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.

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.

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.

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.

The Antinarcolepsy Drug Modafinil Reverses Hypoglycemia Unawareness and Normalizes Glucose Sensing of Orexin Neurons in Male Mice

Perifornical hypothalamus (PFH) orexin glucose-inhibited (GI) neurons that facilitate arousal have been implicated in hypoglycemia awareness. Mice lacking orexin exhibit narcolepsy, and orexin mediates the effect of the antinarcolepsy drug modafinil. Thus, hypoglycemia awareness may require a certain level of arousal for awareness of the sympathetic symptoms of hypoglycemia (e.g., tremors, anxiety). Recurrent hypoglycemia (RH) causes hypoglycemia unawareness. We hypothesize that RH impairs the glucose sensitivity of PFH orexin GI neurons and that modafinil normalizes glucose sensitivity of these neurons and restores hypoglycemia awareness after RH. Using patch-clamp recording, we found that RH enhanced glucose inhibition of PFH orexin GI neurons in male mice, thereby blunting activation of these neurons in low-glucose conditions. We then used a modified conditioned place preference behavioral test to demonstrate that modafinil reversed hypoglycemia unawareness in male mice after RH. Similarly, modafinil restored normal glucose sensitivity to PFH orexin GI neurons. We conclude that impaired glucose sensitivity of PFH orexin GI neurons plays a role in hypoglycemia unawareness and that normalizing their glucose sensitivity after RH is associated with restoration of hypoglycemia awareness. This suggests that the glucose sensitivity of PFH orexin GI neurons is a therapeutic target for preventing hypoglycemia unawareness.

Effects of glucose modulation in lateral hypothalamus on motivated behavior to obtain sucrose in an operant task

Orexin neurons in the Lateral Hypothalamus (LH) play an important role in food seeking behavior. Approximately 60 percent of LH orexin neurons are inhibited by elevated extracellular glucose. It has been shown that elevated LH glucose decreases conditioned place preference for a food associated chamber. However, it has never been shown how modulation of LH extracellular glucose effects a rat's motivation to work for food. In this experiment we used reverse microdialysis to modulate extracellular glucose levels in LH during an operant task. Results from a progressive ratio task demonstrated that 4 mM glucose perfusion significantly decreased the animal's motivation to work for sucrose pellets while not effecting the hedonic value of the pellets. In a second experiment we demonstrated that 4 mM but not 2.5 mM glucose perfusion was sufficient to significantly decrease the number of sucrose pellets earned. Finally, we showed that modulating LH extracellular glucose mid-session from 0.7 mM to 4 mM did not affect behavior. This indicates that once feeding behavior has begun the animal becomes unresponsive to changes in extracellular glucose levels in LH. Taken together these experiments indicate that LH glucose sensing neurons play an important role in motivation to initiate feeding. However, once consumption has begun it is likely that feeding is controlled by brain regions downstream of LH.

Lateral hypothalamus hypocretin/orexin glucose-inhibited neurons promote food seeking after calorie restriction

OBJECTIVE

The present study tests the hypothesis that changes in the glucose sensitivity of lateral hypothalamus (LH) hypocretin/orexin glucose-inhibited (GI) neurons following weight loss leads to glutamate plasticity on ventral tegmental area (VTA) dopamine neurons and drives food seeking behavior.

METHODS

C57BL/6J mice were calorie restricted to a 15% body weight loss and maintained at that body weight for 1 week. The glucose sensitivity of LH hypocretin/orexin GI and VTA dopamine neurons was measured using whole cell patch clamp recordings in brain slices. Food seeking behavior was assessed using conditioned place preference (CPP).

RESULTS

1-week maintenance of calorie restricted 15% body weight loss reduced glucose inhibition of hypocretin/orexin GI neurons resulting in increased neuronal activation with reduced glycemia. The effect of decreased glucose on hypocretin/orexin GI neuronal activation was blocked by pertussis toxin (inhibitor of G-protein coupled receptor subunit Gαi/o) and Rp-cAMP (inhibitor of protein kinase A, PKA). This suggests that glucose sensitivity is mediated by the Gαi/o-adenylyl cyclase-cAMP-PKA signaling pathway. The excitatory effect of the hunger hormone, ghrelin, on hcrt/ox neurons was also blocked by Rp-cAMP suggesting that hormonal signals of metabolic status may converge on the glucose sensing pathway. Food restriction and weight loss increased glutamate synaptic strength (indexed by increased AMPA/NMDA receptor current ratio) on VTA dopamine neurons and the motivation to seek food (indexed by CPP). Chemogenetic inhibition of hypocretin/orexin neurons during caloric restriction and weight loss prevented these changes in glutamate plasticity and food seeking behavior.

CONCLUSIONS

We hypothesize that this change in the glucose sensitivity of hypocretin/orexin GI neurons may drive, in part, food seeking behavior following caloric restriction.

Reducing neuronal nitric oxide synthase (nNOS) expression in the ventromedial hypothalamus (VMH) increases body weight and blood glucose levels while decreasing physical activity in female mice

Glucose-inhibited (GI) neurons of the ventromedial hypothalamus (VMH) depend on neuronal nitric oxide synthase (nNOS) and AMP-activated protein kinase (AMPK) for activation in low glucose. The Lopez laboratory has shown that the effects of estrogen on brown fat thermogenesis and white fat browning require inhibition of VMH AMPK. This effect of estrogen was mediated by downstream lateral hypothalamus (LH) orexin neurons (1,2). We previously showed that estrogen inhibits activation of GI neurons in low glucose by inhibiting AMPK (3). Thus, we hypothesized that VMH AMPK- and nNOS-dependent GI neurons project to and inhibit orexin neurons. Estrogen inhibition of AMPK in GI neurons would then disinhibit orexin neurons and stimulate brown fat thermogenesis and white fat browning, leading to decreased body weight. To test this hypothesis, we reduced VMH nNOS expression using nNOS shRNA in female mice and measured body weight, adiposity, body temperature, white and brown fat uncoupling protein (UCP1; an index of thermogenesis and browning), locomotor activity, and blood glucose levels. Surprisingly, we saw no effect of reduced VMH nNOS expression on body temperature or UCP1. Instead, body weight and adiposity increased by 30% over 2 weeks post injection of nNOS shRNA. This was associated with increased blood glucose levels and decreased locomotor activity. We also found that VMH nNOS-GI neurons project to the LH. However, stimulation of VMH-LH projections increased excitatory glutamate input onto orexin neurons. Thus, our data do not support our original hypothesis. Excitation of orexin neurons has previously been shown to increase physical activity, leading to decreased blood glucose and body weight (4). We now hypothesize that VMH nNOS-GI neurons play a role in this latter function of orexin neurons.