Effects of hindbrain melanin-concentrating hormone and neuropeptide Y administration on licking for water, saccharin, and sucrose solutions

Melanin-concentrating hormone and food intake control: Sites of action, peptide interactions, and appetition

Given the increased prevalence of obesity and its associated comorbidities, understanding the mechanisms through which the brain regulates energy balance is of critical importance. The neuropeptide melanin-concentrating hormone (MCH) is produced in the lateral hypothalamic area and the adjacent incerto-hypothalamic area and promotes both food intake and energy conservation, overall contributing to body weight gain. Decades of research into this system has provided insight into the neural pathways and mechanisms (behavioral and neurobiological) through which MCH stimulates food intake. Recent technological advancements that allow for selective manipulation of MCH neuron activity have elucidated novel mechanisms of action for the hyperphagic effects of MCH, implicating neural "volume" transmission in the cerebrospinal fluid and sex-specific effects of MCH on food intake control as understudied areas for future investigation. Highlighted here are historical and recent findings that illuminate the neurobiological mechanisms through which MCH promotes food intake, including the identification of various specific neural signaling pathways and interactions with other peptide systems. We conclude with a framework that the hyperphagic effects of MCH signaling are predominantly mediated through enhancement of an "appetition" process in which early postoral prandial signals promote further caloric consumption.

The dynamic regulation of appetitive behavior through lateral hypothalamic orexin and melanin concentrating hormone expressing cells

The lateral hypothalamic area (LHA) is a heterogeneous brain structure extensively studied for its potent role in regulating energy balance. The anatomical and molecular diversity of the LHA permits the orchestration of responses to energy sensing cues from the brain and periphery. Two of the primary cell populations within the LHA associated with integration of this information are Orexin (ORX) and Melanin Concentrating Hormone (MCH). While both of these non-overlapping populations exhibit orexigenic properties, the activities of these two systems support feeding behavior through contrasting mechanisms. We describe the anatomical and functional properties as well as interaction with other neuropeptides and brain reward and hedonic systems. Specific outputs relating to arousal, food seeking, feeding, and metabolism are coordinated through these mechanisms. We then discuss how both the ORX and MCH systems harmonize in a divergent yet overall cooperative manner to orchestrate feeding behavior through transitions between various appetitive states, and thus offer novel insights into LHA allostatic control of appetite.

Melanin-concentrating hormone neurons contribute to dysregulation of rapid eye movement sleep in narcolepsy

The lateral hypothalamus contains neurons producing orexins that promote wakefulness and suppress REM sleep as well as neurons producing melanin-concentrating hormone (MCH) that likely promote REM sleep. Narcolepsy with cataplexy is caused by selective loss of the orexin neurons, and the MCH neurons appear unaffected. As the orexin and MCH systems exert opposing effects on REM sleep, we hypothesized that imbalance in this REM sleep-regulating system due to activity in the MCH neurons may contribute to the striking REM sleep dysfunction characteristic of narcolepsy. To test this hypothesis, we chemogenetically activated the MCH neurons and pharmacologically blocked MCH signaling in a murine model of narcolepsy and studied the effects on sleep-wake behavior and cataplexy. To chemoactivate MCH neurons, we injected an adeno-associated viral vector containing the hM3Dq stimulatory DREADD into the lateral hypothalamus of orexin null mice that also express Cre recombinase in the MCH neurons (MCH-Cre::OX-KO mice) and into control MCH-Cre mice with normal orexin expression. In both lines of mice, activation of MCH neurons by clozapine-N-oxide (CNO) increased rapid eye movement (REM) sleep without altering other states. In mice lacking orexins, activation of the MCH neurons also increased abnormal intrusions of REM sleep manifest as cataplexy and short latency transitions into REM sleep (SLREM). Conversely, a MCH receptor 1 antagonist, SNAP 94847, almost completely eliminated SLREM and cataplexy in OX-KO mice. These findings affirm that MCH neurons promote REM sleep under normal circumstances, and their activity in mice lacking orexins likely triggers abnormal intrusions of REM sleep into non-REM sleep and wake, resulting in the SLREM and cataplexy characteristic of narcolepsy.

