Centrally administered galanin-like peptide modifies food intake in the rat: a comparison with galanin

Effects of galanin-like peptide (GALP) on locomotion, reproduction, and body weight in female and male mice

Galanin-like peptide (GALP) has been implicated in the neuroendocrine regulation of both feeding and reproduction. In male rodents and primates, intracerebroventricular (icv) infusions of GALP stimulate luteinizing hormone (LH) release, induce Fos expression in brain areas implicated in feeding and reproduction, and affect food intake and body weight in rodents. In gonad-intact and castrated male rats, icv administration of GALP also stimulates male sexual behavior. While the effects of GALP on male physiology and behavior are well documented, no studies have addressed such a role of GALP in females. We tested the effects of icv GALP infusions on LH release, locomotor activity, motor control, and body weight regulation in adult ovariectomized female mice hormonally primed with estradiol benzoate and progesterone. In addition, sexually-experienced male and female mice were treated with GALP and tested for sexual behavior. In females, GALP reduced open-field locomotor activity, the ability to maintain grip on an accelerating rotarod, and 24-h body weight in a dose-dependent manner. GALP also increased LH secretion in female mice, an effect that was blocked by pre-treatment with Antide, a gonadotropin-releasing hormone (GnRH) type-1 receptor antagonist. GALP infusions slightly decreased the occurrence of lordosis behavior in female mice and significantly increased the latencies with which females displayed receptivity. Unlike previous reports in male rats, GALP inhibited male sexual behavior in mice. Our data indicate that in female mice, GALP stimulates LH release via GnRH, and decreases body weight, motor control, and locomotor activity via GnRH-independent pathways. Furthermore, our sexual behavior and locomotor findings suggest species-specific differences in the mechanism and/or location of GALP action in the brains of rats and mice.

Regulatory roles of leptin in reproduction and metabolism: a comparative review

Leptin plays an important role in signaling nutritional status to the central reproductive axis of mammals and appears to be at least a permissive factor in the initiation of puberty. The expression and secretion of leptin are correlated with body fat mass and are acutely affected by changes in feed intake. Moreover, circulating leptin increases during pubertal development in rodents, human females and heifers. Effects of leptin are mediated mainly via receptor activation of the JAK-STAT pathway; however, activation of alternative pathways, such as MAP kinase, has also been reported. Although the leptin receptor (LR) has not been found on GnRH neurons, leptin stimulates the release of GnRH from rat and porcine hypothalamic explants. Moreover, leptin increases the release of LH in rats and from adenohypophyseal explants and/or cells from full-fed rats and pigs. In contrast, stimulation of the hypothalamic-gonadotropic axis by leptin in cattle and sheep is observed predominantly in animals and tissues pre-exposed to profound negative energy balance. For example, leptin prevents fasting-mediated reductions in the frequency of LH pulses in peripubertal heifers, augments the magnitude of LH and GnRH pulses in fasted cows, and enhances basal secretion of LH in vivo and from adenohypophyseal explants of fasted cows. However, leptin is incapable of accelerating the frequency of LH pulses in prepubertal heifers, regardless of nutrient status, and has no effect on the secretion of GnRH and LH in full-fed cattle or hypothalamic/hypophyseal explants derived thereof. Similar to results obtained with LH, basal secretion of GH from anterior pituitary explants of fasted, but not normal-fed cows, was potentiated acutely by low, but not high, doses of leptin. Mechanisms through which undernutrition hypersensitize the hypothalamic-gonadotropic axis to leptin may involve up-regulation of the LR. However, an increase in LR mRNA expression is not a requisite feature of heightened adenohypophyseal responses in fasted cattle. To date, leptin has not been successful for inducing puberty in ruminants. Future therapeutic uses for recombinant leptin that exploit states of nutritional hypersensitization, and identification of genetic markers for genotypic variation in leptin resistance, are currently under investigation.

