Sensitivity to leptin and susceptibility to seizures of mice lacking neuropeptide Y

Relation of leptin and neuropeptide Y in human blood and cerebrospinal fluid

Leptin and neuropeptide Y (NPY) are involved in the regulation of food intake and body weight. Both hormones act through specific receptors in the central nervous system. The objective of this study was to investigate the relation of leptin and NPY in human plasma and cerebrospinal fluid (CSF). Leptin and NPY in CSF and in serum/plasma were measured by radioimmunoassays in 35 patients. Leptin concentrations in serum were 100-200 fold higher than in CSF. There was a significant correlation between leptin levels in CSF and in serum (r=0.88, P<0.0001). Female patients had significantly higher leptin serum concentrations than males (16.6+/-10.9 microg/l vs. 6.5+/-7.3 microg/l, P=0.002). In contrast, NPY levels were only twofold higher in CSF than in plasma. There was no relation between leptin and NPY in CSF and serum/plasma, respectively. The ratio of CSF and peripheral leptin levels did not correlate with the respective albumin ratio, indicating that leptin did not merely leak into the CSF via a defective blood-CSF barrier. It is concluded that leptin uptake from the circulation into CSF is a regulated process. The NPY concentration in CSF is not directly related to leptin CSF levels.

Sodium/hydrogen exchanger gene defect in slow-wave epilepsy mutant mice

The "housekeeping" sodium/hydrogen exchanger, NHE1, mediates the electroneutral 1:1 exchange of Na+ and H+ across the plasma membrane. NHE1 is ubiquitous and is studied extensively for regulation of pHi, cell volume, and response to growth factors. We describe a spontaneous mouse mutant, slow-wave epilepsy, (swe), with a neurological syndrome including ataxia and a unique epilepsy phenotype consisting of 3/sec absence and tonic-clonic seizures. swe was fine-mapped on Chromosome 4 and identified as a null allele of Nhe1. Mutants show selective neuronal death in the cerebellum and brainstem but otherwise are healthy. This first example of a disease-causing mutation in an Nhe gene provides a new tool for studying the delicate balance of neuroexcitability and cell survival within the CNS.

Leptin, corticotropin-releasing hormone (CRH), and neuropeptide Y (NPY)in free-ranging pregnant bats

Leptin, the product of the obese gene first identified in mice, restores fertility in obese mice, and accelerates puberty in mice. We hypothesized that leptin's putative role in reproduction may extend to pregnancy and lactation. Leptin levels were determined in Myotis lucifugus, the little brown bat, a free-ranging mammal with a seasonal breeding cycle. The present study shows that plasma levels of leptin progressively rise during pregnancy, supporting a potential role for leptin in the maintenancy of pregnancy. In contrast, leptin was significantly lower during lactation, a time when most mammals, including bats, demonstrate reduced fertility. In addition to its possible roles in reproduction, leptin appears important in regulation of energy balance. M. lucifugus spontaneously fasts for up to 16 h each day during the active season, which allowed us to test the hypothesis that acute fasting was associated with decreased leptin. Leptin was significantly lower in fasted (lactating) bats, compared to those that recently returned from nightly foraging. Although postprandial lactating bats had a significantly higher fat index than fasted bats, plasma leptin and body fat were not significantly correlated, and were only weakly correlated (r2 = 0.26) when both pregnant and lactating females were included in the analysis. Similar changes during pregnancy, lactation, and the daily feeding cycle were observed in the hypothalamic neuropeptide, corticotropin-releasing hormone (CRH), which is believed to play an important role in energy balance and reproduction. By contrast, neuropeptide Y (NPY) increased during pregnancy but did not change during fasting. These results suggest that leptin's putative role in reproduction may extend to pregnancy and lactation, and that spontaneous, acute fasting results in decreased circulating levels of leptin in M. lucifugus.

Leptin rapidly lowers food intake and elevates metabolic rates in lean and ob/ob mice

