Depletion of JunB increases adipocyte thermogenic capacity and ameliorates diet-induced insulin resistance

The coexistence of brown adipocytes with low and high thermogenic activity is a fundamental feature of brown adipose tissue heterogeneity and plasticity. However, the mechanisms that govern thermogenic adipocyte heterogeneity and its significance in obesity and metabolic disease remain poorly understood. Here we show that in male mice, a population of transcription factor jun-B (JunB)-enriched (JunB+) adipocytes within the brown adipose tissue exhibits lower thermogenic capacity compared to high-thermogenic adipocytes. The JunB+ adipocyte population expands in obesity. Depletion of JunB in adipocytes increases the fraction of adipocytes exhibiting high thermogenic capacity, leading to enhanced basal and cold-induced energy expenditure and protection against diet-induced obesity and insulin resistance. Mechanistically, JunB antagonizes the stimulatory effects of PPARγ coactivator-1α on high-thermogenic adipocyte formation by directly binding to the promoter of oestrogen-related receptor alpha, a PPARγ coactivator-1α downstream effector. Taken together, our study uncovers that JunB shapes thermogenic adipocyte heterogeneity, serving a critical role in maintaining systemic metabolic health.

FASN deficiency induces a cytosol-to-mitochondria citrate flux to mitigate detachment-induced oxidative stress

Fatty acid synthase (FASN) maintains de novo lipogenesis (DNL) to support rapid growth in most proliferating cancer cells. Lipogenic acetyl-coenzyme A (CoA) is primarily produced from carbohydrates but can arise from glutamine-dependent reductive carboxylation. Here, we show that reductive carboxylation also occurs in the absence of DNL. In FASN-deficient cells, reductive carboxylation is mainly catalyzed by isocitrate dehydrogenase-1 (IDH1), but IDH1-generated cytosolic citrate is not utilized for supplying DNL. Metabolic flux analysis (MFA) shows that FASN deficiency induces a net cytosol-to-mitochondria citrate flux through mitochondrial citrate transport protein (CTP). Previously, a similar pathway has been shown to mitigate detachment-induced oxidative stress in anchorage-independent tumor spheroids. We further report that tumor spheroids show reduced FASN activity and that FASN-deficient cells acquire resistance to oxidative stress in a CTP- and IDH1-dependent manner. Collectively, these data indicate that by inducing a cytosol-to-mitochondria citrate flux, anchorage-independent malignant cells can gain redox capacity by trading off FASN-supported rapid growth.

Becoming less beige with age

The production of beige adipocytes following cold exposure is blocked as mice get older and leads to changes in the expression of metabolic genes.

FASN-deficiency induces a cytosol-to-mitochondria citrate flux to mitigate detachment-induced oxidative stress

Fatty acid synthase (FASN) maintains de novo lipogenesis (DNL) to support rapid growth in most proliferating cancer cells. Lipogenic acetyl-CoA is primarily produced from carbohydrates but can arise from glutamine-dependent reductive carboxylation under hypoxia. Here we show that reductive carboxylation also occurs in the absence of DNL in cells with defective FASN. In this state, reductive carboxylation was mainly catalyzed by isocitrate dehydrogenase-1 (IDH1) in the cytosol, but IDH1-generated citrate was not used for DNL. Metabolic flux analysis (MFA) revealed that FASN-deficiency induced a net cytosol-to-mitochondria citrate flux through citrate transport protein (CTP). A similar pathway was previously shown to mitigate detachment-induced mitochondrial reactive oxygen species (mtROS) in anchorage-independent tumor spheroids. We further demonstrate that FASN-deficient cells acquire resistance to oxidative stress in a CTP- and IDH1-dependent manner. Together with the reduced FASN activity in tumor spheroids, these data indicate that anchorage-independent malignant cells trade FASN-supported rapid growth for a cytosol-to-mitochondria citrate flux to gain redox capacity against detachment-induced oxidative stress.

Adipose Tissue Plasticity in Aging

As a dynamic endocrine organ, white adipose tissue (WAT) stores lipids and plays a critical role in maintaining whole-body energy homeostasis and insulin sensitivity. A large group of the population over 65 years old suffer from increased WAT mass, especially in the visceral location. Visceral adiposity accelerates aging through promoting age-associated chronic conditions, significantly shortening life expectancy. Unlike WAT, brown adipose tissue (BAT) functions as an effective energy sink that burns and disposes of excess lipids and glucose upon activation of thermogenesis. Unfortunately, the thermogenic activity of BAT declines during aging. New appreciation of cellular and functional remodeling of WAT and BAT during aging has emerged in recent years. Efforts are underway to explore the potential underlying mechanisms behind these age-associated alterations in WAT and BAT and the impact of these alterations on whole-body metabolism. Lastly, it is intriguing to translate our knowledge obtained from animal models to the clinic to prevent and treat age-associated metabolic disorders. © 2022 American Physiological Society. Compr Physiol 12: 4119-4132, 2022.

Adipocyte IRE1α promotes PGC1α mRNA decay and restrains adaptive thermogenesis

Adipose tissue undergoes thermogenic remodeling in response to thermal stress and metabolic cues, playing a crucial role in regulating energy expenditure and metabolic homeostasis. Endoplasmic reticulum (ER) stress is associated with adipose dysfunction in obesity and metabolic disease. It remains unclear, however, if ER stress-signaling in adipocytes mechanistically mediates dysregulation of thermogenic fat. Here we show that inositol-requiring enzyme 1α (IRE1α), a key ER stress sensor and signal transducer, acts in both white and beige adipocytes to impede beige fat activation. Ablation of adipocyte IRE1α promotes browning/beiging of subcutaneous white adipose tissue following cold exposure or β3-adrenergic stimulation. Loss of IRE1α alleviates diet-induced obesity and augments the anti-obesity effect of pharmacologic β3-adrenergic stimulation. Notably, IRE1α suppresses stimulated lipolysis and degrades Ppargc1a messenger RNA through its RNase activity to downregulate the thermogenic gene program. Hence, blocking IRE1α bears therapeutic potential in unlocking adipocytes' thermogenic capacity to combat obesity and metabolic disorders.

