Galanin-like peptide mRNA in neural lobe of rat pituitary. Increased expression after osmotic stimulation suggests a role for galanin-like peptide in neuron-glial interactions and/or neurosecretion

The osmoresponsiveness of oxytocin and vasopressin neurones: Mechanisms, allostasis and evolution

In the rat supraoptic nucleus, every oxytocin cell projects to the posterior pituitary, and is involved both in reflex milk ejection during lactation and in regulating uterine contractions during parturition. All are also osmosensitive, regulating natriuresis. All are also regulated by signals that control appetite, including the neural and hormonal signals that arise from the gut after food intake and from the sites of energy storage. All are also involved in sexual behaviour, anxiety-related behaviours and social behaviours. The challenge is to understand how a single population of neurones can coherently regulate such a diverse set of functions and adapt to changing physiological states. Their multiple functions arise from complex intrinsic properties that confer sensitivity to a wide range of internal and environmental signals. Many of these properties have a distant evolutionary origin in multifunctional, multisensory neurones of Urbilateria, the hypothesised common ancestor of vertebrates, insects and worms. Their properties allow different patterns of oxytocin release into the circulation from their axon terminals in the posterior pituitary into other brain areas from axonal projections, as well as independent release from their dendrites.

Involvement of Novel Feeding-Related Peptides in Neuroendocrine Response to Stress

Various stressors are known to cause eating disorders. However, it is not known in detail about the neural network and molecular mechanism that are involved in the stress-induced changes of feeding behavior in the central nervous system. Many novel feeding-regulated peptides such as orexins/hypocretins and ghrelin have been discovered since the discovery of leptin derived from adipocytes as a product of the ob gene. These novel peptides were identified as endogenous ligands of orphan G protein-coupled receptors. The accumulating evidence reveals that these peptides may be involved in stress responses via the central nervous system, as well as feeding behavior. The possible involvement of novel feeding-related peptides in neuroendocrine responses to stress is reviewed here.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Galanin and galanin-like peptide modulate vasopressin and oxytocin release in vitro: the role of galanin receptors

Galanin (Gal) and galanin-like peptide (GALP) may be involved in the mechanisms of the hypothalamo-neurohypophysial system. The aim of the present in vitro study was to compare the influence of Gal and GALP on vasopressin (AVP) and oxytocin (OT) release from isolated rat neurohypophysis (NH) or hypothalamo-neurohypophysial explants (Hth-NH). The effect of Gal/GALP on AVP/OT secretion was also studied in the presence of galantide, the non-selective galanin receptors antagonist. Gal at concentrations of 10(-10 )M and 10(-8 )M distinctly inhibited basal and K(+)-stimulated AVP release from the NH and Hth-NH explants, whereas Gal exerted a similar action on OT release only during basal incubation. Gal added to the incubation medium in the presence of galantide did not exert any action on the secretion of either neurohormone from NH and Hth-NH explants. GALP (10(-10 )M and 10(-9 )M) induced intensified basal AVP release from the NH and Hth-NH complex as well as the release of potassium-evoked AVP from the Hth-NH. The same effect of GALP has been observed in the presence of galantide. GALP added to basal incubation medium was the reason for stimulated OT release from the NH as well as from the Hth-NH explants. However, under potassium-stimulated conditions, OT release from the NH and Hth-NH complexes has been observed to be distinctly impaired. Galantide did not block this inhibitory effect of GALP on OT secretion. It may be concluded that: (i) Gal as well as GALP modulate AVP and OT release at every level of the hypothalamo-neurohypophysial system; (ii) Gal acts in the rat central nervous system as the inhibitory neuromodulator for AVP and OT release via its galanin receptors; (iii) the stimulatory effect of GALP on AVP and OT release is likely to be mediated via an unidentified specific GALP receptor(s).

