Effects of neuropeptide Y on feeding microstructure: Dissociation of appetitive and consummatory actions

Thyrotropin-Releasing Hormone and Food Intake in Mammals: An Update

Thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2) is an intercellular signal produced mainly by neurons. Among the multiple pharmacological effects of TRH, that on food intake is not well understood. We review studies demonstrating that peripheral injection of TRH generally produces a transient anorexic effect, discuss the pathways that might initiate this effect, and explain its short half-life. In addition, central administration of TRH can produce anorexic or orexigenic effects, depending on the site of injection, that are likely due to interaction with TRH receptor 1. Anorexic effects are most notable when TRH is injected into the hypothalamus and the nucleus accumbens, while the orexigenic effect has only been detected by injection into the brain stem. Functional evidence points to TRH neurons that are prime candidate vectors for TRH action on food intake. These include the caudal raphe nuclei projecting to the dorsal motor nucleus of the vagus, and possibly TRH neurons from the tuberal lateral hypothalamus projecting to the tuberomammillary nuclei. For other TRH neurons, the anatomical or physiological context and impact of TRH in each synaptic domain are still poorly understood. The manipulation of TRH expression in well-defined neuron types will facilitate the discovery of its role in food intake control in each anatomical scene.

Y1 receptors modulate taste-related behavioral responsiveness in male mice to prototypical gustatory stimuli

Mammalian taste bud cells express receptors for numerous peptides implicated elsewhere in the body in the regulation of metabolism, nutrient assimilation, and satiety. The perturbation of several peptide signaling pathways in the gustatory periphery results in changes in behavioral and/or physiological responsiveness to subsets of taste stimuli. We previously showed that Peptide YY (PYY) - which is present in both saliva and in subsets of taste cells - can affect behavioral taste responsiveness and reduce food intake and body weight. Here, we investigated the contributions of taste bud-localized receptors for PYY and the related Neuropeptide Y (NPY) on behavioral taste responsiveness. Y1R, but not Y2R, null mice show reduced responsiveness to sweet, bitter, and salty taste stimuli in brief-access taste tests; similar results were seen when wildtype mice were exposed to Y receptor antagonists in the taste stimuli. Finally, mice in which the gene encoding the NPY propeptide was deleted also showed reduced taste responsiveness to sweet and bitter taste stimuli. Collectively, these results suggest that Y1R signaling, likely through its interactions with NPY, can modulate peripheral taste responsiveness in mice.

Selective stimulation of central GABAAα2,3,5 receptors increases intake and motivation to consume sucrose solution in rats

Benzodiazepines are one of the most commonly prescribed anxiolytic drugs in America, and between 2006 and 2015 prescription rates increased by an estimated 27.1%. Weight gain is a common side effect of these drugs and it may result from increased feeding caused by drug-enhanced food palatability. We investigated the role of specific GABA_A receptor subtypes involved with benzodiazepine-induced food consumption through third ventricle injections of L-838,417, a partial agonist of GABA_A α2, α3, and α5 subunits, and a full antagonist of the α1 receptor subunit. A microanalysis of the licking behavior of adult male rats to a sucrose solution was used to isolate drug effects on specific consummatory behaviors that include: hedonic taste evaluation, food approach behavior, and oromotor function. L-838,417 dose-dependently increased intake through increases in the motivation to approach the solution (shorter pause intervals between bouts of licking) and through enhancement of measures associated with hedonic taste evaluation. Oromotor depressant effects previously associated with broad-spectrum benzodiazepine receptor agonists were not observed. These results indicate that nuclei in proximity to the ventricles respond to GABA_A α2, α3, or α5 activation to induce motivation to feed, absent of α1 receptor subunit activation. Furthermore, activation of the α1 subunit is not necessary for benzodiazepine hyperphagia and may instead contribute to the oromotor depressant and sedative properties of classic benzodiazepine agonists. Hypothalamic nuclei such as the paraventricular nucleus may be involved in the benzodiazepine-increased motivation to feed, while the parabrachial nucleus of the hindbrain could contribute to benzodiazepine-induced enhancement of taste palatability.

