The Regulation of Energy Balance

Invited review: An evaluation of the likely effects of individualized feeding of concentrate supplements to pasture-based dairy cows

In pasture-based dairy systems, supplementary feeds are used to increase dry matter intake and milk production. Historically, supplementation involved the provision of the same amount of feed (usually a grain-based concentrate feed) to each cow in the herd during milking (i.e., flat-rate feeding). The increasing availability of computerized feeding and milk monitoring technology in milking parlors, however, has led to increased interest in the potential benefits of feeding individual cows (i.e., individualized or differential feeding) different amounts and types of supplements according to one or more parameters (e.g., breeding value for milk yield, current milk yield, days in milk, body condition score, reproduction status, parity). In this review, we consider the likely benefits of individualized supplementary feeding strategies for pasture-based dairy cows fed supplements in the bail during milking. A unique feature of our review compared with earlier publications is the focus on individualized feeding strategies under practical grazing management. Previous reviews focused primarily on research undertaken in situations where cows were offered ad libitum forage, whereas we consider the likely benefits of individualized supplementary feeding strategies under rotational grazing management, wherein pasture is often restricted to all or part of a herd. The review provides compelling evidence that between-cow differences in response to concentrate supplements support the concept of individualized supplementary feeding.

Ghrelin is an orexigenic peptide and elicits anxiety-like behaviors following administration into discrete regions of the hypothalamus

Previous evidence indicates that peripherally administered ghrelin significantly increases corticotropin releasing hormone (CRH) mRNA and serum corticosterone. In addition, intraventricular administration of ghrelin has been reported to elicit anxiety-like behaviors suggesting that the peptide plays a role in mediating neuroendocrine and behavioral responses to stress. In the present study, we characterized the orexigenic, metabolic, and anxiogenic actions of ghrelin following microinjection into the arcuate nucleus (ARN), paraventricular nucleus (PVN), perifornical hypothalamus (PFH), and ventromedial nucleus (VMN). To assess ghrelin's role in anxiogenic behavior, rats were injected with vehicle or 50-800pmol of ghrelin and then placed in an elevated plus maze (EPM) for 10min. Each test was performed as a single trial per animal. In separate behavioral testing we measured the induction of stereotypic behaviors. Doses of 200pmol or higher administered into the ARN and PVN elicited anxiety-like behaviors, including an increased avoidance of the open arms of the EPM. However, in the PFH and VMN, higher doses of ghrelin (400-800pmol) were required to induce anxiety. Ghrelin doses as low as 50pmol stimulated eating and altered energy substrate oxidation (respiratory quotient; RQ) when injected into the ARN and PVN. Injections into the PFH and VMN elicited more modest effects on eating and RQ at doses of 400pmol or greater. Our findings indicate that regions of the hypothalamus appear to be differentially sensitive and responsive to the feeding-stimulant, metabolic, and anxiogenic actions of ghrelin and that the ARN and PVN, in particular, exert a primary role in mediating these effects.

Ghrelin is an orexigenic and metabolic signaling peptide in the arcuate and paraventricular nuclei

Ghrelin is a 28-amino acid acylated peptide and is the endogenous ligand for the growth hormone secretagogue receptor (GHS-R). The GHS-R is expressed in hypothalamic nuclei, including the arcuate nucleus (Arc) where it is colocalized with neuropeptide Y (NPY) neurons. In the present study, we examined the effects of ghrelin on feeding and energy substrate utilization (respiratory quotient; RQ) following direct injections into either the arcuate or the paraventricular nucleus (PVN) of the hypothalamus. Ghrelin was administered at the beginning of the dark cycle at doses of 15–60 pmol to male and female rats. In feeding studies, food intake was measured 2 and 4 h postinjection. Separate groups of rats were injected with ghrelin, and the RQ (V̇co2/V̇o2) was measured using an open circuit calorimeter over a 4-h period. Both Arc and PVN injections of ghrelin increased food intake in male and female rats. Ghrelin also increased RQ, reflecting a shift in energy substrate utilization in favor of carbohydrate oxidation. Because these effects are similar to those observed after PVN injection of NPY, we then assessed the impact of coinjecting ghrelin with NPY into the PVN. When rats were pretreated with very low doses of ghrelin (2.5–10 pmol), NPY's (50 pmol) effects on eating and RQ were potentiated. Overall, these data are in agreement with evidence suggesting that ghrelin functions as a gut-brain endocrine hormone implicated in the regulation of food intake and energy metabolism. Our findings are also consistent with a possible interactive role of hypothalamic ghrelin and NPY systems.