Characterizing ingestive behavior through licking microstructure: Underlying neurobiology and its use in the study of obesity in animal models

Ingestive behavior is controlled by multiple distinct peripheral and central physiological mechanisms that ultimately determine whether a particular food should be accepted or avoided. As rodents consume a fluid they display stereotyped rhythmic tongue movements, and by analyzing the temporal distribution of pauses of licking, it is possible through analyses of licking microstructure to uncover dissociable evaluative and motivational variables that contribute to ingestive behavior. The mean number of licks occurring within each burst of licking (burst and cluster size) reflects the palatability of the consumed solution, whereas the frequency of initiating novel bouts of licking behavior (burst and cluster number) is dependent upon the degree of gastrointestinal inhibition that accrues through continued fluid ingestion. This review describes the analysis of these measures within a context of the behavioral variables that come to influence the acceptance or avoidance of a fluid, and the neurobiological mechanisms that underlie alterations in the temporal distribution of pauses of licks. The application of these studies to models of obesity in animals is also described.

Levels of Cocaine- and Amphetamine-Regulated Transcript in Vagal Afferents in the Mouse Are Unaltered in Response to Metabolic Challenges

Cocaine- and amphetamine-regulated transcript (CART) is one of the most abundant neuropeptides in vagal afferents, including those involved in regulating feeding. Recent observations indicate that metabolic challenges dramatically alter the neuropeptidergic profile of CART-producing vagal afferents. Here, using confocal microscopy, we reassessed the distribution and regulation of CART(55–102) immunoreactivity in vagal afferents of the male mouse in response to metabolic challenges, including fasting and high-fat-diet feeding. Importantly, the perikarya and axons of vagal C-fibers were labeled using mice expressing channelrodhopsin-2 (ChR2-YFP) in Na_v1.8-Cre–expressing neurons. In these mice, approximately 82% of the nodose ganglion neurons were labeled with ChR2-YFP. Furthermore, ChR2-YFP–labeled axons could easily be identified in the dorsovagal complex. CART(55–102) immunoreactivity was observed in 55% of the ChR2-YFP–labeled neurons in the nodose ganglion and 22% of the ChR2-YFP–labeled varicosities within the area postrema of fed, fasted, and obese mice. The distribution of positive profiles was also identical across the full range of CART staining in fed, fasted, and obese mice. In contrast to previous studies, fasting did not induce melanin-concentrating hormone (MCH) immunoreactivity in vagal afferents. Moreover, prepro-MCH mRNA was undetectable in the nodose ganglion of fasted mice. In summary, this study showed that the perikarya and central terminals of vagal afferents are invariably enriched in CART and devoid of MCH.

Maternal stress and diet may influence affective behavior and stress-response in offspring via epigenetic regulation of central peptidergic function

It has been shown that maternal stress and malnutrition, or experience of other adverse events, during the perinatal period may alter susceptibility in the adult offspring in a time-of-exposure dependent manner. The mechanism underlying this may be epigenetic in nature. Here, we summarize some recent findings on the effects on gene-regulation following maternal malnutrition, focusing on epigenetic regulation of peptidergic activity. Numerous neuropeptides within the central nervous system are crucial components in regulation of homeostatic energy-balance, as well as affective health (i.e. health events related to affective disorders, psychiatric disorders also referred to as mood disorders). It is becoming evident that expression, and function, of these neuropeptides can be regulated via epigenetic mechanisms during fetal development, thereby contributing to the development of the adult phenotype and, possibly, modulating disease susceptibility. Here, we focus on two such neuropeptides, neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH), both involved in regulation of endocrine function, energy homeostasis, as well as affective health. While a number of published studies indicate the involvement of epigenetic mechanisms in CRH-dependent regulation of the offspring adult phenotype, NPY has been much less studied in this context and needs further work.

