A parabrachial-hypothalamic cholecystokinin neurocircuit controls counterregulatory responses to hypoglycemia

A role for leptin-regulated neurocircuitry in subordination stress

The visible burrow system produces a distinct combination of psychological and metabolic stress on, primarily, subordinate individuals that results in pronounced physiologic and behavioral dysfunction. However, the mechanisms underlying the consequences of chronic subordination stress are largely unknown. The simplest mechanistic explanation is that adaptations within brain systems with overlapping functions of both psychological and metabolic control provide immediate benefits that result in lasting susceptibility to diseases, disorders, and increased mortality rates in subordinates. Circuits regulated by leptin adapt to fluctuating levels of energy storage, such that the loss of leptin action within leptin-regulated neurocircuitry results in dysfunction in physiologic and behavioral systems implicated in the consequences of chronic social subordination. Thus, leptin-regulated neurocircuitry may provide a window into understanding the consequences of social subordination stress. This review examines the neural systems of leptin physiology implicated in social subordination stress: energy balance, motivation, HPA axis, and glycemic control.

Central leptin action on euglycemia restoration in type 1 diabetes: Restraining responses normally induced by fasting?

Leptin monotherapy is sufficient to restore euglycemia in insulinopenic type 1 diabetes (T1D), yet the underlying mechanism remains poorly understood. Accumulating evidence demonstrates that the brain mediates the leptin action on euglycemia restoration. Here, we first review evidence supporting that symptoms in T1D resemble an uncontrolled response to fasting. Then, we discuss recent research progress on brain neurons and their neurotransmitters that potentially mediate the leptin action. Finally, peripheral effective pathways, which are normally involved in fasting responses and associated with leptin action on euglycemia restoration in T1D, will also be discussed. This summary complements several previous excellent reviews on this topic (Meek and Morton, 2016; Perry et al., 2016; Fujikawa and Coppari, 2015). A deep understanding of neurocircuitry and the peripheral effective pathways that mediate the leptin action on euglycemia restoration will likely lead to novel targets for an insulin-independent therapeutics against T1D.

Leptin and insulin engage specific PI3K subunits in hypothalamic SF1 neurons

Objective

The ventromedial hypothalamic nucleus (VMH) regulates energy balance and glucose homeostasis. Leptin and insulin exert metabolic effects via their cognate receptors expressed by the steroidogenic factor 1 (SF1) neurons within the VMH. However, detailed cellular mechanisms involved in the regulation of these neurons by leptin and insulin remain to be identified.

Methods

We utilized genetically-modified mouse models and performed patch-clamp electrophysiology experiments to resolve this issue.

Results

We identified distinct populations of leptin-activated and leptin-inhibited SF1 neurons. In contrast, insulin uniformly inhibited SF1 neurons. Notably, we found that leptin-activated, leptin-inhibited, and insulin-inhibited SF1 neurons are distinct subpopulations within the VMH. Leptin depolarization of SF1 neuron also required the PI3K p110β catalytic subunit. This effect was mediated by the putative transient receptor potential C (TRPC) channel. On the other hand, hyperpolarizing responses of SF1 neurons by leptin and insulin required either of the p110α or p110β catalytic subunits, and were mediated by the putative ATP-sensitive K⁺ (K_ATP) channel.

Conclusions

Our results demonstrate that specific PI3K catalytic subunits are responsible for the acute effects of leptin and insulin on VMH SF1 neurons, and provide insights into the cellular mechanisms of leptin and insulin action on VMH SF1 neurons that regulate energy balance and glucose homeostasis.

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Leptin recruits p110β/TRPC channels to depolarize/activate SF1 neurons.

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Leptin recruits p110α/p110β/K_ATP channels to hyperpolarize/inhibit SF1 neurons.

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Insulin recruits p110α/p110β/K_ATP channels to hyperpolarize/inhibit SF1 neurons.

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Acute leptin and insulin responses are segregated to distinct subsets of VMH SF1 neurons.

