Time-dependent effects of starvation on serum, pituitary and hypothalamic leptin levels in rats

Circadian Dysfunction in Adipose Tissue: Chronotherapy in Metabolic Diseases

Essential for survival and reproduction, the circadian timing system (CTS) regulates adaptation to cyclical changes such as the light/dark cycle, temperature change, and food availability. The regulation of energy homeostasis possesses rhythmic properties that correspond to constantly fluctuating needs for energy production and consumption. Adipose tissue is mainly responsible for energy storage and, thus, operates as one of the principal components of energy homeostasis regulation. In accordance with its roles in energy homeostasis, alterations in adipose tissue's physiological processes are associated with numerous pathologies, such as obesity and type 2 diabetes. These alterations also include changes in circadian rhythm. In the current review, we aim to summarize the current knowledge regarding the circadian rhythmicity of adipogenesis, lipolysis, adipokine secretion, browning, and non-shivering thermogenesis in adipose tissue and to evaluate possible links between those alterations and metabolic diseases. Based on this evaluation, potential therapeutic approaches, as well as clock genes as potential therapeutic targets, are also discussed in the context of chronotherapy.

Hypothalamic insulin expression remains unaltered after short-term fasting in female rats

PURPOSE

Our previous study showed that 6-h fasting increased insulin expression in the hypothalamus of male rats. We, therefore, wanted to examine if this phenomenon occurs in female rats and whether it depended on the estrus cycle phase.

METHODS

Female rats in proestrus or diestrus were either exposed to 6-h fasting or had ad libitum access to food. The serum, cerebrospinal fluid, and hypothalamic insulin levels were determined using radioimmunoassay. The hypothalamic insulin mRNA expression was measured by RT-qPCR, while the hypothalamic insulin distribution was assessed immunohistochemically.

RESULTS

Albeit the short-term fasting lowered circulating insulin, both hypothalamic insulin mRNA expression and hypothalamic insulin content remained unaltered. As for the hypothalamic insulin distribution, strong insulin immunopositivity was noted primarily in ependymal cells lining the upper part of the third ventricle and some neurons mainly located within the periventricular nucleus. The pattern of insulin distribution was similar between the controls and the females exposed to fasting regardless of the estrous cycle phase.

CONCLUSION

The findings of this study indicate that the control of insulin expression in the hypothalamus differs from that in the pancreatic beta cells during short-term fasting. Furthermore, they also imply that the regulation of insulin expression in the female hypothalamus is different from males but independent of the estrus cycle phase.

Leptin mediates postprandial increases in body temperature through hypothalamus-adrenal medulla-adipose tissue crosstalk

Meal ingestion increases body temperature in multiple species, an effect that is blunted by obesity. However, the mechanisms responsible for these phenomena remain incompletely understood. Here we show that refeeding increases plasma leptin concentrations approximately 8-fold in 48-hour-fasted lean rats, and this normalization of plasma leptin concentrations stimulates adrenomedullary catecholamine secretion. Increased adrenal medulla-derived plasma catecholamines were necessary and sufficient to increase body temperature postprandially, a process that required both fatty acids generated from adipose tissue lipolysis and β-adrenergic activation of brown adipose tissue (BAT). Diet-induced obese rats, which remained relatively hyperleptinemic while fasting, did not exhibit fasting-induced reductions in temperature. To examine the impact of feeding-induced increases in body temperature on energy balance, we compared rats fed chronically by either 2 carbohydrate-rich boluses daily or a continuous isocaloric intragastric infusion. Bolus feeding increased body temperature and reduced weight gain compared with continuous feeding, an effect abrogated by treatment with atenolol. In summary, these data demonstrate that leptin stimulates a hypothalamus-adrenal medulla-BAT axis, which is necessary and sufficient to induce lipolysis and, as a result, increase body temperature after refeeding.

Food For Thought: Short-Term Fasting Upregulates Glucose Transporters in Neurons and Endothelial Cells, But Not in Astrocytes

Our group previously reported that 6-h fasting increased both insulin II mRNA expression and insulin level in rat hypothalamus. Given that insulin effects on central glucose metabolism are insufficiently understood, we wanted to examine if the centrally produced insulin affects expression and/or regional distribution of glucose transporters, and glycogen stores in the hypothalamus during short-term fasting. In addition to determining the amount of total and activated insulin receptor, glucose transporters, and glycogen, we also studied distribution of insulin receptors and glucose transporters within the hypothalamus. We found that short-term fasting did not affect the astrocytic 45 kDa GLUT1 isoform, but it significantly increased the amount of endothelial 55 kDa GLUT1, and neuronal GLUT3 in the membrane fractions of hypothalamic proteins. The level of GLUT2 whose presence was detected in neurons, ependymocytes and tanycytes was also elevated. Unlike hepatic glycogen which was decreased, hypothalamic glycogen content was not changed after 6-h fasting. Our findings suggest that neurons may be given a priority over astrocytes in terms of glucose supply even during the initial phase of metabolic response to fasting. Namely, increase in glucose influx into the brain extracellular fluid and neurons by increasing the translocation of GLUT1, and GLUT3 in the cell membrane may represent the first line of defense in times of scarcity. The absence of co-localization of these membrane transporters with the activated insulin receptor suggests this process takes place in an insulin-independent manner.

