Some metabolic and behavioral effects of adrenalectomy on obese Zucker rats

Sex differences in the effect of acute fasting on excitatory and inhibitory synapses onto ventral tegmental area dopamine neurons

KEY POINTS

Fasting can increase motivation for food and can energize reward-seeking. Ventral tegmental area (VTA) dopamine neurons respond to motivationally relevant information and fasting can influence mesolimbic dopamine concentration. An acute overnight fast differentially alters food approach behaviours and excitatory synaptic transmission onto VTA dopamine neurons of male or female mice. While inhibitory synapses onto VTA dopamine neurons are not altered by fasting in male or female mice, male mice had strengthened excitatory synapses whereas female mice had increased endocannabinoid-mediated short-term plasticity at excitatory synapses. These results help us understand how fasting differentially influences excitatory synaptic transmission onto dopamine neurons and may inform different strategies for fasting-induced food seeking by male and female mice.

ABSTRACT

Dopamine neurons in the ventral tegmental area (VTA) are important for energizing goal-directed behaviour towards food and are sensitive to changes in metabolic states. Fasting increases the incentive motivation for food and the mobilization of energy stores and has sex-dependent effects. However, it is unknown how acute fasting alters excitatory or inhibitory synaptic transmission onto VTA dopamine neurons. An acute 16 h overnight fast induced increased food-seeking behaviour that was more predominant in male mice. Fasting increased miniature excitatory postsynaptic current frequency and amplitude in male, but not female, mice. This effect was not due to altered release probability as there was no change in the paired pulse ratio, nor was it due to an altered postsynaptic response as there was no change in the AMPA receptor/NMDA receptor ratio or response to glutamate uncaging. However, this effect was consistent with an increase in the number of release sites. In addition, depolarization-induced suppression of excitation, a measure of short-term endocannabinoid-mediated plasticity, was enhanced in female but not male fasted mice. There were no fasting-induced changes at inhibitory synapses onto dopamine neurons of male or female mice. Taken together, these results demonstrate that fasting influences excitatory synapses differentially in male and female mice, but preserves inhibitory synapses onto dopamine neurons, indicating that the mesolimbic circuits of male and female mice respond differently to acute energy deprivation.

The hypothalamic-pituitary-adrenal axis as a substrate for stress resilience: Interactions with the circadian clock

Stress, primarily processed via the hypothalamic-pituitary-adrenal (HPA) axis, engages biological pathways throughout the brain and body which promote adaptation and survival to changing environmental demands. Adaptation to environmental challenges is compromised when these pathways are no longer functioning optimally. The physiological and behavioral mechanisms through which HPA axis function influences stress adaptation and resilience are not fully elucidated. Our understanding of stress biology and disease must take into account the complex interactions between the endocrine system, neural circuits, and behavioral coping strategies. In addition, further consideration must be taken concerning influences of other aspects of physiology, including the circadian clock which is critical for regulation of daily changes in HPA activity. While adding a layer of complexity, it also offers targets for intervention. Understanding the role of HPA function in mediating these diverse biological responses will lead to important insights about how to bolster successful stress adaptation and promote stress resilience.

Metabolic and behavioral parameters of mice with reduced expression of Syndecan-1

Energy balance is essential for all species. Ligand-receptor interactions mediate processes that regulate body activities like reproduction and metabolism based on the energy status. Such receptors are the heparan sulfate proteoglycans and specifically the family of syndecans. Therefore we investigated the differences of metabolic parameters of heterozygous Syndecan 1 mice (Sdc1^+/-) with reduced expression of Sdc1 and the corresponding wild type mice. Sdc1^+/- mice have a reduced body weight although they show increased leptin and decreased corticosterone levels. Furthermore, their food and water intake is increased. This is accompanied with less adipose tissue, smaller adipocytes and thus an increased density of adipocytes. For the detailed analysis of the metabolism the automated PhenoMaster system has been used, which allowed continuous and undisturbed recording of food and water intake, energy expenditure and movement. The reason for the lower body weight was the higher energy expenditure of these animals compared to controls. Additionally, female Sdc1^+/- mice showed an increased locomotor activity. Referring to organs, the intestine in Sdc1^+/- mice was heavier and longer, but no differences at the cellular level could be observed. These findings were independent of normal mating or vice versa embryo transfers of Sdc1^+/- and wild type embryos in recipient females of the other genotype. Herein we showed that the reduced expression of Sdc1 led to an altered metabolism on fetal as well as on maternal side, which may play a role in the growth restriction observed in human pregnancy pathologies and in mice lacking Sdc1.

