Dissociation between brown adipose tissue thermogenesis and sympathetic activity in rats with high plasma levels of oestradiol

It has been shown previously that high plasma levels of oestradiol inhibit brown adipose tissue thermogenesis. Since rats and mice show a close association between thermogenic activity in and sympathetic discharge to brown fat, we measured the noradrenaline turnover in rats with high plasma levels of oestradiol to establish whether the observed inhibition of thermogenic activity is brought about by a reduction in the sympathetic drive to brown adipocytes. Oestradiol-filled Silastic capsules were implanted subcutaneously in female rats previously acclimated either to thermoneutrality or to cold. Control rats received empty implants. After 15 days treatment, noradrenaline turnover was measured by blocking its synthesis with -methyl-p-tyrosine. As expected, noradrenaline turnover was higher in cold-acclimated rats than in rats kept at thermoneutrality. The presence of high plasma oestradiol levels did not alter sympathetic activity in any of the treated groups despite reducing thermogenic activity. This result reveals that oestradiol dissociates the thermogenic activity of brown adipose tissue from its sympathetic activation. Such dissociation has never been previously reported in rats, although it seems to be common in Syrian hamsters. However the causative factor in this species is unknown.A preliminary report of this work was presented to the XXXIIth International Congress of Physiological Sciences, Glasgow, August 1993 and appears as abstract No. 287.6/P.     

Untersuchungen über die Bedeutung des multilokulären Fettgewebes für die Thermogenese des neugeborenen Meerschweinchens

Summary As has been previously shown, non-shivering thermogenesis is the prevailing mechanism of heat production in the newborn guinea pig exposed to a cool environment. In the course of the first two to three weeks of life, non-shivering thermogenesis is reduced and replaced by shivering. In the present study this process has been shown to be linked with morphological and functional alterations in the multilocular adipose tissue. The functional changes have been evidenced by the following experiments: In two groups of guinea pigs, 0–2, and 25 to 42 days of age, respectively, the local temperature and blood flow were measured in the large fat pad located between the scapulae, in addition to gaseous metabolism, electrical activity of the sceletal musculature and colon temperature. In the young animals the rise of O₂-consumption as occuring on cold exposure was accompanied by a temperature rise in the fat tissue (absolute and relative to the colon temperature), whereas electrical activity of the musculature remained low. Blood flow of the fat tissue increased at the same time. In the older animals, the temperature difference between the fat tissue and colon remained low but it occurred shivering. The increase of temperature difference and blood flow was reduced or abolished in the young animals by Alderlin, an adrenergic -receptor blocking agent that has been previously shown to block non-shivering thermogenesis; on the other hand, the blockade of non-shivering thermogenesis caused an onset of shivering. Further, noradrenaline and adrenaline have been shown to increase blood flow of the fat tissue in the young animals. The results indicate. 1. that the brown adipose tissue can be considered an important site of non-shivering thermogenesis in the newborn guinea pig, and 2. that the thermogenic capacity of the brown fat is diminished with increasing age.

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Mit Unterstützung der Deutschen Forschungsgemeinschaft.     

Influence of noradrenaline on blood flow to brown adipose tissue in rats exhibiting diet-induced thermogenesis

1. The influence of noradrenaline on regional blood flow was determined using radioactive microspheres in rats maintained on either stock diet or a palatable cafeteria diet.
2. Cardiac output and blood flow to brain, lungs, liver and skeletal muscle were similar for rats on the two diets.
3. Blood flow to total dissectable brown adipose tissue in control and cafeteria rats represented 1 and 2% of cardiac output respectively but these values rose to 7 and 15.5% during infusion of noradrenaline.
4. Arterial oxygen content was similar for all groups but the oxygen content of venous blood draining the interscapular brown adipose tissue fell to 6 ml O₂/100 ml blood in control rats and 1 ml/100 ml in cafeteria rats after noradrenaline.
5. The total oxygen consumption of brown adipose tissue was calculated and found to account for 42% of the response to noradrenaline in control rats and 74% in cafeteria animals. The increments in the oxygen consumption of other tissues were almost identical in both groups and so all the diet-induced changes in thermogenic capacity can be attributed to increases in brown adipose tissue metabolism.
6. These findings demonstrate the quantitative importance of brown adipose tissue in diet-induced thermogenesis and confirm the similarities between diet and non-shivering thermogenesis.