Peptides and food intake

The mechanisms for controlling food intake involve mainly an interplay between gut, brain, and adipose tissue (AT), among the major organs. Parasympathetic, sympathetic, and other systems are required for communication between the brain satiety center, gut, and AT. These neuronal circuits include a variety of peptides and hormones, being ghrelin the only orexigenic molecule known, whereas the plethora of other factors are inhibitors of appetite, suggesting its physiological relevance in the regulation of food intake and energy homeostasis. Nutrients generated by food digestion have been proposed to activate G-protein-coupled receptors on the luminal side of enteroendocrine cells, e.g., the L-cells. This stimulates the release of gut hormones into the circulation such as glucagon-like peptide-1 (GLP-1), oxyntomodulin, pancreatic polypeptides, peptide tyrosine tyrosine, and cholecystokinin, which inhibit appetite. Ghrelin is a peptide secreted from the stomach and, in contrast to other gut hormones, plasma levels decrease after a meal and potently stimulate food intake. Other circulating factors such as insulin and leptin relay information regarding long-term energy stores. Both hormones circulate at proportional levels to body fat content, enter the CNS proportionally to their plasma levels, and reduce food intake. Circulating hormones can influence the activity of the arcuate nucleus (ARC) neurons of the hypothalamus, after passing across the median eminence. Circulating factors such as gut hormones may also influence the nucleus of the tractus solitarius (NTS) through the adjacent circumventricular organ. On the other hand, gastrointestinal vagal afferents converge in the NTS of the brainstem. Neural projections from the NTS, in turn, carry signals to the hypothalamus. The ARC acts as an integrative center, with two major subpopulations of neurons influencing appetite, one of them coexpressing neuropeptide Y and agouti-related protein (AgRP) that increases food intake, whereas the other subpopulation coexpresses pro-opiomelanocortin (POMC) and cocaine and amphetamine-regulated transcript that inhibits food intake. AgRP antagonizes the effects of the POMC product, α-melanocyte-stimulating hormone (α-MSH). Both populations project to areas important in the regulation of food intake, including the hypothalamic paraventricular nucleus, which also receives important inputs from other hypothalamic nuclei.

Neuronal circuits involving neuropeptide Y in hypothalamic arcuate nucleus-mediated feeding regulation

Neuropeptide Y (NPY) is a 36-amino-acid neuropeptide that was first discovered in porcine brain extracts and later in the porcine intestine. It is widely distributed in both the central and peripheral nervous systems and exerts a powerful orexigenic effect. NPY-producing neuronal cell bodies are abundantly localized in the medial arcuate nucleus of the hypothalamus, this being a brain center that integrates signals for energy homeostasis. Accumulated evidence shows that hypothalamic neuropeptides such as ghrelin, orexin, melanin-concentrating hormone (MCH), galanin-like peptide (GALP) and proopiomelanocortin (POMC) are involved in the regulation of feeding behavior and energy homeostasis via neuronal circuits in the hypothalamus. NPY also forms part of the feeding-regulating neuronal circuitry in conjunction with other feeding-regulating peptide-containing neurons within the hypothalamus. We summarize here current knowledge of the neuronal interactions between NPY and the different types of feeding-regulating peptide-containing neurons in the hypothalamus based on evidence at the immunohistochemicl level and with calcium imaging techniques.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

Galanin receptors and ligands

The neuropeptide galanin was first discovered 30 years ago. Today, the galanin family consists of galanin, galanin-like peptide (GALP), galanin-message associated peptide (GMAP), and alarin and this family has been shown to be involved in a wide variety of biological and pathological functions. The effect is mediated through three GPCR subtypes, GalR1-3. The limited number of specific ligands to the galanin receptor subtypes has hindered the understanding of the individual effects of each receptor subtype. This review aims to summarize the current data of the importance of the galanin receptor subtypes and receptor subtype specific agonists and antagonists and their involvement in different biological and pathological functions.

Galanin-like peptide (GALP) is a hypothalamic regulator of energy homeostasis and reproduction

Galanin-like peptide (GALP) was discovered in 1999 in the porcine hypothalamus and was found to be a 60 amino acid neuropeptide. GALP shares sequence homology to galanin (1-13) in position 9-21 and can bind to, as well as activate, the three galanin receptor subtypes (GalR1-3). GALP-expressing cells are limited, and are mainly found in the arcuate nucleus of the hypothalamus (ARC) and the posterior pituitary. GALP-positive neurons in the ARC project to several brain regions where they appear to make contact with multiple neuromodulators. These neuromodulators are involved in the regulation of energy homeostasis and reproduction, anatomical evidence that suggests a role for GALP in these physiological functions. In support of this idea, GALP gene expression is regulated by several factors that reflect metabolic state including the metabolic hormones leptin and insulin, thyroid hormones, and blood glucose. Considerable evidence now exists to support the hypothesis that GALP has a role in the regulation of energy homeostasis and reproduction; and, that GALP's role may be independent of the known galanin receptors. In this review, we (1) provide an overview of the distribution of GALP, and discuss the potential relationship between GALP and other neuromodulators of energy homeostasis and reproduction, (2) discuss the metabolic factors that regulate GALP expression, (3) review the evidence for the role of GALP in energy homeostasis and reproduction, (4) discuss the potential downstream mediators and mechanisms underlying GALP's effects, and (5) discuss the possibility that GALP may mediate its effects via an as yet unidentified GALP-specific receptor.

