p<0.001) and VE increased (55.4±22.0 vs. 39.8±16.4mL/min, p<0.001) with leptin vs. vehicle administration. The increase in VE was due to a significant increase in VT (0.20±0.06 vs. 0.16±0.05mL, p<0.01) rather than RR. Increases in VE were attributable to increases in mean inspiratory flow (2.5±0.8 vs. 1.8±0.6mL/s, p<0.001) rather than inspiratory duty cycle. Similar increases in VE and its components were observed in non-flow limited breaths during NREM and REM sleep. These responses suggest that leptin stabilized pharyngeal patency and increased drive to both the upper airway and diaphragm during sleep.
This chapter focuses on the mechanisms that influence ingestive behavior, body weight, body-fat content, body-fat distribution, energy expenditure, and fuel homeostasis. Reproductive processes are an important part of the energy balancing system because evolutionary adaptation involves not only survival, but also differential reproductive success. Mechanisms that control ingestive behavior and energy balance did not evolve specifically to ensure a set point for body weight or body composition, or to prevent clinical syndromes, such as obesity or diabetes, but rather to increase survival and optimize reproductive success. A biological perspective on this problem begins with the observation that living cells expend energy constantly, and thus, require a continuous supply of fuels for energy metabolism. A constant supply of glucose as a metabolic substrate is particularly critical for cells in the central nervous system. Food ingestion, digestion, and metabolism are the primary means by which these energetic requirements are met, but food availability and energetic demands fluctuate in most habitats, and most organisms do not eat continuously.
PURPOSE:
To evaluate genes differentially expressed in ovaries from lean (wild type) and obese (ob/ob) female mice and cyclic AMP production in both groups.
METHODS:
The expression on messenger RNA levels of 84 genes concerning obesity was analyzed through the PCR array, and cyclic AMP was quantified by the enzyme immunoassay method.
RESULTS:
The most downregulated genes in the Obesity Group included adenylate cyclase-activating polypeptide type 1, somatostatin, apolipoprotein A4, pancreatic colipase, and interleukin-1 beta. The mean decrease in expression levels of these genes was around 96, 40, 9, 4.2 and 3.6-fold, respectively. On the other hand, the most upregulated genes in the Obesity Group were receptor (calcitonin) activity-modifying protein 3, peroxisome proliferator activated receptor alpha, calcitonin receptor, and corticotropin-releasing hormone receptor 1. The increase means in the expression levels of such genes were 2.3, 2.7, 4.8 and 6.3-fold, respectively. The ovarian cyclic AMP production was significantly higher in ob/ob female mice (2,229±52 fMol) compared to the Control Group (1,814±45 fMol).
CONCLUSIONS:
Obese and anovulatory female mice have reduced reproductive hormone levels and altered ovogenesis. Several genes have their expression levels altered when leptin is absent, especially adenylate cyclase-activating polypeptide type 1.
Obesity and type 2 diabetes mellitus (T2DM) often occur together and affect a growing number of individuals in both the developed and developing worlds. Both are associated with a number of other serious illnesses that lead to increased rates of mortality. There is likely a polygenic mode of inheritance underlying both disorders, but it has become increasingly clear that the pre- and postnatal environments play critical roles in pushing predisposed individuals over the edge into a disease state. This review focuses on the many genetic and environmental variables that interact to cause predisposed individuals to become obese and diabetic. The brain and its interactions with the external and internal environment are a major focus given the prominent role these interactions play in the regulation of energy and glucose homeostasis in health and disease.
Copyright © 2015 the American Physiological Society.
The decline in cardiovagal baroreflex function that occurs with aging is accompanied by an increase in circulating leptin levels. Our previous studies showed that exogenous leptin impairs the baroreflex sensitivity for control of heart rate in younger rats, but the contribution of this hormone to baroreflex dysfunction during aging is unknown. Thus, we assessed the effect of bilateral leptin microinjection (500 fmol/60 nl) within the solitary tract nucleus (NTS) on the baroreflex sensitivity in older (66±2 weeks of age) urethane/chloralose anesthetized Sprague-Dawley rats, with elevated circulating leptin levels. In contrast to the 63% reduction observed in younger rats, leptin did not alter the baroreflex sensitivity for bradycardia evoked by phenylephrine in older rats (0.76±0.19 baseline vs. 0.71±0.15 msec/mm Hg after leptin; p=0.806). We hypothesized that this loss of sensitivity reflected endogenous suppression of the baroreflex by elevated leptin, rather than cardiovascular resistance to the peptide. Indeed, NTS administration of a leptin receptor antagonist (75 pmol/120 nl) improved the baroreflex sensitivity for bradycardia in older rats (0.73±0.13 baseline vs. 1.19±0.26 at 10 min vs. 1.87±0.32 at 60 min vs. 1.22±0.54 msec/mm Hg at 120 min; p=0.002), with no effect in younger rats. There was no effect of the leptin antagonist on the baroreflex sensitivity for tachycardia, responses to cardiac vagal chemosensitive fiber activation, or resting hemodynamics in older rats. These findings suggest that the actions of endogenous leptin within the NTS, either produced locally or derived from the circulation, contribute to baroreflex suppression during aging.
