Key role for AMP-activated protein kinase in the ventromedial hypothalamus in regulating counterregulatory hormone responses to acute hypoglycemia

Glucose transporter-2 regulation of VMN GABA neuron metabolic sensor and transmitter gene expression

Glucose transporter-2 (GLUT2) monitors cellular glucose uptake. Astrocyte GLUT2 controls glucose counterregulatory hormone secretion. In vivo gene silencing and laser-catapult-microdissection tools were used here to investigate whether ventromedial hypothalamic nucleus (VMN) GLUT2 may regulate dorsomedial (VMNdm) and/or ventrolateral (VMNvl) γ-aminobutyric acid (GABA) neurotransmission to control this endocrine outflow in female rats. VMN GLUT2 gene knockdown suppressed or stimulated hypoglycemia-associated glutamate decarboxylase (GAD)1 and GAD2 mRNA expression in VMNdm versus VMNvl GABAergic neurons, respectively. GLUT2 siRNA pretreatment also modified co-expressed transmitter marker gene profiles in each cell population. VMNdm GABA neurons exhibited GLUT2 knockdown-sensitive up-regulated 5'-AMP-activated protein kinase-alpha1 (AMPKα1) and -alpha2 (AMPKα2) transcripts during hypoglycemia. Hypoglycemic augmentation of VMNvl GABA neuron AMPKα2 was refractory to GLUT2 siRNA. GLUT2 siRNA blunted (VMNdm) or exacerbated (VMNvl) hypoglycemic stimulation of GABAergic neuron steroidogenic factor-1 (SF-1) mRNA. Results infer that VMNdm and VMNvl GABA neurons may exhibit divergent, GLUT2-dependent GABA neurotransmission patterns in the hypoglycemic female rat. Data also document differential GLUT2 regulation of VMNdm versus VMNvl GABA nerve cell SF-1 gene expression. Evidence for intensification of hypoglycemic hypercorticosteronemia and -glucagonemia by GLUT2 siRNA infers that VMN GLUT2 function imposes an inhibitory tone on these hormone profiles in this sex.

Molecular mechanisms linking type 2 diabetes mellitus and late-onset Alzheimer's disease: A systematic review and qualitative meta-analysis

Research evidence indicating common metabolic mechanisms through which type 2 diabetes mellitus (T2DM) increases risk of late-onset Alzheimer's dementia (LOAD) has accumulated over recent decades. The aim of this systematic review is to provide a comprehensive review of common mechanisms, which have hitherto been discussed in separate perspectives, and to assemble and evaluate candidate loci and epigenetic modifications contributing to polygenic risk linkages between T2DM and LOAD. For the systematic review on pathophysiological mechanisms, both human and animal studies up to December 2023 are included. For the qualitative meta-analysis of genomic bases, human association studies were examined; for epigenetic mechanisms, data from human studies and animal models were accepted. Papers describing pathophysiological studies were identified in databases, and further literature gathered from cited work. For genomic and epigenomic studies, literature mining was conducted by formalised search codes using Boolean operators in search engines, and augmented by GeneRif citations in Entrez Gene, and other sources (WikiGenes, etc.). For the systematic review of pathophysiological mechanisms, 923 publications were evaluated, and 138 gene loci extracted for testing candidate risk linkages. 3 57 publications were evaluated for genomic association and descriptions of epigenomic modifications. Overall accumulated results highlight insulin signalling, inflammation and inflammasome pathways, proteolysis, gluconeogenesis and glycolysis, glycosylation, lipoprotein metabolism and oxidation, cell cycle regulation or survival, autophagic-lysosomal pathways, and energy. Documented findings suggest interplay between brain insulin resistance, neuroinflammation, insult compensatory mechanisms, and peripheral metabolic dysregulation in T2DM and LOAD linkage. The results allow for more streamlined longitudinal studies of T2DM-LOAD risk linkages.

Sex-Dimorphic Effects of Hypoglycemia on Metabolic Sensor mRNA Expression in Ventromedial Hypothalamic Nucleus-Dorsomedial Division (VMNdm) Growth Hormone-Releasing Hormone Neurons

Growth hormone-releasing hormone (Ghrh) neurons in the dorsomedial ventromedial hypothalamic nucleus (VMNdm) express the metabolic transcription factor steroidogenic factor-1 and hypoglycemia-sensitive neurochemicals of diverse chemical structures, transmission modes, and temporal signaling profiles. Ghrh imposes neuromodulatory control of coexpressed transmitters. Multiple metabolic sensory mechanisms are employed in the brain, including screening of the critical nutrient glucose or the energy currency ATP. Here, combinatory laser-catapult-microdissection/single-cell multiplex qPCR tools were used to investigate whether these neurons possess molecular machinery for monitoring cellular metabolic status and if these biomarkers exhibit sex-specific sensitivity to insulin-induced hypoglycemia. Data show that hypoglycemia up- (male) or downregulated (female) Ghrh neuron glucokinase (Gck) mRNA; Ghrh gene silencing decreased baseline and hypoglycemic patterns of Gck gene expression in each sex. Ghrh neuron glucokinase regulatory protein (Gckr) transcript levels were respectively diminished or augmented in hypoglycemic male vs female rats; this mRNA profile was decreased by Ghrh siRNA in both sexes. Gene transcripts encoding catalytic alpha subunits of the energy monitor 5-AMP-activated protein kinase (AMPK), i.e., Prkaa1 and 2, were increased by hypoglycemia in males, yet only the former mRNA was hypoglycemia-sensitive in females. Ghrh siRNA downregulated baseline and hypoglycemia-associated Prkaa subunit mRNAs in males but elicited divergent changes in Prkaa2 transcripts in eu- vs hypoglycemic females. Results provide unique evidence that VMNdm Ghrh neurons express the characterized metabolic sensor biomarkers glucokinase and AMPK and that the corresponding gene profiles exhibit distinctive sex-dimorphic transcriptional responses to hypoglycemia. Data further document Ghrh neuromodulation of baseline and hypoglycemic transcription patterns of these metabolic gene profiles.

Brain Regulation of Cardiac Function during Hypoglycemia

Hypoglycemia occurs frequently in people with type 1 and type 2 diabetes. Hypoglycemia activates the counter-regulatory response. Besides peripheral glucose sensors located in the pancreas, mouth, gastrointestinal tract, portal vein, and carotid body, many brain regions also contain glucose-sensing neurons that detect this fall in glucose. The autonomic nervous system innervates the heart, and during hypoglycemia, can cause many changes. Clinical and animal studies have revealed changes in electrocardiograms during hypoglycemia. Cardiac repolarization defects (QTc prolongation) occur during moderate levels of hypoglycemia. When hypoglycemia is severe, it can be fatal. Cardiac arrhythmias are thought to be the major mediator of sudden death due to severe hypoglycemia. Both the sympathetic and parasympathetic nervous systems of the brain have been implicated in regulating these arrhythmias. Besides cardiac arrhythmias, hypoglycemia can have profound changes in the heart and most of these changes are exacerbated in the setting of diabetes. A better understanding of how the brain regulates cardiac changes during hypoglycemia will allow for better therapeutic intervention to prevent cardiovascular death associated with hypoglycemia in people with diabetes. The aim of this paper is to provide a narrative review of what is known in the field regarding how the brain regulates the heart during hypoglycemia.

FGF4 alleviates hyperglycemia in diabetes and obesity conditions

Increasing evidence suggests that the brain plays a key role in glucose homeostasis, making it a potential target for the treatment of type 2 diabetes (T2D). Sun et al. recently reported that intracerebroventricular (ICV) administration of a single dose of fibroblast growth factor 4 (FGF4) can induce sustained T2D remission in mouse models in the absence of any risk of hypoglycemia.

Differential G Protein-Coupled Estrogen Receptor-1 Regulation of Counter-Regulatory Transmitter Marker and 5'-AMP-Activated Protein Kinase Expression in Ventrolateral versus Dorsomedial Ventromedial Hypothalamic Nucleus

INTRODUCTION

The ventromedial hypothalamic nucleus (VMN) is an estrogen receptor (ER)-rich structure that regulates glucostasis. The role of nuclear but not membrane G protein-coupled ER-1 (GPER) in that function has been studied.

METHODS

Gene silencing and laser-catapult microdissection/immunoblot tools were used to examine whether GPER regulates transmitter and energy sensor function in dorsomedial (VMNdm) and/or ventrolateral (VMNvl) VMN counter-regulatory nitrergic and γ-Aminobutyric acid (GABA) neurons.

RESULTS

Intra-VMN GPER siRNA administration to euglycemic animals did not affect VMNdm or -vl nitrergic neuron nitric oxide synthase (nNOS), but upregulated (VMNdm) or lacked influence on (VMNvl) GABA nerve cell glutamate decarboxylase65/67 (GAD) protein. Insulin-induced hypoglycemia (IIH) caused GPER knockdown-reversible augmentation of nNOS, 5'-AMP-activated protein kinase (AMPK), and phospho-AMPK proteins in nitrergic neurons in both divisions. IIH had dissimilar effects on VMNvl (unchanged) versus VMNdm (increased) GABAergic neuron GAD levels, yet GPER knockdown affected these profiles. GPER siRNA prevented hypoglycemic upregulation of VMNvl and -dm GABA neuron AMPK without altering pAMPK expression.

CONCLUSIONS

Outcomes infer that GPER exerts differential control of VMNdm versus -vl GABA transmission during glucostasis and is required for hypoglycemic upregulated nitrergic (VMNdm and -vl) and GABA (VMNdm) signaling. Glycogen metabolism is reported to regulate VMN nNOS and GAD proteins. Data show that GPER limits VMNvl glycogen phosphorylase (GP) protein expression and glycogen buildup during euglycemia but mediates hypoglycemic augmentation of VMNvl GP protein and glycogen content; VMNdm glycogen mass is refractory to GPER control. GPER regulation of VMNvl glycogen metabolism infers that this receptor may govern local counter-regulatory transmission in part by astrocyte metabolic coupling.

