Extracellular Hypothalamic Monoamines Measured by In Vivo Microdialysis in a Rat Model of Dietary Fat-Induced Obesity

Ketogenic diet exposure during the juvenile period increases social behaviors and forebrain neural activation in adult Engrailed 2 null mice

Prolonged consumption of ketogenic diets (KD) has reported neuroprotective benefits. Several studies suggest KD interventions could be useful in the management of neurological and developmental disorders. Alterations in the Engrailed (En) genes, specifically Engrailed 2 (En2), have neurodevelopmental consequences and produce autism-related behaviors. The following studies used En2 knockout (KO; En2(-/-)), and wild-type (WT; En2(+/+)), male mice fed either KD (80% fat, 0.1% carbohydrates) or control diet (CD; 10% fat, 70% carbohydrates). The objective was to determine whether a KD fed from weaning at postnatal day (PND) 21 to adulthood (PND 60) would alter brain monoamines concentrations, previously found dysregulated, and improve social outcomes. In WT animals, there was an increase in hypothalamic norepinephrine content in the KD-fed group. However, regional monoamines were not altered in KO mice in KD-fed compared with CD-fed group. In order to determine the effects of juvenile exposure to KD in mice with normal blood ketone levels, separate experiments were conducted in mice removed from the KD or CD and fed standard chow for 2days (PND 62). In a three-chamber social test with a novel mouse, KO mice previously exposed to the KD displayed similar social and self-grooming behaviors compared with the WT group. Groups previously exposed to a KD, regardless of genotype, had more c-Fos-positive cells in the cingulate cortex, lateral septal nuclei, and anterior bed nucleus of the stria terminalis. In the novel object condition, KO mice previously exposed to KD had similar behavioral responses and pattern of c-Fos immunoreactivity compared with the WT group. Thus, juvenile exposure to KD resulted in short-term consequences of improving social interactions and appropriate exploratory behaviors in a mouse model that displays autism-related behaviors. Such findings further our understanding of metabolic-based therapies for neurological and developmental disorders.

Metabolomic analysis of livers and serum from high-fat diet induced obese mice

Liver and serum metabolites of obese and lean mice fed on high fat or normal diets were analyzed using ultraperformance liquid chromatography-quadrupole-time-of-flight mass spectrometry, gas chromatography-mass spectrometry, and partial least-squares-discriminant analysis (PLS-DA). Obese and lean groups were clearly discriminated from each other on PLS-DA score plot and major metabolites contributing to the discrimination were assigned as lipid metabolites (fatty acids, phosphatidylcholines (PCs), and lysophosphatidylcholines (lysoPCs)), lipid metabolism intermediates (betaine, carnitine, and acylcarnitines), amino acids, acidic compounds, monosaccharides, and serotonin. A high-fat diet increased lipid metabolites but decreased lipid metabolism intermediates and the NAD/NADH ratio, indicating that abnormal lipid and energy metabolism induced by a high-fat diet resulted in fat accumulation via decreased β-oxidation. In addition, this study revealed that the levels of many metabolites, including serotonin, betaine, pipecolic acid, and uric acid, were positively or negatively related to obesity-associated diseases. On the basis of these metabolites, we proposed a metabolic pathway related to high-fat diet-induced obesity. These metabolites can be used to better understand obesity and related diseases induced by a hyperlipidic diet. Furthermore, the level changes of these metabolites can be used to assess the risk of obesity and the therapeutic effect of obesity management.

Deficits in gastrointestinal responses controlling food intake and body weight

The gastrointestinal tract serves as a portal sensing incoming nutrients and relays mechanical and chemosensory signals of a meal to higher brain centers. Prolonged consumption of dietary fat causes adaptive changes within the alimentary, metabolic, and humoral systems that promote a more efficient process for energy metabolism from this rich source, leading to storage of energy in the form of adipose tissue. Furthermore, prolonged ingestion of dietary fats exerts profound effects on responses to signals involved in termination of a meal. This article reviews the effects of ingested fat on gastrointestinal motility, hormone release, and neuronal substrates. It focuses on changes in sensitivity to satiation signals resulting from chronic ingestion of high-fat diet, which may lead to disordered appetite and dysregulation of body weight.

