Melanocortin signaling is decreased during neurotoxin-induced transient hyperphagia and increased body-weight gain

Low abundance of NPY in the hypothalamus can produce hyperphagia and obesity

States of increased metabolic demand are associated with up-regulation of NPY and hyperphagia. However, we present some instances of hyperphagia in which NPY is not up-regulated. Ablation or functional disruption of specific sites in the hypothalamus, such as the ventromedial or paraventricular nuclei, or transection of inputs to the hypothalamus from the hindbrain results in hyperphagia and excess body weight gain. However, NPY expression and concentration in these experimental models is either decreased or unchanged. While there is no up-regulation of NPY in these models, there is increased sensitivity to the orexigenic effects of NPY. This enhanced responsiveness to NPY may more than compensate for the reduced levels of NPY and result in hyperphagia and excess body weight gain. The hyper-responsiveness may be due either to an increase in NPY receptors or to other changes in target cells and response pathways that may result from the treatments used in these models.

The hypothalamic paraventricular nucleus is not essential for orexigenic NPY or anorexigenic melanocortin action

Bilateral electrolytic lesions of the paraventricular nucleus of the hypothalamus (PVN) produce hyperphagia with excess weight gain. The orexigenic neuropeptide Y (NPY) system and the anorexigenic melanocortin system act in the PVN to regulate food intake, and participate in mediating the anorexic effects of leptin. We hypothesized that changes in the responsiveness of these systems may contribute to the hyperphagia observed in PVN-lesioned rats. Adult female Sprague-Dawley rats received either sham or electrolytic lesions in the PVN immediately followed by implantation of a guide cannula into the third cerebroventricle. Twenty-five days following surgery groups of sham and hyperphagic PVN-lesioned rats were injected intracerebroventricularly (i.c.v.) with either 118 pmole or 470 pmole of NPY and food intake was measured for 3 h. Food intake in response to NPY was nearly three-fold higher in PVN-lesioned rats as compared to sham rats. However, the response to 5 microg leptin i.c.v. was not different in lesioned versus sham rats. The effect of the melanocortin agonist MTII on food intake was tested in additional rats beginning either 7-14 days or 30-40 days following surgery. Doses of 0.1 nmole or 1.0 nmole of MTII were injected immediately before lights-off and food intake was measured at 2 h, 24 h and 48 h post-injection. Suppression of food intake in PVN-lesioned rats was not different from that in sham-lesioned rats. These data suggest that hyper-responsiveness to NPY may account in part for the hyperphagia observed in PVN-lesioned rats. Furthermore, based on the similarities of responses of PVN-lesioned and sham control rats to the anorexigenic agents MTII and leptin and the hypersensitivity of lesioned rats to NPY, we conclude that the PVN is not essential for NPY stimulation of food intake or for melanocortin suppression of food intake and that NPY and melanocortin receptors outside of the PVN are sufficient to produce these effects.

Differential regulation of synaptic inputs by constitutively released endocannabinoids and exogenous cannabinoids

Endocannabinoid release from a single neuron has been shown to cause presynaptic inhibition of transmitter release at many different sites. Here, we demonstrate that hypothalamic proopiomelanocortin (POMC) neurons release endocannabinoids continuously under basal conditions, unlike other release sites at which endocannabinoid production must be stimulated. The basal endocannabinoid release selectively inhibited GABA release onto POMC neurons, although exogenous administration of cannabinoid agonists also inhibited glutamate release. The CB1 cannabinoid receptor antagonist AM 251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] blocked endocannabinoid-mediated inhibition of GABA release without affecting excitatory synaptic currents, whereas the CB1 receptor agonist WIN 55,212-2 [R-(+)-(2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrol [1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl) methanone monomethanesulfonate] inhibited both inhibitory and excitatory synaptic currents in POMC neurons. These data demonstrate that endogenously released cannabinoids and exogenously applied CB1 receptor agonists can have markedly different effects on synaptic inputs. Furthermore, the data suggest a novel form of endocannabinoid-mediated retrograde inhibition, whereby the regulation of a subset of inputs requires either the removal of tonic presynaptic inhibition caused by endocannabinoids or the engagement of a mechanism that actively inhibits endocannabinoid production.

