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Resume Le glucose est la source principale d’energie metabolique pour le cerveau. Prevenir le developpement d’hypoglycemies est donc necessaire pour la survie de l’individu. L’insuline et le glucagon, des hormones produites, respectivement, par les cellules s et α des ilots de Langerhans et agissant sur le foie, les muscle et le tissu adipeux, jouent un role essentiel dans le maintien de l’homeostasie glucidique. Cependant, ces cellules endocrines, de meme que leurs tissus cibles, sont sous le controle du systeme nerveux autonome. Celui-ci est regule par le glucose, par le biais de neurones sensibles au glucose, actives soit par l’hyperglycemie, soit par l’hypoglycemie. Elucider les proprietes de ces neurones, les circuits neuronaux qu’ils forment, et les fonctions physiologiques qu’ils controlent, est un but important de la recherche sur les maladies metaboliques.

The counterregulatory response to hypoglycemia (CRR), which ensures a sufficient glucose supply to the brain, is an essential survival function. It is orchestrated by incompletely characterized glucose-sensing neurons, which trigger a coordinated autonomous and hormonal response that restores normoglycemia. Here, we investigated the role of hypothalamicTmem117, identified in a genetic screen as a regulator of CRR. We show thatTmem117is expressed in vasopressin magnocellular neurons of the hypothalamus.Tmem117inactivation in these neurons increases hypoglycemia-induced vasopressin secretion leading to higher glucagon secretion, an estrus cycle phase-dependent effect in female mice.Ex vivoelectrophysiological analysis, in-situ hybridization andin vivocalcium imaging reveal thatTmem117inactivation does not affect the glucose-sensing properties of vasopressin neurons but increases ER-stress, ROS production and intracellular calcium levels accompanied by increased AVP production and secretion. Thus,Tmem117in vasopressin neurons is a physiological regulator of glucagon secretion and highlight the role of these neurons in the coordinated response to hypoglycemia.

The counterregulatory response to hypoglycemia that restores normal blood glucose levels is an essential physiological function. It is initiated, in large part, by incompletely characterized brain hypoglycemia sensing neurons that trigger the secretion of counterregulatory hormones, in particular glucagon, to stimulate hepatic glucose production. In a genetic screen of recombinant inbred BXD mice we previously identified Agpat5 as a candidate regulator of hypoglycemia-induced glucagon secretion. Here, using genetic mouse models, we demonstrate that Agpat5 expressed in agouti-related peptide neurons is required for their activation by hypoglycemia, for hypoglycemia-induced vagal nerve activity, and glucagon secretion. We find that inactivation of Agpat5 leads to increased fatty acid oxidation and ATP production and that suppressing Cpt1a-dependent fatty acid import into mitochondria restores hypoglycemia sensing. Collectively, our data show that AgRP neurons are involved in the control of glucagon secretion and that Agpat5, by partitioning fatty acyl-CoAs away from mitochondrial fatty acid oxidation and ATP generation, ensures that the fall in intracellular ATP, which triggers neuronal firing, faithfully reflects changes in glycemia.

The counterregulatory response to hypoglycemia is an essential survival function. It is controlled by an integrated network of glucose-responsive neurons, which trigger endogenous glucose production to restore normoglycemia. The complexity of this glucoregulatory network is, however, only partly characterized. In a genetic screen of a panel of recombinant inbred mice we previously identified Fgf15, expressed in neurons of the dorsomedial hypothalamus (DMH), as a negative regulator of glucagon secretion. Here, we report on the generation of Fgf15 CretdTomato mice and their use to further characterize these neurons. We show that they were glutamatergic and comprised glucose-inhibited and glucose-excited neurons. When activated by chemogenetics, Fgf15 neurons prevented the increase in vagal nerve firing and the secretion of glucagon normally triggered by insulin-induced hypoglycemia. On the other hand, they increased the activity of the sympathetic nerve in the basal state and prevented its silencing by glucose overload. Higher sympathetic tone increased hepatic Creb1 phosphorylation, Pck1 mRNA expression, and hepatic glucose production leading to glucose intolerance. Thus, Fgf15 neurons of the DMH participate in the counterregulatory response to hypoglycemia by a direct adrenergic stimulation of hepatic glucose production while suppressing vagally induced glucagon secretion. This study provides new insights into the complex neuronal network that prevents the development of hypoglycemia.