Maternal Adipocyte Connexin43 Gap Junctions Affect Breastmilk Lactose Levels and Neonate Growth in Mice

Simple Summary

Breastfeeding offers many health benefits for both mothers and infants. However, overnutrition and a steady increase in obesity in the U.S. has made it harder for many mothers to produce and express breastmilk. Moreover, the quality of breastmilk from obese mothers is frequently compromised in that it contains fewer nutrients and more inflammatory components. In this study, we used mice to model this phenomenon. We found that short-term high-fat feeding at the start of breeding reduces litter size and pups’ body weight. It also impairs adipocyte remodeling during lactation. Connexin43 is the primary building block for gap junctions in the adipose tissue. It is postulated to play an essential role in adipose tissue remodeling to accommodate mammary gland development and breastmilk production. Using genetically engineered mice without Connexin43 in their adipocytes, we demonstrated that the deletion of adipocyte Connexin43 affects the disappearance of adipocytes during lactation and affects milk composition, which is postulated to impair the pups’ growth. Altogether, this study suggests that increasing or enhancing adipocyte Connexin43 gap junctions may help obese mothers produce better breastmilk to support their neonates.

Abstract

Breastfeeding offers a broad spectrum of health benefits for infants. However, overnutrition and a steady increase in maternal obesity in the U.S. have made it harder for many mothers to produce and express breastmilk, and the quality of milk from obese mothers is also frequently compromised. Adipocytes, the primary cell type in the non-lactating breast, display a drastic morphological and functional change during lactation in mice. Lipid-filled adipocytes undergo lipolysis, and lipid droplets disappear to provide fatty acids and energy for breastmilk production. Once the animal stops lactation, these lipid-depleted adipocytes return as lipid-laden cells. This dynamic remodeling of the tissue is likely the result of active intercellular communications. Connexin43 (Cx43) is the most abundant connexin in the mammary adipose tissue that makes up the gap junctions for direct intercellular communications. Its expression is increased during lactation and reduced in obese mammary adipose tissue, which is resistant to lactation-induced remodeling. However, whether Cx43 is required for adipocyte remodeling and breastmilk production to support neonates’ growth has not been established. In this study, we used doxycycline-inducible adipocyte-specific Cx43-deleted mice and demonstrated that adipocyte Cx43 played a vital role in determining the carbohydrate levels in breastmilk, which may subsequently affect neonates’ growth.

The B56α subunit of PP2A is necessary for mesenchymal stem cell commitment to adipocyte

Adipose tissue plays a major role in maintaining organismal metabolic equilibrium. Control over the fate decision from mesenchymal stem cells (MSCs) to adipocyte differentiation involves coordinated command of phosphorylation. Protein phosphatase 2A plays an important role in Wnt pathway and adipocyte development, yet how PP2A complexes actively respond to adipocyte differentiation signals and acquire specificity in the face of the promiscuous activity of its catalytic subunit remains unknown. Here, we report the PP2A phosphatase B subunit B56α is specifically induced during adipocyte differentiation and mediates PP2A to dephosphorylate GSK3β, thereby blocking Wnt activity and driving adipocyte differentiation. Using an inducible B56α knock-out mouse, we further demonstrate that B56α is essential for gonadal adipose tissue development in vivo and required for the fate decision of adipocytes over osteoblasts. Moreover, we show B56α expression is driven by the adipocyte transcription factor PPARγ thereby establishing a novel link between PPARγ signaling and Wnt blockade. Overall, our results reveal B56α is a necessary part of the machinery dictating the transition from pre-adipocyte to mature adipocyte and provide fundamental insights into how PP2A complex specifically and actively regulates unique signaling pathway in biology.

Energy metabolism in brown adipose tissue

Brown adipose tissue (BAT) is well known to burn calories through uncoupled respiration, producing heat to maintain body temperature. This 'calorie wasting' feature makes BAT a special tissue, which can function as an 'energy sink' in mammals. While a combination of high energy intake and low energy expenditure is the leading cause of overweight and obesity in modern society, activating a safe 'energy sink' has been proposed as a promising obesity treatment strategy. Metabolically, lipids and glucose have been viewed as the major energy substrates in BAT, while succinate, lactate, branched-chain amino acids, and other metabolites can also serve as energy substrates for thermogenesis. Since the cataplerotic and anaplerotic reactions of these metabolites interconnect with each other, BAT relies on its dynamic, flexible, and complex metabolism to support its special function. In this review, we summarize how BAT orchestrates the metabolic utilization of various nutrients to support thermogenesis and contributes to whole-body metabolic homeostasis.

Brown Adipose Tissue Heterogeneity, Energy Metabolism, and Beyond

Brown adipocyte in brown adipose tissue (BAT) specializes in expending energy through non-shivering thermogenesis, a process that produces heat either by uncoupling protein 1 (UCP1) dependent uncoupling of mitochondrial respiration or by UCP1 independent mechanisms. Apart from this, there is ample evidence suggesting that BAT has an endocrine function. Studies in rodents point toward its vital roles in glucose and lipid homeostasis, making it an important therapeutic target for treating metabolic disorders related to morbidities such as obesity and type 2 diabetes. The rediscovery of thermogenically active BAT depots in humans by several independent research groups in the last decade has revitalized interest in BAT as an even more promising therapeutic intervention. Over the last few years, there has been overwhelming interest in understanding brown adipocyte's developmental lineages and how brown adipocyte uniquely utilizes energy beyond UCP1 mediated uncoupling respiration. These new discoveries would be leveraged for designing novel therapeutic interventions for metabolic disorders.

Chronic cold exposure enhances glucose oxidation in brown adipose tissue

The cultured brown adipocytes can oxidize glucose in vitro, but it is still not fully clear whether brown adipose tissue (BAT) could completely oxidize glucose in vivo. Although positron emission tomography (PET) with ¹⁸F‐fluorodeoxyglucose (¹⁸F‐FDG) showed a high level of glucose uptake in the activated BAT, the non‐metabolizable ¹⁸F‐FDG cannot fully demonstrate intracellular glucose metabolism. Through in vivo [U‐¹³C]glucose tracing, here we show that chronic cold exposure dramatically activates glucose oxidation in BAT and the browning/beiging subcutaneous white adipose tissue (sWAT). Specifically, chronic cold exposure enhances glucose flux into the mitochondrial TCA cycle. Metabolic flux analysis models that β3‐adrenergic receptor (β3‐AR) agonist significantly enhances the flux of mitochondrial pyruvate uptake through mitochondrial pyruvate carrier (MPC) in the differentiated primary brown adipocytes. Furthermore, in vivo MPC inhibition blocks cold‐induced glucose oxidation and impairs body temperature maintenance in mice. Together, mitochondrial pyruvate uptake and oxidation serve an important energy source in the chronic cold exposure activated BAT and beige adipose tissue, which supports a role for glucose oxidation in brown fat thermogenesis.