Distinct Types of Feeding Related Neurons in Mouse Hypothalamus

The last two decades of research provided evidence for a substantial heterogeneity among feeding-related neurons (FRNs) in the hypothalamus. However, it remains unclear how FRNs differ in their firing patterns during food intake. Here, we investigated the relationship between the activity of neurons in mouse hypothalamus and their feeding behavior. Using tetrode-based in vivo recording technique, we identified various firing patterns of hypothalamic FRNs, which, after the initiation of food intake, can be sorted into four types: sharp increase (type I), slow increase (type II), sharp decrease (type III), and sustained decrease (type IV) of firing rates. The feeding-related firing response of FRNs was rigidly related to the duration of food intake and, to a less extent, associated with the type of food. The majority of these FRNs responded to glucose and leptin and exhibited electrophysiological characteristics of putative GABAergic neurons. In conclusion, our study demonstrated the diversity of neurons in the complex hypothalamic network coordinating food intake.

Oxytocin Regulates Stress-Induced Crf Gene Transcription through CREB-Regulated Transcription Coactivator 3

The major regulator of the neuroendocrine stress response in the brain is corticotropin releasing factor (CRF), whose transcription is controlled by CREB and its cofactors CRTC2/3 (TORC2/3). Phosphorylated CRTCs are sequestered in the cytoplasm, but rapidly dephosphorylated and translocated into the nucleus following a stressful stimulus. As the stress response is attenuated by oxytocin (OT), we tested whether OT interferes with CRTC translocation and, thereby, Crf expression. OT (1 nmol, i.c.v.) delayed the stress-induced increase of nuclear CRTC3 and Crf hnRNA levels in the paraventricular nucleus of male rats and mice, but did not affect either parameter in the absence of the stressor. The increase in Crf hnRNA levels at later time points was parallel to elevated nuclear CRTC2/3 levels. A direct effect of Thr(4) Gly(7)-OT (TGOT) on CRTC3 translocation and Crf expression was found in rat primary hypothalamic neurons, amygdaloid (Ar-5), hypothalamic (H32), and human neuroblastoma (Be(2)M17) cell lines. CRTC3, but not CRCT2, knockdown using siRNA in Be(2)M17 cells prevented the effect of TGOT on Crf hnRNA levels. Chromatin-immunoprecipitation demonstrated that TGOT reduced CRTC3, but not CRTC2, binding to the Crf promoter after 10 min of forskolin stimulation. Together, the results indicate that OT modulates CRTC3 translocation, the binding of CRTC3 to the Crf promoter and, ultimately, transcription of the Crf gene.

SIGNIFICANCE STATEMENT

The neuropeptide oxytocin has been proposed to reduce hypothalamic-pituitary-adrenal (HPA) axis activation during stress. The underlying mechanisms are, however, elusive. In this study we show that activation of the oxytocin receptor in the paraventricular nucleus delays transcription of the gene encoding corticotropin releasing factor (Crf), the main regulator of the stress response. It does so by sequestering the coactivator of the transcription factor CREB, CRTC3, in the cytosol, resulting in reduced binding of CRTC3 to the Crf gene promoter and subsequent Crf gene expression. This novel oxytocin receptor-mediated intracellular mechanism might provide a basis for the treatment of exaggerated stress responses in the future.

Sex differences in feeding behavior in rats: the relationship with neuronal activation in the hypothalamus

There is general agreement that the central nervous system in rodents differs between sexes due to the presence of gonadal steroid hormone during differentiation. Sex differences in feeding seem to occur among species, and responses to fasting (i.e., starvation), gonadal steroids (i.e., testosterone and estradiol), and diet (i.e., western-style diet) vary significantly between sexes. The hypothalamus is the center for controlling feeding behavior. We examined the activation of feeding-related peptides in neurons in the hypothalamus. Phosphorylation of cyclic AMP response element-binding protein (CREB) is a good marker for neural activation, as is the Fos antigen. Therefore, we predicted that sex differences in the activity of melanin-concentrating hormone (MCH) neurons would be associated with feeding behavior. We determined the response of MCH neurons to glucose in the lateral hypothalamic area (LHA) and our results suggested MCH neurons play an important role in sex differences in feeding behavior. In addition, fasting increased the number of orexin neurons harboring phosphorylated CREB in female rats (regardless of the estrous day), but not male rats. Glucose injection decreased the number of these neurons with phosphorylated CREB in fasted female rats. Finally, under normal spontaneous food intake, MCH neurons, but not orexin neurons, expressed phosphorylated CREB. These sex differences in response to fasting and glucose, as well as under normal conditions, suggest a vulnerability to metabolic challenges in females.

