Dietary fat stimulates endogenous enkephalin and dynorphin in the paraventricular nucleus: role of circulating triglycerides

A Starch- and Sucrose-Reduced Diet in Irritable Bowel Syndrome Leads to Lower Circulating Levels of PAI-1 and Visfatin: A Randomized Controlled Study

Irritable bowel syndrome (IBS) is characterized by gastrointestinal symptoms. Overweight and increased risk of metabolic syndromes/diabetes are observed in IBS, conditions associated with plasminogen activator inhibitor-1 (PAI-1) and visfatin. The aim of this study was to measure blood levels of AXIN1, cholecystokinin (CCK), enkephalin, ghrelin, neuropeptide Y (NPY), PAI-1, and visfatin before and after a 4-week intervention with a starch- and sucrose-reduced diet (SSRD). A total of 105 IBS patients were randomized to either SSRD (n = 80) or ordinary diet (n = 25). Questionnaires were completed, and blood was analyzed for AXIN1 and hormones. AXIN1 (p = 0.001) and active ghrelin levels (p = 0.025) were lower in IBS than in healthy volunteers at baseline, whereas CCK and enkephalin levels were higher (p < 0.001). In the intervention group, total IBS-symptom severity score (IBS-SSS), specific gastrointestinal symptoms, psychological well-being, and the influence of intestinal symptoms on daily life were improved during the study, and weight decreased (p < 0.001 for all), whereas only constipation (p = 0.045) and bloating (p = 0.001) were improved in the control group. PAI-1 levels tended to be decreased in the intervention group (p = 0.066), with a difference in the decrease between groups (p = 0.022). Visfatin levels were decreased in the intervention group (p = 0.007). There were few correlations between hormonal levels and symptoms. Thus, this diet not only improves IBS symptoms but also seems to have a general health-promoting effect.

Hypothalamic Kisspeptin Neurons and the Control of Homeostasis

Hypothalamic kisspeptin (Kiss1) neurons provide indispensable excitatory transmission to gonadotropin-releasing hormone (GnRH) neurons for the coordinated release of gonadotropins, estrous cyclicity, and ovulation. But maintaining reproductive functions is metabolically demanding so there must be a coordination with multiple homeostatic functions, and it is apparent that Kiss1 neurons play that role. There are 2 distinct populations of hypothalamic Kiss1 neurons, namely arcuate nucleus (Kiss1ARH) neurons and anteroventral periventricular and periventricular nucleus (Kiss1AVPV/PeN) neurons in rodents, both of which excite GnRH neurons via kisspeptin release but are differentially regulated by ovarian steroids. Estradiol (E2) increases the expression of kisspeptin in Kiss1AVPV/PeN neurons but decreases its expression in Kiss1ARH neurons. Also, Kiss1ARH neurons coexpress glutamate and Kiss1AVPV/PeN neurons coexpress gamma aminobutyric acid (GABA), both of which are upregulated by E2 in females. Also, Kiss1ARH neurons express critical metabolic hormone receptors, and these neurons are excited by insulin and leptin during the fed state. Moreover, Kiss1ARH neurons project to and excite the anorexigenic proopiomelanocortin neurons but inhibit the orexigenic neuropeptide Y/Agouti-related peptide neurons, highlighting their role in regulating feeding behavior. Kiss1ARH and Kiss1AVPV/PeN neurons also project to the preautonomic paraventricular nucleus (satiety) neurons and the dorsomedial nucleus (energy expenditure) neurons to differentially regulate their function via glutamate and GABA release, respectively. Therefore, this review will address not only how Kiss1 neurons govern GnRH release, but how they control other homeostatic functions through their peptidergic, glutamatergic and GABAergic synaptic connections, providing further evidence that Kiss1 neurons are the key neurons coordinating energy states with reproduction.

Interplay between adipose tissue secreted proteins, eating behavior and obesity

Purpose

Adipokines may play an important role in the complex etiology of human obesity and its metabolic complications. Here, we analyzed the relationship between 15 adipokines, eating behavior and body-mass index (BMI).

Methods

The study included 557 participants of the Sorbs (62.1% women, 37.9% men) and 3101 participants of the population-based LIFE-Adult cohorts (53.4% women, 46.4% men) who completed the German version of the Three-Factor-Eating Questionnaire to assess the eating behavior types cognitive restraint, disinhibition and hunger. Serum levels of 15 adipokines, including adiponectin, adipocyte fatty acid-binding protein (AFABP), angiopoietin-related growth factor (AGF), chemerin, fibroblast growth factor (FGF)-19, FGF-21, FGF-23, insulin-like growth factor (IGF)-1, interleukin (IL) 10, irisin, progranulin, vaspin, pro-neurotensin (pro-NT), pro-enkephalin (PENK) and leptin were measured. Based on significant correlations between several adipokines with different eating behavior items and BMI, we conducted mediation analyses, considering the eating behavior items as potential mediation variable towards BMI.

Results

Here, we found that the positive association between chemerin, AFABP or leptin and BMI in Sorbian women was mediated by higher restraint or disinhibited eating, respectively. Additionally, in Sorbian women, the negative relation between IGF-1 and BMI was mediated by higher disinhibition and the positive link between AGF and BMI by lower disinhibition. In Sorbian men, the negative relationship between PENK and BMI was mediated by lower disinhibition and hunger, whereas the negative relation between IGF-1 and BMI was mediated by higher hunger. In the LIFE-Adult women´s cohort, associations between chemerin and BMI were mediated by decreased hunger or disinhibition, respectively, whereas relations between PENK and BMI were fully mediated by decreased disinhibition.

Conclusion

Our study suggests that adipokines such as PENK, IGF-1, chemerin, AGF, AFABP and leptin might affect the development of obesity by directly modifying individual eating behavior. Given the observational nature of the study, future experimental or mechanistic work is warranted.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00394-021-02687-w.

Arcuate and Preoptic Kisspeptin Neurons Exhibit Differential Projections to Hypothalamic Nuclei and Exert Opposite Postsynaptic Effects on Hypothalamic Paraventricular and Dorsomedial Nuclei in the Female Mouse

Kisspeptin (Kiss1) neurons provide indispensable excitatory input to gonadotropin-releasing hormone (GnRH) neurons, which is important for the coordinated release of gonadotropins, estrous cyclicity and ovulation. However, Kiss1 neurons also send projections to many other brain regions within and outside the hypothalamus. Two different populations of Kiss1 neurons, one in the arcuate nucleus (Kiss1^ARH) and another in the anteroventral periventricular nucleus (AVPV) and periventricular nucleus (PeN; Kiss1^AVPV/PeN) of the hypothalamus are differentially regulated by ovarian steroids, and are believed to form direct contacts with GnRH neurons as well as other neurons. To investigate the projection fields from Kiss1^AVPV/PeN and Kiss1^ARH neurons in female mice, we used anterograde projection analysis, and channelrhodopsin-assisted circuit mapping (CRACM) to explore their functional input to select target neurons within the paraventricular (PVH) and dorsomedial (DMH) hypothalamus, key preautonomic nuclei. Cre-dependent viral (AAV1-DIO-ChR2 mCherry) vectors were injected into the brain to label the two Kiss1 neuronal populations. Immunocytochemistry (ICC) for mCherry and neuropeptides combined with confocal microscopy was used to determine the projection-fields of both Kiss1 neuronal groups. Whole-cell electrophysiology and optogenetics were used to elucidate the functional input to the PVH and DMH. Our analysis revealed many common but also several clearly separate projection fields between the two different populations of Kiss1 neurons. In addition, optogenetic stimulation of Kiss1 projections to PVH prodynorphin, Vglut2 and DMH CART-expressing neurons, revealed excitatory glutamatergic input from Kiss1^ARH neurons and inhibitory GABAergic input from Kiss1^AVPV/PeN neurons. Therefore, these steroid-sensitive Kiss1 neuronal groups can differentially control the excitability of target neurons to coordinate autonomic functions with reproduction.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Enkephalinase activity is modified and correlates with fatty acids in frontal cortex depending on fish, olive or coconut oil used in the diet

Objective. Enkephalins are neuropeptides involved in functions such as pain modulation and/ or cognitive processes. It has been reported that dietary fat modifies enkephalins in the brain. Since enkephalins are hydrolyzed by enkephalinases, the study of the influence of dietary fats, differing in their degree of saturation, on brain fatty acids content and enkephalinase activity is important to understand its regulatory role on neuropeptides under different type of diets.

Methods. We analyzed enkephalinase activity, assayed with alanine-β-naphthylamide as sub-strate, in frontal cortex of adult male rats fed diets supplemented with fish oil, olive oil or coconut oil, which markedly differed in the saturation of their fatty acids.

Results. Rats fed a diet enriched with coconut oil had lower soluble enkephalinase activity than the group fed olive oil (p<0.01) and fish oil (p<0.05) whereas rats fed a diet enriched with fish oil had lower membrane-bound enkephalinase activity than the group fed with olive (p<0.001) or coconut oil (p<0.05). Significant negative correlations were observed between certain fatty acids and enkephalinase activities in the groups fed with olive and coconut oils. No correlations were observed in the group fed with fish oil.

