Metabolic sensing neurons and the control of energy homeostasis

Smart Bioelectronics for Real-Time Diagnosis and Therapy of Body Organ Functions

Noncommunicable diseases (NCDs) associated with cardiovascular, neurological, and gastrointestinal disorders remain a leading cause of global mortality, sounding the alarm for the urgent need for better diagnostic and therapeutic solutions. Wearable and implantable biointegrated electronics offer a groundbreaking solution, combining real-time, high-resolution monitoring with innovative treatment capabilities tailored to specific organ functions. In this comprehensive review, we focus on the diseases affecting the brain, heart, gastrointestinal organs, bladder, and adrenal gland, along with their associated physiological parameters. Additionally, we provide an overview of the characteristics of these parameters and explore the potential of bioelectronic devices for in situ sensing and therapeutic applications and highlight the recent advancements in their deployment across specific organs. Finally, we analyze the current challenges and prospects of implementing closed-loop feedback control systems in integrated sensor-therapy applications. By emphasizing organ-specific applications and advocating for closed-loop systems, this review highlights the potential of future bioelectronics to address physiological needs and serves as a guide for researchers navigating the interdisciplinary fields of diagnostics, therapeutics, and personalized medicine.

The control costs of human brain dynamics

The human brain is a complex system with high metabolic demands and extensive connectivity that requires control to balance energy consumption and functional efficiency over time. How this control is manifested on a whole-brain scale is largely unexplored, particularly what the associated costs are. Using the network control theory, here, we introduce a novel concept, time-averaged control energy (TCE), to quantify the cost of controlling human brain dynamics at rest, as measured from functional and diffusion MRI. Importantly, TCE spatially correlates with oxygen metabolism measures from the positron emission tomography, providing insight into the bioenergetic footing of resting-state control. Examining the temporal dimension of control costs, we find that brain state transitions along a hierarchical axis from sensory to association areas are more efficient in terms of control costs and more frequent within hierarchical groups than between. This inverse correlation between temporal control costs and state visits suggests a mechanism for maintaining functional diversity while minimizing energy expenditure. By unpacking the temporal dimension of control costs, we contribute to the neuroscientific understanding of how the brain governs its functionality while managing energy expenses.

Dopamine neuron activity evoked by sucrose and sucrose-predictive cues is augmented by peripheral and central manipulations of glucose availability

Food deprivation drives eating through multiple signals and circuits. Decreased glucose availability (i.e., cytoglucopenia) drives eating and also increases the value of sucrose. Ventral tegmental area (VTA) dopamine neurons (DANs) contribute to the evaluation of taste stimuli, but their role in integrating glucoprivic signals remains unknown. We monitored VTA DAN activity via Cre-dependent expression of a calcium indicator with in vivo fibre photometry. In ad libitum fed rats, intraoral sucrose evoked a phasic increase in DAN activity. To manipulate glucose availability, we administered (intraperitoneal, lateral or fourth ventricular) the antiglycolytic agent 5-thio-D-glucose (5TG), which significantly augmented the phasic DAN activity to sucrose. 5TG failed to alter DAN activity to water or saccharin, suggesting the response was selective for caloric stimuli. 5TG enhancement of sucrose-evoked DAN activity was stronger after fourth ventricular administration, suggesting a critical node of action within the hindbrain. As 5TG also increases blood glucose, in a separate study, we used peripheral insulin, which stimulates eating, to decrease blood glucose-which was associated with increased DAN activity to intraoral sucrose. DAN activity developed to a cue predictive of intraoral sucrose. While 5TG augmented cue-evoked DAN activity, its action was most potent when delivered to the lateral ventricle. Together, the studies point to central glucose availability as a key modulator of phasic DAN activity to food and food-cues. As glucose sensing neurons are known to populate the hypothalamus and brainstem, results suggest differential modulation of cue-evoked and sucrose-evoked DAN activity.

The Vagus Nerve Regulates Immunometabolic Homeostasis in the Ovine Fetus near Term: The Impact on Terminal Ileum

BACKGROUND

Glucosensing elements are widely distributed throughout the body and relay information about circulating glucose levels to the brain via the vagus nerve. However, while anatomical wiring has been established, little is known about the physiological role of the vagus nerve in glucosensing. The contribution of the vagus nerve to inflammation in the fetus is poorly understood. Increased glucose levels and inflammation act synergistically when causing organ injury, but their interplay remains incompletely understood. We hypothesized that vagotomy (Vx) will trigger a rise in systemic glucose levels and this will be enhanced during systemic and organ-specific inflammation. Efferent vagus nerve stimulation (VNS) should reverse this phenotype.

METHODS

Near-term fetal sheep (n = 57) were surgically prepared using vascular catheters and ECG electrodes as the control and treatment groups (lipopolysaccharide (LPS), Vx + LPS, Vx + LPS + selective efferent VNS). The experiment was started 72 h postoperatively to allow for post-surgical recovery. Inflammation was induced with LPS bolus intravenously (LPS group, 400 ng/fetus/day for 2 days; n = 23). For the Vx + LPS group (n = 11), a bilateral cervical vagotomy was performed during surgery; of these n = 5 received double the LPS dose, LPS800. The Vx + LPS + efferent VNS group (n = 8) received cervical VNS probes bilaterally distal from Vx in eight animals. Efferent VNS was administered for 20 min on days 1 and 2 +/10 min around the LPS bolus. Fetal arterial blood samples were drawn on each postoperative day of recovery (-72 h, -48 h, and -24 h) as well as at the baseline and seven selected time points (3-54 h) to profile inflammation (ELISA IL-6, pg/mL), insulin (ELISA), blood gas, and metabolism (glucose). At 54 h post-LPS, a necropsy was performed, and the terminal ileum macrophages' CD11c (M1 phenotype) immunofluorescence was quantified to detect inflammation. The results are reported for p < 0.05 and for Spearman R2 > 0.1. The results are presented as the median (IQR).

RESULTS

Across the treatment groups, blood gas and cardiovascular changes indicated mild septicemia. At 3 h in the LPS group, IL-6 peaked. That peak was decreased in the Vx + LPS400 group and doubled in the Vx + LPS800 group. The efferent VNS sped up the reduction in the inflammatory response profile over 54 h. The M1 macrophage activity was increased in the LPS and Vx + LPS800 groups only. The glucose and insulin concentrations in the Vx + LPS group were, respectively, 1.3-fold (throughout the experiment) and 2.3-fold higher vs. control (at 3 h). The efferent VNS normalized the glucose concentrations.

CONCLUSIONS

The complete withdrawal of vagal innervation resulted in a 72-h delayed onset of a sustained increase in glucose for at least 54 h and intermittent hyperinsulinemia. Under the conditions of moderate fetal inflammation, this was related to higher levels of gut inflammation. The efferent VNS reduced the systemic inflammatory response as well as restored both the concentrations of glucose and the degree of terminal ileum inflammation, but not the insulin concentrations. Supporting our hypothesis, these findings revealed a novel regulatory, hormetic, role of the vagus nerve in the immunometabolic response to endotoxin in near-term fetuses.