Dermal adipose tissue has high plasticity and undergoes reversible dedifferentiation in mice

Dermal adipose tissue (also known as dermal white adipose tissue and herein referred to as dWAT) has been the focus of much discussion in recent years. However, dWAT remains poorly characterized. The fate of the mature dermal adipocytes and the origin of the rapidly reappearing dermal adipocytes at different stages remain unclear. Here, we isolated dermal adipocytes and characterized dermal fat at the cellular and molecular level. Together with dWAT's dynamic responses to external stimuli, we established that dermal adipocytes are a distinct class of white adipocytes with high plasticity. By combining pulse-chase lineage tracing and single-cell RNA sequencing, we observed that mature dermal adipocytes undergo dedifferentiation and redifferentiation under physiological and pathophysiological conditions. Upon various challenges, the dedifferentiated cells proliferate and redifferentiate into adipocytes. In addition, manipulation of dWAT highlighted an important role for mature dermal adipocytes for hair cycling and wound healing. Altogether, these observations unravel a surprising plasticity of dermal adipocytes and provide an explanation for the dynamic changes in dWAT mass that occur under physiological and pathophysiological conditions, and highlight the important contributions of dWAT toward maintaining skin homeostasis.

Mitochondrial division inhibitor (mdivi-1) decreases oxidative metabolism in cancer

Background

Previous studies suggested that mdivi-1 (mitochondrial division inhibitor), a putative inhibitor of dynamin-related protein (DRP1), decreased cancer cell proliferation through inducing mitochondrial fusion and altering oxygen consumption. However, the metabolic reprogramming underlying the DRP1 inhibition is still unclear in cancer cells.

Methods

To better understand the metabolic effect of DRP1 inhibition, [U-¹³C]glucose isotope tracing was employed to assess mdivi-1 effects in several cancer cell lines, DRP1-WT (wild-type) and DRP1-KO (knockout) H460 lung cancer cells and mouse embryonic fibroblasts (MEFs).

Results

Mitochondrial staining confirmed that mdivi-1 treatment and DRP1 deficiency induced mitochondrial fusion. Surprisingly, metabolic isotope tracing found that mdivi-1 decreased mitochondrial oxidative metabolism in the lung cancer cell lines H460, A549 and the colon cancer cell line HCT116. [U-¹³C]glucose tracing studies also showed that the TCA cycle intermediates had significantly lower enrichment in mdivi-1-treated cells. In comparison, DRP1-WT and DRP1-KO H460 cells had similar oxidative metabolism, which was decreased by mdivi-1 treatment. Furthermore, mdivi-1-mediated effects on oxidative metabolism were independent of mitochondrial fusion.

Conclusions

Our data suggest that, in cancer cells, mdivi-1, a putative inhibitor of DRP1, decreases oxidative metabolism to impair cell proliferation.

Low- and high-thermogenic brown adipocyte subpopulations coexist in murine adipose tissue

Brown adipose tissue (BAT), as the main site of adaptive thermogenesis, exerts beneficial metabolic effects on obesity and insulin resistance. BAT has been previously assumed to contain a homogeneous population of brown adipocytes. Utilizing multiple mouse models capable of genetically labeling different cellular populations, as well as single-cell RNA sequencing and 3D tissue profiling, we discovered a brown adipocyte subpopulation with low thermogenic activity coexisting with the classical high-thermogenic brown adipocytes within the BAT. Compared with the high-thermogenic brown adipocytes, these low-thermogenic brown adipocytes had substantially lower Ucp1 and Adipoq expression, larger lipid droplets, and lower mitochondrial content. Functional analyses showed that, unlike the high-thermogenic brown adipocytes, the low-thermogenic brown adipocytes have markedly lower basal mitochondrial respiration, and they are specialized in fatty acid uptake. Upon changes in environmental temperature, the 2 brown adipocyte subpopulations underwent dynamic interconversions. Cold exposure converted low-thermogenic brown adipocytes into high-thermogenic cells. A thermoneutral environment had the opposite effect. The recruitment of high-thermogenic brown adipocytes by cold stimulation is not affected by high-fat diet feeding, but it does substantially decline with age. Our results revealed a high degree of functional heterogeneity of brown adipocytes.

Cellular Origins of Beige Fat Cells Revisited

Activated beige adipocytes have therapeutic potential due to their ability to improve glucose and lipid homeostasis. To date, the origin of beige adipocytes remains enigmatic. Whether beige cells arise through de novo differentiation from resident precursors or through reprogramming of mature white adipocytes has been a topic of intense discussion. Here, we offer our perspective on the natural origin of beige adipocytes in mice. In particular, we revisit recent lineage-tracing studies that shed light on this issue and offer new insight into how environmental housing temperatures early in life influence the mode of beige adipocyte biogenesis upon cold exposure later in life. We suggest a unified model in which beige adipocytes (UCP1⁺ multilocular cells) in rodents initially arise predominantly from progenitors (i.e., de novo beige adipogenesis) upon the first exposure to cold temperatures and then interconvert between "dormant beige" and "active beige" phenotypes (i.e., beige cell activation) upon subsequent changes in environmental temperature. Importantly, we highlight experimental considerations needed to visualize de novo adipogenesis versus beige cell activation in mice. A precise understanding of the cellular origins of beige adipocytes emanating in response to physiological and pharmacological stimuli may better inform therapeutic strategies to recruit beige adipocytes in vivo.

Remodeling of Murine Mammary Adipose Tissue during Pregnancy, Lactation, and Involution

White adipocytes in the mammary gland stroma comprise the majority of the mammary gland mass. White adipocytes regulate numerous hormonal and metabolic processes and exhibit compositional and phenotypic plasticity. This plasticity is exemplified by the ability of mammary adipocytes to regress during lactation, when mammary epithelial cells expand to establish sufficient milk-producing alveoli. Upon weaning, the process reverses through mammary involution, during which adipocytes extensively regenerate, and alveolar epithelial cells disappear through cell death, returning the mammary gland to the non-lactating state. Despite intensive studies on the development and involution of the mammary alveolar epithelium, the fate of mammary adipocytes during pregnancy and lactation, and the origins of mammary adipocytes regenerated during mammary involution, is poorly understood. Here, we discuss the recent discoveries of the fate of mammary adipocytes during pregnancy and lactation in a number of different mouse models, and the lineage origin of mammary adipocytes regenerated during involution.

Reversible De-differentiation of Mature White Adipocytes into Preadipocyte-like Precursors during Lactation

Adipose tissue in the mammary gland undergoes dramatic remodeling during reproduction. Adipocytes are replaced by mammary alveolar structures during pregnancy and lactation, then reappear upon weaning. The fate of the original adipocytes during lactation and the developmental origin of the re-appearing adipocyte post involution are unclear. Here, we reveal that adipocytes in the mammary gland de-differentiate into Pdgfrα⁺ preadipocyte- and fibroblast-like cells during pregnancy and remain de-differentiated during lactation. Upon weaning, de-differentiated fibroblasts proliferate and re-differentiate into adipocytes. This cycle occurs over multiple pregnancies. These observations reveal the potential of terminally differentiated adipocytes to undergo repeated cycles of de-differentiation and re-differentiation in a physiological setting.