Conclusions. Dietary fat modifies enkephalinase activity in the frontal cortex depending on the degree of saturation of the used oil. It is postulated that the functions, in which enkephalins are involved, such as pain modulation or cognitive functions, may also be affected according to the type of oil used in the diet.

Effects of Orchidectomy and Testosterone Replacement on Numbers of Kisspeptin-, Neurokinin B-, and Dynorphin A-Immunoreactive Neurones in the Arcuate Nucleus of the Hypothalamus in Obese and Diabetic Rats

Neurones expressing kisspeptin, neurokinin B and dynorphin A, located in the arcuate nucleus of the hypothalamus (ARC), are important regulators of reproduction. Their functions depend on metabolic and hormonal status. We hypothesised that male rats with high-fat diet-induced obesity (DIO) and/or streptozotocin-induced diabetes mellitus type 1 (DM1) and type 2 (DM2) will have alterations in numbers of immunoreactive (-IR) cells: kisspeptin-IR and/or neurokinin B-IR and dynorphin A-IR neurones in the ARC in the sham condition. In addition, orchidectomy alone (ORX) and with testosterone treatment (ORX+T) will unmask possible deficits in the response of these neurones in DIO, and/or DM1 and DM2 rats. Rats were assigned to four groups: a control (C) and one diabetic group (DM1) were fed a regular chow diet, whereas the obese group (DIO) and the other diabetic group (DM2) were fed a high-fat diet. To induce diabetes, streptozotocin was injected. After 6 weeks, each group was divided into three subgroups: ORX, ORX+T and sham. After another 2 weeks, metabolic and hormonal profiles were assessed and immunocytochemistry was performed. We found that: (1) under sham conditions: (i) DM1 and DM2 animals had higher numbers of kisspeptin-IR cells than controls and (ii) DM2 rats had increased numbers of neurokinin B-IR and dynorphin A-IR cells compared to C animals; (2) ORX and ORX+T treatments unmasked deficits of the studied neurones in DM1 and DM2 but not in DIO animals; and (3) DIO, DM1 and DM2 rats had altered metabolic and hormonal profiles, in particular decreased levels of testosterone. We concluded that alterations in numbers of kisspeptin-IR and neurokinin B-IR neurones in the ARC and their response to ORX and ORX+T may account for disruptions of metabolic and reproductive functions in diabetic but not in obese rats.

Eating moderates the link between body mass index and perceived social connection

Some studies have demonstrated that heavier people perceive themselves as lacking social connections, whereas others have not. The current study investigated whether eating alters the link between body mass index (BMI) and perceived social connection, providing one explanation for inconsistencies across previous studies. Participants were instructed to refrain from eating or drinking anything except water after 9 p.m. the prior night. Upon arrival at the lab, participants were assigned to the food (n = 63) or no food (n = 110) condition. They also provided a saliva sample that was assayed for ghrelin (an appetite-relevant hormone), and completed a series of questionnaires about their relationships. Participants with a higher BMI felt more socially disconnected than people with a lower BMI, but only among those who had not recently eaten. BMI and perceived social disconnection were unrelated among people who had recently eaten. These results were consistent across multiple measures of perceived social disconnection, and also across the experimental manipulation and continuously measured ghrelin.

Hypothalamic kappa opioid receptor mediates both diet-induced and melanin concentrating hormone-induced liver damage through inflammation and endoplasmic reticulum stress

The opioid system is widely known to modulate the brain reward system and thus affect the behavior of humans and other animals, including feeding. We hypothesized that the hypothalamic opioid system might also control energy metabolism in peripheral tissues. Mice lacking the kappa opioid receptor (κOR) and adenoviral vectors overexpressing or silencing κOR were stereotaxically delivered in the lateral hypothalamic area (LHA) of rats. Vagal denervation was performed to assess its effect on liver metabolism. Endoplasmic reticulum (ER) stress was inhibited by pharmacological (tauroursodeoxycholic acid) and genetic (overexpression of the chaperone glucose-regulated protein 78 kDa) approaches. The peripheral effects on lipid metabolism were assessed by histological techniques and western blot. We show that in the LHA κOR directly controls hepatic lipid metabolism through the parasympathetic nervous system, independent of changes in food intake and body weight. κOR colocalizes with melanin concentrating hormone receptor 1 (MCH-R1) in the LHA, and genetic disruption of κOR reduced melanin concentrating hormone-induced liver steatosis. The functional relevance of these findings was given by the fact that silencing of κOR in the LHA attenuated both methionine choline-deficient, diet-induced and choline-deficient, high-fat diet-induced ER stress, inflammation, steatohepatitis, and fibrosis, whereas overexpression of κOR in this area promoted liver steatosis. Overexpression of glucose-regulated protein 78 kDa in the liver abolished hypothalamic κOR-induced steatosis by reducing hepatic ER stress.

CONCLUSIONS

This study reveals a novel hypothalamic-parasympathetic circuit modulating hepatic function through inflammation and ER stress independent of changes in food intake or body weight; these findings might have implications for the clinical use of opioid receptor antagonists. (Hepatology 2016;64:1086-1104).

High Level of Fasting Plasma Proenkephalin-A Predicts Deterioration of Kidney Function and Incidence of CKD

High levels of proenkephalin-A (pro-ENK) have been associated with decreased eGFR in an acute setting. Here, we examined whether pro-ENK levels predict CKD and decline of renal function in a prospective cohort of 2568 participants without CKD (eGFR>60 ml/min per 1.73 m²) at baseline. During a mean follow-up of 16.6 years, 31.7% of participants developed CKD. Participants with baseline pro-ENK levels in the highest tertile had significantly greater yearly mean decline of eGFR (P_trend<0.001) and rise of cystatin C (P_trend=0.01) and creatinine (P_trend<0.001) levels. Furthermore, compared with participants in the lowest tertile, participants in the highest tertile of baseline pro-ENK concentration had increased CKD incidence (odds ratio, 1.51; 95% confidence interval, 1.18 to 1.94) when adjusted for multiple factors. Adding pro-ENK to a model of conventional risk factors in net reclassification improvement analysis resulted in reclassification of 14.14% of participants. Genome-wide association analysis in 4150 participants of the same cohort revealed the strongest association of pro-ENK levels with rs1012178 near the PENK gene, where the minor T-allele associated with a 0.057 pmol/L higher pro-ENK level per allele (P=4.67x10^-21). Furthermore, the T-allele associated with a 19% increased risk of CKD per allele (P=0.03) and a significant decrease in the instrumental variable estimator for eGFR (P<0.01) in a Mendelian randomization analysis. In conclusion, circulating plasma pro-ENK level predicts incident CKD and may aid in identifying subjects in need of primary preventive regimens. Additionally, the Mendelian randomization analysis suggests a causal relationship between pro-ENK level and deterioration of kidney function over time.

A contemporary approach to body mass regulation mechanisms

Recently, a lot of factors and mechanisms regulating body mass have been discovered, although there are still many unknowns. Their effect on the development of many diseases related to nutritional disorders (obesity, anorexia, celiac disease, inflammatory bowel disease) means that the understanding of these mechanisms will make it possible to determine new therapeutic goals and create new medicinal products. This is even more important because nowadays there is no effective medication to cure nutritional disorders. It is necessary to conduct further research to evaluate dependencies and relationships between particular hormones and to study newly discovered substances so that we could progress towards achieving the overall objectives while keeping the ultimate goals in mind.

Consumption of Substances of Abuse during Pregnancy Increases Consumption in Offspring: Possible Underlying Mechanisms

Correlative human observational studies on substances of abuse have been highly dependent on the use of rodent models to determine the neuronal and molecular mechanisms that control behavioral outcomes. This is particularly true for gestational exposure to non-illicit substances of abuse, such as excessive dietary fat, ethanol, and nicotine, which are commonly consumed in our society. Exposure to these substances during the prenatal period has been shown in offspring to increase their intake of these substances, induce other behavioral changes, and affect neurochemical systems in several brain areas that are known to control behavior. More importantly, emerging studies are linking the function of the immune system to these neurochemicals and ingestion of these abused substances. This review article will summarize the prenatal rodent models used to study developmental changes in offspring caused by prenatal exposure to dietary fat, ethanol, or nicotine. We will discuss the various techniques used for the administration of these substances into rodents and summarize the published outcomes induced by prenatal exposure to these substances. Finally, this review will cover some of the recent evidence for the role of immune factors in causing these behavioral and neuronal changes.

