Convergence of pre- and postsynaptic influences on glucosensing neurons in the ventromedial hypothalamic nucleus

Pathophysiological significance and modulation of the transient receptor potential canonical 3 ion channel

Transient receptor potential canonical 3 (TRPC3) protein belongs to the TRP family of nonselective cation channels. Its activation occurs by signaling through a G protein-coupled receptor (GPCR) and a phospholipase C-dependent (PLC) pathway. Perturbations in the expression of TRPC3 are associated with a plethora of pathophysiological conditions responsible for disorders of the cardiovascular, immune, and central nervous systems. The recently solved cryo-EM structure of TRPC3 provides detailed inputs about the underlying mechanistic aspects of the channel, which in turn enables more efficient ways of designing small-molecule modulators. Pharmacologically targeting TRPC3 in animal models has demonstrated great efficacy in treating diseases including cancers, neurological disorders, and cardiovascular diseases. Despite extensive scientific evidence supporting some strong correlations between the expression and activity of TRPC3 and various pathophysiological conditions, therapeutic strategies based on its pharmacological modulations have not led to clinical trials. The development of small-molecule TRPC3 modulators with high safety, sufficient brain penetration, and acceptable drug-like profiles remains in progress. Determining the pathological mechanisms for TRPC3 involvement in human diseases and understanding the requirements for a drug-like TRPC3 modulator will be valuable in advancing small-molecule therapeutics to future clinical trials. In this review, we provide an overview of the origin and activation mechanism of TRPC3 channels, diseases associated with irregularities in their expression, and new development in small-molecule modulators as potential therapeutic interventions for treating TRPC3 channelopathies.

Environmental enrichment accentuates glucose-induced feeding suppression and glial cell line-derived neurotrophic factor gene expression in the hypothalamus of mice

The hypothalamus controls food intake by integrating nutrient signals, of which one of the most important is glucose. Consequently, impairments in hypothalamic glucose-sensing mechanisms are associated with hyperphagia and obesity. Environmental enrichment (EE) is an animal housing protocol that provides complex sensory, motor, and social stimulations and has been proven to reduce adiposity in laboratory mice. However, the mechanism by which EE promotes adiposity-suppressing effect remains incompletely understood. Neurotrophic factors play an important role in the development and maintenance of the nervous system, but they are also involved in the hypothalamic regulation of feeding. Brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) are expressed in the hypothalamus and their expression is stimulated by glucose. EE is associated with increased expression of Bdnf mRNA in the hypothalamus. Therefore, we hypothesized that EE potentiates the anorectic action of glucose by altering the expression of neurotrophic factor genes in the hypothalamus. Male C57BL/6 mice were maintained under standard or EE conditions to investigate the feeding response to glucose and the associated expression of feeding-related neurotrophic factor genes in the hypothalamus. Intraperitoneal glucose injection reduced food intake in both control and EE mice with a significantly greater reduction in the EE group compared to the control group. EE caused a significantly enhanced response of Gdnf mRNA expression to glucose without altering basal Gdnf mRNA expression and Bdnf mRNA response to glucose. These findings suggest that EE enhances glucose-induced feeding suppression, at least partly, by enhancing hypothalamic glucose-sensing ability that involves GDNF.

I like therefore I can, and I can therefore I like: the role of self-efficacy and affect in active inference of allostasis

Active inference (AIF) is a theory of the behavior of information-processing open dynamic systems. It describes them as generative models (GM) generating inferences on the causes of sensory input they receive from their environment. Based on these inferences, GMs generate predictions about sensory input. The discrepancy between a prediction and the actual input results in prediction error. GMs then execute action policies predicted to minimize the prediction error. The free-energy principle provides a rationale for AIF by stipulating that information-processing open systems must constantly minimize their free energy (through suppressing the cumulative prediction error) to avoid decay. The theory of homeostasis and allostasis has a similar logic. Homeostatic set points are expectations of living organisms. Discrepancies between set points and actual states generate stress. For optimal functioning, organisms avoid stress by preserving homeostasis. Theories of AIF and homeostasis have recently converged, with AIF providing a formal account for homeo- and allostasis. In this paper, we present bacterial chemotaxis as molecular AIF, where mutual constraints by extero- and interoception play an essential role in controlling bacterial behavior supporting homeostasis. Extending this insight to the brain, we propose a conceptual model of the brain homeostatic GM, in which we suggest partition of the brain GM into cognitive and physiological homeostatic GMs. We outline their mutual regulation as well as their integration based on the free-energy principle. From this analysis, affect and self-efficacy emerge as the main regulators of the cognitive homeostatic GM. We suggest fatigue and depression as target neurocognitive phenomena for studying the neural mechanisms of such regulation.

ADGRL1 is a glucose receptor involved in mediating energy and glucose homeostasis

AIMS/HYPOTHESIS

The brain is a major consumer of glucose as an energy source and regulates systemic glucose as well as energy balance. Although glucose transporters such as GLUT2 and sodium-glucose cotransporter 2 (SGLT2) are known to regulate glucose homeostasis and metabolism, the identity of a receptor that binds glucose to activate glucose signalling pathways in the brain is unknown. In this study, we aimed to discover a glucose receptor in the mouse hypothalamus.

METHODS

Here we used a high molecular mass glucose-biotin polymer to enrich glucose-bound mouse hypothalamic neurons through cell-based affinity chromatography. We then subjected the enriched neurons to proteomic analyses and identified adhesion G-protein coupled receptor 1 (ADGRL1) as a top candidate for a glucose receptor. We validated glucose-ADGRL1 interactions using CHO cells stably expressing human ADGRL1 and ligand-receptor binding assays. We generated and determined the phenotype of global Adgrl1-knockout mice and hypothalamus-specific Adgrl1-deficient mice. We measured the variables related to glucose and energy homeostasis in these mice. We also generated an Adgrl1Cre mouse model to investigate the role of ADGRL1 in sensing glucose using electrophysiology.

RESULTS

Adgrl1 is highly expressed in the ventromedial nucleus of the hypothalamus (VMH) in mice. Lack of Adgrl1 in the VMH in mice caused fasting hyperinsulinaemia, enhanced glucose-stimulated insulin secretion and insulin resistance. In addition, the Adgrl1-deficient mice had impaired feeding responses to glucose and fasting coupled with abnormal glucose sensing and decreased physical activity before development of obesity and hyperglycaemia. In female mice, ovariectomy was necessary to reveal the contribution of ADGRL1 to energy and glucose homeostasis.

CONCLUSIONS/INTERPRETATION

Altogether, our findings demonstrate that ADGRL1 binds glucose and is involved in energy as well as glucose homeostasis in a sex-dependent manner. Targeting ADGRL1 may introduce a new class of drugs for the treatment of type 2 diabetes and obesity.

Serotonergic neurons are involved in the counter-regulatory response to hypoglycemia

OBJECTIVES

Intensive insulin therapy provides optimal glycemic control in patients with diabetes. However, intensive insulin therapy causes so-called iatrogenic hypoglycemia as a major adverse effect. The ventromedial hypothalamus (VMH) has been described as the primary brain area initiating the counter-regulatory response (CRR). Nevertheless, the VMH receives projections from other brain areas which could participate in the regulation of the CRR. In particular, studies suggest a potential role of the serotonin (5-HT) network. Thus, the objective of this study was to determine the contribution of 5-HT neurons in CRR control.