     

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.     

Adipose tissue and heat production in the new-born ox (Bos taurus)

1. A histological examination of adipose tissue from 1- and 6-day-old calves showed a structure typical of white adipose tissue and no evidence of brown adipose tissue.2. Infusion of noradrenaline (1.0 mug/kg.min I.V.) into 1-to 6-day-old calves affected heart rate and respiratory rate but did not increase heat production, rectal temperature, skin temperature or skin evaporative loss.3. Cold exposure led to shivering and an increased oxygen consumption in the 6-day-old calf.4. Blood samples taken from 6-day-old calves in the cold (-1 degrees C) appeared to have a higher proportion of unesterified fatty acids in the total blood lipid than samples taken in an environment of 20 degrees C, but no change in fatty acid composition was found.5. It has been concluded that there is no non-shivering thermogenesis in the young calf.     

Sex-associated differences in cold-induced UCP1 synthesis in rodent brown adipose tissue

 The effects of acute and chronic acclimation to cold on uncoupling protein 1 (UCP1) levels, as well as on GDP-binding to mitochondria, cytochrome c oxidase activity and mitochondrial protein concentration in brown adipose tissue (BAT) of intact male and female rats have been analyzed. Results reveal that females rats are more sensitive to cold because their threshold temperature for the thermogenic response is set at a higher value (around 22°C) than that of males (around 18°C), hence leading to differences in BAT UCP1 levels between the sexes at different environmental temperatures. In vitro experiments showed that steroid hormones, β-estradiol, estrone and progesterone, can reduce norepinephrine-induced UCP1 synthesis in brown adipocytes differentiated in primary culture. Thus the different sex-associated response of cold-induced thermogenesis in rats does not appear to be explained by a direct action of sex steroids upon the adipocyte, implying that other factors in the thermogenic regulatory system must be involved. Received: 23 March 1998 / Received after revision: 20 May 1998 / Accepted: 21 May 1998     

Mechanisms of Modulation of Thermoregulatory Reactions during Cooling in Hypertensive Rats by the Sympathetic Nervous System

Susceptibility of thermoregulatory responses to cold to blockage of α₁- and β-adrenoreceptors differs in health and hypertension. α₁-Adrenoceptor blockade reduces vessel reactivity during cooling and vessel reaction to cold becomes similar to that in intact normotensive rats. Changes in the structure of metabolic response to cold in favor of non-shivering thermogenesis typical of hypertensive animals becomes even more pronounced under conditions of α₁-adrenoceptor blockade due to inhibition of cold shivering. Blockage of β-adrenoceptors does not affect parameters of vascular response to cooling. In hypertensive rats, in contrast to normotensive animals, β-adrenoceptor blockade during cooling increased temperature thresholds for total metabolic reaction and shivering. The maximum shivering intensity also increased, which partially compensated inhibition of non-shivering thermogenesis. In the whole organism, blockade of one type of adrenoceptors during cooling leads to intensification of compensatory mechanisms realized through adrenoceptors of the other type. In hypertensive rats, compensatory capacities of thermogenic processes controlled by α₁- and β-adrenoceptors are impaired in comparison with normotensive animals under conditions of inhibition of both shivering and non-shivering thermogenesis.     

Effect of immobilization stress on in vitro and in vivo thermogenesis of brown adipose tissue.