Chronic and acute effects of stress on energy balance: are there appropriate animal models?

Stress activates multiple neural and endocrine systems to allow an animal to respond to and survive in a threatening environment. The corticotropin-releasing factor system is a primary initiator of this integrated response, which includes activation of the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. The energetic response to acute stress is determined by the nature and severity of the stressor, but a typical response to an acute stressor is inhibition of food intake, increased heat production, and increased activity with sustained changes in body weight, behavior, and HPA reactivity. The effect of chronic psychological stress is more variable. In humans, chronic stress may cause weight gain in restrained eaters who show increased HPA reactivity to acute stress. This phenotype is difficult to replicate in rodent models where chronic psychological stress is more likely to cause weight loss than weight gain. An exception may be hamsters subjected to repeated bouts of social defeat or foot shock, but the data are limited. Recent reports on the food intake and body composition of subordinate members of group-housed female monkeys indicate that these animals have a similar phenotype to human stress-induced eaters, but there are a limited number of investigators with access to the model. Few stress experiments focus on energy balance, but more information on the phenotype of both humans and animal models during and after exposure to acute or chronic stress may provide novel insight into mechanisms that normally control body weight.

Metabolic disturbances connecting obesity and depression

Obesity markedly increases the odds of developing depression. Depressed mood not only impairs motivation, quality of life and overall functioning but also increases the risks of obesity complications. Abdominal obesity is a better predictor of depression and anxiety risk than overall adipose mass. A growing amount of research suggests that metabolic abnormalities stemming from central obesity that lead to metabolic disease may also be responsible for the increased incidence of depression in obesity. As reviewed here, a higher mass of dysfunctional adipose tissue is associated with several metabolic disturbances that are either directly or indirectly implicated in the control of emotions and mood. To better comprehend the development of depression in obesity, this review pulls together select findings addressing the link between adiposity, diet and negative emotional states and discusses the evidence that alterations in glucocorticoids, adipose-derived hormones, insulin and inflammatory signaling that are characteristic of central obesity may be involved.

Acute exposure to a high-fat diet alters meal patterns and body composition

Weight gain and adiposity are often attributed to the overconsumption of unbalanced, high-fat diets however, the pattern of consumption can also contribute to associated body weight and compositional changes. The present study explored the rapid alterations in meal patterns of normal-weight rats given continuous access to high-fat diet and examined body weight and composition changes compared to chow fed controls. Ten Long-Evans rats were implanted with subcutaneous microchips for meal pattern analysis. Animals were body weight matched and separated into two groups: high-fat or chow fed. Each group was maintained on their assigned diet for nine days and monitored for 22 h each day for meal pattern behavior. Body weight was evaluated every other day, and body composition measures were taken prior and following diet exposure. High-fat fed animals gained more weight and adipose tissue than chow fed controls and displayed a reduced meal frequency and increased meal size. Furthermore, meal size was significantly correlated with the gain of adipose tissue. Together, these results suggest that consumption of a high-fat diet can rapidly alter meal patterns, which in turn contribute to the development of adiposity.