Galanin-like peptide and ghrelin increase cytosolic Ca2+ in neurons containing growth hormone-releasing hormone in the arcuate nucleus

Galanin-like peptide (GALP), discovered in the porcine hypothalamus, is expressed predominantly in the arcuate nucleus (ARC), a feeding-controlling center. Intracerebroventricular injection of GALP has been shown to stimulate food intake in the rats. However, the mechanisms underlying the orexigenic effect of GALP are unknown. The present study aimed to determine the target neurons of GALP in the ARC. We investigated the effects of GALP on cytosolic free Ca2+ concentration ([Ca2+]i) in the neurons isolated from the rat ARC, followed by neurochemical identification of these neurons by immunocytochemistry using antisera against growth hormone-releasing hormone (GHRH), neuropeptide Y (NPY) and proopiomelanocortin (POMC), the peptides localized in the ARC. GALP at 10(-10) M increased [Ca2+]i in 11% of single neurons of the ARC, while ghrelin, an orexigenic and GH-releasing peptide, at 10(-10) M increased [Ca2+]i in 35% of the ARC neurons. Some of these GALP- and/or ghrelin-responsive neurons were proved to contain GHRH. In contrast, NPY- and POMC-containing neurons did not respond to GALP. These results indicate that GALP directly targets GHRH neurons, but not NPY and POMC neurons, and that ghrelin directly targets GHRH neurons in the ARC. The former action may be involved in the orexigenic effect of GALP and the latter in the GH-releasing and/or orexigenic effects ghrelin.

Pharmacological and functional characterization of galanin-like peptide fragments as potent galanin receptor agonists

The hypothalamic galanin-like peptide (GALP) was isolated by its ability to activate galanin receptors. The mature porcine GALP is a 60-amino acid neuropeptide proteolytically processed from a 120-amino acid precursor protein. It contains a region identical to the N-terminal 13-amino acids of the neuropeptide galanin. Within the sequence of human GALP (1-60) a potential proteolytic cleavage site between two basic amino acids is present at position 33, which might lead to a shorter C-terminally amidated peptide. In addition, the first two amino acids could be potentially removed via the action of dipeptidase IV. Ligand binding assays using the human neuroblastoma cell line SH-SY5Y transfected with the respective galanin receptors revealed that human GALP (1-60) displayed the highest affinity for the galanin receptor subtype GalR3 (IC50 = 10 nM) followed by GalR2 (IC50 = 28 nM) and GalR1 (IC50 = 77 nM). Ligand binding assays and functional studies showed that the human GALP (3-32) fragment was at least as potent as full length GALP (1-60). Other studies have shown that shorter fragments like human GALP (1-21) and GALP (22-60) were not effective on feeding responses in mice as compared to the full length peptide. Taken together these data suggest that the putative fragment GALP (3-32) might represent the strongest mediator of biological GALP activity. Furthermore it might be a useful tool to study the affinity of GALP to galanin receptors and to search for specific GALP receptors.

Exaggerated feeding response to central galanin-like peptide administration in diet-induced obese rats

Galanin-like peptide (GALP) is a newly identified neuropeptide implicated in the regulation of metabolism and reproduction. GALP gene expression is decreased in the hypothalamus of genetically obese rodents, such as fa/fa rats and ob/ob mice, and central administration of GALP increases feeding in satiated rats. The effect of dietary obesity on GALP-induced feeding is unknown, so this study characterized the effects of central administration of GALP on feeding in a rat model of diet-induced obesity. Male Sprague-Dawley rats (n = 21) were randomly assigned to receive standard laboratory chow (12% fat as kcal) or high-fat cafeteria diet (35% fat) for 12 weeks before intracerebroventricular (icv) cannulae were implanted. Seven days later, rats received 0,0.2 or 0.3 nmol doses of GALP in randomized order at least 48 h apart. Food intake was measured at 0.5,1,2, 4 and 24 h post administration and body weight was measured at 24 h. Rats were maintained on their respective diets throughout the entire feeding experiment. Implementation of the high-fat diet led to significantly greater caloric intake (230%) and body weight (28%) compared to chow-fed control rats. GALP-induced feeding was rapid and maximal in both dietary groups at 30 min post injection. The 0.3 nmol dose of GALP led to significantly larger increases in caloric intake in high-fat fed rats than in chow-fed controls (35.4 +/- 3.7 and 22.1 +/- 1.3 kcal, respectively, at 30 min). It is not known if diet-induced obesity alters endogenous GALP levels, but our data suggest that adaptive responses in GALP signaling might occur during chronic overfeeding. One possible explanation is an increased sensitivity and/or number of specific GALP receptors, although actions of exogenous GALP may also represent pharmacological actions at galanin receptors.