Neurons expressing the leptin receptor (Ob-R) exist within the caudal nucleus of the solitary tract (NTS). Additionally, afferent neurons expressing the Ob-R have been identified within the nodose ganglion and NTS. Furthermore, systemic injections or focal injections of leptin directly into NTS potentiate the response of NTS neurons to carotid chemoreceptor activation. However, the distribution of carotid body afferents in relation to Ob-R containing neurons within NTS is not known. In this study, chemoreceptor afferent fibers were labeled following microinjection of the anterograde tract tracer biotinylated dextran amine (BDA) into the carotid body or petrosal/nodose ganglion of Wistar rats. After a survival period of 10-14 days, the NTS was processed for BDA and Ob-R immunoreactivity. Afferent axons originating in the carotid body were found to project to the lateral (Slt), gelantinosa (Sg), and medial (Sm) subnuclei of the NTS complex. A similar, but more robust distribution of BDA labeled fibers was observed in the NTS complex after injections into the petrosal/nodose ganglion. Carotid body BDA labeled fibers were observed in close apposition to Ob-R immunoreactive neurons in the region of Slt, Sg and Sm. In addition, a small number of carotid body afferents were found to contain both BDA and express Ob-R-like immunoreactivity within the regions of Slt, Sg and Sm. Taken together, these data suggest that leptin may modulate carotid chemoreceptor function not only through direct effects on NTS neurons, but also through a direct effect on carotid body primary afferent fibers that innervate NTS neurons.
The survival of a species in an unpredictable ecosystem as the desert depends among others on the adaptation of its reproductive system, in terms of matching reproductive activity with water and food availability in the habitat, and halting it when these are limited. Integration of the various environmental cues will eventually determine which populations will be able to breed at an appropriate time thus surviving as a species in xeric environments. Osmotic stress was noted as an effective internal signal for reducing reproductive activity in desert-adapted Acomys populations. In the basis of our study lies the assumption that a possible mechanism by which dietary salinity causes an osmotic stress, which affects reproduction in desert adapted common spiny mice Acomys cahirinus in association with osmoregulatory hormones; vasopressin and aldosterone. As leptin regulates energy intake, it seemed of great interest to assess its role in reproduction of desert adapted rodents, inhabiting an unpredictable environment. For a long time energy source storage seemed as the only role of WAT, apart from providing thermal and mechanical insulation. For now WAT in our understanding is a highly dynamic, endocrine tissue, being involved in a wide range of physiological and metabolic processes far beyond the paradigm of fuel storage. White adipocytes secrete several major hormones, protein signals and factors, most notably is the peptide hormone leptin. Circulating leptin levels serve as a signal for the state of body energy repletion to body cells most importantly the central nervous system (CNS) where it suppresses food intake and permits energy expenditure. In addition to its metabolic effects, leptin is well known as a modulator of reproduction.
Leptin microinjections into the nucleus of the solitary tract (NTS) have been shown to elicit sympathoexcitatory responses, and potentiate the cardiovascular responses to activation of the chemoreflex. In this study, experiments were done in Sprague–Dawley rats initially to provide a detailed mapping within the NTS complex of cells containing immunoreactivity to the long form of the leptin receptor (Ob-Rb). In a second series, this NTS region containing Ob-Rb immunoreactive cells was explored for single units antidromically activated by stimulation of pressor sites in the rostral ventrolateral medulla (RVLM). These antidromically identified neurons were then tested for their response to intra-carotid injections of leptin (50–100 ng/0.1 ml), and to activation of peripheral chemoreceptors following an injection of potassium cyanide (KCN) (80 μg/0.1 ml) into the carotid artery. Cells containing Ob-Rb-like immunoreactivity were found predominantly in the caudal NTS: within the medial, commissural and gelatinous (sub-postremal area) subnuclei of the NTS complex. Of 73 single units tested in these NTS regions, 48 were antidromically activated by stimulation of RVLM pressor sites and 25 of these single units responded with an increase in discharge rate after intra-carotid injections of leptin. In addition, 17 of these leptin responsive neurons were excited by the intra-carotid injections of KCN (80 μg/0.1 ml). Furthermore, the excitatory response of these single units to KCN was potentiated (59–83%) immediately following the leptin injection. These data indicate that leptin responsive neurons in NTS mediate chemoreceptor afferent information to pressor sites in the RVLM, and suggest that leptin may act as a facilitator on neuronal circuits within the NTS that potentiates the sympathoexcitatory responses elicited during the reflex activation of arterial chemoreceptors.