Relationship of the Chemokine, CXCL12, to Effects of Dietary Fat on Feeding-Related Behaviors and Hypothalamic Neuropeptide Systems

The intake of a high fat diet (HFD), in addition to stimulating orexigenic neuropeptides in the hypothalamus while promoting overeating and reducing locomotor behavior, is known to increase inflammatory mediators that modulate neuronal systems in the brain. To understand the involvement of chemokines in the effects of a HFD, we examined in rats whether HFD intake affects a specific chemokine, CXCL12, and its receptors, CXCR4 and CXCR7, in the hypothalamus together with the neuropeptides and whether CXCL12 itself acts similarly to a HFD in stimulating the neuropeptides and altering ingestion and locomotor behavior. Compared to low-fat chow, a HFD for 5 days significantly increased the expression of CXCL12 and its receptors, in both the paraventricular nucleus (PVN) where the neuropeptides enkephalin (ENK) and galanin were also stimulated and the perifornical lateral hypothalamus (PFLH) where orexin (OX) and melanin-concentrating hormone (MCH) were increased. In contrast, the HFD had no impact on expression of CXCL12 or its receptors in the arcuate nucleus (ARC) where the carbohydrate-related peptide, neuropeptide Y (NPY), was suppressed. Analysis of protein levels revealed a similar stimulatory effect of a HFD on CXCL12 levels in the PVN and PFLH, as well as in blood, and an increase in the number of CXCR4-positive cells in the PVN. In the ARC, in contrast, levels of CXCL12 and number of CXCR4-positive cells were too low to measure. When centrally administered, CXCL12 was found to have similar effects to a HFD. Injection of CXCL12 into the third cerebral ventricle immediately anterior to the hypothalamus significantly stimulated the ingestion of a HFD, reduced novelty-induced locomotor activity, and increased expression of ENK in the PVN where the CXCR4 receptors were dense. It had no impact, however, on NPY in the ARC or on OX and MCH in the PFLH where the CXCR4 receptors were not detected. These results, showing CXCL12 in the hypothalamus to be stimulated by a HFD and to mimic the effects of the HFD where its receptors are located, suggest that this chemokine system may have a role in mediating both the neuronal and behavioral effects induced by a fat-rich diet.

Prenatal fat-rich diet exposure alters responses of embryonic neurons to the chemokine, CCL2, in the hypothalamus

Maternal consumption of a high-fat diet (HFD) during pregnancy is found to stimulate the genesis of hypothalamic orexigenic peptide neurons in the offspring, while HFD intake in adult animals produces a systemic low-grade inflammation which increases neuroimmune factors that may affect neurogenesis and neuronal migration. Building on this evidence and our recent study showing that the inflammatory chemokine, CCL2, stimulates the migration of hypothalamic neurons and expression of orexigenic neuropeptides, we tested here the possibility that prenatal exposure to a HFD in rats affects this chemokine system, both CCL2 and its receptors, CCR2 and CCR4, and alters its actions on hypothalamic neurons, specifically those expressing the neuropeptides, enkephalin (ENK) and galanin (GAL). Using primary dissociated hypothalamic neurons extracted from embryos on embryonic day 19, we found that prenatal HFD exposure compared to chow control actually reduces the expression of CCL2 in these hypothalamic neurons, while increasing CCR2 and CCR4 expression, and also reduces the sensitivity of hypothalamic neurons to CCL2. The HFD abolished the dose-dependent, stimulatory effect of CCL2 on the number of migrated neurons and even shifted its normal stimulatory effect on migrational velocity and distance traveled by control neurons to an inhibition of migration. Further, it abolished the dose-dependent, stimulatory effect of CCL2 on neuronal expression of ENK and GAL. These results demonstrate that prenatal HFD exposure greatly disturbs the functioning of the CCL2 chemokine system in embryonic hypothalamic neurons, reducing its endogenous levels and ability to promote the migration of neurons and their expression of orexigenic peptides.

Delta Opioid Pharmacology in Relation to Alcohol Behaviors

Delta opioid receptors (DORs) are heavily involved in alcohol-mediated processes in the brain. In this chapter we provide an overview of studies investigating how alcohol directly impacts DOR pharmacology and of early studies indicating DOR modulation of alcohol behavior. We will offer a brief summary of the different animal species used in alcohol studies investigating DORs followed by a broader overview of the types of alcohol behaviors modulated by DORs. We will highlight a small set of studies investigating the relationship between alcohol and DORs in analgesia. We will then provide an anatomical overview linking DOR expression in specific brain regions to different alcohol behaviors. In this section, we will provide two models that try to explain how endogenous opioids acting at DORs may influence alcohol behaviors. Next, we will provide an overview of studies investigating certain new aspects of DOR pharmacology, including the formation of heteromers and biased signaling. Finally, we provide a short overview of the genetics of the DORs in relation to alcohol use disorders (AUDs) and a short statement on the potential of using DOR-based therapeutics for treatment of AUDs.

Endogenous opioids and feeding behavior: A decade of further progress (2004-2014). A Festschrift to Dr. Abba Kastin

Functional elucidation of the endogenous opioid system temporally paralleled the creation and growth of the journal, Peptides, under the leadership of its founding editor, Dr. Abba Kastin. He was prescient in publishing annual and uninterrupted reviews on Endogenous Opiates and Behavior that served as a microcosm for the journal under his stewardship. This author published a 2004 review, "Endogenous opioids and feeding behavior: a thirty-year historical perspective", summarizing research in this field between 1974 and 2003. The present review "closes the circle" by reviewing the last 10 years (2004-2014) of research examining the role of endogenous opioids and feeding behavior. The review summarizes effects upon ingestive behavior following administration of opioid receptor agonists, in opioid receptor knockout animals, following administration of general opioid receptor antagonists, following administration of selective mu, delta, kappa and ORL-1 receptor antagonists, and evaluating opioid peptide and opioid receptor changes in different food intake models.

Mechanisms of the anorexia of aging-a review

Many, even healthy, older people fail to adequately regulate food intake and experience loss of weight. Aging-associated changes in the regulation of appetite and the lack of hunger have been termed as the anorexia of aging. The etiology of the anorexia of aging is multi-factorial and includes a combination of physiological changes associated with aging (decline in smell and taste, reduced central and peripheral drive to eat, delayed gastric emptying), pathological conditions (depression, dementia, somatic diseases, medications and iatrogenic interventions, oral-health status), and social factors (poverty, loneliness). However, exact mechanisms of the anorexia of aging remain to be elucidated. Many neurobiological mechanisms may be secondary to age-related changes in body composition and not associated with anorexia per se. Therefore, further studies on pathophysiological mechanisms of the anorexia of aging should employ accurate measurement of body fat and lean mass. The anorexia of aging is associated with protein-energy malnutrition, sarcopenia, frailty, functional deterioration, morbidity, and mortality. Since this symptom can lead to dramatic consequences, early identification and effective interventions are needed. One of the most important goals in the geriatric care is to optimize nutritional status of the elderly.

Stimulatory role of the chemokine CCL2 in the migration and peptide expression of embryonic hypothalamic neurons

Neuroinflammation is a feedback mechanism against infection, with recent studies suggesting a neuromodulatory role. The chemokine, (C-C motif) ligand 2 (CCL2), and its receptor, (C-C motif) receptor type 2 (CCR2), affect neuromodulation and migration in response to damage. Although CCL2 co-localizes with neuropeptides in the hypothalamus that control voluntary behavior, the function of CCL2/CCR2 is unknown. This led us to consider the possibility that CCL2 acting through CCR2, under natural conditions, may affect the migration and peptide levels of hypothalamic neurons that control voluntary behavior. This study used primary embryonic hypothalamic neurons to examine the effect of CCL2 on migratory behavior and on levels of the peptides, enkephalin (ENK) and galanin. Treatment with CCL2 led to a significant, dose-dependent increase in the number of migrated neurons and an increase in the velocity and distance traveled. CCL2 also significantly increased the number of ENK-expressing and CCR2/ENK co-expressing neurons and the percentage of neurons that contain higher levels of ENK. Lastly, CCL2 produced a dose-dependent increase in expression of ENK and galanin. These results provide evidence for a stimulatory effect of CCL2 on embryonic hypothalamic neurons involving changes in migratory behavior, expression, and synthesis of neuropeptides that function in controlling behavior. Our results demonstrate that the chemokine, CCL2, functions through its receptor, CCR2, to stimulate the migration and expression of the orexigenic peptides, enkephalin (ENK) and galanin (GAL), in developing embryonic hypothalamic neurons that are important for controlling ingestive behavior. This evidence reveals broad effects of CCL2 in the developing hypothalamus, showing this chemokine system to be tightly linked to the hypothalamic peptide neurons.

Common effects of fat, ethanol, and nicotine on enkephalin in discrete areas of the brain

Fat, ethanol, and nicotine share a number of properties, including their ability to reinforce behavior and produce overconsumption. To test whether these substances act similarly on the same neuronal populations in specific brain areas mediating these behaviors, we administered the substances short-term, using the same methods and within the same experiment, and measured their effects, in areas of the hypothalamus (HYPO), amygdala (AMYG), and nucleus accumbens (NAc), on mRNA levels of the opioid peptide, enkephalin (ENK), using in situ hybridization and on c-Fos immunoreactivity (ir) to indicate neuronal activity, using immunofluorescence histochemistry. In addition, we examined for comparison another reinforcing substance, sucrose, and also took measurements of stress-related behaviors and circulating corticosterone (CORT) and triglycerides (TG), to determine if they contribute to these substances' behavioral and physiological effects. Adult Sprague-Dawley rats were gavaged three times daily over 5 days with 3.5 mL of water, Intralipid (20% v/v), ethanol (12% v/v), nicotine (0.01% w/v) or sucrose (22% w/v) (approximately 7 kcal/dose), and tail vein blood was collected for measurements of circulating CORT and TG. On day five, animals were sacrificed, brains removed, and the HYPO, AMYG, and NAc processed for single- or double-labeling of ENK mRNA and c-Fos-ir. Fat, ethanol, and nicotine, but not sucrose, increased the single- and double-labeling of ENK and c-Fos-ir in precisely the same brain areas, the middle parvocellular but not lateral area of the paraventricular nucleus, central but not basolateral nucleus of the AMYG, and core but not shell of the NAc. While having little effect on stress-related behaviors or CORT levels, fat, ethanol, and nicotine all increased circulating levels of TG. These findings suggest that the overconsumption of these three substances and their potential for abuse are mediated by the same populations of ENK-expressing neurons in specific nuclei of the hypothalamus and limbic system.