Sex-dimorphic hindbrain lactate regulation of ventromedial hypothalamic nucleus glucoregulatory neuron 5'-AMP-activated protein kinase activity and transmitter marker protein expression

BACKGROUND

The oxidizable glycolytic end-product L-lactate is a gauge of nerve cell metabolic fuel stability that metabolic-sensory hindbrain A2 noradrenergic neurons impart to the brain glucose-regulatory network. Current research investigated the premise that hindbrain lactate deficiency exerts sex-specific control of energy sensor and transmitter marker protein responses to hypoglycemia in ventromedial hypothalamic nucleus (VMN) glucose-regulatory nitrergic and γ-aminobutyric acid (GABA) neurons.

METHODS

Nitric oxide synthase (nNOS)- or glutamate decarboxylase65/67 (GAD)-immunoreactive neurons were laser-catapult-microdissected from male and female rat VMN after subcutaneous insulin injection and caudal fourth ventricular L-lactate or vehicle infusion for Western blot protein analysis.

RESULTS

Hindbrain lactate repletion reversed hypoglycemia-associated augmentation (males) or inhibition (females) of nitrergic neuron nNOS expression, and prevented up-regulation of phosphorylated AMPK 5'-AMP-activated protein kinase (pAMPK) expression in those neurons. Hypoglycemic suppression of GABAergic neuron GAD protein was averted by exogenous lactate over the rostro-caudal length of the male VMN and in the middle region of the female VMN. Lactate normalized GABA neuron pAMPK profiles in hypoglycemic male (caudal VMN) and female (all VMN segments) rats. Hypoglycemic patterns of norepinephrine (NE) signaling were lactate-dependent throughout the male VMN, but confined to the rostral and middle female VMN.

CONCLUSIONS

Results document, in each sex, regional VMN glucose-regulatory transmitter responses to hypoglycemia that are controlled by hindbrain lactate status. Hindbrain metabolic-sensory regulation of hypoglycemia-correlated nitric oxide or GABA release may entail AMPK-dependent mechanisms in specific VMN rostro-caudal segments in each sex. Additional effort is required to examine the role of hindbrain lactoprivic-sensitive VMN neurotransmitters in lactate-mediated attenuation of hypoglycemic hyperglucagonemia and hypercorticosteronemia in male and female rats.

Sustained remission of type 2 diabetes in rodents by centrally administered fibroblast growth factor 4

Type 2 diabetes (T2D) is a major health and economic burden worldwide. Despite the availability of multiple drugs for short-term management, sustained remission of T2D is currently not achievable pharmacologically. Intracerebroventricular administration of fibroblast growth factor 1 (icvFGF1) induces sustained remission in T2D rodents, propelling intense research efforts to understand its mechanism of action. Whether other FGFs possess similar therapeutic benefits is currently unknown. Here, we show that icvFGF4 also elicits a sustained antidiabetic effect in both male db/db mice and diet-induced obese mice by activating FGF receptor 1 (FGFR1) expressed in glucose-sensing neurons within the mediobasal hypothalamus. Specifically, FGF4 excites glucose-excited (GE) neurons while inhibiting glucose-inhibited (GI) neurons. Moreover, icvFGF4 restores the percentage of GI neurons in db/db mice. Importantly, intranasal delivery of FGF4 alleviates hyperglycemia in db/db mice, paving the way for non-invasive therapy. We conclude that icvFGF4 holds significant therapeutic potential for achieving sustained remission of T2D.

Sex-Dimorphic Octadecaneuropeptide (ODN) Regulation of Ventromedial Hypothalamic Nucleus Glucoregulatory Neuron Function and Counterregulatory Hormone Secretion

Central endozepinergic signaling is implicated in glucose homeostasis. Ventromedial hypothalamic nucleus (VMN) metabolic monitoring governs glucose counter-regulation. VMN glucose-stimulatory nitric oxide (NO) and glucose-inhibitory γ-aminobutyric acid (GABA) neurons express the energy gauge 5'-AMP-activated protein kinase (AMPK). Current research addresses the premise that the astrocyte glio-peptide octadecaneuropeptide (ODN) imposes sex-dimorphic control of metabolic sensor activity and neurotransmitter signaling in these neurons. The ODN G-protein coupled-receptor antagonist cyclo(1-8)[DLeu5]OP (LV-1075) was administered intracerebroventricularly (icv) to euglycemic rats of each sex; additional groups were pretreated icv with the ODN isoactive surrogate ODN11-18 (OP) before insulin-induced hypoglycemia. Western blotting of laser-catapult-microdissected VMN NO and GABA neurons showed that hypoglycemia caused OP-reversible augmentation of phospho-, e.g., activated AMPK and nitric oxide synthase (nNOS) expression in rostral (female) or middle (male) VMN segments or ODN-dependent suppression of nNOS in male caudal VMN. OP prevented hypoglycemic down-regulation of glutamate decarboxylase profiles in female rat rostral VMN, without affecting AMPK activity. LV-1075 treatment of male, not female rats elevated plasma glucagon and corticosterone concentrations. Moreover, OP attenuated hypoglycemia-associated augmentation of these hormones in males only. Results identify, for each sex, regional VMN metabolic transmitter signals that are subject to endozepinergic regulation. Directional shifts and gain-or-loss of ODN control during eu- versus hypoglycemia infer that VMN neuron receptivity to or post-receptor processing of this stimulus may be modulated by energy state. In male, counter-regulatory hormone secretion may be governed principally by ODN-sensitive neural pathways, whereas this endocrine outflow may be controlled by parallel, redundant ODN-dependent and -independent mechanisms in female.

Diazepam Binding Inhibitor Control of Eu- and Hypoglycemic Patterns of Ventromedial Hypothalamic Nucleus Glucose-Regulatory Signaling

Pharmacological stimulation/antagonism of astrocyte glio-peptide octadecaneuropeptide signaling alters ventromedial hypothalamic nucleus (VMN) counterregulatory γ-aminobutyric acid (GABA) and nitric oxide transmission. The current research used newly developed capillary zone electrophoresis-mass spectrometry methods to investigate hypoglycemia effects on VMN octadecaneuropeptide content, along with gene knockdown tools to determine if octadecaneuropeptide signaling regulates these transmitters during eu- and/or hypoglycemia. Hypoglycemia caused dissimilar adjustments in the octadecaneuropeptide precursor, i.e., diazepam-binding-inhibitor and octadecaneuropeptide levels in dorsomedial versus ventrolateral VMN. Intra-VMN diazepam-binding-inhibitor siRNA administration decreased baseline 67 and 65 kDa glutamate decarboxylase mRNA levels in GABAergic neurons laser-microdissected from each location, but only affected hypoglycemic transcript expression in ventrolateral VMN. This knockdown therapy imposed dissimilar effects on eu- and hypoglycemic glucokinase and 5'-AMP-activated protein kinase-alpha1 (AMPKα1) and -alpha2 (AMPKα2) gene profiles in dorsomedial versus ventrolateral GABAergic neurons. Diazepam-binding-inhibitor gene silencing up-regulated baseline (dorsomedial) or hypoglycemic (ventrolateral) nitrergic neuron neuronal nitric oxide synthase mRNA profiles. Baseline nitrergic cell glucokinase mRNA was up- (ventrolateral) or down- (dorsomedial) regulated by diazepam-binding-inhibitor siRNA, but knockdown enhanced hypoglycemic profiles in both sites. Nitrergic nerve cell AMPKα1 and -α2 transcripts exhibited division-specific responses to this genetic manipulation during eu- and hypoglycemia. Results document the utility of capillary zone electrophoresis-mass spectrometric tools for quantification of ODN in small-volume brain tissue samples. Data show that hypoglycemia has dissimilar effects on ODN signaling in the two major neuroanatomical divisions of the VMN and that this glio-peptide imposes differential control of glucose-regulatory neurotransmission in the VMNdm versus VMNvl during eu- and hypoglycemia.

G protein-coupled lactate receptor GPR81 control of ventrolateral ventromedial hypothalamic nucleus glucoregulatory neurotransmitter and 5'-AMP-activated protein kinase expression

Astrocytes store glycogen as energy and promote neurometabolic stability through supply of oxidizable l-lactate. Whether lactate regulates ventromedial hypothalamic nucleus (VMN) glucostatic function as a metabolic volume transmitter is unknown. Current research investigated whether G protein-coupled lactate receptor GPR81 controls astrocyte glycogen metabolism and glucose-regulatory neurotransmission in the ventrolateral VMN (VMNvl), where glucose-regulatory neurons reside. Female rats were pretreated by intra-VMN GPR81 or scramble siRNA infusion before insulin or vehicle injection. VMNvl cell or tissue samples were acquired by laser-catapult- or micropunch microdissection for Western blot protein or uHPLC-electrospray ionization-mass spectrometric glycogen analyses. Data show that GPR81 regulates eu- and/or hypoglycemic patterns of VMNvl astrocyte glycogen metabolic enzyme and 5'-AMP-activated protein kinase (AMPK) protein expression according to VMNvl segment. GPR81 stimulates baseline rostral and caudal VMNvl glycogen accumulation but mediates glycogen breakdown in the former site during hypoglycemia. During euglycemia, GPR81 suppresses the transmitter marker neuronal nitric oxide synthase (nNOS) in rostral and caudal VMNvl nitrergic neurons, but stimulates (rostral VMNvl) or inhibits (caudal VMNvl) GABAergic neuron glutamate decarboxylase65/67 (GAD)protein. During hypoglycemia, GPR81 regulates AMPK activation in nitrergic and GABAergic neurons located in the rostral, but not caudal VMNvl. VMN GPR81 knockdown amplified hypoglycemic hypercorticosteronemia, but not hyperglucagonemia. Results provide novel evidence that VMNvl astrocyte and glucose-regulatory neurons express GPR81 protein. Data identify neuroanatomical subpopulations of VMNvl astrocytes and glucose-regulatory neurons that exhibit differential reactivity to GPR81 input. Heterogeneous GPR81 effects during eu- versus hypoglycemia infer that energy state may affect cellular sensitivity to or postreceptor processing of lactate transmitter signaling.