Melanocortin-4 receptor-regulated energy homeostasis

The melanocortin system provides a conceptual blueprint for the central control of energetic state. Defined by four principal molecular components--two antagonistically acting ligands and two cognate receptors--this phylogenetically conserved system serves as a prototype for hierarchical energy balance regulation. Over the last decade the application of conditional genetic techniques has facilitated the neuroanatomical dissection of the melanocortinergic network and identified the specific neural substrates and circuits that underscore the regulation of feeding behavior, energy expenditure, glucose homeostasis and autonomic outflow. In this regard, the melanocortin-4 receptor is a critical coordinator of mammalian energy homeostasis and body weight. Drawing on recent advances in neuroscience and genetic technologies, we consider the structure and function of the melanocortin-4 receptor circuitry and its role in energy homeostasis.

The role of leptin in diabetes: metabolic effects

While it is well established that the adiposity hormone leptin plays a key role in the regulation of energy homeostasis, growing evidence suggests that leptin is also critical for glycaemic control. In this review we examine the role of the brain in the glucose-lowering actions of leptin and the potential mediators responsible for driving hyperglycaemia in states of uncontrolled insulin-deficient diabetes (uDM). These considerations highlight the possibility of targeting leptin-sensitive pathways as a therapeutic option for the treatment of diabetes. This review summarises a presentation given at the 'Is leptin coming back?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Christoffer Clemmensen and colleagues, DOI: 10.1007/s00125-016-3906-7 , and by Gerald Shulman and colleagues, DOI: 10.1007/s00125-016-3909-4 ) and an overview by the Session Chair, Ulf Smith (DOI: 10.1007/s00125-016-3894-7 ).

Rapid-onset hypoglycemia suppresses Fos expression in discrete parts of the ventromedial nucleus of the hypothalamus

The consensus view of the ventromedial nucleus of the hypothalamus (VMH) is that it is a key node in the rodent brain network controlling sympathoadrenal counterregulatory responses to hypoglycemia. To identify the location of hypoglycemia-responsive neurons in the VMH, we performed a high spatial resolution Fos analysis in the VMH of rats made hypoglycemic with intraperitoneal injections of insulin. We examined Fos expression in the four constituent parts of VMH throughout its rostrocaudal extent and determined their relationship to blood glucose concentrations. Hypoglycemia significantly decreased Fos expression only in the dorsomedial and central parts of the VMH, but not its anterior or ventrolateral parts. Moreover, the number of Fos-expressing neurons was significantly and positively correlated in the two responsive regions with terminal blood glucose concentrations. We also measured Fos responses in the paraventricular nucleus of the hypothalamus (PVH) and in several levels of the periaqueductal gray (PAG), which receives strong projections from the VMH. We found the expected and highly significant increase in Fos in the neuroendocrine PVH, which was negatively correlated to terminal blood glucose concentrations, but no significant differences were seen in any part of the PAG. Our results show that there are distinct populations of VMH neurons whose Fos expression is suppressed by hypoglycemia, and their numbers correlate with blood glucose. These findings support a clear division of glycemic control functions within the different parts of the VMH.

Functional identification of a neurocircuit regulating blood glucose

Previous studies implicate the hypothalamic ventromedial nucleus (VMN) in glycemic control. Here, we report that selective inhibition of the subset of VMN neurons that express the transcription factor steroidogenic-factor 1 (VMN(SF1) neurons) blocks recovery from insulin-induced hypoglycemia whereas, conversely, activation of VMN(SF1) neurons causes diabetes-range hyperglycemia. Moreover, this hyperglycemic response is reproduced by selective activation of VMN(SF1) fibers projecting to the anterior bed nucleus of the stria terminalis (aBNST), but not to other brain areas innervated by VMN(SF1) neurons. We also report that neurons in the lateral parabrachial nucleus (LPBN), a brain area that is also implicated in the response to hypoglycemia, make synaptic connections with the specific subset of glucoregulatory VMN(SF1) neurons that project to the aBNST. These results collectively establish a physiological role in glucose homeostasis for VMN(SF1) neurons and suggest that these neurons are part of an ascending glucoregulatory LPBN→VMN(SF1)→aBNST neurocircuit.