Repetitive intermittent stress such as immobilization has been shown to induce an improved cold tolerance through an enhanced capacity of nonshivering thermogenesis (NST), causing positive cross adaptation between nonthermal stress and cold. In the present study, effect of 3-h-daily immobilization stress for 4-5 weeks was investigated on in vitro and in vivo thermogenesis of interscapular brown adipose tissue (BAT). In vitro thermogenesis was measured in the minced tissue blocks incubated in Krebs-Ringer phosphate buffer with glucose and albumin at 37 degrees C, using a Clark-type oxygen electrode. The stressed rats showed less body weight gain during the experiment. The BAT weight, its protein and DNA contents were significantly greater in the stressed rats. Basal, noradrenaline- and glucagon-stimulated oxygen consumptions were significantly greater in the stressed rats. In vivo thermogenesis was assessed by the changes of temperatures in colon (Tcol), BAT (TBAT), and tail skin (Tsk) induced by noradrenaline or glucagon infusion in the anesthetized rats. Noradrenaline and glucagon increased the TBAT and the extent of increase was greater in the stressed rats. These results indicate that cross adaptation between nonthermal stress and cold may be mediated through an enhanced thermogenic activity of BAT.     

Differential induction of genes in liver and brown adipose tissue regulated by peroxisome proliferator-activated receptor-alpha during fasting and cold exposure in acyl-CoA dehydrogenase-deficient mice

Mice deficient for either long-chain acyl-CoA dehydrogenase (LCAD-/-) or very-long-chain acyl-CoA dehydrogenase (VLCAD-/-) develop hepatic steatosis upon fasting, due to disrupted mitochondrial fatty acid oxidation. Moreover, neither mouse model can maintain core body temperature when exposed to cold. We investigated the effects of fasting and cold exposure on gene expression in these mice. Non-fasted LCAD-/- mice showed gene expression changes indicative of fatty liver, including elevated mRNA levels for peroxisome proliferator-activated receptor-gamma (PPARgamma) and genes involved in lipogenesis. In LCAD-/- and VLCAD-/- mice challenged with fasting and cold exposure, expression of fatty acid oxidation genes was elevated in liver, consistent with increased PPARalpha activity. This effect was not seen in brown adipose tissue, suggesting that expression of these genes may be regulated differently than in liver. The effect of acute cold exposure on expression of fatty acid oxidation genes was measured in peroxisome proliferator-activated receptor (PPAR)-alpha-deficient mice (PPARalpha-/-) and controls. In PPARalpha-/- mice, basal expression of the acyl-CoA dehydrogenases was reduced in liver but was not altered in brown adipose tissue. While cold altered the expression of PPARgamma, sterol-regulatory element binding protein-1 (SREBP-1), ATP citrate lyase, and the uncoupling proteins in brown adipose tissue from both PPARalpha-/- and control mice, fatty acid oxidation genes were unaffected. Thus, while fatty acid oxidation appears critical for non-shivering thermogenesis, expression of the acyl-CoA dehydrogenases is not influenced by cold exposure. Moreover, mitochondrial fatty acid oxidation genes are not regulated by PPARalpha in brown adipose tissue as they are in liver.     

Brown adipose tissue and thermogenesis in hypophysectomized rats in relation to temperature acclimation

The aim of this work was to test the role of pituitary dependent hormones in cold-induced non-shivering thermogenesis. In the 28°C-acclimated rat, hypophysectomy inhibited body growth and led to an atrophy of thyroid and adrenals. In brown adipose tissue (BAT) some alterations were induced which are usually observed after cold acclimation of the animal: increase in relative weight, decreases in the relative amount of lipids, increases in the amounts of protein and DNA and modification of the proportions of several phospholipid fatty acids; moreover, basal lipolysis, in vitro, was enhanced to the same extent as that following cold acclimation of the normal rat. The in vivo stimulation by norepinephrine (NE) of O₂ consumption (test for nonshivering thermogenesis) and of fatty acid release into blood were suppressed.Progressive cold acclimation of the hypophysectomized rats at 15°C led to a hypertrophy of BAT to the same extent as in the sham-operated animals. The in vivo sensitivity to NE was partially restored. The results suggest that hypophysectomy does not suppress the ability to acclimate to moderate cold by means of BAT dependent non-shivering thermogenesis. However, the low ability to produce heat seems to indicate that pituitary or pituitary-dependent hormones are necessary to optimize the cold stimulation of brown fat thermogenesis.     