Neuronal interactions between galanin-like-peptide- and orexin- or melanin-concentrating hormone-containing neurons

Galanin-like peptide (GALP) is a novel orexigenic neuropeptide that is recently isolated from the porcine hypothalamus. GALP-containing neurons predominantly locate in the hypothalamic arcuate nucleus (ARC). The expression of GALP mRNA within the ARC is increased after the administration of leptin. GALP-containing neurons express leptin receptor and contain alpha-melanocyte-stimulating hormone. We have recently reported that neuropeptide Y (NPY)- and orexin-containing axon terminals are in close apposition with GALP-containing neurons in the ARC. In addition, GALP-containing neurons express orexin-1 receptor (OX1-R). Thus, GALP may function under the influence of leptin and orexin. However, the target neurons of GALP have not yet been clarified. To clarify the neuronal interaction between GALP-containing and other feeding regulating neurons, double-immunostaining method using antibodies against GALP- and orexin- or melanin-concentrating hormone (MCH) was performed in the rat lateral hypothalamus (LH). GALP-immunoreactive fibers appeared to project to the LH around the fornix. They were also found from the rostral to the caudal part of the ARC, paraventricular nucleus (PVH), stria terminalis (BST), medial preoptic area (MPA), and lateral septal nucleus (LSV). Moreover, GALP-like immunoreactive nerve fibers were directly contacted with orexin- and melanin-concentrating hormone (MCH)-like immunoreactive neurons in the LH. Our findings strongly suggest that GALP-containing neurons interact with orexin- and/or MCH-containing neurons in the lateral hypothalamus and that it participates in the regulation of feeding behavior in harmony with other feeding-regulating neurons in the hypothalamus.

Electron microscopy examination of galanin-like peptide (GALP)-containing neurons in the rat hypothalamus

Galanin-like peptide (GALP) is a novel peptide which is isolated from the porcine hypothalamus. GALP-containing neurons are present in the arcuate nucleus (ARC), being particularly densely concentrated in medial posterior regions. To observe the ultrastructure and synaptic relationships of GALP-containing neurons in the ARC, light and immunoelectron microscopy techniques were used. At the light microscope level, GALP-containing neurons were observed distributed rostrocaudally throughout the ARC, with the majority present in the posterior, periventricular zones. At the electron microscope level, many immunopositive dense-cored vesicles were evident in the perikarya, dendrites and axon terminals of the GALP-containing neurons. Furthermore, these neurons received synapses from immunonegative axon terminals that were symmetric in the case of synapses made on perikarya, and both asymmetric and symmetric for synapses made on dendrites. Axon terminals of GALP-containing neurons often made synapses on immunonegative dendrites. Such synapses were all symmetric. Synapses were also found between axon terminals and perikarya as well as dendrites of GALP-containing neurons. These findings suggest that the physiological role of the GALP-containing neurons in the ARC is based on complex synaptic relationships between GALP-containing neurons and either GALP-immunopositive or -immunonegative neurons.

Galanin-like peptide in the brain: effects on feeding, energy metabolism and reproduction

The hypothalamus plays an important role in the regulation of feeding behavior, energy metabolism and reproduction. A novel peptide containing 60 amino acid peptide and a non-amidated C-terminus is produced in the hypothalamic arcuate nucleus (ARC) and has been named galanin-like peptide (GALP) on the basis of a portion of this peptide being homologous with galanin. It acts in the central nervous system (CNS), where it is involved in the regulation of feeding behavior. GALP-producing neurons make neuronal networks with several feeding related peptide-producing neurons. Since GALP is involved in the control of food intake and energy balance, it is possible that it plays an important role in the development of obesity. Furthermore, GALP regulates plasma lateral hypothalamus (LH) levels via the activation of gonadotropin-releasing hormone (GnRH)-producing neurons, suggesting that GALP is active in the reproductive system. Thus, interesting findings on the roles of GALP have made across a number of physiological systems. This review will attempt to summarize the research carried out to date on these areas. Because GALP may be involved in feeding behavior, energy metabolism and reproduction, further studies on the morphology and function of GALP-containing neurons in the CNS should increase our understanding of the role of GALP in brain function.