The Role of Leptin in Maintaining Plasma Glucose During Starvation

For 20 years it has been known that concentrations of leptin, a hormone produced by the white adipose tissue (WAT) largely in proportion to body fat, drops precipitously with starvation, particularly in lean humans and animals. The role of leptin to suppress the thyroid and reproductive axes during a prolonged fast has been well defined; however, the impact of leptin on metabolic regulation has been incompletely understood. However emerging evidence suggests that, in starvation, hypoleptinemia increases activity of the hypothalamic-pituitary-adrenal axis, promoting WAT lipolysis, increasing hepatic acetyl-CoA concentrations, and maintaining euglycemia. In addition, leptin may be largely responsible for mediating a shift from a reliance upon glucose metabolism (absorption and glycogenolysis) to fat metabolism (lipolysis increasing gluconeogenesis) which preserves substrates for the brain, heart, and other critical organs. In this way a leptin-mediated glucose-fatty acid cycle appears to maintain glycemia and permit survival in starvation.

Short-term fasting promotes insulin expression in rat hypothalamus

In the hypothalamus, insulin takes on many roles involved in energy homoeostasis. Therefore, the aim of this study was to examine hypothalamic insulin expression during the initial phase of the metabolic response to fasting. Hypothalamic insulin content was assessed by both radioimmunoassay and Western blot. The relative expression of insulin mRNA was examined by qPCR. Immunofluorescence and immunohistochemistry were used to determine the distribution of insulin immunopositivity in the hypothalamus. After 6-h fasting, both glucose and insulin levels were decreased in serum but not in the cerebrospinal fluid. Our study showed for the first time that, while the concentration of circulating glucose and insulin decreased, both insulin mRNA expression and insulin content in the hypothalamic parenchyma were increased after short-term fasting. Increased insulin immunopositivity was detected specifically in the neurons of the hypothalamic periventricular nucleus and in the ependymal cells of fasting animals. These novel findings point to the complexity of mechanisms regulating insulin expression in the CNS in general and in the hypothalamus in particular.

Effect of restricted feeding on nocturnality and daily leptin rhythms in OVLT in aged male Wistar rats

Circadian system has direct relevance to the problems of modern lifestyle, shift workers, jet lag etc. To understand non-photic regulation of biological clock, the effects of restricted feeding (RF) on locomotor activity and daily leptin immunoreactivity (ir) rhythms in three age groups [3, 12 and 24 months (m)] of male Wistar rats maintained in light:dark (LD) 12:12 h conditions were studied. Leptin-ir was examined in the suprachiasmatic nucleus (SCN), the medial preoptic area (MPOA) and organum vasculosum of the lamina terminalis (OVLT). Reversal of feeding time due to restricted food availability during daytime resulted in switching of the animals from nocturnality to diurnality with significant increase in day time activity and decrease in night time activity. The RF resulted in % diurnality of approximately 32, 29 and 73 from % nocturnality of 82, 92 and 89 in control rats of 3, 12 and 24 m age, respectively. The increase in such switching from nocturnality to diurnality with restricted feeding was found to be robust in 24 m rats. The OVLT region showed daily leptin-ir rhythms with leptin-ir maximum at ZT-0 in all the three age groups. However leptin-ir levels were minimum at ZT-12 in 3 and 12 m though at ZT-18 in 24 m. In addition the mean leptin-ir levels decreased with increase in food intake and body weight significantly in RF aged rats. Thus we report here differential effects of food entrained regulation in switching nocturnality to diurnality and daily leptin-ir rhythms in OVLT in aged rats.

The effect of starvation on blood stream cancer cell metastasis to the liver in rat after laparotomy

BACKGROUND

Preoperative malnutrition worsens the prognosis of cancer patients. However, it is not certain how preoperative malnutrition affects postoperative hematogenous metastasis. We examined the influence of preoperative starvation on liver metastasis in rats using intra-vascular injection of AH109A hepatoma cells.