Representing Stimulus Information in an Energy Metabolism Pathway

We explored a computational model of astrocytic energy metabolism and demonstrated the theoretical plausibility that this type of pathway might be capable of coding information about stimuli in addition to its known functions in cellular energy and carbon budgets. Simulation results indicate that glycogenolytic glycolysis triggered by activation of adrenergic receptors can capture the intensity and duration features of a neuromodulator waveform and can respond in a dose-dependent manner, including non-linear state changes that are analogous to action potentials. We show how this metabolic pathway can translate information about external stimuli to production profiles of energy-carrying molecules such as lactate with a precision beyond simple signal transduction or non-linear amplification. The results suggest the operation of a metabolic state-machine from the spatially discontiguous yet interdependent metabolite elements. Such metabolic pathways might be well-positioned to code an additional level of salient information about a cell's environmental demands to impact its function. Our hypothesis has implications for the computational power and energy efficiency of the brain.

Indirect Calorimetry Measurements Compared With Guideline Weight-Based Energy Calculations in Critically Ill Stroke Patients

BACKGROUND

Perturbations in resting energy expenditure (REE) among critically ill stroke patients are ill defined, and guidelines recommend weight-based dosing when indirect calorimetry (IC) is not feasible to estimate daily energy requirements. We aimed to determine whether guideline-recommended weight-based dosing provides adequate energy requirements compared with guidelines recommended IC target.

METHODS

IC data was collected on stroke patients admitted to a neurocritical care unit. We compared low-weight-based dosing (25 kcal/kg) and high (30 kcal/kg) with the IC REE target. Subsequently, we analyzed the effect of stroke subtype on the differences among these measurements.

RESULTS

Seventy-two metabolic studies were performed (45.1% intracerebral hemorrhage [ICH], 18.3% aneurysmal subarachnoid hemorrhage [aSAH], and 36.6% acute ischemic stroke [AIS]). Energy needs, estimated using low-weight-based group, were significantly lower than IC REE target (1496 kcal/day [IQR, 1224-1850] vs 1770 kcal/day [IQR, 1400-2150]; P = .003). High weight-based group energy measurements were similar to IC REE target (1806 kcal/day [IQR, 1530-2236] vs 1770 kcal/day; P = .343). Subgroup analysis showed that low-weight-based calculations were significantly lower than those of IC in ICH and aSAH, but they were similar in AIS (P ≤ .001, .016, and .078, respectively). Linear regression analysis showed that weight, height, and hemorrhagic stroke subtype were associated with IC (P ≤ .001, .024, and .051, respectively).

CONCLUSION

Important differences between weight-based estimation of energy needs and guideline-recommended IC estimation exist for critically ill stroke patients. Low-weight-based calculations of REE underestimate energy needs in ICH and aSAH patients.

Global transcriptome analysis of rat hypothalamic arcuate nucleus demonstrates reversal of hypothalamic gliosis following surgically and diet induced weight loss

The central mechanisms underlying the marked beneficial metabolic effects of bariatric surgery are unclear. Here, we characterized global gene expression in the hypothalamic arcuate nucleus (Arc) in diet-induced obese (DIO) rats following Roux-en-Y gastric bypass (RYGB). 60 days post-RYGB, the Arc was isolated by laser-capture microdissection and global gene expression was assessed by RNA sequencing. RYGB lowered body weight and adiposity as compared to sham-operated DIO rats. Discrete transcriptome changes were observed in the Arc following RYGB, including differential expression of genes associated with inflammation and neuropeptide signaling. RYGB reduced gene expression of glial cell markers, including Gfap, Aif1 and Timp1, confirmed by a lower number of GFAP immunopositive astrocyte profiles in the Arc. Sham-operated weight-matched rats demonstrated a similar glial gene expression signature, suggesting that RYGB and dietary restriction have common effects on hypothalamic gliosis. Considering that RYGB surgery also led to increased orexigenic and decreased anorexigenic gene expression, this may signify increased hunger-associated signaling at the level of the Arc. Hence, induction of counterregulatory molecular mechanisms downstream from the Arc may play an important role in RYGB-induced weight loss.

Hypothalamic mechanisms associated with corticotropin-releasing factor-induced anorexia in chicks

Central administration of corticotropin-releasing factor (CRF), a 41-amino acid peptide, is associated with potent anorexigenic effects in rodents and chickens. However, the mechanism underlying this effect remains unclear. Hence, the objective of the current study was to elucidate the hypothalamic mechanisms that mediate CRF-induced anorexia in 4 day-old Cobb-500 chicks. After intracerebroventricular (ICV) injection of 0.02 nmol of CRF, CRF-injected chicks ate less than vehicle chicks while no effect on water intake was observed at 30 min post-injection. In subsequent experiments, the hypothalamus samples were processed at 60 min post-injection. The CRF-injected chicks had more c-Fos immunoreactive cells in the arcuate nucleus (ARC), dorsomedial nucleus (DMN), ventromedial hypothalamus (VMH), and paraventricular nucleus (PVN) of the hypothalamus than vehicle-treated chicks. CRF injection was associated with decreased whole hypothalamic mRNA abundance of neuropeptide Y receptor sub-type 1 (NPYR1). In the ARC, CRF-injected chicks expressed more CRF and CRF receptor sub-type 2 (CRFR2) mRNA but less agouti-related peptide (AgRP), NPY, and NPYR1 mRNA than vehicle-injected chicks. CRF-treated chicks expressed greater amounts of CRFR2 and mesotocin mRNA than vehicle chicks in the PVN and VMH, respectively. In the DMN, CRF injection was associated with reduced NPYR1 mRNA. In conclusion, the results provide insights into understanding CRF-induced hypothalamic actions and suggest that the anorexigenic effect of CRF involves increased CRFR2-mediated signaling in the ARC and PVN that overrides the effects of NPY and other orexigenic factors.

Glucose Activates Vagal Control of Hyperglycemia and Inflammation in Fasted Mice

Sepsis is a leading cause of death in hospitalized patients. Many experimental treatments may have failed in clinical trials for sepsis, in part, because they focused on immune responses of healthy animals that did not mimic the metabolic settings of septic patients. Epidemiological studies show an association between metabolic and immune alterations and over 1/3 of septic patients are diabetic, but the mechanism linking these systems is unknown. Here, we report that metabolic fasting increased systemic inflammation and worsened survival in experimental sepsis. Feeding and administration of glucose in fasted mice activated the vagal tone without affecting blood pressure. Vagal stimulation attenuated hyperglycemia and serum TNF levels in sham but only hyperglycemia in splenectomized mice. Vagal stimulation induced the production of dopamine from the adrenal glands. Experimental diabetes increased hyperglycemia and systemic inflammation in experimental sepsis. Fenoldopam, a specific dopaminergic type-1 agonist, attenuated hyperglycemia and systemic inflammation in diabetic endotoxemic mice. These results indicate that glucose activates vagal control of hyperglycemia and inflammation in fasted septic mice via dopamine.