Prenatal exposure to dietary fat induces changes in the transcriptional factors, TEF and YAP, which may stimulate differentiation of peptide neurons in rat hypothalamus

Gestational exposure to a high-fat diet (HFD) stimulates the differentiation of orexigenic peptide-expressing neurons in the hypothalamus of offspring. To examine possible mechanisms that mediate this phenomenon, this study investigated the transcriptional factor, transcription enhancer factor-1 (TEF), and co-activator, Yes-associated protein (YAP), which when inactivated stimulate neuronal differentiation. In rat embryos and postnatal offspring prenatally exposed to a HFD compared to chow, changes in hypothalamic TEF and YAP and their relationship to the orexigenic peptide, enkephalin (ENK), were measured. The HFD offspring at postnatal day 15 (P15) exhibited in the hypothalamic paraventricular nucleus a significant reduction in YAP mRNA and protein, and increased levels of inactive and total TEF protein, with no change in mRNA. Similarly, HFD-exposed embryos at embryonic day 19 (E19) showed in whole hypothalamus significantly decreased levels of YAP mRNA and protein and TEF mRNA, and increased levels of inactive TEF protein, suggesting that HFD inactivates TEF and YAP. This was accompanied by increased density and fluorescence intensity of ENK neurons. A close relationship between TEF and ENK was suggested by the finding that TEF co-localizes with this peptide in hypothalamic neurons and HFD reduced the density of TEF/ENK co-labeled neurons, even while the number and fluorescence intensity of single-labeled TEF neurons were increased. Increased YAP inactivity by HFD was further evidenced by a decrease in number and fluorescence intensity of YAP-containing neurons, although the density of YAP/ENK co-labeled neurons was unaltered. Genetic knockdown of TEF or YAP stimulated ENK expression in hypothalamic neurons, supporting a close relationship between these transcription factors and neuropeptide. These findings suggest that prenatal HFD exposure inactivates both hypothalamic TEF and YAP, by either decreasing their levels or increasing their inactive form, and that this contributes to the stimulatory effect of HFD on ENK expression and possibly the differentiation of ENK-expressing neurons.

Prenatal ethanol exposure stimulates neurogenesis in hypothalamic and limbic peptide systems: possible mechanism for offspring ethanol overconsumption

Exposure to ethanol during the prenatal period contributes to increased alcohol consumption and preference in rodents and increased risk for alcoholism in humans. With studies in adult animals showing the orexigenic peptides, enkephalin (ENK), galanin (GAL) and orexin (OX), to stimulate ethanol consumption, the question addressed here is whether prenatal ethanol alters the development in utero of specific neurons that express these peptides. With reports describing suppressive effects of high doses of ethanol, we examined the offspring of dams gavaged from embryonic day 9 to parturition with a control solution or lower ethanol doses, 1 and 3g/kg/day, known to promote ethanol consumption in the offspring. To understand underlying mechanisms, measurements were taken in postnatal offspring of the expression of ENK in the hypothalamic paraventricular nucleus (PVN) and nucleus accumbens (NAc), GAL in the PVN, and OX in the perifornical lateral hypothalamus (PFLH) using real-time qPCR and in situ hybridization, and also of the cell proliferation marker, 5-bromo-2-deoxyuridine (BrdU), and its double-labeling with either neuronal nuclei (NeuN), a marker of mature neurons, or the peptides. On postnatal day 15 (P15), after two weeks without ethanol, the offspring showed increased expression of ENK in the PVN and NAc core but not shell, GAL in the PVN, and OX in the PFLH. In these same areas, prenatal ethanol compared to control increased the density at birth (P0) of neurons expressing these peptides and at P0 and P15 of neurons double-labeling BrdU and NeuN, indicating increased neurogenesis. These BrdU-positive neurons were found to express ENK, GAL and OX, indicating that prenatal ethanol promotes neurogenesis in these specific peptide systems. There were no changes in gliogenesis or apoptosis. This increase in neurogenesis and density of peptide-expressing neurons suggests the involvement of these hypothalamic and accumbal peptide systems in mediating the increased alcohol consumption observed in prenatal ethanol-exposed offspring.

Developmental changes in embryonic hypothalamic neurons during prenatal fat exposure

Maternal consumption of a fat-rich diet during pregnancy, which causes later overeating and weight gain in offspring, has been shown to stimulate neurogenesis and increase hypothalamic expression of orexigenic neuropeptides in these postnatal offspring. The studies here, using an in vitro model that mimics in vivo characteristics after prenatal high-fat diet (HFD) exposure, investigate whether these same peptide changes occur in embryos and if they are specific to neurons. Isolated hypothalamic neurons were compared with whole hypothalamus from embryonic day 19 (E19) embryos that were prenatally exposed to HFD and were both found to show similar increases in mRNA expression of enkephalin (ENK) and neuropeptide Y (NPY) compared with that of chow-exposed embryos, with no change in melanin-concentrating hormone, orexin, or galanin. Further examination using immunofluorescence cytochemistry revealed an increase in the number of cells expressing ENK and NPY. By plotting the fluorescence intensity of each cell as a probability density function, three different populations of neurons with low, medium, or high levels of ENK or NPY were found in both HFD and chow groups. The prenatal HFD shifted the density of neurons from the population containing low peptide levels to the population containing high peptide levels. This study indicates that neuronal culture is a useful in vitro system for studying diet effects on neuronal development and shows that prenatal HFD exposure alters the population of hypothalamic neurons containing ENK and NPY in the embryo. These changes may contribute to the increase in HFD intake and body weight observed in offspring.

Neurochemical heterogeneity of rats predicted by different measures to be high ethanol consumers

BACKGROUND

Alcoholism is a heterogeneous disease, with subjects possibly differing both in the best measure that predicts their excess consumption and in their most effective pharmacotherapy. Two different measures, high novelty-induced activity and high-fat-induced triglycerides (TGs), are known to identify subgroups of animals prone to consuming higher amounts of ethanol (EtOH). The question investigated here is whether these subgroups are, in fact, similar in their neurochemical phenotype that may contribute to their overconsumption.

METHODS

EtOH-naïve, Sprague-Dawley rats were subgrouped based on the 2 predictor measures of activity or TG levels, and then quantitative real-time polymerase chain reaction and digoxigenin-labeled in situ hybridization were used to measure their expression of hypothalamic peptides that affect EtOH intake. In additional subgroups subsequently trained to drink 9% EtOH, the opioid antagonist and alcoholism medication, naltrexone, was tested at a low dose (0.02 mg/kg, s.c.) to determine the rats' sensitivity to its effects.

RESULTS

The 2 measures, while both effective in predicting amount of EtOH intake, were found to identify distinctive subgroups. Rats with high compared to low activity exhibited significantly greater expression of galanin and enkephalin in the paraventricular nucleus (PVN) and of orexin in the perifornical lateral hypothalamus (PFLH), but no difference in melanin-concentrating hormone in PFLH or neuropeptide Y in arcuate nucleus. This contrasts with rats having high TG, which exhibited greater expression only of PVN galanin, along with reduced PFLH orexin. The high-activity rats with elevated enkephalin, but not high-TG rats, were also unusually sensitive to naltrexone, which significantly reduced their alcohol intake.

CONCLUSIONS

In addition to revealing differences in endogenous peptides and drug responsiveness in predicted high EtOH drinkers, this study demonstrates that these disturbances differ markedly between the 2 at-risk subgroups. This indicates that simple tests may be effective in identifying subjects most responsive to a specific pharmacotherapy.

A high-fat meal, or intraperitoneal administration of a fat emulsion, increases extracellular dopamine in the nucleus accumbens

Evidence links dopamine (DA) in the nucleus accumbens (NAc) shell to the ingestion of palatable diets. Less is known, however, about the specific relation of DA to dietary fat and circulating triglycerides (TG), which are stimulated by fat intake and promote overeating. The present experiments tested in Sprague-Dawley rats whether extracellular levels of NAc DA increase in response to acute access to fat-rich food or peripheral injection of a fat emulsion and, if so, whether this is related to caloric intake or elevated circulating lipids. When rats consumed more calories of a high-fat meal compared with a low-fat meal, there was a significant increase in extracellular accumbens DA (155% vs. 119%). Systemic injection of a fat emulsion, which like a high-fat diet raises circulating TG but eliminates the factor of taste and allows for the control of caloric intake, also significantly increased extracellular levels of DA (127%) compared to an equicaloric glucose solution (70%) and saline (85%). Together, this suggests that a rise in circulating TG may contribute to the stimulatory effect of a high-fat diet on NAc DA.