Appetite controlled by a cholecystokinin nucleus of the solitary tract to hypothalamus neurocircuit

The nucleus of the solitary tract (NTS) is a key gateway for meal-related signals entering the brain from the periphery. However, the chemical mediators crucial to this process have not been fully elucidated. We reveal that a subset of NTS neurons containing cholecystokinin (CCK^NTS) is responsive to nutritional state and that their activation reduces appetite and body weight in mice. Cell-specific anterograde tracing revealed that CCK^NTS neurons provide a distinctive innervation of the paraventricular nucleus of the hypothalamus (PVH), with fibers and varicosities in close apposition to a subset of melanocortin-4 receptor (MC4R^PVH) cells, which are also responsive to CCK. Optogenetic activation of CCK^NTS axon terminals within the PVH reveal the satiating function of CCK^NTS neurons to be mediated by a CCK^NTS→PVH pathway that also encodes positive valence. These data identify the functional significance of CCK^NTS neurons and reveal a sufficient and discrete NTS to hypothalamus circuit controlling appetite.

DOI:http://dx.doi.org/10.7554/eLife.12225.001

Obesity primarily results from eating more food than the body requires, the energy from which is then stored as fat. In recent years obesity has become increasingly common, with the resulting health problems presenting one of the major healthcare challenges of the twenty-first century. New ways to tackle the obesity epidemic are therefore required to improve human health on a global scale.

To regulate how much food is eaten, the gut sends chemical messengers to the brain about how much food has been consumed. These messengers activate particular cells in the brain that signal to other brain regions to trigger a decision about whether we’ve had enough food to eat. This raises a question: if we can artificially activate these cells, can we ‘trick’ the brain into thinking that food has been consumed?

A brain region called the nucleus of the solitary tract (NTS) is known to play a key role in receiving signals from the gut about meals. By studying mice, D’Agostino et al. found that cells in the NTS that make a brain hormone called cholecystokinin (CCK) are particularly activated by food.

Further experiments then used a technique called optogenetics to activate these cells in mice that had free access to different types of food. This activation significantly reduced how hungry the mice were, causing them to eat less food and lose weight. D’Agostino et al. also showed that CCK cells relay the signal about food intake to a brain region called the hypothalamus.

Overall, D’Agostino et al. have found a way to trick the brain into thinking that food has been eaten when it actually hasn’t, and for this reason mice eat less without feeling hungry and lose weight. The next step is to try and find a way to activate the CCK cells in obese humans who have health complications associated with excess body weight.

DOI:http://dx.doi.org/10.7554/eLife.12225.002

An Emerging Technology Framework for the Neurobiology of Appetite

Advances in neuro-technology for mapping, manipulating, and monitoring molecularly defined cell types are rapidly advancing insight into neural circuits that regulate appetite. Here, we review these important tools and their applications in circuits that control food seeking and consumption. Technical capabilities provided by these tools establish a rigorous experimental framework for research into the neurobiology of hunger.

Ventromedial hypothalamic expression of Bdnf is required to establish normal patterns of afferent GABAergic connectivity and responses to hypoglycemia

OBJECTIVE

The ventromedial nucleus of the hypothalamus (VMH) controls energy and glucose homeostasis through direct connections to a distributed network of nuclei in the hypothalamus, midbrain, and hindbrain. Structural changes in VMH circuit morphology have the potential to alter VMH function throughout life, however, molecular signals responsible for specifying its neural connections are not fully defined. The VMH contains a high density of neurons that express brain-derived neurotrophic factor (BDNF), a potent neurodevelopmental effector known to regulate neuronal survival, growth, differentiation, and connectivity in a number of neural systems. In the current study, we examined whether BDNF impacts the afferent and efferent connections of the VMH, as well as energy homeostatic function.

METHODS

To determine if BDNF is required for VMH circuit formation, a transgenic mouse model was used to conditionally delete Bdnf from steroidogenic factor 1 (SF1) expressing neurons of the VMH prior to the onset of establishing neural connections with other regions. Projections of SF1 expressing neurons were visualized with a genetically targeted fluorescent label and immunofluorescence was used to measure the density of afferents to SF1 neurons in the absence of BDNF. Physiological changes in body weight and circulating blood glucose were also evaluated in the mutant mice.