Effects of rimonabant (SR141716) on fasting-induced hypothalamic-pituitary-adrenal axis and neuronal activation in lean and obese Zucker rats

The effects of the cannabinoid-1 receptor (CB(1)) antagonist rimonabant on energy metabolism and fasting-induced hypothalamic-pituitary-adrenal (HPA) axis and neuronal activation were investigated. Lean and obese Zucker rats were treated orally with a daily dose of 10 mg/kg rimonabant for 14 days. A comprehensive energy balance profile based on whole-carcass analyses further demonstrated the potential of CB(1) antagonists for decreasing energy gain through reducing food intake and potentially increasing brown adipose tissue thermogenesis. Rimonabant also reduced plasma glucose, insulin, and homeostasis model assessment of insulin resistance, which further confirms the ability of CB(1) antagonists to improve insulin sensitivity. To test the hypothesis that rimonabant attenuates the effect of fasting on HPA axis activation in the obese Zucker model, rats were either ad libitum-fed or food-deprived for 8 h. Contrary to expectation, rimonabant increased basal circulating corticosterone levels and enhanced the HPA axis response to food deprivation in obese rats. Rimonabant also exacerbated the neuronal activation seen in the arcuate nucleus (ARC) after short-term deprivation. In conclusion, the present study demonstrates that CB(1) blockade does not prevent the hypersensitivity to food deprivation occurring at the level of HPA axis and ARC activation in the obese Zucker rats. This, however, does not prevent CB(1) antagonism from exerting beneficial effects on energy and glucose metabolism.

Opposing crosstalk between leptin and glucocorticoids rapidly modulates synaptic excitation via endocannabinoid release

The hypothalamic paraventricular nucleus (PVN) integrates preautonomic and neuroendocrine control of energy homeostasis, fluid balance, and the stress response. We recently demonstrated that glucocorticoids act via a membrane receptor to rapidly cause endocannabinoid-mediated suppression of synaptic excitation in PVN neurosecretory neurons. Leptin, a major signal of nutritional state, suppresses CB(1) cannabinoid receptor-dependent hyperphagia (increased appetite) in fasting animals by reducing hypothalamic levels of endocannabinoids. Here we show that glucocorticoids stimulate endocannabinoid biosynthesis and release via a Galpha(s)-cAMP-protein kinase A-dependent mechanism and that leptin blocks glucocorticoid-induced endocannabinoid biosynthesis and suppression of excitation in the PVN via a phosphodiesterase-3B-mediated reduction in intracellular cAMP levels. We demonstrate this rapid hormonal interaction in both PVN magnocellular and parvocellular neurosecretory cells. Leptin blockade of the glucocorticoid-induced, endocannabinoid-mediated suppression of excitation was absent in leptin receptor-deficient obese Zucker rats. Our findings reveal a novel hormonal crosstalk that rapidly modulates synaptic excitation via endocannabinoid release in the hypothalamus and that provides a nutritional state-sensitive mechanism to integrate the neuroendocrine regulation of energy homeostasis, fluid balance, and the stress response.

Regulation of hypothalamic gene expression by glucocorticoid: implications for energy homeostasis

The present study investigated the hypothalamic gene expressions regulated by glucocorticoids (GC), key hormones in energy homeostasis. Using the serial analysis of gene expression (SAGE) method, we studied the effects of adrenalectomy (ADX) and GC on the transcriptomes of mouse hypothalamus. Approximately 180,000 SAGE tags, which correspond to 50,000 tag species, were isolated from each group of intact or adrenalectomized mice as well as 1, 3, and 24 h after GC injection. ADX upregulated diazepam binding inhibitor gene expression while downregulating vomeronasal 1 receptor D4, genes involved in mitochondrial phosphorylation (cytochrome-c oxidase 1 and NADH dehydrogenase 3), 3beta-hydroxysteroid dehydrogenase-1, and prostaglandin D2 synthase. GC increased the gene expression levels of dehydrogenase/reductase member 3, prostaglandin D2 synthase, solute carrier family 4 member 4, and five cytoskeletal proteins including myosin light chain phosphorylatable fast and troponin C2 fast. On the other hand, GC reduced the mRNA levels of calmodulin 1 and expressed sequence tag similar to calmodulin 2, ATP synthase F0 subunit 6, and solute carrier family 4 member 3. Moreover, 7 uncharacterized and 43 novel transcripts were modulated by ADX and GC. The present study has identified genes that may regulate hypothalamic systems governing energy balance in response to ADX and GC.