METHODS

Complementary approaches have been used to test this hypothesis in quantifying the level of 5-HT in several brain areas by HPLC in response to insulin-induced hypoglycemia, measuring the electrical activity of dorsal raphe (DR) 5-HT neurons in response to insulin or decreased glucose level by patch-clamp electrophysiology; and measuring the CRR hormone glucagon as an index of the CRR to the modulation of the activity of 5-HT neurons using pharmacological or pharmacogenetic approaches.

RESULTS

HPLC measurements show that the 5HIAA/5HT ratio is increased in several brain regions including the VMH in response to insulin-induced hypoglycemia. Patch-clamp electrophysiological recordings show that insulin, but not decreased glucose level, increases the firing frequency of DR 5-HT neurons in the DR. In vivo, both the pharmacological inhibition of 5-HT neurons by intraperitoneal injection of the 5-HT1A receptor agonist 8-OH-DPAT or the chemogenetic inhibition of these neurons reduce glucagon secretion, suggesting an impaired CRR.

CONCLUSION

Taken together, these data highlight a new neuronal network involved in the regulation of the CRR. In particular, this study shows that DR 5-HT neurons detect iatrogenic hypoglycemia in response to the increased insulin level and may play an important role in the regulation of CRR.

History and Function of the Lactate Receptor GPR81/HCAR1 in the Brain: A Putative Therapeutic Target for the Treatment of Cerebral Ischemia

GPR81 is a G-protein coupled receptor (GPCR) discovered in 2001, but deorphanized only 7 years later, when its affinity for lactate as an endogenous ligand was demonstrated. More recently, GPR81 expression and distribution in the brain were also confirmed and the function of lactate as a volume transmitter has been suggested since then. These findings shed light on a new function of lactate acting as a signaling molecule in the central nervous system, in addition to its well-known role as a metabolic fuel for neurons. GPR81 seems to act as a metabolic sensor, coupling energy metabolism, synaptic activity, and blood flow. Activation of this receptor leads to Gi-mediated downregulation of adenylyl cyclase and subsequent reduction in cAMP levels, regulating several downstream pathways. Recent studies have also suggested the potential role of lactate as a neuroprotective agent, mainly under brain ischemic conditions. This effect is usually attributed to the metabolic role of lactate, but the underlying mechanisms need further investigation and could be related to lactate signaling via GPR81. The activation of GPR81 showed promising results for neuroprotection: it modulates many processes involved in the pathophysiology of ischemia. In this review, we summarize the history of GPR81, starting with its deorphanization; then, we discuss GPR81 expression and distribution, signaling transduction cascades, and neuroprotective roles. Lastly, we propose GPR81 as a potential target for the treatment of cerebral ischemia.

Neural Progenitor Cells and the Hypothalamus

Neural progenitor cells (NPCs) are multipotent neural stem cells (NSCs) capable of self-renewing and differentiating into neurons, astrocytes and oligodendrocytes. In the postnatal/adult brain, NPCs are primarily located in the subventricular zone (SVZ) of the lateral ventricles (LVs) and subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). There is evidence that NPCs are also present in the postnatal/adult hypothalamus, a highly conserved brain region involved in the regulation of core homeostatic processes, such as feeding, metabolism, reproduction, neuroendocrine integration and autonomic output. In the rodent postnatal/adult hypothalamus, NPCs mainly comprise different subtypes of tanycytes lining the wall of the 3^rd ventricle. In the postnatal/adult human hypothalamus, the neurogenic niche is constituted by tanycytes at the floor of the 3^rd ventricle, ependymal cells and ribbon cells (showing a gap-and-ribbon organization similar to that in the SVZ), as well as suprachiasmatic cells. We speculate that in the postnatal/adult human hypothalamus, neurogenesis occurs in a highly complex, exquisitely sophisticated neurogenic niche consisting of at least four subniches; this structure has a key role in the regulation of extrahypothalamic neurogenesis, and hypothalamic and extrahypothalamic neural circuits, partly through the release of neurotransmitters, neuropeptides, extracellular vesicles (EVs) and non-coding RNAs (ncRNAs).

Spectro-spatial features in distributed human intracranial activity proactively encode peripheral metabolic activity

Mounting evidence demonstrates that the central nervous system (CNS) orchestrates glucose homeostasis by sensing glucose and modulating peripheral metabolism. Glucose responsive neuronal populations have been identified in the hypothalamus and several corticolimbic regions. However, how these CNS gluco-regulatory regions modulate peripheral glucose levels is not well understood. To better understand this process, we simultaneously measured interstitial glucose concentrations and local field potentials in 3 human subjects from cortical and subcortical regions, including the hypothalamus in one subject. Correlations between high frequency activity (HFA, 70-170 Hz) and peripheral glucose levels are found across multiple brain regions, notably in the hypothalamus, with correlation magnitude modulated by sleep-wake cycles, circadian coupling, and hypothalamic connectivity. Correlations are further present between non-circadian (ultradian) HFA and glucose levels which are higher during awake periods. Spectro-spatial features of neural activity enable decoding of peripheral glucose levels both in the present and up to hours in the future. Our findings demonstrate proactive encoding of homeostatic glucose dynamics by the CNS.

Carotid body: an emerging target for cardiometabolic co-morbidities

NEW FINDINGS

What is the topic of this review? Regarding the global metabolic syndrome crisis, this review focuses on common mechanisms for high blood sugar and high blood pressure. Connections are made between the homeostatic regulation of blood pressure and blood sugar and their dysregulation to reveal signalling mechanisms converging on the carotid body. What advances does it highlight? The carotid body plays a major part in the generation of excessive sympathetic activity in diabetes and also underpins diabetic hypertension. As treatment of diabetic hypertension is notoriously difficult, we propose that novel receptors within the carotid body may provide a novel treatment strategy.

ABSTRACT

The maintenance of glucose homeostasis is obligatory for health and survival. It relies on peripheral glucose sensing and signalling between the brain and peripheral organs via hormonal and neural responses that restore euglycaemia. Failure of these mechanisms causes hyperglycaemia or diabetes. Current anti-diabetic medications control blood glucose but many patients remain with hyperglycemic condition. Diabetes is often associated with hypertension; the latter is more difficult to control in hyperglycaemic conditions. We ask whether a better understanding of the regulatory mechanisms of glucose control could improve treatment of both diabetes and hypertension when they co-exist. With the involvement of the carotid body (CB) in glucose sensing, metabolic regulation and control of sympathetic nerve activity, we consider the CB as a potential treatment target for both diabetes and hypertension. We provide an update on the role of the CB in glucose sensing and glucose homeostasis. Physiologically, hypoglycaemia stimulates the release of hormones such as glucagon and adrenaline, which mobilize or synthesize glucose; however, these counter-regulatory responses were markedly attenuated after denervation of the CBs in animals. Also, CB denervation prevents and reverses insulin resistance and glucose intolerance. We discuss the CB as a metabolic regulator (not just a sensor of blood gases) and consider recent evidence of novel 'metabolic' receptors within the CB and putative signalling peptides that may control glucose homeostasis via modulation of the sympathetic nervous system. The evidence presented may inform future clinical strategies in the treatment of patients with both diabetes and hypertension, which may include the CB.