The effect of loss of the glucose-monitoring neurons in the anterior cingulate cortex: Physiologic challenges induce complex feeding-metabolic alterations after local streptozotocin microinjection in rats

The anterior cingulate cortex (ACC) is interrelated to limbic structures, parts of the central glucose-monitoring (GM) network. GM neurons, postulated to exist here, are hypothesised to participate in regulatory functions, such as the central control of feeding and metabolism. In the present experiments, GM neurons were identified and examined in the ACC by means of the multibarreled microelectrophoretic technique. After bilateral ACC microinjection of streptozotocin (STZ), glucose tolerance tests (GTTs), and determination of relevant plasma metabolite concentrations were performed. Body weights were measured at regular time points during the GTT experiment. Ten percent of the neurons - 30 of 282 recorded cells - responded to the administration of D-glucose, thus, declared to be the GM units. The peak values and dynamics of the GTT blood glucose curves, the plasma metabolite concentrations, and the weight gain were pathologically altered in the STZ treated animals. Our recording experiments revealed the existence of GM neurons in the anterior cingulate cortex. STZ induced selective destruction of these chemosensory cells resulted in feeding and metabolic alterations. The present findings indicate distinguished significance of the cingulate cortical GM neurons in adaptive processes of maintenance of the homeostatic balance.

Neuroanatomical Framework of the Metabolic Control of Reproduction

A minimum amount of energy is required for basic physiological processes, such as protein biosynthesis, thermoregulation, locomotion, cardiovascular function, and digestion. However, for reproductive function and survival of the species, extra energy stores are necessary. Production of sex hormones and gametes, pubertal development, pregnancy, lactation, and parental care all require energy reserves. Thus the physiological systems that control energy homeostasis and reproductive function coevolved in mammals to support both individual health and species subsistence. In this review, we aim to gather scientific knowledge produced by laboratories around the world on the role of the brain in integrating metabolism and reproduction. We describe essential neuronal networks, highlighting key nodes and potential downstream targets. Novel animal models and genetic tools have produced substantial advances, but critical gaps remain. In times of soaring worldwide obesity and metabolic dysfunction, understanding the mechanisms by which metabolic stress alters reproductive physiology has become crucial for human health.

Comparison of Low Glycaemic Index and High Glycaemic Index Potatoes in Relation to Satiety: A Single-Blinded, Randomised Crossover Study in Humans

High glycaemic index (GI) foods have been proposed to reduce satiety and thus promote overweight and obesity. Generally, potatoes have a high GI, but they also provide many beneficial nutrients and they are a highly important food source globally. In this study, we investigated how a low GI potato affected subjective satiety as compared to a high GI potato. Twenty healthy men (aged 18–40 years; body mass index (BMI) 18–27 kg/m²) participated in this single-blinded, controlled, randomised crossover trial. On each of the two trial days, the subjects were given a 500-gram portion of either a low or high GI potato variety (Carisma^® low GI and Arizona high GI). Subjective appetite sensations were measured at baseline and at +15 min, +45 min, +75 min, +105 min, and +135 min after consumption of the test meal until an ad libitum meal was served at +150 min. No significant differences in the primary endpoint, satiety, were found between the two potato varieties (all p > 0.05). Furthermore, no significant differences were found in the secondary endpoints; hunger, fullness, and prospective food consumption, or ad libitum energy intake (all p > 0.05). In conclusion, the results of this study do not indicate that the GI of potatoes is important for satiety in normal-weight men.

Weaning stage hyperglycemia induces glucose-insensitivity in arcuate POMC neurons and hyperphagia in type 2 diabetic GK rats

Hyperphagia triggers and accelerates diabetes, and prevents proper dietary control of glycemia. Inversely, the impact of hyperglycemia on hyperphagia and possible mechanistic cause common for these two metabolic disorders in type 2 diabetes are less defined. The present study examined the precise developmental process of hyperglycemia and hyperphagia and explored the alterations in the hypothalamic arcuate nucleus (ARC), the primary feeding and metabolic center, in Goto-Kakizaki (GK) rats with type 2 diabetes and nearly normal body weight. At mid 3 to 4 weeks of age, GK rats first exhibited hyperglycemia, and then hyperphagia and reduced mRNA expressions for anorexigenic pro-opiomelanocortin (POMC) and glucokinase in ARC. Furthermore, [Ca²⁺]i responses to high glucose in ARC POMC neurons were impaired in GK rats at 4 weeks. Treating GK rats from early 3 to mid 6 weeks of age with an anti-diabetic medicine miglitol not only suppressed hyperglycemia but ameliorated hyperphagia and restored POMC mRNA expression in ARC. These results suggest that the early hyperglycemia occurring in weaning period may lead to impaired glucose sensing and neuronal activity of POMC neurons, and thereby induce hyperphagia in GK rats. Correction of hyperglycemia in the early period may prevent and/or ameliorate the progression of hyperphagia in type 2 diabetes.

Dietary influences on cognition

Obesity is a world-wide crisis with profound healthcare and socio-economic implications and it is now clear that the central nervous system (CNS) is a target for the complications of metabolic disorders like obesity. In addition to decreases in physical activity and sedentary lifestyles, diet is proposed to be an important contributor to the etiology and progression of obesity. Unfortunately, there are gaps in our knowledge base related to how dietary choices impact the structural and functional integrity of the CNS. For example, while chronic consumption of hypercaloric diets (increased sugars and fat) contribute to increases in body weight and adiposity characteristic of metabolic disorders, the mechanistic basis for neurocognitive deficits in obesity remains to be determined. In addition, studies indicate that acute consumption of hypercaloric diets impairs performance in a wide variety of cognitive domains, even in normal non-obese control subjects. These results from the clinical and basic science literature indicate that diet can have rapid, as well as long lasting effects on cognitive function. This review summarizes our symposium at the 2017 Society for the Study of Ingestive Behavior (SSIB) meeting that discussed these effects of diet on cognition. Collectively, this review highlights the need for integrated and comprehensive approaches to more fully determine how diet impacts behavior and cognition under physiological conditions and in metabolic disorders like type 2 diabetes mellitus (T2DM) and obesity.

Development of diabetes does not alter behavioral and molecular circadian rhythms in a transgenic rat model of type 2 diabetes mellitus

Metabolic state and circadian clock function exhibit a complex bidirectional relationship. Circadian disruption increases propensity for metabolic dysfunction, whereas common metabolic disorders such as obesity and type 2 diabetes (T2DM) are associated with impaired circadian rhythms. Specifically, alterations in glucose availability and glucose metabolism have been shown to modulate clock gene expression and function in vitro; however, to date, it is unknown whether development of diabetes imparts deleterious effects on the suprachiasmatic nucleus (SCN) circadian clock and SCN-driven outputs in vivo. To address this question, we undertook studies in aged diabetic rats transgenic for human islet amyloid polypeptide, an established nonobese model of T2DM (HIP rat), which develops metabolic defects closely recapitulating those present in patients with T2DM. HIP rats were also cross-bred with a clock gene reporter rat model (Per1:luciferase transgenic rat) to permit assessment of the SCN and the peripheral molecular clock function ex vivo. Utilizing these animal models, we examined effects of diabetes on 1) behavioral circadian rhythms, 2) photic entrainment of circadian activity, 3) SCN and peripheral tissue molecular clock function, and 4) melatonin secretion. We report that circadian activity, light-induced entrainment, molecular clockwork, as well as melatonin secretion are preserved in the HIP rat model of T2DM. These results suggest that despite the well-characterized ability of glucose to modulate circadian clock gene expression acutely in vitro, SCN clock function and key behavioral and physiological outputs appear to be preserved under chronic diabetic conditions characteristic of nonobese T2DM.