Induction of galanin-like peptide gene expression in the arcuate nucleus of the rat after acute but not chronic inflammatory stress

Galanin-like peptide (GALP) has been recently isolated from the porcine hypothalamus. The GALP mRNA is restricted to neurons in the hypothalamic arcuate nucleus (Arc) and pituicytes in the posterior pituitary gland (PP), but physiological functions of the GALP remains unclear in both areas. We examined the effects of acute and chronic inflammatory stresses on the GALP mRNA levels in the rat Arc using in situ hybridization histochemistry. Intraperitoneal (i.p.) injection of bacterial endotoxin lipopolysaccharide (LPS) caused a marked increase of the GALP mRNA levels in the Arc. The effects of i.p. injection of LPS on the GALP mRNA levels in the Arc were significantly attenuated by pretreatment with i.p. injection of indomethacin cyclooxygenase inhibitor. Adjuvant arthritis caused by subcutaneous (s.c.) injection of heat-killed Mycobacterium butyricum as chronic inflammatory stress did not affect the GALP mRNA levels in the Arc, though the GALP mRNA levels in the pituicytes of the PP were markedly increased by two peaks at 12 h and 15 days after s.c. injection of heat-killed M. butyricum. Enzymeimmunoassay showed that the plasma concentration of GALP was not affected by these inflammatory stresses. These results suggest that acute inflammatory stress might be a potent stimulant to increase the GALP mRNA levels in the Arc of the rat via synthesis of prostaglandins.

Galanin-Like peptide elicits a robust discharge of growth hormone in the rhesus monkey (Macaca mulatta)

Galanin-like peptide (GALP) stimulates the release of gonadotropin-releasing hormone in rodent and primate species. The widespread distribution of GALP fibers in the hypothalamus suggests that this neuropeptide may influence hypophysiotropic factors that control other aspects of adenohypophysial function. Here we studied the effects of intracerebroventricular administration of GALP on serum levels of growth hormone (GH) and prolactin (PRL) in adult male monkeys. The animals (n = 5) were orchidectomized and implanted with testosterone-containing Silastic capsules to maintain the circulating testosterone levels (approximately 9 ng/ml) within the physiological range. The animals were implanted with an intracerebroventricular cannula and venous catheter for continuous access to the cerebroventricular and the venous circulation, respectively. GALP (500 microg), or vehicle alone, was administered as a bolus intracerebroventricular injection, and sequential blood samples were collected at 20-min intervals for 3 h before and after the injections. Within 20 min following GALP injection, the GH concentrations increased 3.5-fold, and a peak level (12.9 +/- 2.7 ng/ml) was observed 40 min after injection. The GH levels remained elevated until 60 min after injection and thereafter declined to values similar to those observed at 0 min. The GH concentrations were not changed by vehicle alone. A decline in PRL levels was observed following GALP administration, with significantly reduced concentrations occurring between 60 and 120 min following the injection of the neuropeptide. We conclude that in the monkey GALP is a potent secretagogue for GH and an inhibitor of PRL secretion and that GALP may, therefore, interact with the hypothalamic circuitry involved in the regulation of these pituitary hormones.

Stimulation of sexual behavior in the male rat by galanin-like peptide

Galanin-like peptide (GALP) is a recently described neuropeptide, which shares a partial sequence identity with galanin but is derived from a separate gene. Central injections of GALP stimulate the secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) and induce the expression of Fos in several brain areas known to regulate male sexual behavior in the rat. We postulated that GALP may also stimulate sexual behavior in concert with its stimulatory effect on the hypothalamic-pituitary-gonadal (HPG) axis. To test this hypothesis, we administered GALP, galanin, or the vehicle (artificial cerebrospinal fluid, aCSF) alone to sexually experienced male rats and assessed the effects of these agents on sexual behavior. We observed that compared to aCSF alone, GALP significantly increased all aspects of male-typical sexual behavior, whereas galanin inhibited all of these same behaviors. To examine whether the stimulatory effects of GALP on sexual behavior were mediated by GALP's stimulatory effects on the HPG axis, we castrated the same male rats and repeated the behavioral experiment. We found that GALP maintained its inductive action on male-typical sexual behaviors in the castrated animals, suggesting that the effects of GALP on sexual behavior are not the result of GALP's ability to stimulate testosterone secretion. These observations suggest that GALP neurons are part of the hypothalamic circuitry controlling sexual behavior in the male rat.