Integration of hypothalamic feeding and metabolic signals: focus on neuropeptide Y

Research is reviewed on effects of neuropeptide Y (NPY) on energy substrate utilization and central interactions among NPY, serotonin and urocortin, particularly in neurons of the paraventricular nucleus of the hypothalamus.

Comparison of central and peripheral administration of C75 on food intake, body weight, and conditioned taste aversion

Mice respond to fatty acid synthase (FAS) inhibitors by profoundly reducing their food intake and body weight. Evidence indicates that the central nervous system (CNS) may be the critical site of action; however, a peripheral contribution cannot be ruled out. We compared doses of the FAS inhibitor C75 in the CNS (third ventricle [i3vt]) and periphery (intraperitoneal [IP]) to reduce food intake and body weight in rats. Centrally, the threshold dose was 3 micro g, whereas a dose of 10 mg/kg was required peripherally. Such data argue for FAS activity in the CNS as a potent target for the actions of C75. To control for nonspecific effects of FAS inhibition, we compared C75 administration in two models of illness, conditioned taste aversion and need-induced sodium appetite. Our results suggest the anorexia produced by IP C75 is accompanied by visceral illness, whereas the anorexia produced by i3vt is not. In addition, we placed animals in an indirect calorimeter after an IP injection of C75. We found that consistent with behavioral measures of visceral illness, peripheral C75 reduced heat expenditure and resulted in animals losing less weight than fasted control animals, suggesting that peripherally administered C75 has aversive properties. Understanding the mechanisms by which FAS inhibition in the CNS reduces food intake could lead to specific targets for the manipulation of energy balance and the treatment of obesity.

Food intake and the regulation of body weight

This chapter reviews the recent literature on hormonal and neural signals critical to the regulation of individual meals and body fat. Rather than eating in response to acute energy deficits, animals eat when environmental conditions (social and learned factors, food availability, opportunity, etc.) are optimal. Hence, eating patterns are idiosyncratic. Energy homeostasis, the long-term matching of food intake to energy expenditure, is accomplished via controls over the size of meals. Individuals who have not eaten sufficient food to maintain their normal weight have lower levels of adiposity signals (leptin and insulin) in the blood and brain, and one consequence is that meal-generated signals (such as CCK) are less efficacious at reducing meal size. The converse is true if individuals are above their normal weight, when they tend to eat smaller meals. The final section reviews how these signals are received and integrated by the CNS, as well as the neural circuits and transmitters involved.

Neuroendocrine regulation of food intake

Maintenance of appropriate stores of metabolic fuels depends on carefully matching caloric intake to caloric expenditure. Achieving such 'energy balance' is a product of complex interactions of peripheral hormones with effector systems in the central nervous system (CNS) that regulate food intake and energy expenditure. Leptin is a hormone that is made in the adipocytes, circulates in the blood and interacts with receptors in the CNS. These receptors can be found in two different types of systems. One effector system is termed 'anabolic' and is activated by low levels of leptin during negative energy balance. This system (exemplified by the hypothalamic neuropeptide Y system) increases food intake and decreases energy expenditure to facilitate the regaining of lost energy stores. The other effector system is termed 'catabolic' and is activated by high levels of leptin during positive energy balance. This system (exemplified by the hypothalamic melanocortin and corticotrophin-releasing hormone systems) decreases food intake and increases energy expenditure to facilitate the loss of excess energy stores. Further understanding of these systems is necessary to develop adequate treatments for disorders of energy balance, such as obesity and wasting.

Leptin disinhibits nonshivering thermogenesis in infants after maternal separation

Prolonged maternal separation inhibits endogenous heat production in infant mammals exposed to cold. This inhibition of thermogenesis occurs many hours before energy stores have been fully depleted. The need to protect energy resources during separation-induced starvation may be signaled by declining levels of leptin, a hormone that acts as a "fat signal" and a regulator of energy utilization; in fact, starvation reduces leptin levels in adult mice and infant rats. It is not known, however, whether leptin has a functional role during starvation in infants. Such a role may be found in the regulation of nonshivering thermogenesis by brown adipose tissue (BAT), a specialized organ that provides heat to infant mammals, including humans, during cold exposure. Heat produced by BAT allows the cold-exposed infant to prevent the detrimental effects of hypothermia on physiology and behavior and, ultimately, growth. Here we show that leptin disinhibits BAT thermogenesis during cold exposure in infant rats after 18 h of maternal separation. This finding demonstrates that leptin is more than simply an adipostat for the regulation of body weight; specifically, leptin modulates thermogenesis and energy utilization in the early postnatal period.