Effects of leptin and cholecystokinin in rats with a null mutation of the leptin receptor Lepr(fak)

Cardiac Ion Channel Regulation in Obesity and the Metabolic Syndrome: Relevance to Long QT Syndrome and Atrial Fibrillation

Obesity and its associated metabolic dysregulation leading to metabolic syndrome is an epidemic that poses a significant public health problem. More than one-third of the world population is overweight or obese leading to enhanced risk of cardiovascular disease (CVD) incidence and mortality. Obesity predisposes to atrial fibrillation, ventricular, and supraventricular arrhythmias; conditions that are underlain by dysfunction in electrical activity of the heart. To date, current therapeutic options for cardiomyopathy of obesity are limited, suggesting that there is considerable room for development of therapeutic interventions with novel mechanisms of action that will help normalize rhythm in obese patients. Emerging candidates for modulation by obesity are cardiac ion channels and Ca handling proteins. However, the underlying molecular mechanisms of the impact of obesity on these channels/Ca handling proteins remain incompletely understood. Obesity is marked by accumulation of adipose tissue associated with a variety of adverse adaptations including dyslipidemia (or abnormal levels of serum free fatty acids), increased secretion of pro-inflammatory cytokines, fibrosis, hyperglycemia, and insulin resistance, that will cause electrical remodeling and thus predispose to arrhythmias. Further, adipose tissue is also associated with the accumulation of subcutaneous and visceral fat, which are marked by distinct signaling mechanisms. Thus, there may also be functional differences in the outcome of regional distribution of fat deposits on ion channel/Ca handling proteins expression. Evaluating alterations in their functional expression in obesity will lead to progress in the knowledge about the mechanisms responsible for obesity-related arrhythmias. These advances are likely to reveal new targets for pharmacological modulation. The objective of this article is to review cardiac ion channel/Ca handling proteins remodeling that predispose to arrhythmias. Understanding how obesity and related mechanisms lead to cardiac electrical remodeling is likely to have a significant medical and economic impact.

Irbesartan treatment up-regulates hepatic expression of PPARalpha and its target genes in obese Koletsky (fa(k)/fa(k)) rats: a link to amelioration of hypertriglyceridaemia

BACKGROUND AND PURPOSE

Hypertriglyceridaemia is associated with an increased risk of cardiovascular disease. Irbesartan, a well-established angiotensin II type 1 receptor (AT(1)) blocker, improves hypertriglyceridaemia in rodents and humans but the underlying mechanism of action is unclear.

EXPERIMENTAL APPROACH

Male obese Koletsky (fa(k)/fa(k)) rats, which exhibit spontaneous hypertension and metabolic abnormalities, received irbesartan (40 mg x kg(-1) x day(-1)) or vehicle by oral gavage over 7 weeks. Adipocyte-derived hormones in plasma were measured by ELISA. Gene expression in liver and other tissues was assessed by real-time PCR and Western immunoblotting.

KEY RESULTS

In Koletsky (fa(k)/fa(k)) rats irbesartan lowered plasma concentrations of triglycerides and non-esterified fatty acids, and decreased plasma insulin concentrations and the homeostasis model assessment of insulin resistance index. However, this treatment did not affect food intake, body weight, epididymal white adipose tissue weight, adipocyte size and plasma leptin concentrations, although plasma adiponectin was decreased. Irbesartan up-regulated hepatic expression of mRNAs corresponding to peroxisome proliferator-activated receptor (PPAR)alpha and its target genes (carnitine palmitoyltransferase-1a, acyl-CoA oxidase and fatty acid translocase/CD36) that mediate hepatic fatty acid uptake and oxidation; the increase in hepatic PPARalpha expression was confirmed at the protein level. In contrast, irbesartan did not affect expression of adipose PPARgamma and its downstream genes or hepatic genes that mediate fatty acid synthesis.

CONCLUSIONS AND IMPLICATIONS

These findings demonstrate that irbesartan treatment up-regulates PPARalpha and several target genes in liver of obese spontaneously hypertensive Koletsky (fa(k)/fa(k)) rats and offers a novel insight into the lipid-lowering mechanism of irbesartan.

