Victoria Cano

High-fat diets impair spatial learning in the radial-arm maze in mice

It has been suggested that hyperglycemia and insulin resistance triggered by energy-dense diets can account for hippocampal damage and deficits of cognitive behaviour. We wonder if the impairment of learning and memory processes detected in diet-induced obese (DIO) mice is linked to diet composition itself. With this purpose we have evaluated learning performance in mice undergoing a short-term high-fat (HF) treatment, which leads to a pre-obese state characterized by increased adiposity without significant changes of glucose and insulin plasma levels. C57BL/6J mice were fed either a HF (45 kcal% from fat) or control diet (10 kcal% from fat) during 8 weeks. Learning performance was evaluated by using the four-arm baited version of the eight-arm radial maze test (RAM). Mice were trained to learn the RAM protocol and then memory was tested at different time-points. Time spent to consume food placed in baited arms and errors committed to find them were measured in all sessions. DIO mice significantly spent more time in learning the task and made a greater number of errors than controls. Moreover, retention tests revealed that both working and total memory errors were also more numerous in DIO mice. The current results show that short-term DIO impairs spatial learning and suggest that impairment of hippocampal learning elicited by HF diets might be perceptible before metabolic alterations linked to obesity develop.

High-fat diets induce changes in hippocampal glutamate metabolism and neurotransmission.

Obesity and high-fat (HF) diets have a deleterious impact on hippocampal function and lead to impaired synaptic plasticity and learning deficits. Because all of these processes need an adequate glutamatergic transmission, we have hypothesized that nutritional imbalance triggered by these diets might eventually concern glutamate (Glu) neural pathways within the hippocampus. Glu is withdrawn from excitatory synapses by specific uptake mechanisms involving neuronal (EAAT-3) and glial (GLT-1, GLAST) transporters, which regulate the time that synaptically released Glu remains in the extracellular space and, consequently, the duration and location of postsynaptic receptor activation. The goal of the present study was to evaluate in mouse hippocampus the effect of a short-term high-fat dietary treatment on 1) Glu uptake kinetics, 2) the density of Glu carriers and Glu-degrading enzymes, 3) the density of Glu receptor subunits, and 4) synaptic transmission and plasticity. Here, we show that HF diet triggers a 50% decrease of the Michaelis-Menten constant together with a 300% increase of the maximal velocity of the uptake process. Glial Glu carriers GLT-1 and GLAST were upregulated in HF mice (32 and 27%, respectively), whereas Glu-degrading enzymes glutamine synthase and GABA-decarboxilase appeared to be downregulated in these animals. In addition, HF diet hippocampus displayed diminished basal synaptic transmission and hindered NMDA-induced long-term depression (NMDA-LTD). This was coincident with a reduced density of the NR2B subunit of NMDA receptors. All of these results are compatible with the development of leptin resistance within the hippocampus. Our data show that HF diets upregulate mechanisms involved in Glu clearance and simultaneously impair Glu metabolism. Neurochemical changes occur concomitantly with impaired basal synaptic transmission and reduced NMDA-LTD. Taken together, our results suggest that HF diets trigger neurochemical changes, leading to a desensitization of NMDA receptors within the hippocampus, which might account for cognitive deficits.

Adaptative Nitric Oxide Overproduction in Perivascular Adipose Tissue during Early Diet-Induced Obesity

Perivascular adipose tissue (PVAT) plays a paracrine role in regulating vascular tone. We hypothesize that PVAT undergoes adaptative mechanisms during initial steps of diet-induced obesity (DIO) which contribute to preserve vascular function. Four-week-old male C57BL/6J mice were assigned either to a control [low-fat (LF); 10% kcal from fat] or to a high-fat diet (HF; 45% kcal from fat). After 8 wk of dietary treatment vascular function was analyzed in the whole perfused mesenteric bed (MB) and in isolated mesenteric arteries cleaned of PVAT. Relaxant responses to acetylcholine (10(-9)-10(-4) m) and sodium nitroprusside (10(-12)-10(-5) m) were significantly ameliorated in the whole MB from HF animals. However, there was no difference between HF and LF groups in isolated mesenteric arteries devoid of PVAT. The enhancement of relaxant responses detected in HF mice was not attributable to an increased release of nitric oxide (NO) from the endothelium nor to an increased sensitivity and/or activity of muscular guanilylcyclase. Mesenteric PVAT of HF animals showed an increased bioavailability of NO, detected by 4,5-diaminofluorescein diacetate (DAF2-DA) staining, which positively correlated with plasma leptin levels. DAF-2DA staining was absent in PVAT from ob/ob mice but was detected in these animals after 4-wk leptin replacement. The main finding in this study is that adaptative NO overproduction occurs in PVAT during early DIO which might be aimed at preserving vascular function.

