Orexin A (hypocretin 1) injected into hypothalamic paraventricular nucleus and spontaneous physical activity in rats

Regulation by orexin of feeding behaviour and locomotor activity in the goldfish

Orexin is a hypothalamic neuropeptide that is implicated in the regulation of feeding behaviour and the sleep-wakefulness cycle in mammals. However, in spite of a growing body of knowledge concerning orexin in mammals, the orexin system and its function have not been well studied in lower vertebrates. In the present study, we first examined the effect of feeding status on the orexin-like immunoreactivity (orexin-LI) and the expression of orexin mRNA in the goldfish brain. The number of cells showing orexin-LI in the hypothalamus of goldfish brain showed a significant increase in fasted fish and a significant decrease in glucose-injected fish. The expression level of orexin mRNA in the brains of fasted fish increased compared to that of fed fish. We also examined the effect of an i.c.v. injection of orexin or an anti-orexin serum on food intake and locomotor activity in the goldfish. Administration of orexin by i.c.v. injection induced a significant increase of food intake and locomotor activity, whereas i.p. injection of glucose or i.c.v. injection of anti-orexin serum decreased food consumption. These results indicate that the orexin functions as an orexigenic factor in the goldfish brain.

Central orexin sensitivity, physical activity, and obesity in diet-induced obese and diet-resistant rats

Nonexercise activity thermogenesis (NEAT), the most variable component of energy expenditure, can account for differential capacities for human weight gain. Also highly variable, spontaneous physical activity (SPA) may similarly affect weight balance in animals. In the following study, we utilized the rat model of obesity, the diet-induced obese (DIO) rat, as well as the diet-resistant (DR) rat strain, to investigate how access to a high-fat diet alters SPA and the associated energy expenditure (i.e., NEAT). DIO and DR rats showed no differences in the amount of SPA before access to the high-fat diet. After 29 days on a high-fat diet, the DIO rats showed significant decreases in SPA, whereas the DR rats did not. Next, we wanted to determine whether the DIO and DR rats showed differential sensitivity to microinjections of orexin into the paraventricular nucleus of the hypothalamus (PVN). Unilateral guide cannulae were implanted, aimed at the PVN. Orexin A (0, 0.125, 0.25, and 1.0 nmol in 500 nl) was microinjected through the guide cannula into the PVN, then SPA and energy expenditure were measured for 2 h. Using the response to vehicle as a baseline, the DR rats showed significantly greater increase in NEAT compared with the DIO rats. These data indicate that diet-induced obesity is associated with decreases in SPA and a lack of increase in NEAT. A putative mechanism for changes in NEAT that accompany obesity is a decreased sensitivity to the NEAT-activating effects of neuropeptides such as orexin.

NEAT--non-exercise activity thermogenesis--egocentric & geocentric environmental factors vs. biological regulation

Non-exercise activity thermogenesis (NEAT) is the energy expenditure of all physical activities other than volitional sporting-like exercise. NEAT includes all those activities that render us vibrant, unique and independent beings such as going to work, playing guitar, toe-tapping and dancing. The factors that account for the 2000 kcal day(-1) variability of NEAT can be categorized as environmental or biological. The environmental determinants of NEAT can be view using one of two models. In the egocentric model we consider a single person as the focus, e.g. 'my job'. In the geocentric model we consider the 'environment' as the focus, e.g. well-lit and safe walk ways. These models provide us with a theoretical framework to understand NEAT and how best to intervene to promote NEAT. As well as environmental effectors of NEAT, there are also biological regulatory mechanisms that enable us to account for three-quarters of the biological variance in susceptibility and resistance to fat gain with human over-feeding. NEAT is likely to be regulated through a central mechanism that integrates NEAT with energy intake and energy stores so that NEAT is activated with over-feeding and suppressed with under-feeding. In conclusion, NEAT is likely to serve as a crucial thermoregulatory switch between energy storage and dissipation that is biologically regulated and influenced, and perhaps over-ridden, by environment. Deciphering the role of NEAT may lead to a better understanding of the pathogenesis, prevention and treatment of obesity.