Corticosterone-dependent metabolic and neuroendocrine abnormalities in obese Zucker rats in relation to feeding

The obese Zucker (fa/fa) rat is characterized by hyperphagia, hyperinsulinemia, an increase in fat deposition, and a hyperactivity in the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis in fa/fa rats is hypersensitive to stressful experimental conditions. Food deprivation even leads to a stress reaction in obese fa/fa rats. The present study was conducted to investigate the role of corticosterone in obese rats on the basal, fasting, and postprandial metabolic rate as well as on the central expression of the thyrotropin-releasing hormone (TRH) in these conditions. In addition, the study was aimed at clarifying whether the high levels of corticosterone in obese rats are responsible for the induction of the stress reaction to food deprivation in these animals. The present results demonstrate that whole body fat oxidation and postprandial metabolic responses in obese Zucker rats were improved by adrenalectomy (ADX). At the level of the central nervous system, ADX reversed a decrease in TRH mRNA expression in the paraventricular hypothalamus (PVH) detected in fasting animals. Considering all feeding conditions, the obese rats demonstrated lower TRH mRNA levels compared with lean animals. ADX resulted in an enhanced postprandial activation of the parvocellular PVH. In contrast, the magnocellular part of the PVH was less responsive to refeeding in ADX animals. Finally, ADX failed to prevent the stress response of obese rats to food deprivation. The present results provide evidence that the removal of adrenals resolve some of the metabolic defects encountered in obese Zucker rats. They also demonstrate that not all the abnormalities of the obese Zucker rats are attributable to the hyperactivity of the HPA axis.

Enhanced circadian ACTH release in obese premenopausal women: reversal by short-term acipimox treatment

Several studies suggest that the hypothalamo-pituitary-adrenal (HPA) axis is exceedingly active in obese individuals. Experimental studies show that circulating free fatty acids (FFAs) promote the secretory activity of the HPA axis and that human obesity is associated with high circulating FFAs. We hypothesized that HPA axis activity is enhanced and that lowering of circulating FFAs by acipimox would reduce spontaneous secretion of the HPA hormonal ensemble in obese humans. To evaluate these hypotheses, diurnal ACTH and cortisol secretion was studied in 11 obese and 9 lean premenopausal women (body mass index: obese 33.5 +/- 0.9 vs. lean 21.2 +/- 0.6 kg/m(2), P < 0.001) in the early follicular stage of their menstrual cycle. Obese women were randomly assigned to treatment with either acipimox (inhibitor of lipolysis, 250 mg orally four times daily) or placebo in a double-blind crossover design, starting one day before admission until the end of the blood-sampling period. Blood samples were taken during 24 h with a sampling interval of 10 min for assessment of plasma ACTH and cortisol concentrations. ACTH and cortisol secretion rates were estimated by multiparameter deconvolution analysis. Daily ACTH secretion was substantially higher in obese than in lean women (7,950 +/- 1,212 vs. 2,808 +/- 329 ng/24 h, P = 0.002), whereas cortisol was not altered (obese 36,362 +/- 5,639 vs. lean 37,187 +/- 4,239 nmol/24 h, P = 0.912). Acipimox significantly reduced ACTH secretion in the obese subjects (acipimox 5,850 +/- 769 ng/24 h, P = 0.039 vs. placebo), whereas cortisol release did not change (acipimox 33,542 +/- 3,436 nmol/24 h, P = 0.484 vs. placebo). In conclusion, spontaneous ACTH secretion is enhanced in obese premenopausal women, whereas cortisol production is normal. Reduction of circulating FFA concentrations by acipimox blunts ACTH release in obese women, which suggests that FFAs are involved in the pathophysiology of this neuroendocrine anomaly.