Leptin, the ob gene product, is released from adipose tissue and likely acts in the central nervous system, particularly within the hypothalamus, to exert many of its effects. Obesity in C57BL/6J ob/ob mice is caused by a mutation in the ob gene resulting in a lack of functional leptin. In this study, we first compared effects of a single intracerebroventricular (ICV) injection of 3 pmol (50 ng) or 60 pmol (1 microg) leptin on food intake and oxygen consumption of lean and ob/ob mice deprived of food for 4 h during the 48-h period postinjection. Injection of 3 pmol leptin minimally lowered food intake in these mice without influencing oxygen consumption. Injection of 60 pmol of leptin rapidly lowered food intake within 30 min in both lean and ob/ob mice, with effects persisting for 24 h. Lean and ob/ob mice treated with leptin consumed 40 and 60% less food, respectively, in 24 h than vehicle-treated controls. Injection of leptin (60 pmol ICV) suppressed food intake of adrenalectomized mice as well (by 25 and 40% in lean mice and by 20 and 68% in ob/ob mice at 3 and 24 h, respectively), indicating that glucocorticoids are not essential for leptin to suppress food intake. Leptin increased oxygen consumption in conditions in which diet-induced thermogenesis was low, i.e., in fed ob/ob mice and in food-deprived lean mice, but not in fed adrenalectomized ob/ob mice or in fed lean mice. ICV injection of 60 pmol leptin along with 230 pmol (2 microg) of neuropeptide Y (NPY) attenuated NPY-induced feeding in ob/ob, but not in lean mice, suggesting an enhanced potential for crosstalk between the leptin and NPY signaling systems in ob/ob mice lacking endogenous leptin. Leptin exerts rapid-onset actions within the central nervous system to coordinate control of food intake and metabolic rate.

Leptin interacts with glucagon-like peptide-1 neurons to reduce food intake and body weight in rodents

The adipose tissue hormone, leptin, and the neuropeptide glucagon-like peptide-1 (7-36) amide (GLP-1) both reduce food intake and body weight in rodents. Using dual in situ hybridization, long isoform leptin receptor (OB-Rb) was localized to GLP-1 neurons originating in the nucleus of the solitary tract. ICV injection of the specific GLP-1 receptor antagonist, exendin(9-39), at the onset of dark phase, did not affect feeding in saline pre-treated controls, but blocked the reduction in food intake and body weight of leptin pre-treated rats. These findings suggest that GLP-1 neurons are a potential target for leptin in its control of feeding.

Development of obesity and insulin resistance in the Israeli sand rat (Psammomys obesus). Does leptin play a role?

The Israeli Sand Rat (Psammomys obesus) is an excellent polygenic model for the study of obesity and diabetes. The metabolic characteristics and the heterogeneous development of these defects, including elevated leptin levels, mimic those found in susceptible human populations. Interestingly, only animals that develop metabolic abnormalities demonstrate hyperleptinemia and, in these animals, leptin administration at the same dose that is effective in ob/ob mice is ineffective in reducing food intake or body weight. Perhaps leptin resistance needs to develop in Israeli Sand Rats to allow the development of obesity and, in fact, leptin resistance may be the "thrifty gene" that predisposes individuals to the development of obesity and subsequent metabolic abnormalities. However, there remain many unanswered questions about the physiological actions of leptin. The widespread tissue location of receptors and the actions of leptin independent of food intake highlight the need for further research aimed at determining the major physiological action of this newly discovered and exciting hormone.

Neuropeptide Y suppresses epileptiform activity in rat hippocampus in vitro

Neuropeptide Y (NPY) potently inhibits glutamate-mediated synaptic transmission in areas CA1 and CA3 of the rat hippocampus without affecting other synaptic inputs onto principal cells of the hippocampal formation, suggesting that its biological role may include the regulation of excitability within the hippocampus. Here we examine NPY's actions in three in vitro models of epilepsy [0 Mg2+-, picrotoxin-, and stimulus-train-induced bursting (STIB)] with the use of extracellular and whole cell patch-clamp recordings from rat hippocampal-entorhinal cortex slices. Perfusion of the slice with saline that had Mg2+ omitted (0 Mg2+) or that had picrotoxin (100 microM) added resulted in brief spontaneous bursts (SBs) resembling interictal discharges. SB frequency is significantly reduced in both models by 1 microM NPY and by the Y2-preferring agonists peptide (P)YY(3-36) (1 microM) and 1-4-(6-aminohexanoic acid)-25-36 ([ahx(5-24)] NPY; 3 microM). The Y1-preferring agonist Leu31-Pro34NPY (1 microM) is considerably less potent, but also reduces burst frequency, even in the presence of the selective Y1 receptor antagonist GR231118, suggesting the involvement of a different receptor. In STIB, high-frequency stimulus trains to stratum radiatum of area CA2/CA3 result in clonic or tonic-clonic ictaform primary afterdischarges (primary ADs) as well as longer, spontaneous secondary ictaform discharges and SBs similar to those in the other models. Primary AD duration is greatly reduced or abolished by Y2- but not Y1-preferring agonists. SBs, although variable, were inhibited by both Y1 and Y2 agonists. In single and dual whole cell recordings from CA3 pyramidal cells, we frequently observed spontaneous, rhythmic synchronous events (SRSEs) arising after several STIB stimuli. Once established, SRSEs persist in the absence of further stimuli and are insensitive to the application of NPY. SRSEs in pyramidal cells typically occur at 2-4 Hz, are outward currents when cells are clamped near rest (>100 pA at a holding potential of -55 mV), reverse between -60 and -70 mV, and are inhibited by 100 microM picrotoxin, indicating involvement of gamma-aminobutyric acid-A receptors. They are inhibited by blockers of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) but not N-methyl-D-aspartate receptors. Whole cell patch-clamp recordings from interneurons in CA3 after STIB reveal NPY-insensitive, rhythmic, inward AMPA-receptor-mediated currents that are similar in frequency to SRSEs seen in pyramidal cells. We conclude that NPY, acting predominantly via Y2 receptors, can dramatically inhibit epileptiform activity in three fundamentally different in vitro models of epilepsy without affecting endogenous inhibitory activity. The results also provide support for the hypothesis that endogenous NPY may normally control excitability in the hippocampus and suggest the potential for NPY receptors as targets for anticonvulsant therapy.