To ingest or rest? Specialized roles of lateral hypothalamic area neurons in coordinating energy balance

Survival depends on an organism’s ability to sense nutrient status and accordingly regulate intake and energy expenditure behaviors. Uncoupling of energy sensing and behavior, however, underlies energy balance disorders such as anorexia or obesity. The hypothalamus regulates energy balance, and in particular the lateral hypothalamic area (LHA) is poised to coordinate peripheral cues of energy status and behaviors that impact weight, such as drinking, locomotor behavior, arousal/sleep and autonomic output. There are several populations of LHA neurons that are defined by their neuropeptide content and contribute to energy balance. LHA neurons that express the neuropeptides melanin-concentrating hormone (MCH) or orexins/hypocretins (OX) are best characterized and these neurons play important roles in regulating ingestion, arousal, locomotor behavior and autonomic function via distinct neuronal circuits. Recently, another population of LHA neurons containing the neuropeptide Neurotensin (Nts) has been implicated in coordinating anorectic stimuli and behavior to regulate hydration and energy balance. Understanding the specific roles of MCH, OX and Nts neurons in harmonizing energy sensing and behavior thus has the potential to inform pharmacological strategies to modify behaviors and treat energy balance disorders.

Immunohistochemical mapping of neuropeptide Y in the tree shrew brain

Day-active tree shrews are promising animals as research models for a variety of human disorders. Neuropeptide Y (NPY) modulates many behaviors in vertebrates. Here we examined the distribution of NPY in the brain of tree shrews (Tupaia belangeri chinensis) using immunohistochemical techniques. The differential distribution of NPY-immunoreactive (-ir) cells and fibers were observed in the rhinencephalon, telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon of tree shrews. Most NPY-ir cells were multipolar or bipolar in shape with triangular, fusiform, and/or globular perikarya. The densest cluster of NPY-ir cells were found in the mitral cell layer of the main olfactory bulb (MOB), arcuate nucleus of the hypothalamus, and pretectal nucleus of the thalamus. The MOB presented a unique pattern of NPY immunoreactivity. Laminar distribution of NPY-ir cells was observed in the MOB, neocortex, and hippocampus. Compared to rats, the tree shrews exhibited a particularly robust and widespread distribution of NPY-ir cells in the MOB, bed nucleus of the stria terminalis, and amygdala as well as the ventral lateral geniculate nucleus and pretectal nucleus of the thalamus. By contrast, a low density of neurons were scattered in the striatum, neocortex, polymorph cell layer of the dentate gyrus, superior colliculus, inferior colliculus, and dorsal tegmental nucleus. These findings provide the first detailed mapping of NPY immunoreactivity in the tree shrew brain and demonstrate species differences in the distribution of this neuropeptide, providing an anatomical basis for the participation of the NPY system in the regulation of numerous physiological and behavioral processes.

Hypothalamic peptides controlling alcohol intake: differential effects on microstructure of drinking bouts

Different alcohol drinking patterns, involving either small and frequent drinking bouts or large and long-lasting bouts, are found to differentially affect the risk for developing alcohol-related diseases, suggesting that they have different underlying mechanisms. Such mechanisms may involve orexigenic peptides known to stimulate alcohol intake through their actions in the hypothalamic paraventricular nucleus (PVN). These include orexin (OX), which is expressed in the perifornical lateral hypothalamus, and galanin (GAL) and enkephalin (ENK), which are expressed within as well as outside the PVN. To investigate the possibility that these peptides affect different aspects of consumption, a microstructural analysis of ethanol drinking behavior was performed in male, Sprague-Dawley rats trained to drink 7% ethanol and implanted with guide shafts aimed at the PVN. While housed in specialized cages containing computerized intake monitors (BioDAQ Laboratory Intake Monitoring System, Research Diets Inc., New Brunswick, NJ) that measure bouts of ethanol drinking, these rats were given PVN injections of OX (0.9 nmol), GAL (1.0 nmol), or the ENK analog D-Ala2-met-enkephalinamide (DALA) (14.2 nmol), as compared to saline vehicle. Results revealed clear differences between the effects of these peptides. While all 3 stimulated ethanol intake, they had distinct effects on patterns of drinking, with OX increasing the number of drinking bouts, GAL increasing the size of the drinking bouts, and DALA increasing both the size and duration of the bouts. In contrast, these peptides had little impact on water or food intake. These results support the idea that different peptides can increase ethanol consumption by promoting distinct aspects of the ethanol drinking response. The stimulatory effect of OX on drinking frequency may be related to its neuronally stimulatory properties, while the stimulatory effect of GAL and ENK on bout size and duration may reflect a suppressive effect of these neuronally inhibitory peptides on the satiety-controlling PVN.

