Metabolic effects of galanin injections into the paraventricular nucleus of the hypothalamus

Unified Classification of Molecular, Network, and Endocrine Features of Hypothalamic Neurons

Peripheral endocrine output relies on either direct or feed-forward multi-order command from the hypothalamus. Efficient coding of endocrine responses is made possible by the many neuronal cell types that coexist in intercalated hypothalamic nuclei and communicate through extensive synaptic connectivity. Although general anatomical and neurochemical features of hypothalamic neurons were described during the past decades, they have yet to be reconciled with recently discovered molecular classifiers and neurogenetic function determination. By interrogating magnocellular as well as parvocellular dopamine, GABA, glutamate, and phenotypically mixed neurons, we integrate available information at the molecular, cellular, network, and endocrine output levels to propose a framework for the comprehensive classification of hypothalamic neurons. Simultaneously, we single out putative neuronal subclasses for which future research can fill in existing gaps of knowledge to rationalize cellular diversity through function-determinant molecular marks in the hypothalamus.

Effects of Fluoxetine Administration on Regional Galanin Expression in Obese Zucker Rat Hypothalamus

The aim of the present work was to study the potential involvement of hypothalamic galanin system in the anorectic mechanism of fluoxetine in obese Zucker rats. Male obese Zucker (fa/fa) rats were administered fluoxetine (10 mg/kg; i.p.) daily for two weeks. The control group was given 0.9% NaCl solution. Significant decreases in food intake, final body weight and total body fat were observed after fluoxetine treatment. Although fluoxetine-treated rats showed a decrease in urine elimination, this effect was not enough to compensate decreased water intake, leading to dehydration, as showed by decreased body water content. Chronic fluoxetine administration increased the numbers of galanin positively immunostained neural cells in medial and lateral preoptic areas, lateral hypothalamic area and paraventricular nucleus (rostral and magnocellular regions), without changes in dorsomedial, ventromedial, supraoptic, suprachiasmatic and arcuate nuclei. Taken into account that galanin stimulates appetite, these results could represent rather a compensatory response against reduced food intake than a direct anorectic mechanism. Changes in the magnocellular region of the hypothalamic paraventricular nucleus suggest a role for galanin neural circuits at this level in fluoxetine-induced hydro-osmotic impairment.

Leptin receptor neurons in the mouse hypothalamus are colocalized with the neuropeptide galanin and mediate anorexigenic leptin action

Leptin acts centrally via leptin receptor (LepRb)-expressing neurons to regulate food intake, energy expenditure, and other physiological functions. LepRb neurons are found throughout the brain, and several distinct populations contribute to energy homeostasis control. However, the function of most LepRb populations remains unknown, and their contribution to regulate energy homeostasis has not been studied. Galanin has been hypothesized to interact with the leptin signaling system, but literature investigating colocalization of LepRb and galanin has been inconsistent, which is likely due to technical difficulties to visualize both. We used reporter mice with green fluorescent protein expression from the galanin locus to recapitulate the colocalization of galanin and leptin-induced p-STAT3 as a marker for LepRb expression. Here, we report the existence of two populations of galanin-expressing LepRb neurons (Gal-LepRb neurons): in the hypothalamus overspanning the perifornical area and adjacent dorsomedial and lateral hypothalamus [collectively named extended perifornical area (exPFA)] and in the brainstem (nucleus of the solitary tract). Surprisingly, despite the known orexigenic galanin action, leptin induces galanin mRNA expression and stimulates LepRb neurons in the exPFA, thus conflicting with the expected anorexigenic leptin action. However, we confirmed that intra-exPFA leptin injections were indeed sufficient to mediate anorexic responses. Interestingly, LepRb and galanin-expressing neurons are distinct from orexin or melanin-concentrating hormone (MCH)-expressing neurons, but exPFA galanin neurons colocalized with the anorexigenic neuropeptides neurotensin and cocaine- and amphetamine-regulated transcript (CART). Based on galanin's known inhibitory function, we speculate that in exPFA Gal-LepRb neurons galanin acts inhibitory rather than orexigenic.

Possible involvement of hypothalamic nucleobindin-2 in hyperphagic feeding in Tsumura Suzuki obese diabetes mice

The aim of this study was to clarify the hypothalamic neuropeptides that are associated with hyperphagic feeding in Tsumura Suzuki Obese Diabetes (TSOD) mice, a model of type 2 diabetes with polygenic abnormalities. TSOD mice showed an increase in body weight and hyperleptinemia from 1 month of age and hyperphagic feeding, hyperglycemia, hyperlipidemia and hyperinsulinemia from 3 to 12 months of age compared with age-matched non-diabetic control Tsumura Suzuki Non Obesity (TSNO) mice. The mRNA level of nucleobindin-2 (NUCB2), the precursor of the anorexigenic neuropeptide nesfatin-1, was significantly decreased in the hypothalamus of TSOD mice compared with that in TSNO mice from 3 to 12 months of age. The protein level of NUCB2 was significantly decreased in the hypothalamus of TSOD mice compared with that in TSNO mice at 3 months of age. The mRNA levels of galanin, melanin-concentrating hormone, neuropeptide Y, and pro-opiomelanocortin were significantly changed in the hypothalamus in TSOD mice at several time points. Another model of type 2 diabetes, db/db mice, which is a mutant mouse that lacks a functional leptin receptor, showed hyperphagic feeding but no change in hypothalamic NUCB2 mRNA compared with non-diabetic control db/+ mice. The results suggest that the disrupted control of hypothalamic NUCB2-mediated signaling may contribute to hyperphagic feeding in TSOD mice. In addition, the mechanism for the development of hyperphagic feeding in TSOD mice is different than that in db/db mice.