Remembrance of things past: The consequences of recurrent hypoglycaemia in diabetes

AIMS

People with type 1 and type 2 diabetes still frequently experience hypoglycaemia, which can be severe, leading to loss of consciousness. This review will examine the cellular consequences of recurrent hypoglycaemia.

METHODS

This review, based on the Dorothy Hodgkin Lecture given at the Diabetes UK 2022 annual symposium by the author, will discuss our current understanding of the mechanisms by which hypoglycaemia is detected and the consequences of recurrent exposure to hypoglycaemia.

RESULTS

Glucose-responsive cells found in the periphery as well as multiple areas of the brain are organised in a classical sensori-motor integrative network encompassing peripheral, hindbrain and hypothalamic components. The mechanism used by glucose-responsive neurons to detect hypoglycaemia parallel those of the classical glucose sensor the pancreatic ß-cell, namely in their use of glucokinase, K_ATP channels and AMP-activated protein kinase. Recurrent exposure to hypoglycaemia results in a series of cellular adaptations that may be designed to increase the resilience of cells to future hypoglycaemia. This review also highlights how hypoglycaemia, as an oxidative stressor, may also exacerbate chronic hyperglycaemia-induced increases in oxidative stress and inflammation, leading to damage to vulnerable brain regions.

CONCLUSIONS

Impaired awareness of hypoglycaemia follows the adaptation of central glucose-responsive neurons to repeated hypoglycaemia and may represent a form of memory called habituation. In diabetes, recurrent hypoglycaemia may have tissue consequences as a result of a profound disruption in the cellular response to a hypoglycaemic challenge that increases vulnerability to oxidative damage.

Singular versus combinatory glucose-sensitive signal control of metabolic sensor protein profiles in hypothalamic astrocyte cultures from each sex

Brain metabolic-sensory targets for modulatory glucose-sensitive endocrine and neurochemical signals remain unidentified. A hypothalamic astrocyte primary culture model was here used to investigate whether glucocorticoid receptor (GR) and noradrenergic signals regulate astrocyte glucose (glucose transporter-2 [GLUT2], glucokinase) and/or energy (5′-AMP-activated protein kinase [AMPK]) sensor reactivity to glucoprivation by sex. Glucose-supplied astrocytes of each sex showed increased GLUT2 expression after incubation with the GR agonist dexamethasone (DEX) or norepinephrine (NE); DEX plus NE (DEX/NE) augmented GLUT2 in the female, but not in male. Glucoprivation did not alter GLUT2 expression, but eliminated NE regulation of this protein in both sexes. Male and female astrocyte glucokinase profiles were refractory to all drug treatments, but were down-regulated by glucoprivation. Glucoprivation altered AMPK expression in male only, and caused divergent sex-specific changes in activated, i.e., phosphoAMPK (pAMPK) levels. DEX or DEX/NE inhibited (male) or stimulated (female) AMPK and pAMPK proteins in both glucose-supplied and -deprived astrocytes. In male, NE coincidently up-regulated AMPK and inhibited pAMPK profiles in glucose-supplied astrocytes; these effects were abolished by glucoprivation. In female, AMPK profiles were unaffected by NE irrespective of glucose status, whereas pAMPK expression was up-regulated by NE only during glucoprivation. Present outcomes document, for each sex, effects of glucose status on hypothalamic astrocyte glucokinase, AMPK, and pAMPK protein expression and on noradrenergic control of these profiles. Data also show that DEX and NE regulation of GLUT2 is sex-monomorphic, but both stimuli impose divergent sex-specific effects on AMPK and pAMPK. Further effort is warranted to characterize mechanisms responsible for sex-dimorphic GR and noradrenergic governance of hypothalamic astrocyte energy sensory function.

Modulation of hypothalamic AMPK and hypothalamic neuropeptides in the control of eating behavior: A systematic review

Eating behavior is regulated by central and peripheral signals, which interact to modulate the response to nutrient intake. Central control is mediated by the hypothalamus through neuropeptides that activate the orexigenic and anorexigenic pathways. Energy homeostasis depends on the efficiency of these regulatory mechanisms. This neuroendocrine regulation of hunger and appetite can be modulated by nutritional sensors such as adenosine monophosphate-activated protein kinase (AMPK). Thus, this systematic review discusses the literature on correlations between AMPK and hypothalamic neuropeptides regarding control of eating behavior. Lilacs, PubMed/Medline, ScienceDirect, and Web of Science were searched for articles published from 2009 to 2021 containing combinations of the following descriptors: "eating behavior," "hypothalamus," "neuropeptide," and "AMPK." Of the 1330 articles found initially, 27 were selected after application of the inclusion and exclusion criteria. Of the selected articles, 15 reported decreased AMPK activity, due to interventions using angiotensin II infusion, fructose, glucose, cholecystokinin, leptin, or lipopolysaccharide (LPS) injection; dietary control through a low-protein diet or a high-fat diet (60 % fat); induction of hyperthyroidism; or injection of AMPK inhibitors. Seven studies showed a decrease in neuropeptide Y (NPY) through CV4 AICAR administration; fructose, glucose, leptin, or angiotensin II injections; or infusion of LPS from Escherichia coli and liver kinase B1 (LKB1) overexpression. Eleven studies reported a decrease in food consumption due to a decrease in AMPK activity and/or hypothalamic neuropeptides such as NPY. The results indicate that there is a relationship between AMPK and the control of eating behavior: a decrease in AMPK activity due to a dietary or non-dietary stimulus is associated with a consequent decrease in food intake. Furthermore, AMPK activity can be modulated by glucose, thyroid hormones, estradiol, leptin, and ghrelin.

Signaling pathways in obesity: mechanisms and therapeutic interventions

Obesity is a complex, chronic disease and global public health challenge. Characterized by excessive fat accumulation in the body, obesity sharply increases the risk of several diseases, such as type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease, and is linked to lower life expectancy. Although lifestyle intervention (diet and exercise) has remarkable effects on weight management, achieving long-term success at weight loss is extremely challenging, and the prevalence of obesity continues to rise worldwide. Over the past decades, the pathophysiology of obesity has been extensively investigated, and an increasing number of signal transduction pathways have been implicated in obesity, making it possible to fight obesity in a more effective and precise way. In this review, we summarize recent advances in the pathogenesis of obesity from both experimental and clinical studies, focusing on signaling pathways and their roles in the regulation of food intake, glucose homeostasis, adipogenesis, thermogenesis, and chronic inflammation. We also discuss the current anti-obesity drugs, as well as weight loss compounds in clinical trials, that target these signals. The evolving knowledge of signaling transduction may shed light on the future direction of obesity research, as we move into a new era of precision medicine.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Using Intermittent Fasting as a Non-pharmacological Strategy to Alleviate Obesity-Induced Hypothalamic Molecular Pathway Disruption

Intermittent fasting (IF) is a popular intervention used to fight overweight/obesity. This condition is accompanied by hypothalamic inflammation, limiting the proper signaling of molecular pathways, with consequent dysregulation of food intake and energy homeostasis. This mini-review explored the therapeutic modulation potential of IF regarding the disruption of these molecular pathways. IF seems to modulate inflammatory pathways in the brain, which may also be correlated with the brain-microbiota axis, improving hypothalamic signaling of leptin and insulin, and inducing the autophagic pathway in hypothalamic neurons, contributing to weight loss in obesity. Evidence also suggests that when an IF protocol is performed without respecting the circadian cycle, it can lead to dysregulation in the expression of circadian cycle regulatory genes, with potential health damage. In conclusion, IF may have the potential to be an adjuvant treatment to improve the reestablishment of hypothalamic responses in obesity.

Hypothalamic Irak4 is a genetically-controlled regulator of hypoglycemia-induced glucagon secretion

OBJECTIVES

Glucagon secretion to stimulate hepatic glucose production is a first line of defense against hypoglycemia. This response is triggered by so far incompletely characterized central hypoglycemia sensing mechanisms, which control autonomous nervous activity and hormone secretion. The objective of this study was to identify novel hypothalamic genes controlling insulin-induced glucagon secretion.

METHODS

To obtain new information about the mechanisms of hypothalamic hypoglycemia sensing, we combined genetic and transcriptomic analysis of the glucagon response to insulin-induced hypoglycemia in a panel of BXD recombinant inbred mice.

RESULTS

We identified two QTLs, on chromosome 8 and chromosome 15. We further investigated the role of Irak4 and Cpne8, both located in the chromosome 15 QTL, in C57BL/6J and DBA/2J mice, the BXD mouse parental strains. We found that the poor glucagon response of DBA/2J mice was associated with higher hypothalamic expression of Irak4, which encodes a kinase acting downstream of the interleukin-1 receptor (Il-1R), and of Il-ß when compared to C57BL/6J mice. We showed that intracerebroventricular administration of an Il-1R antagonist in DBA/2J restored insulin-induced glucagon secretion; this was associated with increased c-fos expression in the arcuate and paraventricular nuclei of the hypothalamus and with higher activation of both branches of the autonomous nervous system. Whole body inactivation of Cpne8, which encodes a Ca⁺⁺-dependent regulator of membrane trafficking and exocytosis had, however, no impact on insulin-induced glucagon secretion.

