Regional and temporal differences in leptin signaling in rat brain

Leptin coordinates efferent sympathetic outflow to the white adipose tissue through the midbrain centrally-projecting Edinger-Westphal nucleus in male rats

The centrally-projecting Edinger-Westphal nucleus (EWcp) hosts a large population of neurons expressing urocortin 1 (Ucn1) and about half of these neurons also express the leptin receptor (LepRb). Previously, we have shown that the peripheral adiposity hormone leptin signaling energy surfeit modulates EWcp neurons' activity. Here, we hypothesized that Ucn1/LepRb neurons in the EWcp would act as a crucial neuronal node in the brain-white adipose tissue (WAT) axis modulating efferent sympathetic outflow to the WAT. We showed that leptin bound to neurons of the EWcp stimulated STAT3 phosphorylation, and increased Ucn1-production in a time-dependent manner. Besides, retrograde transneuronal tract-tracing using pseudorabies virus (PRV) identified EWcp Ucn1 neurons connected to WAT. Interestingly, reducing EWcp Ucn1 contents by ablating EWcp LepRb-positive neurons with leptin-saporin, did not affect food intake and body weight gain, but substantially (+26%) increased WAT weight accompanied by a higher plasma leptin level and changed plasma lipid profile. We also found that ablation of EWcp Ucn1/LepRb neurons resulted in lower respiratory quotient and oxygen consumption one week after surgery, but was comparable to sham values after 3 and 5 weeks of surgery. Taken together, we report that EWcp/LepRb/Ucn1 neurons not only respond to leptin signaling but also control WAT size and fat metabolism without altering food intake. These data suggest the existence of a EWcp-WAT circuitry allowing an organism to recruit fuels without being able to eat in situations such as the fight-or-flight response.

Perinatal free-choice of a high-calorie low-protein diet affects leptin signaling through IRS1 and AMPK dephosphorylation in the hypothalami of female rat offspring in adulthood

AIM

We aimed to investigate whether a dysregulated maternal diet during gestation and lactation induces long-lasting changes in the hypothalamic control of feeding behavior in the offspring and whether this effect is sex specific.

METHODS

The study included an analysis of appetite-regulating metabolic hormones and hypothalamic signaling in male and female offspring in adulthood after exposure to a free-choice high-calorie palatable low-protein (P) diet or standard chow (C) during (pre)gestation/lactation (maternal) and/or postweaning (offspring).

RESULTS

Maternal exposure to the P diet resulted in decreased protein intake and body weight gain in dams and decreased body weight gain in offspring during lactation. The maternal P diet (PC) specifically increased feed efficacy and decreased body weight and cholesterol levels in the female offspring in adulthood, but no changes in adiposity or leptin levels were found. In contrast, P diet exposure after weaning (CP and PP) increased caloric intake, adiposity and circulating levels of leptin in the male and female offspring in adulthood. The hypothalami of the female offspring exposed to the maternal P diet (PC and PP) expressed high levels of the phospho-leptin receptor and low levels of SOCS3, phospho-IRS1 and phospho-AMPK, regardless of the postweaning diet. The hypothalami of the female rats in the PC group also showed increased levels of STAT3 and the orexigenic neuropeptide Agrp.

CONCLUSIONS

Maternal exposure to a free-choice high-calorie low-protein diet induces a long-term feed efficacy associated with changes in leptin signaling through IRS-1 and AMPK dephosphorylation in the hypothalami of female offspring in adulthood.

Leptin signaling in the rainbow trout central nervous system is modulated by a truncated leptin receptor isoform