Aliskiren reduces body-weight gain, adiposity and plasma leptin during diet-induced obesity

Background and purpose:  Overfeeding increases adipose tissue mass and leptin production and up‐regulates the renin‐angiotensin system in adipose tissue in rodents. Here, we determined the effect of chronic treatment with the renin inhibitor, aliskiren, in a model of diet‐induced obesity in mice, on: (i) body weight, adipose tissue weight and plasma leptin; (ii) food intake and caloric efficiency; and (iii) angiotensin II (Ang II) in adipose tissue.

Shift of Circadian Feeding Pattern by High-Fat Diets Is Coincident with Reward Deficits in Obese Mice

Recent studies provide evidence that high-fat diets (HF) trigger both i) a deficit of reward responses linked to a decrease of mesolimbic dopaminergic activity, and ii) a disorganization of circadian feeding behavior that switch from a structured meal-based schedule to a continuous snacking, even during periods normally devoted to rest. This feeding pattern has been shown to be a cause of HF-induced overweight and obesity. Our hypothesis deals with the eventual link between the rewarding properties of food and the circadian distribution of meals. We have investigated the effect of circadian feeding pattern on reward circuits by means of the conditioned-place preference (CPP) paradigm and we have characterized the rewarding properties of natural (food) and artificial (cocaine) reinforcers both in free-feeding ad libitum HF mice and in HF animals submitted to a re-organized feeding schedule based on the standard feeding behavior displayed by mice feeding normal chow (“forced synchronization”). We demonstrate that i) ad libitum HF diet attenuates cocaine and food reward in the CPP protocol, and ii) forced synchronization of feeding prevents this reward deficit. Our study provides further evidence that the rewarding impact of food with low palatability is diminished in mice exposed to a high-fat diet and strongly suggest that the decreased sensitivity to chow as a positive reinforcer triggers a disorganized feeding pattern which might account for metabolic disorders leading to obesity.

A cholecystokinin-1 receptor agonist (CCK-8) mediates increased permeability of brain barriers to leptin

Leptin regulates energy expenditure and body weight by acting both on the hypothalamus and on peripheral targets. Central actions of leptin are enhanced by cholecystokinin (CCK). The interaction between leptin and CCK makes physiological sense, as rats lacking CCK1 receptors are resistant to peripheral leptin but not to leptin directly infused into the brain. We have recently reported that CCK enhances leptin effects by increasing the entry of leptin into the CNS. The aim of this work was to further characterize the effect of CCK (10 μg kg−1) on leptin kinetics as well as the CCK receptor subtype involved in the interaction between CCK and leptin.

Circadian Feeding Drive of Metabolic Activity in Adipose Tissue and not Hyperphagia Triggers Overweight in Mice: Is There a Role of the Pentose-Phosphate Pathway?

High-fat (HF) diets trigger an increase in adipose tissue and body weight (BW) and disordered eating behavior. Our study deals with the hypothesis that circadian distribution of energy intake is more relevant for BW dynamics than diet composition. Four-week-old mice were exposed for 8 wk to a HF diet and compared with animals receiving control chow. HF mice progressively increased BW, decreased the amount of nocturnal (1800-0900 h) calories (energy or food intake) (30%) and increased diurnal (0900-1800 h) caloric intake (energy or food intake), although total daily intake was identical between groups. Animals were killed at 3-h intervals and plasma insulin, leptin, corticosterone, glucose, and fatty acid levels quantified. Adipose tissue was weighed, and enzymatic activities integral to the pentose phosphate pathway (PPP) assayed in lumbar adipose tissue. Phosphorylated AMP-dependent protein kinase and fatty acid synthase were quantified by Western blotting. In HF mice, there was a shift in the circadian oscillations of plasma parameters together with an inhibition of PPP activity and a decrease in phosphorylated AMP-dependent protein kinase and fatty acid synthase. In a second experiment, HF mice were forced to adhere to a circadian pattern of food intake similar to that in control animals. In this case, BW, adipose tissue, morning plasma parameters and PPP activity appeared to be normal. These data indicate that disordered feeding behavior can trigger BW gain independently of food composition and daily energy intake. Because PPP is the main source of reduced nicotinamide adenine dinucleotide phosphate, we suggest that PPP inhibition might be an early marker of adipose dysfunction in diet-induced obesity.