Changes in hypothalamic corticotropin-releasing hormone, neuropeptide Y, and proopiomelanocortin gene expression during chronic rapid eye movement sleep deprivation of rats

Chronic rapid eye movement (paradoxical) sleep deprivation (REM-SD) of rats leads to two conspicuous pathologies: hyperphagia coincident with body weight loss, prompted by elevated metabolism. Our goals were to test the hypotheses that 1) as a stressor, REM-SD would increase CRH gene expression in the hypothalamus and that 2) to account for hyperphagia, hypothalamic gene expression of the orexigen neuropeptide Y (NPY) would increase, but expression of the anorexigen proopiomelanocortin (POMC) would decrease. Enforcement of REM-SD of adult male rats for 20 d with the platform (flowerpot) method led to progressive hyperphagia, increasing to approximately 300% of baseline; body weight steadily declined by approximately 25%. Consistent with changes in food intake patterns, NPY expression rapidly increased in the hypothalamic arcuate nucleus by d 5 of REM-SD, peaking at d 20; by contrast, POMC expression decreased progressively during REM-SD. CRH expression was increased by d 5, both in mRNA and ability to detect neuronal perikaryal staining in paraventricular nucleus with immunocytochemistry, and it remained elevated thereafter with modest declines. Taken together, these data indicate that changes in hypothalamic neuropeptides regulating food intake are altered in a manner consistent with the hyperphagia seen with REM-SD. Changes in CRH, although indicative of REM-SD as a stressor, suggest that the anorexigenic actions of CRH are ineffective (or disabled). Furthermore, changes in NPY and POMC agree with current models of food intake behavior, but they are opposite to their acute effects on peripheral energy metabolism and thermogenesis.

Estradiol and orexin-2 saporin actions on multiple forms of behavioral arousal in female mice

Estrogens modulate almost all aspects of female behavioral arousal; however, apart from that of sexual behavior, the neurobiology of female arousal remains unclear. Because orexins-hypocretins are neurotransmitters known to be important for behavioral arousal, the authors hypothesized that orexins may be a target for estrogen. Gonadectomized female mice received an intracerebral injection of either phosphate-buffered saline, the neurotoxin saporin (SAP), or the orexin-2-saporin conjugate (OXSAP) in the lateral hypothalamus. SAP- and OXSAP-treated mice were also divided into groups receiving either estradiol capsules or oil capsules. Mice were tested in 3 behavioral tests measuring different modes of arousal: sensory responsiveness, running wheel activity, and fearfulness. OXSAP mice showed decreases in sensory responsiveness and fearfulness concomitant with a reduction in orexin cell number. Estradiol affected all behaviors tested but decreased fearfulness only when combined with OXSAP treatment. These data indicate that estrogens modulate orexins' effects on fearfulness.

Central relaxin-3 administration causes hyperphagia in male Wistar rats

Relaxin-3 (INSL-7) is a recently discovered member of the insulin superfamily. Relaxin-3 mRNA is expressed in the nucleus incertus of the brainstem, which has projections to the hypothalamus. Relaxin-3 binds with high affinity to the LGR7 receptor and to the previously orphan G protein-coupled receptor GPCR135. GPCR135 mRNA is expressed predominantly in the central nervous system, particularly in the paraventricular nucleus (PVN). The presence of relaxin-3 and these receptors in the PVN led us to investigate the effect of central administration of relaxin-3 on food intake in male Wistar rats. The receptor involved in mediating these effects was also investigated. Intracerebroventricular injections of human relaxin-3 (H3) to satiated rats significantly increased food intake 1 h post administration in the early light phase [0.96 +/- 0.16 g (vehicle) vs. 1.81 +/- 0.21 g (180 pmol H3), P < 0.05] and the early dark phase [2.95 +/- 0.45 g (vehicle) vs. 4.39 +/- 0.39 g (180 pmol H3), P < 0.05]. Intra-PVN H3 administration significantly increased 1-h food intake in satiated rats in the early light phase [0.34 +/- 0.16 g (vehicle) vs. 1.23 +/- 0.30 g (18 pmol H3), P < 0.05] and the early dark phase [4.43 +/- 0.32 g (vehicle) vs. 6.57 +/- 0.42 g (18 pmol H3), P < 0.05]. Feeding behavior increased after intra-PVN H3. Equimolar doses of human relaxin-2, which binds the LGR7 receptor but not GPCR135, did not increase feeding. Hypothalamic neuropeptide Y, proopiomelanocortin, or agouti-related peptide mRNA expression did not change after acute intracerebroventricular H3. These results suggest a novel role for relaxin-3 in appetite regulation.