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 AMP-activated protein kinase (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 down-regulated 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 ROS production. In physiological studies, absence of GI neuron activity following AMPK suppression in the VMN had no impact on the counterregulatory hormone response to hypoglycemia nor on feeding. Thus, AMPK is required for GI neuron activity by controlling the expression of the anti-oxidant 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 gluco-detection systems that are connected to the VMN.

Feeding behavior is governed by homeostatic needs and motivational drive to obtain palatable foods. Here, we identify a population of glutamatergic neurons in the paraventricular thalamus of mice that express the glucose transporter Glut2 (encoded by Slc2a2) and project to the nucleus accumbens. These neurons are activated by hypoglycemia and, in freely moving mice, their activation by optogenetics or Slc2a2 inactivation increases motivated sucrose-seeking but not saccharin-seeking behavior. These neurons may control sugar overconsumption in obesity and diabetes.

Glucose transporter 2 ( Glut2)-positive cells are sparsely distributed in brain and play an important role in the stimulation of glucagon secretion in response to hypoglycemia. We aimed to determine if Glut2-positive cells can influence another response to hypoglycemia, i.e. increased cerebral blood flow (CBF). CBF of adult male mice devoid of Glut2, either globally ( ripglut1:glut2−/−) or in the nervous system only (NG2KO), and their respective controls were studied under basal glycemia and insulin-induced hypoglycemia using quantitative perfusion magnetic resonance imaging at 9.4 T. The effect on CBF of optogenetic activation of hypoglycemia responsive Glut2-positive neurons of the paraventricular thalamic area was measured in mice expressing channelrhodopsin2 under the control of the Glut2 promoter. We found that in both ripglut1:glut2−/− mice and NG2KO mice, CBF in basal conditions was higher than in their respective controls and not further activated by hypoglycemia, as measured in the hippocampus, hypothalamus and whole brain. Conversely, optogenetic activation of Glut2-positive cells in the paraventricular thalamic nucleus induced a local increase in CBF similar to that induced by hypoglycemia. Thus, Glut2 expression in the nervous system is required for the control of CBF in response to changes in blood glucose concentrations.

Brain glucose sensing plays an essential role in the regulation of energy homeostasis. Recent publications report that neurons expressing glucose transporter GLUT2 act as glucose sensors in different regions of the brain and contribute to the control of glucose homeostasis and feeding behavior. In this chapter we describe the methods used to explore glucose sensing in genetically tagged GLUT2-expressing neurons with slice electrophysiology.

Brain glucose sensing plays an essential role in the regulation of energy homeostasis. Recent publications report that neurons expressing glucose transporter GLUT2 act as glucose sensors in different regions of the brain and contribute to the control of glucose homeostasis and feeding behavior. In this chapter we describe the methods used to explore glucose sensing in genetically tagged GLUT2-expressing neurons with slice electrophysiology.

Glucose sensing neurons in the brainstem participate in the regulation of energy homeostasis but have been poorly characterized because of the lack of specific markers to identify them. Here we show that GLUT2 expressing neurons of the nucleus of the tractus solitarius form a distinct population of hypoglycemia activated neurons. Their response to low glucose is mediated by reduced intracellular glucose metabolism increased AMP activated protein kinase activity and closure of leak K+ channels. These are GABAergic neurons that send projections to the vagal motor nucleus. Light induced stimulation of channelrhodospin expressing GLUT2 neurons in vivo led to increased parasympathetic nerve firing and glucagon secretion. Thus GLUT2 neurons of the nucleus tractus solitarius link hypoglycemia detection to counterregulatory response. These results may help identify the cause of hypoglycemia associated autonomic failure a major threat in the insulin treatment of diabetes. © 2014 Elsevier Inc.

Benzodiazepines are widely used in clinics and for recreational purposes, but will lead to addiction in vulnerable individuals. Addictive drugs increase the levels of dopamine and also trigger long-lasting synaptic adaptations in the mesolimbic reward system that ultimately may induce the pathological behaviour. The neural basis for the addictive nature of benzodiazepines, however, remains elusive. Here we show that benzodiazepines increase firing of dopamine neurons of the ventral tegmental area through the positive modulation of GABA(A) (gamma-aminobutyric acid type A) receptors in nearby interneurons. Such disinhibition, which relies on alpha1-containing GABA(A) receptors expressed in these cells, triggers drug-evoked synaptic plasticity in excitatory afferents onto dopamine neurons and underlies drug reinforcement. Taken together, our data provide evidence that benzodiazepines share defining pharmacological features of addictive drugs through cell-type-specific expression of alpha1-containing GABA(A) receptors in the ventral tegmental area. The data also indicate that subunit-selective benzodiazepines sparing alpha1 may be devoid of addiction liability.