Taste as a marker for behavioral energy regulation:Replication and extension of meal pattern evidence from selectively bred rats

A key feature of energy regulation among species that eat discrete meals is meal patterning - meal frequency, size, and duration. Such animals can adjust to internal states and external circumstances with changes in one or more of those meal parameters, with or without altering total food intake. Relatively little is known about individual differences in meal patterning. We previously reported meal patterning differences between rat lines selectively bred for differential saccharin solution intake, lines that also differ in sensitivity to metabolic challenges: Relative to high-saccharin-consuming counterparts (HiS), male low-saccharin-consuming rats (LoS) ate smaller, more frequent meals. Those findings provided evidence of an association between taste and short term satiety. Twenty generations later, we describe systematic replication of the line difference in meal patterns in males and females using two different kinds of reinforcer pellet. The previous study was further extended by examining meal parameters (1) with bi- and multivariate analyses and (2) after acute food restriction and a moderate stressor. Results are discussed within a behavior-systems framework incorporating taste as a marker for behavioral energy regulation.

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.

Parabrachial Nucleus Contributions to Glucagon-Like Peptide-1 Receptor Agonist-Induced Hypophagia

Exendin-4 (Ex4), a glucagon-like peptide-1 receptor (GLP-1R) agonist approved to treat type 2 diabetes mellitus, is well known to induce hypophagia in human and animal models. We evaluated the contributions of the hindbrain parabrachial nucleus (PBN) to systemic Ex4-induced hypophagia, as the PBN receives gustatory and visceral afferent relays and descending input from several brain nuclei associated with feeding. Rats with ibotenic-acid lesions targeted to the lateral PBN (PBNx) and sham controls received Ex4 (1 μg/kg) before 24 h home cage chow or 90 min 0.3 M sucrose access tests, and licking microstructure was analyzed to identify components of feeding behavior affected by Ex4. PBN lesion efficacy was confirmed using conditioned taste aversion (CTA) tests. As expected, sham control but not PBNx rats developed a CTA. In sham-lesioned rats, Ex4 reduced chow intake within 4 h of injection and sucrose intake within 90 min. PBNx rats did not show reduced chow or sucrose intake after Ex4 treatment, indicating that the PBN is necessary for Ex4 effects under the conditions tested. In sham-treated rats, Ex4 affected licking microstructure measures associated with hedonic taste evaluation, appetitive behavior, oromotor coordination, and inhibitory postingestive feedback. Licking microstructure responses in PBNx rats after Ex4 treatment were similar to sham-treated rats with the exception of inhibitory postingestive feedback measures. Together, the results suggest that the PBN critically contributes to the hypophagic effects of systemically delivered GLP-1R agonists by enhancing visceral feedback.

Microstructural analysis of rat ethanol and water drinking patterns using a modified operant self-administration model

BACKGROUND

Ethanol drinking pattern has emerged as an important factor in the development, maintenance, and health consequences of alcohol use disorders in humans. The goal of these studies was to further our understanding of this important factor through refinement of an operant rodent model of ethanol consumption capable of drinking pattern microstructural analysis. We evaluated measures of total consumption, appetitive behavior, and drinking microstructure for ethanol and water at baseline and assessed alterations induced by two treatments previously shown to significantly alter gross ethanol appetitive and consummatory behaviors in opposing directions.

METHODS

Male Long-Evans rats were trained on an FR1 operant paradigm which allowed for continuous liquid access until an 8 second pause in consumption resulted in termination of liquid access. Total appetitive and consummatory behaviors were assessed in addition to microstructural drinking pattern for both ethanol and water during a five day baseline drinking period, after chronic intermittent ethanol vapor exposure, and following administration of a cannabinoid receptor antagonist SR141716a.

RESULTS

As in previous operant studies, ethanol vapor exposure resulted in increases in ethanol-directed responding, total consumption, and rate of intake. Further, striking differential alterations to ethanol and water bout size, duration, and lick pattern occurred consistent with alterations in hedonic evaluation. Vapor additionally specifically reduced the number of ethanol-directed lever presses which did not result in subsequent consumption. SR141716a administration reversed many of these effects.

CONCLUSIONS

The addition of microstructural analysis to operant self-administration by rodents provides a powerful and translational tool for the detection of specific alterations in ethanol drinking pattern which may enable insights into neural mechanisms underlying specific components of drug consumption.

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.