CONCLUSIONS

Collectively, our data identify Irak4 as a genetically controlled regulator of hypoglycemia-activated hypothalamic neurons and glucagon secretion.

Attenuation of the Counter-Regulatory Glucose Response in CVLM C1 Neurons: A Possible Explanation for Anorexia of Aging

This study aimed to determine the effect of age on CVLM C1 neuron glucoregulatory proteins in the feeding pathway. Male Sprague Dawley rats aged 3 months and 24 months old were divided into two subgroups: the treatment group with 2-deoxy-d-glucose (2DG) and the control group. Rat brains were dissected to obtain the CVLM region of the brainstem. Western blot was used to determine protein expression of tyrosine hydroxylase (TH), phosphorylated TH at Serine40 (pSer40TH), AMP-activated protein kinase (AMPK), phosphorylated AMPK (phospho AMPK), and neuropeptide Y Y5 receptors (NPY5R) in CVLM samples. Immunofluorescence was used to determine TH-, AMPK-, and NPY5R-like immunoreactivities among other brain coronal sections. Results obtained denote a decrease in basal TH phosphorylation levels and AMPK proteins and an increase in TH proteins among aged CVLM neurons. Increases in the basal immunoreactivity of TH+, AMPK+, NPY5R+, TH+/AMPK+, and TH+/NPY5R+ were also observed among old rats. Young treatment-group rats saw a decrease in TH phosphorylation and AMPK proteins following 2DG administration, while an increase in AMPK phosphorylation and a decrease in TH proteins were found among the old-treatment-group rats. These findings suggest the participation of CVLM C1 neurons in counter-regulatory responses among young and old rats. Altering protein changes in aged CVLM C1 neurons may attenuate responses to glucoprivation, thus explaining the decline in food intake among the elderly.

Hypoglycaemia: Still the main drawback of insulin 100 years on: "From man to mouse"

One hundred years on from the initial discovery of insulin, we take this opportunity to reflect on the scientific discoveries that have improved so many lives. From its original crude form, insulin therapy has improved significantly over the past century. Despite this, hypoglycaemia remains an ever-present fear for people with Type 1 diabetes. As such, it is essential that research now looks to minimise the frequency and severity of insulin-induced hypoglycaemia and its complications, some of which can be life-threatening. Over the last century, one thing that has become apparent is the success and need for translational diabetes research. From its origin in dogs, insulin treatment has revolutionised the lives of those with Type 1 diabetes through the coordinated effort of scientists and clinicians. In this review, we recount the more recent research that uses a mouse-to-man approach, specifically in hypoglycaemia research.

Glycogen Phosphorylase Isoform Regulation of Ventromedial Hypothalamic Nucleus Gluco-Regulatory Neuron 5'-AMP-Activated Protein Kinase and Transmitter Marker Protein Expression

Brain glycogen is remodeled during metabolic homeostasis and provides oxidizable L-lactate equivalents. Brain glycogen phosphorylase (GP)-brain (GPbb; AMP-sensitive) and -muscle (GPmm; norepinephrine-sensitive) type isoforms facilitate stimulus-specific control of glycogen disassembly. Here, a whole animal model involving stereotactic-targeted delivery of GPmm or GPbb siRNA to the ventromedial hypothalamic nucleus (VMN) was used to investigate the premise that these variants impose differential control of gluco-regulatory transmission. Intra-VMN GPmm or GPbb siRNA administration inhibited glutamate decarboxylate_65/67 (GAD), a protein marker for the gluco-inhibitory transmitter γ--aminobutyric acid (GABA), in the caudal VMN. GPbb knockdown, respectively overturned or exacerbated hypoglycemia-associated GAD suppression in rostral and caudal VMN. GPmm siRNA caused a segment-specific reversal of hypoglycemic augmentation of the gluco-stimulatory transmitter indicator, neuronal nitric oxide synthase (nNOS). In both cell types, GP siRNA down-regulated 5′-AMP-activated protein kinase (AMPK) during euglycemia, but hypoglycemic suppression of AMPK was reversed by GPmm targeting. GP knockdown elevated baseline GABA neuron phosphoAMPK (pAMKP) content, and amplified hypoglycemic augmentation of pAMPK expression in each neuron type. GPbb knockdown increased corticosterone secretion in eu- and hypoglycemic rats. Outcomes validate efficacy of GP siRNA delivery for manipulation of glycogen breakdown in discrete brain structures in vivo, and document VMN GPbb control of local GPmm expression. Results document GPmm and/or -bb regulation of GABAergic and nitrergic transmission in discrete rostro-caudal VMN segments. Contrary effects of glycogenolysis on metabolic-sensory AMPK protein during eu- versus hypoglycemia may reflect energy state-specific astrocyte signaling. Amplifying effects of GPbb knockdown on hypoglycemic stimulation of pAMPK infer that glycogen mobilization by GPbb limits neuronal energy instability during hypoglycemia.

Ventrolateral ventromedial hypothalamic nucleus GABA neuron adaptation to recurring Hypoglycemia correlates with up-regulated 5'-AMP-activated protein kinase activity

Gamma-aminobutyric acid (GABA) acts on ventromedial hypothalamic targets to suppress counter-regulatory hormone release, thereby lowering blood glucose. Maladaptive up-regulation of GABA signaling is implicated in impaired counter-regulatory outflow during recurring insulin-induced hypoglycemia (RIIH). Ventromedial hypothalamic nucleus (VMN) GABAergic neurons express the sensitive energy gauge 5'-AMP-activated protein kinase (AMPK). Current research used high-neuroanatomical resolution single-cell microdissection tools to address the premise that GABAergic cells in the VMNvl, the primary location of 'glucose-excited' metabolic-sensory neurons in the VMN, exhibit attenuated sensor activation during RIIH. Data show that during acute hypoglycemia, VMNvl glutamate decarboxylase65/67 (GAD)-immunoreactive neurons maintain energy stability, yet a regional subset of this population exhibited decreased GAD content. GABA neurons located along the rostrocaudal length of the VMNvl acclimated to RIIH through a shift to negative energy imbalance, e.g. increased phosphoAMPK expression, alongside amplification/gain of inhibition of GAD profiles. Acquisition of negative GAD sensitivity may involve altered cellular receptivity to noradrenergic input via α2-AR and/or β1-AR. Suppression of VMNvl GABA nerve cell signaling during RIIH may differentiate this neuroanatomical population from other, possibly non-metabolic-sensory GABA neurons in the MBH. Data here also provide novel evidence that VMNvl GABA neurons are direct targets of glucocorticoid control, and show that glucocorticoid receptors may inhibit RIIH-associated GAD expression in rostral VMNvl GABAergic cells through AMPK-independent mechanisms.

Ventromedial hypothalamic nucleus glycogen regulation of metabolic-sensory neuron AMPK and neurotransmitter expression: role of lactate

Astrocyte glycogen is dynamically remodeled during metabolic stability and provides oxidizable l-lactate equivalents during neuroglucopenia. Current research investigated the hypothesis that ventromedial hypothalamic nucleus (VMN) glycogen metabolism controls glucostimulatory nitric oxide (NO) and/or glucoinhibitory gamma-aminobutyric acid (GABA) neuron 5'-AMP-activated protein kinase (AMPK) and transmitter marker, e.g., neuronal nitric oxide synthase (nNOS), and glutamate decarboxylase65/67 (GAD) protein expression. Adult ovariectomized estradiol-implanted female rats were injected into the VMN with the glycogen phosphorylase inhibitor 1,4-dideoxy-1,4-imino-d-arabinitol (DAB) before vehicle or l-lactate infusion. Western blot analysis of laser-catapult-microdissected nitrergic and GABAergic neurons showed that DAB caused lactate-reversible upregulation of nNOS and GAD proteins. DAB suppressed or increased total AMPK content of NO and GABA neurons, respectively, by lactate-independent mechanisms, but lactate prevented drug enhancement of pAMPK expression in nitrergic neurons. Inhibition of VMN glycogen disassembly caused divergent changes in counter-regulatory hormone, e.g. corticosterone (increased) and glucagon (decreased) secretion. Outcomes show that VMN glycogen metabolism controls local glucoregulatory transmission by means of lactate signal volume. Results implicate glycogen-derived lactate deficiency as a physiological stimulus of corticosterone release. Concurrent normalization of nitrergic neuron nNOS and pAMPK protein and corticosterone secretory response to DAB by lactate infers that the hypothalamic-pituitary-adrenal axis may be activated by VMN NO-mediated signals of cellular energy imbalance.