The rise, fall, and resurrection of the ventromedial hypothalamus in the regulation of feeding behavior and body weight

Early researchers found that lesions of the ventromedial hypothalamus (VMH) resulted in hyperphagia and obesity in a variety of species including humans, which led them to designate the VMH as the brain's "satiety center." Many researchers later dismissed a role for the VMH in feeding behavior when Gold claimed that lesions restricted to the VMH did not result in overeating and that obesity was observed only with lesions or knife cuts that extended beyond the borders of the VMH and damaged or severed the ventral noradrenergic bundle (VNAB) or paraventricular nucleus (PVN). However, anatomical studies done both before and after Gold's study did not replicate his results with lesions, and in nearly every published direct comparison of VMH lesions vs. PVN or VNAB lesions, the group with VMH lesions ate substantially more food and gained twice as much weight. Several other important differences have also been found between VMH and both PVN and VNAB lesion-induced obesity. Concerns regarding (a) motivation to work for food and (b) the effects of nonirritative lesions have also been addressed and answered in many studies. Lesion studies with weanling rats and adult pair-tube-fed rats, as well as recent studies of knockout mice deficient in the orphan nuclear receptor steroidogenic factor 1, indicate that VMH lesion-induced obesity is in large part a metabolic obesity (due to autonomic nervous system disorders) independent of hyperphagia. However, there is ample evidence that the VMH also plays a primary role in feeding behavior. Neuroimaging studies in humans have shown a marked increase in activity in the area of the VMH during feeding. The VMH has a large population of glucoresponsive neurons that dynamically respond to blood glucose levels and numerous histamine, dopamine, serotonin, and GABA neurons that respond to feeding-related stimuli. Recent studies have implicated melanocortins in the VMH regulation of feeding behavior: food intake decreases when arcuate nucleus pro-opiomelanocortin (POMC) neurons activate VMH brain-derived neurotrophic factor (BDNF) neurons. Moderate hyperphagia and obesity have also been observed in female rats with damage to the efferent projections from the posterodorsal amygdala to the VMH. Hypothalamic obesity can result from damage to either the POMC or BDNF neurons. The concept of hypothalamic feeding and satiety centers is outdated and unnecessary, and progress in understanding hypothalamic mechanisms of feeding behavior will be achieved only by appreciating the different types of neural and blood-borne information received by the various nuclei, and then attempting to determine how this information is integrated to obtain a balance between energy intake and energy output.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Chronic exercise lowers the defended body weight gain and adiposity in diet-induced obese rats

The effects of running wheel exercise and caloric restriction on the regulation of body weight, adiposity, and hypothalamic neuropeptide expression were compared in diet-induced obese male rats over 6 wk. Compared with sedentary controls, exercising rats had reduced body weight gain (24%), visceral (4 fat pads; 36%) and carcass (leptin; 35%) adiposity but not insulin levels. Hypothalamic arcuate nucleus (ARC) proopiomelanocortin (POMC) mRNA expression was 25% lower, but ARC neuropeptide Y (NPY), agouti- related peptide, dorsomedial nucleus (DMN) NPY, and paraventricular nucleus (PVN) corticotropin- releasing hormone (CRH) expression was comparable to controls. Sedentary rats calorically restricted to 85% of control body weight reduced their visceral adiposity (24%), leptin (64%), and insulin (21%) levels. ARC NPY (23%) and DMN NPY (60%) were increased, while ARC POMC (40%) and PVN CRH (14%) were decreased. Calorically restricted exercising rats an half as much as ad libitum-fed exercising rats and had less visceral obesity than comparably restricted sedentary rats. When sedentary restricted rats were refed after 4 wk, they increased intake and regained the weight gain and adiposity of sedentary controls. While refed exercising rats and sedentary rats ate comparable amounts, refed exercising rats regained weight and adiposity only to the level of ad libitum-fed exercising rats. Thus exercise lowers the defended level of weight gain and adiposity without a compensatory increase in intake and with a very different profile of hypothalamic neuropeptide expression from calorically restricted rats. This may be due to exercise-related factors other than plasma insulin and leptin.