RESULTS

Our findings suggest that BDNF is required to establish normal densities of GABAergic afferents onto SF1 neurons located in the ventrolateral part of the VMH. Furthermore, loss of BDNF from VMH SF1 neurons results in impaired physiological responses to insulin-induced hypoglycemia.

CONCLUSION

The results of this study indicate that BDNF is required for formation and/or maintenance of inhibitory inputs to SF1 neurons, with enduring effects on glycemic control.

Genetic identity of thermosensory relay neurons in the lateral parabrachial nucleus

The parabrachial nucleus is important for thermoregulation because it relays skin temperature information from the spinal cord to the hypothalamus. Prior work in rats localized thermosensory relay neurons to its lateral subdivision (LPB), but the genetic and neurochemical identity of these neurons remains unknown. To determine the identity of LPB thermosensory neurons, we exposed mice to a warm (36°C) or cool (4°C) ambient temperature. Each condition activated neurons in distinct LPB subregions that receive input from the spinal cord. Most c-Fos+ neurons in these LPB subregions expressed the transcription factor marker FoxP2. Consistent with prior evidence that LPB thermosensory relay neurons are glutamatergic, all FoxP2+ neurons in these subregions colocalized with green fluorescent protein (GFP) in reporter mice for Vglut2, but not for Vgat. Prodynorphin (Pdyn)-expressing neurons were identified using a GFP reporter mouse and formed a caudal subset of LPB FoxP2+ neurons, primarily in the dorsal lateral subnucleus (PBdL). Warm exposure activated many FoxP2+ neurons within PBdL. Half of the c-Fos+ neurons in PBdL were Pdyn+, and most of these project into the preoptic area. Cool exposure activated a separate FoxP2+ cluster of neurons in the far-rostral LPB, which we named the rostral-to-external lateral subnucleus (PBreL). These findings improve our understanding of LPB organization and reveal that Pdyn-IRES-Cre mice provide genetic access to warm-activated, FoxP2+ glutamatergic neurons in PBdL, many of which project to the hypothalamus.

Minireview: CNS Mechanisms of Leptin Action

Leptin is an adipocytokine that circulates in proportion to body fat to signal the repletion of long-term energy stores. Leptin acts via its receptor, LepRb, on specialized neuronal populations in the brain (mainly in the hypothalamus and brainstem) to alter motivation and satiety, as well as to permit energy expenditure and appropriate glucose homeostasis. Decreased leptin, as with prolonged caloric restriction, promotes a powerful orexigenic signal, decreases energy use via a number of neuroendocrine and autonomic axes, and disrupts glucose homeostasis. Here, we review what is known about cellular leptin action and focus on the roles for specific populations of LepRb-expressing neurons for leptin action.

Developmental specification of metabolic circuitry

The hypothalamus contains a core circuitry that communicates with the brainstem and spinal cord to regulate energy balance. Because metabolic phenotype is influenced by environmental variables during perinatal development, it is important to understand how these neural pathways form in order to identify key signaling pathways that are responsible for metabolic programming. Recent progress in defining gene expression events that direct early patterning and cellular specification of the hypothalamus, as well as advances in our understanding of hormonal control of central neuroendocrine pathways, suggest several key regulatory nodes that may represent targets for metabolic programming of brain structure and function. This review focuses on components of central circuitry known to regulate various aspects of energy balance and summarizes what is known about their developmental neurobiology within the context of metabolic programming.

Sex-dependent changes in metabolism and behavior, as well as reduced anxiety after eliminating ventromedial hypothalamus excitatory output

Objectives

The ventromedial hypothalamic nucleus (VMH) regulates energy homeostasis as well as social and emotional behaviors. Nearly all VMH neurons, including those in the sexually dimorphic ventrolateral VMH (VMHvl) subregion, release the excitatory neurotransmitter glutamate and use the vesicular glutamate transporter 2 (Vglut2). Here, we asked how glutamatergic signaling contributes to the collective metabolic and behavioral responses attributed to the VMH and VMHvl.

