Hypothalamic Glucose Research Articles

Role of AMP‐Activated Protein Kinase in the Control of Appetite

AMP-activated protein kinase is a key enzyme in the regulation of energy metabolism. Its activation has pleiotropic effects in multiple tissues, including increased fatty acid oxidation, glucose uptake and glycolysis, as well as the inhibition of fatty acid and glycogen synthesis and gluconeogenesis, and stimulation of mitochondrial biogenesis. Recently, the AMP-activated protein kinase (AMPK) has also emerged as a regulator of appetite, contributing to the control of energy metabolism at both cell and the whole body levels. Pharmacological and genetic activation or inhibition of hypothalamic AMPK lead to increased or reduced food intake, respectively. AMPK appears to play a role in hypothalamic glucose and nutrient sensing and numerous studies have suggested a role for AMPK in mediating the orexigenic or anorexigenic effects of various endogenous and exogenous substances.

Isocaloric versus hypercaloric fat intake: Differential impact on hypothalamic NPY, POMC, glucose tolerance and adiposity in the rat

Isocaloric versus hypercaloric fat intake: Differential impact on hypothalamic NPY, POMC, glucose tolerance and adiposity in the rat

Neuronal Glucose Sensing: Still a Physiological Orphan?

Although hypothalamic glucose sensing is a long-established phenomenon, its physiological role remains unclear. New studies (Parton et al., 2007; Claret et al., 2007) disrupting glucose sensing in pro-opiomelanocortin neurons via differing methods have yielded disparate energy and glucose homeostasis phenotypes, suggesting that neuronal glucose sensing is not critical for these processes.

Antecedent Hindbrain Glucoprivation Does Not Impair the Counterregulatory Response to Hypoglycemia

Recurrent hypoglycemia impairs hormonal counterregulatory responses (CRRs) to further bouts of hypoglycemia. The hypothalamus and hindbrain are both critical for sensing hypoglycemia and triggering CRRs. Hypothalamic glucose sensing sites are implicated in the pathogenesis of defective CRRs; however, the contribution of hindbrain glucose sensing has not been elucidated. Using a rat model, we compared the effect of antecedent glucoprivation targeting hindbrain or hypothalamic glucose sensing sites with the effect of antecedent recurrent hypoglycemia on CRR to hypoglycemia induced 24 h later. Recurrent hypoglycemia decreased sympathoadrenal (1,470 +/- 325 vs. 3,811 +/- 540 pg/ml in controls [t = 60 min], P = 0.001) and glucagon secretion (222 +/- 43 vs. 494 +/- 56 pg/ml in controls [t = 60]), P = 0.003) in response to hypoglycemia. Antecedent 5-thio-glucose (5TG) injected into the hindbrain did not impair sympathoadrenal (3,806 +/- 344 pg/ml [t = 60]) or glucagon (513 +/- 56 pg/ml [t = 60]) responses to subsequent hypoglycemia. However, antecedent 5TG delivered into the third ventricle was sufficient to blunt CRRs to hypoglycemia. These results show that hindbrain glucose sensing is not involved in the development of defective CRRs. However, neural substrates surrounding the third ventricle are particularly sensitive to glucoprivic stimulation and may contribute importantly to the development of defective CRRs.

Changes in extracellular hypothalamic glucose in relation to feeding

The aim of the present in vivo microdialysis study was to investigate the relation between feeding and changes in glucose concentrations in the rat ventromedial hypothalamus (VMH). Absolute ambient glucose concentrations in VMH were 1.43 mm in non-deprived rats as compared to 0.94 mm after 24-h food deprivation. To examine whether feeding influences hypothalamic glucose, changes of glucose concentration over time were determined relative to a baseline. Experiments were conducted in relation to both, nutritional state (food-deprived rats vs. non-deprived rats) and feeding conditions throughout the experiment (freely feeding rats vs. rats without access to food). The results of this microdialysis study show clearly that glucose concentration in the VMH of rats increases significantly in relation to food intake. The data demonstrate that a 24-h food deprivation before the experiment further augments this increase (up to 350% from baseline) as compared to non-deprived conditions (up to 60% from baseline). However, the magnitude of food related increase in VMH glucose does not correlate with the individual amount of food eaten. In conclusion, the present study shows for the first time that VMH glucose concentrations increase with food intake in the early dark phase, indicating that such changes do not only occur after pharmacological treatment, but also under physiological feeding conditions. The results further indicate that the feeding related increase in VMH glucose depends on the nutritional state of the organism.