Neuropeptide Y: a physiological orexigen modulated by the feedback action of ghrelin and leptin

Neuropeptide Y (NPY), a 36-amino-acid neuropeptide is the most potent physiological appetite transducer known. Episodic NPY neurosecretion in hypothalamic target sites is temporally linked with onset of the daily feeding pattern. Upregulation of NPY signaling in the arcuate nucleus-paraventricular nucleus (ARC-PVN) neural axis is responsible for the hyperphagia evoked by dieting, fasting, hormonal and genetic factors, and disruption in intrahypothalamic signaling. Clusters of NPY-producing neurons in the ARC that coexpress gamma- amino butyric acid and agouti-related peptide, and those in the brain stem (BS) that coexpress catecholamines and galanin, participate in disparate manners to regulate appetitive behavior. NPY receptors, Y1, Y2, and Y5, expressed by various components of the NPY network, mediate NPY-induced feeding. Imbalance in NPY signaling due either to high or low abundance of NPY at target sites elicits hyperphagia leading to increased fat accretion and obesity. Recent studies show that intermittent, feedback action of opposing afferent hormonal signals-leptin from adipose tissue and ghrelin from stomach-regulate the episodic secretion of orexigenic NPY in the PVN-ARC. Apparently, the hypothalamic NPY network is the primary common pathway intimately involved in genesis of appetite- stimulating impulses.

Rhythmic, reciprocal ghrelin and leptin signaling: new insight in the development of obesity

The hypothalamus integrates metabolic, neural and hormonal signals to evoke an intermittent appetitive drive in the daily management of energy homeostasis. Three major players identified recently in the feedback communication between the periphery and hypothalamus are leptin, ghrelin and neuropeptide Y (NPY). We propose that reciprocal circadian and ultradian rhythmicities in the afferent humoral signals, anorexigenic leptin from adipocytes and orexigenic ghrelin from stomach, encode a corresponding discharge pattern in the appetite-stimulating neuropeptide Y network in the hypothalamus. An exquisitely intricate temporal relationship among these signaling modalities with varied sites of origin is paramount in sustenance of weight control on a daily basis. Our model envisages that subtle and progressive derangements in temporal communication, imposed by environmental shifts in energy intake, impel a positive energy balance culminating in excessive weight gain and obesity. This conceptual advance provides a new target for designing pharmacologic or gene transfer therapies that would normalize the rhythmic patterns of afferent hormonal and efferent neurochemical messages.

Evidence for involvement of neuropeptide Y and melanocortin systems in the hyperphagia of lactation in rats

Hypothalamic neuropeptide Y (NPY) systems are upregulated during lactation in rats. Because NPY is central to the hypothalamic control of energy balance, the present studies tested the hypothesis that NPY contributes to the marked hyperphagia during lactation. A 4-day infusion of [D-tyr (27,36), D-thr (32)] NPY (27-36) (D-NPY(27-36)), a peptide analogue of NPY that antagonizes NPY-induced feeding, into the third ventricle at 1 microg/h transiently inhibited nocturnal feeding in nonlactating female rats. However, this antagonist had no effect on nocturnal feeding, but did transiently reduce food intake during the light hours, when infused into the third ventricle at the same dose in lactating females. An essentially similar pattern of results was obtained with chronic infusion into the third ventricle of the anorexigenic peptide alpha-melanocyte-stimulating hormone (alpha-MSH, 1 microg/h), in nonlactating and lactating rats. Both D-NPY(27-36) and alpha-MSH transiently reduced nocturnal food intake in lactating rats by approximately 10% when infused at the higher dose of 5 microg/h, and a marked inhibition of approximately 40% of both nocturnal and diurnal feeding was produced by a combined infusion of both at 5 microg/h. These results provide the first pharmacological evidence implicating specific neuromessengers in mediating the hyperphagia of lactation, and suggest that, while an action of NPY may contribute to the increased food intake seen in lactating animals, other systems are also involved. In particular, a reduction in melanocortin signaling during lactation may allow for an increased orexigenic influence of the agouti-related protein (AgRP), which is co-expressed with NPY.