Interleukin 1 alpha increases serum leptin concentrations in humans

Leptin, the protein product of the ob gene, regulates appetite and body weight in animals. Endotoxin and cytokines, induced by endotoxin, interleukin (IL) 1 and tumor necrosis factor, increase expression of leptin in mice and hamsters. We measured serum leptin concentrations in patients with cancer before and after administration of recombinant human IL-1 alpha. Fourteen patients received IL-1 alpha at one of three dose levels (0.03, 0.1, or 0.3 microgram/kg.day) for 5 days. Serum leptin concentrations increased in all but two patients within 24 h after the first dose. The increase in leptin was correlated directly with IL-1 alpha dose (P = 0.0030). Despite continued administration of IL-1 alpha, serum leptin concentrations returned to pretreatment levels by day 5 of therapy. An increase in serum leptin concentrations may be one mechanism by which anorexia is induced by IL-1 alpha. However, tachyphylaxis of the leptin response suggests that other mechanisms also are involved.

The bioactive conformation of neuropeptide Y analogues at the human Y2-receptor

Several attempts to investigate the bioactive conformation of neuropeptide Y have been made so far. As cyclic peptides are much more rigid than linear ones, we decided to synthesise cyclic analogues of the C-terminal dodekapeptide amide neuropeptide Y Ac-25-36. Cyclisation was performed by side chain lactamisation of ornithine or lysine and glutamic or aspartic acid. The affinity of the 19 peptides ranged from Ki 0.6 nM to greater than 10,000 nM. We found that the size, position, orientation, configuration. and the location of the cycle plays an important role for receptor recognition. Circular dichroic studies have been performed to characterise the secondary structure of each peptide. Receptor binding studies were carried out on human neuroblastoma cell lines SK-N-MC (Y1) and SMS-KAN (Y2), and on rabbit kidney membranes (Y2). The pharmacological and spectral data showed that the alpha-helix content was not the predominant factor for high Y2-receptor affinity. Instead, the location and the size of the hydrophobic lactam bridge, and the conserved C-terminal tetrapeptide (Arg-Glu-Arg-Tyr) seemed to be the main parameters. Using molecular dynamics, the structures of four cyclic peptides (i,i+4) have been investigated and compared with the previously published NMR structure of one of the cyclic peptide analogues. Significant differences have been found in the overall three-dimensional fold of the peptides. The distances between the N- and the C-terminus allow discrimination between peptides with high binding affinity and those with low binding affinity, because of the correlation that was found with the measured affinity. Thus, this study suggests that a turn-like structure and the orientation of the C-terminus towards the N-terminus play major roles for high affinity binding of cyclic dodecapeptides to the Y2-receptor. None of the cyclic segments exhibits significant affinity to the Y1-receptor. Thus, these results support the hypothesis of a discontinuous binding site of neuropeptide Y at the Y1-receptor.

Direct gene transfer into human epileptogenic hippocampal tissue with an adeno-associated virus vector: implications for a gene therapy approach to epilepsy

PURPOSE

Virus vectors capable of transferring genetic information into human cells provide hope for improved therapy in several neurological diseases, including epilepsy. We evaluated the ability of an adeno-associated virus (AAV) vector to transfer and cause expression of a lacZ marker gene in brain slices obtained from patients undergoing temporal lobectomy for control of medically intractable seizures.

METHODS

Human brain slices were injected with an AAV vector (AAVlacZ) encoding Escherichia coli beta-galactosidase and incubated for as long as 24 h. The presence of lacZ mRNA. beta-galactosidase protein and enzymatic activity were assayed by reverse transcriptase polymerase chain reaction (rtPCR), immunocytochemistry, and the X-Gal technique, respectively.

RESULTS

AAVlacZ directed the expression in human epileptogenic brain of E. coli beta-galactosidase that had functional activity. Expression was observed in < or =5 h and was sustained for as long as the slices were viable. Morphological analysis indicated that neurons were preferentially transfected, and there was no evidence of cytotoxicity.