Many questions must be considered with regard to consuming food, including when to eat, what to eat and how much to eat. Although eating is often thought to be a homeostatic behaviour, little evidence exists to suggest that eating is an automatic response to an acute shortage of energy. Instead, food intake can be considered as an integrated response over a prolonged period of time that maintains the levels of energy stored in adipocytes. When we eat is generally determined by habit, convenience or opportunity rather than need, and meals are preceded by a neurally-controlled coordinated secretion of numerous hormones that prime the digestive system for the anticipated caloric load. How much we eat is determined by satiation hormones that are secreted in response to ingested nutrients, and these signals are in turn modified by adiposity hormones that indicate the fat content of the body. In addition, many nonhomeostatic factors, including stress, learning, palatability and social influences, interact with other controllers of food intake. If a choice of food is available, what we eat is based on pleasure and past experience. This article reviews the hormones that mediate and influence these processes.
Leptin receptors have been identified within the nucleus of the solitary tract (NTS) and leptin injections into the caudal NTS inhibit the baroreceptor reflex. However, whether plasma leptin alters the discharge of NTS neurons mediating aortic baroreceptor reflex activity is not known. A series of electrophysiological single unit recording experiments was done in the urethane-chloralose anesthetized, paralyzed and artificially ventilated Wistar and Zucker obese rat with either their neuroaxis intact or with mid-collicular transections. Single units in NTS antidromically activated by electrical stimulation of depressor sites in the caudal ventrolateral medulla (CVLM) were found to display a cardiac cycle-related rhythmicity. These units were tested for their responses to stimulation of the aortic depressor nerve (ADN) and intra-carotid injections of leptin (50-200ng/0.1ml). Of 63 single units tested in NTS, 33 were antidromically activated by stimulation of CVLM depressor sites and 18 of these single units responded with a decrease in discharge rate after intracarotid injections of leptin. Thirteen of these leptin responsive neurons (∼72%) were excited by ADN stimulation. Furthermore, the excitatory response of these single units to ADN stimulation was attenuated by about 50% after the intracarotid leptin injection. Intracarotid injections of leptin (200ng/0.1ml) in the Zucker obese rat did not alter the discharge rate of NTS-CVLM projecting neurons. These data suggest that leptin exerts a modulatory effect on brainstem neuronal circuits that control cardiovascular responses elicited during the reflex activation of arterial baroreceptors.
Recent data suggests that neurons expressing the long form of the leptin receptor form at least two distinct groups within the caudal nucleus of the solitary tract (NTS): a group within the lateral NTS (Slt) and one within the medial (Sm) and gelantinosa (Sg) NTS. Discrete injections of leptin into Sm and Sg, a region that receives chemoreceptor input, elicit increases in arterial pressure (AP) and renal sympathetic nerve activity (RSNA). However, the effect of microinjections of leptin into Slt, a region that receives baroreceptor input is unknown. Experiments were done in the urethane-chloralose anesthetized, paralyzed and artificially ventilated Wistar or Zucker obese rat to determine leptin's effect in Slt on heart rate (HR), AP and RSNA during electrical stimulation of the aortic depressor nerve (ADN). Depressor sites within Slt were first identified by the microinjection of L-glutamate (Glu; 0.25M; 10 nl) followed by leptin microinjections. In the Wistar rat leptin microinjection (50ng; 20 nl) into depressor sites within the lateral Slt elicited increases in HR and RSNA, but no changes in AP. Additionally, leptin injections into Slt prior to Glu injections at the same site or to stimulation of the ADN were found to attenuate the decreases in HR, AP and RSNA to both the Glu injection and ADN stimulation. In Zucker obese rats, leptin injections into NTS depressor sites did not elicit cardiovascular responses, nor altered the cardiovascular responses elicited by stimulation of ADN. Those data suggest that leptin acts at the level of NTS to alter the activity of neurons that mediate the cardiovascular responses to activation of the aortic baroreceptor reflex.
To investigate whether the metabolically important visceral adipose tissue (VAT) relates differently to structural and functional brain changes in comparison with body weight measured as body mass index (BMI). Moreover, we aimed to investigate whether these effects change with age.
Cross-sectional, exploratory.
University Clinic, Integrative Research and Treatment Centre.
We included 100 (mean BMI=26.0 kg/m², 42 women) out of 202 volunteers randomly invited by the city's registration office, subdivided into two age groups: young-to-mid-age (n=51, 20-45 years of age, mean BMI=24.9, 24 women) versus old (n=49, 65-70 years of age, mean BMI=27.0, 18 women).
VAT, BMI, subcutaneous abdominal adipose tissue, brain structure (grey matter density), functional brain architecture (eigenvector centrality, EC).
We discovered a loss of cerebellar structure with increasing VAT in the younger participants, most significantly in regions involved in motor processing. This negative correlation disappeared in the elderly. Investigating functional brain architecture showed again inverse VAT-cerebellum correlations, whereas now regions involved in cognitive and emotional processing were significant. Although we detected similar results for EC using BMI, significant age interaction for both brain structure and functional architecture was only found using VAT.
Visceral adiposity is associated with cerebellar changes of both structure and function, whereas the regions involved contribute to motor, cognitive and emotional processes. Furthermore, these associations seem to be age dependent, with younger adults' brains being adversely affected.