Chronic increase of circulating galanin levels induces obesity and marked alterations in lipid metabolism similar to metabolic syndrome

OBJECTIVE

Galanin (GAL) has a role in the regulation of food intake by way of acting on the central nervous system in rodents. High serum GAL levels have been observed in obese human subjects, suggesting that peripheral GAL has a role in the regulation of energy balance and that elevated circulating GAL levels contribute to the development of obesity and obesity-associated metabolic impairments. Currently, it is not known how chronically increased levels of circulating GAL affect energy balance. The purpose of this study is to clarify the importance of chronically increased levels of circulating GAL on energy balance in a transgenic mouse model.

RESEARCH DESIGN AND METHODS

Male wild-type and homozygous galanin transgenic (GAL-Tg) mice were used to study the peripheral effects of a 10-fold increase in circulating GAL on food intake, body weight, lipid metabolism, hepatic steatosis, glucose homeostasis and energy expenditure.

RESULTS

In the absence of an orexigenic effect, GAL-Tg mice had increased body weight, visceral adiposity, total serum cholesterol, total serum triglycerides and hyperinsulinemia, as well as impaired glucose tolerance. Compared with wild-type mice, the obese phenotype observed in the GAL-Tg mice was attributed to decreased oxygen consumption and carbon dioxide production, and this effect was independent of any changes in food intake or horizontal activity. In this obese model, GAL contributed to the development of fatty liver disease, which was associated with impaired glucose tolerance, as well as a reduction in heat production and metabolic rate.

CONCLUSIONS

Chronically elevated GAL may regulate body weight, metabolic rate, and lipid and carbohydrate metabolism through a mechanism that is independent of feeding regulation. The obese phenotype in the GAL-Tg mice is related to the reduced energy expenditure and insulin resistance. These findings support the hypothesis that increased circulating GAL levels contribute to the development of metabolic syndrome.

Distinct phenotypes of obesity-prone AKR/J, DBA2J and C57BL/6J mice compared to control strains

OBJECTIVE

To characterize and compare three obesity-prone inbred strains, AKR/J, DBA/2J and C57BL/6J, to three control strains, C3H/HeJ, BALB/cByJ and C57L/J, selected based on their normal eating patterns and moderate weight gain on high-calorie diets.

METHODS AND PROCEDURES

These six strains were examined at 5 weeks of age while still of normal body weight, and they were maintained for 1 day or 3 weeks on different feeding paradigms with macronutrient diets. Measurements were taken of macronutrient intake, body weight and body fat accrual, circulating hormones and metabolites, and the hypothalamic peptide, galanin.

RESULTS

The three control strains each selected a balanced diet with 50% carbohydrate and 15-25% fat when given a choice of macronutrients, and they had similar, normal range of scores for the measures of body weight, adiposity, the hormones, insulin and leptin, and the metabolites, glucose and triglycerides. When compared to this control baseline, the obesity-prone strains with similar total caloric intake to controls selected a diet with significantly more fat (30-40%) and less carbohydrate (<40%). They also had greater adiposity, with the largest differences detected for the AKR/J and DBA/2J strains. These two obesity-prone strains compared to control strains had elevated levels of insulin and leptin. They also had higher triglyceride levels and increased expression and levels of galanin in the hypothalamic paraventricular nucleus. A very different pattern was detected in the obesity-prone C57BL/6J strain, which exhibited a stronger preference for protein as well as fat, normal levels of insulin, leptin and triglycerides, hyperglycemia relative to all other strains, and a small increase in galanin.

CONCLUSION

These comparisons to control strains revealed a distinct phenotype in the two obesity-prone strains, AKR/J and DBA/2J, which is very similar to that described in obesity-prone, outbred rats. They also identified a clearly different phenotype in the obesity-prone C57BL/6J strain.

Model for predicting and phenotyping at normal weight the long-term propensity for obesity in Sprague–Dawley rats

Tests were conducted to determine whether weight gain or nutrient intake measures during the first week of exposure to a macronutrient diet can accurately predict an animal's long-term propensity towards obesity. In multiple groups of normal-weight Sprague-Dawley rats (n=35-70/group), daily weight gain during the first 5 days on a high-fat diet (45-60% fat) was found to be strongly, positively correlated (r=+0.71 to r=+0.82) with accumulated body fat in 4 dissected depots after 4-6 weeks on the diet. This measure consistently identified obesity-prone (OP) rats which, relative to the obesity-resistant (OR) rats, were only slightly heavier (+15 g, 4%) and hyperphagic (+9 kcal, 8%) after 5 days but markedly heavier (+70g) with up to 2-fold greater fat mass after several weeks on the diet. Other dietary conditions and measures revealed weaker relationships to ultimate body fat accrual. The OP rats identified by their 5-day weight-gain score exhibited at this early stage clear disturbances characteristic of markedly obese rats. These included elevated leptin, insulin, triglycerides and glucose, along with increased lipoprotein lipase activity (LPL) in adipose tissue and galanin expression in the paraventricular nucleus. Most notable were significant reductions in muscle of LPL activity and ratio of beta-hydroxyacyl-CoA dehydrogenase to citrate synthase activity, indicating a decline in lipid transport and capacity of muscle to metabolize lipids. By occurring early with initial weight gain, these hypothalamic and metabolic disturbances in OP rats, favoring fat storage in adipose tissue over fat oxidation in muscle, may have causal relationships to long-term accumulation of body fat.