CONCLUSIONS

Our results confirm the feasibility of using AAV vectors to transfer genes into the human CNS and in particular, into neurons. Replacement of the lacZ gene with a functional gene modulating hippocampal neuronal physiology, might allow a localized genetic intervention for focal seizures based on the stereotaxic or endovascular delivery of such a vector system into the appropriate brain region.

Powerful inhibition of kainic acid seizures by neuropeptide Y via Y5-like receptors

Neuropeptide Y (NPY) is widely distributed in interneurons of the central nervous system (CNS), including the hippocampus and cerebral cortex, in concentrations exceeding those of any other known neuropeptides. Sequence data comparing different species show that NPY is highly conserved. This suggests a critical role in regulation of regional neuronal excitability. Kainic acid, a glutamate agonist at kainic acid receptors, causes severe limbic motor seizures culminating in status epilepticus. We here report that NPY administered into the lateral ventricle is a powerful inhibitor of motor as well as electroencephalographic (EEG) seizures induced by kainic acid. This effect was mediated via receptors with a pharmacological profile similar to the recently cloned rat Y5 receptor. The present study is the first to demonstrate that NPY possesses anticonvulsant activity. This is consistent with the concept that NPY is an endogenous anticonvulsant and suggests that agonists acting at Y5-like receptors may constitute a novel group of drugs in antiepileptic therapy.

Attenuation of leptin-mediated effects by monosodium glutamate-induced arcuate nucleus damage

Leptin is a protein secreted by adipocytes that is important in regulating appetite and adiposity. Recent studies have suggested the presence of leptin receptors in the arcuate nucleus of the hypothalamus (ANH). Neonatal administration of monosodium glutamate (MSG) damages the ANH, resulting in obesity and neuroendocrine dysfunction. Neonatal administration of MSG was utilized to test the hypothesis that the anatomic site for many of leptin's actions is the ANH. Female control (n = 6) and MSG-treated rats (n = 7) were implanted for 14 days with osmotic minipumps containing phosphate-buffered saline or leptin (1 mg.kg-1.day-1). Leptin suppressed (P < 0.05) body weight gain in controls but did not suppress weight gain in MSG-treated rats. Leptin decreased (P < 0.05) fat depots in controls but had no effect in MSG-treated rats. Night feeding was suppressed (P < 0.05) in leptin-treated control rats. MSG-treated rats showed a suppression in food intake that was of a smaller magnitude and appeared later in the course of leptin treatment. These findings suggest that leptin mediates some physiological actions related to fat mobilization via receptors located in the ANH.

Endocrine function of neuropeptide Y knockout mice

Among its many proposed functions, neuropeptide Y (NPY) is thought to modulate the hypothalamic-pituitary axis. Specifically, increased hypothalamic NPY signaling may be critical in mediating the neuroendocrine response to fasting. To determine the consequences of NPY deficiency on endocrine physiology, multiple hormones were quantitated in wildtype and NPY-knockout mice under fed and fasted conditions. Serum concentrations of leptin, corticosterone, thyroxine, and testosterone were normal in NPY-knockout males fed ad libitum. A 48-hour fast resulted in a 50% reduction in leptin, a 60% reduction in thyroxine, a 75% reduction in testosterone, and a 12-fold increase in corticosterone in both wildtype and NPY-knockout mice. Fasting also increased the estrous cycle length by 3 days in both wildtype and NPY-deficient female mice. We conclude that NPY is not essential for appropriate function of the gonadotropic, thyrotropic, or corticotropic axes under ad lib fed conditions or in response to acute fasting.

Deficiency of a protein-repair enzyme results in the accumulation of altered proteins, retardation of growth, and fatal seizures in mice

L-Asparaginyl and L-aspartyl residues in proteins are subject to spontaneous degradation reactions that generate isomerized and racemized aspartyl derivatives. Proteins containing L-isoaspartyl and D-aspartyl residues can have altered structures and diminished biological activity. These residues are recognized by a highly conserved cytosolic enzyme, the protein L-isoaspartate(D-aspartate) O-methyltransferase (EC 2.1.1.77). The enzymatic methyl esterification of these abnormal residues in vitro can lead to their conversion (i.e., repair) to normal L-aspartyl residues and should therefore prevent the accumulation of potentially dysfunctional proteins in vivo as cells and tissues age. Particularly high levels of the repair methyltransferase are present in the brain, although enyzme activity is present in all vertebrate tissues. To define the physiological relevance of this protein-repair pathway and to determine whether deficient protein repair would cause central nervous system dysfunction, we used gene targeting in mouse embryonic stem cells to generate protein L-isoaspartate(D-aspartate) O-methyltransferase-deficient mice. Analyses of tissues from methyltransferase knockout mice revealed a striking accumulation of protein substrates for this enzyme in the cytosolic fraction of brain, heart, liver, and erythrocytes. The knockout mice showed significant growth retardation and succumbed to fatal seizures at an average of 42 days after birth. These results suggest that the ability of mice to repair L-isoaspartyl- and D-aspartyl-containing proteins is essential for normal growth and for normal central nervous system function.