Neural and metabolic regulation of macronutrient intake and selection

There is considerable disagreement regarding what constitutes a healthy diet. Ever since the influential work of Cannon and Richter, it was debated whether the 'wisdom of the body' will automatically direct us to the foods we need for healthy lives or whether we must carefully learn to eat the right foods, particularly in an environment of plenty. Although it is clear that strong mechanisms have evolved to prevent consumption of foods that have previously made us sick, it is less clear whether reciprocal mechanisms exist that reinforce the consumption of healthy diets. Here, we review recent progress in providing behavioural evidence for the regulation of intake and selection of proteins, carbohydrates and fats. We examine new developments in sensory physiology enabling recognition of macronutrients both pre- and post-ingestively. Finally, we propose a general model for central neural processing of nutrient-specific appetites. We suggest that the same basic neural circuitry responsible for the homoeostatic regulation of total energy intake is also used to control consumption of specific macro- and micronutrients. Similar to salt appetite, specific appetites for other micro- and macronutrients may be encoded by unique molecular changes in the hypothalamus. Gratification of such specific appetites is then accomplished by engaging the brain motivational system to assign the highest reward prediction to exteroceptive cues previously associated with consuming the missing ingredient. A better understanding of these nutrient-specific neural processes could help design drugs and behavioural strategies that promote healthier eating.

Regulation of drug and palatable food overconsumption by similar peptide systems

This review is aimed at understanding some of the common neurochemical, behavioral and physiological determinants of drug and food overconsumption. Much current work has been devoted to determining the similarities between the brain circuits controlling excessive use of addictive drugs and the overconsumption of palatable foods. The brain systems involved likely include peptides of both mesolimbic and hypothalamic origin. Evidence gathered from expression and injection studies suggests that the consumption of drugs, such as ethanol and nicotine, and also of palatable foods rich in fat is stimulated by different orexigenic peptides, such as enkephalin, galanin, orexin, and melaninconcentrating hormone, acting within the hypothalamus or various limbic structures, while another peptide, neuropeptide Y, is closely related to carbohydrate consumption and shows an inverse relationship with ethanol and nicotine consumption. Moreover, studies in animal models suggest that a propensity to overconsume these reinforcing substances may result from preexisting disturbances in these same peptide systems. These neurochemical disturbances, in turn, may also be closely linked to specific behaviors associated with excessive consummatory behavior, such as hyperactivity or novelty-seeking, palatable food preference, and also fluctuations in circulating lipid levels. Clear understanding of the relationship between these various determinants of consummatory behavior will allow researchers to effectively predict and examine at early stages of exposure animals that are prone to drug and food overconsumption. This work may ultimately aid in the identification of inherent traits that increase the risk for drug abuse and palatable food overconsumption.

Neurobiology of consummatory behavior: mechanisms underlying overeating and drug use

Consummatory behavior is driven by both caloric and emotional need, and a wide variety of animal models have been useful in research on the systems that drive consumption of food and drugs. Models have included selective breeding for a specific trait, manipulation of gene expression, forced or voluntary exposure to a substance, and identification of biomarkers that predict which animals are prone to overconsuming specific substances. This research has elucidated numerous brain areas and neurochemicals that drive consummatory behavior. Although energy homeostasis is primarily mediated by the hypothalamus, reinforcement is more strongly mediated by nuclei outside the hypothalamus, in mesocorticolimbic regions. Orexigenic neurochemicals that control food intake can provide a general signal for promoting caloric intake or a more specific signal for stimulating consumption of a particular macronutrient, fat, carbohydrate, or protein. The neurochemicals involved in controlling fat ingestion--galanin, enkephalin, orexin, melanin-concentrating hormone, and the endocannabinoids--show positive feedback with this macronutrient, as these peptides both increase fat intake and are further stimulated by its intake. This positive association offers some explanation for why foods high in fat are so often overconsumed. Consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by the neurochemical systems involved in fat intake, according to evidence that closely relates fat and ethanol consumption. Further understanding of the systems involved in consummatory behavior will enable the development of effective therapies for the treatment of both overeating and drug abuse.

Galanin and the orexin 2 receptor as possible regulators of enkephalin in the paraventricular nucleus of the hypothalamus: relation to dietary fat

Recent studies show that the non-opioid peptides, galanin (GAL) and orexin (OX), are similar to the opioid enkephalin (ENK) in being stimulated by dietary fat and also in enhancing the consumption of a high-fat diet (HFD). This suggests that, when an HFD is provided, these non-opioids may stimulate the opioid system to promote excess consumption of this diet. Using single- and double-labeling immunohistochemistry, the present study sought to identify possible neuroanatomical substrates for this close relationship. Focusing on the hypothalamic paraventricular nucleus (PVN), and particularly its anterior (aPVN), middle (mPVN) and posterior (pPVN) parts, the experiments examined whether GAL itself or the receptors for GAL and OX are stimulated by an HFD in the same areas and possibly the same neurons as ENK. Compared to animals fed a standard chow diet, rats consuming an HFD exhibited an increased density of medial parvocellular neurons immunoreactive (IR) for GAL in the mPVN and aPVN and for ENK in the mPVN and pPVN, distinguishing the mPVN as an area where both peptides were affected. While showing little evidence for GAL and ENK colocalization with a chow diet, double-labeling studies in HFD-fed rats revealed significant colocalization specifically in medial parvocellular neurons of the mPVN. Immediately posterior to this site, further analyses revealed a similar relationship between the OX 2 receptor (OX(2)R) and ENK in HFD-treated animals. While increasing the density of neurons immunoreactive for OX(2)R as well as for the GAL 1 receptor but not OX 1 receptor, HFD consumption increased the colocalization only of OX(2)R and ENK, specifically in the medial parvocellular neurons of the pPVN. These changes in HFD-fed rats, showing GAL and OX(2)R to colocalize with ENK exclusively in neurons of the medial parvocellular mPVN and pPVN, respectively, suggest possible neural substrates through which the non-opioid peptides may functionally interact with ENK when exposed to an HFD.

Similarities in hypothalamic and mesocorticolimbic circuits regulating the overconsumption of food and alcohol

Historically, studies of food intake regulation started with the hypothalamus and gradually expanded to mesocorticolimbic regions, while studies of drug use began with mesocorticolimbic regions and now include the hypothalamus. As research on ingestive behavior has progressed, it has uncovered more and more similarities between the regulation of palatable food and drug intake. It has also identified specific neurochemicals involved in palatable food and drug intake. Hypothalamic orexigenic neurochemicals specifically involved in controlling fat ingestion, including galanin, enkephalin, orexin and melanin-concentrating hormone, show positive feedback with this macronutrient, with these peptides both increasing fat intake and being further stimulated by its intake. This positive relationship offers some explanation for why foods high in fat are so often overconsumed. Research in Bart Hoebel's laboratory in conjunction with our own has shown that consumption of ethanol, a drug of abuse that also contains calories, is similarly driven by these neurochemical systems involved in fat intake, consistent with evidence closely relating fat and ethanol consumption. Both fat and ethanol intake are also regulated by dopamine and acetylcholine acting in mesocorticolimbic nuclei. This close relationship of fat and ethanol is likely driven in part by circulating lipids, which are increased by fat and ethanol intake, known to increase expression and levels of the neurochemicals, and found to promote further intake of fat and ethanol. Compellingly, recent studies suggest that these systems may already be dysregulated in animals prone to consuming excess fat or ethanol, even before they have ever been exposed to these substances. Further understanding of these systems involved in consummatory behavior will allow researchers to develop effective therapies for the treatment of overeating as well as drug abuse.

Opioids as facilitators of feeding: can any food be rewarding?

Palatability is one of the most rewarding aspects of consummatory behavior. Opioids, potent facilitators of intake of sweet and fat tastants, are thought to mediate hedonics of feeding. However, the rewarding context of consumption is not limited to palatability, and gratification can be achieved through other means, e.g., eating to satisfy hunger. The current review discusses the role of opioid peptides in food intake regulation by incorporating this expanded concept of feeding reward. We present evidence that, aside from increasing sugar/fat consumption, opioids propel the intake of diets whose gustatory value is low but are nonetheless consumed under circumstances allowing feeding gratification to occur. Opioids enhance reward-driven consumption by acting within the classical reward circuitry and also by signaling reward at sites that regulate other aspects of food intake, such as satiety and aversion. We conclude that, due to the complexity of neural and functional interactions, opioids are capable of enhancing pleasure of eating any food--palatable, bland or even aversive--making any meal into a more rewarding experience, despite possible consequences.

Increased orexin and melanin-concentrating hormone expression in the perifornical lateral hypothalamus of rats prone to overconsuming a fat-rich diet

The goal of this study is to examine the expression pattern of orexigenic peptides, orexin (OX) and melanin-concentrating hormone (MCH), in the perifornical lateral hypothalamus (PFLH) in subpopulations of Sprague-Dawley rats differing in their propensity to overconsume a high-fat diet. Immediately after an initial 5-day screening test that predicts long-term consumption, rats identified as high-fat consumers (HFC), ingesting 35% more calories of a high-fat relative to low-fat chow diet, had significantly elevated mRNA expression of OX in the perifornical but not lateral hypothalamic area and of MCH mRNA in both areas, when compared to control rats that consume similar amounts of these diets. This same OX and MCH expression pattern was seen in HFC rats maintained for two weeks on a low-fat chow diet, indicating that increased expression of these orexigenic peptides, occurring independently of the high-fat diet, may be an inherent characteristic of these rats. These HFC rats were also more active and slightly more anxious than controls, as measured by line crossings and time spent in the periphery or middle segments of an open field. Together, these results demonstrate that animals prone to overeating a high-fat diet show a baseline increase in orexigenic peptide expression in the PFLH along with higher behavioral arousal, which together may contribute to their increased consummatory behavior.