METHODS

Male donryu rats were divided into Fasting and Control groups. Rats received laparotomy and (125)I-iodo-deoxyuridine labeled AH109A hepatoma cells were inoculated via superior mesenteric vein. Radioactivity in the organs, macroscopic liver metastasis, white blood cell count, leukocyte count, NK cell activity, endogenous serum corticosterone and ACTH concentration and mRNA expression of cytokine in the liver and brain were evaluated at certain time points.

RESULTS

48hours preoperative starvation reduced body weight and induced a state of malnutrition. Accumulation of radioactivity in the liver was more than 4 times higher, and the number of liver metastases was 3.5 times higher in the Fasting than in the Control group. Preoperative starvation caused an almost 2 fold increase in plasma endogenous corticosterone levels and a 66% reduction in white blood cell and lymphocyte counts. Postoperative hypothalamus pituitary adrenal axis response was preserved. In addition, inflammatory cytokine expression in the liver was suppressed in the starved animals, suggesting that preoperative starvation led to a state of cellular immunosuppression, which would be an important factor for liver metastasis.

CONCLUSION

Preoperative malnutrition by 48 hours starvation reduced inflammatory cytokine response and cellular immunity, resulting in an increase in hematogenous liver metastasis.

Impact of thoracic surgery on cardiac morphology and function in small animal models of heart disease: a cardiac MRI study in rats

Background

Surgical procedures in small animal models of heart disease might evoke alterations in cardiac morphology and function. The aim of this study was to reveal and quantify such potential artificial early or long term effects in vivo, which might account for a significant bias in basic cardiovascular research, and, therefore, could potentially question the meaning of respective studies.

Methods

Female Wistar rats (n = 6 per group) were matched for weight and assorted for sham left coronary artery ligation or control. Cardiac morphology and function was then investigated in vivo by cine magnetic resonance imaging at 7 Tesla 1 and 8 weeks after the surgical procedure. The time course of metabolic and inflammatory blood parameters was determined in addition.

Results

Compared to healthy controls, rats after sham surgery showed a lower body weight both 1 week (267.5±10.6 vs. 317.0±11.3 g, n<0.05) and 8 weeks (317.0±21.1 vs. 358.7±22.4 g, n<0.05) after the intervention. Left and right ventricular morphology and function were not different in absolute measures in both groups 1 week after surgery. However, there was a confined difference in several cardiac parameters normalized to the body weight (bw), such as myocardial mass (2.19±0.30/0.83±0.13 vs. 1.85±0.22/0.70±0.07 mg left/right per g bw, p<0.05), or enddiastolic ventricular volume (1.31±0.36/1.21±0.31 vs. 1.14±0.20/1.07±0.17 µl left/right per g bw, p<0.05). Vice versa, after 8 weeks, cardiac masses, volumes, and output showed a trend for lower values in sham operated rats compared to controls in absolute measures (782.2±57.2/260.2±33.2 vs. 805.9±84.8/310.4±48.5 mg, p<0.05 for left/right ventricular mass), but not normalized to body weight. Matching these findings, blood testing revealed only minor inflammatory but prolonged metabolic changes after surgery not related to cardiac disease.

Conclusion

Cardio-thoracic surgical procedures in experimental myocardial infarction cause distinct alterations upon the global integrity of the organism, which in the long term also induce circumscribed repercussions on cardiac morphology and function. This impact has to be considered when analyzing data from respective animal studies and transferring these findings to conditions in patients.

Leptin and the hypothalamo-pituitary-adrenal stress axis

Leptin is a 16-kDa protein mainly produced and secreted by white adipose tissue and informing various brain centers via leptin receptor long and short forms about the amount of fat stored in the body. In this way leptin exerts a plethora of regulatory functions especially related to energy intake and metabolism, one of which is controlling the activity of the hypothalamo-pituitary-adrenal (HPA) stress axis. First, this review deals with the basic properties of leptin's structure and signaling at the organ, cell and molecule level, from lower vertebrates to humans but with emphasis on rodents because these have been investigated in most detail. Then, attention is given to the various interactions of adipose leptin with the HPA-axis, at the levels of the hypothalamus (especially the paraventricular nucleus), the anterior lobe of the pituitary gland (action on corticotropes) and the adrenal gland, where it releases corticosteroids needed for adequate stress adaptation. Also, possible local production and autocrine and paracrine actions of leptin at the hypothalamic and pituitary levels of the HPA-axis are being considered. Finally, a schematic model is presented showing the ways peripherally and centrally produced leptin may modulate, via the HPA-axis, stress adaptation in conjunction with the control of energy homeostasis.