The metabolic syndrome (MetS) is characterized by obesity concomitant with other metabolic abnormalities such as hypertriglyceridemia, reduced high-density lipoprotein levels, elevated blood pressure and raised fasting glucose levels. The precise definition of MetS, the relationships of its metabolic features, and what initiates it, are debated. However, obesity is on the rise worldwide, and its association with these metabolic symptoms increases the risk for diabetes and cardiovascular disease (among many other diseases). Research needs to determine the mechanisms by which obesity and MetS increase the risk of disease. In light of this growing epidemic, it is imperative to develop animal models of MetS. These models will help determine the pathophysiological basis for MetS and how MetS increases the risk for other diseases. Among the various animal models available to study MetS, mice are the most commonly used for several reasons. First, there are several spontaneously occurring obese mouse strains that have been used for decades and that are very well characterized. Second, high-fat feeding studies require only months to induce MetS. Third, it is relatively easy to study the effects of single genes by developing transgenic or gene knockouts to determine the influence of a gene on MetS. For these reasons, this review will focus on the benefits and caveats of the most common mouse models of MetS. It is our hope that the reader will be able to use this review as a guide for the selection of mouse models for their own studies.

Hypothalamic pro-opiomelanocortin (POMC) neurons help regulate long-term energy stores. POMC neurons are also found in the nucleus tractus solitarius (NTS), a region regulating satiety. We show here that mouse brainstem NTS POMC neurons are activated by cholecystokinin (CCK) and feeding-induced satiety and that activation of the neuronal melanocortin-4 receptor (MC4-R) is required for CCK-induced suppression of feeding; the melanocortin system thus provides a potential substrate for integration of long-term adipostatic and short-term satiety signals.

Summary The global obesity epidemic has heightened the need for an improved understanding of how body weight is controlled, and research using mouse models is critical to this effort. In this perspective, we provide a conceptual framework for investigation of feeding behavior in this species, with an emphasis on factors that influence study design, data interpretation, and relevance to feeding behavior in humans. Although we focus on the mouse, the principles presented can be applied to most other animal models. This document represents the current consensus view of investigators from the National Institutes of Health (NIH)-funded Mouse Metabolic Phenotyping Centers (MMPCs).

The organization of timing in mammalian circadian clocks optimally coordinates behavior and physiology with daily environmental cycles. Chronic consumption of a high-fat diet alters circadian rhythms, but the acute effects on circadian organization are unknown. To investigate the proximate effects of a high-fat diet on circadian physiology, we examined the phase relationship between central and peripheral clocks in mice fed a high-fat diet for 1 week. By 7 days, the phase of the liver rhythm was markedly advanced (by 5 h), whereas rhythms in other tissues were not affected. In addition, immediately upon consumption of a high-fat diet, the daily rhythm of eating behavior was altered. As the tissue rhythm of the suprachiasmatic nucleus was not affected by 1 week of high-fat diet consumption, the brain nuclei mediating the effect of a high-fat diet on eating behavior are likely to be downstream of the suprachiasmatic nucleus.

Peptide YY(3-36) (PYY(3-36)), a peptide released postprandially by the gut, has been demonstrated to inhibit food intake. Little is known about the mechanism by which PYY(3-36) inhibits food intake, although the peptide has been shown to increase hypothalamic proopiomelanocortin (POMC) mRNA in vivo and to activate POMC neurons in an electrophysiological slice preparation. Understanding the physiology of PYY(3-36) is further complicated by the fact that some laboratories have had difficulty demonstrating inhibition of feeding by the peptide in rodents. We demonstrate here that, like cholecystokinin, PYY(3-36) dose-dependently inhibits food intake by approximately 20-45% over a 3- to 4-h period post ip administration, with no effect on 12-h food intake. This short-lived satiety effect is not seen in animals that are not thoroughly acclimated to handling and ip injection, thus potentially explaining the difficulty in reproducing the effect. Surprisingly, PYY(3-36) was equally efficacious in inducing satiety in wild-type and melanocortin-4 receptor (MC4-R)-deficient mice and thus does not appear to be dependent on MC4-R signaling. The expression of c-Fos, an indirect marker of neuronal activation, was also examined in forebrain and brainstem neurons after ip treatment with a dose of PYY(3-36) shown to induce satiety. The peptide induced no significant neuronal activation in the brainstem by this assay, and only modest activation of hypothalamic POMC neurons. Thus, unlike cholecystokinin, PYY(3-36)-induced satiety is atypical, because it does not produce detectable activation of brainstem satiety centers and is not dependent on MC4-R signaling.