Glucokinase (Gck) is a critical regulator of glucose-induced insulin secretion by pancreatic β-cells. It has been suggested to also play an important role in glucose signaling in neurons of the ventromedial hypothalamic nucleus (VMN), a brain nucleus involved in the control of glucose homeostasis and feeding. To test the role of Gck in VMN glucose sensing and physiological regulation, we studied mice with genetic inactivation of the Gck gene in Sf1 neurons of the VMN (Sf1Gck−/− mice). Compared with control littermates, Sf1Gck−/− mice displayed increased white fat mass and adipocyte size, reduced lean mass, impaired hypoglycemia-induced glucagon secretion, and a lack of parasympathetic and sympathetic nerve activation by neuroglucopenia. However, these phenotypes were observed only in female mice. To determine whether Gck was required for glucose sensing by Sf1 neurons, we performed whole-cell patch clamp analysis of brain slices from control and Sf1Gck−/− mice. Absence of Gck expression did not prevent the glucose responsiveness of glucose-excited or glucose-inhibited Sf1 neurons in either sex. Thus Gck in the VMN plays a sex-specific role in the glucose-dependent control of autonomic nervous activity; this is, however, unrelated to the control of the firing activity of classical glucose-responsive neurons.

© 2015 Elsevier Ltd. 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.

Obesity has drastically increased over the last few decades. Obesity is associated with elevated insulin levels, which can gain access to the brain, including into dopamine neurons of the ventral tegmental area (VTA), a brain region critical for mediating reward-seeking behavior. Synaptic plasticity of VTA dopamine neurons is associated with altered motivation to obtain reinforcing substances such as food and drugs of abuse. Under physiological circumstances, insulin in the VTA can suppress excitatory synaptic transmission onto VTA dopamine neurons and reduce aspects of palatable feeding behavior. However, it is unknown how insulin modulates excitatory synaptic transmission in pathological circumstances such as hyperinsulinemia. Using patch-clamp electrophysiology, we demonstrate that, in a hyperinsulinemic mouse model, insulin has reduced capacity to cause a synaptic depression of VTA dopamine neurons, although both low-frequency stimulation-induced long-term depression and cannabinoid-induced depression were normal. These results suggest that insulin action in the VTA during pathological hyperinsulinemia is disrupted and may lead to increased feeding behavior.

SummarySalient but aversive stimuli inhibit the majority of dopamine (DA) neurons in the ventral tegmental area (VTA) and cause conditioned place aversion (CPA). The cellular mechanism underlying DA neuron inhibition has not been investigated and the causal link to behavior remains elusive. Here, we show that GABA neurons of the VTA inhibit DA neurons through neurotransmission at GABAA receptors. We also observe that GABA neurons increase their firing in response to a footshock and provide evidence that driving GABA neurons with optogenetic effectors is sufficient to affect behavior. Taken together, our data demonstrate that synaptic inhibition of DA neurons drives place aversion.

Agonists of GABA(B) receptors exert a bi-directional effect on the activity of dopamine (DA) neurons of the ventral tegmental area, which can be explained by the fact that coupling between GABA(B) receptors and G protein-gated inwardly rectifying potassium (GIRK) channels is significantly weaker in DA neurons than in GABA neurons. Thus, low concentrations of agonists preferentially inhibit GABA neurons and thereby disinhibit DA neurons. This disinhibition might confer reinforcing properties on addictive GABA(B) receptor agonists such as gamma-hydroxybutyrate (GHB) and its derivatives. Here we show that, in DA neurons of mice, the low coupling efficiency reflects the selective expression of heteromeric GIRK2/3 channels and is dynamically modulated by a member of the regulator of G protein signaling (RGS) protein family. Moreover, repetitive exposure to GHB increases the GABA(B) receptor-GIRK channel coupling efficiency through downregulation of RGS2. Finally, oral self-administration of GHB at a concentration that is normally rewarding becomes aversive after chronic exposure. On the basis of these results, we propose a mechanism that might underlie tolerance to GHB.

Orexin/hypocretin in psychiatric disorders: present state of knowledge and future potential