The history of discovery of leptin is given and current knowledge on the factors that regulate ob gene expression and leptin protein secretion is summarized. The chapter focuses on the metabolic effects of leptin, the pathways involved in the suppression of food intake and stimulation of energy expenditure are described. The role of central activation of sympathetic outflow to peripheral tissues, compared with direct effects of leptin on insulin-sensitive tissues, in mediating metabolic responses to leptin is discussed. Recent work elucidating the cellular basis for the shift in metabolism of adipose and non-adipose tissue to lipid oxidation and triglyceride depletion is reviewed.
1. The adipose tissue-derived hormone leptin reduces food intake and bodyweight via leptin receptors (Ob-R) in the hypothalamus.
2. Leptin receptor immunoreactivity, demonstrated with an antiserum recognizing all Ob-R isoforms, is present in hypothalamic neurons of the medial and lateral preoptic area, organum vasculosum lamina terminalis, subfornical organ, periventricular, suprachiasmatic, supraoptic (SON), paraventricular (PVN), arcuate (ARC), dorsomedial, ventromedial hypothalamic and tuberomammillary nuclei and lateral hypothalamic area. In the brainstem, Ob-R immunoreactivity is present in the area postrema, nucleus tractus solitarius, hypoglossal nucleus and dorsal motor nucleus of the vagus nerve.
3. Leptin receptor immunoreactivity is present in magnocellular vasopressin and oxytocin neurons of the SON and PVN, in parvocellular corticotropin-releasing hormone neurons of the PVN, neuropeptide Y and pro-opiomelanocortin neurons of the ARC and in melanin-concentrating hormone neurons of the lateral hypothalamic area.
4. The passage of leptin across the blood–brain barrier and the chemical mediators of the action of leptin in the hypothalamus are discussed.
Protein tyrosine phosphatase 1B (PTP1B) is a ubiquitously expressed tyrosine phosphatase implicated in the negative regulation of leptin and insulin receptor signaling. PTP1B(-/-) mice possess a lean metabolic phenotype attributed at least partially to improved hypothalamic leptin sensitivity. Interestingly, mice lacking both leptin and PTP1B (ob/ob:PTP1B(-/-)) have reduced body weight compared with mice lacking leptin only, suggesting that PTP1B may have important leptin-independent metabolic effects. We generated mice with PTP1B deficiency specifically in leptin receptor (LepRb)-expressing neurons (LepRb-PTP1B(-/-)) and compared them with LepRb-Cre-only wild-type (WT) controls and global PTP1B(-/-) mice. Consistent with PTP1B's role as a negative regulator of leptin signaling, our results show that LepRb-PTP1B(-/-) mice are leptin hypersensitive and have significantly reduced body weight when maintained on chow or high-fat diet (HFD) compared with WT controls. LepRb-PTP1B(-/-) mice have a significant decrease in adiposity on HFD compared with controls. Notably, the extent of attenuated body weight gain on HFD, as well as the extent of leptin hypersensitivity, is similar between LepRb-PTP1B(-/-) mice and global PTP1B(-/-) mice. Overall, these results demonstrate that PTP1B deficiency in LepRb-expressing neurons results in reduced body weight and adiposity compared with WT controls and likely underlies the improved metabolic phenotype of global and brain-specific PTP1B-deficient models. Subtle phenotypic differences between LepRb-PTP1B(-/-) and global PTP1B(-/-) mice, however, suggest that PTP1B independent of leptin signaling may also contribute to energy balance in mice.
Leptin regulates energy balance through central circuits that control food intake and energy expenditure, including proopiomelanocortin (POMC) neurons. POMC neuron-specific deletion of protein tyrosine phosphatase 1B (PTP1B) (Ptpn1(loxP/loxP) POMC-Cre), a negative regulator of CNS leptin signaling, results in resistance to diet-induced obesity and improved peripheral leptin sensitivity in mice, thus establishing PTP1B as an important component of POMC neuron regulation of energy balance. POMC neurons are expressed in the pituitary, the arcuate nucleus of the hypothalamus (ARH), and the nucleus of the solitary tract (NTS) in the hindbrain, and it is unknown how each population might contribute to the phenotype of POMC-Ptp1b(-/-) mice. It is also unknown whether improved leptin sensitivity in POMC-Ptp1b(-/-) mice involves altered melanocortin receptor signaling. Therefore, we examined the effects of hindbrain administration (4th ventricle) of leptin (1.5, 3, and 6 μg) or the melanocortin 3/4R agonist melanotan II (0.1 and 0.2 nmol) in POMC-Ptp1b(-/-) (KO) and control PTP1B(fl/fl) (WT) mice on food intake, body weight, spontaneous physical activity (SPA), and core temperature (T(C)). The results show that KO mice were hypersensitive to hindbrain leptin- and MTII-induced food intake and body weight suppression and SPA compared with WT mice. Greater increases in leptin- but not MTII-induced T(C) were also observed in KO vs. WT animals. In addition, KO mice displayed elevated hindbrain and hypothalamic MC4R mRNA expression. These studies are the first to show that hindbrain administration of leptin or a melanocortin receptor agonist alters energy balance in mice likely via participation of hindbrain POMC neurons.