Roux-en-Y gastric bypass does not affect daily water intake or the drinking response to dipsogenic stimuli in rats

Bariatric surgery is currently the most effective treatment for severe obesity, and Roux-en-Y gastric bypass (RYGB) is the most common approach in the United States and worldwide. Many studies have documented the changes in body weight, food intake, and glycemic control associated with the procedure. Although dehydration is commonly listed as a postoperative complication, little focus has been directed to testing the response to dipsogenic treatments after RYGB. Accordingly, we used a rat model of RYGB to test for procedure-induced changes in daily water intake and in the response to three dipsogenic treatments: central administration of ANG II, peripheral injection of hypertonic saline, and overnight water deprivation. We did not find any systematic differences in daily water intake of sham-operated and RYGB rats, nor did we find any differences in the response to the dipsogenic treatments. The results of these experiments suggest that RYGB does not impair thirst responses and does not enhance any satiating effect of water intake. Furthermore, these data support the current view that feedback from the stomach is unnecessary for the termination of drinking behavior and are consistent with a role of orosensory or postgastric feedback.

Moxonidine into the lateral parabrachial nucleus modifies postingestive signals involved in sodium intake control

The activation of α2-adrenoceptors with bilateral injections of moxonidine (α2-adrenoceptor and imidazoline receptor agonist) into the lateral parabrachial nucleus (LPBN) increases 1.8% NaCl intake induced by treatment with furosemide (FURO)+captopril (CAP) subcutaneously. In the present study, we analyzed licking microstructure during water and 1.8% NaCl intake to investigate the changes in orosensory and postingestive signals produced by moxonidine injected into the LPBN. Male Sprague-Dawley rats were treated with FURO+CAP combined with bilateral injections of vehicle or moxonidine (0.5 nmol/0.2 μl) into the LPBN. Bilateral injections of moxonidine into the LPBN increased FURO+CAP-induced 1.8% NaCl intake, without changing water intake. Microstructural analysis of licking behavior found that this increase in NaCl intake was a function of increased number of licking bursts from 15 to 75 min of the test (maximum of 49±9 bursts/bin, vs. vehicle: 2±2 bursts/bin). Analysis of the first 15 min of the test, when most of the licking behavior occurred, found no effect of moxonidine on the number of licks/burst for sodium intake (24±5 licks/burst, vs. vehicle: 27±8 licks/burst). This finding suggests that activation of α2-adrenoceptors in the LPBN affects postingestive signals that are important to inhibit and limit sodium intake by FURO+CAP-treated rats.

When do we eat? Ingestive behavior, survival, and reproductive success

The neuroendocrinology of ingestive behavior is a topic central to human health, particularly in light of the prevalence of obesity, eating disorders, and diabetes. The study of food intake in laboratory rats and mice has yielded some useful hypotheses, but there are still many gaps in our knowledge. Ingestive behavior is more complex than the consummatory act of eating, and decisions about when and how much to eat usually take place in the context of potential mating partners, competitors, predators, and environmental fluctuations that are not present in the laboratory. We emphasize appetitive behaviors, actions that bring animals in contact with a goal object, precede consummatory behaviors, and provide a window into motivation. Appetitive ingestive behaviors are under the control of neural circuits and neuropeptide systems that control appetitive sex behaviors and differ from those that control consummatory ingestive behaviors. Decreases in the availability of oxidizable metabolic fuels enhance the stimulatory effects of peripheral hormones on appetitive ingestive behavior and the inhibitory effects on appetitive sex behavior, putting a new twist on the notion of leptin, insulin, and ghrelin "resistance." The ratio of hormone concentrations to the availability of oxidizable metabolic fuels may generate a critical signal that schedules conflicting behaviors, e.g., mate searching vs. foraging, food hoarding vs. courtship, and fat accumulation vs. parental care. In species representing every vertebrate taxa and even in some invertebrates, many putative "satiety" or "hunger" hormones function to schedule ingestive behavior in order to optimize reproductive success in environments where energy availability fluctuates.