Leptin secretion after a high-fat meal in normal-weight rats: strong predictor of long-term body fat accrual on a high-fat diet

The objective of this study was to investigate meal-related endocrine changes that permit one to identify Sprague-Dawley rats at normal weight that are prone (OP) vs. resistant (OR) to obesity. In blood collected via chronic cardiac catheters, a 2-h high-fat meal (HFM, 50% fat, 40 kcal) at dark onset caused a significant increase in leptin, insulin, and triglycerides compared with premeal levels. Similar to patterns in already obese compared with lean rats on a high-fat diet, these meal-induced endocrine changes in normal-weight rats on lab chow were almost twofold larger in OP rats that, compared with OR rats, subsequently accumulated 100% more fat mass on a chronic high-fat diet. These exaggerated endocrine changes were similarly observed in blood collected using a simpler tail vein puncture procedure. In three separate experiments, the HFM-induced rise in leptin was found to be the strongest, positive correlate (r = +0.58, +0.62 and +0.64) of long-term body fat accrual. The lowest (2-5 ng/ml) vs. highest (6-9 ng/ml) scores for this post-HFM leptin measurement identified distinct OR and OP subgroups, respectively, when they were similar in body weight (340-350 g), premeal leptin (2.6-3.4 ng/ml), and meal size (40 kcal). Subsequent tests in these normal-weight OP rats revealed a distinct characteristic compared with OR rats, namely, exaggerated HFM-induced rise in expression of the orexigenic peptide galanin in the paraventricular nucleus. Thus, with this HFM-induced leptin measurement, OP rats can be identified while still at normal weight and then investigated for mechanisms that contribute to their excessive body fat accrual on a high-fat diet.

PVN galanin increases fat storage and promotes obesity by causing muscle to utilize carbohydrate more than fat

To understand the function of the feeding-stimulatory peptide, galanin (GAL), in eating and body weight regulation, the present experiments tested the effects of both acute and chronic injections of this peptide into the paraventricular nucleus (PVN) of rats. With food absent during the test, acute injection of GAL (300 pmol/0.3 microl) significantly increased phosphofructokinase activity in muscle, suggesting enhanced capacity to metabolize carbohydrate, and reduced circulating glucose levels. It also decreased beta-hydroxyacyl-CoA dehydrogenase activity in muscle, indicating reduced fat oxidation, while increasing circulating non-esterified fatty acids (NEFA) and lipoprotein lipase activity in adipose tissue (aLPL). Chronic PVN injections of GAL (300 pmol/0.3 microl/injection) versus saline over 7-10 days significantly stimulated daily caloric intake and increased the weight of four dissected fat depots by 30-40%. These effects, accompanied by elevated levels of leptin, triglycerides, NEFA and aLPL activity, were evident only in rats on a diet with at least 35% fat. Thus, by favoring carbohydrate over fat metabolism in muscle and reversing hyperglycemia, PVN GAL may have a function in counteracting the metabolic disturbances induced by a high-fat diet. As a consequence of these actions, GAL can promote the partitioning of lipids away from oxidation in muscle towards storage in adipose tissue.

Different forms of obesity as a function of diet composition

OBJECTIVE

To characterize the phenotype of obesity on a high-carbohydrate diet (HCD) as compared to a high-fat diet (HFD) or moderate-fat diet (MFD).

METHODS AND PROCEDURES

In four experiments, adult Sprague-Dawley rats (275-300 g) were maintained for several weeks on a: (1) HFD with 50% fat; (2) balanced MFD with 25% fat; or (3) HCD with 10% fat/65% carbohydrate. Then, based on the amount of body fat accumulated in four dissected fat pads, the animals were subgrouped as lean (lowest tertile) or obese (highest tertile) and characterized with multiple measures.

RESULTS

The obese rats of these diet groups, with 70-80% greater body fat than the lean animals, exhibited elevated levels of leptin and insulin and increased activity of lipoprotein lipase in adipose tissue (aLPL), with no change in muscle LPL. Characteristics common to the obese rats on the HFD or MFD, but not seen on the HCD, were hyperphagia, elevated circulating levels of triglycerides (TG), nonesterified fatty acids (NEFA) and glucose, and a significant increase in beta-hydroxyacyl-CoA dehydrogenase (HADH) activity in muscle, reflecting its greater capacity to metabolize fat. This was accompanied by a significant increase in expression of the peptide, galanin (GAL), in the paraventricular nucleus (PVN), as measured by in situ hybridization and real-time quantitative PCR, and also in GAL peptide immunoreactivity. These measures of GAL were consistently, positively correlated with circulating TG levels and also with HADH activity in muscle. In contrast to these fat-associated changes, rats that became obese on an HCD maintained normal caloric intake and levels of TG, NEFA, and glucose. They also showed no change in PVN GAL mRNA or peptide. Instead, they exhibited a significant reduction in HADH activity compared to the lean animals, along with increased activity of phosphofructokinase in muscle, a key enzyme in glycolysis.

CONCLUSION

Specific characteristics of obesity, including expression of hypothalamic peptides, are dependent upon diet composition. Whereas obesity on an HFD is associated with hyperphagia and elevated lipids, fat metabolism in muscle, and fat-stimulated peptides such as GAL, obesity on an HCD with a similar increase in body fat shows none of these characteristics and instead exhibits a metabolic pattern in muscle that favors carbohydrate over fat oxidation. These results suggest the existence of multiple forms of obesity with different underlying mechanisms that are diet dependent.

Regulation and effects of hypothalamic galanin: relation to dietary fat, alcohol ingestion, circulating lipids and energy homeostasis

Galanin (GAL) is known to stimulate feeding behavior. This peptide has different properties and functions from other feeding stimulants, e.g., neuropeptide Y and agouti-related protein. Hypothalamic GAL is relatively unresponsive to food deprivation and to changes in corticosterone, glucose utilization, dietary carbohydrate and leptin. This indicates that this peptide is not essential under conditions when food is scarce or low-energy, high-carbohydrate diets are being consumed. In contrast, recent evidence suggests that GAL in the paraventricular nucleus (PVN) functions in close relation to dietary fat and alcohol. In particular, it mediates functions that allow animals to adapt to conditions of positive energy balance involving excess consumption of these nutrients. This peptide in the PVN is stimulated by a high-fat diet and also by alcohol. It is stimulated by an increase in circulating lipids caused by a fat-rich meal or alcohol consumption, and it rises during the middle of the active feeding cycle, when fat consumption and triglycerides naturally rise. When centrally injected, GAL in the PVN increases the consumption of food and alcohol. Moreover, it produces a significantly stronger feeding response in rats maintained on a fat-rich diet, which also promotes alcohol intake. This evidence supports the existence of non-homeostatic, positive feedback circuits between GAL and both dietary fat and alcohol. These circuits are believed to contribute to the large meal size, over-consumption of alcohol, and obesity which are generally associated with fat-rich foods.