Short-Term Versus Long-Term Effects of Adipocyte Toll-Like Receptor 4 Activation on Insulin Resistance in Male Mice

Chronic exposure to high-saturated fat diets (HFDs) increases the prevalence of obesity and contributes to the development of low-grade inflammation and insulin resistance. A possible mediator accounting for obesity-associated inflammation and insulin resistance is Toll-like receptor 4 (TLR4). We investigated the role of adipocyte TLR4 in lipid and glucose homeostasis through an inducible, adipocyte-specific deletion of TLR4 in a mouse model that is referred to as the "Tadipo" mouse. Consistent with a critical role for inflammation as a positive force for healthy adipose tissue expansion, chronic HFD exposure results in exacerbated whole-body and muscle insulin resistance in the absence of TLR4 in the adipocyte. Elimination of TLR4 in adipocytes affects TLR4 expression in other tissues, with reduced TLR4 expression in peritoneal macrophages and in the liver. In contrast, TLR4 deletion from adipocytes protects whole-body insulin sensitivity after an acute lipid challenge during a hyperinsulinemic euglycemic clamp. Our results therefore demonstrate dichotomous effects of TLR4 on adipose tissue functionality, with an important positive role of TLR4 during a chronic HFD challenge due to the lack of adipose tissue remodeling and a negative role of TLR4 as a mediator of insulin resistance in the adipocyte during an acute challenge with saturated fatty acids.

Zfp423 Maintains White Adipocyte Identity through Suppression of the Beige Cell Thermogenic Gene Program

The transcriptional regulators Ebf2 and Prdm16 establish and maintain the brown and/or beige fat cell identity. However, the mechanisms operating in white adipocytes to suppress the thermogenic gene program and maintain an energy-storing phenotype are less understood. Here, we report that the transcriptional regulator Zfp423 is critical for maintaining white adipocyte identity through suppression of the thermogenic gene program. Zfp423 expression is enriched in white versus brown adipocytes and suppressed upon cold exposure. Doxycycline-inducible inactivation of Zfp423 in mature adipocytes, combined with β-adrenergic stimulation, triggers a conversion of differentiated adiponectin-expressing inguinal and gonadal adipocytes into beige-like adipocytes; this reprogramming event is sufficient to prevent and reverse diet-induced obesity and insulin resistance. Mechanistically, Zfp423 acts in adipocytes to inhibit the activity of Ebf2 and suppress Prdm16 activation. These data identify Zfp423 as a molecular brake on adipocyte thermogenesis and suggest a therapeutic strategy to unlock the thermogenic potential of white adipocytes in obesity.

MitoNEET-Parkin Effects in Pancreatic α- and β-Cells, Cellular Survival, and Intrainsular Cross Talk

Mitochondrial metabolism plays an integral role in glucose-stimulated insulin secretion (GSIS) in β-cells. In addition, the diabetogenic role of glucagon released from α-cells plays a major role in the etiology of both type 1 and type 2 diabetes because unopposed hyperglucagonemia is a pertinent contributor to diabetic hyperglycemia. Titrating expression levels of the mitochondrial protein mitoNEET is a powerful approach to fine-tune mitochondrial capacity of cells. Mechanistically, β-cell-specific mitoNEET induction causes hyperglycemia and glucose intolerance due to activation of a Parkin-dependent mitophagic pathway, leading to the formation of vacuoles and uniquely structured mitophagosomes. Induction of mitoNEET in α-cells leads to fasting-induced hypoglycemia and hypersecretion of insulin during GSIS. MitoNEET-challenged α-cells exert potent antiapoptotic effects on β-cells and prevent cellular dysfunction associated with mitoNEET overexpression in β-cells. These observations identify that reduced mitochondrial function in α-cells exerts potently protective effects on β-cells, preserving β-cell viability and mass.

Autonomous interconversion between adult pancreatic α-cells and β-cells after differential metabolic challenges

Background

Evidence hints at the ability of β-cells to emerge from non-β-cells upon genetic or pharmacological interventions. However, their quantitative contributions to the process of autonomous β-cell regeneration without genetic or pharmacological manipulations remain to be determined.

Methods & results

Using PANIC-ATTAC mice, a model of titratable, acute β-cell apoptosis capable of autonomous, and effective islet mass regeneration, we demonstrate that an extended washout of residual tamoxifen activity is crucial for β-cell lineage tracing studies using the tamoxifen-inducible Cre/loxP systems. We further establish a doxycycline-inducible system to label different cell types in the mouse pancreas and pursued a highly quantitative assessment to trace adult β-cells after various metabolic challenges. Beyond proliferation of pre-existing β-cells, non-β-cells contribute significantly to the post-challenge regenerated β-cell pool. α-cell trans-differentiation is the predominant mechanism upon post-apoptosis regeneration and multiparity. No contributions from exocrine acinar cells were observed. During diet-induced obesity, about 25% of α-cells arise de novo from β-cells. Ectopic expression of Nkx6.1 promotes α-to-β conversion and insulin production.

Conclusions

We identify the origins and fates of adult β-cells upon post-challenge upon autonomous regeneration of islet mass and establish the quantitative contributions of the different cell types using a lineage tracing system with high temporal resolution.

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Insufficient washout for tamoxifen leads to β-cell lineage tracing artifacts.

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Inter-conversion between α- and β-cells after differential metabolic challenges.

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Developmental distance and precursor population determine conversion efficiency.

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Nkx6.1 promotes α-to-β conversion.

Human Beige Adipocytes: Epiphenomenon or Drivers of Metabolic Improvements?

There is a lack of direct evidence that human brown or beige adipocytes improve whole-body energy homeostasis. In a recent study, Min et al. implanted proliferated human adipocyte progenitors from a 3D culture system in mice. Implants responded to adipogenic stimuli and had a beige-like phenotype upon induction that significantly improved systemic metabolic parameters.

