Effect of sibutramine on macronutrient selection in male and female rats

Can We Selectively Reduce Appetite for Energy-Dense Foods? An Overview of Pharmacological Strategies for Modification of Food Preference Behavior

Excessive intake of food, especially palatable and energy-dense carbohydrates and fats, is largely responsible for the growing incidence of obesity worldwide. Although there are a number of candidate antiobesity drugs, only a few of them have been proven able to inhibit appetite for palatable foods without the concurrent reduction in regular food consumption. In this review, we discuss the interrelationships between homeostatic and hedonic food intake control mechanisms in promoting overeating with palatable foods and assess the potential usefulness of systemically administered pharmaceuticals that impinge on the endogenous cannabinoid, opioid, aminergic, cholinergic, and peptidergic systems in the modification of food preference behavior. Also, certain dietary supplements with the potency to reduce specifically palatable food intake are presented. Based on human and animal studies, we indicate the most promising therapies and agents that influence the effectiveness of appetite-modifying drugs. It should be stressed, however, that most of the data included in our review come from preclinical studies; therefore, further investigations aimed at confirming the effectiveness and safety of the aforementioned medications in the treatment of obese humans are necessary.

Serotonin signaling mediates protein valuation and aging

Research into how protein restriction improves organismal health and lengthens lifespan has largely focused on cell-autonomous processes. In certain instances, however, nutrient effects on lifespan are independent of consumption, leading us to test the hypothesis that central, cell non-autonomous processes are important protein restriction regulators. We characterized a transient feeding preference for dietary protein after modest starvation in the fruit fly, Drosophila melanogaster, and identified tryptophan hydroxylase (Trh), serotonin receptor 2a (5HT2a), and the solute carrier 7-family amino acid transporter, JhI-21, as required for this preference through their role in establishing protein value. Disruption of any one of these genes increased lifespan up to 90% independent of food intake suggesting the perceived value of dietary protein is a critical determinant of its effect on lifespan. Evolutionarily conserved neuromodulatory systems that define neural states of nutrient demand and reward are therefore sufficient to control aging and physiology independent of food consumption.

DOI:http://dx.doi.org/10.7554/eLife.16843.001

Limiting the amount of protein eaten, while still eating enough to avoid starving, has an unexpected effect: it can slow down aging and extend the lifespan in many animals from flies to mice. Previous work suggests that how an animal perceives food can also influence how fast the animal ages. For example, both flies and worms actually have shorter lifespans if their food intake is reduced when they can still “smell” food in their environment. However, the sensory cues that trigger changes in lifespan and the molecular mechanisms behind these effects are largely unknown.

Ro et al. therefore asked whether fruit flies recognize protein in their food, and if so, whether such a recognition system would influence how the flies age. Flies that had been deprived of food for a brief period tended to eat more protein than other flies that had not been starved. Ro et al. then revealed that serotonin, a brain chemical that can alter the activity of nerve cells, plays a key role in how fruit flies decide to feed specifically on foods that contain protein. Further experiments revealed also that flies age faster when they are allowed to interact with protein in their diet independently from other nutrients, despite eating the same amount. Disrupting any of several components involved in serotonin signaling protected the flies from this effect and led to them living almost twice as long under these conditions.

Ro et al. propose that the components of the recognition system work together to determine the reward associated with consuming protein by enhancing how much an animal values the protein in its food. As such, it is this protein reward or value – rather than just eating protein itself – that influences how quickly the fly ages. Further work is now needed to understand how the brain mechanisms that allow animals to perceive and evaluate food act to control lifespan and aging.

DOI:http://dx.doi.org/10.7554/eLife.16843.002

JTT-130, a novel intestine-specific inhibitor of microsomal triglyceride transfer protein, reduces food preference for fat

Microsomal triglyceride transfer protein (MTP) is involved in the assembly and secretion of triglyceride-rich lipoproteins from enterocytes and hepatocytes. JTT-130 is a novel intestine-specific MTP inhibitor, which has been shown to be useful in the prevention and treatment of dyslipidemia, obesity, and diabetes. JTT-130 has also been shown to suppress food intake in a dietary fat-dependent manner in rats. However, whether JTT-130 enables changes in food preference and nutrient consumption remains to be determined. Therefore, the aim of the present study was to investigate the effects of JTT-130 on food preference in rat under free access to two different diets containing 3.3% fat (low-fat diet, LF diet) and 35% fat (high-fat diet, HF diet). JTT-130 decreased HF diet intake and increased LF diet intake, resulting in a change in ratio of caloric intake from LF and HF diets to total caloric intake. In addition, macronutrient analysis revealed that JTT-130 did not affect carbohydrate consumption but significantly decreased fat consumption (P < 0.01). These findings suggest that JTT-130 not only inhibits fat absorption, but also suppresses food intake and specifically reduces food preference for fat. Therefore, JTT-130 is expected to provide a new option for the prevention and treatment of obesity and obesity-related metabolic disorders.