Increased enkephalin in brain of rats prone to overconsuming a fat-rich diet

Recent studies have shown that the opioid enkephalin (ENK), acting in part through the hypothalamic paraventricular nucleus (PVN), can stimulate consumption of a high-fat diet. The objective of the present study was to examine sub-populations of Sprague-Dawley rats naturally prone to overconsuming a high-fat diet and determine whether endogenous ENK, in different brain regions, is altered in these animals and possibly contributes to their behavioral phenotype. An animal model, involving a measure of initial high-fat diet intake during a few days of access that predicts long-term intake, was designed to classify rats at normal weight that are either high-fat consumers (HFC), which ingest 35% more calories of the high-fat than low-fat chow diet, or controls, which consume similar calories of these two diets. Immediately after their initial access to the diet, the HFC compared to control rats exhibited significantly greater expression of ENK mRNA, in the PVN, nucleus accumbens and central nucleus of the amygdala, but not the arcuate nucleus or basolateral amygdala. This site-specific increase in ENK persisted even when the HFC rats were maintained on a chow diet, suggesting that it reflects an inherent characteristic that can be expressed independently of the diet. It was also accompanied by a greater responsiveness of the HFC rats to the stimulatory effect of a PVN-injected, ENK analogue, D-ala2-met-enkephalinamide, compared to saline on consumption of the high-fat diet. Thus, normal-weight rats predicted to overconsume a fat-rich diet exhibit disturbances in endogenous ENK expression and functioning that may contribute to their long-term, behavioral phenotype.

Reduced accumbens dopamine in Sprague-Dawley rats prone to overeating a fat-rich diet

Obese humans and animals exhibit reduced functioning of the dopamine (DA) system in the nucleus accumbens (NAc). The question addressed here is whether this change in NAc DA can be detected in Sprague-Dawley rats that are prone to obesity on a fat-rich diet but still at normal body weight. Rats were subgrouped as "obesity-prone" (OP) or "obesity-resistant" (OR), based on their weight gain during 5days of access to a high-fat diet, and were then shifted to a lower-fat chow diet before microdialysis testing was performed. The OP rats compared to OR rats exhibited markedly reduced basal levels of DA in the NAc. After a high-fat challenge meal, both OP and OR rats showed a significant increase in extracellular DA and its metabolites; however, the NAc DA of the OP rats still remained at reduced levels. Also, the increase in DA and metabolite levels observed in OR rats after systemic administration of a fat emulsion was not evident in the OP rats, which instead showed no change in DA and a decrease in its metabolites. These results demonstrate, first, that fat can stimulate accumbal DA release and, second, that outbred rats prone to overeating and becoming obese on a palatable, fat-rich diet exhibit reduced signaling in the mesolimbic DA system while still at normal weight, suggesting that it may be causally related to their excess consummatory behavior.

Predictors of ethanol consumption in adult Sprague-Dawley rats: relation to hypothalamic peptides that stimulate ethanol intake

To investigate mechanisms in outbred animals that increase the propensity to consume ethanol, it is important to identify and characterize these animals before or at early stages in their exposure to ethanol. In the present study, different measures were examined in adult Sprague-Dawley rats to determine whether they can predict long-term propensity to overconsume ethanol. Before consuming 9% ethanol with a two-bottle choice paradigm, rats were examined with the commonly used behavioral measures of novelty-induced locomotor activity and anxiety, as assessed during 15 min in an open-field activity chamber. Two additional measures, intake of a low 2% ethanol concentration or circulating triglyceride (TG) levels after a meal, were also examined with respect to their ability to predict chronic 9% ethanol consumption. The results revealed significant positive correlations across individual rats between the amount of 9% ethanol ultimately consumed and three of these different measures, with high scores for activity, 2% ethanol intake, and TGs identifying rats that consume 150% more ethanol than rats with low scores. Measurements of hypothalamic peptides that stimulate ethanol intake suggest that they contribute early to the greater ethanol consumption predicted by these high scores. Rats with high 2% ethanol intake or high TGs, two measures found to be closely related, had significantly elevated expression of enkephalin (ENK) and galanin (GAL) in the hypothalamic paraventricular nucleus (PVN) but no change in neuropeptide Y (NPY) in the arcuate nucleus (ARC). This is in contrast to rats with high activity scores, which in addition to elevated PVN ENK expression showed enhanced NPY in the ARC but no change in GAL. Elevated ENK is a common characteristic related to all three predictors of chronic ethanol intake, whereas the other peptides differentiate these predictors, with GAL enhanced with high 2% ethanol intake and TG measures but NPY related to activity.

High-fructose corn syrup causes characteristics of obesity in rats: increased body weight, body fat and triglyceride levels

High-fructose corn syrup (HFCS) accounts for as much as 40% of caloric sweeteners used in the United States. Some studies have shown that short-term access to HFCS can cause increased body weight, but the findings are mixed. The current study examined both short- and long-term effects of HFCS on body weight, body fat, and circulating triglycerides. In Experiment 1, male Sprague-Dawley rats were maintained for short term (8 weeks) on (1) 12 h/day of 8% HFCS, (2) 12 h/day 10% sucrose, (3) 24 h/day HFCS, all with ad libitum rodent chow, or (4) ad libitum chow alone. Rats with 12-h access to HFCS gained significantly more body weight than animals given equal access to 10% sucrose, even though they consumed the same number of total calories, but fewer calories from HFCS than sucrose. In Experiment 2, the long-term effects of HFCS on body weight and obesogenic parameters, as well as gender differences, were explored. Over the course of 6 or 7 months, both male and female rats with access to HFCS gained significantly more body weight than control groups. This increase in body weight with HFCS was accompanied by an increase in adipose fat, notably in the abdominal region, and elevated circulating triglyceride levels. Translated to humans, these results suggest that excessive consumption of HFCS may contribute to the incidence of obesity.

Effect of chronic ethanol on enkephalin in the hypothalamus and extra-hypothalamic areas

BACKGROUND

Ethanol may be consumed for reasons such as reward, anxiety reduction, or caloric content, and the opioid enkephalin (ENK) appears to be involved in many of these functions. Previous studies in Sprague-Dawley rats have demonstrated that ENK in the hypothalamic paraventricular nucleus (PVN) is stimulated by voluntary consumption of ethanol. This suggests that this opioid peptide may be involved in promoting the drinking of ethanol, consistent with our recent findings that PVN injections of ENK analogs stimulate ethanol intake. To broaden our understanding of how this peptide functions throughout the brain to promote ethanol intake, we measured, in rats trained to drink 9% ethanol, the expression of the ENK gene in additional brain areas outside the hypothalamus, namely, the ventral tegmental area (VTA), nucleus accumbens shell (NAcSh) and core (NAcC), medial prefrontal cortex (mPFC), and central nucleus of the amygdala (CeA).

METHODS

In the first experiment, the brains of rats chronically drinking 1 g/kg/d ethanol, 3 g/kg/d ethanol, or water were examined using real-time quantitative polymerase chain reaction (qRT-PCR). In the second experiment, a more detailed, anatomic analysis of changes in gene expression, in rats chronically drinking 3 g/kg/d ethanol compared to water, was performed using radiolabeled in situ hybridization (ISH). The third experiment employed digoxigenin-labeled ISH (DIG) to examine changes in the density of cells expressing ENK and, for comparison, dynorphin (DYN) in rats chronically drinking 3 g/kg/d ethanol versus water.

RESULTS

With qRT-PCR, the rats chronically drinking ethanol plus water compared to water alone showed significantly higher levels of ENK mRNA, not only in the PVN but also in the VTA, NAcSh, NAcC, and mPFC, although not in the CeA. Using radiolabeled ISH, levels of ENK mRNA in rats drinking ethanol were found to be elevated in all areas examined, including the CeA. The experiment using DIG confirmed this effect of ethanol, showing an increase in density of ENK-expressing cells in all areas studied. It additionally revealed a similar change in DYN mRNA in the PVN, mPFC, and CeA, although not in the NAcSh or NAcC.

CONCLUSIONS

While distinguishing the NAc as a site where ENK and DYN respond differentially, these findings lead us to propose that these opioids, in response to voluntary ethanol consumption, are generally elevated in extra-hypothalamic as well as hypothalamic areas, possibly to carry out specific area-related functions that, in turn, drive animals to further consume ethanol. These functions include calorie ingestion in the PVN, reward and motivation in the VTA and NAcSh, response-reinforcement learning in the NAcC, stress reduction in the CeA, and behavioral control in the mPFC.

