Lack of diet-induced thermogenesis following lesions of paraventricular nucleus in rats

Modulation of the thermosensory system by oxytocin

Oxytocin (OT) is a neurohormone involved early in neurodevelopment and is implicated in multiple functions, including sensory modulation. Evidence of such modulation has been observed for different sensory modalities in both healthy and pathological conditions. This review summarizes the pleiotropic modulation that OT can exercise on an often overlooked sensory system: thermosensation. This system allows us to sense temperature variations and compensate for the variation to maintain a stable core body temperature. Oxytocin modulates autonomic and behavioral mechanisms underlying thermoregulation at both central and peripheral levels. Hyposensitivity or hypersensitivity for different sensory modalities, including thermosensitivity, is a common feature in autism spectrum disorder (ASD), recapitulated in several ASD mouse models. These sensory dysregulations occur early in post-natal development and are correlated with dysregulation of the oxytocinergic system. In this study, we discussed the potential link between thermosensory atypia and the dysregulation of the oxytocinergic system in ASD.

Knockout of the X-linked Fgf13 in the hypothalamic paraventricular nucleus impairs sympathetic output to brown fat and causes obesity

Fibroblast growth factor (FGF)13, a nonsecreted, X-linked, FGF homologous factor, is differentially expressed in adipocytes in response to diet, yet Fgf13's role in metabolism has not been explored. Heterozygous Fgf13 knockouts fed normal chow and housed at 22°C showed hyperactivity accompanying reduced core temperature and obesity when housed at 30°C. Those heterozygous knockouts showed defects in thermogenesis even at 30°C and an inability to protect core temperature. Surprisingly, we detected trivial FGF13 in adipose of wild-type mice fed normal chow and no obesity in adipose-specific heterozygous knockouts housed at 30°C, and we detected an intact brown fat response through exogenous β3 agonist stimulation, suggesting a defect in sympathetic drive to brown adipose tissue. In contrast, hypothalamic-specific ablation of Fgf13 recapitulated weight gain at 30°C. Norepinephrine turnover in brown fat was reduced at both housing temperatures. Thus, our data suggest that impaired CNS regulation of sympathetic activation of brown fat underlies obesity and thermogenesis in Fgf13 heterozygous knockouts fed normal chow.-Sinden, D. S., Holman, C. D., Bare, C. J., Sun, X., Gade, A. R., Cohen, D. E., Pitt, G. S. Knockout of the X-linked Fgf13 in the hypothalamic paraventricular nucleus impairs sympathetic output to brown fat and causes obesity.

Role of leptin in energy expenditure: the hypothalamic perspective

The adipocyte-derived hormone leptin is a peripheral signal that informs the brain about the metabolic status of an organism. Although traditionally viewed as an appetite-suppressing hormone, studies in the past decade have highlighted the role of leptin in energy expenditure. Leptin has been shown to increase energy expenditure in particular through its effects on the cardiovascular system and brown adipose tissue (BAT) thermogenesis via the hypothalamus. The current review summarizes the role of leptin signaling in various hypothalamic nuclei and its effects on the sympathetic nervous system to influence blood pressure, heart rate, and BAT thermogenesis. Specifically, the role of leptin signaling on three different hypothalamic nuclei, the dorsomedial hypothalamus, the ventromedial hypothalamus, and the arcuate nucleus, is reviewed. It is known that all of these brain regions influence the sympathetic nervous system activity and thereby regulate BAT thermogenesis and the cardiovascular system. Thus the current work focuses on how leptin signaling in specific neuronal populations within these hypothalamic nuclei influences certain aspects of energy expenditure.

Neuronal Control of Adaptive Thermogenesis

The obesity epidemic continues rising as a global health challenge, despite the increasing public awareness and the use of lifestyle and medical interventions. The biomedical community is urged to develop new treatments to obesity. Excess energy is stored as fat in white adipose tissue (WAT), dysfunction of which lies at the core of obesity and associated metabolic disorders. By contrast, brown adipose tissue (BAT) burns fat and dissipates chemical energy as heat. The development and activation of "brown-like" adipocytes, also known as beige cells, result in WAT browning and thermogenesis. The recent discovery of brown and beige adipocytes in adult humans has sparked the exploration of the development, regulation, and function of these thermogenic adipocytes. The central nervous system drives the sympathetic nerve activity in BAT and WAT to control heat production and energy homeostasis. This review provides an overview of the integration of thermal, hormonal, and nutritional information on hypothalamic circuits in thermoregulation.