A preclinical model of binge eating elicited by yo-yo dieting and stressful exposure to food: effect of sibutramine, fluoxetine, topiramate, and midazolam

RATIONALE

Preclinical models are needed to investigate the neurobiology and psychobiology of binge eating and to identify innovative pharmacotherapeutic strategies.

OBJECTIVES

A modification of the model based on the combination of cyclic caloric restrictions and acute stress was developed to further increase its face validity and reliability and, for the first time, to assess its predictive value.

MATERIALS AND METHODS

Four groups of female rats were employed: group 1 was normally fed and not stressed on the test day (25th); group 2 was fed normally but was exposed to an acute stress on day 25; group 3 was exposed to three cycles (4 days 66% of chow intake + 4 days food ad libitum) of yo-yo dieting but not stressed; and group 4 was exposed to cyclic yo-yo dieting and then stressed. All groups were fed highly palatable food (HPF) for 2 h on days 5-6 and 13-14. Acute stress was elicited by exposing rats to HPF, but preventing them from access to it for 15 min.

RESULTS

The combination of cyclic food restriction and stressful exposure to food markedly increased HPF intake. Sibutramine and fluoxetine inhibited food intake in all conditions. Topiramate selectively inhibited compulsive HPF intake in rats submitted to caloric restriction and stress. Midazolam increased HPF intake.

CONCLUSIONS

Pharmacological results suggest that this model, in addition to face validity as an isomorphic model of human binge eating, is endowed with good predictive validity.

Effects of sibutramine on the appetitive and consummatory aspects of feeding in non-human primates

This study examined how sibutramine (0.06-4.0 mg/kg, i.m.), a clinically effective weight-loss medication which increases extracellular serotonin and norepinephrine levels, affected the appetitive and consummatory aspects of feeding of non-human primates. The effects were compared to the effects of the positive control dexfenfluramine (2.0-6.0 mg/kg, p.o.), which primarily increases extracellular serotonin levels. Baboons had access to food 24 h each day, but they had to complete a two-phase operant procedure in order to eat. Responding on one lever during a 30-min appetitive phase was required before animals could start a consumption phase, where responding on another lever led to food delivery, i.e., a meal. Responding during the appetitive phase resulted in presentations of food-related stimuli only. Sibutramine increased the latency to the first meal of the session in females, and decreased consummatory behavior without affecting other appetitive behavior in males and females. In contrast, dexfenfluramine, increased the latency to the first meal of the session, and decreased both appetitive and consummatory behavior in males and females. The behavioral mechanism by which sibutramine decreases food intake is distinct from other anorectic drugs, including dexfenfluramine, that have been tested in this paradigm.

The anorectic effect of fenfluramine is influenced by sex and stage of the estrous cycle in rats

The controls of food intake differ in male and female rats. Daily food intake is typically greater in male rats, relative to female rats, and a decrease in food intake, coincident with the estrous stage of the ovarian reproductive cycle, is well documented in female rats. This estrous-related decrease in food intake has been attributed to a transient increase in the female rat's sensitivity to satiety signals generated during feeding bouts. Here, we investigated whether sex or stage of the estrous cycle modulate the satiety signal generated by fenfluramine, a potent serotonin (5-HT) releasing agent. To examine this hypothesis, food intake was monitored in male, diestrous female, and estrous female rats after intraperitoneal injections of 0, 0.25, and 1.0 mg/kg d-fenfluramine. The lower dose of fenfluramine decreased food intake only in diestrous and estrous females, suggesting that the minimally effective anorectic dose of fenfluramine is lower in female rats, relative to male rats. Although the larger dose of fenfluramine decreased food intake in both sexes, the duration of anorexia was greater in diestrous and estrous female rats, relative to male rats. Moreover, the magnitude of the anorectic effect of the larger dose of fenfluramine was greatest in estrous rats, intermediate in diestrous rats, and least in male rats. Thus our findings indicate that the anorectic effect of fenfluramine is modulated by gonadal hormone status.