The roles of melanin-concentrating hormone in energy balance and reproductive function: Are they connected?

Melanin-concentrating hormone (MCH) is an anabolic neuropeptide with multiple and diverse physiological functions including a key role in energy homoeostasis. Rodent studies have shown that the ablation of functional MCH results in a lean phenotype, increased energy expenditure and resistance to diet-induced obesity. These findings have generated interest among pharmaceutical companies vigilant for potential anti-obesity agents. Nutritional status affects reproductive physiology and behaviours, thereby optimising reproductive success and the ability to meet energetic demands. This complex control system entails the integration of direct or indirect peripheral stimuli with central effector systems and involves numerous mediators. A role for MCH in the reproductive axis has emerged, giving rise to the premise that MCH may serve as an integratory mediator between those discrete systems that regulate energy balance and reproductive function. Hence, this review focuses on published evidence concerning i) the role of MCH in energy homoeostasis and ii) the regulatory role of MCH in the reproductive axis. The question as to whether the MCH system mediates the integration of energy homoeostasis with the neuroendocrine reproductive axis and, if so, by what means has received limited coverage in the literature; evidence to date and current theories are summarised herein.

Estrogen-related receptor β deletion modulates whole-body energy balance via estrogen-related receptor γ and attenuates neuropeptide Y gene expression

Estrogen-related receptors (ERRs) α, β and γ are orphan nuclear hormone receptors with no known ligands. Little is known concerning the role of ERRβ in energy homeostasis, as complete ERRβ-null mice die mid-gestation. We generated two viable conditional ERRβ-null mouse models to address its metabolic function. Whole-body deletion of ERRβ in Sox2-Cre:ERRβ(lox/lox) mice resulted in major alterations in body composition, metabolic rate, meal patterns and voluntary physical activity levels. Nestin-Cre:ERRβ(lox/lox) mice exhibited decreased expression of ERRβ in hindbrain neurons, the predominant site of expression, decreased neuropeptide Y (NPY) gene expression in the hindbrain, increased lean body mass, insulin sensitivity, increased energy expenditure, decreased satiety and decreased time between meals. In the absence of ERRβ, increased ERRγ signaling decreased satiety and the duration of time between meals, similar to meal patterns observed for both the Sox2-Cre:ERRβ(lox/lox) and Nestin-Cre:ERRβ(lox/lox) strains of mice. Central and/or peripheral ERRγ signaling may modulate these phenotypes by decreasing NPY gene expression. Overall, the relative expression ratio between ERRβ and ERRγ may be important in modulating ingestive behavior, specifically satiety, gene expression, as well as whole-body energy balance.

The role of melanin-concentrating hormone and its receptors in energy homeostasis

Extensive studies in rodents with melanin-concentrating hormone (MCH) have demonstrated that the neuropeptide hormone is a potent orexigen. Acutely, MCH causes an increase in food intake, while chronically it leads to increased weight gain, primarily as an increase in fat mass. Multiple knockout mice models have confirmed the importance of MCH in modulating energy homeostasis. Animals lacking MCH, MCH-containing neurons, or the MCH receptor all are resistant to diet-induced obesity. These genetic and pharmacologic studies have prompted a large effort to identify potent and selective MCH receptor antagonists, initially as tool compounds to probe pharmacology in models of obesity, with an ultimate goal to identify novel anti-obesity drugs. In animal models, MCH antagonists have consistently shown efficacy in reducing food intake acutely and inhibiting body-weight gain when given chronically. Five compounds have proceeded into clinical testing. Although they were reported as well-tolerated, none has advanced to long-term efficacy and safety studies.