Central leptin (Lep) signaling is important in control of appetite and energy balance in mammals, but information on Lep signaling and physiological roles in early vertebrates is still lacking. To elucidate fish Lep signaling activation and modulation, a long-form Lep receptor (LepRL) and a truncated LepR (LepRT) are functionally characterized from rainbow trout. The receptors generated in alternatively splicing events have identical extracellular and transmembrane domains but differ in the intracellular sequence, both in length and identity. Gene transfection experiments show that LepRL is expressed as a 125-kDa protein in rainbow trout hepatoma cell line RTH-149, whereas LepRT is 100 kDa; both receptors specifically bind Lep. Homogenous Lep induces tyrosine phosphorylation of Janus kinase 2 and signal transducer and activation of transcription 3 in LepRL-expressing RTH-149 cells. This response is diminished in cells coexpressing LepRL and LepRT, suggesting that the LepRT which lacks these kinase-associated motifs competes with the LepRL for Lep availability, thus attenuating the Lep signal. Both receptor genes are highly expressed in the central nervous system. The mRNA levels of LepRT in hypothalamus, but not LepRL, change postprandially, with decreased transcription at 2 hours postfeeding and then elevated at 8 hours, concomitant with changes in proopiomelanocortin-A1 transcription. However, both receptors have no change in mRNA levels during 3 weeks of fasting. These data indicate that LepRT transcription is more likely a mechanism for modulating Lep effects on short-term feed intake than in regulating energy balance in the long term. In vitro and physiological characterization of LepR isoforms indicates divergent Lep signaling modulation patterns among vertebrates with different life histories and metabolic profiles.

Leptin induces hippocampal synaptogenesis via CREB-regulated microRNA-132 suppression of p250GAP

Leptin acts in the hippocampus to enhance cognition and reduce depression and anxiety. Cognitive and emotional disorders are associated with abnormal hippocampal dendritic spine formation and synaptogenesis. Although leptin has been shown to induce synaptogenesis in the hypothalamus, its effects on hippocampal synaptogenesis and the mechanism(s) involved are not well understood. Here we show that leptin receptors (LepRs) are critical for hippocampal dendritic spine formation in vivo because db/db mice lacking the long form of the leptin receptor (LepRb) have reduced spine density on CA1 and CA3 neurons. Leptin promotes the formation of mature spines and functional glutamate synapses on hippocampal pyramidal neurons in both dissociated and slice cultures. These effects are blocked by short hairpin RNAs specifically targeting the LepRb and are absent in cultures from db/db mice. Activation of the LepR leads to cAMP response element-binding protein (CREB) phosphorylation and initiation of CREB-dependent transcription via the MAPK kinase/Erk pathway. Furthermore, both Mek/Erk and CREB activation are required for leptin-induced synaptogenesis. Leptin also increases expression of microRNA-132 (miR132), a well-known CREB target, which is also required for leptin-induced synaptogenesis. Last, leptin suppresses the expression of p250GAP, a miR132 target, and this suppression is obligatory for leptin's effects as is the downstream target of p250GAP, Rac1. LepRs appear to be critical in vivo as db/db mice have lowered hippocampal miR132 levels and elevated p250GAP expression. In conclusion, we identify a novel signaling pathway by which leptin increases synaptogenesis through inducing CREB transcription and increasing microRNA-mediated suppression of p250GAP activity, thus removing a known inhibitor of Rac1-stimulated synaptogenesis.

Ghrelin, neuropeptide Y, and other feeding-regulatory peptides active in the hippocampus: role in learning and memory

The hippocampus is a brain region of primary importance for neurogenesis, which occurs during early developmental states as well as during adulthood. Increases in neuronal proliferation and in neuronal death with age have been associated with drastic changes in memory and learning. Numerous neurotransmitters are involved in these processes, and some neuropeptides that mediate neurogenesis also modulate feeding behavior. Concomitantly, feeding peptides, which act primarily in the hypothalamus, are also present in the hippocampus. This review aims to ascertain the role of several important feeding peptides in cognitive functions, either through their local synthesis in the hippocampus or through their actions via specific receptors in the hippocampus. A link between neurogenesis and the orexigenic or anorexigenic properties of feeding peptides is discussed.