Effect of maternal protein restriction during pregnancy and postweaning high-fat feeding on diet-induced thermogenesis in adult mouse offspring

PURPOSE

Prenatal undernutrition followed by postweaning feeding of a high-fat diet results in obesity in the adult offspring. In this study, we investigated whether diet-induced thermogenesis is altered as a result of such nutritional mismatch.

METHODS

Female MF-1 mice were fed a normal protein (NP, 18% casein) or a protein-restricted (PR, 9% casein) diet throughout pregnancy and lactation. After weaning, male offspring of both groups were fed either a high-fat diet (HF; 45% kcal fat) or standard chow (C, 7% kcal fat) to generate the NP/C, NP/HF, PR/C and PR/HF adult offspring groups (n = 7-11 per group).

RESULTS

PR/C and NP/C offspring have similar body weights at 30 weeks of age. Postweaning HF feeding resulted in significantly heavier NP/HF offspring (P < 0.01), but not in PR/HF offspring, compared with their chow-fed counterparts. However, the PR/HF offspring exhibited greater adiposity (P < 0.01) v the NP/HF group. The NP/HF offspring had increased energy expenditure and increased mRNA expression of uncoupling protein-1 and β-3 adrenergic receptor in the interscapular brown adipose tissue (iBAT) compared with the NP/C mice (both at P < 0.01). No such differences in energy expenditure and iBAT gene expression were observed between the PR/HF and PR/C offspring.

CONCLUSIONS

These data suggest that a mismatch between maternal diet during pregnancy and lactation, and the postweaning diet of the offspring, can attenuate diet-induced thermogenesis in the iBAT, resulting in the development of obesity in adulthood.

Role of oxytocin in energy metabolism

The basic mechanisms that lead obesity are not fully understood; however, several peptides undoubtedly play a role in regulating body weight. Obesity, a highly complex metabolic disorder, involves central mechanisms that control food intake and energy expenditure. Previous studies have shown that central or peripheral oxytocin administration induces anorexia. Recently, in an apparent discrepancy, rodents that were deficient in oxytocin or the oxytocin receptor were shown to develop late-onset obesity without changing their total food intake, which indicates the physiological importance of oxytocin to body metabolism. Oxytocin is synthesized not only within magnocellular and parvocellular neurons but also in several organs, including the ovary, uterus, placenta, testis, thymus, kidney, heart, blood vessels, and skin. The presence of oxytocin receptors in neurons, the myometrium and myoepithelial cells is well recognized; however, this receptor has also been identified in other tissues, including the pancreas and adipose tissue. The oxytocin receptor is a typical class I G protein-coupled receptor that is primarily linked to phospholipase C-β via Gq proteins but can also be coupled to other G proteins, leading to different functional effects. In this review, we summarize the present knowledge of the effects of oxytocin on controlling energy metabolism, focusing primarily on the role of oxytocin on appetite regulation, thermoregulation, and metabolic homeostasis.

Ablation of Sim1 neurons causes obesity through hyperphagia and reduced energy expenditure