Leptin, secreted by white adipocytes, has profound feeding, metabolic, and neuroendocrine effects. Leptin acts on the brain, but the specific anatomic sites and pathways responsible for mediating these effects are still unclear. We have systematically examined distributions of mRNA of leptin receptor isoforms in the rat brain by using a probe specific for the long form and a probe recognizing all known forms of the leptin receptor. The mRNA for the long form of the receptor (OB-Rb) localized to selected nuclear groups in the rat brain. Within the hypothalamus, dense hybridization was observed in the arcuate, dorsomedial, ventromedial, and ventral premamillary nuclei. Within the dorsomedial nucleus, particularly intense hybridization was observed in the caudal regions of the nucleus ventral to the compact formation. Receptors were preferentially localized to the dorsomedial division of the ventromedial nucleus. Hybridization accumulated throughout the arcuate nucleus, extending from the retrochiasmatic region to the posterior periventricular region. Moderate hybridization was observed in the periventricular hypothalamic nucleus, lateral hypothalamic area, medial mammillary nucleus, posterior hypothalamic nucleus, nucleus of the lateral olfactory tract, and within substantia nigra pars compacta. Several thalamic nuclei were also found to contain dense hybridization. These groups included the mediodorsal, ventral anterior, ventral medial, submedial, ventral posterior, and lateral dorsal thalamic nuclei. Hybridization was also observed in the medial and lateral geniculate nuclei. Intense hybridization was observed in the Purkinje and granular cell layers of the cerebellum. A probe recognizing all known forms of the leptin receptor hybridized to all of these sites within the brain. In addition, intense hybridization was observed in the choroid plexus, meninges, and also surrounding blood vessels. These findings indicate that circulating leptin may act through hypothalamic nuclear groups involved in regulating feeding, body weight, and neuroendocrine function. The localization of leptin receptor mRNA in extrahypothalamic sites in the thalamus and cerebellum suggests that leptin may act on specific sensory and motor systems. Leptin receptors localized in nonneuronal cells in the meninges, choroid plexus, and blood vessels may be involved in transport of leptin into the brain and in the clearance of leptin from the cerebrospinal fluid. J. Comp. Neurol. 395:535–547, 1998. © 1998 Wiley-Liss, Inc.
There may be a genetically predetermined, inherent drive to eat that is only periodically neutralized by satiety, sleep, or
other competing drives (Berthoud, 2002). In infants, and perhaps also in adults, disinhibition of feeding motor outputs by
factors that remove, compete with, or otherwise neutralize inhibitory controls of feeding could be enough to initiate and
maintain food intake without the need for special “hunger” stimuli (Stricker, 1984). Research reviewed in this chapter supports
the view that behavioral responses to such direct and indirect controls of feeding might generally be effected through DVC
neural circuits. The intrinsic components and output pathways of these circuits are accessed by numerous afferent inputs in
mature rats, but by a more limited set of inputs in neonatal rats. Our understanding of these central neural systems will
be enhanced by continued examination of behavioral and physiological responses to treatments that affect food intake differently
in developing and mature animals. The presence or absence of responses to a given stimulus or control presumably reflects
the functional integrity of neural circuits that receive and process the signal, and those that organize and execute the response.
As new behavioral responses emerge during postnatal development, one may infer maturation of the neural systems and, importantly,
the functional interactions among neural systems that support these responses.
The mechanisms that balance food intake and energy expenditure determine who will be obese and who will be lean. One of the molecules that regulates energy balance in the mouse is the obese (ob) gene. Mutation of ob results in profound obesity and type II diabetes as part of a syndrome that resembles morbid obesity in humans. The ob gene product may function as part of a signalling pathway from adipose tissue that acts to regulate the size of the body fat depot.