Hypothalamic NPY and agouti-related protein are increased in human illness but not in Prader-Willi syndrome and other obese subjects

Animal studies have demonstrated the importance of orexigenic NPY and agouti-related protein (AGRP) hypothalamic neurons, which are inhibited by the adipocyte hormone leptin, in the regulation of body weight and neuroendocrine secretion. We have examined NPY and AGRP neurons in postmortem human hypothalami from controls, Prader-Willi syndrome and other obese subjects, using quantitative immunocytochemistry (ICC) and in situ hybridization, to identify causes of leptin resistance in human obesity. Using combined ICC and in situ hybridization, AGRP, but not POMC, was colocalized with NPY in infundibular nucleus neurons. Infundibular nucleus (including median eminence) NPY ICC staining or mRNA expression, and AGRP ICC staining, increased with premorbid illness duration. NPY ICC staining and mRNA expression were reduced in obese subjects, but AGRP ICC staining was unchanged, correcting for illness duration. This suggests normal responses of NPY and AGRP neurons to peripheral signals, such as leptin and insulin, in human illness and obesity. The pathophysiology of obesity and illness-associated anorexia appear to lie in downstream or separate neuronal circuits, but the infundibular neurons may mediate neuroendocrine responses to illness. The implications for pharmacological treatment of human obesity are discussed.

Dose-dependent effects of central leptin gene therapy on genes that regulate body weight and appetite in the hypothalamus

We have examined the dose-dependent effects and central action of intraventricular administration of a recombinant adeno-associated virus encoding rat leptin (rAAV-leptin) in suppressing body weight (BW) gain in adult female rats. A low dose of rAAV-leptin (5x10(10) particles) suppressed weight gain (15%) without changing daily food intake (FI), but a twofold higher dose decreased BW by 30% along with a reduction in daily FI. Reduced BW was due to a loss in body adiposity because serum leptin was reduced. Serum insulin levels were decreased (96%) by only the high dose along with a slight reduction in glucose. Uncoupling protein-1 (UCP-1) mRNA expression in brown adipose tissue (BAT), reflecting energy expenditure through thermogenesis, was upregulated to the same magnitude by the two rAAV-leptin doses. We analyzed by in situ hybridization the expression in the hypothalamus of genes encoding the appetite-regulating neuropeptides. Only the high dose decreased expression of neuropeptide Y (NPY), the orexigenic peptide, and increased proopiomelanocortin (POMC), precursor of the an orexigenic peptide, alpha-MSH. Our studies show for the first time that increased availability of leptin within the hypothalamus through central leptin gene therapy dose-dependently decreases weight gain, adiposity, and serum insulin by increasing energy expenditure and decreasing FI. The decrease in FI occurs only when NPY is reduced and alpha-MSH is increased in the hypothalamus by the high dose of rAAV-leptin. Delivery of the leptin gene centrally through rAAV vectors is a viable therapeutic modality for long-term control of weight and metabolic hormones.

Activity of body energy regulatory pathways in inflammation-induced anorexia

Previous research has shown that reductions in body weight prior to induction of acute inflammation can attenuate inflammation-induced anorexia in male rats. In the current study, potential mechanisms responsible for this observation were examined. Specifically, the effect of a 12% prior reduction in body weight on serum leptin, insulin, and corticosterone; levels of interleukin-1 (IL-1), interleukin-6 (IL-6) in the serum, liver, and spleen; neuropeptide Y (NPY) and POMC mRNA levels in the arcuate nucleus (ARC) of the hypothalamus were examined 8 h after induction of acute inflammation. Rats with prior weight reduction had significantly lower serum leptin levels and gene expression of POMC in the ARC than normal-weight rats. In contrast, prior weight reduction altered neither NPY mRNA in the ARC, nor IL-1alpha, IL-1beta, and IL-6 levels in the serum, liver, and spleen. These results suggest that the attenuation of inflammation-induced anorexia by prior weight reduction is not due to altered cytokine activity, but rather to changes in energy regulatory systems that moderate the anorexic actions of IL-1beta and IL-6. One potential change may be reduced activity of the CNS melanocortin system induced by decreased circulating leptin.