Liver glycogen-induced enhancements in hypoglycemic counterregulation require neuroglucopenia

Iatrogenic hypoglycemia is a prominent barrier to achieving optimal glycemic control in patients with diabetes, in part due to dampened counterregulatory hormone responses. It has been demonstrated that elevated liver glycogen content can enhance these hormonal responses through signaling to the brain via afferent nerves, but the role that hypoglycemia in the brain plays in this liver glycogen effect remains unclear. During the first 4 h of each study, the liver glycogen content of dogs was increased by using an intraportal infusion of fructose to stimulate hepatic glucose uptake (HG; n = 13), or glycogen was maintained near fasting levels with a saline infusion (NG; n = 6). After a 2-h control period, during which the fructose/saline infusion was discontinued, insulin was infused intravenously for an additional 2 h to bring about systemic hypoglycemia in all animals, whereas brain euglycemia was maintained in a subset of the HG group by infusing glucose bilaterally into the carotid and vertebral arteries (HG-HeadEu; n = 7). Liver glycogen content was markedly elevated in the two HG groups (43 ± 4, 73 ± 3, and 75 ± 7 mg/g in NG, HG, and HG-HeadEu, respectively). During the hypoglycemic period, arterial plasma glucose levels were indistinguishable between groups (53 ± 2, 52 ± 1, and 51 ± 1 mg/dL, respectively), but jugular vein glucose levels were kept euglycemic (88 ± 5 mg/dL) only in the HG-HeadEu group. Glucagon and epinephrine responses to hypoglycemia were higher in HG compared with NG, whereas despite the increase in liver glycogen, neither increased above basal in HG-HeadEu. These data demonstrate that the enhanced counterregulatory hormone secretion that accompanies increased liver glycogen content requires hypoglycemia in the brain.NEW & NOTEWORTHY It is well known that iatrogenic hypoglycemia is a barrier to optimal glycemic regulation in patients with diabetes. Our data confirm that increasing liver glycogen content 75% above fasting levels enhances hormonal responses to insulin-induced hypoglycemia and demonstrate that this enhanced hormonal response does not occur in the absence of hypoglycemia in the brain. These data demonstrate that information from the liver regarding glycogen availability is integrated in the brain to optimize the counterregulatory response.

Insulin actions on hypothalamic glucose-sensing neurones

Subsequent to the discovery of insulin 100 years ago, great strides have been made in understanding its function, especially in the brain. It is now clear that insulin is a critical regulator of the neuronal circuitry controlling energy balance and glucose homeostasis. This review focuses on the effects of insulin and diabetes on the activity and glucose sensitivity of hypothalamic glucose-sensing neurones. We highlight the role of electrophysiological data in understanding how insulin regulates glucose-sensing neurones. A brief introduction describing the benefits and limitations of the major electrophysiological techniques used to investigate glucose-sensing neurones is provided. The mechanisms by which hypothalamic neurones sense glucose are discussed with an emphasis on those glucose-sensing neurones already shown to be modulated by insulin. Next, the literature pertaining to how insulin alters the activity and glucose sensitivity of these hypothalamic glucose-sensing neurones is described. In addition, the effects of impaired insulin signalling during diabetes and the ramifications of insulin-induced hypoglycaemia on hypothalamic glucose-sensing neurones are covered. To the extent that it is known, we present hypotheses concerning the mechanisms underlying the effects of these insulin-related pathologies. To conclude, electrophysiological data from the hippocampus are evaluated aiming to provide clues regarding how insulin might influence neuronal plasticity in glucose-sensing neurones. Although much has been accomplished subsequent to the discovery of insulin, the work described in our review suggests that the regulation of central glucose sensing by this hormone is both important and understudied.

Sex differences in ventromedial hypothalamic nucleus glucoregulatory transmitter biomarker protein during recurring insulin-induced hypoglycemia

Recurring insulin-induced hypoglycemia (RIIH) in males correlates with maladaptive glucose counter-regulatory collapse and acclimated expression of ventromedial hypothalamic nucleus (VMN) nitric oxide (NO) and γ-aminobutyric acid (GABA) metabolic transmitter biomarkers, e.g., neuronal nitric oxide synthase (nNOS) and glutamate decarboxylase_65/67 (GAD). Hindbrain noradrenergic neurons innervate the VMN, where norepinephrine regulates nNOS and GAD expression. Current research investigated the hypothesis that antecedent hypoglycemia (AH) exposure causes sex-dimorphic habituation of VMN glucoregulatory biomarker proteins between and/or during serial hypoglycemic bouts, and that hindbrain catecholaminergic (CA) signaling may control sex-specific adaptation of one or more of these proteins. Data show that upon recovery from AH, females exhibit CA-mediated reductions in baseline VMN nNOS, GAD, steroidogenic factor-1 (SF-1), and brain-derived neurotrophic factor (BNDF) expression compared to euglycemic profiles. In males, however, AH caused 6-OHDA-insensitive suppression of only basal SF-1 levels in the VMN. VMN transmitter protein acclimation to RIIH was sex-contingent, as differential nNOS, GAD, SF-1, and BDNF responses to a single vs final bout of hypoglycemia occur in males, whereas females show acclimated reactivity of GAD and SF-1 only to renewed hypoglycemia. CA-mediated and -independent habituation of distinctive VMN protein profiles occurred in each sex. Further research is necessary to evaluate, in each sex, effects of altered baseline VMN metabolic neurotransmitter signals on glucose homeostasis as well as non-metabolic functions under the control of those neurochemicals. It would also be insightful to learn if and how sex-contingent habituation of VMN transmitter responses to hypoglycemia contribute to sex-dimorphic patterns of glucose counter-regulation during RIIH.

Neuroestradiol regulation of ventromedial hypothalamic nucleus 5'-AMP-activated protein kinase activity and counterregulatory hormone secretion in hypoglycemic male versus female rats

Hypoglycemia activates the ultra-sensitive energy gauge 5′-AMP-activated protein kinase (AMPK) in ventromedial hypothalamic nucleus (VMN) gluco-regulatory neurons. The VMN is exemplified by high levels of expression of the enzyme aromatase, which converts testosterone to estradiol. This study examined the hypothesis that neuroestradiol imposes sex-dimorphic control of VMN AMPK activity during eu- and/or hypoglycemia. VMN tissue corresponding to distinct rostro-caudal segments was obtained by micropunch dissection from testes-intact male and estradiol-replaced ovariectomized female rats that were infused intracerebroventricularly with the aromatase inhibitor letrozole (Lz) before subcutaneous insulin (INS) injection. In euglycemic rats, Lz treatment elevated (male) or decreased (female) middle VMN phosphoAMPK content, with concurrent effects on total AMPK expression. Lz prevented hypoglycemic up-regulation of the mean pAMPK/AMPK ratio in rostral and middle segments of the male VMN, and significantly inhibited this proportion throughout the VMN of hypoglycemic female rats. Lz prevented glucagon secretion in hypoglycemic rats of each sex, and abolished hypoglycemic hypercorticosteronemia in males. Results show that neuroestradiol regulation of VMN AMPK activity during euglycemia is region-specific and gender-divergent, e.g. inhibitory in males versus stimulatory in females. Intra-VMN distribution of hypoglycemia-activated AMPK varies between sexes, but in each sex, locally-generated estradiol is critical for sensor reactivity to this stimulus. Coincident Lz attenuation of VMN AMPK and counter-regulatory hormone responses to hypoglycemia infers a possible cause-and-effect association. Further effort is needed to elucidate the cellular and molecular mechanisms that underlie sex-dimorphic neuroestradiol control of VMN total AMPK and phosphoAMPK expression during distinct metabolic states.

Brain-Body Control of Glucose Homeostasis-Insights From Model Organisms

Tight regulation of blood glucose is essential for long term health. Blood glucose levels are defended by the correct function of, and communication between, internal organs including the gastrointestinal tract, pancreas, liver, and brain. Critically, the brain is sensitive to acute changes in blood glucose level and can modulate peripheral processes to defend against these deviations. In this mini-review we highlight select key findings showcasing the utility, strengths, and limitations of model organisms to study brain-body interactions that sense and control blood glucose levels. First, we discuss the large platform of genetic tools available to investigators studying mice and how this field may yet reveal new modes of communication between peripheral organs and the brain. Second, we discuss how rats, by virtue of their size, have unique advantages for the study of CNS control of glucose homeostasis and note that they may more closely model some aspects of human (patho)physiology. Third, we discuss the nascent field of studying the CNS control of blood glucose in the zebrafish which permits ease of genetic modification, large-scale measurements of neural activity and live imaging in addition to high-throughput screening. Finally, we briefly discuss glucose homeostasis in drosophila, which have a distinct physiology and glucoregulatory systems to vertebrates.

Brain Permeable AMP-Activated Protein Kinase Activator R481 Raises Glycaemia by Autonomic Nervous System Activation and Amplifies the Counterregulatory Response to Hypoglycaemia in Rats

AIM

We evaluated the efficacy of a novel brain permeable "metformin-like" AMP-activated protein kinase activator, R481, in regulating glucose homeostasis.

MATERIALS AND METHODS

We used glucose sensing hypothalamic GT1-7 neuronal cells and pancreatic αTC1.9 α-cells to examine the effect of R481 on AMPK pathway activation and cellular metabolism. Glucose tolerance tests and hyperinsulinemic-euglycemic and hypoglycemic clamps were used in Sprague-Dawley rats to assess insulin sensitivity and hypoglycemia counterregulation, respectively.

RESULTS

In vitro, we demonstrate that R481 increased AMPK phosphorylation in GT1-7 and αTC1.9 cells. In Sprague-Dawley rats, R481 increased peak glucose levels during a glucose tolerance test, without altering insulin levels or glucose clearance. The effect of R481 to raise peak glucose levels was attenuated by allosteric brain permeable AMPK inhibitor SBI-0206965. This effect was also completely abolished by blockade of the autonomic nervous system using hexamethonium. During hypoglycemic clamp studies, R481 treated animals had a significantly lower glucose infusion rate compared to vehicle treated controls. Peak plasma glucagon levels were significantly higher in R481 treated rats with no change to plasma adrenaline levels. In vitro, R481 did not alter glucagon release from αTC1.9 cells, but increased glycolysis. Non brain permeable AMPK activator R419 enhanced AMPK activity in vitro in neuronal cells but did not alter glucose excursion in vivo.

CONCLUSIONS

These data demonstrate that peripheral administration of the brain permeable "metformin-like" AMPK activator R481 increases blood glucose by activation of the autonomic nervous system and amplifies the glucagon response to hypoglycemia in rats. Taken together, our data suggest that R481 amplifies the counterregulatory response to hypoglycemia by a central rather than a direct effect on the pancreatic α-cell. These data provide proof-of-concept that central AMPK could be a target for future drug development for prevention of hypoglycemia in diabetes.