Galanin- but not galanin-like peptide-containing axon terminals innervate hypophysiotropic TRH-synthesizing neurons in the hypothalamic paraventricular nucleus

Galanin and galanin-like peptide (GALP) are both orexigenic peptides involved in the regulation of food intake and energy metabolism. To determine whether these peptides may directly influence the hypophysiotropic thyrotropin-releasing hormone (TRH)-synthesizing neurons, double-labeling immunocytochemistry was performed at light and electron microscopic levels using antisera against proTRH, galanin and GALP. Galanin-IR axons densely innervated all of the major parvocellular subdivisions of the PVN where proTRH neurons were identified. The periventricular and anterior parvocellular subdivisions exhibited a prominent network of galaninergic nerve fibers, while the density of fibers was less intense in the medial parvocellular subdivision. Galanin-immunoreactive (IR) axon varicosities were juxtaposed to the majority of TRH-synthesizing neurons in the anterior, medial and periventricular subdivisions of the PVN. Ultrastucturally, galanin-IR nerve terminals established symmetric type synapses with the perikarya of proTRH-IR neurons, suggesting an inhibitory nature of these contacts. In contrast, GALP immunoreactive fibers and nerve terminals concentrated primarily in the anterior parvocellular subdivision of the PVN and were found in association with only few proTRH-IR neurons in the periventricular and medial parvocellular subdivisions. In conclusion, the dense innervation of TRH neurons in all subdivisions of the PVN by galanin-IR axons indicates that galanin may be involved in the central regulation of the hypothalamic-pituitary-thyroid axis. In contrast, the relative paucity of GALP-containing axons in juxtapsoition to TRH neurons in the medial and periventricular parvocellular subdivisions of the PVN, the origin of hypophysiotropic TRH neurons, makes it unlikely that GALP similarly exerts direct regulatory effects on hypophysiotropic TRH neurons.

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.

Effects of galanin-like peptide on food intake and the hypothalamo-pituitary-thyroid axis

Galanin-like peptide (GALP) is a novel hypothalamic peptide synthesised in neurons in the arcuate nucleus which project to the paraventricular nucleus (PVN). GALP has recently been identified as an orexigenic peptide. In this study we aimed to further characterise the hypothalamic action of this peptide in energy homeostasis. Firstly, we investigated the orexigenic effect of GALP in the PVN and compared its effects with galanin and galanin 2-29. Secondly, we examined the effect of PVN administration of GALP and galanin on circulating thyroid-stimulating hormone (TSH). PVN administration of GALP significantly increased the food intake of satiated rats 1 h after administration at doses of 0.3, 1 and 3 nmol. In comparison with paraventricular administration of galanin, GALP was a more potent orexigen, whereas galanin 2-29, the relatively selective GAL R2 agonist, had no effect on food intake. Both GALP and galanin administration (1 nmol) into the PVN significantly decreased the level of circulating TSH. To investigate the mechanism of these effects, we examined the effect of GALP and galanin application on neuropeptide release from hypothalamic explants in vitro. GALP peptide (100 nM) stimulated the release of the orexigenic peptide neuropeptide Y from hypothalamic explants and decreased the release of the anorectic peptide cocaine-and-amphetamine-regulated transcript, whereas galanin (100 nM) peptide had no significant effect on the release of either peptide. Both GALP (100 nM) and galanin (100 nM) inhibited the release thyrotrophin-releasing hormone. These data suggest that in the PVN, GALP may play a role in energy homeostasis by stimulating food intake and suppressing TSH release.

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

Galanin-like peptide (GALP) is a recently identified neuropeptide that shares sequence homology with the orexigenic neuropeptide, galanin. In contrast to galanin, GALP is reported to bind preferentially to the galanin receptor 2 subtype (GalR2) compared to GalR1. The aim of this study was to determine the effect of GALP on feeding, body weight and core body temperature after central administration in rats compared to the effects of galanin. Intracerebroventricular (i.c.v.) injection of GALP (1 micro g-10 micro g) significantly stimulated feeding at 1 h in both satiated and fasted Sprague-Dawley rats. However, 24 h after GALP injection, body weight gain was significantly reduced and food intake was also usually decreased. In addition, i.c.v. GALP caused a dose-related increase in core body temperature, which lasted until 6-8 h after injection, and was reduced by peripheral administration of the cyclooxygenase inhibitor, flurbiprofen (1 mg/kg). Similar to GALP, i.c.v. injection of galanin (5 micro g) significantly increased feeding at 1 h in satiated rats. However, there was no difference in food intake and body weight at 24 h, and galanin only caused a transient rise in body temperature. Thus, similar to galanin, GALP has an acute orexigenic effect on feeding. However, GALP also has an anorectic action, which is apparent at a later time. Therefore, GALP has complex opposing actions on energy homeostasis.

Effects of peptides on animal and human behavior: a review of studies published in the first twenty years of the journal Peptides

This review catalogs effects of peptides on various aspects of animal and human behavior as published in the journal Peptides in its first twenty years. Topics covered include: activity levels, addiction behavior, ingestive behaviors, learning and memory-based behaviors, nociceptive behaviors, social and sexual behavior, and stereotyped and other behaviors. There are separate tables for these behaviors and a short introduction for each section.