Overexpression of neuropeptide Y in the dorsomedial hypothalamus increases trial initiation but does not significantly alter concentration-dependent licking to sucrose in a brief-access taste test

Evidence in the literature raises the possibility that alterations in neuropeptide Y (NPY) in the dorsomedial hypothalamus (DMH) may contribute to hyperphagia leading to body weight gain. Previously, we have shown that compared to AAVGFP controls, adeno-associated virus (AAV)-mediated overexpression of NPY in the DMH of lean rats resulted in significantly higher body weight gain that was attributed to increased food intake, and this was further exacerbated by a high-fat diet. Here, we tested AAVNPY and AAVGFP control rats in a brief-access taste procedure (10-s trials, 30-min sessions) to an array of sucrose concentrations under ad libitum and partial food and water access conditions. The test allows for some segregation of the behavioral components by providing a measure of trial initiation (appetitive) and unconditioned licks at each concentration (consummatory). Consistent with previous findings suggesting that NPY has a primary effect on appetitive function, overexpression of DMH NPY did not significantly alter concentration-dependent licking response to sucrose but when tested in a non-restricted food and water schedule, AAVNPY rats initiated significantly more sucrose trials compared to AAVGFP controls in a brief-access taste test.

Activation of membrane-associated estrogen receptors decreases food and water intake in ovariectomized rats

Estradiol (E2) decreases food and water intake in a variety of species, including rats. Available evidence suggests that this is mediated by genomic mechanisms that are most often attributed to nuclear estrogen receptors. More recent studies indicate that membrane-associated estrogen receptors (mERs) also can influence gene expression through the activation of transcription factors, yet it is unclear whether mERs are involved in mediating the hypophagic and antidipsetic effects of E2. In the present experiments, we injected E2 or a membrane-impermeable form of E2 (E2-BSA) into the lateral cerebral ventricle of ovariectomized female rats and evaluated the effect on 23 h food and water intake. First, we found that higher doses of E2 were necessary to reduce water intake than were sufficient to reduce food intake. Analysis of drinking microstructure revealed that the decrease in water intake after E2 treatment was mediated by both a decrease in burst number and burst size. Next, the activation of mERs with E2-BSA decreased both overnight food and water intake and analysis of drinking microstructure indicated that the decreased water intake resulted from a decrease in burst number. Finally, E2-BSA did not condition a taste aversion, suggesting that the inhibitory effects on food and water intake were not secondary to malaise. Together these findings suggest that activation of mERs is sufficient to decrease food and water intake in female rats.

Inactivation of the median raphe nucleus increases intake of sucrose solutions: a microstructural analysis

Previous studies have shown that microinjections of the GABA-A agonist muscimol into the median raphe nucleus (MR) result in large increases in the intake of solid foods. In the current study, we used microstructural techniques to characterize the effects of intra-MR muscimol injections on the consumption of either a 0.05 M or a 0.29 M sucrose solution. After injections of either saline or muscimol, animals consumed more of the 0.29 M than the 0.05 M solution, an effect which resulted primarily from increases in the initial rate of consumption with no change in the rate at which licking decayed across the test session. In contrast, intra-MR muscimol injections had little effect on the initial licking rate, but greatly increased meal duration, indicating that this treatment affected ingestion in a different way than did altering the sucrose concentration. Muscimol injections produced a significantly larger increase in the intake of the 0.29 M than of the 0.05 M solution. Intra-MR muscimol injections did not alter the within burst rate of licking, suggesting that they did not affect the functioning of the licking pattern generator. In contrast, these injections did increase the number of licks contained within "clusters," that is groups of licks separated from each other by intervals of more than 0.5 sec. These findings show that inactivation of the MR produces a powerful effect on the intake of liquid diets, and that the nature of this effect is different from that produced here by changes in sucrose concentration and from those reported after pharmacological manipulations of a number of other brain systems. We additionally discuss several theoretical issues arising in the interpretation of microstructural data. (PsycINFO Database Record (c) 2011 APA, all rights reserved).

Centrally injected kisspeptin reduces food intake by increasing meal intervals in mice

Kisspeptin is distributed not only in brain areas for regulating reproduction but also in nuclei involved in feeding control. Whether kisspeptin alters food intake is unknown in mice. We examined how kisspeptin-10 influences feeding after intracerebroventricular injection in mice using automated monitoring. Kisspeptin-10 (0.3, 1, and 3 μg/mouse) dose-dependently inhibited the feeding response to an overnight fast by 50, 95, and 90% respectively, during the 2-3 h period postinjection. The 1μg/mouse dose reduced the 4-h cumulative food intake by 28% whereas intraperitoneal injection (10 μg/mouse) did not. The decreased 4-h food intake was due to reduced meal frequency (-45%/4 h), whereas meal size and gastric emptying were not altered. These data suggest that kisspeptin may be a negative central regulator of feeding by increasing satiety.

