Protein appetite is increased after central leptin-induced fat depletion

Learning of food preferences: mechanisms and implications for obesity & metabolic diseases

Omnivores, including rodents and humans, compose their diets from a wide variety of potential foods. Beyond the guidance of a few basic orosensory biases such as attraction to sweet and avoidance of bitter, they have limited innate dietary knowledge and must learn to prefer foods based on their flavors and postoral effects. This review focuses on postoral nutrient sensing and signaling as an essential part of the reward system that shapes preferences for the associated flavors of foods. We discuss the extensive array of sensors in the gastrointestinal system and the vagal pathways conveying information about ingested nutrients to the brain. Earlier studies of vagal contributions were limited by nonselective methods that could not easily distinguish the contributions of subsets of vagal afferents. Recent advances in technique have generated substantial new details on sugar- and fat-responsive signaling pathways. We explain methods for conditioning flavor preferences and their use in evaluating gut-brain communication. The SGLT1 intestinal sugar sensor is important in sugar conditioning; the critical sensors for fat are less certain, though GPR40 and 120 fatty acid sensors have been implicated. Ongoing work points to particular vagal pathways to brain reward areas. An implication for obesity treatment is that bariatric surgery may alter vagal function.

Homeostatic responses to palatable food consumption in satiated rats

OBJECTIVE

Energy intake is regulated by overlapping homeostatic and hedonic systems. Consumption of palatable foods has been implicated in weight gain, but this assumes that homeostatic control systems do not accurately detect this hedonically driven energy intake. This study tested this assumption, hypothesizing that satiated rats would reduce their voluntary food intake and maintain a stable body weight after consuming a palatable food.

METHODS

Lean rats or rats previously exposed to an obesogenic diet were schedule-fed with fixed or varying amounts of palatable sweetened condensed milk (SCM) daily, and their voluntary energy intake and body weight were monitored.

RESULTS

During scheduled feeding of SCM, rats voluntarily reduced bland food consumption and maintained a stable body weight. This behavior was also seen in rats with access to an obesogenic diet and was independent of the predictability of SCM access. However, lean rats offered large amounts of SCM showed an increase in total energy intake. To test whether a nutrient deficiency drove this under-compensatory behavior, SCM was enriched with protein. However, no effect was seen on voluntary energy intake.

CONCLUSIONS

In schedule-fed rats, compensatory reductions in voluntary energy intake were seen, but under-compensation was observed if large amounts of SCM were consumed.

Evidence that leptin-induced weight loss requires activation of both forebrain and hindbrain receptors

Previous studies with chronic decerebrate rats and rats infused with leptin into the 4th ventricle suggest that hindbrain leptin receptors attenuate the catabolic effect of forebrain leptin receptor activation. To test this further, rats were fitted with both 3rd and 4th ventricle cannulae. They were infused for 12 days with different combinations of saline, low dose leptin or leptin receptor antagonist (leptin mutein protein). Infusion of 0.1 μg leptin/day into the 3rd ventricle or 0.6 μg leptin/day into the 4th ventricle had no significant effect on food intake, energy expenditure or body composition. Infusion of 2 μg mutein/day into either ventricle caused a small, but significant weight gain. When mutein was infused into one ventricle and leptin into the other, the rats lost weight irrespective of which combination was applied. Surprisingly, rats that received leptin infusions into both ventricles showed an initial hypophagia, no change in energy expenditure, but a 75% loss of carcass fat after 12 days. These data suggest that neuronal pathways activated by leptin receptors in either the forebrain or hindbrain modulate each other's effects. In normal conditions hindbrain leptin may attenuate the catabolic effect of forebrain leptin, but if activity in one area is blocked with mutein, then the catabolic response to leptin in the other ventricle is exaggerated. When receptors in both areas are activated there is an integration of response to produce negative energy balance. This may ensure that leptin causes a loss of fat only when leptin is elevated in both the CSF and periphery.

The lipoprivic control of feeding is governed by fat metabolism, not by leptin or adipose depletion

A lipoprivic control of feeding has been proposed based on the finding that appetite is stimulated by drugs such as beta-mercaptoacetate (MA) that reduce fatty acid oxidation. The adipose-derived hormone, leptin, has effects on feeding and fat oxidation that are opposite those produced by MA. However, effects of this hormone on MA-induced feeding are not known. Here we examined the effects of endogenous leptin levels and of acute central and peripheral leptin administration on MA-induced feeding. We also examined leptin-induced changes in feeding, body weight, and plasma fuels after capsaicin-induced deletion of the lipoprivic control. MA-induced feeding was not altered under any of these conditions, and leptin's effects were not altered by capsaicin. We then examined MA-induced feeding during chronic leptin treatment. Because chronic leptin produces several distinct metabolic states as body adiposity is reduced, we tested MA before, during, and after leptin treatment at times that coincided with these states. MA-induced feeding was unchanged on d 3 of leptin treatment when rats were in a lipolytic state and rapidly metabolizing body fat stores but reduced on d 10 when they were adipose deplete and their level of fat oxidation was reduced. Together results suggest that the lipoprivic control is normally less active in the fat deplete state than during states associated with fat availability. If so, its insensitivity to leptin would enable the lipoprivic control to operate when dietary fat, adiposity, and leptin levels are elevated. The role played by the lipoprivic control under such conditions remains uncertain.