Single-minded 1 (Sim1) is a transcription factor necessary for development of the paraventricular nucleus of the hypothalamus (PVH). This nucleus is a critical regulator of appetite, energy expenditure and body weight. Previously we showed that Sim1^+/− mice and conditional postnatal Sim1^−/− mice exhibit hyperphagia, obesity, increased linear growth and susceptibility to diet-induced obesity, but no decrease in energy expenditure. Bilateral ablation of the PVH causes obesity due to hyperphagia and reduced energy expenditure. It remains unknown whether Sim1 neurons regulate energy expenditure. In this study, Sim1cre mice were bred to homozygous inducible diphtheria toxin receptor (iDTR) mice to generate mice expressing the simian DTR in Sim1 cells. In these mice, Sim1 neuron ablation was performed by intracerebroventricular (ICV) injection of diphtheria toxin. Compared to controls, mice with Sim1 neuron ablation became obese (with increased fat mass) on a chow diet due to increased food intake and reduced energy expenditure. In post-injection mice, we observed a strong inverse correlation between the degree of obesity and hypothalamic Sim1 expression. The reduction in baseline energy expenditure observed in these mice was accompanied by a reduction in activity. This reduction in activity did not fully account for the reduced energy expenditure as these mice exhibited decreased resting energy expenditure, decreased body temperature, decreased brown adipose tissue temperature, and decreased UCP1 expression suggesting an impairment of thermogenesis. In injected mice, hypothalamic gene expression of Sim1, oxytocin (OXT) and thyrotropin releasing hormone (TRH) was reduced by about 50%. These results demonstrate that Sim1 neurons in adult mice regulate both food intake and energy expenditure. Based on the body of work in the field, feeding regulation by Sim1 neurons likely occurs in both the PVH and medial amygdala, in contrast to energy expenditure regulation by Sim1 neurons, which likely is localized to the PVH.

Central control of thermogenesis

In mammals and birds, conservation of body heat at around 37 °C is vital to life. Thermogenesis is the production of this heat which can be obligatory, as in basal metabolic rate, or it can be facultative such as the response to cold. A complex regulatory system has evolved which senses environmental or core temperature and integrates this information in hypothalamic regions such as the preoptic area and dorsomedial hypothalamus. These areas then send the appropriate signals to generate and conserve heat (or dissipate it). In this review, the importance of the sympathetic nervous system is discussed in relation to its role in basal metabolic rate and adaptive thermogenesis with a particular emphasis to human obesity. The efferent sympathetic pathway does not uniformly act on all tissues; different tissues can receive different levels of sympathetic drive at the same time. This is an important concept in the discussion of the pharmacotherapy of obesity. Despite decades of work the medicine chest contains only one pill for the long term treatment of obesity, orlistat, a lipase inhibitor that prevents the absorption of lipid from the gut and is itself not systemically absorbed. The central controlling system for thermogenesis has many potential intervention points. Several drugs, previously marketed, awaiting approval or in the earlier stages of development may have a thermogenic effect via activation of the sympathetic nervous system at some point in the thermoregulatory circuit and are discussed in this review. If the balance is weighted to the "wrong" side there is the burden of increased cardiovascular risk while a shift to the "right" side, if possible, will afford a thermogenic benefit that is conducive to weight loss maintenance. This article is part of a Special Issue entitled 'Central Control Food Intake'

Melanocortin-4 receptor mRNA expressed in sympathetic outflow neurons to brown adipose tissue: neuroanatomical and functional evidence

A precise understanding of neural circuits controlling lipid mobilization and thermogenesis remains to be determined. We have been studying the sympathetic nervous system (SNS) contributions to white adipose tissue (WAT) lipolysis largely in Siberian hamsters. Central melanocortins are implicated in the control of the sympathetic outflow to WAT, and, moreover, the melanocortin 4 receptors (MC4-R) appear to be principally involved. We previously found that acute third ventricular melanotan II (MTII; an MC3/4-R agonist) injections increase sympathetic drive (norepinephrine turnover) to interscapular brown adipose tissue (IBAT) and IBAT temperature. Here we tested whether MC4-R mRNA is expressed in IBAT SNS outflow neurons using in situ hybridization for the former and injections of the transneuronal viral retrograde tract tracer, pseudorabies virus (PRV) into IBAT, for the latter. Significant numbers of double-labeled cells for PRV and MC4-R mRNA were found across the neuroaxis (mean of all brain sites approximately 60%), including the hypothalamic paraventricular nucleus (PVH; approximately 80%). Acute parenchymal MTII microinjections into the PVH of awake, freely-moving hamsters, using doses below those able to increase IBAT temperature when injected into the third ventricle, increased IBAT temperature for as long as 4 h, as measured by temperature transponders implanted below the tissue. Collectively, these data add significant support to the view that central melanocortins are important in controlling IBAT thermogenesis via the SNS innervation of this tissue, likely through the MC4-Rs.