Correction of the obese state induced by genetic leptin deficiency reduces elevated levels of both blood glucose and hypothalamic neuropeptide Y (NPY) mRNA in ob/ob mice. To determine whether these responses are due to a specific action of leptin or to the reversal of the obese state, we investigated the specificity of the effect of systemic leptin administration to ob/ob mice (n = 8) on levels of plasma glucose and insulin and on hypothalamic expression of NPY mRNA. Saline-treated controls were either fed ad libitum (n = 8) or pair-fed to the intake of the leptin-treated group (n = 8) to control for changes of food intake induced by leptin. The specificity of the effect of leptin was further assessed by 1) measuring NPY gene expression in db/db mice (n = 6) that are resistant to leptin, 2) measuring NPY gene expression in brain areas outside the hypothalamus, and 3) measuring the effect of leptin administration on hypothalamic expression of corticotropin-releasing hormone (CRH) mRNA. Five daily intraperitoneal injections of recombinant mouse leptin (150 micrograms) in ob/ob mice lowered food intake by 56% (P < 0.05), body weight by 4.1% (P < 0.05), and levels of NPY mRNA in the hypothalamic arcuate nucleus by 42.3% (P < 0.05) as compared with saline-treated controls. Pair-feeding of ob/ob mice to the intake of leptin-treated animals produced equivalent weight loss, but did not alter expression of NPY mRNA in the arcuate nucleus. Leptin administration was also without effect on food intake, body weight, or NPY mRNA levels in the arcuate nucleus of db/db mice. In ob/ob mice, leptin did not alter NPY mRNA levels in cerebral cortex or hippocampus or the expression of CRH mRNA in the hypothalamic paraventricular nucleus (PVN). Leptin administration to ob/ob mice also markedly reduced serum glucose (8.3 +/- 1.2 vs. 24.5 +/- 3.8 mmol/l; P < 0.01) and insulin levels (7,263 +/- 1,309 vs. 3,150 +/- 780 pmol/l), but was ineffective in db/db mice. Pair-fed mice experienced reductions of glucose and insulin levels that were < 60% of the reduction induced by leptin. The results suggest that in ob/ob mice, systemic administration of leptin inhibits NPY gene overexpression through a specific action in the arcuate nucleus and exerts a hypoglycemic action that is partly independent of its weight-reducing effects. Furthermore, both effects occur before reversal of the obesity syndrome. Defective leptin signaling due to either leptin deficiency (in ob/ob mice) or leptin resistance (in db/db mice) therefore leads directly to hyperglycemia and the overexpression of hypothalamic NPY that is implicated in the pathogenesis of the obesity syndrome.
Recently, glucagon-like peptide-1-(7-36) amide (GLP-1) and leptin have been implicated in the regulation of food intake. In the present study, we compared the effects of third ventricular administration (i3vt) of leptin (3.5 micrograms) and GLP-1 (10.0 micrograms) on short-term food intake and c-Fos-like immunoreactivity (c-FLI) in hypothalamic, limbic, and hindbrain areas in the rat. Relative to controls, infusion of leptin or GLP-1 (3 h before lights off) significantly reduced food intake over the first 2 h in the dark phase (53 and 63%, respectively). In different rats, infusion of leptin or GLP-1 elevated c-FLI in the paraventricular hypothalamus and central amygdala. Furthermore, leptin selectively elevated c-FLI in the dorsomedial hypothalamus, whereas GLP-1 selectively elevated c-FLI in the nucleus of the solitary tract, area postrema, lateral parabrachial nucleus, and arcuate hypothalamic nucleus. The fact that most of the c-FLI after leptin or GLP-1 administration was observed in separate regions within the central nervous system (CNS) suggests different roles for leptin and GLP-1 in the CNS regulation of food intake and body weight.
Leptin's effects are mediated by interactions with a receptor that is alternatively spliced, resulting in at least five different murine forms: Ob-Ra, Ob-Rb, Ob-Rc, Ob-Rd, and Ob-Re. A mutation in one splice form, Ob-Rb, results in obesity in mice. Northern blots, RNase protection assays, and PCR indicate that Ob-Rb is expressed at a relatively high level in hypothalamus and low level in several other tissues. Ob-Ra is expressed ubiquitously, whereas Ob-Rc, -Rd, and -Re RNAs are only detectable using PCR. In hypothalamus, Ob-Rb is present in the arcuate, ventromedial, dorsomedial, and lateral hypothalamic nuclei but is not detectable in other brain regions. These nuclei are known to regulate food intake and body weight. The level of Ob-Rb in hypothalamus is reduced in mice rendered obese by gold thioglucose (GTG), which causes hypothalamic lesions. The obesity in GTG-treated mice is likely to be caused by ablation of Ob-Rb-expressing neurons, which results in leptin resistance.
The recent positional cloning of the mouse ob gene and its human homology has provided the basis to investigate the potential
role of the ob gene product in body weight regulation. A biologically active form of recombinant mouse OB protein was overexpressed
and purified to near homogeneity from a bacterial expression system. Peripheral and central administration of microgram doses
of OB protein reduced food intake and body weight of ob/ob and diet-induced obese mice but not in db/db obese mice. The behavioral
effects after brain administration suggest that OB protein can act directly on neuronal networks that control feeding and
energy balance.
The gene product of the ob locus is important in the regulation of body weight. The ob product was shown to be present as a 16-kilodalton protein in mouse and human plasma but was undetectable in plasma from C57BL/6J ob/ob mice. Plasma levels of this protein were increased in diabetic (db) mice, a mutant thought to be resistant to the effects of ob. Daily intraperitoneal injections of either mouse or human recombinant OB protein reduced the body weight of ob/ob mice by 30 percent after 2 weeks of treatment with no apparent toxicity but had no effect on db/db mice. The protein reduced food intake and increased energy expenditure in ob/ob mice. Injections of wild-type mice twice daily with the mouse protein resulted in a sustained 12 percent weight loss, decreased food intake, and a reduction of body fat from 12.2 to 0.7 percent. These data suggest that the OB protein serves an endocrine function to regulate body fat stores.