In Vivo Photometry Reveals Insulin and 2-Deoxyglucose Maintain Prolonged Inhibition of VMH Vglut2 Neurons in Male Mice

The ventromedial hypothalamic (VMH) nucleus is a well-established hub for energy and glucose homeostasis. In particular, VMH neurons are thought to be important for initiating the counterregulatory response to hypoglycemia, and ex vivo electrophysiology and immunohistochemistry data indicate a clear role for VMH neurons in sensing glucose concentration. However, the temporal response of VMH neurons to physiologically relevant changes in glucose availability in vivo has been hampered by a lack of available tools for measuring neuronal activity over time. Since the majority of neurons within the VMH are glutamatergic and can be targeted using the vesicular glutamate transporter Vglut2, we expressed cre-dependent GCaMP7s in Vglut2 cre mice and examined the response profile of VMH to intraperitoneal injections of glucose, insulin, and 2-deoxyglucose (2DG). We show that reduced available glucose via insulin-induced hypoglycemia and 2DG-induced glucoprivation, but not hyperglycemia induced by glucose injection, inhibits VMH Vglut2 neuronal population activity in vivo. Surprisingly, this inhibition was maintained for at least 45 minutes despite prolonged hypoglycemia and initiation of a counterregulatory response. Thus, although VMH stimulation, via pharmacological, electrical, or optogenetic approaches, is sufficient to drive a counterregulatory response, our data suggest VMH Vglut2 neurons are not the main drivers required to do so, since VMH Vglut2 neuronal population activity remains suppressed during hypoglycemia and glucoprivation.

Glial cells as integrators of peripheral and central signals in the regulation of energy homeostasis

Communication between the periphery and the brain is key for maintaining energy homeostasis. To do so, peripheral signals from the circulation reach the brain via the circumventricular organs (CVOs), which are characterized by fenestrated vessels lacking the protective blood-brain barrier (BBB). Glial cells, by virtue of their plasticity and their ideal location at the interface of blood vessels and neurons, participate in the integration and transmission of peripheral information to neuronal networks in the brain for the neuroendocrine control of whole-body metabolism. Metabolic diseases, such as obesity and type 2 diabetes, can disrupt the brain-to-periphery communication mediated by glial cells, highlighting the relevance of these cell types in the pathophysiology of such complications. An improved understanding of how glial cells integrate and respond to metabolic and humoral signals has become a priority for the discovery of promising therapeutic strategies to treat metabolic disorders. This Review highlights the role of glial cells in the exchange of metabolic signals between the periphery and the brain that are relevant for the regulation of whole-body energy homeostasis.

Glucose-sensing glucagon-like peptide-1 receptor neurons in the dorsomedial hypothalamus regulate glucose metabolism

Glucagon-like peptide-1 (GLP-1) regulates energy homeostasis via activation of the GLP-1 receptors (GLP-1Rs) in the central nervous system. However, the mechanism by which the central GLP-1 signal controls blood glucose levels, especially in different nutrient states, remains unclear. Here, we defined a population of glucose-sensing GLP-1R neurons in the dorsomedial hypothalamic nucleus (DMH), by which endogenous GLP-1 decreases glucose levels via the cross-talk between the hypothalamus and pancreas. Specifically, we illustrated the sufficiency and necessity of DMHGLP-1R in glucose regulation. The activation of the DMHGLP-1R neurons is mediated by a cAMP-PKA-dependent inhibition of a delayed rectifier potassium current. We also dissected a descending control of DMHGLP-1R -dorsal motor nucleus of the vagus nerve (DMV)-pancreas activity that can regulate glucose levels by increasing insulin release. Thus, our results illustrate how central GLP-1 action in the DMH can induce a nutrient state-dependent reduction in blood glucose level.

The ventromedial hypothalamic nucleus: watchdog of whole-body glucose homeostasis

The brain, particularly the ventromedial hypothalamic nucleus (VMH), has been long known for its involvement in glucose sensing and whole-body glucose homeostasis. However, it is still not fully understood how the brain detects and responds to the changes in the circulating glucose levels, as well as brain-body coordinated control of glucose homeostasis. In this review, we address the growing evidence implicating the brain in glucose homeostasis, especially in the contexts of hypoglycemia and diabetes. In addition to neurons, we emphasize the potential roles played by non-neuronal cells, as well as extracellular matrix in the hypothalamus in whole-body glucose homeostasis. Further, we review the ionic mechanisms by which glucose-sensing neurons sense fluctuations of ambient glucose levels. We also introduce the significant implications of heterogeneous neurons in the VMH upon glucose sensing and whole-body glucose homeostasis, in which sex difference is also addressed. Meanwhile, research gaps have also been identified, which necessities further mechanistic studies in future.

The Solitary Nucleus Connectivity to Key Autonomic Regions in Humans MRI and Literature based Considerations

The nucleus tractus solitarius (NTS), is a key brainstem structure relaying interoceptive peripheral information to the interrelated brain centers for eliciting rapid autonomic responses and for shaping longer-term neuroendocrine and motor patterns. Structural and functional NTS' connectivity has been extensively investigated in laboratory animals. But there is limited information about NTS' connectome in humans. Using MRI, we examined diffusion and resting state data from 20 healthy participants in the Human Connectome Project. The regions within the brainstem (n=8), subcortical (n=6), cerebellar (n=2) and cortical (n=5) parts of the brain were selected via a systematic review of the literature and their white matter NTS connections were evaluated via probabilistic tractography along with functional and directional (i.e., Granger-causality) analyses. The underlying study confirms previous results from animal models and provides novel aspects on NTS integration in humans. Two key findings can be summarized: (i) the NTS predominantly processes afferent input and (ii) a lateralization towards a predominantly left-sided NTS processing. Our results lay the foundations for future investigations into the NTS' tripartite role comprised of interoreceptors' input integration, the resultant neurochemical outflow and cognitive/affective processing. The implications of these data add to the understanding of NTS' role in specific aspects of autonomic functions.

Spatial distribution of beta-klotho mRNA in the mouse hypothalamus, hippocampal region, subiculum, and amygdala

Beta-klotho (KLB) is a co-receptor required for endocrine fibroblast growth factor (FGF) 15/19 and FGF21 signaling in the brain. Klb is prominent within the hypothalamus, which is consistent with its metabolic functions, but diverse roles for Klb are now emerging. Central Klb expression is low but discrete and may govern FGF-targeted sites. However, given its low expression, it is unclear if Klb mRNA is more widespread. We performed in situ hybridization to label Klb mRNA to generate spatial maps capturing the distribution and level of Klb within the mouse hypothalamus, hippocampal region, subiculum, and amygdala. Semi-quantitative analysis revealed that Klb-labeled cells may express low, medium, or high levels of Klb mRNA. Hypothalamic Klb hybridization was heterogeneous and varied rostrocaudally within the same region. Most Klb-labeled cells were found in the lateral hypothalamic zone, but the periventricular hypothalamic region, including the suprachiasmatic nucleus, contained the greatest proportion of cells expressing medium or high Klb levels. We also found heterogeneous Klb hybridization in the amygdala and subiculum, where Klb was especially distinct within the central amygdalar nucleus and ventral subiculum, respectively. By contrast, Klb-labeled cells in the hippocampal region only expressed low levels of Klb and were typically found in the pyramidal layer of Ammon's horn or dentate gyrus. The Klb-labeled regions identified in this study are consistent with reported roles of Klb in metabolism, taste preference, and neuroprotection. However, additional identified sites, including within the hypothalamus and amygdala, may suggest novel roles for FGF15/19 or FGF21 signaling. The central expression of beta-klotho (Klb) is essential for the physiological actions of endocrine fibroblast growth factors. Klb mRNA was widely expressed throughout the hypothalamus, hippocampus, and amygdala. However, the level of Klb expression varied between cells and contributed to a distinctive pattern of distribution within each brain structure. This article is protected by copyright. All rights reserved.