Obesity and the neuroendocrine control of energy homeostasis: the role of spontaneous locomotor activity

Obesity represents one of the most urgent global health threats as well as one of the leading causes of death throughout industrialized nations. Efficacious and safe therapies remain at large. Attempts to decrease fat mass via pharmacological reduction of energy intake have had limited potency or intolerable side effects. Increasingly widespread sedentary lifestyle is often cited as a major contributor to the increasing prevalence of obesity. Moreover, low levels of spontaneous physical activity (SPA) are a major predictor of fat mass accumulation during overfeeding in humans, pointing to a substantial role for SPA in the control of energy balance. Despite this, very little is known about the molecular mechanisms by which SPA is regulated. The overview will attempt to summarize available information on neuroendocrine factors regulating SPA.

Aging muscle

Age causes structural and functional changes in skeletal muscle in a wide range of species, including humans. Muscle changes in humans start in the fourth decade of life and cause frailty and disabilities. Associated changes in body composition form the basis of many metabolic disorders, such as insulin resistance, type 2 diabetes, hypertension, and hyperlipidemia, which result in an increased incidence of cardiovascular death. Decreases in the synthesis rates of many muscle proteins, specifically of myosin heavy chain and mitochondrial proteins, occur with age. The underlying causes of the reduction in mitochondrial biogenesis and ATP production seem to be decreases in mitochondrial DNA and messenger RNA. Reduced ATP production could be the basis of reduced muscle protein turnover, which requires energy. Both aerobic exercise and resistance exercise enhance muscle protein synthesis and mitochondrial biogenesis. Insulin and amino acids have also been shown to enhance muscle mitochondrial biogenesis and mitochondrial protein synthesis. However, the insulin-induced increase in muscle mitochondrial ATP production is defective in type 2 diabetic patients with insulin resistance. Moreover, a dissociation between increases in muscle mitochondrial biogenesis and insulin sensitivity after exercise has been noted in older persons. It remains to be determined whether muscle mitochondrial dysfunction causes or results from insulin resistance. Exercise seems to enhance the efficiency of muscle mitochondrial DNA in rodents. Reduced physical activity as a contributor of age-related mitochondrial dysfunction remains to be determined. It is proposed that a reduction in tissue mitochondrial ATP production signals the hypothalamic centers to reduce spontaneous physical activities. Voluntary physical activity is regulated by cognitive centers and could attenuate the progressive decline in mitochondrial functions that occurs with age.

Caloric restriction and physical activity in zebrafish (Danio rerio)

Understanding the mechanism of energy flux may be critical for explaining how obesity has emerged as a public health epidemic. It is known that changes in caloric intake predictably alter physical activity levels (PA) in mammals. Here, our goal was to test the hypothesis that fasting induces a biphasic pattern of change in PA by measuring PA before and after long-term food deprivation in zebrafish. Compared to control-fed fish, food-deprived fish showed a significant increase in PA levels during the first 2 days of food deprivation. Subsequently, however, fasted fish showed a significant chronic decrease in PA compared to fish fed at weight-maintenance levels. These data are comparable to those seen with mammals, which also show a biphasic response of PA to caloric restriction. In a separate group of fish, long-term food deprivation, associated with decreases in PA, induced a significant increase in brain preproorexin mRNA levels compared to fed controls. No change in orexin mRNA was seen after 2 days of food deprivation. The finding that orexin mRNA expression is altered only after long-term starvation suggests that orexin may be coupled with the changes in PA seen at this time. Thus, the association between negative energy balance and reductions in PA occurs across genera in biology and is associated with predictable neurological changes in brain gene expression.