Galanin-like peptide as a link in the integration of metabolism and reproduction

The arcuate nucleus is a hypothalamic center that couples energetics and reproduction. Peptide-releasing neurons in the arcuate nucleus receive and process humoral signals from the periphery and relay this information to other nuclei in the hypothalamus and preoptic area. Galanin-like peptide (GALP) is expressed in the arcuate nucleus, and GALP-containing neurons are targets for the action of leptin. GALP-containing neurons are closely apposed to gonadotropin-releasing hormone (GnRH) neurons in the preoptic area, and CNS injections of GALP stimulate GnRH-mediated secretion of luteinizing hormone. These observations indicate that GALP is a molecular signal that couples circulating indices of metabolism to the neuroendocrine reproductive system and, thus, regulates reproductive activity as a function of the energy state. In this article, we describe the involvement of GALP in metabolism and reproduction, and in the coupling between these two processes.

Galanin-like peptide gene expression in the hypothalamus and posterior pituitary of the obese fa/fa rat

We examined the galanin-like peptide (GALP) gene expression in the arcuate nucleus (ARC) and posterior pituitary (PP) in 6- and 18-week-old male obese fa/fa rats. GALP mRNA in the ARC in fa/fa rats was significantly decreased in 6- and 18-week-old and GALP mRNA in the PP in fa/fa rats was significantly increased in 18-week-old compared to lean Fa/? rats. Insulin treatment in hyperglycemic fa/fa rats partially reversed those changes. These results suggest that the GALP gene expression in fa/fa rats might be regulated in part by leptin-independent mechanisms.

Effects of diabetes and insulin on the expression of galanin-like peptide in the hypothalamus of the rat

Galanin-like peptide (GALP) is produced in a small population of neurons in the arcuate nucleus of the hypothalamus, and leptin stimulates the hypothalamic expression of GALP mRNA. Because insulin and leptin share common signaling pathways in the brain, we reasoned that GALP neurons might also be responsive to changes in circulating concentrations of insulin. To test this hypothesis, we first studied the effect of insulin deficiency on the expression of GALP by comparing levels of GALP mRNA between normal and diabetic animals. Streptozotocin-induced diabetes was associated with a significant reduction in the expression of GALP mRNA, which was reversed by treatment with either insulin or leptin. Second, we examined the effect of insulin administered directly into the brain on the expression of GALP mRNA in fasted rats. Hypothalamic levels of GALP mRNA were lower in animals after a 48-h fast, and central treatment with insulin reversed this effect. These results suggest that GALP neurons are direct targets for regulation by insulin and implicate these cells for a role in the metabolic and behavioral sequelae of type 1 diabetes.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

Galanin-like peptide mRNA alterations in arcuate nucleus and neural lobe of streptozotocin-diabetic and obese zucker rats. Further evidence for leptin-dependent and independent regulation