Neuropeptide expression in rat paraventricular hypothalamic neurons that project to the spinal cord

The paraventricular hypothalamic nucleus (PVH) exerts many of its regulatory functions through projections to spinal cord neurons that control autonomic and sensory functions. By using in situ hybridization histochemistry in combination with retrograde tract tracing, we analyzed the peptide expression among neurons in the rat PVH that send axons to the spinal cord. Projection neurons were labeled by immunohistochemical detection of retrogradely transported cholera toxin subunit B, and radiolabeled long riboprobes were used to identify neurons containing dynorphin, enkephalin, or oxytocin mRNA. Of the spinally projecting neurons in the PVH, approximately 40% expressed dynorphin mRNA, 40% expressed oxytocin mRNA, and 20% expressed enkephalin mRNA. Taken together with our previous findings on the distribution of vasopressin-expressing neurons in the PVH (Hallbeck and Blomqvist [1999] J. Comp. Neurol. 411:201-211), the results demonstrated that the different PVH subdivisions display distinct peptide expression patterns among the spinal cord-projecting neurons. Thus, the lateral parvocellular subdivision contained large numbers of spinal cord-projecting neurons that express any of the four investigated peptides, whereas the ventral part of the medial parvocellular subdivision displayed a strong preponderance for dynorphin- and vasopressin-expressing cells. The dorsal parvocellular subdivision almost exclusively contained dynorphin- and oxytocin-expressing spinal cord-projecting neurons. This parcellation of the peptide-expressing neurons suggested a functional diversity among the spinal cord-projecting subdivisions of the PVH that provide an anatomic basis for its various and distinct influences on autonomic and sensory processing at the spinal level.

Differential functions of hypothalamic galanin cell grows in the regulation of eating and body weight

Evidence suggests that hypothalamic galanin (GAL) has a variety of functions related to energy and nutrient balance, reproduction, water balance, and neuroendocrine regulation. The focus of this chapter is the role of GAL in eating and body weight regulation. Findings described herein demonstrate that GAL, in a cell group of the anterior region of the paraventricular nucleus (aPVN) that projects to the median eminence, has a role in the control of fat intake, fat metabolism, and body fat. This function of aPVN GAL neurons is carried out in close relation to circulating insulin and glucose. Galanin-expressing perikarya in the medial preoptic area (MPOA) have a similar function, although GAL here operates in association with the female steroids estrogen and progesterone. These GAL cell groups of the aPVN and MPOA contrast with those in the arcuate nucleus as well as the magnocellular vasopressin-containing neurons of the PVN and supraoptic nucleus, which show no relation to fat balance. This evidence reveals differential functions for the distinct GAL neuronal cell groups of the hypothalamus.

Hypothalamic galanin: control by signals of fat metabolism

The peptide, galanin (GAL), is known to stimulate eating behavior, reduce energy expenditure and affect the release of metabolic hormones. Further, the activity of this peptide in the hypothalamus is modulated, in turn, by these hormones as well as by the ingestion of nutrients. The focus of this investigation is on signals related to nutrient metabolism that may also affect GAL production and, through these neurochemical events, control the ingestion of specific nutrients. Three experiments were performed in normal-weight male, Sprague-Dawley rats. In Experiment 1, the impact of food deprivation (24 and 48 h) was examined. Experiment 2 tested the effects of the compound, 2-deoxy-D-glucose (2-DG, 200 and 400 mg/kg), which blocks glucose utilization, whereas Experiment 3 studied mercaptoacetate (MA, 200 and 600 micromol/kg), which blocks fatty acid oxidation. Eating behavior was examined in some rats, whereas hypothalamic GAL activity was measured in others using radioimmunoassay, immunohistochemistry and in situ hybridization. Both food deprivation and MA (600 micromol/kg), but not 2-DG, affected GAL in the hypothalamus, in one specific area. This is the anterior parvocellular region of the paraventricular nucleus (aPVN), which has a dense concentration of GAL-containing neurons and terminals. GAL gene expression and peptide immunoreactivity in this area is enhanced by food deprivation; in contrast, it is reduced by injection of MA. Other hypothalamic sites with dense concentrations of GAL-containing neurons or fibers are unaffected by food deprivation or MA, and the antimetabolite 2-DG has no impact on GAL in any area. Behavioral measurements indicate that these shifts in GAL activity are accompanied by specific changes in eating behavior. Food deprivation which enhances aPVN GAL produces a marked increase in fat ingestion, whereas MA which reduces aPVN GAL causes a specific reduction in fat ingestion along with a stimulation of protein intake. In contrast, 2-DG preferentially enhances ingestion of carbohydrate. These findings suggest a possible relationship between GAL activity in the aPVN and the metabolic and behavioral processes of fat metabolism and ingestion.

An immunohistochemical study of temperature-related changes in galanin and nitric oxide synthase immunoreactivity in the hypothalamus of the toad

Galanin (GAL) and nitric oxide synthase (NOS) have been implicated in the control of thermogenesis in mammals. An experimental protocol was designed to determine whether or not the expression of these molecules in the hypothalamus of the toad could be related to environmental temperature changes. Exposure of the animals to low temperature increased the number and intensity of NOS-positive neurons in the magnocellular hypothalamic region, in contrast to a weak immunoreactivity observed in control animals kept in a natural environment at a spring-summer temperature (23-27 degrees C). Also a significantly higher number of GAL-immunoreactive (-IR) cells was observed in the preoptic area as compared to that observed in controls, while no difference in the intensity of GAL immunostaining intensity was detected. These results show a temperature-related expression of GAL and NOS in the hypothalamus and preoptic area of the toad. The results suggest a possible role of GAL and NOS in the regulation of hibernation in these animals.