Effects of dietary fat and enterostatin on dopamine and 5-hydroxytrytamine release from rat striatal slices

Studies have demonstrated defects of DA and 5HT neurotransmission in dietary fat induced obese animals. In the present study, we used a perfusion system to assay the release of DA and 5HT from striatal slices preloaded with [(3)H]-DA or [(3)H]-5HT. The release of both DA and 5HT from striatal slices of rats fed a high fat diet for 10 days, but not 3 days, was reduced when compared to striatal slices taken from rats fed a low fat diet. Enterostatin, an endogenous pentapeptide inhibits dietary fat intake when administered peripherally and centrally in animals. The central mechanism for the action of enterostatin is not yet determined even though several mechanisms have been suggested. We have shown that enterostatin enhanced [(3)H]-DA release, but not [(3)H]-5HT release from striatal slices of rats that had been adapted to high fat diet for 10 days. The enterostatin-induced increase in [(3)H]-DA release was blocked by nomifensine. Enterostatin did not alter [(3)H]-DA or [(3)H]-5HT release from striatal slices of rats adapted to high fat or low fat diet feeding for 3 days. These findings suggest that enterostatin may inhibit dietary fat intake by blocking dopamine reuptake transport to increase central striatal DA release from rats that have acquired diminished dopamine signal after an adaptive period of fat consumption.

Tesofensine, a novel triple monoamine reuptake inhibitor, induces appetite suppression by indirect stimulation of alpha1 adrenoceptor and dopamine D1 receptor pathways in the diet-induced obese rat

Tesofensine is a novel monoamine reuptake inhibitor that inhibits both norepinephrine, 5-HT, and dopamine (DA) reuptake function. Tesofensine is currently in clinical development for the treatment of obesity, however, the pharmacological basis for its strong effect in obesity management is not clarified. Using a rat model of diet-induced obesity (DIO), we characterized the pharmacological mechanisms underlying the appetite suppressive effect of tesofensine. DIO rats treated with tesofensine (2.0 mg/kg, s.c.) for 16 days showed significantly lower body weights than vehicle-treated DIO rats, being reflected by a marked hypophagic response. Using an automatized food intake monitoring system during a 12 h nocturnal test period, tesofensine-induced hypophagia was investigated further by studying the acute interaction of a variety of monoamine receptor antagonists with tesofensine-induced hypophagia in the DIO rat. Tesofensine (0.5-3.0 mg/kg, s.c.) induced a dose-dependent and marked decline in food intake with an ED(50) of 1.3 mg/kg. The hypophagic response of tesofensine (1.5 mg/kg, s.c.) was almost completely reversed by co-administration of prazosin (1.0 mg/kg, alpha(1) adrenoceptor antagonist) and partially antagonized by co-administration of SCH23390 (0.03 mg/kg, DA D(1) receptor antagonist). In contrast, tesofensine-induced hypophagia was not affected by RX821002 (0.3 mg/kg, alpha(2) adrenoceptor antagonist), haloperidol (0.03 mg/kg, D(2) receptor antagonist), NGB2904 (0.1 mg/kg, D(3) receptor antagonist), or ritanserin (0.03 mg/kg, 5-HT(2A/C) receptor antagonist). Hence, the mechanism underlying the suppression of feeding by tesofensine in the obese rat is dependent on the drug's ability to indirectly stimulate alpha(1) adrenoceptor and DA D(1) receptor function.

Olfactory bulbectomy increases food intake and hypothalamic neuropeptide Y in obesity-prone but not obesity-resistant rats