Galanin-like peptide: a key player in the homeostatic regulation of feeding and energy metabolism?

The hypothalamus has a critical role in the regulation of feeding behavior, energy metabolism and reproduction. Galanin-like peptide (GALP), a novel 60 amino-acid peptide with a nonamidated C-terminus, was first discovered in porcine hypothalamus. GALP is mainly produced in the hypothalamic arcuate nucleus and is involved in the regulation of feeding behavior and energy metabolism, with GALP-containing neurons forming networks with several feeding-regulating peptide-containing neurons. The effects of GALP on food intake and body weight are complex. In rats, the central effect of GALP is to first stimulate and then reduce food intake, whereas in mice, GALP has an anorectic function. Furthermore, GALP regulates plasma luteinizing hormone levels through activation of gonadotropin-releasing hormone-producing neurons, suggesting that it is also involved in the reproductive system. This review summarizes the research on these topics and discusses current evidence regarding the function of GALP, particularly in relation to feeding and energy metabolism. We also discuss the effects of GALP activity on food intake, body weight and locomotor activity after intranasal infusion, a clinically viable mode of delivery.

Altered response to metabolic challenges in mice with genetically targeted deletions of galanin-like peptide

Galanin-like peptide (GALP) is expressed in the arcuate nucleus and is implicated in the neuroendocrine regulation of metabolism and reproduction. To investigate the physiological significance of GALP, we generated and characterized a strain of mice with a genetically targeted deletion in the GALP gene [GALP knockout (KO) mice]. We report that GALP KO mice have a subtle, but notable, metabolic phenotype that becomes apparent during adaptation to changes in nutrition. GALP KO mice are indistinguishable from wild-type (WT) controls in virtually all aspects of growth, sexual development, body weight, food and water consumption, and motor behaviors, when they are allowed unlimited access to standard rodent chow. However, GALP KO mice have an altered response to changes in diet. 1) Male GALP KO mice consumed less food during refeeding after a fast than WT controls (P < 0.01). 2) GALP KO mice of both sexes gained less weight on a high-fat diet than WT controls (P < 0.01), despite both genotypes having consumed equal amounts of food. We conclude that although GALP signaling may not be essential for the maintenance of energy homeostasis under steady-state nutritional conditions, GALP may play a role in readjusting energy balance under changing nutritional circumstances.

Galanin-like peptide: a role in the homeostatic regulation of energy balance?

Galanin-like peptide (GALP) is a neuropeptide that has been proposed to play a role in the regulation of food intake behaviour and body weight. However, the actions of GALP on energy balance are complex. In rats, it appears to impel both appetite stimulating and suppressing effects, whereas in mice, the only effect is a reduction in food intake. Thus, it is currently unclear whether GALP is important in the homeostatic regulation of energy balance, or if it produces effects on appetite and body weight by non-specific actions. This review discusses current evidence of the role of GALP with respect to energy balance, and the mechanisms involved in its regulation. We describe recent evidence that suggests that GALP may elicit differential effects in different rodent species. Furthermore, we provide an insight into a potential novel role for GALP in inflammation, and discuss how this may relate to the non-homeostatic regulation of energy balance.