C57BL/6J mice with a mutation in the obese (ob) gene are obese, diabetic, and exhibit reduced activity, metabolism, and body temperature. Daily intraperitoneal injection of these mice with recombinant OB protein lowered their body weight, percent body fat, food intake, and serum concentrations of glucose and insulin. In addition, metabolic rate, body temperature, and activity levels were increased by this treatment. None of these parameters was altered beyond the level observed in lean controls, suggesting that the OB protein normalized the metabolic status of the ob/ob mice. Lean animals injected with OB protein maintained a smaller weight loss throughout the 28-day study and showed no changes in any of the metabolic parameters. These data suggest that the OB protein regulates body weight and fat deposition through effects on metabolism and appetite.
The ob gene product, leptin, is an important circulating signal for the regulation of body weight. To identify high affinity leptin-binding sites, we generated a series of leptin-alkaline phosphatase (AP) fusion proteins as well as [125I]leptin. After a binding survey of cell lines and tissues, we identified leptin-binding sites in the mouse choroid plexus. A cDNA expression library was prepared from mouse choroid plexus and screened with a leptin-AP fusion protein to identify a leptin receptor (OB-R). OB-R is a single membrane-spanning receptor most related to the gp130 signal-transducing component of the IL-6 receptor, the G-CSF receptor, and the LIF receptor. OB-R mRNA is expressed not only in choroid plexus, but also in several other tissues, including hypothalamus. Genetic mapping of the gene encoding OB-R shows that it is within the 5.1 cM interval of mouse chromosome 4 that contains the db locus.
OB-R is a high affinity receptor for leptin, an important circulating signal for the regulation of body weight. We identified an alternatively spliced transcript that encodes a form of mouse OB-R with a long intracellular domain. db/db mice also produce this alternatively spliced transcript, but with a 106 nt insertion that prematurely terminates the intracellular domain. We further identified G --> T point mutation in the genomic OB-R sequence in db/db mice. This mutation generates a donor splice site that converts the 106 nt region to a novel exon retained in the OB-R transcript. We predict that the long intracellular domain form of OB-R is crucial for initiating intracellular signal transduction, and as a corollary, the inability to produce this form of OB-R leads to the severe obese phenotype found in db/db mice.
Mutations in the mouse diabetes (db) gene result in obesity and diabetes in a syndrome resembling morbid human obesity. Previous data suggest that the db gene encodes the receptor for the obese (ob) gene product, leptin. A leptin receptor was recently cloned from choroid plexus and shown to map to the same 6-cM interval on mouse chromosome 4 as db. This receptor maps to the same 300-kilobase interval as db, and has at least six alternatively spliced forms. One of these splice variants is expressed at a high level in the hypothalamus, and is abnormally spliced in C57BL/Ks db/db mice. The mutant protein is missing the cytoplasmic region, and is likely to be defective in signal transduction. This suggests that the weight-reducing effects of leptin may be mediated by signal transduction through a leptin receptor in the hypothalamus.
Expression of the leptin receptor gene has been examined in mouse hypothalamus and other brain regions by in situ hybridization. With a probe recognizing all the known splice variants, receptor mRNA was evident in several brain regions (cortex, hippocampus, thalamus), with strong expression in the hypothalamus (arcuate, ventromedial, paraventricular and ventral premammillary nuclei), choroid plexus and leptomeninges. A probe specific to the long splice variant of the leptin receptor (Ob-Rb), containing the putative intracellular signaling domain, again revealed strong expression in the hypothalamus; there was, however, minimal hybridization to choroid plexus and leptomeninges. These results indicate that the hypothalamus is a key site of leptin action, although other brain regions are also targeted.
Leptin, or OB protein, is produced by fat cells and may regulate body weight by acting on the brain. To reach the brain, circulating leptin must cross the blood-brain barrier (BBB). Intravenously injected radioiodinated leptin (125I-leptin) had an influx constant (Ki) into brain of (5.87)10(-4) ml/g-min, a rate 20 times greater than that of labeled albumin. Unlabeled leptin inhibited the influx of 125I-leptin in a dose-dependent manner whereas unlabeled tyrosine and insulin, which have saturable transport systems, were without effect. HPLC and acid precipitation showed that the radioactivity in brain and serum represented intact 125I-leptin. About 75% of the extravascular 125I-leptin in brain completely crossed the BBB to reach brain parenchyma. Autoradiography detected uptake at the choroid plexus, arcuate nuclei of the hypothalamus, and the median eminence. Saturable transport did not occur out of the brain. The results show that leptin is transported intact from blood to brain by a saturable system.
This article is designed as an introduction to the major theoretical models in the field of regulation of eating behavior, and a selective review of relevant neurobiological data. We first critically consider the paradigm of homeostasis as it relates to body energy content, and argue that additional theoretical constructs will be needed to account for the complexity of eating behavior in both nonhumans and humans. We then summarize some of the methods available to the neuroscientist in this area, and address some of their limitations. We review treatments and potential mechanisms that increase food intake, including deprivation, antimetabolites, norepinephrine, and several peptides including neuropeptide Y. We next review treatments that decrease food intake, including a variety of humoral, gastrointestinal, and pancreatic factors, as well as examine central pathways of satiety. This includes a discussion of leptin and other potential anorectic agents. We conclude with a discussion of human obesity and anorexias, and prospects for pharmacotherapy of eating disorders. We emphasize throughout that most regions of the human brain probably make some contribution to feeding behavior, and so a focus on any one area of transmitter/hormone is an unrealistic approach both in basic and applied areas.