The importance of the central melanocortin system in the regulation of energy balance is highlighted by studies in transgenic animals and humans with defects in this system. Mice that are engineered to be deficient for the melanocortin-4 receptor (MC4R) or pro-opiomelanocortin (POMC) and those that overexpress agouti or agouti-related protein (AgRP) all have a characteristic obese phenotype typified by hyperphagia, increased linear growth, and metabolic defects. Similar attributes are seen in humans with haploinsufficiency of the MC4R. The central melanocortin system modulates energy homeostasis through the actions of the agonist, ␣-melanocyte-stimulating hormone (␣-MSH), a POMC cleavage product, and the endogenous antagonist AgRP on the MC3R and MC4R. POMC is expressed at only two locations in the brain: the arcuate nucleus of the hypothalamus (ARC) and the nucleus of the tractus solitarius (NTS) of the brainstem. This chapter will discuss these two populations of POMC neurons and their contribution to energy homeostasis. We will examine the involvement of the central melanocortin system in the incorporation of information from the adipostatic hormone leptin and acute hunger and satiety factors such as peptide YY ) and ghrelin via a neuronal network involving POMC/cocaine and amphetamine-related transcript (CART) and neuropeptide Y (NPY)/AgRP neurons. We will discuss evidence for the existence of a similar network of neurons in the NTS and propose a model by which this information from the ARC and NTS centers may be integrated directly or via adipostatic centers such as the paraventricular nucleus of the hypothalamus (PVH).

ObjectiveIntroduction of a high-fat diet to mice results in a period of voracious feeding, known as hyperphagia, before homeostatic mechanisms prevail to restore energy intake to an isocaloric level. Acute high-fat diet hyperphagia induces astrocyte activation in the rodent hypothalamus, suggesting a potential role of these cells in the homeostatic response to the diet. The objective of this study was to determine physiologic role of astrocytes in the acute homeostatic response to high-fat feeding.MethodsWe bred a transgenic mouse model with doxycycline-inducible inhibition of NFkappaB (NFκB) signaling in astrocytes to determine the effect of loss of NFκB-mediated astrocyte activation on acute high-fat hyperphagia. ELISA was used to measure the levels of markers of astrocyte activation, glial-fibrillary acidic protein (GFAP) and S100B, in the medial basal hypothalamus.ResultsInhibition of NFκB signaling in astrocytes prevented acute high-fat diet-induced astrocyte activation and resulted in a 15% increase in caloric intake (P < 0.01) in the first 24 h after introduction of the diet.ConclusionsThese data reveal a novel homeostatic role for astrocytes in the acute physiologic regulation of food intake in response to high-fat feeding.

The central melanocortin 4 receptor (MC4R) plays a critical role in energy homeostasis, although little is known regarding its role in the regulation of adaptive thermogenesis of brown adipose tissue (BAT). Here we show using retrograde transsynaptic tracing with attenuated pseudorabies virus coupled with dual-label immunohistochemistry that specific subsets of MC4R-expressing neurons in multiple nuclei of the central nervous system known to regulate sympathetic outflow polysynaptically connect with interscapular BAT (IBAT). Furthermore, we show that MC4R-/- and agouti-related peptide-treated mice are defective in HF diet-induced up-regulation of uncoupling protein 1 in IBAT. Additionally, MC4R-/- mice exposed to 4 C for 4 h exhibit a defect in up-regulation of uncoupling protein 1 levels in IBAT. Our results provide a neuroanatomic substrate for MC4R regulating sympathetically mediated IBAT thermogenesis and demonstrate that the MC4R is critically required for acute high-fat- and cold-induced IBAT thermogenesis.