A proposed test battery and constellations of specific behavioral paradigms to investigate the behavioral phenotypes of transgenic and knockout mice

Behavioral phenotyping of transgenic and knockout mice requires rigorous, formal analyses. Well-characterized paradigms can be chosen from the established behavioral neuroscience literature. This review describes (1) a series of neurological and neuropsychological tests which are effectively used as a first screen for behavioral abnormalities in mutant mice, and (2) a series of specific behavioral paradigms, clustered by category. Included are multiple paradigms for each category, including learning and memory, feeding, analgesia, aggression, anxiety, depression, schizophrenia, and drug abuse models. Examples are given from the experiences of the authors, in applying these experimental designs to transgenic and knockout mice. Extensive references for each behavioral paradigm are provided, to allow new investigators to access the relevant literature on behavioral methodology.

The genetics of obesity

Despite the influence of obesity in predisposing to many diseases, and evidence for high heritability, efforts to identify human genes with major effects on bodyweight have not yet been successful. In contrast, remarkable progress has been made in the identification and characterization of the genes mutated in five monogenic mouse models of obesity. These genes have led to new insights into the etiology of obesity and provide promising targets for therapeutic intervention.

Energy balance: role of genetics and activity

It is clear from the above discussion that much remains to be learned about both energy intake and energy expenditure. Obesity in humans is not likely to be caused by one gene, as it is in certain rodent models of obesity, such as the ob/ob mouse and the fa/fa rat. It is likely to be a polygenic condition in which numerous genes interact with each other and the environment to express the obesity phenotype. It is likely that genes that affect energy intake as well as genes that affect energy expenditure are involved. The role of leptin as a putative factor in signaling the extent of the fat mass to the central nervous system and controlling both food intake and energy expenditure is unclear at this time. The genetics of energy expenditure are also not clear at this time. There are mechanisms of nutrient partitioning, such as respiratory quotient and lipoprotein lipase activity, that are being discovered to be important. In addition, insulin sensitivity is likely to play a role in the etiology of obesity. Much more investigation will be required before we have a clearer picture of how the above-named factors interact, what the genetic contribution to this is, and how important each factor is to the overall phenotypic expression of obesity.

Regulation of neurotransmission in the arcuate nucleus of the rat by different neuropeptide Y receptors

We examined the effects of peptides of the neuropeptide Y (NPY)/pancreatic polypeptide (PP) family on synaptic transmission in the arcuate nucleus in rat hypothalamic slices. Application of NPY produced two effects. In some cells NPY produced an outward current that had the properties of a K+ current. NPY also inhibited the evoked glutamatergic EPSC recorded in these arcuate neurons by a presynaptic mechanism. Although the effects of NPY on the K+ current reversed within a few minutes of washout of the peptide, its effects on the EPSC frequently were longer lasting (>30 min). Similar effects were observed using peptide YY or the NPY analog [Leu31, Pro34]NPY. Although K+ current activation by [Leu31,Pro34]NPY was blocked by the selective Y1 antagonist BIBP 3226, inhibition of the EPSC was blocked only partially. Other NPY-related peptides such as NPY(13-36), PP, and [D-Trp32]NPY also inhibited the EPSC. However, none of these peptides produced activation of the K+ current. Thus, activation of more than one NPY receptor produces synaptic inhibition in the arcuate nucleus. A Y1 receptor activates a K+ current postsynaptically, and several receptor types appear to inhibit the EPSC by a presynaptic mechanism.

Induction of tolerance and mossy fibre neuropeptide-Y expression in the contralateral hippocampus following a unilateral intrahippocampal kainic acid injection in the rat

We have previously reported an ectopic expression of neuropeptide-Y (NPY) immunoreactivity in mossy fibres (MFs) in the contralateral hippocampus following a unilateral intrahippocampal (IH) injection of kainic acid (KA). In the present study we report that, in addition to MF NPY expression, unilateral IH KA injections also induce tolerance towards a subsequent intracerebroventricular (ICV) contralateral KA injection, resulting in a reduction in the number of overt seizures and degree of cell loss.