Galanin-like peptide (GALP) is a 60-amino-acid peptide with structural similarities to galanin and a high affinity for galanin receptors. GALP is expressed by a discrete population of neurons in the arcuate nucleus (ARC) and median eminence of the hypothalamus of several species, including the rat. GALP neurons express leptin receptors and GALP mRNA levels are decreased slightly in fasted rats and stimulated significantly by acute leptin treatment in combination with fasting. In studies to further explore the leptin dependence of GALP expression, we examined GALP mRNA levels in the hypothalamus of obese Zucker and streptozotocin-induced diabetic (STZ-DM) rats. In leptin receptor-deficient obese Zucker rats, with 75% higher body weight than lean littermates, GALP mRNA levels in the ARC were decreased by 75%, while neuropeptide Y (NPY) mRNA levels were increased 7-fold (n = 5, p < 0.001), consistent with earlier reports. In hypoleptinemic diabetic rats with 4.5-fold higher blood glucose and 15% lower body weight than controls, GALP mRNA levels in the ARC were decreased by 90%, while NPY mRNA levels were increased 9-fold (n = 5, p < 0.001). GALP is also expressed by pituicytes in the neural lobe of the rat pituitary gland and GALP expression is increased by osmotic stimulation such as dehydration and salt loading. Thus, in STZ-DM rats that are in a hyperosmotic state with elevated plasma vasopressin levels, GALP mRNA levels were increased by approximately 20-fold in the neural lobe relative to control (n = 4, p < 0.001). The current findings are consistent with a strong tonic influence of leptin receptor signalling on hypothalamic GALP expression under normal conditions, and possible abnormalities in GALP neuronal signalling and their putative targets, thyrotropin-releasing hormone and gonadotropin hormone-releasing hormone neurons, under pathophysiological conditions such as diabetes and obesity. Our data in STZ-DM rats also clearly demonstrate that GALP gene expression is differentially regulated in neurons and pituicytes.

Regulation of galanin-like peptide gene expression by pituitary hormones and their downstream targets

Galanin-like peptide (GALP) mRNA is expressed in neurones of the hypothalamic arcuate nucleus and within pituicytes in the neurohypophysis. Several neuropeptides that are expressed in the arcuate nucleus participate in the neuroendocrine regulation of pituitary hormone secretion. Our objective was to determine the extent to which GALP might be a target for regulation by pituitary hormones or their downstream targets in the rat. The expression of GALP mRNA in the arcuate nucleus was reduced by hypophysectomy as determined by in situ hybridization. However, this did not appear to be attributable to the loss of either gonadal or adrenal steroids because castrated, ovariectomized and adrenalectomized rats had GALP mRNA expression that was indistinguishable from their respective controls. Next, we investigated the effects of growth hormone deficiency on GALP mRNA expression by studying dwarf rats and found that GALP gene expression was not different between dwarf rats and controls. We found that thyroidectomy led to a significant reduction in GALP mRNA expression compared to intact controls, and thyroidectomized rats implanted with thyroxine pellets had GALP mRNA expression that was similar to intact controls. Thus, the reduction of GALP mRNA expression seen in hypophysectomized animals may reflect, in part, a selective loss of thyroid hormone. We also found that the expression of GALP mRNA was increased in the neurohypophysis of lactating rats compared to nonlactating rats, whereas GALP mRNA expression in the arcuate nucleus was unaffected by lactation. This suggests that the induction of GALP gene expression in pituicytes is physiologically associated with activation of oxytocin and vasopressin secretion during lactation.

Analysis of the contribution of galanin receptors 1 and 2 to the central actions of galanin-like peptide

Galanin-like peptide (GALP) shares partial sequence identity with galanin and exhibits agonistic activity at two of the galanin receptor subtypes (GALR1 and GALR2) in vitro. The goal of these experiments was to determine whether galanin receptors mediate the effects of central GALP administration on food intake, body weight, and luteinizing hormone (LH) secretion in the mouse. We first evaluated the effects of intracerebroventricular injections of GALP or its vehicle alone in GALR1 knockout mice, GALR2 knockout mice, and their respective wild-type controls. GALP reduced food intake and body weight after 24 h to a similar degree in wild-type, GALR1 knockout, and GALR2 knockout mice. The wild-type, GALR1 knockout, and GALR2 knockout mice also exhibited significant increases in serum levels of LH following the GALP injections. To help delineate the biologically active moiety of the GALP molecule, we injected wild-type mice with shorter fragments of the full-length GALP peptide. Neither GALP((1-21)) (the fragment containing the galanin-homologous sequence) nor GALP((22-60)) (the C-terminal portion of the GALP molecule lacking sequence identity with galanin) had any discernable effect on food intake, body weight or circulating LH. These observations demonstrate that neither GALR1 nor GALR2 are essential for mediating the effects of GALP on feeding, body weight or LH secretion. Furthermore, the galanin-homologous region of the GALP molecule is not sufficient to mimic the effects of full-length GALP. Together, these findings argue against the hypothesis that GALP signals solely through galanin receptors in vivoand suggest the existence of a yet-to-be-identified GALP-specific receptor.