Interindividual variation in posture allocation: possible role in human obesity

Obesity occurs when energy intake exceeds energy expenditure. Humans expend energy through purposeful exercise and through changes in posture and movement that are associated with the routines of daily life [called nonexercise activity thermogenesis (NEAT)]. To examine NEAT's role in obesity, we recruited 10 lean and 10 mildly obese sedentary volunteers and measured their body postures and movements every half-second for 10 days. Obese individuals were seated, on average, 2 hours longer per day than lean individuals. Posture allocation did not change when the obese individuals lost weight or when lean individuals gained weight, suggesting that it is biologically determined. If obese individuals adopted the NEAT-enhanced behaviors of their lean counterparts, they might expend an additional 350 calories (kcal) per day.

Metabolic consequences of overfeeding in humans

PURPOSE OF REVIEW

Overfeeding leads to obesity and metabolic disorders, including impaired glucose homeostasis, lipid disorders, and hepatic steatosis. The consequences of standardized overfeeding on body weight have shown, however, considerable interindividual variability, which suggests that it also leads to adaptative changes in energy expenditure, in some individuals at least. The present review is mainly focused on the recent developments regarding the effects of overfeeding on energy expenditure.

RECENT FINDINGS

Individuals who gain the less body weight during overfeeding are those who experience a greater increase in total energy expenditure. This increase in energy expenditure has been attributed to stimulation of nonexercise physical activity. Recent developments regarding adaptative increases in physical activity are critically reviewed. Overfeeding also alters the pathways used for carbohydrate storage after a glucose load, by increasing de-novo lipogenesis in the liver and adipose tissue at the expense of glycogen storage. The sympathetic nervous system is a good candidate for energy expenditure increase during overfeeding. The increases in energy expenditure observed during acute stimulation of the sympathetic nervous system were however found to be unaltered by short-term overfeeding.

SUMMARY

The mechanisms by which some individuals protect themselves against body weight gain remain poorly understood. Nonvoluntary physical activity may allow one to increase energy expenditure during overfeeding, and may therefore constitute a regulatory factor in body weight control. The biological determinant of spontaneous, nonvoluntary physical activity, however, remains to be investigated.

Nonexercise activity thermogenesis (NEAT): environment and biology

Nonexercise activity thermogenesis (NEAT) is the energy expended for everything that is not sleeping, eating, or sports-like exercise. It includes the energy expended walking to work, typing, performing yard work, undertaking agricultural tasks, and fidgeting. NEAT can be measured by one of two approaches. The first is to measure or estimate total NEAT. Here, total daily energy expenditure is measured, and from it "basal metabolic rate-plus-thermic effect of food" is subtracted. The second is the factoral approach, whereby the components of NEAT are quantified, and total NEAT is calculated by summing these components. The amount of NEAT that humans perform represents the product of the amount and types of physical activities and the thermogenic cost of each activity. The factors that impact a human's NEAT are readily divisible into environmental factors, such as occupation or dwelling within a "concrete jungle," and biological factors such as weight, gender, and body composition. The combined impact of these factors explains the substantial variance in human NEAT. The variability in NEAT might be viewed as random, but human and animal data contradict this. It appears that changes in NEAT subtly accompany experimentally induced changes in energy balance and are important in the physiology of weight change. Inadequate modulation of NEAT plus a sedentary lifestyle may thus be important in obesity. It then becomes intriguing to dissect mechanistic studies that delineate how NEAT is regulated into neural, peripheral, and humoral factors. A scheme is described in this review in which NEAT corresponds to a carefully regulated "tank" of physical activity that is crucial for weight control.