Reductive carboxylation supports redox homeostasis during anchorage-independent growth

Cells receive growth and survival stimuli through their attachment to an extracellular matrix (ECM). Overcoming the addiction to ECM-induced signals is required for anchorage-independent growth, a property of most malignant cells. Detachment from ECM is associated with enhanced production of reactive oxygen species (ROS) owing to altered glucose metabolism. Here we identify an unconventional pathway that supports redox homeostasis and growth during adaptation to anchorage independence. We observed that detachment from monolayer culture and growth as anchorage-independent tumour spheroids was accompanied by changes in both glucose and glutamine metabolism. Specifically, oxidation of both nutrients was suppressed in spheroids, whereas reductive formation of citrate from glutamine was enhanced. Reductive glutamine metabolism was highly dependent on cytosolic isocitrate dehydrogenase-1 (IDH1), because the activity was suppressed in cells homozygous null for IDH1 or treated with an IDH1 inhibitor. This activity occurred in absence of hypoxia, a well-known inducer of reductive metabolism. Rather, IDH1 mitigated mitochondrial ROS in spheroids, and suppressing IDH1 reduced spheroid growth through a mechanism requiring mitochondrial ROS. Isotope tracing revealed that in spheroids, isocitrate/citrate produced reductively in the cytosol could enter the mitochondria and participate in oxidative metabolism, including oxidation by IDH2. This generates NADPH in the mitochondria, enabling cells to mitigate mitochondrial ROS and maximize growth. Neither IDH1 nor IDH2 was necessary for monolayer growth, but deleting either one enhanced mitochondrial ROS and reduced spheroid size, as did deletion of the mitochondrial citrate transporter protein. Together, the data indicate that adaptation to anchorage independence requires a fundamental change in citrate metabolism, initiated by IDH1-dependent reductive carboxylation and culminating in suppression of mitochondrial ROS.

Pdgfrβ+ Mural Preadipocytes Contribute to Adipocyte Hyperplasia Induced by High-Fat-Diet Feeding and Prolonged Cold Exposure in Adult Mice

The expansion of white adipose tissue (WAT) in obesity involves de novo differentiation of new adipocytes; however, the cellular origin of these cells remains unclear. Here, we utilize Zfp423(GFP) reporter mice to characterize adipose mural (Pdgfrβ(+)) cells with varying levels of the preadipocyte commitment factor Zfp423. We find that adipose tissue contains distinct mural populations, with levels of Zfp423 distinguishing adipogenic from inflammatory-like mural cells. Using our "MuralChaser" lineage tracking system, we uncover adipose perivascular cells as developmental precursors of adipocytes formed in obesity, with adipogenesis and precursor abundance regulated in a depot-dependent manner. Interestingly, Pdgfrβ(+) cells do not significantly contribute to the initial cold-induced recruitment of beige adipocytes in WAT; it is only after prolonged cold exposure that these cells differentiate into beige adipocytes. These results provide genetic evidence for a mural cell origin of white adipocytes in obesity and suggest that beige adipogenesis may originate from multiple sources.

Impact of tamoxifen on adipocyte lineage tracing: Inducer of adipogenesis and prolonged nuclear translocation of Cre recombinase

Background

The selective estrogen receptor modulator tamoxifen, in combination with the Cre-ER^T2 fusion protein, has been one of the mainstream methods to induce genetic recombination and has found widespread application in lineage tracing studies.

Methods & results

Here, we report that tamoxifen exposure at widely used concentrations remains detectable by mass-spectrometric analysis in adipose tissue after a washout period of 10 days. Surprisingly, its ability to maintain nuclear translocation of the Cre-ER^T2 protein is preserved beyond 2 months of washout. Tamoxifen treatment acutely leads to transient lipoatrophy, followed by de novo adipogenesis that reconstitutes the original fat mass. In addition, we find a “synthetically lethal” phenotype for adipocytes when tamoxifen treatment is combined with adipocyte-specific loss-of-function mutants, such as an adipocyte-specific PPARγ knockout. This is observed to a lesser extent when alternative inducible approaches are employed.

Conclusions

These findings highlight the potential for tamoxifen-induced adipogenesis, and the associated drawbacks of the use of tamoxifen in lineage tracing studies, explaining the discrepancy in lineage tracing results from different systems with temporal control of gene targeting.

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Tamoxifen treatment induces acute fat loss and de novo adipogenesis in mice.

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PPARγ knockout exacerbates tamoxifen-induced lipoatrophy.

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Tamoxifen-driven Cre-ER^T2 nuclear translocation lasts beyond two months in adipose.

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De novo adipogenesis upon cold exposure is confirmed with the mTmG reporter.

Adiponectin-mediated antilipotoxic effects in regenerating pancreatic islets

Pathways that stimulate β-cell regeneration remain of great clinical interest, yet effective therapeutic avenues that promote survival or reconstitution of β-cell mass remain elusive. Using a mouse model with inducible β-cell apoptosis followed by adiponectin-mediated regeneration, we aimed to identify key molecules boosting β-cell viability. In the regenerating pancreatic islets, we examined changes within the transcriptome and observed an extensive up-regulation of genes encoding proteins involved in lipid transport and metabolism. The most prominent targets were further confirmed by quantitative PCR and immunofluorescence. Among the upstream regulators predicted by pathway analysis of the transcriptome, we detected enhanced levels of 2 key transcription factors, Hepatocyte Nuclear Factor 4α and Peroxisome Proliferator-Activated Receptorα. Our data suggest that improving pancreatic islet lipid metabolism as an important antilipotoxic phenomenon to boost β-cell regeneration. This is primarily mediated by the adipokine adiponectin that exerts its action on both the beta-cell directly as well as on the adipocyte. Adiponectin induces lipid metabolism gene expression in regenerating islets through Hepatocyte Nuclear Factor 4α and Peroxisome Proliferator-Activated Receptorα. Adiponectin also modulates leptin levels via preserving adipose tissue mass in the insulinopenic state.

Adiponectin is essential for lipid homeostasis and survival under insulin deficiency and promotes β-cell regeneration

As an adipokine in circulation, adiponectin has been extensively studied for its beneficial metabolic effects. While many important functions have been attributed to adiponectin under high-fat diet conditions, little is known about its essential role under regular chow. Employing a mouse model with inducible, acute β-cell ablation, we uncovered an essential role of adiponectin under insulinopenic conditions to maintain minimal lipid homeostasis. When insulin levels are marginal, adiponectin is critical for insulin signaling, endocytosis, and lipid uptake in subcutaneous white adipose tissue. In the absence of both insulin and adiponectin, severe lipoatrophy and hyperlipidemia lead to lethality. In contrast, elevated adiponectin levels improve systemic lipid metabolism in the near absence of insulin. Moreover, adiponectin is sufficient to mitigate local lipotoxicity in pancreatic islets, and it promotes reconstitution of β-cell mass, eventually reinstating glycemic control. We uncovered an essential new role for adiponectin, with major implications for type 1 diabetes.