10 lessons learned by a misguided physician

It was a great and humbling honor to receive the 2016 Distinguished Career Award from my SSIB colleagues. This paper summarizes the major points of my DCA talk at the 2016 annual meeting. It is a reflection on my 50year medical and research career and 10 lessons I have learned over those years which might be of help to young investigators near the beginning of their own research careers. These lessons include: the value of being receptive to the opportunities provided you; how clinician-scientists can serve as critical role models for young investigators like me and a history of how my career developed as a result of their influence; the importance of carefully examining your own data, particularly when it doesn't agree with your preconceived ideas; the critical role that students, postdocs and PhD (and even veterinarian) colleagues can play in developing one's career; the likelihood that your career path will have many interesting twists and turns determined by changes in your own scientific interests and how rewarding various areas of research focus are to you; the importance of building a close-knit laboratory staff family; the fact that science and romance can mix. Finally, I offer 3 somewhat self-evident free pieces of advice for building and maintaining a rewarding career.

Impaired glucose tolerance after streptozotocin microinjection into the mediodorsal prefrontal cortex of the rat

The mediodorsal prefrontal cortex (mdPFC) is a key structure of the central glucose-monitoring (GM) neural network. Previous studies indicate that intracerebral streptozotocin (STZ) microinjection-induced destruction of local chemosensory neurons results in feeding and metabolic alterations. The present experiments aimed to examine whether STZ microinjection into the mdPFC causes metabolic deficits. To do so, glucose tolerance test (GTT) and measurements of plasma metabolites were performed in STZ-treated or control rats. Intraperitoneal D-glucose load was delivered 20 min or 4 weeks following the intracerebral microinjection of STZ or saline (acute or subacute GTT, respectively). The STZ-treated rats displayed acute glucose intolerance: at the 120th min of the test, blood glucose level of these rats was significantly higher than that of the ones in the control group. When determining the plasma level of various metabolites, 30 min following the intracerebral STZ or saline microinjection, the triglyceride concentration of the STZ-treated rats was found to be reduced compared with that of the control rats. The GM neurons of the mdPFC are suggested to be involved in the organization of complex metabolic processes by which these chemosensory cells contribute to adaptive control mechanisms of the maintenance of homeostasis.

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

Evidence obtained in recent years in a few species, especially rainbow trout, supports the presence in fish of nutrient sensing mechanisms. Glucosensing capacity is present in central (hypothalamus and hindbrain) and peripheral [liver, Brockmann bodies (BB, main accumulation of pancreatic endocrine cells in several fish species), and intestine] locations whereas fatty acid sensors seem to be present in hypothalamus, liver and BB. Glucose and fatty acid sensing capacities relate to food intake regulation and metabolism in fish. Hypothalamus is as a signaling integratory center in a way that detection of increased levels of nutrients result in food intake inhibition through changes in the expression of anorexigenic and orexigenic neuropeptides. Moreover, central nutrient sensing modulates functions in the periphery since they elicit changes in hepatic metabolism as well as in hormone secretion to counter-regulate changes in nutrient levels detected in the CNS. At peripheral level, the direct nutrient detection in liver has a crucial role in homeostatic control of glucose and fatty acid whereas in BB and intestine nutrient sensing is probably involved in regulation of hormone secretion from endocrine cells.

Insights into the role of neuronal glucokinase

Glucokinase is a key component of the neuronal glucose-sensing mechanism and is expressed in brain regions that control a range of homeostatic processes. In this review, we detail recently identified roles for neuronal glucokinase in glucose homeostasis and counterregulatory responses to hypoglycemia and in regulating appetite. We describe clinical implications from these advances in our knowledge, especially for developing novel treatments for diabetes and obesity. Further research required to extend our knowledge and help our efforts to tackle the diabetes and obesity epidemics is suggested.

Clinical Management of Obesity in Women: Addressing a Lifecycle of Risk

The World Health Organization estimates that nearly 2 billion people worldwide are overweight, 600 million of whom are obese. The increasing prevalence of this condition in women is of particular concern given its impact on reproductive health and mortality. Burgeoning data implicating maternal obesity in fetal programming and the metabolic health of future generations further suggest that obesity in women is one of the most pressing public health concerns of the twenty-first century. However, health care professionals are infrequently engaged in obesity management. This article provides a conceptual understanding of obesity and a rational approach to treatment.

Anti-Diabetic Activities of Gastrodia elata Blume Water Extracts Are Mediated Mainly by Potentiating Glucose-Stimulated Insulin Secretion and Increasing β-Cell Mass in Non-Obese Type 2 Diabetic Animals

The brain is an important modulator of glucose metabolism, and is known to respond Gastrodia elata Blume water extract (GEB). Therefore, we examined whether long-term administration of GEB has hypoglycemic activity, and its action mechanism was explored in partially-pancreatectomized rats that exhibit similar characteristics as Asian type 2 diabetes, non-obese insulin-insufficient diabetes. The rats were provided high-fat diets supplemented with either of (1) 0.5% GEB (GEB-L), (2) 2% GEB (GEB-H), (3) 2% dextrin (control), or (4) 2% dextrin with rosiglitazone (20 mg/kg body weight; positive-control) for eight weeks. GEB dose-dependently improved hypothalamic insulin signaling, enhanced whole-body insulin sensitivity during hyperinsulinemic euglycemic clamp, and reduced hepatic glucose output in a hyperinsulinemic state. GEB dose-dependently increased the area under the curve of the serum insulin levels at the first and second phases during hyperglycemic clamp compared to the control, whereas the positive control had no effect. Insulin sensitivity during the hyperglycemic state also improved, dose-dependently, in response to GEB compared with that of the control, but was less than the positive control. GEB-H increased the mass of β-cells by potentiating proliferation and decreasing apoptosis. In conclusion, GEB could be a therapeutic agent for treating Asian type 2 diabetes.