Leptin signaling in the hypothalamus: emphasis on energy homeostasis and leptin resistance

Leptin, the long-sought satiety factor of adipocytes origin, has emerged as one of the major signals that relay the status of fat stores to the hypothalamus and plays a significant role in energy homeostasis. Understanding the mechanisms of leptin signaling in the hypothalamus during normal and pathological conditions, such as obesity, has been the subject of intensive research during the last decade. It is now established that leptin action in the hypothalamus in regulation of food intake and body weight is mediated by a neural circuitry comprising of orexigenic and anorectic signals, including NPY, MCH, galanin, orexin, GALP, alpha-MSH, NT, and CRH. In addition to the conventional JAK2-STAT3 pathway, it has become evident that PI3K-PDE3B-cAMP pathway plays a critical role in leptin signaling in the hypothalamus. It is now established that central leptin resistance contributes to the development of diet-induced obesity and ageing associated obesity. Central leptin resistance also occurs due to hyperleptinimia produced by exogenous leptin infusion. A defective nutritional regulation of leptin receptor gene expression and reduced STAT3 signaling may be involved in the development of leptin resistance in DIO. However, leptin resistance in the hypothalamic neurons may occur despite an intact JAK2-STAT3 pathway of leptin signaling. Thus, in addition to defective JAK2-STAT3 pathway, defects in other leptin signaling pathways may be involved in leptin resistance. We hypothesize that defective regulation of PI3K-PDE3B-cAMP pathway may be one of the mechanisms behind the development of central leptin resistance seen in obesity.

Hypothalamic neuronal networks and feeding-related peptides involved in the regulation of feeding

The hypothalamus is a region of the brain that plays a critical role in feeding regulation. It has been revealed by various physiological experiments that the feeding-regulating center is confined to the ventromedial hypothalamus, lateral hypothalamus (LH) and arcuate nucleus (ARC). Many kinds of neurons in these areas of the hypothalamus express factors such as melanin-concentrating hormone (MCH), neuropeptide Y (NPY), proopiomelanocortin (POMC), orexin (OX) and ghrelin, which have been implicated in feeding regulation. In tissues of the periphery, two critical factors involved in feeding regulation, leptin and ghrelin, have been identified. Both hormone peptides are secreted mainly from adipose and stomach tissue, respectively, and are considered to function via their receptors mainly through several hypothalamic nuclei that play important roles in the regulation of appetite. The present review looks mainly at the functional significance of feeding-regulation factors, such as those described above, and the humoral and neuronal interactions among these compounds in the hypothalamus by drawing on published reports of morphological and physiological analyses. Immunohistochemical and in situ hybridization experiments indicate that both leptin and ghrelin receptors are distributed in the hypothalamus and that there are reciprocal interactions between MCH and OX neurons in the LH. Morphological and physiological studies on single living cells isolated from fresh rat hypothalamus or with receptor agonist and antagonist combined with immunohistochemisry clearly demonstrate that both leptin and OX reciprocally regulate NPY- and POMC-containing neurons in the ARC and that ghrelin may regulate feeding status independently through direct OX and NPY pathways. In this way, cross-talking systems in the hypothalamus play a role in determining feeding states.

Intracerebroventricular galanin-like peptide induces different brain activation compared with galanin

Like galanin, the 60-amino-acid peptide, galanin-like peptide (GALP), has orexigenic actions, demonstrated by an acute increase in feeding after central injection in rodents. However, in contrast to galanin, GALP causes a prolonged rise in core body temperature and a reduction in body weight over 24 h. In an attempt to identify potential explanations for the observed differences between GALP and galanin, this study examined which brain areas were activated by these peptides. Intracerebroventricular injection of GALP into conscious rats significantly stimulated feeding over 0-1 h, increased core body temperature, but reduced body weight gain over 24 h. Immunohistochemistry to detect c-fos demonstrated that intracerebroventricular injection of GALP or galanin activated several brain regions in common, including the dorsomedial nucleus of the hypothalamus, lateral hypothalamus, and nucleus tractus solitarius of the brainstem. However, GALP also induced c-fos expression in the periventricular hypothalamic region and supraoptic hypothalamic nucleus. Cell activation induced by GALP in the supraoptic hypothalamic nucleus and nucleus tractus solitarius was dependent on food intake but independent of food consumption in all other brain regions. Double immunohistochemistry indicated that small cells expressing c-fos in the periventricular hypothalamic region after GALP were astrocytes and not microglia.