Hypothalamic injection of non-opioid peptides increases gene expression of the opioid enkephalin in hypothalamic and mesolimbic nuclei: Possible mechanism underlying their behavioral effects

The peptides galanin (GAL) and orexin (OX) share common features with the opioid enkephalin (ENK) in their relationship to ingestive behavior, stimulating consumption of a fat-rich diet and ethanol when injected into the hypothalamus. Since receptors for GAL and OX are dense in areas where ENK-expressing neurons are concentrated, these non-opioid peptides may exert their effects, in part, through the stimulation of endogenous ENK. This study was conducted to determine whether injection of GAL or OX affects the expression of ENK in hypothalamic and mesolimbic nuclei involved in consummatory behavior. Rats were injected with GAL (1 microg), OX-A (1 microg), or saline vehicle just dorsal to the hypothalamic paraventricular nucleus (PVN). They were sacrificed 1h later for analysis of ENK mRNA levels in the PVN, ventral tegmental area (VTA), central nucleus of the amygdala (CeA), and nucleus accumbens (NAc). Both GAL and OX had similar effects, significantly increasing ENK mRNA expression in each of these areas, except for the NAc. This enhanced ENK expression in the PVN, VTA and CeA was demonstrated with real-time quantitative polymerase chain reaction and confirmed in separate groups using radiolabeled and digoxigenin-labeled in situ hybridization. These findings demonstrate that the non-opioid peptides, GAL or OX, which have similar effects on consummatory behavior, are also similar in their effect on endogenous ENK. In light of published findings showing an opioid antagonist to block GAL- and OX-induced feeding, these results provide additional evidence that ENK is involved in mediating the common behavioral effects of these peptides.

Opioids in the hypothalamus control dopamine and acetylcholine levels in the nucleus accumbens

The experimental question is whether hypothalamic opioids, known to stimulate consummatory behavior, control a link to the nucleus accumbens (NAc). It was hypothesized that opioids injected in the hypothalamic paraventricular nucleus (PVN) alter the balance of dopamine (DA) and acetylcholine (ACh) in the NAc in a manner that fosters appetite for food or ethanol. Rats were implanted with two guide shafts, one in the NAc to measure extracellular DA and ACh by microdialysis and the other in the PVN for microinjection of opioid mu- and delta-agonists, an antagonist, or saline vehicle. The compounds tested were morphine, the mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin (DAMGO), the delta-receptor agonist D-Ala-Gly-Phe-Met-NH2 (DALA), and the opioid antagonist naloxone methiodide (m-naloxone). Morphine in the PVN increased the release of accumbens DA (+41%) and decreased ACh (-35%). Consistent with this, the opioid antagonist m-naloxone decreased DA (-24%) and increased ACh (+19%). In terms of receptor involvement, DAMGO dose-dependently increased DA to up to 209% of baseline. Simultaneously, ACh levels were markedly decreased to 55% of baseline. The agonist DALA produced a smaller but significant, 34% increase in DA, without affecting ACh. In contrast, control injections of saline had no significant effect. These results demonstrate that mu- and delta-opioids in the PVN contribute to the control of accumbens DA and ACh release and suggest that this circuit from the PVN to the NAc may be one of the mechanisms underlying opiate-induced ingestive behavior as well as naltrexone therapy for overeating and alcoholism.

Opioids in the hypothalamic paraventricular nucleus stimulate ethanol intake

BACKGROUND

Specialized hypothalamic systems that increase food intake might also increase ethanol intake. To test this possibility, morphine and receptor-specific opioid agonists were microinjected in the paraventricular nucleus (PVN) of rats that had learned to drink ethanol. To cross-validate the results, naloxone methiodide (m-naloxone), an opioid antagonist, was microinjected with the expectation that it would have the opposite effect of morphine and the specific opioid agonists.

METHODS

Sprague-Dawley rats were trained, without sugar, to drink 4 or 7% ethanol and were then implanted with chronic brain cannulas aimed at the PVN. After recovery, those drinking 7% ethanol, with food and water available, were injected with 2 doses each of morphine or m-naloxone. To test for receptor specificity, 2 doses each of the mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin (DAMGO), delta-receptor agonist D-Ala-Gly-Phe-Met-NH2 (DALA), or kappa-receptor agonist U-50,488H were injected. DAMGO was also tested in rats drinking 4% ethanol without food or water available. As an anatomical control for drug reflux, injections were made 2 mm dorsal to the PVN.

RESULTS

A main result was a significant increase in ethanol intake induced by PVN injection of morphine. The opposite effect was produced by m-naloxone. The effects of morphine and m-naloxone were exclusively on intake of ethanol, even though food and water were freely available. In the analysis with specific receptor agonists, PVN injection of the delta-agonist DALA significantly increased 7% ethanol intake without affecting food or water intake. This is in contrast to the kappa-agonist U-50,488H, which decreased ethanol intake, and the mu-agonist DAMGO, which had no effect on ethanol intake in the presence or absence of food and water. In the anatomical control location 2 mm dorsal to the PVN, no drug caused any significant changes in ethanol, food, or water intake, providing evidence that the active site was close to the cannula tip.

CONCLUSIONS

The delta-opioid receptor agonist in the PVN increased ethanol intake in strong preference over food and water, while the kappa-opioid agonist suppressed ethanol intake. Prior studies show that learning to drink ethanol stimulates PVN expression and production of the peptides enkephalin and dynorphin, which are endogenous agonists for the delta- and kappa-receptors, respectively. These results suggest that enkephalin via the delta-opioid system can function locally within a positive feedback circuit to cause ethanol intake to escalate and ultimately contribute to the abuse of ethanol. This is in contrast to dynorphin via the kappa-opioid system, which may act to counter this escalation. Naltrexone therapy for alcoholism may act, in part, by blocking the enkephalin-triggered positive feedback cycle.

kappa-Opioid receptors control the metabolic response to a high-energy diet in mice

General opioid receptor antagonists reduce food intake and body weight in rodents, but the contributions of specific receptor subtypes are unknown. We examined whether genetic deletion of the kappa-opioid receptor (KOR) in mice alters metabolic physiology. KOR-knockout (KO) and wild-type (WT) mice were fed a high-energy diet (HED) for 16 wk. KO mice had 28% lower body weight and 45% lower fat mass when compared to WT mice fed an HED. No differences in caloric intake were found. An HED reduced energy expenditure in WT mice, but not in KO mice. KOR deficiency led to an attenuation of triglyceride synthesis in the liver. Malonyl CoA levels were also reduced in response to an HED, thereby promoting hepatic beta-oxidation. Glycemic control was also found to be improved in KO mice. These data suggest a key role for KORs in the central nervous system regulation of the metabolic adaptation to an HED, as we were unable to detect expression of KOR in liver, white adipose tissue, or skeletal muscle in WT mice. This study provides the first evidence that KORs play an essential physiological role in the control of hepatic lipid metabolism, and KOR activation is a permissive signal toward fat storage.-Czyzyk, T. A., Nogueiras, R., Lockwood, J. F., McKinzie, J. H., Coskun, T., Pintar, J. E., Hammond, C., Tschöp, M. H., Statnick, M. A. kappa-Opioid receptors control the metabolic response to a high-energy diet in mice.

Hypothalamic FTO is associated with the regulation of energy intake not feeding reward

Background

Polymorphism in the FTO gene is strongly associated with obesity, but little is known about the molecular bases of this relationship. We investigated whether hypothalamic FTO is involved in energy-dependent overconsumption of food. We determined FTO mRNA levels in rodent models of short- and long-term intake of palatable fat or sugar, deprivation, diet-induced increase in body weight, baseline preference for fat versus sugar as well as in same-weight animals differing in the inherent propensity to eat calories especially upon availability of diverse diets, using quantitative PCR. FTO gene expression was also studied in organotypic hypothalamic cultures treated with anorexigenic amino acid, leucine. In situ hybridization (ISH) was utilized to study FTO signal in reward- and hunger-related sites, colocalization with anorexigenic oxytocin, and c-Fos immunoreactivity in FTO cells at initiation and termination of a meal.

Results

Deprivation upregulated FTO mRNA, while leucine downregulated it. Consumption of palatable diets or macronutrient preference did not affect FTO expression. However, the propensity to ingest more energy without an effect on body weight was associated with lower FTO mRNA levels. We found that 4-fold higher number of FTO cells displayed c-Fos at meal termination as compared to initiation in the paraventricular and arcuate nuclei of re-fed mice. Moreover, ISH showed that FTO is present mainly in hunger-related sites and it shows a high degree of colocalization with anorexigenic oxytocin.

Conclusion

We conclude that FTO mRNA is present mainly in sites related to hunger/satiation control; changes in hypothalamic FTO expression are associated with cues related to energy intake rather than feeding reward. In line with that, neurons involved in feeding termination express FTO. Interestingly, baseline FTO expression appears linked not only with energy intake but also energy metabolism.

Mice lacking the G protein gamma3-subunit show resistance to opioids and diet induced obesity

Contributing to the obesity epidemic, there is increasing evidence that overconsumption of high-fat foods may be analogous to drug addiction in that the palatability of these foods is associated with activation of specific reward pathways in the brain. With this perspective, we report that mice lacking the G protein gamma(3)-subunit (Gng3(-/-) mice) show resistance to high-fat diet-induced weight gain over the course of a 12-wk study. Compared with Gng3(+/+) controls, female Gng3(-/-) mice exhibit a 40% reduction in weight gain and a 53% decrease in fat pad mass, whereas male Gng3(-/-) mice display an 18% reduction in weight gain and no significant decrease in fat pad mass. The basis for the lowered weight gain is related to reduced food consumption for female and male Gng3(-/-) mice of 13% and 14%, respectively. Female Gng3(-/-) mice also show a lesser preference for high-fat chow than their female Gng3(+/+) littermates, suggesting an attenuated effect on a reward pathway associated with overconsumption of fat. One possible candidate is the micro-opioid receptor (Oprm1) signaling cascade. Supporting a defect in this signaling pathway, Gng3(-/-) mice show marked reductions in both acute and chronic morphine responsiveness, as well as increases in endogenous opioid mRNA levels in reward-related regions of the brain. Taken together, these data suggest that the decreased weight gain of Gng3(-/-) mice may be related to a reduced rewarding effect of the high-fat diet resulting from a defect in Oprm1 signaling and loss of the G protein gamma(3)-subunit.