Obese individuals often suffer from depression. The olfactory bulbectomy (OBX) model is an animal model of depression that produces behavioral, physiological, and neurochemical alterations resembling clinical depression. The OBX model was employed to assess depression-related changes in food intake in obesity-prone, Osborne-Mendel (OM) rats and obesity-resistant, S5B/Pl rats. OBX increased food intake in OM rats beginning 7 days following surgery, however, OBX did not alter food intake in S5B/Pl rats at any time point. Fourteen days following surgery, OBX significantly increased locomotor activity (total lines crossed and rears) in the openfield test in OM and S5B/Pl rats. Fifteen days following surgery, prepro-neuropeptide Y (NPY) mRNA levels were significantly increased in the hypothalamus of bulbectomized OM rats and in the medial nucleus of the amygdala of bulbectomized OM and S5B/Pl rats. OBX decreased NPY Y2 receptor mRNA levels in the hypothalamus and medial nucleus of the amygdala in OM rats, while increasing NPY Y2 receptor mRNA levels in the medial nucleus of the amygdala of S5B/Pl rats. These data indicate that though both obesity-prone and obesity-resistant strains were susceptible to the locomotor effects of OBX, food intake and hypothalamic prepro-NPY mRNA were only increased in OM rats. Therefore, strain specific alterations in hypothalamic NPY may account for increased food intake in the obesity-prone rats following OBX, and suggests a potential mechanism to explain the comorbidity of obesity and depression.

Effect of meta-chlorophenylpiperazine and cholecystokinin on food intake of Osborne-Mendel and S5B/P1 rats

OBJECTIVE

To investigate whether there is a difference in sensitivity to a serotonin agonist, meta-chlorophenylpiperazine (mCPP), or cholecystokinin (CCK-8), an intestinal hormone that inhibits food intake, between the Osborne-Mendel (OM) rat, which becomes obese eating a high-fat diet, and the S5B/Pl (S5B) rat, which is resistant to dietary-induced obesity.

RESEARCH METHODS AND PROCEDURES

OM and S5B rats were adapted to either a high-saturated-fat diet (56% energy as fat) or a low-fat diet (10% energy as fat) or to both for 14 days and then treated with several doses of mCPP or CCK-8.

RESULTS

Treatment with mCPP reduced food intake in both strains of rats. The dose-response curve showed that the OM rats had an increased sensitivity to the serotonergic agonist. Animals eating the high-fat diet had less response to mCPP; and in the S5B rats, the response was significantly reduced. After treatment with CCK-8, there was a similar dose-related suppression of food intake in both the OM and S5B rats.

DISCUSSION

These data are consistent with the hypothesis that the serotonin system in the S5B rat has a greater activity that could act to inhibit fat intake. The response to CCK was not significantly affected by strain or diet.

Molecular mechanisms associated with leptin resistance: n-3 polyunsaturated fatty acids induce alterations in the tight junction of the brain

High-fat diets cause peripheral leptin resistance, and dietary lipid composition affects sensitivity to leptin. We examined the role of n-3 polyunsaturated fatty acid (PUFA) in peripheral leptin resistance. Dietary PUFAs (0.4% wt/wt) caused insensitivity to peripherally but not intracerebroventricularly administered leptin. n-3 PUFA increased body weight, associated with a significant reduction of leptin concentration in the cerebrospinal fluid. Dietary n-3 PUFA reduced transport of endogenous or exogenously administered leptin into the brain, associated with increased expression of hypothalamic occludin, but caused no change in expression of leptin receptors, proteins associated with leptin signaling or other tight junction proteins. Continuous intracerebroventricular infusion of an antisense morpholino oligonucleotide targeted to occludin mRNA reversed n-3 PUFA-induced insensitivity to peripherally administered leptin. We conclude that n-3 PUFA induces peripheral leptin resistance via an increase in the expression of hypothalamic occludin, reducing paracellular transport of leptin into the brain.

Effects of a high-fat diet and strain on hypothalamic gene expression in rats

OBJECTIVE

This study was designed to investigate whether dietary fat and genetic background might differentially alter the expression of hypothalamic genes involved in food intake.

RESEARCH METHODS AND PROCEDURES

Three-month-old Osborne-Mendel (OM) and S5B/Pl rats were fed either a high-fat or a low-fat diet for 14 days. mRNA for neuropeptide Y (NPY), corticotrophin-releasing hormone, NPY Y-1 receptor and Y-5 receptor, and serotonin 2c (5-HT2c) receptors were measured using Northern blotting or ribonuclease protection assays.

RESULTS

OM rats showed an increased expression of NPY and corticotrophin-releasing hormone compared with S5B/Pl rats. The expression of NPY-Y1 and -Y5 receptor mRNA was significantly higher in the hypothalamus of OM rats compared with S5B/Pl rats. The expression of 5HT-2c receptor mRNA was significantly reduced in both strains of rats eating a high-fat diet when compared with the animals eating the low-fat diet.