CNS origins of the sympathetic nervous system outflow to brown adipose tissue

Brown adipose tissue (BAT) plays a critical role in cold- and diet-induced thermogenesis. Although BAT is densely innervated by the sympathetic nervous system (SNS), little is known about the central nervous system (CNS) origins of this innervation. The purpose of the present experiment was to determine the neuroanatomic chain of functionally connected neurons from the CNS to BAT. A transneuronal viral tract tracer, Bartha's K strain of the pseudorabies virus (PRV), was injected into the interscapular BAT of Siberian hamsters. The animals were killed 4 and 6 days postinjection, and the infected neurons were visualized by immunocytochemistry. PRV-infected neurons were found in the spinal cord, brain stem, midbrain, and forebrain. The intensity of labeled neurons in the forebrain varied from heavy infections in the medial preoptic area and paraventricular hypothalamic nucleus to few infections in the ventromedial hypothalamic nucleus, with moderate infections in the suprachiasmatic and lateral hypothalamic nuclei. These results define the SNS outflow from the brain to BAT for the first time in any species.

Norepinephrine Injection into the Paraventricular Nucleus Induces a Reduced Modification of Eating Behavior and Thermogenesis in Brattleboro Rats

Intake of carbohydrates, lipids, proteins and total calories, temperature of interscapular brown adipose tissue, and oxygen consumption were monitored in vasopressin-containing and vasopressin-deficient rats. These variables were measured after a 20 nmol norepinephrine (NE) or saline injection into the paraventricular nucleus (PVN) of the hypothalamus. NE increased the intake of carbohydrates, lipids and total calories, decreased brown adipose tissue temperature and oxygen consumption in vasopressin-containing rats. NE reduced the intake of carbohydrates, while it increased the consumption of lipids in vasopressin-deficient rats. These findings indicate that vasopressin is involved in the modifications of eating behavioral and thermogenesis induced by NE injection into the hypothalamic PVN.

Procaine injection into the paraventricular nucleus reduces sympathetic and thermogenic activation induced by frontal cortex stimulation in the rat

These experiments were designed to test the effect of procaine injection into the paraventricular nucleus on the sympathetic and thermogenic changes induced by frontal cortex stimulation. Oxygen consumption, firing rate of the sympathetic nerves to interscapular brown adipose tissue (IBAT), along with IBAT and colonic temperatures (T(IBAT) and T(C)) were monitored in fasted male Sprague-Dawley rats before and 25 min after an electrical stimulation of the frontal cortex. The same variables were monitored in rats with administration of procaine into the paraventricular nucleus. The results show that cortical stimulation raises oxygen consumption, sympathetic neuron firing rates, T(IBAT), and T(C). This increase is reduced by procaine injection. These findings suggest that the paraventricular nucleus plays a key role in the sympathetic and thermogenic changes induced by cortical stimulation.

Lesions of the ventromedial hypothalamus reduce postingestional thermogenesis

The aim of this experiment was to evaluate the effects of ventromedial hypothalamus lesions on the thermogenic changes that follow food intake. Four groups of six Sprague-Dawley male rats were used. Under anesthesia with pentobarbital, the animals in the first and second groups received lesions at the ventromedial hypothalamus, and animals in the third and fourth groups received sham lesions. Body weight and food intake were monitored daily until the experimental procedure began. Twenty days after lesion, oxygen consumption, firing rate of sympathetic nerves to interscapular brown adipose tissue (IBAT), and IBAT temperature were monitored for 45 min both before and after 5 g food intake in 24 h fasted rats from the first and third groups. The same variables were measured in the animals of the second and fourth groups 50 days after receiving the lesions. Lesion placements were histologically verified. The results showed that lesions produced hyperphagia and obesity. Firing rate of nerves to IBAT, IBAT temperature, and oxygen consumption increased after food intake in sham-lesioned rats. This increase was significantly reduced by the lesion at both the 20- and 50-day time points. These findings indicate that the ventromedial hypothalamus controls postingestional activation of sympathetic discharge to IBAT. The reduction of postingestional thermogenesis could be involved in the development of obesity induced by lesion of the ventromedial hypothalamus.