Postnatal development of galanin-like peptide mRNA expression in rat hypothalamus

We examined the developmental change of GALP mRNA in male and female rat hypothalamus during postnatal day 1 to 60, using in situ hybridization histochemistry. Neuropeptide Y (NPY) and proopiomelanocortin (POMC) mRNA in the hypothalamus were also examined because they are important in the regulation of food intake. GALP mRNA was first detected in the arcuate nucleus (ARC) on day 8. GALP mRNA was gradually increased between day 8 and 14 and markedly increased between day 14 and 40, which is the weaning and pubertal period in rats. After day 40, there were no significant differences in GALP mRNA. In contrast to GALP, NPY and POMC mRNAs were detected in the ARC from day 1 and lasted to day 60. There was no sexual dimorphism in GALP, NPY and POMC mRNAs during postnatal development. Next, we examined the effect of the milk deprivation for 24 h on GALP, NPY and POMC mRNA in pups. GALP mRNA did not change by milk deprivation on day 9 and 15, while milk deprivation had a significant effect on NPY and POMC mRNA on day 15. These results suggest that the development of GALP may be associated with developmental changes such as weaning, feeding and maturation of reproductive functions. The regulatory mechanism of GALP mRNA is different from that of the NPY and POMC genes during postnatal development.

Induction of the galanin-like peptide gene expression in the posterior pituitary gland after acute osmotic stimulus in rats

Galanin-like peptide (GALP) is a 60 amino-acid peptide, and the GALP mRNA is restricted to pituicytes in the posterior pituitary gland (PP) and neurons in the hypothalamic arcuate nucleus (Arc). We examined whether the GALP gene expression in the PP and Arc would be induced after intraperitoneal (i.p.) administration of hypertonic saline, that is, acute osmotic stimulus, in rats. The dose-response (2.8, 4.5, 6.0 and 9.0% NaCl) and time-course (6.0% NaCl, 1, 3, 6, 12 and 24h) effects of acute osmotic stimulus on GALP mRNA levels in the PP and Arc were examined in rats by using in situ hybridization histochemistry. Plasma osmolality and plasma sodium concentration increased significantly at 1h, and returned to control level at 6h after i.p. administration of hypertonic saline (6.0% NaCl). The GALP mRNA level in the PP increased significantly 3 and 6h after i.p. administration of hypertonic saline (6.0% NaCl), but the level in the Arc did not change. These results showed that acute osmotic stimulus-induced GALP gene expression in the pituicyte of the PP, but not in the neurons in the Arc, and the gene expression in the pituicyte might be regulated by plasma osmolality and/or plasma sodium concentration.

Neural interaction between galanin-like peptide (GALP)- and luteinizing hormone-releasing hormone (LHRH)-containing neurons

Galanin-like peptide (GALP), commonly known as an appetite-regulating peptide, has been shown to increase plasma luteinizing hormone (LH) through luteinizing hormone-releasing hormone (LHRH). This led us to investigate, using both light and electron microscopy, whether GALP-containing neurons in the rat brain make direct inputs to LHRH-containing neurons. As LHRH-containing neurons are very difficult to demonstrate immunohistochemically with LHRH antiserum without colchicine treatment, we used a transgenic rat in which LHRH tagged with enhanced green fluorescence protein facilitated the precise detection of LHRH-producing neuronal cell bodies and processes. This is the first study to report on synaptic inputs to LHRH-containing neurons at the ultrastructural level using this transgenic model. We also used immunohistochemistry to investigate the neuronal interaction between GALP- and LHRH-containing neurons. The experiments revealed that GALP-containing nerve terminals lie in close apposition with LHRH-containing cell bodies and processes in the medial preoptic area and the bed nucleus of the stria terminalis. At the ultrastructural level, the GALP-positive nerve terminals were found to make axo-somatic and axo-dendritic synaptic contacts with the EGFP-positive neurons in these areas. These results strongly suggest that GALP-containing neurons provide direct input to LHRH-containing neurons and that GALP plays a crucial role in the regulation of LH secretion via LHRH.

Galanin and its binding sites in the brain of eels subjected to different osmolar conditions

A galanin (GAL)-like peptidergic system was investigated in the brain of Anguilla anguilla subjected to hyperosmolar and hypoosmolar conditions, by using antisera raised against porcine 1-29 GAL. A group of immunoreactive perikaria was identified in the periventricular hypothalamus, in the ventral thalamus, in the pretectal areas and in the optic tectum. Immunoreactive perikaria were present in the nucleus lateralis of the torus semicircularis in seawater (SW) adapted eels, and were absent in the freshwater (FW) adapted eels. Galaninergic fibres were observed in many areas of the brain. The immunoreactive perikaria and fibres, when localised in the same areas, were more strongly labelled and numerous in the SW adapted eels in comparison to the FW adapted animals. GAL-specific binding sites, investigated by autoradiography using iodinated porcine GAL, occurred in several regions of the brain, in particular in the dorsal telencephalon, in the dorsal thalamus and in the torus semicircularis, where a high density of binding sites was observed in the SW adapted eels. Conversely, a higher density of binding sites was observed in the caudal substantia reticularis and in the corpus cerebelli in the FW adapted eels. We conclude that different distributions and intensity in immunoreactive elements occur in response to the two environmental conditions.