Alterations in the Vaginal Microbiome by Maternal Stress Are Associated With Metabolic Reprogramming of the Offspring Gut and Brain

The neonate is exposed to the maternal vaginal microbiota during parturition, providing the primary source for normal gut colonization, host immune maturation, and metabolism. These early interactions between the host and microbiota occur during a critical window of neurodevelopment, suggesting early life as an important period of cross talk between the developing gut and brain. Because perturbations in the prenatal environment such as maternal stress increase neurodevelopmental disease risk, disruptions to the vaginal ecosystem could be a contributing factor in significant and long-term consequences for the offspring. Therefore, to examine the hypothesis that changes in the vaginal microbiome are associated with effects on the offspring gut microbiota and on the developing brain, we used genomic, proteomic and metabolomic technologies to examine outcomes in our mouse model of early prenatal stress. Multivariate modeling identified broad proteomic changes to the maternal vaginal environment that influence offspring microbiota composition and metabolic processes essential for normal neurodevelopment. Maternal stress altered proteins related to vaginal immunity and abundance of Lactobacillus, the prominent taxa in the maternal vagina. Loss of maternal vaginal Lactobacillus resulted in decreased transmission of this bacterium to offspring. Further, altered microbiota composition in the neonate gut corresponded with changes in metabolite profiles involved in energy balance, and with region- and sex-specific disruptions of amino acid profiles in the developing brain. Taken together, these results identify the vaginal microbiota as a novel factor by which maternal stress may contribute to reprogramming of the developing brain that may predispose individuals to neurodevelopmental disorders.

Insulin and GH-IGF-I axis: endocrine pacer or endocrine disruptor?

Growth hormone/insulin-like growth factor (IGF) axis may play a role in maintaining glucose homeostasis in synergism with insulin. IGF-1 can directly stimulate glucose transport into the muscle through either IGF-1 or insulin/IGF-1 hybrid receptors. In severely decompensated diabetes including diabetic ketoacidosis, plasma levels of IGF-1 are low and insulin delivery into the portal system is required to normalize IGF-1 synthesis and bioavailability. Normalization of serum IGF-1 correlated with the improvement of glucose homeostasis during insulin therapy providing evidence for the use of IGF-1 as biomarker of metabolic control in diabetes. Taking apart the inherent mitogenic discussion, diabetes treatment using insulins with high affinity for the IGF-1 receptor may act as an endocrine pacer exerting a cardioprotective effect by restoring the right level of IGF-1 in bloodstream and target tissues, whereas insulins with low affinity for the IGF-1 receptor may lack this positive effect. An excessive and indirect stimulation of IGF-1 receptor due to sustained and chronic hyperinsulinemia over the therapeutic level required to overtake acute/chronic insulin resistance may act as endocrine disruptor as it may possibly increase the cardiovascular risk in the short and medium term and mitogenic/proliferative action in the long term. In conclusion, normal IGF-1 may be hypothesized to be a good marker of appropriate insulin treatment of the subject with diabetes and may integrate and make more robust the message coming from HbA1c in terms of prediction of cardiovascular risk.

The role of dopamine in the pursuit of nutritional value

Acquiring enough food to meet energy expenditure is fundamental for all organisms. Thus, mechanisms have evolved to allow foods with high nutritional value to be readily detected, consumed, and remembered. Although taste is often involved in these processes, there is a wealth of evidence supporting the existence of taste-independent nutrient sensing. In particular, post-ingestive mechanisms arising from the arrival of nutrients in the gut are able to drive food intake and behavioural conditioning. The physiological mechanisms underlying these effects are complex but are believed to converge on mesolimbic dopamine signalling to translate post-ingestive sensing of nutrients into reward and reinforcement value. Discerning the role of nutrition is often difficult because food stimulates sensory systems and post-ingestive pathways in concert. In this mini-review, I discuss the various methods that may be used to study post-ingestive processes in isolation including sham-feeding, non-nutritive sweeteners, post-ingestive infusions, and pharmacological and genetic methods. Using this structure, I present the evidence that dopamine is sensitive to nutritional value of certain foods and examine how this affects learning about food, the role of taste, and the implications for human obesity.

Gene-environment interactions controlling energy and glucose homeostasis and the developmental origins of obesity

Obesity and type 2 diabetes mellitus (T2DM) often occur together and affect a growing number of individuals in both the developed and developing worlds. Both are associated with a number of other serious illnesses that lead to increased rates of mortality. There is likely a polygenic mode of inheritance underlying both disorders, but it has become increasingly clear that the pre- and postnatal environments play critical roles in pushing predisposed individuals over the edge into a disease state. This review focuses on the many genetic and environmental variables that interact to cause predisposed individuals to become obese and diabetic. The brain and its interactions with the external and internal environment are a major focus given the prominent role these interactions play in the regulation of energy and glucose homeostasis in health and disease.

Involvement of orexin-A neurons but not melanin-concentrating hormone neurons in the short-term regulation of food intake in rats

In order to elucidate the involvement of melanin-concentrating hormone (MCH) and orexin-A (ORX-A) neurons of the perifornical/lateral hypothalamic areas (PF/LH) in the regulation of food intake induced by acutely reduced glucose availability, we examined the food intake response and c-Fos expression in the MCH and ORX-A neurons in the PF/LH during 2-deoxy-D-glucose (2DG)-induced glucoprivation (400 mg/kg; i.v.) and systemic insulin-induced hypoglycemia (5 U/kg; s.c.) in male Wistar rats. The administration of both 2DG and insulin stimulated food intake and induced c-Fos expression in the ORX-A neurons corresponding to food intake, but not in the MCH neurons. These data indicate that ORX-A neurons, but not MCH neurons, play a role in the short-term regulation of food intake, and that the input signals for the neurons containing MCH and ORX-A are different, and these neurons play different roles in the regulation of feeding behavior.

CRF and urocortin peptides as modulators of energy balance and feeding behavior during stress

Early on, corticotropin-releasing factor (CRF), a hallmark brain peptide mediating many components of the stress response, was shown to affect food intake inducing a robust anorexigenic response when injected into the rodent brain. Subsequently, other members of the CRF signaling family have been identified, namely urocortin (Ucn) 1, Ucn 2, and Ucn 3 which were also shown to decrease food intake upon central or peripheral injection. However, the kinetics of feeding suppression was different with an early decrease following intracerebroventricular injection of CRF and a delayed action of Ucns contrasting with the early onset after systemic injection. CRF and Ucns bind to two distinct G-protein coupled membrane receptors, the CRF1 and CRF2. New pharmacological tools such as highly selective peptide CRF1 or CRF2 agonists or antagonists along with genetic knock-in or knock-out models have allowed delineating the primary role of CRF2 involved in the anorexic response to exogenous administration of CRF and Ucns. Several stressors trigger behavioral changes including suppression of feeding behavior which are mediated by brain CRF receptor activation. The present review will highlight the state-of-knowledge on the effects and mechanisms of action of CRF/Ucns-CRF1/2 signaling under basal conditions and the role in the alterations of food intake in response to stress.