EEG arousal, sympathetic activity, and brown adipose tissue thermogenesis after conditioned taste aversion

Conditioned taste aversion was induced in rats by pairing saccharin with intraperitoneal LiCl injection. Animals injected with NaCl served as controls. After evaluating the preference levels for saccharin in rats of both groups, the animals were anesthetized with urethane and the duration of EEG desynchronization, firing rate of sympathetic nerves innervating interscapular brown adipose tissue (BAT), and temperature of the same tissues were recorded, before and after oral stimulation with saccharin or water. The EEG desynchronization was longer in conditioned rats after stimulation with saccharin. Firing rate of sympathetic nerves was higher in conditioned rats after presentation of saccharin. BAT temperature, decreased in conditioned rats after saccharin stimulus, was unchanged in the three other conditions. In a second experiment temperature and firing rate of sympathetic nerves of BAT were recorded after oral presentation of water or saccharin in rats treated as in the first experiment and injected with the alpha 1-adrenergic blocker prazosin. As in the first experiment, saccharin presentation in conditioned animals enhanced the neural sympathetic activity, whereas differently from the first experiment it increased BAT temperature. No changes were found in the same measurements in the three other conditions. The drop in interscapular BAT temperature found in the first experiment, an unexpected finding, probably depends on the use of lithium as unconditioned stimulus, because LiCl interacting with adrenergic receptors changes the two-phase response, normally seen in interscapular BAT after increased sympathetic activity, in a single-phase response.

Septal lesions block sympathetic activation following frontal cortex stimulation in the rat

The rates of oxygen consumption, colonic and interscapular brown adipose tissue temperature, and discharge of sympathetic nerves innervating the same tissue were recorded before and after orbital frontal cortex stimulation in three groups of rats. These groups consisted of animals with electrolytic lesions of the nucleus medialis septi, with lesions of nucleus lateralis septi or with sham lesions. The values of all the variables considered were similar during the pre-stimulation time in the three groups of rats. There was an increase in all the parameters in sham-lesioned animals after electrical stimulation, while no changes were found in the two groups of injured rats. The results showed that the frontal cortex is involved in the control of thermogenesis through the activation of the sympathetic nervous system. Medial and lateral septal nuclei, in spite of their neurochemical and pharmacological differences, share a common role in the pathway from the frontal cortex to the hypothalamic area and the amygdala, which in turn drive the sympathetic nervous system.

Postprandial thermogenesis and conditioned taste aversion or preference

Postprandial thermogenesis is under the control of the autonomic nervous system and alimentary conditioned stimuli change sympathetic and parasympathetic activity. Here we studied the effect of conditioned taste aversion on postprandial thermogenesis in rats. Two groups of animals were used, rats of the first group were controls, these were placed on a standard diet and, for some days, on two other different diets: one thiamine-free and the other thiamine-rich. Each diet had a different taste. The treated animals belonged to the second group, these were fed with the same three diets but for different lengths of times: thiamine-free diet for the first 5 wk afterwards, with thiamine-rich diet for 3 wk, and finally with laboratory standard diet for a few days. After a preference test with the three familiar diets, oxygen consumption rate and brown adipose tissue temperature were evaluated three times in both groups after ingestion of a test meal, each time with one of the three different diets. The preference test was unvaried for the three different familiar foods in controls, while the treated animals showed a lower preference for thiamine-free food than for the other two. Treated rats had a significantly higher increase in O2 consumption rate than controls. In this group intake of thiamine-free food induced a significantly lower increase in O2 consumption than the other two. The increase in brown adipose tissue temperature was also higher in treated than in control animals but in treated rats this was lower after intake of thiamine-free food than after the intake of the other two.(ABSTRACT TRUNCATED AT 250 WORDS)