Homeostatic Regulation of Glucose Metabolism by the Central Nervous System

Evidence for involvement of the central nervous system (CNS) in the regulation of glucose metabolism dates back to the 19th century, although the majority of the research on glucose metabolism has focused on the peripheral metabolic organs. Due to recent advances in neuroscience, it has now become clear that the CNS is indeed vital for maintaining glucose homeostasis. To achieve normoglycemia, specific populations of neurons and glia in the hypothalamus sense changes in the blood concentrations of glucose and of glucoregulatory hormones such as insulin, leptin, glucagon-like peptide 1, and glucagon. This information is integrated and transmitted to other areas of the brain where it eventually modulates various processes in glucose metabolism (i.e., hepatic glucose production, glucose uptake in the brown adipose tissue and skeletal muscle, pancreatic insulin and glucagon secretion, renal glucose reabsorption, etc.). Errors in these processes lead to hyper- or hypoglycemia. We here review the current understanding of the brain regulation of glucose metabolism.

Lactate activates hypothalamic POMC neurons by intercellular signaling

Previous studies indicate that the activity of hypothalamic POMC neurons can be regulated by glucose via intracellular mechanisms, but its regulation by lactate is poorly understood. In addition to its energetic role, lactate acts as a signaling molecule. In this study, we evaluated the function and location of the lactate receptor, hydroxycarboxylic acid receptor 1 (HCAR1). We used a conditional genetic approach to label POMC neurons and evaluated their sensitivity to lactate using patch-clamp recordings. l-Lactate and 3-chloro-5-hydroxybenzoic acid (3Cl-HBA), HCAR1 specific agonist depolarized POMC neurons and the increase in excitability was abolished by pertussis toxin (PTX), indicating the involvement of Gαi/o-protein-coupled receptors. In addition, the depolarization of a subset of POMC neurons was sensitive to α-cyano-4-hydroxycinnamate (4-CIN), a lactate transporter blocker, suggesting that the depolarization induced by l-lactate can also occur by direct intracellular action. Surprisingly, HCAR1 was not detected in POMC neurons, but instead localized in astrocytes. These results suggest a new lactate-mediated mechanism for astrocyte-neuron intercellular communication.

Ventromedial Nucleus of the Hypothalamus Neurons Under the Magnifying Glass

The ventromedial nucleus of the hypothalamus (VMH) is a complex brain structure that is integral to many neuroendocrine functions, including glucose regulation, thermogenesis, and appetitive, social, and sexual behaviors. As such, it is of little surprise that the nucleus is under intensive investigation to decipher the mechanisms which underlie these diverse roles. Developments in genetic and investigative tools, for example the targeting of steroidogenic factor-1-expressing neurons, have allowed us to take a closer look at the VMH, its connections, and how it affects competing behaviors. In the current review, we aim to integrate recent findings into the literature and contemplate the conclusions that can be drawn.

Melanocortin 3 receptor-expressing neurons in the ventromedial hypothalamus promote glucose disposal

The ventromedial hypothalamus (VMH) is a critical neural node that senses blood glucose and promotes glucose utilization or mobilization during hypoglycemia. The VMH neurons that control these distinct physiologic processes are largely unknown. Here, we show that melanocortin 3 receptor (Mc3R)-expressing VMH neurons (VMH^MC3R) sense glucose changes both directly and indirectly via altered excitatory input. We identify presynaptic nodes that potentially regulate VMH^MC3R neuronal activity, including inputs from proopiomelanocortin (POMC)-producing neurons in the arcuate nucleus. We find that VMH^MC3R neuron activation blunts, and their silencing enhances glucose excursion following a glucose load. Overall, these findings demonstrate that VMH^MC3R neurons are a glucose-responsive hypothalamic subpopulation that promotes glucose disposal upon activation; this highlights a potential site for targeting dysregulated glycemia.

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.

ATP-sensitive K+ channels control the spontaneous firing of a glycinergic interneuron in the auditory brainstem

KEY POINTS

Cartwheel neurons provide potent inhibition to fusiform neurons in the dorsal cochlear nucleus (DCN). Most cartwheel neurons fire action potentials spontaneously, but the ion channels responsible for this intrinsic activity are unknown. We investigated the ion channels responsible for the intrinsic firing of cartwheel neurons and the stable resting membrane potential found in a fraction of these neurons (quiet neurons). Among the ion channels controlling membrane potential of cartwheel neurons, the presence of open ATP-sensitive potassium channels (K_ATP ) is responsible for the existence of quiet neurons. Our results pinpoint K_ATP channel modulation as a critical factor controlling the firing of cartwheel neurons. Hence, it is a crucial channel influencing the balance of excitation and inhibition in the DCN.

ABSTRACT

Cartwheel neurons from the dorsal cochlear nucleus (DCN) are glycinergic interneurons and the primary source of inhibition on the fusiform neurons, the DCN's principal excitatory neuron. Most cartwheel neurons present spontaneous firing (active neurons), producing a steady inhibitory tone on fusiform neurons. In contrast, a small fraction of these neurons do not fire spontaneously (quiet neurons). Hyperactivity of fusiform neurons is seen in animals with behavioural evidence of tinnitus. Because of its relevance in controlling the excitability of fusiform neurons, we investigated the ion channels responsible for the spontaneous firing of cartwheel neurons in DCN slices from rats. We found that quiet neurons presented an outward conductance not seen in active neurons, which generates a stable resting potential. This current was sensitive to tolbutamide, an ATP-sensitive potassium channel (K_ATP ) antagonist. After inhibition with tolbutamide, quiet neurons start to fire spontaneously, while the active neurons were not affected. On the other hand, in active neurons, K_ATP agonist diazoxide activated a conductance similar to quiet neurons' K_ATP conductance and stopped spontaneous firing. According to the effect of K_ATP channels on cartwheel neuron firing, glycinergic neurotransmission in DCN was increased by tolbutamide and decreased by diazoxide. Our results reveal a role of K_ATP channels in controlling the spontaneous firing of neurons not involved in fuel homeostasis.

Attenuated Induction of the Unfolded Protein Response in Adult Human Primary Astrocytes in Response to Recurrent Low Glucose

AIMS/HYPOTHESIS

Recurrent hypoglycaemia (RH) is a major side-effect of intensive insulin therapy for people with diabetes. Changes in hypoglycaemia sensing by the brain contribute to the development of impaired counterregulatory responses to and awareness of hypoglycaemia. Little is known about the intrinsic changes in human astrocytes in response to acute and recurrent low glucose (RLG) exposure.