Hypothalamic neuropeptides and the regulation of appetite

Neuropeptides released by hypothalamic neurons play a major role in the regulation of feeding, acting both within the hypothalamus, and at other appetite regulating centres throughout the brain. Where classical neurotransmitters signal only within synapses, neuropeptides diffuse over greater distances affecting both nearby and distant neurons expressing the relevant receptors, which are often extrasynaptic. As well as triggering a behavioural output, neuropeptides also act as neuromodulators: altering the response of neurons to both neurotransmitters and circulating signals of nutrient status. The mechanisms of action of hypothalamic neuropeptides with established roles in feeding, including melanin-concentrating hormone (MCH), the orexins, α-melanocyte stimulating hormone (α-MSH), agouti-gene related protein (AgRP), neuropeptide Y, and oxytocin, are reviewed in this article, with emphasis laid on both their effects on appetite regulating centres throughout the brain, and on examining the evidence for their physiological roles. In addition, evidence for the involvement of several putative appetite regulating hypothalamic neuropeptides is assessed including, ghrelin, cocaine and amphetamine-regulated transcript (CART), neuropeptide W and the galanin-like peptides. This article is part of a Special Issue entitled 'Central control of Food Intake'.

Anatomical dissociation of melanocortin receptor agonist effects on taste- and gut-sensitive feeding processes

Injections of the melanocortin 3/4 receptor (MCR) agonist melanotan II (MTII) to a variety of brain structures produces anorexia, suggesting distributed brain MCR control of food intake. We performed a detailed analysis of feeding behavior (licking microstructure analysis) after a range of MTII doses (0.005 nM to 1 nM) was targeted to the forebrain (third ventricle, 3V) or hindbrain (fourth ventricle, 4V) regions. MTII (0.1 nM and 1 nM) delivered to the 3V or 4V significantly reduced 0.8 M sucrose intake. The anorexia was mediated by reductions in the number of licking bursts in the meal, intrameal ingestion rate, and meal duration; these measures have been associated with postingestive feedback inhibition of feeding. Anorexia after 3V but not 4V MTII injection was also associated with a reduced rate of licking in the first minute (initial lick rate) and reduced mean duration of licking bursts; these measures have been associated with taste evaluation. MTII effects on taste evaluation were further explored: In experiment 2, 3V MTII (1 nM) significantly reduced intake of noncaloric 4 mM saccharin and 0.1 M and 1 M sucrose solutions, but not water. The anorexia was again associated with reduced number of licking bursts, ingestion rate, meal duration, initial lick rate, and mean burst duration. In experiments 3 and 4, brief access (20 s) licking responses for sweet sucrose (0.015 M to 0.25 M) and bitter quinine hydrochloride (0.01 mM to 1 mM) solutions were evaluated. Licking responses for sucrose were suppressed, whereas those for quinine solutions were increased after 3V MTII, but not after 4V MTII injections (0.1 nM and 1 nM). The results suggest that multiple brain MCR sites influence sensitivity to visceral feedback, whereas forebrain MCR stimulation is necessary to influence taste responsiveness, possibly through attenuation of the perceived intensity of taste stimuli.

An analysis of licking microstructure in three strains of mice

Mouse models of feeding provide a useful tool for elucidating the molecular pathways of energy regulation. The majority of studies in mice have been limited to intake analyses conducted over extended periods of time, which fail to distinguish between a variety of factors that influence nutrient intake. Using licking microstructure analyses we examined both the size and number of licking bursts for water, polycose, sucrose and lecithin in three strains of mice (C57BL/6J, 129Sv/ImJ and C57129F1 hybrids), using pause criteria (250-500, >500 and >1000 ms) that have previously been described in the rat. Burst size and number varied both as a function of tastant concentration and mouse strain; however, these differences were most evident with the >1000 ms pause criterion. Consistent with previous reports, during water consumption C57 mice showed longer mean interlick intervals, a larger number of bursts but reduced burst size relative to the two other strains. F1 mice showed larger burst sizes for polycose, while C57 mice displayed a greater number of bursts for both polycose and sucrose. Both 129 and F1 mice were insensitive to sucrose concentration, whereas C57 mice showed attenuated lecithin intake influenced by a reduction in the size of bursts for this tastant. These results suggest that these strains of mice display differences in the pattern of licking that are most evident with the use of larger pause criteria. These differences in licking behavior might reflect influences of genetic background on pre- and post-ingestive factors controlling intake, the reinforcing properties of each tastant, or native differences in licking style.