Basal ganglia morphology links the metabolic syndrome and depressive symptoms

The metabolic syndrome (MetS) is a clustering of cardiovascular and cerebrovascular risk factors that are often comorbid with depressive symptoms. Individual components of the MetS also covary with the morphology of basal ganglia regions that are altered by depression. However, it remains unknown whether the covariation between the MetS and depressive symptomatology can be accounted for in part by morphological changes in the basal ganglia. Accordingly, we tested the hypothesis that increased depressive symptoms among individuals with the MetS might be statistically mediated by reduced gray matter volume in basal ganglia regions. The presence of the MetS was determined in 147 middle-aged adults using the criteria of the National Cholesterol Education Program, Adult Treatment Panel III. Basal ganglia volumes were determined on an a priori basis by automated segmentation of high-resolution magnetic resonance images. Depressive symptoms were assessed using the Patient Health Questionnaire. Even after controlling for demographic and other confounding factors, having the MetS and meeting more MetS criteria covaried with reduced globus pallidus volume. Meeting more MetS criteria and reduced pallidal volume were also related to depressive symptoms. Moreover, the MetS-depression association was statistically mediated by pallidal volume. In summary, reduced globus pallidus volume is a neural correlate of the MetS that may partly account for its association with depressive symptoms.

Enantiomer-specific selection of amino acids

Dietary intake of L-amino acids impacts on several physiological functions, including the control of gastrointestinal motility, pancreatic secretion, and appetite. However, the biological mechanisms regulating behavioral predilections for certain amino acid types remain poorly understood. We tested the hypothesis that, in mice, the potency with which a given glucogenic amino acid increases glucose utilization reflects its rewarding properties. We have found that: (1) during long-, but not short-, term preference tests, L-alanine and L-serine were preferred over their D-enantiomer counterparts, while no such effect was observed for L-threonine vs. D-threonine; (2) these behavioral patterns were closely associated with the ability of L-amino acids to promote increases in respiratory exchange ratios such that those, and only those, L-amino acids able to promote increases in respiratory exchange ratios were preferred over their D-isomers; (3) these behavioral preferences were independent of gustatory influences, since taste-deficient Trpm5 knockout mice displayed ingestive responses very similar to those of their wild-type counterparts. We conclude that the ability to promote increases in respiratory exchange ratios enhances the reward value of nutritionally relevant amino acids and suggest a mechanistic link between substrate utilization and amino acid preferences.

Enantiomer-specific selection of amino acids

Dietary intake of L-amino acids impacts on several physiological functions, including the control of gastrointestinal motility, pancreatic secretion, and appetite. However, the biological mechanisms regulating behavioral predilections for certain amino acid types remain poorly understood. We tested the hypothesis that, in mice, the potency with which a given glucogenic amino acid increases glucose utilization reflects its rewarding properties. We have found that: (1) during long-, but not short-, term preference tests, L-alanine and L-serine were preferred over their D-enantiomer counterparts, while no such effect was observed for L-threonine vs. D-threonine; (2) these behavioral patterns were closely associated with the ability of L-amino acids to promote increases in respiratory exchange ratios such that those, and only those, L-amino acids able to promote increases in respiratory exchange ratios were preferred over their D-isomers; (3) these behavioral preferences were independent of gustatory influences, since taste-deficient Trpm5 knockout mice displayed ingestive responses very similar to those of their wild-type counterparts. We conclude that the ability to promote increases in respiratory exchange ratios enhances the reward value of nutritionally relevant amino acids and suggest a mechanistic link between substrate utilization and amino acid preferences.

CNS control of glucose metabolism: response to environmental challenges

Over the last 15 years, considerable work has accumulated to support the role of the CNS in regulating postprandial glucose levels. As discussed in the first section of this review, the CNS receives and integrates information from afferent neurons, circulating hormones, and postprandially generated nutrients to subsequently direct changes in glucose output by the liver and glucose uptake by peripheral tissues. The second major component of this review focuses on the effects of external pressures, including high fat diet and changes to the light:dark cycle on CNS-regulating glucose homeostasis. We also discuss the interaction between these different pressures and how they contribute to the multifaceted mechanisms that we hypothesize contribute to the dysregulation of glucose in type 2 diabetes mellitus (T2DM). We argue that while current peripheral therapies serve to delay the progression of T2DM, generating combined obesity and T2DM therapies targeted at the CNS, the primary site of dysfunction for both diseases, would lead to a more profound impact on the progression of both diseases.

Appetite control and gastrointestinal hormonal behavior (CCK, GLP-1, PYY 1-36) following low doses of a whey protein-rich nutraceutic

Whey proteins represent the most satiating nutrients. In particular, their effects are due to enterohormonal changes (CCK, GLP-1 and PYY 1–36) observed after their exclusive ingestion. Glucomannan has important satiety property due to volume increase following gelification. The aim of the study is the evaluation of subjective rate of hunger and enterohormone concentrations (CCK, GLP-1, PYY 1–36) following oral loading of a mixture containing WP (8 g) or casein (8 g) plus glucomannan (1 g) (Colordiet^®, Inpha DUEMILA Srl Lecco, Italy). The study was conducted as a double-blind crossover with five healthy volunteers (BMI 22–26 kg/m² aging 18–65 years) in acute and a wash-out period of 1 week between the first and the second evaluation. From the analysis of the data, we observe that the load with WP induces a significant decrease in the desire to eat after 90 min (P < 0.0446) when compared with casein. As far as plasma hormones are concerned, there was a significant increase only in GLP-1 at 90 min after WP (P < 0.00166) and 180 min after casein (T0 vs. T180 P = 0.000129). There is a significant correlation between the increase in GLP-1 and decrease of desire to eat (R = −0.93). There is a tendency to the increasing of CCK after 90 min, which is not significant (P = 0.091). These results could be due to (a) the low number of cases or (b) the low dose of protein used. The present study suggests that a mixture of WP plus glucomannan exerts a decrease in the desire to eat which is correlated to enterohormonal modification (GLP-1 increase) despite the low content of protein (8 g) and the presence of glucomannan, which could reduce the fast absorption of WP in relation to the net forming during the gelification of the gastric environment.

From conditioned hypoglycemia to obesity: following the data

While a graduate student in the late 1960s I trained rats to lower their blood glucose in response to an arbitrary cue, a phenomenon called conditioned hypoglycemia. Over many years as my colleagues and I attempted to understand the underlying physiology of conditioned insulin secretion and conditioned hypoglycemia, it became clear that there were many implications that were highly important, including the entry of insulin into the brain, the existence of insulin receptors in certain brain areas, neural reflexes that project to insulin-secreting B-cells in the pancreas, the entrainment of those reflexes to improve the efficiency of meal-taking, and the possibility of adiposity signals from the body to the brain that influence behavior and metabolism. This article summarizes how we tackled each of these areas of research.