Galanin-like peptide is target for regulation by orexin in the rat hypothalamus

Galanin-like peptide (GALP) is a newly discovered 60 amino acid peptide from the porcine hypothalamus. GALP has been shown to be expressed predominantly in the arcuate nucleus (ARC) of the rat hypothalamus, a region considered to be one of the most important feeding-regulating centers in the brain. GALP-containing neurons in the ARC express leptin receptors, but relationships between GALP and other feeding-regulating neurons have not yet been fully elucidated. Given that Orexin (OX)-containing neurons make synaptic inputs to the ARC, we thus examined the relationship between GALP and OX in the ARC by use of a dual immunostaining technique. OX-immunoreactive fibers appeared to be closely apposed to GALP-immunoreactive cell bodies and their processes. We also examined whether the OX receptor, OX(1)-R was expressed in the GALP-containing neurons. Immunoreactivity for both OX(1)-R and GALP was detectable in 9.6 % neurons (range 4.2-14.6%) in the ARC. These findings strongly suggest that GALP may participate in the regulation of feeding behavior under the influence of leptin and OX.

Differential patterns of Fos induction in the hypothalamus of the rat following central injections of galanin-like peptide and galanin

Galanin and its newly discovered relative galanin-like peptide (GALP) are neuropeptides that are implicated in the neuroendocrine regulation of body weight and reproduction. GALP has been shown to bind in vitro to galanin receptor subtypes 1 and 2, but whether it has its own specific receptor(s) is unknown. We reasoned that if GALP acts through a receptor that is distinct from galanin receptors, then GALP should activate central pathways that are different from those activated by galanin. The purpose of this study was to determine whether galanin and GALP produce different patterns of neuronal activation within the hypothalamus. Quantitative analysis of Fos immunoreactivity showed that galanin induced a significantly greater number of Fos-positive nuclei in the paraventricular nucleus compared with GALP (P < 0.001); however, compared with galanin, GALP induced significantly more Fos-positive cells in the horizontal limb of the diagonal band of Broca, caudal preoptic area, arcuate nucleus, and median eminence (P < 0.05). These observations suggest that GALP and galanin act through different receptor-mediated pathways to exert their effects on the regulation of body weight and reproduction and identify target cells for GALP's specific actions in the hypothalamus, including the preoptic area, paraventricular and arcuate nuclei, and the median eminence.

A role for galanin-like peptide in the integration of feeding, body weight regulation, and reproduction in the mouse

Galanin-like peptide (GALP) shares sequence homology with galanin and binds to galanin receptors in vitro. GALP neurons in the arcuate nucleus coexpress leptin receptors, and GALP mRNA expression is up-regulated by leptin. Based on these observations, we postulated that GALP plays a role in mediating leptin's inhibitory effects on food intake (FI) and body weight (BW), as well as its stimulatory effect on the reproductive axis. To test these hypotheses, we performed several studies in which mice received intracerebroventricular injections of either GALP or vehicle. Acute GALP treatment elicited a dose-dependent suppression of FI and BW. Long-term treatment with GALP caused only transient reductions in FI and BW, demonstrating that the mice became refractory to continued exposure to GALP. GALP inhibited FI as early as 1 h post injection. Central injection of GALP suppressed locomotor activity and elicited the formation of a conditioned taste aversion. In male mice, serum levels of LH and testosterone were increased by GALP administration. Although we cannot rule out possible nonspecific effects of GALP on FI, the present observations are consistent with the argument that GALP is a downstream effector of leptin's actions within the central nervous system.