Methods

Using Sf1-Cre and a Vglut2 floxed allele, Vglut2 was knocked-out in SF-1 VMH neurons (Vglut2^Sf1-Cre). Metabolic and neurobehavioral assays were carried out initially on Vglut2^fl/fl and Vglut2^Sf1-Cre mice in a mixed, and then in the C57BL/6 genetic background, which is prone to hyperglycemia and diet induced obesity (DIO).

Results

Several phenotypes observed in Vglut2^Sf1-Cre mice were largely unexpected based on prior studies that have perturbed VMH development or VMH glutamate signaling. In our hands, Vglut2^Sf1-Cre mice failed to exhibit the anticipated increase in body weight after high fat diet (HFD) or the impaired glucose homeostasis after fasting. Instead, there was a significant sex-dependent attenuation of DIO in Vglut2^Sf1-Cre females. Vglut2^Sf1-Cre males also display a sex-specific loss of conditioned-fear responses and aggression accompanied by more novelty-associated locomotion. Finally, unlike the higher anxiety noted in Sf1^Nestin-Cre mice that lack a fully formed VMH, both male and female Vglut2^Sf1-Cre mice were less anxious.

Conclusions

Loss of VMH glutamatergic signaling sharply decreased DIO in females, attenuated aggression and learned fear in males, and was anxiolytic in males and females. Collectively, our findings demonstrate that while glutamatergic output from the VMH appears largely dispensable for counter regulatory responses to hypoglycemia, it drives sex-dependent differences in metabolism and social behaviors and is essential for adaptive responses to anxiety-provoking stimuli in both sexes.

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Excitatory VMH output controls sex-dependent metabolic and behavioral phenotypes.

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Vglut2^Sf1-Cre mice are not prone to diet-induced obesity or glucose misregulation.

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Loss of VMH glutamatergic signaling leads to negative energy state in females.

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Aggression and learned fear are lower in males lacking VMH excitatory output.

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VMH glutamatergic signaling drives normal anxiety responses in both sexes.

Brain glucose sensing in homeostatic and hedonic regulation

Glucose homeostasis as well as homeostatic and hedonic control of feeding is regulated by hormonal, neuronal, and nutrient-related cues. Glucose, besides its role as a source of metabolic energy, is an important signal controlling hormone secretion and neuronal activity, hence contributing to whole-body metabolic integration in coordination with feeding control. Brain glucose sensing plays a key, but insufficiently explored, role in these metabolic and behavioral controls, which when deregulated may contribute to the development of obesity and diabetes. The recent introduction of innovative transgenic, pharmacogenetic, and optogenetic techniques allows unprecedented analysis of the complexity of central glucose sensing at the molecular, cellular, and neuronal circuit levels, which will lead to a new understanding of the pathogenesis of metabolic diseases.

A neural basis for melanocortin-4 receptor-regulated appetite

Pro-opiomelanocortin (POMC)- and agouti-related peptide (AgRP)-expressing neurons are oppositely regulated by caloric depletion and co-ordinately stimulate and inhibit homeostatic satiety, respectively. This bimodality is principally underscored by the antagonistic actions of these ligands at downstream melanocortin-4 receptors (MC4R) within the paraventricular nucleus of the hypothalamus. Although this population is critical to energy balance the underlying neural circuitry remains unknown. Enabled by mice expressing Cre-recombinase in MC4R neurons, we demonstrate bidirectional control of feeding following real-time activation and inhibition of PVH^MC4R neurons and further identify these cells as a functional exponent of ARC^AgRP neuron-driven hunger. Moreover, we reveal this function to be mediated by a PVH^MC4R→lateral parabrachial nucleus (LPBN) pathway. Activation of this circuit encodes positive valence, but only in calorically depleted mice. Thus, the satiating and appetitive nature of PVH^MC4R→LPBN neurons supports the principles of drive reduction and highlights this circuit as a promising target for anti-obesity drug development.

Expanding the brain glucosensing territory

Brain glucosensing neurons monitor extracellular glucose concentrations and act to defend normoglycemia. To date, the majority of these neurons have been ascribed to hypothalamic and hindbrain centers. In this issue, Garfield and colleagues (2014) demonstrate that cholecystokinin-expressing neurons in the rodent parabrachial nucleus function as glucosensors that counter-regulate hypoglycemia.