Thermoregulatory and metabolic phenotypes of mice lacking noradrenaline and adrenaline

Adrenaline and noradrenaline, the main effectors of the sympathetic nervous system and adrenal medulla, respectively, are thought to control adiposity and energy balance through several mechanisms. They promote catabolism of triglycerides and glycogen, stimulate food intake when injected into the central nervous system, activate thermogenesis in brown adipose tissue, and regulate heat loss through modulation of peripheral vasoconstriction and piloerection. Thermogenesis in brown adipose tissue occurs in response to cold and overeating (diet induced), and there is an inverse relationship between diet-induced thermogenesis and obesity both in humans and in animal models. As a potential model for obesity, we generated mice that cannot synthesize noradrenaline or adrenaline by inactivating the gene that encodes dopamine beta-hydroxylase. These mice are cold intolerant because they have impaired peripheral vasoconstriction and are unable to induce thermogenesis in brown adipose tissue through uncoupling protein (UCP1). The mutants have increased food intake but do not become obese because their basal metabolic rate is also elevated. The unexpected increase in basal metabolic rate is not due to hyperthyroidism, compensation by the widely expressed uncoupling protein UCP2, or shivering.

Pediatric obesity. An overview of etiology and treatment

Pediatric obesity is a chronic and growing problem for which new ideas about the biologic basis of obesity offer hope for effective solutions. Prevalence of pediatric and adult obesity is increasing despite a bewildering array of treatment programs and severe psychosocial and economic costs. The definition of obesity as an increase in fat mass, not just an increase in body weight, has profound influence on the understanding and treatment of obesity. In principle, body weight is determined by a balance between energy expenditure and energy intake, but this observation does not by itself explain obesity. There is surprisingly little evidence that the obese overeat and only some evidence that the obese are more sedentary. Understanding of the biologic basis of obesity has grown rapidly in the last few years, especially with the identification of a novel endocrine pathway involving the adipose tissue secreted hormone leptin and the leptin receptor that is expressed in the hypothalamus. Plasma leptin levels are strongly correlated with body fat mass and are regulated by feeding and fasting, insulin, glucocorticoids, and other factors, consistent with the hypothesis that leptin is involved in body weight regulation and may even be a satiety factor (Fig. 2, Table 1). Leptin injections have been shown to reduce body weight of primates, although human clinical trials will not be reported until summer 1997. So many peptides influencing feeding have been described that one or more may have therapeutic potential (Fig. 2, Table 1). Although the complexity of pathways regulating body weight homeostasis slowed the pace of understanding underlying mechanisms, these complexities now offer many possibilities for novel therapeutic interventions (Fig. 2). Obesity is a major risk factor for insulin resistance and diabetes, hypertension, cancer, gallbladder disease, and atherosclerosis. In particular, adults who were obese as children have increased mortality independent of adult weight. Thus, prevention programs for children and adolescents will have long-term benefits. Treatment programs focus on modification of energy intake and expenditure through decreased calorie intake and exercise programs. Behavior-modification programs have been developed to increase effectiveness of these intake and exercise programs. These programs can produce short-term weight loss. Long-term losses are more modest but achieved more successfully in children than in adults. Several drug therapies for obesity treatment recently have been approved for adults that produce sustained 5% to 10% weight losses but experience with their use in children is limited. Identification of the biochemical pathways causing obesity by genetic approaches could provide the theoretic foundation for novel, safe, and effective obesity treatments. The cloning of leptin in 1994 has already led to testing the efficacy of leptin in clinical trials that are now underway. Although novel treatments of obesity are being developed as a result of the new biology of obesity, prevention of obesity remains an important goal.

Characterization of a selective antagonist of neuropeptide Y at the Y2 receptor. Synthesis and pharmacological evaluation of a Y2 antagonist

Neuropeptide Y (NPY) is a potent inhibitor of neurotransmitter release through the Y2 receptor subtype. Specific antagonists for the Y2 receptors have not yet been described. Based on the concept of template-assembled synthetic proteins we have used a cyclic template molecule containing two beta-turn mimetics for covalent attachment of four COOH-terminal fragments RQRYNH2 (NPY 33-36), termed T4-[NPY(33-36)]4. This structurally defined template-assembled synthetic protein has been tested for binding using SK-N-MC and LN319 cell lines that express the Y1 and Y2 receptor, respectively. T4-[NPY(33-36)]4 binds to the Y2 receptor with high affinity (IC50 = 67.2 nM) and has poor binding to the Y1 receptor. This peptidomimetic tested on LN319 cells at concentrations up to 10 microM shows no inhibitory effect on forskolin-stimulated cAMP levels (IC50 for NPY = 2.5 nM). Furthermore, we used confocal microscopy to examine the NPY-induced increase in intracellular calcium in single LN319 cells. Preincubation of the cells with T4-[NPY(33-36)]4 shifted to the right the dose-response curves for intracellular mobilization of calcium induced by NPY at concentrations ranging from 0.1 nM to 10 microM. Finally, we assessed the competitive antagonistic properties of T4-[NPY(33-36)]4 at presynaptic peptidergic Y2 receptors modulating noradrenaline release. the compound T4-[NPY(33-36)]4 caused a marked shift to the right of the concentration-response curve of NPY 13-36, a Y2-selective fragment, yielding a pA2 value of 8.48. Thus, to our best knowledge, T4-[NPY(33-36)]4 represents the first potent and selective Y2 antagonist.