Chronic l-menthol-induced browning of white adipose tissue hypothesis: A putative therapeutic regime for combating obesity and improving metabolic health

IntroductionObesity constitutes a serious global health concern reaching pandemic prevalence rates. The existence of functional brown adipose tissue (BAT) in adult humans has provoked intense research interest in the role of this metabolically active tissue in whole-body energy balance and body weight regulation. A number of environmental, physiological, pathological, and pharmacological stimuli have been proposed to induce BAT-mediated thermogenesis and functional thermogenic BAT-like activity in white adipose tissue (WAT), opening new avenues for therapeutic strategies based on enhancing the number of beige adipocytes in WAT.HypothesisRecent evidence support a role of l-menthol cooling, mediated by TRPM8 receptor, on UCP1-dependent thermogenesis and BAT-like activity in classical WAT depots along with the recruitment of BAT at specific anatomical sites. l-Menthol-induced BAT thermogenesis has been suggested to occur by a β-adrenergic-independent mechanism, avoiding potential side-effects due to extensive β-adrenergic stimulation mediated by available beta receptor agonists. l-Menthol has been also linked to the activation of the cold-gated ion channel TRPA1. However, its role in l-menthol-induced UCP1-dependent thermogenic activity in BAT and WAT remains undetermined. White adipose tissue plasticity has important clinical implications for obesity prevention and/or treatment because higher levels of UCP1-dependent thermogenesis can lead to enhanced energy expenditure at a considerable extent. We hypothesize that chronic dietary l-menthol treatment could induce TRPM8- and TRPA1-dependent WAT adaptations, resembling BAT-like activity, and overall improve whole-body metabolic health in obese and overweight individuals.ConclusionsThe putative impact of chronic l-menthol dietary treatment on the stimulation of BAT-like activity in classical WAT depots in humans remains unknown. A detailed experimental design has been proposed to investigate the hypothesized l-menthol-induced browning of WAT. If our hypothesis was to be confirmed, TRPM8/TRPA1-induced metabolic adaptations of WAT to BAT-like activity could provide a promising novel therapeutic approach for increasing energy expenditure, regulating body weight, and preventing obesity and its related co-morbidities in humans.     

Effect of acute cold-exposure on norepinephrine turnover and thermogenesis in brown adipose tissue and metabolic rate in MSG-induced obese mice

Mice treated neonatally with monosodium-L-glutamate (MSG) are known to develop into obese adults without hyperphagia, which are characterized by the reduced levels in the resting metabolic rate (RMR) and the thermogenesis of brown adipose tissue (BAT) in the thermoneutral environment. The present study revealed that an acute cold-exposure (5 degrees C, 1 h) of these animals resulted in the increase in norepinephrine turnover and mitochondrial-5'-diphosphate (GDP) binding in the interscapular BAT as well as the guanosine RMR, suggesting a normal thermogenic responsiveness of BAT to cold.     