Crosstalk between diabetes and brain: glucagon-like peptide-1 mimetics as a promising therapy against neurodegeneration

According to World Health Organization estimates, type 2 diabetes (T2D) is an epidemic (particularly in under development countries) and a socio-economic challenge. This is even more relevant since increasing evidence points T2D as a risk factor for Alzheimer's disease (AD), supporting the hypothesis that AD is a "type 3 diabetes" or "brain insulin resistant state". Despite the limited knowledge on the molecular mechanisms and the etiological complexity of both pathologies, evidence suggests that neurodegeneration/death underlying cognitive dysfunction (and ultimately dementia) upon long-term T2D may arise from a complex interplay between T2D and brain aging. Additionally, decreased brain insulin levels/signaling and glucose metabolism in both pathologies further suggests that an effective treatment strategy for one disorder may be also beneficial in the other. In this regard, one such promising strategy is a novel successful anti-T2D class of drugs, the glucagon-like peptide-1 (GLP-1) mimetics (e.g. exendin-4 or liraglutide), whose potential neuroprotective effects have been increasingly shown in the last years. In fact, several studies showed that, besides improving peripheral (and probably brain) insulin signaling, GLP-1 analogs minimize cell loss and possibly rescue cognitive decline in models of AD, Parkinson's (PD) or Huntington's disease. Interestingly, exendin-4 is undergoing clinical trials to test its potential as an anti-PD therapy. Herewith, we aim to integrate the available data on the metabolic and neuroprotective effects of GLP-1 mimetics in the central nervous system (CNS) with the complex crosstalk between T2D-AD, as well as their potential therapeutic value against T2D-associated cognitive dysfunction.

Involvement of the parasympathetic nervous system in the initiation of regeneration of pancreatic β-cells

The mechanism that initiates regeneration of pancreatic β-cells is not clear at present. The vagal nerve is implicated in the regulation of gastrointestinal functions, glucose metabolism and proliferation of pancreatic β-cells under physiological conditions. To elucidate the triggering mechanism of the regeneration of pancreatic β-cells, we examined the involvement of the vagal nerve. To this end, we employed a rat pancreatic duct ligation (DL) model, in which profound β-cell neogenesis and β-cell proliferation were observed within a week. We administered atropine to block the vagal nerve. Administration of atropine inhibited proliferation of β-cells in both islets and islet-like cell clusters (ICC), without affecting ductal cell proliferation in the ligated pancreas. The numbers of PDX-1 and MafB-positive cells in or attaching to the ducts were significantly reduced by atropine. MafB/glucagon and MafB/insulin double-positive cells were also decreased by atropine. Finally, atropine reduced the number of MafA-positive ductal cells, all of which were positive for insulin, by 50% on day 5. These results strongly suggest that the vagal nerve is involved in β-cell proliferation, induction of endocrine progenitors and neogenesis of α- and β-cells.

Wired on sugar: the role of the CNS in the regulation of glucose homeostasis

Obesity and type 2 diabetes mellitus (T2DM)--disorders of energy homeostasis and glucose homeostasis, respectively--are tightly linked and the incidences of both conditions are increasing in parallel. The CNS integrates information regarding peripheral nutrient and hormonal changes and processes this information to regulate energy homeostasis. Recent findings indicate that some of the neural circuits and mechanisms underlying energy balance are also essential for the regulation of glucose homeostasis. We propose that disruption of these overlapping pathways links the metabolic disturbances associated with obesity and T2DM. A better understanding of these converging mechanisms may lead to therapeutic strategies that target both T2DM and obesity.

A role for inducible 6-phosphofructo-2-kinase in the control of neuronal glycolysis

Increased glycolysis is the result of the sensing of glucose by hypothalamic neurons. The biochemical mechanisms underlying the control of hypothalamic glycolysis, however, remain to be elucidated. Here we showed that PFKFB3, the gene that encodes for inducible 6-phosphofructo-2-kinase (iPFK2), was expressed at high abundance in both mouse hypothalami and clonal hypothalamic neurons. In response to re-feeding, PFKFB3 mRNA levels were increased by 10-fold in mouse hypothalami. In the hypothalamus, re-feeding also decreased the phosphorylation of AMP-activated protein kinase (AMPK) (Thr172) and the mRNA levels of agouti-related protein (AgRP), and increased the mRNA levels of cocaine-amphetamine-related transcript (CART). Similar results were observed in N-43/5 clonal hypothalamic neurons upon treatment with glucose and/or insulin. In addition, knockdown of PFKFB3/iPFK2 in N-43/5 neurons caused a decrease in rates of glycolysis, which was accompanied by increased AMPK phosphorylation, increased AgRP mRNA levels and decreased CART mRNA levels. In contrast, overexpression of PFKFB3/iPFK2 in N-43/5 neurons caused an increase in glycolysis, which was accompanied by decreased AMPK phosphorylation and decreased AgRP mRNA levels and increased CART mRNA levels. Together, these results suggest that PFKFB3/iPFK2 responds to re-feeding, which in turn stimulates hypothalamic glycolysis and decreases hypothalamic AMPK phosphorylation and alters neuropeptide expression in a pattern that is associated with suppression of food intake.

Brainstem nutrient sensing in the nucleus of the solitary tract inhibits feeding

Direct detection of circulating nutrients by the central nervous system has been implicated in the regulation of energy balance, and the mediobasal hypothalamus is considered as the primary sensing site mediating these effects. Neurons sensitive to energyrelated signals have also been identified outside the hypothalamus, particularly within the caudomedial nucleus of the solitary tract (cmNTS) in brainstem, but the consequences of direct cmNTS nutrient detection on energy balance remain poorly characterized. Here we determined the behavioral and metabolic consequences of direct L-leucine detection by the cmNTS and investigated the intracellular signaling and neurochemical pathways implicated in cmNTS L-leucine sensing in rats. Our results support the distributed nature of central nutrient detection, evidence a role for the cmNTS S6K1 pathway in the regulation of meal size and body weight, and suggest that the cmNTS integrates direct cmNTS nutrient detection with gut-derived, descending forebrain, and adiposity signals of energy availability to regulate food intake.

Hindbrain neurons as an essential hub in the neuroanatomically distributed control of energy balance

This Review highlights the processing and integration performed by hindbrain nuclei, focusing on the inputs received by nucleus tractus solitarius (NTS) neurons. These inputs include vagally mediated gastrointestinal satiation signals, blood-borne energy-related hormonal and nutrient signals, and descending neural signals from the forebrain. We propose that NTS (and hindbrain neurons, more broadly) integrate these multiple energy status signals and issue-output commands controlling the behavioral, autonomic, and endocrine responses that collectively govern energy balance. These hindbrain-mediated controls are neuroanatomically distributed; they involve endemic hindbrain neurons and circuits, hindbrain projections to peripheral circuits, and projections to and from midbrain and forebrain nuclei.

High calorie diet triggers hypothalamic angiopathy

Obesity, type 2 diabetes, and related diseases represent major health threats to modern society. Related pathophysiology of impaired neuronal function in hypothalamic control centers regulating metabolism and body weight has been dissected extensively and recent studies have started focusing on potential roles of astrocytes and microglia. The hypothalamic vascular system, however, which maintains the microenvironment necessary for appropriate neuronal function, has been largely understudied. We recently discovered that high fat/high sucrose diet exposure leads to increased hypothalamic presence of immunoglobulin G (IgG1). Investigating this phenomenon further, we have discovered a significant increase in blood vessel length and density in the arcuate nucleus (ARC) of the hypothalamus in mice fed a high fat/high sucrose diet, compared to matched controls fed standard chow diet. We also found a clearly increased presence of α-smooth muscle actin immunoreactive vessels, which are rarely present in the ARC and indicate an increase in the formation of new arterial vessels. Along the blood brain barrier, an increase of degenerated endothelial cells are observed. Moreover, such hypothalamic angiogenesis was not limited to rodent models. We also found an increase in the number of arterioles of the infundibular nucleus (the human equivalent of the mouse ARC) in patients with type 2 diabetes, suggesting angiogenesis occurs in the human hypothalamus of diabetics. Our discovery reveals novel hypothalamic pathophysiology, which is reminiscent of diabetic retinopathy and suggests a potential functional involvement of the hypothalamic vasculature in the later stage pathogenesis of metabolic syndrome.