Obesity on a high-fat diet: role of hypothalamic galanin in neurons of the anterior paraventricular nucleus projecting to the median eminence

Previous studies have suggested that the peptide galanin (GAL) in the hypothalamus is related to the preference of an animal for dietary fat. The present report investigates this relationship further to identify the specific GAL-synthesizing cell groups involved and to characterize their association to circulating glucose or hormones and their possible contribution to body fat deposition. Male albino Sprague Dawley rats were tested in different feeding paradigms with diets containing the macronutrients, fat, carbohydrate, or protein. These studies, using multiple techniques, identify a cell group in the hypothalamus that expresses GAL and that shows a shift in peptide activity in close relation to dietary fat, circulating glucose, and body fat. In all paradigms, a rise in fat intake, from 10 to 30%, is associated with reduced levels of insulin and corticosterone and normal glucose levels, whereas a further increase in fat ingestion (>30%) leads to hyperglycemia along with greater adiposity. In the hypothalamus, GAL gene expression, peptide production, and peptide release rise significantly (by 40%) in association with fat ingestion, showing no relation to either carbohydrate or protein ingestion. This change is highly site specific, evident predominantly in GAL-synthesizing neurons in the anterior parvocellular region of the paraventricular nucleus (aPVN) and in GAL-containing terminals in the external zone of the median eminence (ME). Positive correlations detected between mRNA abundance in the aPVN and GAL peptide in the ME support the existence of an aPVN-ME projection system related to fat intake and fat deposition. When activated by dietary fat, the contribution of this projection to body fat is suggested by consistent positive correlations between aPVN-ME GAL and either dietary fat, circulating glucose, or body fat and by significantly higher GAL levels (+30%) in obesity-prone compared with obesity-resistant rats. This evidence supports a role for this hypothalamic GAL projection system in the development of obesity produced by the overconsumption of fat.

Hypothalamic neuropeptide Y and galanin in overweight rats fed a cafeteria diet

We evaluated neuropeptide Y (NPY) and galanin (GAL) immunoreactivity (IR) and mRNA in the paraventricular and arcuate nucleus, respectively, in rats that became overweight (Ov) or not (NOv) when fed a cafeteria diet. After 2 months of diet, NOv rats showed a significant increase in NPY IR, whereas Ov rats showed a significant increase in GAL mRNA levels. None of these changes was present in rats overfed for 6.5 months. These differential changes in hypothalamic GAL and NPY transmissions may contribute to the different susceptibility of the two rat subpopulations to the weight-promoting effects of the hypercaloric diet.

Brain peptides and obesity: pharmacologic treatment

Obesity results from an imbalance between nutrient ingestion and metabolism, with more calories being ingested than utilized. The brain plays an important role in coordinating these complex behavioral and physiological functions, operating through multiple neurochemical systems with distinct properties. This review focuses on two hypothalamic peptide systems, neuropeptide Y (NPY) and galanin (GAL), that illustrate how the brain operates through different mechanisms to control the body's nutrient stores, in different states or conditions. These peptides have different behavioral and physiological effects and are, themselves, differentially responsive to feedback signals from circulating steroids, peptides, and nutrients. They can be distinguished by their relation to natural feeding patterns and endogenous hormones and by their specificity of action in relation to natural biological rhythms. The neuroanatomical substrates involved in these actions of NPY and GAL are also distinct. The neurocircuit mediating NPY's actions originates in the arcuate nucleus and terminates in the medial portion of the paraventricular nucleus; the GAL-containing neurons, in contrast, are concentrated in the lateral portion of the paraventricular nucleus, in addition to the medial preoptic area, which contribute to local GAL innervation as well as projections to the median eminence. Regarding their distinct functions, the evidence suggests that the NPY system is more closely related to patterns of carbohydrate ingestion and carbohydrate utilization, channeling nutrients towards the synthesis of fat. It is most strongly activated at the start of the active feeding cycle or after weaning, in close association with the adrenal steroid, corticosterone. The GAL system, in contrast, is more closely associated with patterns of fat consumption and signals related to fat oxidation. This peptide system is most active during the middle of the feeding cycle or immediately after puberty, in close association with the gonadal steroids. The gene expression and synthesis of these peptides in their respective neuronal cell groups is inhibited by circulating insulin and altered by dietary nutrients. Disturbances in sensitivity to insulin and steroid feedback regulation in the brain are believed to be involved in producing abnormal patterns of peptide function that result in overeating and body weight gain.

Specificity of hypothalamic peptides in the control of behavioral and physiological processes

This review summarizes two model systems for understanding how brain neurochemicals, in conjunction with peripheral endocrine and metabolic processes, may be active in controlling very different functions in relation to energy and nutrient balance. As proposed, these systems are unquestionably oversimplified; however, they generate testable hypotheses for future investigations that will help to advance and revise these working models, as well as those of other peptide systems in the brain. Under normal conditions, these peptide systems are behaviorally and endocrinologically specific, and they are activated at very different periods of the daily cycle and at different stages of development. However, under pathologic conditions, their specificity and rhythmicity may be greatly disturbed. This occurs in states involving hypercortisolemia along with hyperinsulinemia or insulin deficiency, when these peptide systems become chronically activated. To determine whether this increased activity actually contributes to conditions of hyperphagia and obesity, and, thus, whether a reversal of this neurochemical activity may help in the treatment of these conditions, critical studies with various pharmacological manipulations are required. Of equal importance are investigations examining the development of these pathologic conditions, from birth to maturity, and their associated disturbances in neurochemical and endocrine processes. A thorough understanding of gene expression in localized brain areas and the contribution of various transcription factors to this process should allow the identification and development of methods that are useful in the treatment, as well as prevention, of disturbed patterns of nutrient intake, fat deposition, and body weight gain.