Blockade of body weight gain and plasma corticosterone levels in Zucker fatty rats using an orally active neuropeptide Y Y1 antagonist

1. An experiment was conducted to examine whether a potent, orally active and highly selective neuropeptide Y Y1 receptor antagonist attenuates hyperphagia and obesity in genetically obese Zucker fatty rats. 2. Oral administration of the Y1 antagonist (30 and 100 mg x kg(-1), once daily for 2 weeks) significantly suppressed the daily food intake and body weight gain in Zucker fatty rats accompanied with a reduction of fat cell size and plasma corticosterone levels. 3. Despite the fact that food intake was gradually returned to near the control level, the body weight of the treated animals remained significantly less when compared to that of the controls for the duration of the treatment. 4. These results suggest that the Y1 receptor, at least in part, participate in pathophysiological feeding and/or fat accumulation observed in Zucker fatty rats. Y1 antagonists might be useful for the treatment of obesity.

Differential stress responsivity in diet-induced obese and resistant rats

The relationship between stress and obesity was assessed in male rats selectively bred to develop either diet-induced obesity (DIO) or diet resistance (DR) when fed a high-energy, 31% fat diet for 3 wk followed by 2 wk on a hyperphagic liquid diet (Ensure). One-half of the rats of each phenotype were subjected to moderate daily, unpredictable stress (cage changing, exposure to conspecific, swim, and immobilization stress, intraperitoneal saline injection) during the 5 wk. Both stressed and unstressed DIO rats were 26% heavier and ate 27% more than comparable DR rats at onset and had 48% lower basal morning plasma corticosterone levels. Stressed DR rats gained less weight and had significant elevations of basal morning corticosterone but reduced basal sympathetic activity (24-h urine norepinephrine) over 5 wk compared with their unstressed DR controls. Terminally, there was a 35% increase in the paraventricular nucleus corticotropin-releasing hormone mRNA expression. On the other hand, stressed DIO rats showed only a transient early increase in open-field activity and a terminal increase in basal corticosterone levels as the only effects of stress. Thus DIO rats are hyporesponsive to chronic stress compared with DR rats. This is in keeping with several other known differences in hypothalamopituitary and autonomic function in this model.

Heritable variation in food preferences and their contribution to obesity

What an animal chooses to eat can either induce or retard the development of obesity; this review summarizes what is known about the genetic determinants of nutrient selection and its impact on obesity in humans and rodents. The selection of macronutrients in the diet appears to be, in part, heritable. Genes that mediate the consumption of sweet-tasting carbohydrate sources have been mapped and are being isolated and characterized. Excessive dietary fat intake is strongly tied to obesity, and several studies suggest that a preference for fat and the resulting obesity are partially genetically determined. Identifying genes involved in the excess consumption of dietary fat will be an important key to our understanding of the genetic disposition toward common dietary obesity.

Corticosterone facilitation of inhibition of fat intake by enterostatin (Val-Pro-Asp-Pro-Arg)

Enterostatin or Val-Pro-Asp-Pro-Arg (VPDPR) is the amino-terminal pentapeptide of procolipase; VPDPR is generated during tryptic activation of procolipase to lipase. In rodents, exogenous VPDPR has been shown to cause a selective decrement in fat appetite. To understand the mechanism(s) underlying the action of this peptide, we have studied the effects of corticosterone, an adrenal hormone known to modulate caloric intake, on VPDPR-mediated inhibition of appetite. The results of this study show a significant increase in the inhibition of total caloric intake by 250 micrograms/kg VPDPR following corticosterone treatment (control, 2.3%; corticosterone treated, 22.7%). Furthermore, the decrement in the caloric intake in corticosterone-treated rats was exclusively due to the loss of fat intake.