METHODS

Human primary astrocytes (HPA) were exposed to zero, one, three or four bouts of low glucose (0.1 mmol/l) for three hours per day for four days to mimic RH. On the fourth day, DNA and RNA were collected. Differential gene expression and ontology analyses were performed using DESeq2 and GOseq, respectively. DNA methylation was assessed using the Infinium MethylationEPIC BeadChip platform.

RESULTS

24 differentially expressed genes (DEGs) were detected (after correction for multiple comparisons). One bout of low glucose exposure had the largest effect on gene expression. Pathway analyses revealed that endoplasmic-reticulum (ER) stress-related genes such as HSPA5, XBP1, and MANF, involved in the unfolded protein response (UPR), were all significantly increased following low glucose (LG) exposure, which was diminished following RLG. There was little correlation between differentially methylated positions and changes in gene expression yet the number of bouts of LG exposure produced distinct methylation signatures.

CONCLUSIONS/INTERPRETATION

These data suggest that exposure of human astrocytes to transient LG triggers activation of genes involved in the UPR linked to endoplasmic reticulum (ER) stress. Following RLG, the activation of UPR related genes was diminished, suggesting attenuated ER stress. This may be a consequence of a successful metabolic adaptation, as previously reported, that better preserves intracellular energy levels and a reduced necessity for the UPR.

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.

Repeated hypoglycemia remodels neural inputs and disrupts mitochondrial function to blunt glucose-inhibited GHRH neuron responsiveness

Hypoglycemia is a frequent complication of diabetes, limiting therapy and increasing morbidity and mortality. With recurrent hypoglycemia, the counterregulatory response (CRR) to decreased blood glucose is blunted, resulting in hypoglycemia-associated autonomic failure (HAAF). The mechanisms leading to these blunted effects are only poorly understood. Here, we report, with ISH, IHC, and the tissue-clearing capability of iDISCO+, that growth hormone releasing hormone (GHRH) neurons represent a unique population of arcuate nucleus neurons activated by glucose deprivation in vivo. Repeated glucose deprivation reduces GHRH neuron activation and remodels excitatory and inhibitory inputs to GHRH neurons. We show that low glucose sensing is coupled to GHRH neuron depolarization, decreased ATP production, and mitochondrial fusion. Repeated hypoglycemia attenuates these responses during low glucose. By maintaining mitochondrial length with the small molecule mitochondrial division inhibitor-1, we preserved hypoglycemia sensitivity in vitro and in vivo. Our findings present possible mechanisms for the blunting of the CRR, significantly broaden our understanding of the structure of GHRH neurons, and reveal that mitochondrial dynamics play an important role in HAAF. We conclude that interventions targeting mitochondrial fission in GHRH neurons may offer a new pathway to prevent HAAF in patients with diabetes.

GHRH neurons in the arcuate nucleus are activated by glucose deprivation; however, repeated hypoglycemia blunts activation, remodels inputs, and disrupts mitochondrial fusion.

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.

Cellular and systemic mechanisms for glucose sensing and homeostasis

Glucose is a major source of energy in animals. Maintaining blood glucose levels within a physiological range is important for facilitating glucose uptake by cells, as required for optimal functioning. Glucose homeostasis relies on multiple glucose-sensing cells in the body that constantly monitor blood glucose levels and respond accordingly to adjust its glycemia. These include not only pancreatic β-cells and α-cells that secrete insulin and glucagon, but also central and peripheral neurons regulating pancreatic endocrine function. Different types of cells respond distinctively to changes in blood glucose levels, and the mechanisms involved in glucose sensing are diverse. Notably, recent studies have challenged the currently held views regarding glucose-sensing mechanisms. Furthermore, peripheral and central glucose-sensing cells appear to work in concert to control blood glucose level and maintain glucose and energy homeostasis in organisms. In this review, we summarize the established concepts and recent advances in the understanding of cellular and systemic mechanisms that regulate glucose sensing and its homeostasis.

Essential and sex-specific effects of mGluR5 in ventromedial hypothalamus regulating estrogen signaling and glucose balance

The ventromedial hypothalamus (VMH) plays chief roles regulating energy and glucose homeostasis and is sexually dimorphic. We discovered that expression of metabotropic glutamate receptor subtype 5 (mGluR5) in the VMH is regulated by caloric status in normal mice and reduced in brain-derived neurotrophic factor (BDNF) mutants, which are severely obese and have diminished glucose balance control. These findings led us to investigate whether mGluR5 might act downstream of BDNF to critically regulate VMH neuronal activity and metabolic function. We found that mGluR5 depletion in VMH SF1 neurons did not affect energy balance regulation. However, it significantly impaired insulin sensitivity, glycemic control, lipid metabolism, and sympathetic output in females but not in males. These sex-specific deficits are linked to reductions in intrinsic excitability and firing rate of SF1 neurons. Abnormal excitatory and inhibitory synapse assembly and elevated expression of the GABAergic synthetic enzyme GAD67 also cooperate to decrease and potentiate the synaptic excitatory and inhibitory tone onto mutant SF1 neurons, respectively. Notably, these alterations arise from disrupted functional interactions of mGluR5 with estrogen receptors that switch the normally positive effects of estrogen on SF1 neuronal activity and glucose balance control to paradoxical and detrimental. The collective data inform an essential central mechanism regulating metabolic function in females and underlying the protective effects of estrogen against metabolic disease.

17β-estradiol promotes acute refeeding in hungry mice via membrane-initiated ERα signaling

Objective

Estrogen protects animals from obesity through estrogen receptor α (ERα), partially by inhibiting overeating in animals fed ad libitum. However, the effects of estrogen on feeding behavior in hungry animals remain unclear. In this study, we examined the roles of 17β-estradiol (E2) and ERα in the regulation of feeding in hungry female animals and explored the underlying mechanisms.

Methods

Wild-type female mice with surgical depletion of endogenous estrogens were used to examine the effects of E2 supplementation on acute refeeding behavior after starvation. ERα-C451A mutant mice deficient in membrane-bound ERα activity and ERα-AF2⁰ mutant mice lacking ERα transcriptional activity were used to further examine mechanisms underlying acute feeding triggered by either fasting or central glucopenia (induced by intracerebroventricular injections of 2-deoxy-D-glucose). We also used electrophysiology to explore the impact of these ERα mutations on the neural activities of ERα neurons in the hypothalamus.

Results

In the wild-type female mice, ovariectomy reduced fasting-induced refeeding, which was restored by E2 supplementation. The ERα-C451A mutation, but not the ERα-AF2⁰ mutation, attenuated acute feeding induced by either fasting or central glucopenia. The ERα-C451A mutation consistently impaired the neural responses of hypothalamic ERα neurons to hypoglycemia.

Conclusion

In addition to previous evidence that estrogen reduces deviations in energy balance by inhibiting eating at a satiated state, our findings demonstrate the unexpected role of E2 that promotes eating in hungry mice, also contributing to the stability of energy homeostasis. This latter effect specifically requires membrane-bound ERα activity.

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Endogenous E2 is required to maintain acute refeeding in hungry female mice after starvation.