DOI:http://dx.doi.org/10.7554/eLife.03851.001

Fat tissue is essential for health. Fat cells store energy and release it when it is needed; they also release hormones that are important for the health of our heart and for regulating our metabolism. One of these hormones, adiponectin, helps cells to remove fat molecules from the bloodstream. This allows the body to maintain appropriate cholesterol levels and prevents fatty build-ups from blocking blood vessels, which is associated with heart disease. Adiponectin also helps cells respond to insulin, a hormone that regulates blood sugar levels, and thus helps to prevent diabetes.

Despite this hormone's important roles in health, mice that lack adiponectin can thrive under normal conditions. Adiponectin becomes essential, however, when blood sugar or fat levels are considerably altered. For example, when mice without adiponectin are fed a high fat-content diet, they become insulin-resistant. Moreover, certain diabetes drugs that boost insulin sensitivity only work if adiponectin is present in the body.

Adiponectin helps to keep the β-cells that produce insulin alive. In patients with diabetes, β-cells slowly die, and this leads to a poor insulin response and an imbalance in the amount of fats and sugars in the cells. This is toxic to the β-cells; and as more β-cells die, less insulin is produced to control sugar levels, and the condition worsens. Adiponectin appears to protect the β-cells against this vicious cycle, but the details of how it does so are unclear.

Ye et al. used a mouse model in which β-cells were destroyed to see what adiponectin does when insulin is in short supply. When insulin levels were extremely low, adiponectin was found to help one type of fat tissue absorb fat molecules from the bloodstream, which reduced blood cholesterol levels. It also protects fat cells from being destroyed when insulin levels are low. Ye et al. also found that mice that lack both insulin and adiponectin lose excessive amounts of fat tissue and develop high blood cholesterol levels, which lead to death.

Increasing adiponectin levels in insulin-deficient mice, however, improves their blood cholesterol levels and protects β-cells from being destroyed. This allows the β-cells to begin regenerating. As the β-cells regenerate, the insulin-deficient mice developed better control over their blood sugar.

Many people with type-1 diabetes (which is caused by their own immune system destroying their β-cells) currently rely on insulin injections and restricted diets to manage their condition. Ye et al.'s findings might lead to new strategies to restore β-cell production and blood sugar control; as such these findings will have important implications for the management of type-1 diabetes.

DOI:http://dx.doi.org/10.7554/eLife.03851.002

Brown adipose tissue derived VEGF-A modulates cold tolerance and energy expenditure

We recently reported that local overexpression of VEGF-A in white adipose tissue (WAT) protects against diet-induced obesity and metabolic dysfunction. The observation that VEGF-A induces a “brown adipose tissue (BAT)-like” phenotype in WAT prompted us to further explore the direct function of VEGF-A in BAT. We utilized a doxycycline (Dox)-inducible, brown adipocyte-specific VEGF-A transgenic overexpression model to assess direct effects of VEGF-A in BAT in vivo. We observed that BAT-specific VEGF-A expression increases vascularization and up-regulates expression of both UCP1 and PGC-1α in BAT. As a result, the transgenic mice show increased thermogenesis during chronic cold exposure. In diet-induced obese mice, introducing VEGF-A locally in BAT rescues capillary rarefaction, ameliorates brown adipocyte dysfunction, and improves deleterious effects on glucose and lipid metabolism caused by a high-fat diet challenge. These results demonstrate a direct positive role of VEGF-A in the activation and expansion of BAT.

Improved methodologies for the study of adipose biology: insights gained and opportunities ahead

Adipocyte differentiation and function have become areas of intense focus in the field of energy metabolism; however, understanding the role of specific genes in the establishment and maintenance of fat cell function can be challenging and complex. In this review, we offer practical guidelines for the study of adipocyte development and function. We discuss improved cellular and genetic systems for the study of adipose biology and highlight recent insights gained from these new approaches.

The AdipoChaser mouse: A model tracking adipogenesis in vivo

White adipose tissue is considered to have high plasticity. Dynamic, abnormal expansion of white adipose tissue leads to obesity and associated metabolic disorders. Due to technical limitations, the life cycle and turnover rates of different white adipose depots during development and under various physiological conditions and environmental challenges has been assessed only through highly indirect approaches. We have recently described a system for the inducible, permanent labeling of mature adipocytes, the "AdipoChaser" mouse. Utilizing this AdipoChaser mouse model, we found that epididymal fat depots initiate adipogenesis after prolonged high fat diet feeding, whereas subcutaneous fat depots merely undergo hypertrophy and have a very low rate of adipogenesis. During cold exposure or β-3 agonist-induced "browning" of subcutaneous fat depot, most of the beige adipocytes arise from de novo adipogenesis. We also found that cold exposure or β-3 agonist stimulation induces massive white adipogenesis in the epididymal fat depot. Developmentally, adipocytes in the gonadal fat depot are differentiated postnatally, between birth and sexual maturation, while adipocytes in the subcutaneous fat are differentiated between embryonic days 14-18. Our study shed new insights into the developmental aspects of adipose tissue and its dynamics under a number of different physiological challenges.

Differential transendothelial transport of adiponectin complexes

Background

Adiponectin’s effects on systemic physiology and cell-specific responses are well-defined, but little is known about how this insulin-sensitizing and anti-inflammatory adipokine reaches its target cells. All molecules face active and passive transport limitations, but adiponectin is particularly noteworthy due to the diverse size range and high molecular weights of its oligomers. Additionally, its metabolic target organs possess a range of endothelial permeability.

Methods

Full-length recombinant murine adiponectin was produced and oligomer fractions isolated by gel filtration. Adiponectin complex sizes were measured by dynamic light scattering to determine Stokes radii. Transendothelial transport of purified oligomers was quantitatively assessed under a number of different conditions in vitro using murine endothelial cells and in vivo using several mouse models of altered endothelial function.

Results

Adiponectin oligomers exhibit large transport radii that limit transendothelial transport. Oligomerization is a significant determinant of flux across endothelial monolayers in vitro; low molecular weight adiponectin is preferentially transported. In vivo sampled sera from the heart, liver, and tail vein demonstrated significantly different complex distribution of lower molecular weight oligomers. Pharmacological interventions, such as PPARγ agonist treatment, differentially affect adiponectin plasma clearance and tissue uptake. Exercise induces enhanced adiponectin uptake to oxidative skeletal muscles, wherein adiponectin potently lowers ceramide levels. In total, endothelial barriers control adiponectin transport in a cell- and tissue-specific manner.