Neuropeptides responding to leptin

Leptin, the circulating protein that inhibits food intake and energy expenditure, was thought to function through inhibition of the hypothalamic neuropeptide Y (NPY), a stimulator of food intake. However, mouse mutants lacking NPY are normal, suggesting that alternative neuromodulators of food intake must exist. Recently, melanocortin, a neuropeptide acting on the hypothalamic receptor melanocortin4-R, was discovered in mice, controlling energy regulation. This receptor is antagonized by the "agouti" protein in the mutant obese agouti mouse.

Leptin receptor mRNA is expressed in ewe anterior pituitary and adipose tissues and is differentially expressed in hypothalamic regions of well-fed and feed-restricted ewes

Infertility associated with suboptimal nutrition is a major concern among livestock producers. Recently, much effort has been put into understanding the role of the protein leptin in regulating feed intake and reproduction. Leptin, produced by adipocytes, has receptors in the hypothalamus, but more precise locations of leptin receptor-expressing cell bodies have not been reported in a livestock species. The leptin receptor transcript has several splice variants in the mouse and human, but only the "long-form" product (OBRL) is capable of signal transduction. A partial ovine long-form leptin receptor cDNA was cloned and used to evaluate OBRL mRNA expression within hypothalamic, anterior pituitary, and adipose tissues of ovariectomized adult ewes. Expression was detected in reverse transcription-polymerase chain reaction products of all tissues examined. OBRL mRNA was detected by in situ hybridization in the ventromedial and arcuate nuclei of the hypothalamus. In ewes that had been feed restricted for 3 wk before tissue collection, the expression of OBRL mRNA in these areas was greater (P < 0.05) than that found in well-fed ewes. These findings provide evidence that the full-length leptin receptor is expressed in hypothalamic, anterior pituitary, and adipose tissue (the latter proffering an autoregulatory mechanisms for leptin) and that within the hypothalamus, this receptor form is differentially expressed in well-fed vs. feed-restricted animals.

The new biology of body weight regulation

A growing body of evidence suggests that energy balance (the difference between energy intake and expenditure) and body fuel stores in the form of adipose tissue are maintained by the body within a narrow range. This regulation of adiposity is mediated by the secretion of hormonal signals into the circulation in proportion to body adipose stores and their subsequent actions on brain systems that control caloric intake and energy expenditure. As a result, changes in energy balance sufficient to alter fuel stores elicit compensatory changes in energy intake and expenditure that return fat stores to their regulated level. Recent scientific break-through have identified the key components of this physiologic system. These include the circulating signals, leptin (the hormone encoded by the ob gene that is secreted by fat cells) and the pancreatic hormone insulin; and brain peptides such as neuropeptide Y, which is released from nerve terminals in the hypothalamus to elicit changes in feeding behavior and energy expenditure that mediate adaptive changes in energy balance. This article reviews the discovery of leptin and its receptor and discusses the interaction of leptin and insulin with the hypothalamic neuropeptide Y system. These observations provide a basis for understanding how weight lost during a period of negative energy balance (because of the inability to consume and/or store sufficient energy to meet ongoing energy demands) is eventually recovered. As our understanding of this weight-regulatory system increases, new insights into the causes of human obesity are likely to follow. Such insights may yield improvements in the medical and nutrition management of obese patients.

Synergy between leptin and cholecystokinin (CCK) to control daily caloric intake

Both cholecystokinin (CCK), a short-term meal-related satiety signal, and the ob protein leptin, a postulated long-term adiposity hormone, are thought to be important signals in the multiple interacting systems that control appetite and adiposity. We hypothesized that these hormones may synergistically interact to suppress feeding. Following IP administration of leptin (two doses of 50 micrograms each) and CCK (2, 4, 8, or 16 micrograms) total daily caloric intake was significantly reduced by leptin and CCK compared to leptin alone. These results support the hypothesis that CCK and leptin may synergistically interact to control long-term feeding.

Neuropeptide Y family of hormones: receptor subtypes and antagonists

Neuropeptide Y (NPY) is the most abundant peptide present in the mammalian central and peripheral nervous system. NPY exhibits a variety of potent central and peripheral effects including those on feeding, memory, blood pressure, cardiac contractility and intestinal secretions. Classical pharmacological studies have shown that NPY effects are mediated by four different receptor subtypes, Y-1, Y-1-like, Y-2, and Y-3. However, the existence of numerous atypical activities provide strong evidence for the occurrence of additional NPY receptor subtypes. Pharmacological studies have further been facilitated by the recent cloning and expression of Y-1, Y-2, Y-4 (PP-1) and Y-5 receptors. Moreover, the cloned Y-5 receptor has been suggested to be the long awaited Y-1-like receptor involved in feeding. Structure-activity studies have laid a good foundation towards the development of receptor selective compounds, and to date potent Y-1 selective peptide and nonpeptide antagonists have been developed. The need to clone numerous receptor subtypes and to develop receptor selective compounds for physiological and perhaps clinical use is expected to keep NPY research active for many years to come.