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Membrane-bound ERα activity in female mice is required for efficient refeeding after starvation.

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Membrane-bound ERα activity is required for hypothalamic ERα neurons to respond to hypoglycemia.

Circadian-timed dopamine agonist treatment reverses high-fat diet-induced diabetogenic shift in ventromedial hypothalamic glucose sensing

INTRODUCTION

Within the ventromedial hypothalamus (VMH), glucose inhibitory (GI) neurons sense hypoglycaemia while glucose excitatory (GE) neurons sense hyperglycaemia to initiate counter control mechanisms under normal conditions. However, potential electrophysiological alterations of these two neuronal types in vivo in insulin-resistant states have never been simultaneously fully documented. Further, the anti-diabetic effect of dopamine agonism on this VMH system under insulin resistance has not been studied.

METHODS

This study examined the impact of a high-fat diet (HFD) on in vivo electrophysiological recordings from VMH GE and GI neurons and the ability of circadian-timed dopamine agonist therapy to reverse any adverse effect of the HFD on such VMH activities and peripheral glucose metabolism.

RESULTS

HFD significantly inhibited VMH GE neuronal electrophysiological response to local hyperglycaemia (36.3%) and augmented GI neuronal excitation response to local hypoglycaemia (47.0%). Bromocriptine (dopamine agonist) administration at onset of daily activity (but not during the daily sleep phase) completely reversed both VMH GE and GI neuronal aberrations induced by HFD. Such timed treatment also normalized glucose intolerance and insulin resistance. These VMH and peripheral glucose metabolism effects of circadian-timed bromocriptine may involve its known effect to reduce elevated VMH noradrenergic activity in insulin-resistant states as local VMH administration of norepinephrine was observed to significantly inhibit VMH GE neuronal sensing of local hyperglycaemia in insulin-sensitive animals on regular chow diet (52.4%).

CONCLUSIONS

HFD alters VMH glucose sensing in a manner that potentiates hyperglycaemia and this effect on the VMH can be reversed by appropriately circadian-timed dopamine agonist administration.

Estrogen receptor-α expressing neurons in the ventrolateral VMH regulate glucose balance

Brain glucose-sensing neurons detect glucose fluctuations and prevent severe hypoglycemia, but mechanisms mediating functions of these glucose-sensing neurons are unclear. Here we report that estrogen receptor-α (ERα)-expressing neurons in the ventrolateral subdivision of the ventromedial hypothalamic nucleus (vlVMH) can sense glucose fluctuations, being glucose-inhibited neurons (GI-ERαvlVMH) or glucose-excited neurons (GE-ERαvlVMH). Hypoglycemia activates GI-ERαvlVMH neurons via the anoctamin 4 channel, and inhibits GE-ERαvlVMH neurons through opening the ATP-sensitive potassium channel. Further, we show that GI-ERαvlVMH neurons preferentially project to the medioposterior arcuate nucleus of the hypothalamus (mpARH) and GE-ERαvlVMH neurons preferentially project to the dorsal Raphe nuclei (DRN). Activation of ERαvlVMH to mpARH circuit and inhibition of ERαvlVMH to DRN circuit both increase blood glucose. Thus, our results indicate that ERαvlVMH neurons detect glucose fluctuations and prevent severe hypoglycemia in mice.

Changes in neuronal activity across the mouse ventromedial nucleus of the hypothalamus in response to low glucose: Evaluation using an extracellular multi-electrode array approach

The hypothalamic ventromedial nucleus (VMN) is involved in maintaining systemic glucose homeostasis. Neurophysiological studies in rodent brain slices have identified populations of VMN glucose-sensing neurones: glucose-excited (GE) neurones, cells which increased their firing rate in response to increases in glucose concentration, and glucose-inhibited (GI) neurones, which show a reduced firing frequency in response to increasing glucose concentrations. To date, most slice electrophysiological studies characterising VMN glucose-sensing neurones in rodents have utilised the patch clamp technique. Multi-electrode arrays (MEAs) are a state-of-the-art electrophysiological tool enabling the electrical activity of many cells to be recorded across multiple electrode sites (channels) simultaneously. We used a perforated MEA (pMEA) system to evaluate electrical activity changes across the dorsal-ventral extent of the mouse VMN region in response to alterations in glucose concentration. Because intrinsic (ie, direct postsynaptic sensing) and extrinsic (ie, presynaptically modulated) glucosensation were not discriminated, we use the terminology 'GE/presynaptically excited by an increase (PER)' and 'GI/presynaptically excited by a decrease (PED)' in the present study to describe responsiveness to changes in extracellular glucose across the mouse VMN. We observed that 15%-60% of channels were GE/PER, whereas 2%-7% were GI/PED channels. Within the dorsomedial portion of the VMN (DM-VMN), significantly more channels were GE/PER compared to the ventrolateral portion of the VMN (VL-VMN). However, GE/PER channels within the VL-VMN showed a significantly higher basal firing rate in 2.5 mmol l^-1 glucose than DM-VMN GE/PER channels. No significant difference in the distribution of GI/PED channels was observed between the VMN subregions. The results of the present study demonstrate the utility of the pMEA approach for evaluating glucose responsivity across the mouse VMN. pMEA studies could be used to refine our understanding of other neuroendocrine systems by examining population level changes in electrical activity across brain nuclei, thus providing key functional neuroanatomical information to complement and inform the design of single-cell neurophysiological studies.

Integration of energy homeostasis and stress by parvocellular neurons in rat hypothalamic paraventricular nucleus

KEY POINTS

Central regulation of energy homeostasis and stress are believed to be reciprocally regulated, i.e. excessive food intake suppresses, while prolonged hunger exacerbates, stress responses in vivo. This relationship may be mediated by neuroendocrine parvocellular corticotropin-releasing hormone (CRH) neurons in the hypothalamic paraventricular nucleus that receive both stress- and feeding-related input. We find that hunger strongly and selectively potentiates, while re-feeding suppresses, a cellular analogue of a stress response induced by acute glucopenia in CRH neurons in rat hypothalamic slices. Neuronal activation in response to glucopenia was mediated synaptically, via the relative enhancement of glutamate over GABA input. These results illustrate how acute stress responses may be initiated in vivo and show that it is reciprocally integrated with energy balance via local hypothalamic mechanisms acting at the level of CRH neurons and their afferent terminals.

ABSTRACT

Increased food intake is a common response to help cope with stress, implying the existence of a previously postulated but imperfectly understood, inverse relationship between the regulation of feeding and stress. We have identified components of the neural circuitry that can integrate these homeostatic responses. Prior fasting (∼24 h) potentiates, and re-feeding suppresses, excitatory responses to acute glucopenia in about half of the corticotropin releasing hormone (CRH)-expressing, putatively neurosecretory, stress-related neurons in the paraventricular nucleus of the hypothalamus studied. Glucoprivation stress ex vivo resulted from a preferential relative increase in excitatory (glutamatergic) over inhibitory (GABAergic) inputs. Putative preautonomic cells were less sensitive to fasting, and showed a predominant inhibition to acute glucopenia. We conclude that hunger may sensitize hypothalamic stress responses by acting via local mechanisms, at the level of CRH neurons and their presynaptic inputs. Those mechanisms involve neither presynaptic ATP-sensitive potassium channels nor postsynaptic ATP levels.