Human torpor: translating insights from nature into manned deep space expedition

During a long-duration manned spaceflight mission, such as flying to Mars and beyond, all crew members will spend a long period in an independent spacecraft with closed-loop bioregenerative life-support systems. Saving resources and reducing medical risks, particularly in mental heath, are key technology gaps hampering human expedition into deep space. In the 1960s, several scientists proposed that an induced state of suppressed metabolism in humans, which mimics 'hibernation', could be an ideal solution to cope with many issues during spaceflight. In recent years, with the introduction of specific methods, it is becoming more feasible to induce an artificial hibernation-like state (synthetic torpor) in non-hibernating species. Natural torpor is a fascinating, yet enigmatic, physiological process in which metabolic rate (MR), body core temperature (T_b ) and behavioural activity are reduced to save energy during harsh seasonal conditions. It employs a complex central neural network to orchestrate a homeostatic state of hypometabolism, hypothermia and hypoactivity in response to environmental challenges. The anatomical and functional connections within the central nervous system (CNS) lie at the heart of controlling synthetic torpor. Although progress has been made, the precise mechanisms underlying the active regulation of the torpor-arousal transition, and their profound influence on neural function and behaviour, which are critical concerns for safe and reversible human torpor, remain poorly understood. In this review, we place particular emphasis on elaborating the central nervous mechanism orchestrating the torpor-arousal transition in both non-flying hibernating mammals and non-hibernating species, and aim to provide translational insights into long-duration manned spaceflight. In addition, identifying difficulties and challenges ahead will underscore important concerns in engineering synthetic torpor in humans. We believe that synthetic torpor may not be the only option for manned long-duration spaceflight, but it is the most achievable solution in the foreseeable future. Translating the available knowledge from natural torpor research will not only benefit manned spaceflight, but also many clinical settings attempting to manipulate energy metabolism and neurobehavioural functions.

Sex-dimorphic Rostro-caudal Patterns of 5'-AMP-activated Protein Kinase Activation and Glucoregulatory Transmitter Marker Protein Expression in the Ventrolateral Ventromedial Hypothalamic Nucleus (VMNvl) in Hypoglycemic Male and Female Rats: Impact of Estradiol

The ventromedial hypothalamic nucleus-ventrolateral part (VMNvl) is an estradiol-sensitive structure that controls sex-specific behavior. Electrical reactivity of VMNvl neurons to hypoglycemia infers that cellular energy stability is monitored there. Current research investigated the hypothesis that estradiol elicits sex-dimorphic patterns of VMNvl metabolic sensor activation and gluco-regulatory neurotransmission during hypoglycemia. Rostral-, middle-, and caudal-VMNvl tissue was separately micropunch-dissected from letrozole (Lz)- or vehicle-injected male and estradiol- or vehicle-implanted ovariectomized (OVX) female rats for Western blot analysis of total and phosphorylated 5'-AMP-activated protein kinase (AMPK) protein expression and gluco-stimulatory [neuronal nitric oxide synthase (nNOS); steroidogenic factor-1 (SF1) or -inhibitory (glutamate decarboxylase_65/67 (GAD)] transmitter marker proteins after sc insulin (INS) or vehicle injection. In both sexes, hypoglycemic up-regulation of phosphoAMPK was estradiol-dependent in rostral and middle, but not caudal VMNvl. AMPK activity remained elevated after recovery from hypoglycemia over the rostro-caudal VMNvl in female, but only in the rostral segment in male. In each sex, hypoglycemia correspondingly augmented or suppressed nNOS profiles in rostral and middle versus caudal VMNvl; these segmental responses persisted longer in female. Rostral and middle segment SF1 protein was inhibited by estradiol-independent mechanisms in hypoglycemic males, but increased by estradiol-reliant mechanisms in female. After INS injection, GAD expression was inhibited in the male rostral VMNvl without estradiol involvement, but this hormone was required for broader suppression of this profile in the female. Neuroanatomical variability of VMNvl metabolic transmitter reactivity to hypoglycemia underscores the existence of functionally different subgroups in that structure. The regional distribution and estradiol sensitivity of hypoglycemia-sensitive VMNvl neurons of each neurochemical phenotype evidently vary between sexes.

Hypoglycemia-Sensing Neurons of the Ventromedial Hypothalamus Require AMPK-Induced Txn2 Expression but Are Dispensable for Physiological Counterregulation

The ventromedial nucleus of the hypothalamus (VMN) is involved in the counterregulatory response to hypoglycemia. VMN neurons activated by hypoglycemia (glucose-inhibited [GI] neurons) have been assumed to play a critical although untested role in this response. Here, we show that expression of a dominant negative form of AMPK or inactivation of AMPK α1 and α2 subunit genes in Sf1 neurons of the VMN selectively suppressed GI neuron activity. We found that Txn2, encoding a mitochondrial redox enzyme, was strongly downregulated in the absence of AMPK activity and that reexpression of Txn2 in Sf1 neurons restored GI neuron activity. In cell lines, Txn2 was required to limit glucopenia-induced reactive oxygen species production. In physiological studies, absence of GI neuron activity after AMPK suppression in the VMN had no impact on the counterregulatory hormone response to hypoglycemia or on feeding. Thus, AMPK is required for GI neuron activity by controlling the expression of the antioxidant enzyme Txn2. However, the glucose-sensing capacity of VMN GI neurons is not required for the normal counterregulatory response to hypoglycemia. Instead, it may represent a fail-safe system in case of impaired hypoglycemia sensing by peripherally located glucose detection systems that are connected to the VMN.

Interaction of glucose sensing and leptin action in the brain

Background

In response to energy abundant or deprived conditions, nutrients and hormones activate hypothalamic pathways to maintain energy and glucose homeostasis. The underlying CNS mechanisms, however, remain elusive in rodents and humans.

Scope of review

Here, we first discuss brain glucose sensing mechanisms in the presence of a rise or fall of plasma glucose levels, and highlight defects in hypothalamic glucose sensing disrupt in vivo glucose homeostasis in high-fat fed, obese, and/or diabetic conditions. Second, we discuss brain leptin signalling pathways that impact glucose homeostasis in glucose-deprived and excessed conditions, and propose that leptin enhances hypothalamic glucose sensing and restores glucose homeostasis in short-term high-fat fed and/or uncontrolled diabetic conditions.

Major conclusions

In conclusion, we believe basic studies that investigate the interaction of glucose sensing and leptin action in the brain will address the translational impact of hypothalamic glucose sensing in diabetes and obesity.

Hypoglycemic thalamic activation in type 1 diabetes is associated with preserved symptoms despite reduced epinephrine

Brain responses to low plasma glucose may be key to understanding the behaviors that prevent severe hypoglycemia in type 1 diabetes. This study investigated the impact of long duration, hypoglycemia aware type 1 diabetes on cerebral blood flow responses to hypoglycemia. Three-dimensional pseudo-continuous arterial spin labeling magnetic resonance imaging was performed in 15 individuals with type 1 diabetes and 15 non-diabetic controls during a two-step hyperinsulinemic glucose clamp. Symptom, hormone, global cerebral blood flow and regional cerebral blood flow responses to hypoglycemia were measured. Epinephrine release during hypoglycemia was attenuated in type 1 diabetes, but symptom score rose comparably in both groups. A rise in global cerebral blood flow did not differ between groups. Regional cerebral blood flow increased in the thalamus and fell in the hippocampus and temporal cortex in both groups. Type 1 diabetes demonstrated lesser anterior cingulate cortex activation; however, this difference did not survive correction for multiple comparisons. Thalamic cerebral blood flow change correlated with autonomic symptoms, and anterior cingulate cortex cerebral blood flow change correlated with epinephrine response across groups. The thalamus may thus be involved in symptom responses to hypoglycemia, independent of epinephrine action, while anterior cingulate cortex activation may be linked to counterregulation. Activation of these regions may have a role in hypoglycemia awareness and avoidance of problematic hypoglycemia.

Norepinephrine Regulation of Ventromedial Hypothalamic Nucleus Metabolic-Sensory Neuron 5'-AMP-Activated Protein Kinase Activity: Impact of Estradiol

The mediobasal hypothalamus (MBH) shapes the neural regulation of glucostasis by 5′-AMP-activated protein kinase (AMPK)-dependent mechanisms. Yet, the neurochemical identity and neuroanatomical distribution of MBH neurons that express glucoprivic-sensitive AMPK remain unclear. The neurotransmitters γ-aminobutyric acid (GABA) and nitric oxide (NO) act within the MBH to correspondingly inhibit or stimulate glucose counter-regulation. The current review highlights recent findings that GABA and NO, neurons located in the ventromedial hypothalamic nucleus (VMN), a distinct important element of the MBH, are direct targets of noradrenergic regulatory signaling, and thereby, likely operate under the control of hindbrain metabolic-sensory neurons. The ovarian hormone estradiol acts within the VMN to govern energy homeostasis. Discussed here is current evidence that estradiol regulates GABA and NO nerve cell receptivity to norepinephrine and moreover, controls the noradrenergic regulation of AMPK activity in each cell type. Future gains in insight on mechanisms underpinning estradiol’s impact on neurotransmitter communication between the hindbrain and hypothalamic AMPKergic neurons are expected to disclose viable new molecular targets for the therapeutic simulation of hormonal enhancement of neuro-metabolic stability during circumstances of diminished endogenous estrogen secretion or glucose dysregulation.