Neuropeptide Y in normal eating and in genetic and dietary-induced obesity

Neuropeptide Y (NPY) is one the most potent orexigenic peptides found in the brain. It stimulates food intake with a preferential effect on carbohydrate intake. It decreases latency to eat, increases motivation to eat and delays satiety by augmenting meal size. The effects on feeding are mediated through at least two receptors, the Y1 and Y5 receptors. The NPY system for feeding regulation is mostly located in the hypothalamus. It is formed of the arcuate nucleus (ARC), where the peptide is synthesized, and the paraventricular (PVN), dorsomedial (DMN) and ventromedial (VMN) nuclei and perifornical area where it is active. This activity is modulated by the hindbrain and limbic structures. It is dependent on energy availability, e.g. upregulation with food deprivation or restriction, and return to baseline with refeeding. It is also sensitive to diet composition with variable effects of carbohydrates and fats. Leptin signalling and glucose sensing which are directly linked to diet type are the most important factors involved in its regulation. Absence of leptin signalling in obesity models due to gene mutation either at the receptor level, as in the Zucker rat, the Koletsky rat or the db/db mouse, or at the peptide level, as in ob/ob mouse, is associated with increased mRNA abundance, peptide content and/or release in the ARC or PVN. Other genetic obesity models, such as the Otsuka-Long-Evans-Tokushima Fatty rat, the agouti mouse or the tubby mouse, are characterized by a diminution in NPY expression in the ARC nucleus and by a significant increase in the DMN. Further studies are necessary to determine the exact role of NPY in these latter models. Long-term exposure to high-fat or high-energy palatable diets leads to the development of adiposity and is associated with a decrease in hypothalamic NPY content or expression, consistent with the existence of a counter-regulatory mechanism to diminish energy intake and limit obesity development. On the other hand, an overactive NPY system (increased mRNA expression in the ARC associated with an upregulation of the receptors) is characteristic of rats or rodent strains sensitive to dietary-induced obesity. Finally, NPY appears to play an important role in body weight and feeding regulation, and while it does not constitute the only target for drug treatment of obesity, it may nevertheless provide a useful target in conjunction with others.

Leptin action in the forebrain regulates the hindbrain response to satiety signals

The capacity to adjust energy intake in response to changing energy requirements is a defining feature of energy homeostasis. Despite the identification of leptin as a key mediator of this process, the mechanism whereby changes of body adiposity are coupled to adaptive, short-term adjustments of energy intake remains poorly understood. To investigate the physiological role of leptin in the control of meal size and the response to satiety signals, and to identify brain areas mediating this effect, we studied Koletsky (fa(k)/fa(k)) rats, which develop severe obesity due to the genetic absence of leptin receptors. Our finding of markedly increased meal size and reduced satiety in response to the gut peptide cholecystokinin (CCK) in these leptin receptor-deficient animals suggests a critical role for leptin signaling in the response to endogenous signals that promote meal termination. To determine if the hypothalamic arcuate nucleus (ARC) (a key forebrain site of leptin action) mediates this leptin effect, we used adenoviral gene therapy to express either functional leptin receptors or a reporter gene in the area of the ARC of fa(k)/fa(k) rats. Restoration of leptin signaling to this brain area normalized the effect of CCK on the activation of neurons in the nucleus of the solitary tract and area postrema, key hindbrain areas for processing satiety-related inputs. This intervention also reduced meal size and enhanced CCK-induced satiety in fa(k)/fa(k) rats. These findings demonstrate that forebrain signaling by leptin, a long-term regulator of body adiposity, limits food intake on a meal-to-meal basis by regulating the hindbrain response to short-acting satiety signals.