Conclusions

Adiponectin oligomer efficacy in a given tissue may therefore be endothelial transport mediated. Targeting endothelial dysfunction in the metabolic syndrome through exercise and pharmaceuticals may afford an effective approach to increasing adiponectin’s beneficial effects.

Explaining Zipf's law via a mental lexicon

Zipf's law is the major regularity of statistical linguistics that has served as a prototype for rank-frequency relations and scaling laws in natural sciences. Here we show that Zipf's law-together with its applicability for a single text and its generalizations to high and low frequencies including hapax legomena-can be derived from assuming that the words are drawn into the text with random probabilities. Their a priori density relates, via the Bayesian statistics, to the mental lexicon of the author who produced the text.

Tracking adipogenesis during white adipose tissue development, expansion and regeneration

White adipose tissue displays high plasticity. We developed a system for the inducible, permanent labeling of mature adipocytes that we called the AdipoChaser mouse. We monitored adipogenesis during development, high-fat diet (HFD) feeding and cold exposure. During cold-induced 'browning' of subcutaneous fat, most 'beige' adipocytes stem from de novo-differentiated adipocytes. During HFD feeding, epididymal fat initiates adipogenesis after 4 weeks, whereas subcutaneous fat undergoes hypertrophy for a period of up to 12 weeks. Gonadal fat develops postnatally, whereas subcutaneous fat develops between embryonic days 14 and 18. Our results highlight the extensive differences in adipogenic potential in various fat depots.

Time course of histomorphological changes in adipose tissue upon acute lipoatrophy

BACKGROUND AND AIMS

Crown-like structures (CLS) are characteristic histopathology features of inflamed adipose tissues in obese mice and humans. In previous work, we suggested that these cells derived from macrophages primarily involved in the reabsorption of dead adipocytes. Here, we used a well-characterized transgenic mouse model in which the death of adipocytes in adult mice is inducible and highly synchronized. In this "FAT ATTAC" model, apoptosis is induced through forced dimerization of a caspase-8 fusion protein.

METHODS AND RESULTS

0, 0.5, 1, 2, 3 and 10 days post induction of adipocyte cell death, we analyzed mesenteric and epididymal adipose depots by histology, immunohistochemistry and electron microscopy. Upon induction of caspase-8 dimerization, numerous adipocytes lost immunoreactivity for perilipin, a marker for live adipocytes. In the same areas, we found adipocytes with hypertrophic mitochondria and signs of organelle degeneration. Neutrophils and lymphocytes were the main inflammatory cells present in the tissue, and the macrophages were predominantly Mac-2 negative. Over the course of ablation, Mac-2 positive macrophages substituted for Mac-2 negative macrophages, followed by CLS formation. All perilipin negative, dead adipocytes were surrounded by CLS structures. The time course of histopathology was similar in both fat pads studied, but occurred at earlier stages and was more gradual in mesenteric fat.

CONCLUSION

Our data demonstrate that CLS formation results as a direct consequence of adipocyte death, and that infiltrating macrophages actively uptake remnant lipids of dead adipocytes. Upon induction of adipocyte apoptosis, inflammatory cells infiltrate adipose tissue initially consisting of neutrophils followed by macrophages that are involved in CLS formation.

Dual modeling of political opinion networks

We present the result of a dual modeling of opinion networks. The model complements the agent-based opinion models by attaching to the social agent (voters) network a political opinion (party) network having its own intrinsic mechanisms of evolution. These two subnetworks form a global network, which can be either isolated from, or dependent on, the external influence. Basically, the evolution of the agent network includes link adding and deleting, with the opinion changes influenced by social validation, the political climate, the attractivity of the parties, and the interaction between them. The opinion network is initially composed of numerous nodes representing opinions or parties that are located on a one dimensional axis according to their political positions. The mechanism of evolution includes union, splitting, change of position, and attractivity, taking into account the pairwise node interaction decaying with node distance in power law. The global evolution ends in a stable distribution of the social agents over a quasistable and fluctuating stationary number of remaining parties. Empirical study on the lifetime distribution of numerous parties and vote results is carried out to verify numerical results.

Targeted deletion of adipocytes by apoptosis leads to adipose tissue recruitment of alternatively activated M2 macrophages

Obesity is frequently associated with an infiltration of macrophages into adipose tissue. Adipocyte dysfunction causes a phenotypic switch of macrophages from an alternatively activated M2-like phenotype towards a proinflammatory M1 phenotype. The cross talk between adipocytes and infiltrating immune cells, in particular macrophages, is thought to contribute to local and eventually systemic inflammation. Here, we tested the phenotypic impact of a lack of adipocytes on the inflammatory status of macrophages. We took advantage of the fat apoptosis through targeted activation of caspase-8 (FAT-ATTAC) mouse model that allows for the inducible system-wide elimination of adipocytes through a proapoptotic mechanism and followed the degree and type of inflammatory response upon ablation of live adipocytes. Analysis of depots 2 wk after elimination of adipocytes resulted in markedly reduced levels of adipose tissue and a robust down-regulation of circulating adipokines. Quantitative PCR and immunohistochemistry on epididymal and inguinal fat depots revealed an increase of the macrophage markers F4/80 and CD11c. Using polychromatic flow cytometry, we observed an up-regulation of alternatively activated M2 macrophage markers (CD206 and CD301) on the majority of F4/80 positive cells. Apoptosis of adipocytes is sufficient to initiate a large influx of macrophages into the remnant fat pads. However, these macrophages are alternatively activated, antiinflammatory M2 macrophages and not M1 cells. We conclude that adipocyte death is sufficient to initiate macrophage infiltration, and live adipocytes are required to initiate and/or sustain a proinflammatory response within the infiltrating macrophages in adipose tissue.

Identification and characterization of a promoter cassette conferring adipocyte-specific gene expression

The adipocyte-specific secretory molecule adiponectin has found widespread acceptance as a systemic marker that effectively integrates a number of signals associated with metabolic dysfunction at the level of adipose tissue. The widely used aP2 promoter cassette, which is frequently chosen to achieve adipocyte-specific expression of transgenes, conveys transcription in cell types other than adipocytes, such as macrophages and cardiomyocytes. To improve our ability to drive transgene expression in a more adipocyte-specific way, we aimed to define the minimal promoter segment from the adiponectin genomic locus. We generated a series of transgenic animals in which the expression of reporter genes and Cre recombinase was driven by 2, 4.9, and 5.4 kb of adiponectin promoter sequences. We found that the 5.4-kb adiponectin promoter fragment is the most effective cassette conveying adipocyte-specific expression of target genes. We therefore define a novel promoter cassette that ensures adipocyte-specific expression of passenger genes and may be used in the generation of transgenic mouse models to study gene function in vivo.