AMPK in the Ventromedial Nucleus of the Hypothalamus: A Key Regulator for Thermogenesis

Obesity has become a global health issue, but effective therapies remain very limited. Adaptive thermogenesis promotes weight loss by dissipating energy in the form of heat, thereby representing a promising target to counteract obesity. Notably, the regulation of thermogenesis is tightly orchestrated by complex neuronal networks, especially those in the hypothalamus. Recent evidence highlights the importance of adenosine monophosphate-activated protein kinase (AMPK) within the ventromedial nucleus of the hypothalamus (VMH) in modulating thermogenesis. Various molecules, such as GLP-1, leptin, estradiol, and thyroid hormones, have been reported to act on the VMH to inhibit AMPK, which subsequently increases thermogenesis through the activation of the sympathetic nervous system (SNS). In this review, we summarize the critical role of AMPK within the VMH in the control of energy balance, focusing on its contribution to thermogenesis and the associated mechanisms.

Ventromedial hypothalamus glucose-inhibited neurones: A role in glucose and energy homeostasis?

The ventromedial hypothalamus (VMH) plays a complex role in glucose and energy homeostasis. The VMH is necessary for the counter-regulatory response to hypoglycaemia (CRR) that increases hepatic gluconeogenesis to restore euglycaemia. On the other hand, the VMH also restrains hepatic glucose production during euglycaemia and stimulates peripheral glucose uptake. The VMH is also important for the ability of oestrogen to increase energy expenditure. This latter function is mediated by VMH modulation of the lateral/perifornical hypothalamic area (lateral/perifornical hypothalamus) orexin neurones. Activation of VMH AMP-activated protein kinase (AMPK) is necessary for the CRR. By contrast, VMH AMPK inhibition favours decreased basal glucose levels and is required for oestrogen to increase energy expenditure. Specialised VMH glucose-sensing neurones confer the ability to sense and respond to changes in blood glucose levels. Glucose-excited (GE) neurones increase and glucose-inhibited (GI) neurones decrease their activity as glucose levels rise. VMH GI neurones, in particular, appear to be important in the CRR, although a role for GE neurones cannot be discounted. AMPK mediates glucose sensing in VMH GI neurones suggesting that, although activation of these neurones is important for the CRR, it is necessary to silence them to lower basal glucose levels and enable oestrogen to increase energy expenditure. In support of this, we found that oestrogen reduces activation of VMH GI neurones in low glucose by inhibiting AMPK. In this review, we present the evidence underlying the role of the VMH in glucose and energy homeostasis. We then discuss the role of VMH glucose-sensing neurones in mediating these effects, with a strong emphasis on oestrogenic regulation of glucose sensing and how this may affect glucose and energy homeostasis.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

Growth hormone enhances the recovery of hypoglycemia via ventromedial hypothalamic neurons

Growth hormone (GH) is secreted during hypoglycemia, and GH-responsive neurons are found in brain areas containing glucose-sensing neurons that regulate the counter-regulatory response (CRR). However, whether GH modulates the CRR to hypoglycemia via specific neuronal populations is currently unknown. Mice carrying ablation of GH receptor (GHR) either in leptin receptor (LepR)- or steroidogenic factor-1 (SF1)-expressing cells were studied. We also investigated the importance of signal transducer and activator of transcription 5 (STAT5) signaling in SF1 cells for the CRR. GHR ablation in LepR cells led to impaired capacity to recover from insulin-induced hypoglycemia and to a blunted CRR caused by 2-deoxy-d-glucose (2DG) administration. GHR inactivation in SF1 cells, which include ventromedial hypothalamic neurons, also attenuated the CRR. The reduced CRR was prevented by parasympathetic blockers. Additionally, infusion of 2DG produced an abnormal hyperactivity of parasympathetic preganglionic neurons, whereas the 2DG-induced activation of anterior bed nucleus of the stria terminalis neurons was reduced in mice without GHR in SF1 cells. Mice carrying ablation of Stat5a/b genes in SF1 cells showed no defects in the CRR. In summary, GHR expression in SF1 cells is required for a normal CRR, and these effects are largely independent of STAT5 pathway.-Furigo, I. C., de Souza, G. O., Teixeira, P. D. S., Guadagnini, D., Frazão, R., List, E. O., Kopchick, J. J., Prada, P. O., Donato, J., Jr. Growth hormone enhances the recovery of hypoglycemia via ventromedial hypothalamic neurons.

Central Mechanisms of Glucose Sensing and Counterregulation in Defense of Hypoglycemia

Glucose homeostasis requires an organism to rapidly respond to changes in plasma glucose concentrations. Iatrogenic hypoglycemia as a result of treatment with insulin or sulfonylureas is the most common cause of hypoglycemia in humans and is generally only seen in patients with diabetes who take these medications. The first response to a fall in glucose is the detection of impending hypoglycemia by hypoglycemia-detecting sensors, including glucose-sensing neurons in the hypothalamus and other regions. This detection is then linked to a series of neural and hormonal responses that serve to prevent the fall in blood glucose and restore euglycemia. In this review, we discuss the current state of knowledge about central glucose sensing and how detection of a fall in glucose leads to the stimulation of counterregulatory hormone and behavior responses. We also review how diabetes and recurrent hypoglycemia impact glucose sensing and counterregulation, leading to development of impaired awareness of hypoglycemia in diabetes.

Norepinephrine regulation of ventromedial hypothalamic nucleus metabolic transmitter biomarker and astrocyte enzyme and receptor expression: Impact of 5' AMP-activated protein kinase

The ventromedial hypothalamic energy sensor AMP-activated protein kinase (AMPK) maintains glucostasis via neurotransmitter signals that diminish [γ-aminobutyric acid] or enhance [nitric oxide] counter-regulation. Ventromedial hypothalamic nucleus (VMN) 'fuel-inhibited' neurons are sensitive to astrocyte-generated metabolic substrate stream. Norepinephrine (NE) regulates astrocyte glycogen metabolism in vitro, and hypoglycemia intensifies VMN NE activity in vivo. Current research investigated the premise that NE elicits AMPK-dependent adjustments in VMN astrocyte glycogen metabolic enzyme [glycogen synthase (GS); glycogen phosphorylase (GP)] and gluco-regulatory neuron biomarker [glutamate decarboxylase_65/67 (GAD); neuronal nitric oxide synthase (nNOS); SF-1] protein expression in male rats. We also examined whether VMN astrocytes are directly receptive to NE and if noradrenergic input regulates cellular sensitivity to the neuro-protective steroid estradiol. Intra-VMN NE correspondingly augmented or reduced VMN tissue GAD and nNOS protein despite no change in circulating glucose, data that imply that short-term exposure to NE promotes persistent improvement in VMN nerve cell energy stability. The AMPK inhibitor Compound C (Cc) normalized VMN nNOS, GS, and GP expression in NE-treated animals. NE caused AMPK-independent down-regulation of alpha₂-, alongside Cc-reversible augmentation of beta₁-adrenergic receptor protein profiles in laser-microdissected astrocytes. NE elicited divergent adjustments in astrocyte estrogen receptor-beta (AMPK-unrelated reduction) and GPR-30 (Cc-revocable increase) proteins. Outcomes implicate AMPK in noradrenergic diminution of VMN nitrergic metabolic-deficit signaling and astrocyte glycogen shunt activity. Differentiating NE effects on VMN astrocyte adrenergic and estrogen receptor variant expression suggest that noradrenergic regulation of glycogen metabolism may be mediated, in part, by one or more receptors characterized here by sensitivity to this catecholamine.

Adipose Tissue, Inter-Organ Communication, and the Path to Type 2 Diabetes: The 2016 Banting Medal for Scientific Achievement Lecture

My scientific career has focused on understanding the mechanisms underlying insulin resistance with the goal of developing new strategies to prevent and treat type 2 diabetes. My early studies focused on understanding how insulin promotes glucose transport into adipocytes, a classic model of highly insulin-responsive target cells. When we found changes in adipocyte glucose transport in altered metabolic states, we were highly motivated to understand the consequences of this on whole-body glucose homeostasis. In the late 1980s, when GLUT4, the major insulin-regulated glucose transporter, was identified, my lab observed that it was downregulated in adipocytes but not in skeletal muscle in insulin-resistant states, such as obesity and type 2 diabetes, in humans and rodents. We investigated the role of GLUT4 in adipose tissue and muscle in whole-body insulin sensitivity, making tissue-specific GLUT4-overexpressing and GLUT4 knockout mice. These studies led to the discovery that adipocytes, and specifically glucose transport into adipocytes, regulate whole-body glucose homeostasis. As adipocytes take up relatively little glucose, we investigated the underlying mechanisms. In the 1990s, we performed DNA microarrays on adipose tissue from adipose-specific GLUT4-overexpressing and GLUT4 knockout mice to find reciprocally regulated genes, and we identified several molecules that were not previously known to regulate systemic insulin sensitivity and/or energy balance. More recently, with Alan Saghatelian's lab, we discovered a novel class of lipids with antidiabetes and anti-inflammatory effects. We also investigated the effects of the adipose-secreted hormone, leptin, on insulin sensitivity. We found that the AMP-activated protein kinase (AMPK) pathway mediates leptin's effects on fatty acid oxidation in muscle and also plays a role in leptin's anorexigenic effects in the hypothalamus. These studies transformed AMPK from a "fuel gauge" that regulates energy supply at the cellular level to a sensing and signaling pathway that regulates organismal energy balance. Overall, these studies have expanded our understanding of the multifaceted role of adipose tissue in metabolic health and how adipose dysfunction increases the risk for type 2 diabetes.