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.

Inhibitory effects of different galanin compounds and fragments on osmotically and histamine-induced enhanced vasopressin secretion in rats

The effects of rat, porcine and human galanin, and the human 1-16 and human 16-30 terminal galanin fragments on vasopressin secretion were studied in rat. The plasma vasopressin level was determined by radioimmunoassay (RIA). There were no changes in the basal vasopressin secretion after galanin administration. A significant increase in vasopressin concentration was detected following 2.5% NaCl or histamine administration. I.c.v. injected rat, porcine or human galanin or the 1-16 N-terminal galanin fragment prevented the plasma vasopressin level enhancement. Following the i.v. administration of rat galanin or the i.c.v. injected 16-30 C-terminal galanin fragment, the vasopressin concentration did not return to the normal level. Administration of the galanin antagonist galantid (M15) i.c.v. before the rat galanin i.c.v. injection prevented the inhibitory effect on the increased plasma vasopressin level following 2.5% NaCl solution or histamine administration. The results indicate that there is no significant difference in the inhibitory effect of rat, porcine or human galanin or the 1-16 galanin fragment on the enhanced plasma vasopressin secretion induced by hyperosmosis or histamine administration. Our findings suggest that galanin, as a peptide modulator, is physiologically involved in the regulation of vasopressin release following different forms of stimulation: an osmotic response or histamine administration.

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 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.

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.

Induction of galanin-like peptide gene expression in the rat posterior pituitary gland during endotoxin shock and adjuvant arthritis

Galanin-like peptide (GALP) was recently isolated from the porcine hypothalamus. The GALP mRNA is restricted in the hypothalamic arcuate nucleus and pituicytes in the posterior pituitary grand (PP) of the rat. The physiological function of the GALP is not clear in both areas. We focused on the regulation of the GALP gene expression to determine the role of GALP in the PP. We examined the effects of potent stressors to modulate a pituitary function, nociceptive stimuli and acute and chronic inflammatory stresses on the expression of the GALP gene in the PP using in situ hybridization histochemistry. Subcutaneous (s.c.) injection of 5% formalin in the bilateral hind paws caused a moderate increase of GALP gene expression in the PP. Intraperitoneal (i.p.) injection of lipopolysaccharide (LPS) also caused a marked increase of GALP gene expression in the PP. Effects of i.p. injection of LPS on the expression of the GALP gene in the PP were attenuated by pretreatment with i.p. injection of indomethacin. The levels of GALP mRNA in the PP were markedly increased by two peaks at 24 h and 15 days after s.c. injection of heat-inactivated M. butyricum that induced adjuvant arthritis. These results suggested that inflammatory stresses may be a potent stimulant to induce the expression of the GALP gene in the PP.

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.

Inhibitory effect of galanin on dopamine-induced enhanced vasopressin secretion in rat neurohypophyseal tissue cultures

The effect of galanin (GAL) on vasopressin (VP) secretion was studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP content of the supernatant was determined by radioimmunoassay (RIA) after a 1- or 2-h incubation. A significantly decreased content of VP was detected following the administration of 10(-6)-10(-9) M doses of GAL. Dopamine (DA) and the DA-active drugs apomorphine (APM) and Pro-Lys-Gly (PLG) (10(-6) M in each medium) increased the VP level of NH tissue cultures. This VP concentration elevation could be blocked by the administration of GAL together with DA, APM or PLG. The DA-blocking effect of GAL was prevented by previous treatment with the GAL receptor antagonist galantid (M15). The results indicate that VP release is directly influenced by the GAL-ergic system. The GAL-ergic control of VP secretion from NH tissue in rats can occur independently of the hypothalamus, at the level of the posterior pituitary.