Attenuation of the obesity syndrome of ob/ob mice by the loss of neuropeptide Y

The obesity syndrome of ob/ob mice results from lack of leptin, a hormone released by fat cells that acts in the brain to suppress feeding and stimulate metabolism. Neuropeptide Y (NPY) is a neuromodulator implicated in the control of energy balance and is overproduced in the hypothalamus of ob/ob mice. To determine the role of NPY in the response to leptin deficiency, ob/ob mice deficient for NPY were generated. In the absence of NPY, ob/ob mice are less obese because of reduced food intake and increased energy expenditure, and are less severely affected by diabetes, sterility, and somatotropic defects. These results suggest that NPY is a central effector of leptin deficiency.

Glucose intolerance but normal satiety in mice with a null mutation in the glucagon-like peptide 1 receptor gene

Glucagon-like peptide 1 (GLP1) is postulated to regulate blood glucose and satiety, but the biological importance of GLP1 as an incretin and neuropeptide remains controversal. The regulation of nutrient-induced insulin secretion is dependent on the secretion of incretins, gut-derived peptides that potentiate insulin secretion from the pancreatic islets. To ascertain the relative physiological importance of GLP1 as a regulator of feeding behavior and insulin secretion, we have generated mice with a targeted disruption of the GLP1 receptor gene (GLP1R). These GLP1R-/- mice are viable, develop normally but exhibit increased levels of blood glucose following oral glucose challenge in association with diminished levels of circulating insulin. It is surprising that they also exhibit abnormal levels of blood glucose following intraperitoneal glucose challenge. Intracerebroventricular administration of GLP1 inhibited feeding in wild-type mice but not in GLP1R-/- mice; however, no evidence for abnormal body weight or feeding behavior was observed in GLP1R-/- mice. These observations demonstrate that GLP1 plays a central role in the regulation of glycemia; however, disruption of GLP1/GLP1R signaling in the central nervous system is not associated with perturbation of feeding behavior or obesity in vivo.

Modified, cyclic dodecapeptide analog of neuropeptide Y is the smallest full agonist at the human Y2 receptor

In order to stabilize the C-terminal dodecapeptide of neuropeptide Y (NPY) we replaced Leu28 and Thr32 by Lys and Glu, respectively, and subsequently linked these residues by lactamization. This peptide analog of NPY shows a more than 100-fold increase in affinity compared to the C-terminal linear dodecapeptide in receptor binding studies performed at human neuroblastoma cells SMS-KAN, which exclusively express the Y2 receptor subtype. Signal transduction was investigated by measuring Ca2+ current inhibition in human SH-SY5Y cells and cyclic [Lys28-Glu32] NPY Ac-25-36 and NPY were shown to be equipotent in this assay. Thus, this molecule is the smallest Y2 receptor selective full agonist of NPY. Using 2D-NMR experiments and molecular modelling techniques, the structures of the linear and cyclic peptides have been investigated and significant differences have been found, which may explain the improvement in biological activity. Thus, a model of the bioactive conformation of NPY at the human Y2 receptor is suggested.

Shedding new light on obesity. Appetite regulation

Several genes involved in the regulation of appetite and energy metabolism have been cloned and characterized recently. Each seems to form part of the complex regulatory network, centred in the hypothalamus, that is responsible for striking a balance between food intake and energy expenditure.

Role of leptin in the neuroendocrine response to fasting

A total deficiency in or resistance to the protein leptin causes severe obesity. As leptin levels rise with increasing adiposity in rodents and man, it is proposed to act as a negative feedback 'adipostatic signal' to brain centres controlling energy homeostasis, limiting obesity in times of nutritional abundance. Starvation is also a threat to homeostasis that triggers adaptive responses, but whether leptin plays a role in the physiology of starvation is unknown. Leptin concentration falls during starvation and totally leptin-deficient ob/ob mice have neuroendocrine abnormalities similar to those of starvation, suggesting that this may be the case. Here we show that preventing the starvation-induced fall in leptin with exogenous leptin substantially blunts the changes in gonadal, adrenal and thyroid axes in male mice, and prevents the starvation-induced delay in ovulation in female mice. In contrast, leptin repletion during this period of starvation has little or no effect on body weight, blood glucose or ketones. We propose that regulation of the neuroendocrine system during starvation could be the main physiological role of leptin.