Effects of leptin deficiency and replacement on cerebellar response to food-related cues

Leptin affects eating behavior partly by altering the response of the brain to food-related stimuli. The effects of leptin on brain structure have been observed in the cerebellum, where leptin receptors are most densely expressed, but the function of leptin in the cerebellum remains unclear. We performed a nonrandomized, prospective interventional study of three adults with genetically mediated leptin deficiency. FMRI was recorded three times each year during years 5 and 6 of leptin replacement treatment. Session 1 of each year occurred after 10 months of continuous daily replacement, session 2 after 33-37 days without leptin, and session 3 at 14-23 days after daily replacement was restored. Statistical parametric mapping software (SPM5) was employed to contrast the fMRI blood oxygenation level-dependent response to images of high-calorie foods versus images of brick walls. Covariate analyses quantified the effects of the duration of leptin replacement and concomitant changes in body mass on the cerebral responses. Longer duration of replacement was associated with more activation by food images in a ventral portion of the posterior lobe of the cerebellum, while simultaneous decreases in body mass were associated with decreased activation in a more dorsal portion of the same lobe. These findings indicate that leptin replacement reversibly alters neural function within the posterior cerebellum and modulates plasticity-dependent brain physiology in response to food cues. The results suggest an underexplored role for the posterior cerebellum in the regulation of leptin-mediated processes related to food intake.

Leptin-induced downregulation of the rat hippocampal somatostatinergic system may potentiate its anorexigenic effects

The learning and memory mechanisms in the hippocampus translate hormonal signals of energy balance into behavioral outcomes involved in the regulation of food intake. As leptin and its receptors are expressed in the hippocampus and somatostatin (SRIF), an orexigenic neuropeptide, may inhibit leptin-mediated suppression of food intake in other brain areas, we asked whether chronic leptin infusion induces changes in the hippocampal somatostatinergic system and whether these modifications are involved in leptin-mediated effects. We studied 18 male Wistar rats divided into three groups: controls (C), treated intracerebroventricularly (icv) with leptin (12 μg/day) for 14 days (L) and a pair-fed group (PF) that received the same amount of food consumed by the L group. Food restriction increased whereas leptin decreased the hippocampal SRIF receptor density, due to changes in SRIF receptor 2 protein levels. These changes in the PF group were concurrent with an increase of hippocampal G protein-coupled receptor kinase 2 protein levels and activation of Akt and cyclic AMP response element binding protein. The inhibitory effect of SRIF on adenylyl cyclase (AC) activity, however, was decreased in L rats, coincident with lower G inhibitory α3 and higher AC-I levels as well as signal transducer and activator of transcription factor 3 activation. In addition, 20 male Wistar rats were included to analyze whether the leptin antagonist L39A/D40A/F41A and the SRIF receptor agonist SMS 201-995 modify SRIF signaling and food intake, respectively. Administration of L39A/D40A/F41A reversed changes in SRIF signaling, whereas SMS 201-995 ameliorated food consumption in L. Altogether, these results suggest that increased somatostatinergic tone in PF rats may be a mechanism to improve the hippocampal orexigenic effects in a situation of metabolic demand, whereas down-regulation of this system in L rats may represent a mechanism to enhance the anorexigenic effects of leptin.

Cortical neurons develop insulin resistance and blunted Akt signaling: a potential mechanism contributing to enhanced ischemic injury in diabetes

Patients with diabetes are at higher risk of stroke and experience increased morbidity and mortality after stroke. We hypothesized that cortical neurons develop insulin resistance, which decreases neuroprotection via circulating insulin and insulin-like growth factor-I (IGF-I). Acute insulin treatment of primary embryonic cortical neurons activated insulin signaling including phosphorylation of the insulin receptor, extracellular signal-regulated kinase (ERK), Akt, p70S6K, and glycogen synthase kinase-3β (GSK-3β). To mimic insulin resistance, cortical neurons were chronically treated with 25 mM glucose, 0.2 mM palmitic acid (PA), or 20 nM insulin before acute exposure to 20 nM insulin. Cortical neurons pretreated with insulin, but not glucose or PA, exhibited blunted phosphorylation of Akt, p70S6K, and GSK-3β with no change detected in ERK. Inhibition of the phosphatidylinositol 3-kinase (PI3-K) pathway during insulin pretreatment restored acute insulin-mediated Akt phosphorylation. Cortical neurons in adult BKS-db/db mice exhibited higher basal Akt phosphorylation than BKS-db(+) mice and did not respond to insulin. Our results indicate that prolonged hyperinsulinemia leads to insulin resistance in cortical neurons. Decreased sensitivity to neuroprotective ligands may explain the increased neuronal damage reported in both experimental models of diabetes and diabetic patients after ischemia-reperfusion injury.