Selective sexual differentiation of neurone populations may contribute to sex-specific outputs of the ventromedial nucleus of the hypothalamus

Sex differences among neurones in the ventrolateral region of the ventromedial hypothalamic nucleus (VMHvl) allow for the display of a diversity of sex-typical behaviours and physiological responses, ranging from mating behaviour to metabolism. Here, we review recent studies that interrogate the relationship between sex-typical responses and changes in cellular phenotypes. We discuss technologies that increase the resolution of molecular profiling or targeting of cell populations, including single-cell transcriptional profiling and conditional viral genetic approaches to manipulate neurone survival or activity. Overall, emerging studies indicate that sex-typical functions of the VMH may be mediated by phenotypically distinct and sexually differentiated neurone populations within the VMHvl. Future studies in this and other brain regions could exploit cell-type-specific tools to reveal the cell populations and molecular mediators that modulate sex-typical responses. Furthermore, cell-type-specific analyses of the effects of sexually differentiating factors, including sex hormones, can test the hypothesis that distinct cell types within a single brain region vary with respect to sexual differentiation.

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.

Hypothalamic Fatty Acids and Ketone Bodies Sensing and Role of FAT/CD36 in the Regulation of Food Intake

The obesity and type-2 diabetes epidemic is escalating and represents one of the costliest biomedical challenges confronting modern society. Moreover, the increasing consumption of high fat food is often correlated with an increase in body mass index. In people predisposed to be obese or already obese, the impaired ability of the brain to monitor and respond to alterations in fatty acid (FA) metabolism is increasingly recognized as playing a role in the pathophysiological development of these disorders. The brain senses and regulates metabolism using highly specialized nutrient-sensing neurons located mainly in the hypothalamus. The same neurons are able to detect variation in the extracellular levels of glucose, FA and ketone bodies as a way to monitor nutrient availability and to alter its own activity. In addition, glial cells such as astrocytes create major connections to neurons and form a tight relationship to closely regulate nutrient uptake and metabolism. This review will examine the different pathways by which neurons are able to detect free fatty acids (FFA) to alter its activity and how high fat diet (HFD)-astrocytes induced ketone bodies production interplays with neuronal FA sensing. The role of HFD-induced inflammation and how FA modulate the reward system will also be investigated here.

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.

A voltage-dependent depolarization induced by low external glucose in neurons of the nucleus of the tractus solitarius: interaction with KATP channels

KEY POINTS

Neurons from the brainstem nucleus of the tractus solitarius (NTS) participate in the counter-regulatory mechanisms in response to hypoglycaemia. ATP-sensitive potassium (K_ATP ) channels are expressed in NTS neurons, and are partially open at rest in normoglycaemic 5 mM glucose. In normoglycaemic conditions, most NTS neurons depolarize in response to low external glucose (0.5 mM), via a voltage-dependent mechanism. Conversely, most NTS neurons incubated in hyperglycaemic 10 mM glucose do not respond to low glucose due to a more positive resting membrane potential caused by the closure of K_ATP channels following increased intracellular metabolic ATP. Our findings show that in hyperglycaemic conditions, NTS neurons failed to sense rapid changes in external glucose, which could be related to hypoglycaemia-associated autonomic failure.

ABSTRACT

The nucleus of the tractus solitarius (NTS) is an integrative centre for autonomic counter-regulatory responses to hypoglycaemia. K_ATP channels link the metabolic status of the neuron to its excitability. Here we investigated the influence of K_ATP channels on the membrane potential of NTS neurons in normo- and hyperglycaemic external glucose concentrations, and after switching to a hypoglycaemic concentration, using in vitro electrophysiological recordings in brainstem slices. We found that in normoglycaemic (5 mM) glucose, tolbutamide, a K_ATP channel antagonist, depolarized the membrane of most neurons, and this effect was observed in more hyperpolarized neurons. All neurons hyperpolarized after pharmacological activation of K_ATP channels. Most NTS neurons depolarized in the presence of low glucose (0.5 mM), and this effect was only seen in hyperpolarized neurons. The effect of glucose was caused by a cationic current with a reversal potential around -50 mV. In the presence of hyperglycaemic glucose (10 mM), neurons were more depolarized, and fewer neurons responded to K_ATP blockage. Application of 0.5 mM glucose solution to these neurons depolarized the membrane only in more hyperpolarized neurons. We conclude that NTS neurons present with K_ATP channels open at rest in normoglycaemic conditions, and their membrane potential is affected by extracellular glucose. Moreover, NTS neurons depolarize the membrane in response to the application of a low glucose solution, but this effect is occluded by membrane depolarization triggered by K_ATP blockage. Our data suggest a homeostatic regulation of the membrane potential by external glucose, and a possible mechanism related to the hypoglycaemia-associated autonomic failure.

Effects of Feeding-Related Peptides on Neuronal Oscillation in the Ventromedial Hypothalamus

The ventromedial hypothalamus (VMH) plays an important role in feeding behavior, obesity, and thermoregulation. The VMH contains glucose-sensing neurons, the firing of which depends on the level of extracellular glucose and which are involved in maintaining the blood glucose level via the sympathetic nervous system. The VMH also expresses various receptors of the peptides related to feeding. However, it is not well-understood whether the action of feeding-related peptides mediates the activity of glucose-sensing neurons in the VMH. In the present study, we examined the effects of feeding-related peptides on the burst-generating property of the VMH. Superfusion with insulin, pituitary adenylate cyclase-activating polypeptide, corticotropin-releasing factor, and orexin increased the frequency of the VMH oscillation. In contrast, superfusion with leptin, cholecystokinin, cocaine- and amphetamine-regulated transcript, galanin, ghrelin, and neuropeptide Y decreased the frequency of the oscillation. Our findings indicated that the frequency changes of VMH oscillation in response to the application of feeding-related peptides showed a tendency similar to changes of sympathetic nerve activity in response to the application of these substances to the brain.

Activation of µ-opioid receptors in the rostral ventrolateral medulla blocks the sympathetic counterregulatory response to glucoprivation

Activation of neurons in the rostral ventrolateral medulla (RVLM) following glucoprivation initiates sympathoadrenal activation, adrenaline release, and increased glucose production. Here, we aimed to determine the role of RVLM µ-opioid receptors in the counterregulatory response to systemic glucoprivation. Experiments were performed in pentobarbital sodium anesthetized male Sprague-Dawley rats ( n = 30). Bilateral activation of RVLM µ-opioid receptors with [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) (8 mM, 50 nl) depressed adrenal sympathetic nerve activity for ~60 min ( n = 6; Δ49.9 ± 5.8%, P < 0.05). The counterregulatory response to glucoprivation (measured by adrenal sympathetic efferent nerve activity) induced by 2-deoxyglucose (2-DG) ( n = 6; Δ63.6 ± 16.5%, P < 0.05) was completely blocked 60 min after DAMGO microinjections ( n = 6; Δ10.2 ± 3.5%, P < 0.05). Furthermore, DAMGO pretreatment attenuated the increase in blood glucose levels after 2-DG infusion ( n = 6; 6.1 ± 0.7mmol/l vs. baseline 5.2 ± 0.3mmol/l, P > 0.05) compared with 2-DG alone ( n = 6; 7.6 ± 0.4mmol/l vs. baseline 6.0 ± 0.4mmol/l, P < 0.05). Thus, activation of RVLM µ-opioid receptors attenuated the neural efferent response to glucoprivation and reduced glucose production.