Galanin-like peptide is co-localized with alpha-melanocyte stimulating hormone but not with neuropeptide Y in the rat brain

Galanin-like peptide (GALP), recently isolated from the hypothalamus, is a novel peptide of 60 amino acid residues. GALP is an endogenous ligand of the orphan receptor and shows a high affinity to its specific receptor GalR2. GALP mRNA was 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. According to recent reports of morphological and physiological experiments, GALP-containing neurons express leptin receptors and respond to leptin treatment by increasing mRNA expression. However, the relationships between GALP and other feeding-regulating neurons have not yet been proven. In this study, we examined the relationships between GALP- and neuropeptide Y (NPY)- or alpha-melanocyte stimulating hormone ( MSH)-containing neurons by using a dual immunostaining technique. We found that many NPY-immunoreactive fibers were in close apposition with GALP-immunoreactive cell bodies. Furthermore, immunoreactivity for GALP and alpha-MSH was detectable in the same neurons (3.3-11.8%) in the ARC. However, the co-existence of GALP and NPY was never demonstrated. These findings strongly suggest that GALP may participate in the regulation of feeding behavior in harmony with alpha-MSH.

Galanin/GALP and galanin receptors: role in central control of feeding, body weight/obesity and reproduction?

Scientific and commercial pharmacological interest in the role of galanin and galanin receptors in the regulation of food intake, energy balance, and obesity has waned recently, following initial enthusiasm during the 1980-1990s. It has been replaced by efforts to understand the role of newly discovered peptide systems such as the hypocretin/orexins, melanocortins and cocaine- and amphetamine-regulated transcript (CART) and their relationship to the important hormones, leptin and insulin. Thus, while numerous studies have revealed the ability of galanin to stimulate food intake via actions at sites within the hypothalamus, and shown reliable changes in hypothalamic galanin synthesis in response to food ingestion; findings including the lack of a 'body weight/obesity' phenotype in galanin transgenic mouse strains and a lack of agonists/antagonists for galanin receptor subtypes have probably served to reduce enthusiasm. However, as more is learnt about the general and galanin-related neurochemistry of brain pathways involved in feeding, metabolism and body weight control, the potential importance of galanin systems is again in focus. Studies of the newly discovered galanin family peptide, 'galanin-like peptide' (GALP), highlight the likely role of galanin peptides and receptors in the physiological coupling of body weight, adiposity and reproductive function. GALP is produced by a discrete population of neurons within the basomedial arcuate nucleus (and median eminence) that send projections to the anterior paraventricular nucleus and that make close contacts with leutinizing hormone-releasing hormone (LHRH) neurons in basal forebrain. Furthermore, GALP neurons express leptin receptors and respond to leptin treatment by increasing their expression of GALP mRNA. Centrally administered GALP activates LHRH-immunoreactive neurons and increases plasma LH levels. These findings suggest a direct stimulatory action of endogenous GALP on gonadotropin secretion via actions within the hypothalamus/basal forebrain, with leptin actions linking this system to body adipose levels.

Cloning and distribution of galanin-like peptide mRNA in the hypothalamus and pituitary of the macaque

Galanin-like peptide (GALP) is a newly discovered hypothalamic neuropeptide, which is regulated by leptin and implicated in the regulation of GnRH secretion in the rodent. We searched the human genome database and determined that the human GALP gene comprises six exons, as has been shown for human galanin. We used rapid amplification of cDNA ends to clone a full-length cDNA (802 bp) of the macaque homologue of GALP and found it to be highly conserved between human and macaque at both the nucleotide (93%) and peptide (94%) levels. Mature GALP is predicted to be 60 amino acids in the macaque as in other species, and the region of GALP (9-21) that shows homology to the N-terminal 13 amino acids of galanin is perfectly conserved. We mapped the distribution of GALP mRNA in the hypothalamus and pituitary of the macaque by in situ hybridization and observed that, as in rodent species, the expression of GALP mRNA is confined to the arcuate nucleus, median eminence, and neurohypophysis. Using double-label in situ hybridization, we found that nearly all (98%) GALP mRNA-expressing cells in the arcuate nucleus also express mRNA for the long form of the leptin receptor. These findings suggest that a leptin-GALP signaling pathway exists in a primate species.