Control of metabolism by nutrient-regulated nuclear receptors acting in the brain

Today, we are witnessing a rising incidence of obesity worldwide. This increase is due to a sedentary life style, an increased caloric intake and a decrease in physical activity. Obesity contributes to the appearance of type 2 diabetes, dyslipidemia and cardiovascular complications due to atherosclerosis, and nephropathy. Therefore, the development of new therapeutic strategies may become a necessity. Given the metabolism controlling properties of nuclear receptors in peripheral organs (such as liver, adipose tissues, pancreas) and their implication in various processes underlying metabolic diseases, they constitute interesting therapeutic targets for obesity, dyslipidemia, cardiovascular disease and type 2 diabetes. The recent identification of the central nervous system as a player in the control of peripheral metabolism opens new avenues to our understanding of the pathophysiology of obesity and type 2 diabetes and potential novel ways to treat these diseases. While the metabolic functions of nuclear receptors in peripheral organs have been extensively investigated, little is known about their functions in the brain, in particular with respect to brain control of energy homeostasis. This review provides an overview of the relationships between nuclear receptors in the brain, mainly at the hypothalamic level, and the central regulation of energy homeostasis. In this context, we will particularly focus on the role of PPARα, PPARγ, LXR and Rev-erbα.

Novel pharmacological approaches to the treatment of type 2 diabetes

The huge increase in type 2 diabetes is a burden worldwide. Many marketed compounds do not address relevant aspects of the disease; they may already compensate for defects in insulin secretion and insulin action, but loss of secreting cells (β-cell destruction), hyperglucagonemia, gastric emptying, enzyme activation/inhibition in insulin-sensitive cells, substitution or antagonizing of physiological hormones and pathways, finally leading to secondary complications of diabetes, are not sufficiently addressed. In addition, side effects for established therapies such as hypoglycemias and weight gain have to be diminished. At present, nearly 1000 compounds have been described, and approximately 180 of these are going to be developed (already in clinical studies), some of them directly influencing enzyme activity, influencing pathophysiological pathways, and some using G-protein-coupled receptors. In addition, immunological approaches and antisense strategies are going to be developed. Many compounds are derived from physiological compounds (hormones) aiming at improving their kinetics and selectivity, and others are chemical compounds that were obtained by screening for a newly identified target in the physiological or pathophysiological machinery. In some areas, great progress is observed (e.g., incretin area); in others, no great progress is obvious (e.g., glucokinase activators), and other areas are not recommended for further research. For all scientific areas, conclusions with respect to their impact on diabetes are given. Potential targets for which no chemical compound has yet been identified as a ligand (agonist or antagonist) are also described.

Insulin in central nervous system: more than just a peripheral hormone

Insulin signaling in central nervous system (CNS) has emerged as a novel field of research since decreased brain insulin levels and/or signaling were associated to impaired learning, memory, and age-related neurodegenerative diseases. Thus, besides its well-known role in longevity, insulin may constitute a promising therapy against diabetes- and age-related neurodegenerative disorders. More interestingly, insulin has been also faced as the potential missing link between diabetes and aging in CNS, with Alzheimer's disease (AD) considered as the "brain-type diabetes." In fact, brain insulin has been shown to regulate both peripheral and central glucose metabolism, neurotransmission, learning, and memory and to be neuroprotective. And a future challenge will be to unravel the complex interactions between aging and diabetes, which, we believe, will allow the development of efficient preventive and therapeutic strategies to overcome age-related diseases and to prolong human "healthy" longevity. Herewith, we aim to integrate the metabolic, neuromodulatory, and neuroprotective roles of insulin in two age-related pathologies: diabetes and AD, both in terms of intracellular signaling and potential therapeutic approach.

Central prolactin modulates insulin sensitivity and insulin secretion in diabetic rats

BACKGROUND

Prolactin secretion is self-regulating as it acts upon hypothalamic dopaminergic systems which inhibit prolactin release from the anterior pituitary. Circulating prolactin improves glucose homeostasis by increasing insulin action and secretion, but central prolactin effects on glucose homeostasis have not been examined. Here, we determined that chronic central infusion of prolactin modulates insulin resistance and β-cell function and mass in 90% of pancreatectomized diabetic male rats.

METHODS

Diabetic rats were divided into three groups according to the dose of intracerebroventricular infusion of prolactin during 4 weeks: (1) low-dose prolactin (Low-PRL; 0.1 µg/h), (2) high-dose prolactin (High-PRL; 1 µg/h) and (3) vehicle only (cerebrospinal fluid). Nondiabetic rats were centrally infused with the vehicle.

RESULTS

Chronic intracerebroventricular infusion of Low-PRL lowered body weight and epididymal fat pads by increasing hypothalamic dopamine levels that reduced serum prolactin levels and potentiated leptin signaling. However, High-PRL slightly exacerbated energy dysregulation, decreased hypothalamic dopamine levels, and elevated serum prolactin levels. Both dosages promoted β-cell mass but in a different manner: Low-PRL decreased β-cell apoptosis, whereas High-PRL increased its proliferation. However, only Low-PRL enhanced first-phase insulin secretion and improved insulin sensitivity at a hyperglycemic state in comparison to the control. Low-PRL also increased glucose infusion rates and decreased hepatic glucose output in hyperinsulinemic states, signifying an improvement in hepatic insulin sensitivity. However, High-PRL exacerbated hepatic insulin resistance compared with the control diabetic rats.

CONCLUSIONS

In contrast to the exacerbation of insulin resistance caused by High-PRL, Low-PRL may improve energy and glucose metabolism by increasing hypothalamic dopamine levels in diabetic rats.

Regulation of gonadotropin-releasing hormone neurons by glucose

Reproduction is influenced by energy balance, but the physiological pathways mediating their relationship have not been fully elucidated. As the central regulators of fertility, gonadotropin-releasing hormone (GnRH) neurons integrate numerous physiological signals, including metabolic cues. Circulating glucose levels regulate GnRH release and might in part mediate the effects of negative energy balance on fertility. Existing evidence suggests that neural pathways originating in the hindbrain, as well as in the hypothalamic feeding nuclei, transmit information concerning glucose availability to GnRH neurons. Here we review recent evidence suggesting that GnRH neurons might directly sense changes in glucose availability by a mechanism involving AMP-activated protein kinase. These findings expand our understanding of how metabolic signaling in the brain regulates reproduction.