Neurobiological characteristics underlying metabolic differences between males and females

The hypothalamus is the main integrating center for metabolic control. Our understanding of how hypothalamic circuits function to control appetite and energy expenditure has increased dramatically in recent years, due to the rapid rise in the incidence of obesity and the search for effective treatments. Increasing evidence indicates that these treatments will most likely differ between males and females. Indeed, sex differences in metabolism have been demonstrated at various levels, including in two of the most studied neuronal populations involved in metabolic control: the anorexigenic proopiomelanocortin neurons and the orexigenic neuropeptide Y/Agouti-related protein neurons. Here we review what is known to date regarding the sex differences in these two neuronal populations, as well as other neuronal populations involved in metabolic control and glial cells.

Alpha2delta-1 in SF1+ Neurons of the Ventromedial Hypothalamus Is an Essential Regulator of Glucose and Lipid Homeostasis

The central mechanisms controlling glucose and lipid homeostasis are inadequately understood. We show that α2δ-1 is an essential regulator of glucose and lipid balance, acting in steroidogenic factor-1 (SF1) neurons of the ventromedial hypothalamus (VMH). These effects are body weight independent and involve regulation of SF1⁺ neuronal activity and sympathetic output to metabolic tissues. Accordingly, mice with α2δ-1 deletion in SF1 neurons exhibit glucose intolerance, altered lipolysis, and decreased cholesterol content in adipose tissue despite normal energy balance regulation. Profound reductions in the firing rate of SF1 neurons, decreased sympathetic output, and elevated circulating levels of serotonin are associated with these alterations. Normal calcium currents but reduced excitatory postsynaptic currents in mutant SF1 neurons implicate α2δ-1 in the promotion of excitatory synaptogenesis separate from its canonical role as a calcium channel subunit. Collectively, these findings identify an essential mechanism that regulates VMH neuronal activity and glycemic and lipid control and may be a target for tackling metabolic disease.

Lateral hypothalamic orexin glucose-inhibited neurons may regulate reward-based feeding by modulating glutamate transmission in the ventral tegmental area

Glucose inhibits ∼60% of lateral hypothalamic (LH) orexin neurons. Fasting increases the activation of LH orexin glucose-inhibited (GI) neurons in low glucose. Increases in spontaneous glutamate excitatory postsynaptic currents (sEPSCs) onto putative VTA DA neurons in low glucose are orexin dependent (Sheng et al., 2014). VTA DA neurons modulate reward-based feeding. We tested the hypothesis that increased activation of LH orexin-GI neurons in low glucose increases glutamate signaling onto VTA DA neurons and contributes to reward-based feeding in food restricted animals. N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) currents on putative VTA DA neurons were measured using whole cell voltage clamp recording in horizontal brain slices containing the LH and VTA. Decreased glucose increased the NMDA receptor current for at least one hour after returning glucose to basal levels (P < 0.05; N = 8). The increased current was blocked by an orexin 1 receptor antagonist (P < 0.05; N = 5). Low glucose caused a similar persistent enhancement of AMPA receptor currents (P < 0.05; N = 7). An overnight fast increased the AMPA/NMDA receptor current ratio, an in vivo index of glutamate plasticity, on putative VTA DA neurons. Conditioned place preference (CPP) for palatable food was measured during LH dialysis with glucose. CPP score was negatively correlated with increasing LH glucose (P < 0.05; N = 20). These data suggest that increased activation of LH orexin-GI neurons in low glucose after weight loss, leads to enhanced glutamate signaling on VTA DA neurons, increases the drive to eat rewarding food, and may contribute to weight regain.

Hypoglycemia: Role of Hypothalamic Glucose-Inhibited (GI) Neurons in Detection and Correction

Hypoglycemia is a profound threat to the brain since glucose is its primary fuel. As a result, glucose sensors are widely located in the central nervous system and periphery. In this perspective we will focus on the role of hypothalamic glucose-inhibited (GI) neurons in sensing and correcting hypoglycemia. In particular, we will discuss GI neurons in the ventromedial hypothalamus (VMH) which express neuronal nitric oxide synthase (nNOS) and in the perifornical hypothalamus (PFH) which express orexin. The ability of VMH nNOS-GI neurons to depolarize in low glucose closely parallels the hormonal response to hypoglycemia which stimulates gluconeogenesis. We have found that nitric oxide (NO) production in low glucose is dependent on oxidative status. In this perspective we will discuss the potential relevance of our work showing that enhancing the glutathione antioxidant system prevents hypoglycemia associated autonomic failure (HAAF) in non-diabetic rats whereas VMH overexpression of the thioredoxin antioxidant system restores hypoglycemia counterregulation in rats with type 1 diabetes.We will also address the potential role of the orexin-GI neurons in the arousal response needed for hypoglycemia awareness which leads to behavioral correction (e.g., food intake, glucose administration). The potential relationship between the hypothalamic sensors and the neurocircuitry in the hindbrain and portal mesenteric vein which is critical for hypoglycemia correction will then be discussed.

BAD and KATP channels regulate neuron excitability and epileptiform activity

Brain metabolism can profoundly influence neuronal excitability. Mice with genetic deletion or alteration of Bad (BCL-2 agonist of cell death) exhibit altered brain-cell fuel metabolism, accompanied by resistance to acutely induced epileptic seizures; this seizure protection is mediated by ATP-sensitive potassium (K_ATP) channels. Here we investigated the effect of BAD manipulation on K_ATP channel activity and excitability in acute brain slices. We found that BAD’s influence on neuronal K_ATP channels was cell-autonomous and directly affected dentate granule neuron (DGN) excitability. To investigate the role of neuronal K_ATP channels in the anticonvulsant effects of BAD, we imaged calcium during picrotoxin-induced epileptiform activity in entorhinal-hippocampal slices. BAD knockout reduced epileptiform activity, and this effect was lost upon knockout or pharmacological inhibition of K_ATP channels. Targeted BAD knockout in DGNs alone was sufficient for the antiseizure effect in slices, consistent with a ‘dentate gate’ function that is reinforced by increased K_ATP channel activity.

Shared pathological pathways of Alzheimer's disease with specific comorbidities: current perspectives and interventions

Alzheimer's disease (AD) belongs to one of the most multifactorial, complex and heterogeneous morbidity-leading disorders. Despite the extensive research in the field, AD pathogenesis is still at some extend obscure. Mechanisms linking AD with certain comorbidities, namely diabetes mellitus, obesity and dyslipidemia, are increasingly gaining importance, mainly because of their potential role in promoting AD development and exacerbation. Their exact cognitive impairment trajectories, however, remain to be fully elucidated. The current review aims to offer a clear and comprehensive description of the state-of-the-art approaches focused on generating in-depth knowledge regarding the overlapping pathology of AD and its concomitant ailments. Thorough understanding of associated alterations on a number of molecular, metabolic and hormonal pathways, will contribute to the further development of novel and integrated theranostics, as well as targeted interventions that may be beneficial for individuals with age-related cognitive decline.