Basal fatty acid oxidation increases after recurrent low glucose in human primary astrocytes

AIMS/HYPOTHESIS

Hypoglycaemia is a major barrier to good glucose control in type 1 diabetes. Frequent hypoglycaemic episodes impair awareness of subsequent hypoglycaemic bouts. Neural changes underpinning awareness of hypoglycaemia are poorly defined and molecular mechanisms by which glial cells contribute to hypoglycaemia sensing and glucose counterregulation require further investigation. The aim of the current study was to examine whether, and by what mechanism, human primary astrocyte (HPA) function was altered by acute and recurrent low glucose (RLG).

METHODS

To test whether glia, specifically astrocytes, could detect changes in glucose, we utilised HPA and U373 astrocytoma cells and exposed them to RLG in vitro. This allowed measurement, with high specificity and sensitivity, of RLG-associated changes in cellular metabolism. We examined changes in protein phosphorylation/expression using western blotting. Metabolic function was assessed using a Seahorse extracellular flux analyser. Immunofluorescent imaging was used to examine cell morphology and enzymatic assays were used to measure lactate release, glycogen content, intracellular ATP and nucleotide ratios.

RESULTS

AMP-activated protein kinase (AMPK) was activated over a pathophysiologically relevant glucose concentration range. RLG produced an increased dependency on fatty acid oxidation for basal mitochondrial metabolism and exhibited hallmarks of mitochondrial stress, including increased proton leak and reduced coupling efficiency. Relative to glucose availability, lactate release increased during low glucose but this was not modified by RLG. Basal glucose uptake was not modified by RLG and glycogen levels were similar in control and RLG-treated cells. Mitochondrial adaptations to RLG were partially recovered by maintaining euglycaemic levels of glucose following RLG exposure.

CONCLUSIONS/INTERPRETATION

Taken together, these data indicate that HPA mitochondria are altered following RLG, with a metabolic switch towards increased fatty acid oxidation, suggesting glial adaptations to RLG involve altered mitochondrial metabolism that could contribute to defective glucose counterregulation to hypoglycaemia in diabetes.

Hindbrain dorsal vagal complex AMPK controls hypothalamic gluco-regulatory transmitter and counter-regulatory hormone responses to hypoglycemia

Pharmacologic activation of the hindbrain dorsal vagal complex energy sensor 5'-adenosine monophosphate-activated protein kinase (AMPK) causes site-specific adjustments in hypothalamic AMPK activity. DVC A2 noradrenergic neurons are a likely source of metabolo-sensory cues to downstream network components as they express substrate fuel-sensitive AMPK. This study investigated the hypothesis that DVC AMPK controls hypothalamic sensor, metabolic effector transmitter, and counter-regulatory hormone responses to insulin-induced hypoglycemia. Male rats were injected into the caudal fourth ventricle with the AMPK inhibitor compound C (Ccor vehicle before hypoglycemia. Arcuate (ARH), ventromedial (VMN), and dorsomedial (DMN) nuclei and lateral hypothalamic area (LHA) were micropunch-dissected for norepinephrine ELISA and Western blot analyses. Hypoglycemic stimulation of norepinephrine activity in each site was impeded by compound C. Hypoglycemia caused drug-revocable (ARH) or -refractory (VMN, DMN) reductions in AMPK, alongside hindbrain AMPK-dependent augmentation of phospho-AMPK expression in each location. Compound C prevented hypoglycemic augmentation of gluco-stimulatory ARH neuropeptide Y, VMN neuronal nitric oxide synthase, and LHA orexin-A expression, while hypoglycemic suppression of the catabolic neuron protein markers ARH pro-opiomelanocortin and VMN glutamate decarboxylase_65/67 was respectively averted or unaffected by drug treatment. DMN RFamide-related peptide-1 and -3 profiles were correspondingly amplified or suppressed hindbrain AMPK-reliant mechanisms during hypoglycemia. Results show that DVC AMPK is required for hypoglycemic intensification of norepinephrine activity in characterized hypothalamic gluco-regulatory structures, and that this sensor regulates AMPK activation and metabolic effector transmission in those sites.

SF1-Specific AMPKα1 Deletion Protects Against Diet-Induced Obesity

AMPK is a cellular gauge that is activated under conditions of low energy, increasing energy production and reducing energy waste. Current evidence links hypothalamic AMPK with the central regulation of energy balance. However, it is unclear whether targeting hypothalamic AMPK has beneficial effects in obesity. Here, we show that genetic inhibition of AMPK in the ventromedial nucleus of the hypothalamus (VMH) protects against high-fat diet (HFD)-induced obesity by increasing brown adipose tissue (BAT) thermogenesis and subsequently energy expenditure. Notably, this effect depends upon the AMPKα1 isoform in steroidogenic factor 1 (SF1) neurons of the VMH, since mice bearing selective ablation of AMPKα1 in SF1 neurons display resistance to diet-induced obesity, increased BAT thermogenesis, browning of white adipose tissue, and improved glucose and lipid homeostasis. Overall, our findings point to hypothalamic AMPK in specific neuronal populations as a potential druggable target for the treatment of obesity and associated metabolic disorders.

Hypothalamic Control of Systemic Glucose Homeostasis: The Pancreas Connection

Maintenance of glucose homeostasis is mandatory for organismal survival. It is accomplished by complex and coordinated interplay between glucose detection mechanisms and multiple effector systems. The brain, in particular homeostatic regions such as the hypothalamus, plays a crucial role in orchestrating such a highly integral response. We review here current understanding of how the hypothalamus senses glucose availability and participates in systemic glucose homeostasis. We provide an update of the relevant signaling pathways and neuronal subsets involved, as well as of the mechanisms modulating metabolic processes in peripheral tissues such as liver, skeletal muscle, fat, and especially the pancreas. We also discuss the relevance of these networks in human biology and prevalent metabolic conditions such as diabetes and obesity.

Inhibition of glycogen phosphorylase stimulates ventromedial hypothalamic nucleus AMP-activated protein kinase: Activity and neuronal nitric oxide synthase protein expression in male rats

The glucose polymer glycogen is a vital fuel reserve in the brain. The mediobasal hypothalamic energy sensor AMP‐activated protein kinase (AMPK) maintains glucostasis via neurotransmitter mechanisms that suppress [γ‐aminobutyric acid; GABA] or stimulate [nitric oxide; steroidogenic factor‐1 (SF1)] counter‐regulatory outflow. This study investigated whether glycogen‐derived fuel supply is a critical screened variable in ventromedial hypothalamic nucleus (VMN) monitoring of neuro‐metabolic stability during glucostasis and/or insulin (I)‐induced hypoglycemia. Adult male rats were pretreated by intra‐VMN infusion of the glycogen phosphorylase inhibitor 1,4‐dideoxy‐1,4‐imino‐D‐arabinitol (DAB) before sc vehicle or I injection. Western blot analyses of micropunch‐dissected VMN tissue from euglycemic animals showed DAB augmentation of phosphoAMPK (pAMPK), neuronal nitric oxide synthase (nNOS), and SF‐1, but not glutamate decarboxylase_65/67 (GAD) protein. Combinatory DAB/I treatment did not further enhance AMPK activity but significantly amplified nNOS expression relative to DAB alone. Hypoglycemic stimulation of corticosterone, but not glucagon release was prevented by DAB. Results imply that glycogen‐derived substrate fuel provision represses VMN AMPK activity and neurotransmitter signals of metabolic deficiency. Progressive augmentation of nNOS protein by DAB/I versus DAB/V intimates that “fuel‐inhibited” nitrergic neurons may exhibit increasing sensitivity to disrupted glycogen breakdown during glucoprivation versus glucostasis. nNOS and GAD reactivity to DAB/I, but not I implies that acute glycogen utilization during hypoglycemia may be sufficiently robust to avert effects on local metabolic sensory signaling. DAB/I upregulation of GAD alongside prevention of hypercorticosteronemia suggests that indicators of metabolic sufficiency may occur secondary to local compensatory adaptations to severe restriction of glucose‐derived energy.

Central regulation of energy metabolism by estrogens

BACKGROUND

Estrogenic actions in the brain prevent obesity. Better understanding of the underlying mechanisms may facilitate development of new obesity therapies.

SCOPE OF REVIEW

This review focuses on the critical brain regions that mediate effects of estrogens on food intake and/or energy expenditure, the molecular signals that are involved, and the functional interactions between brain estrogens and other signals modulating metabolism. Body weight regulation by estrogens in male brains will also be discussed.

MAJOR CONCLUSIONS

17β-estradiol acts in the brain to regulate energy homeostasis in both sexes. It can inhibit feeding and stimulate brown adipose tissue thermogenesis. A better understanding of the central actions of 17β-estradiol on energy balance would provide new insight for the development of therapies against obesity in both sexes.

Impaired hypoglycaemia awareness in type 1 diabetes: lessons from the lab

Hypoglycaemia remains the most common metabolic adverse effect of insulin and sulfonylurea therapy in diabetes. Repeated exposure to hypoglycaemia leads to a change in the symptom complex that characterises hypoglycaemia, culminating in a clinical phenomenon referred to as impaired awareness of hypoglycaemia (IAH). IAH effects approximately 20-25% of people with type 1 diabetes and increases the risk of severe hypoglycaemia. This review focuses on the mechanisms that are responsible for the much higher frequency of hypoglycaemia in people with diabetes compared with those without, and subsequently how repeated exposure to hypoglycaemia leads to the development of IAH. The mechanisms that result in IAH development are incompletely understood and likely to reflect changes in multiple aspects of the counterregulatory response to hypoglycaemia, from adaptations within glucose and non-glucose-sensing cells to changes in the integrative networks that govern glucose homeostasis. Finally, we propose that the general process that incorporates many of these changes and results in IAH following recurrent hypoglycaemia is a form of adaptive memory called 'habituation'.