Leptin action in the forebrain regulates the hindbrain response to satiety signals

The capacity to adjust energy intake in response to changing energy requirements is a defining feature of energy homeostasis. Despite the identification of leptin as a key mediator of this process, the mechanism whereby changes of body adiposity are coupled to adaptive, short-term adjustments of energy intake remains poorly understood. To investigate the physiological role of leptin in the control of meal size and the response to satiety signals, and to identify brain areas mediating this effect, we studied Koletsky (fa(k)/fa(k)) rats, which develop severe obesity due to the genetic absence of leptin receptors. Our finding of markedly increased meal size and reduced satiety in response to the gut peptide cholecystokinin (CCK) in these leptin receptor-deficient animals suggests a critical role for leptin signaling in the response to endogenous signals that promote meal termination. To determine if the hypothalamic arcuate nucleus (ARC) (a key forebrain site of leptin action) mediates this leptin effect, we used adenoviral gene therapy to express either functional leptin receptors or a reporter gene in the area of the ARC of fa(k)/fa(k) rats. Restoration of leptin signaling to this brain area normalized the effect of CCK on the activation of neurons in the nucleus of the solitary tract and area postrema, key hindbrain areas for processing satiety-related inputs. This intervention also reduced meal size and enhanced CCK-induced satiety in fa(k)/fa(k) rats. These findings demonstrate that forebrain signaling by leptin, a long-term regulator of body adiposity, limits food intake on a meal-to-meal basis by regulating the hindbrain response to short-acting satiety signals.

Effects of continuous lumbar intrathecal infusion of leptin in rats on weight regulation

Intracranial leptin alters food consumption and body weight. To systematically characterize the effects of extended continuous spinal intrathecal delivery on such regulation, female rats received continuous lumbar spinal infusion (14 days) through catheters connected to osmotic minipumps of a vehicle or one of several doses of recombinant murine leptin (0.03-10 microg/day). The following observations were made. (1) Leptin resulted in a dose-dependent suppression in body weight and food consumption at doses above 0.3 microg/day. (2) Food consumption was initially reduced. Weight fell for 7 days and then plateaued at a level proportional to dose. (3) The ratio of food consumed to body weight was constant for control animals across the study. Leptin-infused rats slowed the initial fall in weight by increasing food consumption, such that the food to body weight ratio returned to that of control values. Rats were thus regulating food consumption to sustain body weight as defined by leptin dose. (4) On day 14, cisternal cerebrospinal fluid was obtained and leptin measured. Concentrations covaried in a log linear fashion with infusion dose. Body weight and food consumption covaried similarly with cisternal leptin concentrations across dose groups. Our data suggest that steady state infusions of leptin induce a degree of appetite suppression that leads to a steady state level of body weight loss and not simply to a simple block of consumatory behavior. The unexpected potency of the observed effects of intrathecal leptin relative to doses that are required after i.c.v. delivery suggests that at least a portion of the effects of intrathecal leptin may reflect a medullary action. The observed correlation of cisternal leptin levels with the behavioral effects is also consistent with a reliable distribution of the infused leptin to target supraspinal sites.

Corticotropin-releasing factor decreases meal size by decreasing cluster number in Koletsky (LA/N) rats with and without a null mutation of the leptin receptor

The homozygous mutant Koletsky rat is a monogenic form of obesity and hyperphagia due to a null mutation of the leptin receptor (lepr(fak)). To investigate if the lack of leptin action on the brain of homozygous mutants affected the inhibitory potency of corticotropin-releasing factor (CRF) on meal size, artificial cerebrospinal fluid or one of five doses of CRF was administered through third ventricular cannulas in 8 +/+, 10 +/fa(k), and 8 fa(k)/fa(k) rats 15 min before access to 20% sucrose in lickometer tubes for 30 min. CRF had equivalent inhibitory potency in fa(k)/fa(k) and +/+ rats. Thus, the complete lack of leptin action in fa(k)/fa(k) rats did not change the inhibitory potency of CRF. CRF was significantly more potent, however, in +/fa(k) rats than in the other two genotypes. Thus, the heterozygote condition of this mutation did not function as a classical recessive mutation for this behavioral phenotype. Despite these differences in potency, microstructural analysis revealed that CRF decreased intakes in all three genotypes by decreasing the number of clusters of licking without changing the size of clusters.