The role of uncoupling proteins in the regulation of metabolism

Investigations of variations in metabolic efficiency and thermogenesis have a short and turbulent history. In small animals, non-shivering thermogenesis and diet-induced thermogenesis have a great impact on overall body weight, and the question is whether mechanisms to waste energy have evolved also in human energy metabolism. The candidate molecules for this adaptive thermogenesis are the uncoupling proteins. This is a newly discovered family of proteins, consisting of at least five proteins, namely UCP1, UCP2, UCP3, UCP4 and UCP5. Although a role for UCP1 in thermogenesis is unequivocal, the physiological function of the newer uncoupling proteins is as yet unclear. UCP1 is present in brown adipose tissue and has a well-documented role in cold-induced thermogenesis. The targeted disruption of the UCP1-gene rendered animals that were cold sensitive, but not obese. UCP2 mRNA has a ubiquitous distribution in tissue, namely, in skeletal muscle, white and brown adipose tissue, the gastro-intestinal tract, the lung and the spleen. By targeting the UCP2-gene there was no effect on whole body energy metabolism, but instead, a reduced ability to protect against free-radical oxygen species. UCP2 has also been shown to act as a negative regulator for insulin secretion. UCP3 is present in skeletal muscle. Targeted disruption of the UCP3-gene gave no effect on whole body energy metabolism, but showed the mitochondria in muscle to be more coupled. In conclusion, the uncoupling proteins may be important in various specific ways, as protectors of free radical oxygen species and as regulators of ATP-dependent processes.     

Brown adipose tissue of mice with gold thioglucose-induced obesity: effect of cold and diet

Gold thioglucose (GTG)-obese mice have a larger than normal amount of brown adipose tissue (BAT) with ultrastructurally normal mitochondria. The tissue grows normally when the mice adapt to cafeteria feeding or to cold (8 degrees C). Acute exposure to cold causes a fairly normal thermogenic activation of BAT mitochondria of GTG-obese mice, both in dynamic and static phases of their obesity. However, chow-fed GTG-obese mice have BAT mitochondria that are in a low state of thermogenic activation, and these mice fail to respond to eating a cafeteria diet for 3 wk by a normal thermogenic activation of their BAT mitochondria. More prolonged cafeteria feeding for 11-13 wk, into the static phase of obesity, is associated with thermogenic activation of BAT mitochondria of GTG-obese mice. The capacity of GTG-obese mice to respond to noradrenaline (norepinephrine) by an increase in metabolic rate is greater than that of lean mice and is further enhanced by cold acclimation. It is concluded that BAT of the GTG-obese mouse is inherently functional, as is control of its thermogenic function and growth during cold exposure and cold acclimation. Dietary influences on BAT thermogenic function are, however, defective in the GTG-obese mouse at least during the dynamic phase of its obesity. The resulting failure of diet-induced thermogenesis would be expected to contribute to the known high metabolic efficiency of the GTG-obese mouse and, together with the hyperphagia, to the obesity induced by GTG.     

The effect of environmental conditions on the growth and function of brown adipose tissue

1. Rabbits were reared from birth under one of two contrasting conditions. They were either well-fed and kept in a cool environment, or kept warm but fed on alternate days only. When they were a week old their metabolic responses to cold or the net exchanges of glucose, free fatty acid and glycerol by their brown adipose tissue in vivo before and during noradrenaline infusions were measured.2. The cold-reared rabbits grew normally, the under-fed rabbits were undersized; in both groups the brown adipose tissue wet weights were less than those normally found at birth. In cold-reared rabbits this was due largely to the low fat content, in the under-fed rabbits it was not.3. The rabbits' rates of oxygen consumption at a thermoneutral temperature and in response to cold was greater in the cold-reared rabbits.4. The increase in temperature over brown adipose tissue and the increase in blood flow in response to noradrenaline infusion was also greater in the cold-reared rabbits.5. Although the brown adipose tissue of cold-reared rabbits contained 30% fat it did not release fatty acids 24 hr after a feed. In contrast the brown adipose tissue of the under-fed rabbits released fatty acids at a high rate.6. It was concluded that both prolonged cold exposure and undernutrition influenced the function and development of brown adipose tissue during the first week of life. Cold exposure increases the thermogenic capacity of the tissue, undernutrition reduces it. The net release of fatty acids from brown adipose tissue, is, in part, dependent on the tissues' own intracellular requirements.     