Galanin-containing neurons in the paraventricular nucleus: a neurochemical marker for fat ingestion and body weight gain

The physiological function of the peptide galanin (Gal) remains to be established. It is known to exist in high concentrations within the hypothalamus and to modulate the secretion of specific hormones, as well as to potentiate food consumption. Our study provides evidence for an essential function of neuronal Gal, within a specific hypothalamic area, in stimulating the behavioral process of fat ingestion and body weight gain. Through analyses of peptide levels via RIA and of gene expression via in situ hybridization, a close positive association is established between Gal in the paraventricular nucleus (PVN), particularly its midlateral region, and fat ingestion. No such relationship is detected for Gal in other brain areas or between PVN Gal and ingestion of carbohydrate or protein, supporting the behavioral and anatomical specificity of this relationship. Through PVN injection studies with antisense oligonucleotides to Gal mRNA, a dramatic decline in fat ingestion and body weight suggests that endogenous Gal contributes to the natural appetite for fat. Thus, Gal in the PVN is identified as a neurochemical marker for fat ingestion and, consequently, body weight gain.

The possible involvement of galanin in the modulation of the function of rat pituitary-adrenocortical axis under basal and stressful conditions

The effect of a s.c. bolus injection of 2 micrograms galanin on the hypothalamo-pituitary-adrenocortical (HPA) axis were investigated in both normal and ether-stressed (2 min ether-vapor inhalation) or cold-stressed (20 min at 4 degrees C) rats. Blood concentrations of ACTH, aldosterone (ALDO) and corticosterone (B) were measured by specific RIA, 1, 2 or 4 h after galanin injection. Galanin administration to normal rats resulted in a marked rise in the blood levels of ACTH, ALDO and B at 1h and 2 h, the values returned to the baseline after 4 h. Ether and cold stresses notably raised the blood levels of ACTH, ALDO and B, and these rises lasted unchanged until 4 h. Galanin markedly potentiated ACTH and ALDO responses to ether stress at 1 and 2 h, but B response remained unchanged. ACTH response to cold stress was not affected by galanin; however, galanin magnified ALDO response to cold stress at 4 h, and enhanced at 1 h and depressed at 2 h that of B. In light of these findings the following conclusions can be drawn: (i) galanin exerts a stimulatory effect on HPA axis of rats under basal conditions; (ii) under our experimental conditions, ether stress exerts a stronger stimulation of HPA axis than cold stress; (iii) the galaninergic mechanisms involved in the stimulation of ACTH release do not interfere with ether stress-activated ones controlling ACTH secretion, and are probably similar to those underlying the effect of cold stress; (iv) steroidogenic capacity of adrenal cortex, at least in term of glucocorticoid hormones, is a rate-limiting step in the response of rat HPA axis to severe stresses; and (v) galanin exerts a direct secretory action of the rat adrenal gland, that can manifest itself only in the case of submaximally cold stress-stimulated HPA axis.

Chronic cerebroventricular galanin does not induce sustained hyperphagia or obesity

Acute central administration of galanin has been reported to increase fat consumption. These experiments were designed to test the hypothesis that repeated injections of galanin would elicit hyperphagia and weight gain and that this response would depend on the available diet. Male Sprague-Dawley rats were fed high (56% energy) or low (10% energy) fat diets. Galanin (300 pmol) or saline vehicle was injected into the third ventricle twice daily for 7 days and three times daily for another 6 days. On both the high-carbohydrate and high-fat diets, twice daily galanin increased daytime food intake, but there was a compensatory decrease in nighttime intake. The addition of a third, nighttime injection of galanin was ineffective in producing an increase in total 24-h intake. There was no significant increase in body weight during galanin treatment in rats eating either diet although animals eating the high-fat diet gained more weight as reflected by a significant increase in epididymal fat pad weight. Galanin treatment had no effects on serum insulin, glucose or corticosterone concentrations, measured at the end of the experiment. However, animals fed the high-fat diet had significantly higher insulin concentrations at the time of sacrifice. Although repeated central infusions of galanin reliably stimulated daytime intake of both diets, they failed to increase total daily energy intake or body weight.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Somatostatin and the paraventricular hypothalamus: modulation of energy balance

The effects of somatostatin injections (0.1, 1 and 5 micrograms) into the paraventricular nucleus of the hypothalamus (PVN) were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. The 0.1 and 1 microgram doses produced large and long-lasting decreases in respiratory quotient, which indicates the preferential utilization of fats as an energy substrate. The 5 micrograms dose produced a brief decrease in energy expenditure. Locomotor activity was not affected by any treatment which indicates that the effects on respiratory quotient and energy expenditure are not secondary to changes in activity. These findings demonstrate that somatostatin in the PVN inhibits thermogenesis and induces the preferential utilization of fats while sparing carbohydrate reserves. However, it is significant that the effects on energy expenditure and energy substrate utilisation occurred at different doses. These data constitute the first evidence that somatostatin in the PVN produces a primary modulation of the metabolic parameters central to energy balance. In separate experiments, all three doses of somatostatin increased blood glucose concentration over a one hour period, and the 5 micrograms dose decreased body weight over a 24 h period. Food and water intake were not affected by the somatostatin injections. Taken together, these findings are interpreted in a model in which somatostatin is a signal to the PVN of increased body fat. This mobilizes sympathetic mechanisms which increase fat utilization and blood glucose levels.