Leptin resistance develops spontaneously in mice during adult life in a tissue-specific manner. Consequences for hepatic steatosis.

Leptin is an adipocyte-derived hormone which stimulates β-oxidation in peripheral tissues and prevents steatosis. Because leptin production naturally increases during adult life, we have hypothesized that leptin receptors might undergo a physiological and gradual desensitization during ageing. Therefore we have characterized in three- five- and ten-month old mice i) the weight of different white adipose pads, heart and liver, ii) lipid content in these tissues/organs, and iii) responsiveness to acute leptin, measured in terms of phosphorylation of signal transducer and activator of transcription 3 (STAT3) and protein kinase B (Akt). In this study we have detected that leptin-mediated STAT3 phosphorylation appears to be preserved in cardiac tissue even in 10-month old animals but not in adipose tissue and liver of five- and ten-month old mice, respectively. Nevertheless, leptin increased pAkt content in the liver of these mice. In a parallel study we have analyzed the functionality of leptin signalling pathways in 10-month old obese mice and we have observed that the STAT3 pathway appears to be only operative in the heart whereas the Akt pathway remains functional both in heart and liver. Nevertheless, hepatic lipids increased almost 300% compared to age-matched lean controls. Our data demonstrate that during adult life there is a lost of leptin receptor functionality which is tissue-dependent and mainly affects the STAT3 pathway. Otherwise we demonstrate that the antisteatotic effect of leptin is independent of the Akt signalling pathway.

Effect of peripheral cholecystokinin receptor agonists on c-Fos expression in brain sites mediating food consumption in rats

Peripheral cholecystokinin (CCK) elicits satiety by acting on hypothalamic nuclei. The anoretic effect of CCK is mediated by the vagus nerve and involves brainstem areas receiving vagal inputs, such as the nucleus tractus solitarius (NTS) and the area postrema (AP). This work aims to analyze, by measuring c-Fos expression, the effect of selective CCK receptor agonists on brain areas involved in food-intake/satiety process. We observed that SR-146,131, a CCK(1)R agonist, increased c-Fos expression in NTS and AP as well as in some hypothalamic nuclei. CCK-4, a CCK(2)R agonist which does not cross the blood-brain barrier (BBB), only was effective in the hypothalamus. Our data show that the activation of the brainstem is not a requisite to obtain a hypothalamic effect of peripheral CCK and suggest that CCK-4 may indirectly stimulate hypothalamic areas endowed with BBB, without previous activation of neither NTS nor AP.

Sensitivity of cardiac carnitine palmitoyltransferase to malonyl-CoA is regulated by leptin: similarities with a model of endogenous hyperleptinemia.

Acute leptin increase as well as endogenous hyperleptinemia evoked by high-fat diets (HF) activate fatty acid metabolism in nonadipose tissues. This supports the notion that hyperleptinemia is pivotal to prevent/delay steatosis during periods of positive energy balance. We have previously shown that long-term HF spares ectopic accumulation of lipids specifically in the miocardium. Because carnitine palmitoyltransferase I (CPT-I) allows mitochondrial uptake/oxidation of fatty acids, we have hypothesized that leptin drives cardiac CPT-I activity. In the current study, hyperleptinemia was induced in C57BL/6J mice either by exogenous leptin administration or by means of HF, and the ability of malonyl-coenzyme A (malonyl-CoA) (the main endogenous inhibitor of CPT-I) to inhibit cardiac CPT was analyzed. IC(50) values of malonyl-CoA were 8.1 +/- 1.5 micromol/liter in controls vs. 69.3 +/- 5.2 micromol/liter (P < 0.01) in leptin-treated mice. This effect was also observed in cardiac explants incubated with leptin and was blocked by triciribine, a compound shown to inhibit protein kinase B (Akt) phosphorylation (pAkt). In accordance, acute leptin evoked an increase of cardiac pAkt levels, which correlated with CPT sensitivity to malonyl-CoA. Otherwise, the inhibitory effect of malonyl-CoA was hindered in HF hyperleptinemic mice, and in this case, pAkt levels also correlated with CPT sensitivity to malonyl-CoA. Our data show that leptin reduces the sensitivity of cardiac CPT-I to malonyl-CoA and suggest the involvement of an Akt-related signaling pathway in this effect. This mechanism appears to be sensitive to both acute and chronic hyperleptinemia. We conclude that this action of leptin is pivotal to drive cardiac metabolism under situations associated to hyperleptinemia.