Leptin, the protein product of the adipose tissue-specific ob (obese) gene (1), reduces the body weight, adiposity and food intake of obese ob/ob mice on peripheral or central injection (2, 3, 4). [125I]leptin binding has been detected in mouse choroid plexus (5), from which a leptin receptor gene was expression cloned (5). The gene has at least 6 splice variants (6, 7). Leptin receptor mRNA was localized in the hypothalamus by in situ hybridization being particularly abundantly expressed in the arcuate nucleus (8). There is evidence linking the physiological effects of injected leptin with hypothalamic neuropeptide Y (9, 10) (NPY), which has potent central effects on food intake and energy balance (11), and is also expressed in the arcuate nucleus. Here we report dual in situ hybridization studies for leptin receptor and NPY gene expression in the mouse arcuate nucleus, where the majority of cells examined expressed both genes. This provides the first direct evidence that leptin acts on cells that express NPY mRNA.
Leptin receptor gene expression has been measured in arcuate and ventromedial hypothalamic nuclei. Receptor mRNA in both hypothalamic areas was higher in obese mice than in lean littermates. Twice daily leptin administration for 7 days profoundly affected food intake, reduced leptin receptor mRNA in the arcuate nucleus, and had a similar effect on neuropeptide Y gene expression. A single leptin injection was ineffective. Exposure of lean mice to cold for 24 h caused an induction of leptin receptor and NPY mRNA which was normalized when animals were returned to the warm. Regulation of receptor gene expression may be an important component in the reading of the leptin signal.
Saper CB 1997 Leptin activates neurons in ventrobasal hypothalamus and brainstem
- Jk Elmquist
- Rs Ahima
- Maratos
- E Flier
- Flier
Elmquist JK, Ahima RS, Maratos-Flier E, Flier JS, Saper CB 1997 Leptin activates neurons in ventrobasal hypothalamus and brainstem. Endocrinology 138:839 – 842
BaskinDG1996Identification of targets of leptin action in rat hypothalamus
- Schwartzmw
- Seeleyrj
- Campfieldla
SchwartzMW,SeeleyRJ,CampfieldLA,BurnP,BaskinDG1996Identification of targets of leptin action in rat hypothalamus. J Clin Invest 98:1101–110
Morgenstern JP 1996Evidencethatthediabetesgeneencodestheleptinreceptor:identification of a mutation in the leptin receptor gene in db/db mice
- H Chen
- O Charlat
- La Tartaglia
- Ea Woolf
- X Weng
- Sj Ellis
- Nd Lakey
- J Culpepper
- Kj Moore
- Re Breitbart
- Gm Duyk
- Tepper
Chen H, Charlat O, Tartaglia LA, Woolf EA, Weng X, Ellis SJ, Lakey ND, Culpepper J, Moore KJ, Breitbart RE, Duyk GM, Tepper RI, Morgenstern JP 1996Evidencethatthediabetesgeneencodestheleptinreceptor:identification of a mutation in the leptin receptor gene in db/db mice. Cell 84:491–49
Central infusions of leptin and GLP- 1-(7–35) amide differentially stimulate c-FLI in the rat brain
- G Van Dijk
- Te Thiele
- Jck Donahey
- La Campfield
- Fj Smith
- P Burn
- Il Burnstein
- Sc Woods
- Rj Seely
Van Dijk G, Thiele TE, Donahey JCK, Campfield LA, Smith FJ, Burn P,
Burnstein IL, Woods SC, Seely RJ 1996 Central infusions of leptin and GLP-
1-(7–35) amide differentially stimulate c-FLI in the rat brain. Am J Physiol
271:R1096 –R1100
Leptin enters the brain by a saturable system independent of insulin
- Wa Banks
- Aj Kastin
- W Huang
- Jb Jaspan
- Lm Maness
Banks WA, Kastin AJ, Huang W, Jaspan JB, Maness LM 1996 Leptin enters
the brain by a saturable system independent of insulin. Peptides 17:305–311
- Jk Elmquist
- Rs Ahima
- E Maratos-Flier
- Js Flier
- Cb Saper
Elmquist JK, Ahima RS, Maratos-Flier E, Flier JS, Saper CB 1997 Leptin
activates neurons in ventrobasal hypothalamus and brainstem. Endocrinology
138:839 – 842
Identification of targets of leptin action in rat hypothalamus
- Mw Schwartz
- Rj Seeley
- La Campfield
- P Burn
- Dg Baskin
Schwartz MW, Seeley RJ, Campfield LA, Burn P, Baskin DG 1996 Identification
of targets of leptin action in rat hypothalamus. J Clin Invest 98:1101–1106