Glucagon and the paraventricular hypothalamus: modulation of energy balance

The effects of glucagon injections (25, 100 and 200 ng) into the paraventricular nucleus of the hypothalamus (PVN) were investigated in an open-circuit calorimeter. Wistar rats were tested, with no food available during the tests. High doses of glucagon (100 and 200 ng) produced small and short-lasting increases in energy expenditure. The independence of these changes from changes in locomotor activity suggests that the thermogenesis represents a primary modulation and is not secondary to increased locomotion. All three doses of glucagon produced long-lasting and dose-related increases in respiratory quotient which were unrelated to any changes in locomotor activity. As with the changes in energy expenditure, this dissociation indicates that the effects are not secondary to changes in locomotor activity. These data constitute the first evidence that glucagon in the PVN modulates the metabolic parameters central to energy balance. In separate experiments, the three doses of glucagon increased blood glucose concentration over a one hour period, but they did not affect food and water intake and body weight over 24 h. These findings suggest that glucagon normally acts on the PVN in conditions of increased body fat to initiate autonomic mechanisms which increase glycemic levels, thermogenesis and carbohydrate utilization. These data constitute the first direct evidence for the involvement of the PVN in the regulation of energy balance by glucagon. The main effect of glucagon in the PVN is anabolic in that it increases dependence on carbohydrates as an energy substrate which results in a sparing of fat reserves.

Galanin antagonists block galanin-induced feeding in the hypothalamus and amygdala of the rat

Galanin significantly increased food intake when microinjected into the region of the central nucleus of the amygdala as well as into the paraventricular nucleus of the hypothalamus. In the amygdala this effect was specific to feeding; no change in grooming, resting, or other behaviour was observed after galanin treatment. These results provide evidence that the amygdala may be an important site in the mediation of galanin-induced feeding. The galanin receptor antagonists, C7 and M40, antagonized galanin-induced feeding, while having no effect alone on food consumption in free-feeding rats. These new galanin receptor antagonists provide useful tools for further investigating the role of endogenous galanin in the regulation of feeding.

Metabolic modulation by amino acid stimulation of the paraventricular nucleus of the hypothalamus

The role of the paraventricular nucleus of the hypothalamus (PVN) in the regulation of energy expenditure and energy substrate utilization was investigated after the injection of the excitatory amino acid D,L-homocysteic acid (DLH) or its vehicle. Male Wistar rats with chronic PVN cannulae were tested for 1 h with no food available in an open-circuit calorimeter. Whereas low (0.5 nmol), excitatory doses of DLH increased energy expenditure, the thermogenic effect became smaller and then vanished as the DLH dose was increased to inhibitory levels (7 and 50 nmol). None of these doses affected motor activity, indicating a primary thermogenic effect. The highest dose (100 nmol) increased energy expenditure, but this appeared to be secondary to increased locomotor activity. The increased locomotor activity produced by the highest dose of DLH constitutes the first demonstration of an activity effect induced by stimulating the PVN. However, this effect likely reflects the activation of neighboring areas. Only the 50 nmol dose of DLH increased respiratory quotient, indicating a shift toward the preferential utilization of carbohydrates as an energy substrate. These data complement our findings with neuropeptide Y and insulin in showing that different doses of the same substance injected into the PVN may produce qualitatively different effects. Furthermore, the present study demonstrates that exciting PVN neurons activates catabolic forces, whereas inhibiting them activates anabolic forces.

Critical effects of aging and nutritional state on hypothalamic neuropeptide Y and galanin gene expression in lean and genetically obese Zucker rats

We have investigated, by Northern blot analysis, the hypothalamic gene expression [messenger RNA (mRNA)] of two appetite stimulating neuropeptides, neuropeptide Y (NPY) and galanin (GAL) in lean (+/+) and genetically obese (fa/fa) Zucker rats at 11, 24 and 40 weeks of age and their responsiveness to food deprivation. At 11 weeks of age, hypothalamic NPY mRNA levels of fa/fa rats were similar to those observed in lean littermates. However, NPY mRNA levels of fa/fa rats were significantly greater than those of lean rats at 24 (+126%; P < 0.01) and 40 (+65%; P < 0.05) weeks of age. Food deprivation caused a significant increase in NPY mRNA levels in both lean and fa/fa Zucker rats at 11 and 24 weeks of age, but not at 40 weeks old rats. Hypothalamic GAL mRNA showed a different pattern of change. The relative content of GAL mRNA in 11 week old obese rats was significantly lower (-68%; P < 0.05) than that of lean rats, while GAL mRNA was significantly higher in 40 week old (+57%; P < 0.05) obese rats compared to their lean littermates. At 24 weeks of age, hypothalamic GAL mRNA levels did not differ between lean and obese rats. Food deprivation induced no change in hypothalamic GAL mRNA in lean rats of all 3 ages; however, it caused an increase of GAL mRNA in obese rats at 11 (+60%; P < 0.05) and 24 (+44%; P < 0.05) weeks, but not at 40 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Impact of a galanin antagonist on exogenous galanin and natural patterns of fat ingestion

The peptide galanin (GAL) has a potent stimulatory effect on fat ingestion after administration into the hypothalamic paraventricular nucleus (PVN). This study examined a newly synthesized GAL antagonist, M40, in two separate experiments involving: (1) PVN injections of M40 alone in freely feeding animals, to investigate the importance of endogenous GAL receptor activity in determining natural patterns of fat ingestion, and (2) PVN injections of M40 in combination with exogenous GAL, to determine whether endogenous GAL receptors mediate this peptide-induced response. The results demonstrate that PVN injection of M40 by itself dose-dependently (2-108 pmol) reduces spontaneous ingestion of the fat diet. This phenomenon is robust, behaviorally specific and opposite to that induced by GAL itself. Moreover, the stimulatory effect of PVN-injected GAL on fat ingestion can be blocked by prior PVN administration of M40 at relatively low doses (2-6 pmol), indicating that M40 is a potent antagonist of GAL receptors in the hypothalamus. Together, these results provide the first evidence for the existence of endogenous GAL receptors in mediating the action of exogenous GAL in the hypothalamus. They also constitute a crucial step in demonstrating a physiological function of these PVN GAL receptors in controlling natural patterns of fat ingestion.