Dehydration-anorexia derives from a reduction in meal size, but not meal number

The anorexia that results from extended periods of cellular dehydration is an important physiological adaptation that limits the intake of osmolytes from food and helps maintain the integrity of fluid compartments. The ability to experimentally control both the development and reversal of anorexia, together with the understanding of underlying hormonal and neuropeptidergic signals, makes dehydration (DE)-anorexia a powerful model for exploring the interactions of neural networks that stimulate and inhibit food intake. However, it is not known which meal parameters are affected by cellular dehydration to generate anorexia. Here we use continuous and high temporal resolution recording of food and fluid intake, together with a drinking-explicit method of meal pattern analysis to explore which meal parameters are modified during DE-anorexia. We find that the most important factor responsible for DE-anorexia is the failure to maintain feeding behavior once a meal has started, rather than the ability to initiate a meal, which remains virtually intact. This outcome is consistent with increased sensitivity to satiation signals and post-prandial satiety mechanisms. We also find that DE-anorexia significantly disrupts the temporal distribution of meals across the day so that the number of nocturnal meals gradually decreases while diurnal meal number increases. Surprisingly, once DE-anorexia is reversed this temporal redistribution is maintained for at least 4 days after normal food intake has resumed, which may allow increased daily food intake even after normal satiety mechanisms are reinstated. Therefore, DE-anorexia apparently develops from a selective targeting of those neural networks that control meal termination, whereas meal initiation mechanisms remain viable.

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.

Hyperphagia induced by sucrose: relation to circulating and CSF glucose and corticosterone and orexigenic peptides in the arcuate nucleus

Sucrose-rich diets compared to starch-rich diets are known to stimulate overeating under chronic conditions. The present study in normal-weight rats established an acute "preload-to-test meal" paradigm for demonstrating sucrose-induced hyperphagia and investigating possible mechanisms that mediate this behavioral phenomenon. In this acute paradigm, the rats were first given a small (15 kcal) sucrose preload (30% sucrose) for 30 min compared to an equicaloric, starch preload (25% starch with 5% sucrose) and then allowed to freely consume a subsequent test meal of lab chow. The sucrose preload, when compared to a starch preload equal in energy density and palatability, consistently increased food intake in the subsequent test meal occurring between 60 and 120 min after the end of the preload. Measurements of hormones, metabolites and hypothalamic peptides immediately preceding this hyperphagia revealed marked differences between the sucrose vs starch groups that could contribute to the increase in food intake. Whereas the sucrose group compared to the starch group immediately after the preload (at 10 min) had elevated levels of glucose in serum and cerebrospinal fluid (CSF) along with reduced expressions of neuropeptide Y (NPY) and agouti-related protein (AgRP) in the arcuate nucleus (ARC), the subsequent effects (at 30-60 min) just preceding the test meal hyperphagia were the reverse. Along with lower levels of glucose, they included markedly elevated serum and CSF levels of corticosterone and mRNA levels of NPY and AgRP in the ARC. In addition to establishing an animal model for sucrose-induced hyperphagia, these results demonstrate peripheral and central mechanisms that may mediate this behavioral phenomenon.

Site-specific attenuation of food intake but not the latency to eat after hypothalamic injections of neuropeptide Y in dehydrated-anorexic rats

Anorexia that accompanies cellular dehydration in rats (DE-anorexia) offers a relatively simple model for investigating the functional organization of neural mechanisms that can suppress feeding during dehydration. Previous studies strongly suggest that the inputs that drive ingestive behavior control neurons in the paraventricular nucleus of the hypothalamus (PVH) and lateral hypothalamic area (LHA) remain active during DE-anorexia. Here we examine whether these two regions retain their sensitivity to neuropeptide Y (NPY). NPY is an important component in two major feeding-related inputs from the arcuate nucleus and the hindbrain. We found that intake responses to NPY injections in the LHA and PVH were suppressed in DE-anorexia, but the PVH remained less sensitive to the effects of NPY than the LHA in DE-anorexic animals. Indeed the higher dose of NPY (238 pmol) completely overcame shorter periods of DE-anorexia when injected into the LHA but not the PVH. However, the latency to eat after NPY injections remained unchanged from control animals, regardless of NPY dose, injection location, or intensity of anorexia. Furthermore, the onset and size of the strong and rapidly induced compensatory feeding that follows the return of water to DE-anorexic animals was also unaffected by any NPY injections. These data support the hypothesis that DE-anorexia develops as a consequence of the premature termination of regularly initiated meals, which perhaps involves processes that alter the sensitivity of satiety mechanisms downstream to the PVH and LHA.