Orexin-A hyperphagia: hindbrain participation in consummatory feeding responses

Orexin-A (ORXA) is an orexigenic neuropeptide produced by the lateral hypothalamus that increases food intake when injected into the brain ventricles or forebrain nuclei. We used a licking microstructure analysis to evaluate hindbrain and forebrain ORXA effects in intact and hindbrain-lesioned rats, to identify the motivational and anatomical bases of ORXA hyperphagia. Intact rats with cannulas in the fourth brain ventricle (4V) received vehicle (artificial cerebrospinal fluid) or ORXA (0.1, 0.4, 1, or 10 nm) injections before 90 min access to 0.1 m sucrose. Meal size and frequency were increased in a double-dissociated manner by the 1 and 10 nm doses, respectively. In experiment 2, 4V 1 nm ORXA was applied to rats offered solutions varied in caloric and gustatory intensity (water and 0.1 and 1 m sucrose). ORXA increased meal frequency for all tastants. ORXA increased meal size only for 0.1 m sucrose, by prolonging the meal without affecting early ingestion rate or lick burst size, suggesting that 4V ORXA influenced inhibitory postingestive feedback rather than taste evaluation. In experiment 3, rats with cannulas in the third ventricle (3V) received dorsal medullary lesions centered on the area postrema (APX group) or sham procedures, and licking for water and 0.1 and 1 m sucrose was evaluated after 1 nm 3V ORXA/artificial cerebrospinal fluid injections. The 3V ORXA increased 0.1 m sucrose meal size and meal frequency for all tastants in the sham group, as observed after 4V ORXA in experiment 2. In the APX group, 3V ORXA injections influenced meal frequency, but they no longer increased meal size. However, the APX rats increased meal size for 0.1 m sucrose after food and water deprivation and after 3V angiotensin II injection. They also showed meal size suppression after 3V injection of the melanocortin-3/4 receptor agonist melanotan II (1 nm). These findings suggest that the area postrema and subjacent nucleus of the solitary tract are necessary for increases in consummatory (meal size) but not appetitive (meal frequency) responses to 3V ORXA. The meal size increases may be due to reduced postingestive feedback inhibition induced by ORXA delivered to either the hindbrain or forebrain ventricles.

Behavioral neuroendocrinology and treatment of anorexia nervosa

Outcome in anorexia nervosa remains poor and a new way of looking at this condition is therefore needed. To this aim, we review the effects of food restriction and starvation in humans. It is suggested that body weight remains stable and relatively low when the access to food requires a considerable amount of physical activity. In this condition, the human homeostatic phenotype, body fat content is also low and as a consequence, the synthesis and release of brain neurotransmitters are modified. As an example, the role of neuropeptide Y is analyzed in rat models of this state. It is suggested that the normal behavioral role of neuropeptide Y is to facilitate the search for food and switch attention from sexual stimuli to food. Descriptive neuroendocrine studies on patients with anorexia nervosa have not contributed to the management of the patients and the few studies in which hormones have been administered have, at best, reversed an endocrine consequence secondary to starvation. In a modified framework for understanding the etiology and treatment of anorexia nervosa it is suggested that the condition emerges because neural mechanisms of reward and attention are engaged. The neural neuropeptide Y receptor system may be involved in the maintenance of the behavior of eating disorder patients because the localization of these receptors overlaps with the neural systems engaged in cue-conditioned eating in limbic and cortical areas. The eating behavior of patients with anorexia nervosa, and other eating disorders as well, is viewed as a cause of the psychological changes of the patients. Patients are trained to re-learn normal eating habits using external support and as they do, their symptoms, including the psychological symptoms, dissolve.