DISCUSSION

These data suggest that over activity of the NPY system may contribute to the development of obesity in OM rats and that expression of the 5HT-2c receptor gene may be modulated by dietary fat.

The influence of different fats and fatty acids on obesity, insulin resistance and inflammation

Dietary fat and its relation to obesity has been a controversial issue for several years. In this review, several kinds of data relating to this issue are presented. There are epidemiological cross-country data and data within countries showing an effect. However, in the United States, the intake of fat appears to be declining, whereas the prevalence of obesity rises-the American Paradox. Clinical studies show that trans fatty acids can increase insulin resistance and that exercise can enhance the rate of adaptation to a high fat diet by increasing the rate of fat oxidation. The differences in response of inflammatory signals and of insulin resistance to different fatty acids indicate that not all fatty acids are the same. There are also experimental data showing that most, but not all, animals consuming a high fat diet will become obese. A number of mechanisms have been postulated for this difference, including differential sensitivities to neurotransmitters, to the intestinal peptide, enterostatin, and to individual fatty acids. One important conclusion from this review is that both total fat and individual fatty acids have to be considered when reaching conclusions about dietary fat and obesity.

Effect of a serotonin 1-A agonist on food intake of Osborne-Mendel and S5B/P1 rats

The effect on food intake of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a serotonin 1-A agonist, has been evaluated in two strains of rats that differ in their sensitivity to becoming obese while eating a high-fat diet. Male Osborne-Mendel (OM) and S5B/Pl rats were tested at 8 weeks and 16 weeks of age. Both strains were adapted to choose between two diets-a 56% fat energy diet, and a 10% fat energy diet-which were equicaloric for protein (24% energy). Daily food intake was measured for 2 weeks before injection of 8-OH-DPAT. The younger OM rats had no diet preference, while the older OM rats preferred the high-fat diet. The younger S5B/P1 rats preferred the low-fat diet, while the older S5B/P1 rats had no diet preference. Satiated rats were injected subcutaneously with 8-OH-DPAT at doses of 0.3, 1.0, or 3.0 mg/kg or vehicle. During the light phase, subcutaneous 8-OH-DPAT increased the intake of the high-fat diet in the 16-week-old OM rats but not the 8-week-old OM rats. 8-OH-DPAT had no effect on the low-fat diet intake in either age OM rat. The 8-week-old S5B/P1 rats showed a significant decrease in intake of the high-fat diet in response to 8-OH-DPAT, whereas an increase in the intake of the low-fat diet was observed in the older S5B/P1 rats. These data are consistent with the hypothesis that increased fat preference in Osborne-Mendel rats may result in part from altered serotonin activity of 5-HT(1A) receptors.

Feeding response to mercaptoacetate in Osborne-Mendel and S5B/PL rats

The purpose of this experiment was to determine if Osborne-Mendel (OM) rats, which are susceptible to dietary-induced obesity, and S5B/PL (S5B) rats, which are resistant to dietary-induced obesity, differ in their feeding responses to mercaptoacetate (MA), which blocks fatty acid oxidation, or 2-deoxy-D-glucose (2DG), which blocks glucose utilization. 2DG (100 mg/kg or 200 mg/kg) increased food intake in both strains of rats on a high-fat diet (56% energy from fat). Mercaptoacetate (600 mumol/kg) increased food intake in OM but not S5B rats on a high-fat diet. When maintained on a low-fat diet (10% energy from fat), MA (400 mumol/kg or 600 mumol/kg) stimulated food intake in OM rats, whereas S5B rats increased food intake only after the highest dose of MA (600 mumol/kg). MA stimulated carbohydrate and protein intake in OM rats maintained on a macronutrient selection diet, whereas S5B rats maintained on this diet did not significantly increase intake of any macronutrient after MA. These results demonstrate that OM and S5B rats have a similar food intake response to 2DG but a dissimilar response to MA. The variable response to MA in these strains may be due to a difference in peripheral or central signaling systems related to fatty acid oxidation or a difference in metabolic environments between the strains, which in turn affects the feeding response to MA. These studies suggest that a difference in control of fatty acid oxidation may account for the difference in susceptibility to obesity when eating a high-fat diet.