Clinical application of a patient-controlled apparatus for ventricular administration of morphine in intractable pain: report of 28 cases

The authors developed a patient-controlled apparatus (L-224) for ventricular administration of pain-relieving medication and utilized it in the treatment of 28 patients with intractable pain. The initial dose of morphine hydrochloride was 1 mg, which afforded pain relief for an average of 170 hours. At an average dose of 1.8 mg, the average time of pain relief obtained was 137 hours. Therefore, they conclude that L-224 is a satisfactory apparatus for the ventricular administration of pain medication; it is patient controlled, safe, and reliable in clinical application.

Clinical and laboratory observations on polysaccharide sulphate (PSS) in 282 cases of ischemic cerebrovascular disease

Polysaccharide sulphate (PSS) is a new heparinoid drug. The therapeutic effect and laboratory findings in the treatment of 282 cases with ischemic cerebrovascular disease by PSS were analysed in this study. In treating patients with acute cerebral infarction, the effective rate (93.2%) and highly effective rate (62.9%) were both significantly higher in comparison with controls. Excellent results were also obtained in treating patients with cerebral infarction at late and sequela stage and patients with transient ischemic attack. Laboratory observations and animal experiments showed that PSS has anticoagulative and vasodilatory effects. It can also reduce blood viscosity and serum lipids. So PSS is an effective drug for the prevention and treatment of ischemic cerebrovascular diseases.

[Cox regression model analysis of influence of the transitional mucosa adjacent to rectal carcinoma on anastomotic recurrence after anterior resection]

The influence of nine clinico-pathological factors on anastomotic recurrence of rectal cancer after anterior resection was analyzed using Cox regression model. Among the nine factors, the mucin histochemical changes in mucosa at the resection margins, Dukes' stage and histological type of the cancer were of significance (P less than 0.1). The most important variable for anastomotic recurrence was mucin histochemical changes at the resection margins according to the Wald statistic value. It is suggested that the transitional mucosa adjacent to rectal carcinoma in which mucin histochemical changes take place bears great weight on the further malignant transformation after anterior resection. The biological characteristics of the transitional mucosa and clinical significance of the application of Cox regression model for prognostic analysis are discussed.

The clinical and biological significance of the transitional mucosa adjacent to colorectal cancer

The underlying nature of the transitional mucosa adjacent to colorectal cancer is defined and the evidence for and against the statement that this transitional mucosa involves primary premalignant change presented in this article. An association between mucin histochemical changes at the margins of resection and a poorer clinical outcome of patients has been recognized in patients with colorectal cancer after surgery. The retained transitional mucosa at the margins of resection appears to correlate with tumor recurrence and a poorer survival in patients who have undergone radical resection. It is considered that the transitional mucosa adjacent to colorectal cancer and its presence at the margins of resection may be an important prognostic marker for patients with large bowel cancer following radical resection.

Analgesia from electrical stimulation of the hypothalamic arcuate nucleus in pentobarbital-anesthetized rats

Inhibition of noxious heat-induced tail flick by electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) was examined and characterized in pentobarbital-anesthetized rats. Systematic mapping studies revealed that inhibition of the tail flick reflex could be induced by stimulating widespread areas in the ventromedial parts of the hypothalamus, which include the paraventricular nucleus, ventromedial nucleus, dorsomedial nucleus, anterior hypothalamic area as well as the ARH areas. The ARH stimulation-produced tail flick suppression could be completely blocked by systemic naloxone (2 mg/kg) which shows the involvement of an opiate mechanism in this effect. Although the tail flick reflex in the lightly anesthetized state is of significantly shorter latency than in the unanesthetized state, thresholds of the ARH stimulation for suppressing spinal nociceptive reflexes in the lightly anesthetized state were not significantly different from the thresholds at the same ARH sites in the awake state.

The role of periaqueductal gray in mediation of analgesia produced by different frequencies electroacupuncture stimulation in rats

The marked suppression of noxious heat-evoked tail flick reflex was produced in conscious rats by low (2 Hz) or high (100 Hz) frequency electroacupuncture (EA) stimulation at acupoints S36 and Sp6. Electrolytic and kainate lesions in the ventral periaqueductal gray (vPAG) led to significant attenuation of the low and high frequency EA analgesia as measured 4 and 6 days following the lesion. In sham-operated animals, analgesia induced by either low or high frequency EA remained intact. The role of vPAG in organization of different EA analgesia is discussed in detail.

A GABAergic mechanism in the inhibition of cardiac vagal reflexes

A depressor response and bradycardia were produced by aortic nerve stimulation in urethane-chloralose-anaesthetized, paralyzed and artificially ventilated rabbits. Stimulation of the superficial peroneal nerve (SP) or rostral ventrolateral medulla (rVLM) inhibited the aortic nerve stimulation-evoked bradycardia, which was also inhibited by direct microinjection of GABA into the dorsal vagus nucleus (DVN). Application of bicuculline methiodide into the same medullary area antagonized the effect of GABA and partially or completely abolished the SP or rVLM stimulation-produced inhibition. However, strychnine hydrochloride had no effect on the SP or rVLM stimulation-produced inhibition. These observations indicate that GABAergic system present at the DVN is possibly involved in the inhibition of evoked bradycardia during somatic afferent and rVLM stimulation.

The inhibitory effect of somatic inputs on the excitatory responses of vagal cardiomotor neurones to stimulation of the nucleus tractus solitarius in rabbits

Experiments were done in 41 rabbits anaesthetized with urethane and chloralose, paralyzed with Flaxedil and ventilated artificially. Extracellular recordings of 142 units were made in the dorsal vagal nucleus (DVN) and the nucleus ambiguus (NA), identified by antidromic response to stimulation of the cervical vagus nerve. In total 63.5% of them exhibited spontaneous activity and 22 units (17 in DVN and 5 in NA) showed a cardiac rhythm; their antidromic conduction velocity was 3.7-12.5 m/s, which suggests their having axons in the range of B fibres. These neurones were classified as vagal cardiomotor neurones. A total of 16 DVN and 4 NA vagal cardiomotor neurones were excited orthodromically by electrical stimulation of the contralateral nucleus tractus solitarius (NTS). Electrical stimulation of the superficial peroneal nerve (SP) with low intensity or the deep peroneal nerve (DP) with high intensity which activated C fibres inhibited excitatory responses of 16 neurones (14 in DVN and 2 in NA). The other 4 neurones were unaffected by SP inputs. These results provide electrophysiological evidence for the inhibitory effect of somatic inputs on the evoked discharges of vagal cardiomotor neurones in the DVN and the NA.