Central melanocortin system modulates energy intake and expenditure of obese and lean Zucker rats

Melanocortins play a critical role in appetite and body weight regulation, because manipulations of this pathway can lead to the development of obesity in several animal models. The purpose of this study was to use a melanocortin receptor agonist and antagonist to evaluate the involvement of melanocortins in feeding, energy metabolism, and body weight regulation in lean and obese Zucker rats. Central administration of a melanocortin receptor antagonist (SHU9119) elevated food intake and body weight of lean Zucker rats but had little effect in obese Zucker rats. In contrast, the melanocortin receptor agonist MTII reduced food intake in both lean and obese rats but was more potent in the obese Zucker rats. These data indicate the existence of functional melanocortin receptors in both lean and obese Zucker rats but suggest that obese Zucker rats have reduced endogenous melanocortin tone. In addition to its effects on food intake, MTII infusion elevated oxygen consumption and decreased respiratory quotient dose dependently during the light cycle. Our data suggest that a melanocortin receptor agonist can induce weight loss by increasing energy expenditure and promoting body fat utilization in addition to its inhibitory effects on food intake in both obese and lean Zucker rats.

Leptin deficiency induced by fasting impairs the satiety response to cholecystokinin

Leptin administration potentiates the satiety response to signals such as cholecystokinin (CCK), that are released from the gut during a meal. To investigate the physiological relevance of this observation, we hypothesized that leptin deficiency, induced by fasting, attenuates the satiety response to CCK. To test this hypothesis, 48-h-fasted or fed rats were injected with i.p. saline or CCK. Fasting blunted the satiety response to 3.0 microg/kg CCK, such that 30-min food intake was suppressed by 65.1% (relative to saline-treated controls) in fasted rats vs. 85.9% in the fed state (P < 0.05). In a subsequent experiment, rats were divided into three groups: 1) vehicle/fed; 2) vehicle/fasted; and 3) leptin-replaced/fasted; and each group received 3.0 microg/kg i.p. CCK. As expected, the satiety response to CCK was attenuated by fasting in vehicle-treated rats (30-min food intake: vehicle/fed, 0.3 +/- 0.1 g; vehicle/fasted, 1.7 +/- 0.4 g; P < 0.01), and this effect was prevented by leptin replacement (0.7 +/- 0.2 g, P < 0.05 vs. vehicle/fasted; P = not significant vs. vehicle/fed). To investigate whether elevated neuropeptide Y (NPY) signaling plays a role in the effect of leptin deficiency to impair the response to CCK, we measured the response to 3.0 microg/kg i.p. CCK after treatment with 7.5 microg intracerebroventricular NPY. We found that both CCK-induced satiety and its ability to increase c-Fos-like-immunoreactivity in key brainstem-feeding centers were attenuated by NPY pretreatment. We conclude that an attenuated response to meal-related satiety signals is triggered by leptin deficiency and may contribute to increased food intake.

The controls of eating: a shift from nutritional homeostasis to behavioral neuroscience

The experimental analysis of the controls of eating has undergone a paradigmatic shift in the past decade. Instead of seeing meals as a problem of how intake serves metabolism and nutritional homeostasis, meals are now seen as a problem in behavioral neuroscience. The major developments underlying this significant change are behavioral, neurological, and theoretical. Behavioral analysis has shown that a central pattern generator in the caudal brainstem organizes eating movements and that the size of a liquid meal is determined by the number and size of clusters of licking. Neurologic analysis has shown that eating is under orosensory positive-feedback control and postingestive, preabsorptive, negative-feedback control. These feedback controls are activated by food ingested during a meal. The sensory information of the feedbacks is carried by afferent fibers that project to the caudal brainstem. The new theory is based on the fact that the feedback controls are stimulated by food acting directly on mucosal receptors along the gastrointestinal tract, from the mouth to the end of the small intestine. Thus, they are referred to as direct controls, and the caudal brainstem is sufficient for organizing their action. All other controls, such as metabolic, rhythmic, and ecologic, that do not contact the mucosal receptors are indirect controls. Indirect controls act by modulating the potency of the central effects of the direct controls, and they require the forebrain and its reciprocal connections with the caudal brainstem for their control of eating and meal size.