Obesity/hyperleptinemic phenotype impairs structural and functional plasticity in the rat hippocampus

Epidemiological studies estimate that greater than 60% of the adult US population may be categorized as either overweight or obese, and there is a growing appreciation that the complications of obesity extend to the central nervous system (CNS). While the vast majority of these studies have focused on the hypothalamus, more recent studies suggest that the complications of obesity may also affect the structural and functional integrity of the hippocampus. A potential contributor to obesity-related CNS abnormalities is the adipocyte-derived hormone leptin. In this regard, decreases in CNS leptin activity may contribute to deficits in hippocampal synaptic plasticity and suggest that leptin resistance, a well-described phenomenon in the hypothalamus, may also be observed in the hippocampus. Unfortunately, the myriad of metabolic and endocrine abnormalities in diabetes/obesity phenotypes makes it challenging to assess the role of leptin in hippocampal neuroplasticity deficits associated with obesity models. To address this question, we examined hippocampal morphological and behavioral plasticity following lentivirus-mediated downregulation of hypothalamic insulin receptors (hypo-IRAS). Hypo-IRAS rats exhibit increases in body weight, adiposity, plasma leptin and triglyceride levels. As such, hypo-IRAS rats develop a phenotype that is consistent with features of the metabolic syndrome. In addition, hippocampal morphological plasticity and performance of hippocampal-dependent tasks are adversely affected in hypo-IRAS rats. Leptin-mediated signaling is also decreased in hypo-IRAS rats. We will discuss these findings in the context of how hyperleptinemia and hypertriglyceridemia may represent mechanistic mediators of the neurological consequences of impaired hippocampal synaptic plasticity in obesity.

Relationships between Brain Structure and Metabolic Changes in Schizophrenia Patients Treated with Olanzapine: A Voxel-Based Morphometric Study

Introduction. Second-generation antipsychotics treatment is associated with weight gain and metabolic disturbances. Although much research has been done on the topic, the precise mechanisms underlying such side effects are still not well understood. Method. We followed over 16 weeks a group of 17 schizophrenia patients who were treated with olanzapine and monitored biometric, clinical, and metabolic data, including ghrelin and leptin levels. All patients had a structural cerebral magnetic resonance imaging examination during the first week of their followup and at the end of the study. Results. We found positive and negative significant correlations between grey matter volumes of several brain regions and variations of body weight as well as of ghrelin and leptin levels. The right frontal operculum, bilateral precuneus, and bilateral hippocampal regions were found to be significantly associated with those changes. Conclusion. Our results suggest associations between brain structure and metabolic variations in schizophrenia patients taking olanzapine.

Leptin controls ketone body utilization in hypothalamic neuron

Leptin is an appetite-controlling peptide secreted from adipose tissue. Previously, we showed that the gene expression of acetoacetyl-CoA synthetase (AACS), the ketone body-utilizing enzyme for lipid synthesis, was suppressed by leptin deficiency-induced obesity in white adipose tissue. In this study, to clarify the effects of leptin on ketone body utilization in the central nervous system, we examined the effects of leptin signaling on AACS expression. In situ hybridization analysis of ob/ob and db/db mice revealed that AACS mRNA level was reduced by leptin deficiency in the arcuate nucleus (Arc) and ventromedial hypothalamic nucleus (VMH) in hypothalamus but not in other brain regions. Moreover, AACS mRNA level was increased by leptin treatment both in primary cultured neural cells and in N41 neural-like cells. In N41 cells, AACS level was decreased by AMPK inducer but increased by AMPK inhibitor. These results suggest that the up-regulation of AACS expression by leptin is due to the suppression of AMPK activity via neural leptin signaling and that the deficiency of this regulation may be responsible for neurological disorders in central appetite control.