Brown adipose tissue response to cafeteria diet-feeding involves induction of the UCP2 gene and is impaired in female rats as compared to males

Noradrenaline-dependent brown adipose tissue (BAT) thermogenesis is activated by the cold and excess energy intake, largely depends on the activity of the uncoupling protein 1 (UCP1), and is mediated mainly through the beta3-adrenoceptor (beta3-AR). We investigated the expression of ucp2, a gene that encodes a putative UCP1-like uncoupling protein, along with that of ucp1 and beta3-ar, in the interscapular BAT (IBAT) of male and female rats chronically fed a cafeteria diet. After 3 months on this diet, male rats attained a 34% excess body mass and showed IBAT hypertrophy and increased IBAT thermogenic potential, in terms of both UCP1 and UCP2 mRNA expression (both by 1.6-fold), UCP1 protein expression (by 1.75-fold) and GDP binding to IBAT mitochondria (by 2.2-fold); female rats attained a larger excess body weight (50%) and their IBAT, although hypertrophied, showed no signs of increased thermogenic potential per gram of tissue. Interestingly, the IBAT of female rats was already activated compared to males. Treatment of mouse brown adipocytes in primary culture with noradrenaline also triggered a dose-dependent increase of the levels of UCP1 mRNA and UCP2 mRNA. Retroregulatory down-regulation of the beta3-AR mRNA levels was found in the two models used. The results support a physiological role for UCP2, along with UCP1, in rodent BAT thermogenesis.     

Prostaglandins, brown fat and weight loss

In obesity, a situation is created in which energy intake exceeds energy expenditure. The three components of energy expenditure are resting metabolism, physical activity, and thermogenesis. Increasing attention is being paid to the role of impaired energy expenditure in obesity. Evidence indicates that impairment in activity of the sympathetic nervous system, which stimulates thermogenic processes, contributes to the etiology of obesity. In addition, insulin resistance, a well-recognized metabolic consequence of obesity, appears to interfere with feeding-related, insulin-mediated increases in thermogenesis in brown adipose tissue. This thermogenic defect results in reduced energy buffering by brown adipose tissue leading to deficient energy expenditure and an increased efficiency in weight gain. A unique weight loss program, The Princeton Metabolic Diet Program, is presented. The Program stimulates metabolism by stimulating the sympathetic nervous system and correcting insulin resistance, thereby enhancing thermogenesis in brown adipose tissue. Methods include: 1) alternating diet composition and caloric intake and, 2) the use of nutritional metabolic stimulants. This type of non-toxic therapy, directed at correcting biochemical defects, will enhance metabolic mechanisms and induce weight loss.     

Pituitary and autonomic responses to cold exposures in man

This review presents hormonal responses to various cold exposures and their calorigenic effects in man and some animals. Previous studies in rats have shown that cold exposures activate the hypothalamic-pituitary-thyroid axis. Increased thyroid hormone concentrations lead to heat production via general stimulation of metabolism (obligatory thermogenesis) and possibly via activation of thyroid hormone receptors and uncoupling protein 1 (UCP 1) and deiodinase enzyme genes in the brown adipose tissue (BAT). In human subjects long-term cold exposures do not seem to activate the pituitary-thyroid axis, but rather accelerate the elimination of triiodothyronine (T3), leading to low serum concentrations of free T3 hormone. In corollary to this a hypothyreotic condition with increased serum thyroid-stimulating hormone and impaired mood and cognitive performance can be observed after long-term cold exposures such as wintering. During cold exposures the sympathetic nerve system is activated and noradrenaline is released to blood circulation and to BAT, where it leads to production of cAMP, lipolysis and free fatty acids. Free fatty acids open the mitochondrial proton channel protein in BAT. Protons enter the mitochondria and inhibit ATP synthesis (uncoupling). By this way energy is transformed into heat (facultatory or adaptive thermogenesis). In adult human subjects the amount of BAT is small and adaptive thermogenesis (non-shivering thermogenesis) has a smaller role. UCP 1 with other uncoupling proteins may have other functions in the control of body weight, sugar balance and formation of reactive oxygen species.