Positive relationship between dietary fat, ethanol intake, triglycerides, and hypothalamic peptides: counteraction by lipid-lowering drugs

Studies in both humans and animals suggest a positive relationship between the intake of ethanol and intake of fat, which may contribute to alcohol abuse. This relationship may be mediated, in part, by hypothalamic orexigenic peptides such as orexin (OX), which stimulate both consumption of ethanol and fat, and circulating triglycerides (TGs), which stimulate these peptides and promote consummatory behavior. The present study investigated this vicious cycle between ethanol and fat, to further characterize its relation to TGs and to test the effects of lowering TG levels. In Experiment 1, the behavioral relationship between fat intake and ethanol was confirmed. Adult male Sprague-Dawley rats, chronically injected intraperitoneally with ethanol (1g/kg) and tested in terms of their preference for a high-fat diet (HFD) compared with low-fat diet (LFD), showed a significant increase in their fat preference, compared with rats injected with saline, in measures of 2h and 24h intake. Experiment 2 tested the relationship of circulating TGs in this positive association between ethanol and fat, in rats chronically consuming 9% ethanol versus water and given acute meal tests (25kcal) of a HFD versus LFD. Levels of TGs were elevated in response to both chronic drinking of ethanol versus water and acute eating of a high-fat versus low-fat meal. Most importantly, ethanol and a HFD showed an interaction effect, whereby their combination produced a considerably larger increase in TG levels (+172%) compared to ethanol with a LFD (+111%). In Experiment 3, a direct manipulation of TG levels was found to affect ethanol intake. After intragastric administration of gemfibrozil (50mg/kg) compared with vehicle, TG levels were lowered by 37%, and ethanol intake was significantly reduced. In Experiment 4, the TG-lowering drug gemfibrozil also caused a significant reduction in the expression of the orexigenic peptide, OX, in the perifornical lateral hypothalamus. These results support the existence of a vicious cycle between ethanol and fat, whereby each nutrient stimulates intake of the other. Within this vicious cycle, ethanol and fat act synergistically to increase TG levels, which in turn stimulate peptides that promote further consumption, and these phenomena are reversed by gemfibrozil, which lowers TG levels.

Circulating triglycerides after a high-fat meal: predictor of increased caloric intake, orexigenic peptide expression, and dietary obesity

Recent studies in normal-weight rats have linked circulating triglycerides (TG), when elevated by a high-fat (HF) compared to equicaloric low-fat (LF) meal, to an increase in subsequent food intake and hypothalamic expression of orexigenic peptides. The present study tested whether natural variations between rats in their TG levels after a small HF meal can also be related to their individual patterns of eating and peptide expression. In tail vein blood collected on three separate days 60 min after a HF meal, levels of TG were found to be strongly, positively correlated within rats from day to day but were highly variable between rats (75-365 mg/dl), allowing distinct subgroups (33% lowest or highest) to be formed. Compared to "Low-TG responders" with post-meal levels averaging 109 mg/dl, "High-TG responders" with 240 mg/dl showed in two separate experiments a significant increase in caloric intake in a subsequent laboratory chow meal. Before this larger meal, these rats with elevated TG consistently exhibited higher expression levels and synthesis of the orexigenic peptides, enkephalin, orexin and melanin-concentrating hormone, as revealed using real-time quantitative PCR, radiolabeled in situ hybridization, and immunofluorescence histochemistry. Over the long-term, the High-TG responders also showed an increased propensity to overeat, gain weight and accumulate excess body fat on a chronic HF diet. This simple measure of TG levels after a HF meal may offer a useful tool for identifying subpopulations with increased risk for overeating and dietary obesity and detecting early signs of brain disturbances that may contribute to this high-risk phenotype.

From taste hedonics to motivational drive: central μ-opioid receptors and binge-eating behaviour

Endogenous opioids and μ-opioid receptors (MORs) have long been implicated in the mechanism of appetite control and, in particular, hedonic processes associated with food evaluation, consumption and orosensory reward processes. In animal models of binge eating, selective MOR antagonists suppress food consumption. In humans, non-selective opioid receptor antagonists reduce hedonic taste preferences and food intake, particularly for palatable foods, and cause short-term weight loss. These effects have been linked to direct stimulation of MORs and modulation of dopamine release within the reward circuitry including the nucleus accumbens. These findings suggest that reduction of MOR-mediated hedonic and motivation processes driving consumption of highly palatable foods may be a promising therapeutic approach and provide a strong rationale for developing safer and more selective MOR antagonists or inverse agonists for disorders of 'appetitive motivation' including obesity and binge-eating disorder.

Puberty onset in female rats: relationship with fat intake, ovarian steroids and the peptides, galanin and enkephalin, in the paraventricular and medial preoptic nuclei

Puberty is a time of rapid change, including a marked increase in fat consumption and body fat accrual, particularly in females. The mechanisms underlying these changes are unknown. Building on the results obtained in adult rats, the present study in pubertal rats focused on the orexigenic peptides, galanin (GAL) and enkephalin (ENK), in the paraventricular nucleus (PVN) and medial preoptic nucleus (MPN), which are known to be responsive to female steroids and have a role in both energy balance and reproductive function. The present study examined female rats maintained on pure macronutrient diets from before weaning (day 15) to day 70. After an initial burst in protein intake (days 21-35), rats showed an increase, specifically in preference for fat, from 15% to 30%. In rats examined at different ages before (day 30) and after (days 45 and 60) puberty, this rise in fat intake was associated with a marked increase, from days 30-45, in levels of oestradiol and progesterone and in GAL and ENK mRNA or peptide levels, specifically in the PVN and MPN, but not other hypothalamic areas examined. This positive relationship with increased fat intake, steroids and peptides across ages was also observed when comparing pubertal rats that naturally preferred fat (> 25% of total diet) with those consuming little fat (< 15%) or rats that reached puberty at an early age (days 30-34) with those that were late (days 37-40). These rats with early puberty onset exhibited a strong fat preference 3-4 days before vaginal opening, which was positively related to steroid levels, GAL, fat intake and body fat accrual after puberty. These findings suggest that, in addition to providing a signal for puberty onset, early fat ingestion acting through mechanisms involving the steroids and orexigenic peptides may be related to long-term patterns of eating and body weight regulation.

Maternal high-fat diet and fetal programming: increased proliferation of hypothalamic peptide-producing neurons that increase risk for overeating and obesity

Recent studies in adult and weanling rats show that dietary fat, in close association with circulating lipids, can stimulate expression of hypothalamic peptides involved in controlling food intake and body weight. In the present study, we examined the possibility that a fat-rich diet during pregnancy alters the development of these peptide systems in utero, producing neuronal changes in the offspring that persist postnatally in the absence of the diet and have long-term consequences. The offspring of dams on a high-fat diet (HFD) versus balanced diet (BD), from embryonic day 6 to postnatal day 15 (P15), showed increased expression of orexigenic peptides, galanin, enkephalin, and dynorphin, in the paraventricular nucleus and orexin and melanin-concentrating hormone in the perifornical lateral hypothalamus. The increased density of these peptide-expressing neurons, evident in newborn offspring as well as P15 offspring cross-fostered at birth to dams on the BD, led us to examine events that might be occurring in utero. During gestation, the HFD stimulated the proliferation of neuroepithelial and neuronal precursor cells of the embryonic hypothalamic third ventricle. It also stimulated the proliferation and differentiation of neurons and their migration toward hypothalamic areas where ultimately a greater proportion of the new neurons expressed the orexigenic peptides. This increase in neurogenesis, closely associated with a marked increase in lipids in the blood, may have a role in producing the long-term behavioral and physiological changes observed in offspring after weaning, including an increase in food intake, preference for fat, hyperlipidemia, and higher body weight.

Molecular mechanisms of fat preference and overeating

Obesity is recognized as a worldwide health problem. Overconsumption of fatty foods contributes significantly to this phenomenon. Rodents, like humans, display preferences for lipid-rich foods. Rodents thus provide useful models to explore the mechanisms responsible for this complex feeding behavior resulting from the integration of multiple oral and postoral signals. Over the last decades, the lipid-mediated regulation of food intake has received considerable attention. By contrast, orosensory lipid perception was long thought to involve only textural and olfactory cues. Recent findings have challenged this limited viewpoint. These recent data strongly suggest that the sense of taste also plays significant roles in the spontaneous preference for fatty foods. This paper provides a brief overview of postoral regulation of food intake by lipids and then highlights recent data suggesting the existence of a "fatty taste" which might contribute to lipid overeating and hence to the risk of obesity.