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.

The taste of sugars

Sugars evoke a distinctive perceptual quality ("sweetness" in humans) and are generally highly preferred. The neural basis for these phenomena is reviewed for rodents, in which detailed electrophysiological measurements have been made. A receptor has been identified that binds sweeteners and activates G-protein-mediated signaling in taste receptor cells, which leads to changes in neural firing rates in the brain, where perceptions of taste quality, intensity, and palatability are generated. Most cells in gustatory nuclei are broadly tuned, so quality perception presumably arises from patterns of activity across neural populations. However, some manipulations affect only the most sugar-oriented cells, making it useful to consider them as a distinct neural subtype. Quality perception may also arise partly due to temporal patterns of activity to sugars, especially within sugar-oriented cells that give large but delayed responses. Non-specific gustatory neurons that are excited by both sugars and unpalatable stimuli project to ventral forebrain areas, where neural responses provide a closer match with behavioral preferences. This transition likely involves opposing excitatory and inhibitory influences by different subgroups of gustatory cells. Sweeteners are generally preferred over water, but the strength of this preference can vary across time or between individuals, and higher preferences for sugars are often associated with larger taste-evoked responses.

Differential effects of recombinant adeno-associated virus-mediated neuropeptide Y overexpression in the hypothalamic paraventricular nucleus and lateral hypothalamus on feeding behavior

It is well known that neuropeptide Y (NPY) increases food intake. The hypothalamic paraventricular nucleus (PVN) and the lateral hypothalamus (LH) are both involved in the acute, hyperphagic effects of NPY. Although it is obvious that increased energy intake may lead to obesity, it is less understood which aspects of feeding behavior are affected and whether one or multiple neural sites mediate the effects of long-term increased NPY signaling. By long-term overexpressing NPY in either the PVN or the LH, we uncovered brain site-specific effects of NPY on meal frequency, meal size, and diurnal feeding patterns. In rats injected with adeno-associated virus-NPY in the PVN, increased food intake resulted from an increase in the amount of meals consumed, whereas in rats injected in the LH, increased food intake was attributable to increased meal size. Interestingly, food intake and body weight gain were only temporarily increased in PVN-injected rats, whereas in LH-injected rats hyperphagia and body weight gain remained for the entire 50 d. Moreover, in LH-NPY rats, but not in PVN-NPY rats, diurnal rhythmicity with regard to food intake and body core temperature was lost. These data clearly show that the NPY system differentially regulates energy intake and energy expenditure in the PVN and LH, which together adjust energy balance.

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

Melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are orexigenic peptides found in hypothalamic neurons that project throughout the forebrain and hindbrain. The effects of fourth ventricle (4V) infusions of NPY (5 microg) and MCH (5 microg) on licking for water, 4 mM saccharin, and sucrose (0.1 and 1.0 M) solutions were compared to identify the contributions of each peptide to hindbrain-stimulated feeding. NPY increased mean meal size only for the sucrose solutions, suggesting that caloric feedback or taste quality is pertinent to the orexigenic effect; MCH infusions under identical testing conditions failed to produce increases for any tastant. A second experiment also observed no intake or licking effects after MCH doses up to 15 microg, supporting the conclusion that MCH-induced orexigenic responses require forebrain stimulation. A third experiment compared the 4V NPY results with those obtained after NPY infusions (5 microg) into the third ventricle (3V). In contrast to the effects observed after the 3V NPY injections and previously reported forebrain intracerebroventricular (ICV) NPY infusion studies, 4V NPY failed to increase meal frequency for any taste solution or ingestion rate in the early phases of the sucrose meals. Overall, 4V NPY responses were limited to intrameal behavioral processes, whereas forebrain ICV NPY stimulation elicited both consummatory and appetitive responses. The dissociation between MCH and NPY effects observed for 4V injections is consistent with reports that forebrain ICV injections of MCH and NPY produced nearly dichotomous effects on the pattern of licking microstructure, and, collectively, the results indicate that the two peptides have separate sites of feeding action in the brain.