Cerebral glucose transporters expression and spatial learning in the K-ATP Kir6.2 −/− knockout mice

K-ATP channels formed of the Sur and Kir subunits are widely distributed in the brain. Sur1–Kir6.2 is the most common combination of K-ATP channel subunits in the brain and Kir6.2 plays an important role in glucose metabolism through pancreatic insulin secretion or hypothalamic glucose sensing. K-ATP channels have also been reported to play a role in memory processing. Therefore, the aim of the present experiment is to assess the gene and protein expression of GLUT1, GLUT3 and GLUT4 in various brain regions of Kir6.2 −/− K-ATP knockout mice and to test their working memory performance. GLUT4 was measured using two antibodies, one recognizing an intracellular epitope and the other, an extracellular epitope. Relative to their corresponding wild type, semi-quantitative immunohistochemistry showed that GLUT4 protein expression as measured by a GLUT4 antibody recognizing an extracellular epitope was increased in the Kir6.2 −/− K-ATP mice. However, there was only a small increase in GLUT4 labeling using the GLUT4 antibody recognizing the intracellular epitope. These results suggest a compensatory higher GLUT4 inclusion at the cellular neuronal membrane in the cerebral cortex, hippocampus and cerebellum of the Kir6.2 −/− K-ATP knockout mice. However, there was no change in GLUT4 gene expression assessed by TaqMan PCR except for a decrease in the cerebellum of these mice. Working memory performance of the Kir6.2 −/− K-ATP mice was disrupted at age of 12 weeks but not at 5 weeks. The mild glucose intolerance that is observed in the Kir6.2 knockout mice is unlikely to have created the memory deficits observed. Rather, in light of the effects of K-ATP channel modulators on memory, the memory deficits in the Kir6.2 −/− K-ATP mice are more likely due to the absence of the Kir6.2 and possible disruption of the GLUT4 activity in the brain.

Effects of ghrelin on hypothalamic glucose responding neurons in rats

Ghrelin is an endogenous ligand of the growth hormone secretagogue receptor (GHS-R) with potent stimulatory effects on food intake. The aim of the present study was to investigate the effects of ghrelin on neuronal activity of hypothalamic glucose responding neurons. Single unit discharges in the lateral hypothalamic area (LHA), the ventromedial hypothalamic nucleus (VMH), and the parvocellular part of the paraventricular nucleus(pPVN) were recorded extracellularly by means of four-barrel glass micropipettes in anesthetized rats. The activity of glucose-sensitive neurons (GSNs) in the LHA, pPVN, and of glucoreceptor neurons (GRNs) in the VMH modulated by administration of ghrelin was analyzed. In the LHA, the majority of GSNs (17/25) increased in frequency due to ghrelin. Whereas the majority of VMH-GRNs (27/33) and pPVN-GSNs (9/13) was inhibited. The responses to ghrelin were abolished by pretreatment of [ d-Lys-3]-GHRP-6, ghrelin receptor antagonist. These data indicate that the glucose responding neurons in the LHA, VMH, and pPVN are also involved in the orexigenic actions of ghrelin in the hypothalamic circuits, although AgRP/NPY neurons in the arcuate nucleus (ARC) are the primary targets of ghrelin.

Regulation of Blood Glucose by Hypothalamic Pyruvate Metabolism

The brain keenly depends on glucose for energy, and mammalians have redundant systems to control glucose production. An increase in circulating glucose inhibits glucose production in the liver, but this negative feedback is impaired in type 2 diabetes. Here we report that a primary increase in hypothalamic glucose levels lowers blood glucose through inhibition of glucose production in rats. The effect of glucose requires its conversion to lactate followed by stimulation of pyruvate metabolism, which leads to activation of adenosine triphosphate (ATP)-sensitive potassium channels. Thus, interventions designed to enhance the hypothalamic sensing of glucose may improve glucose homeostasis in diabetes.

Effect of 5-HT1A receptor activation on hypothalamic glucose

The somatodendritic 5-HT1A agonist 8-OH-DPAT reduces serotonergic activity and stimulates feeding in freely feeding rats. Interactions between circulating glucose and 5-HT1A receptor expression related to feeding have been described. The aim of the present microdialysis study was to (1) describe the relation between feeding and glucose in the LH, (2) to investigate if peripherally administered 8-OH-DPAT itself has an effect on extracellular glucose in the lateral hypothalamus (LH) of conscious rats. Baseline glucose concentrations were significantly different in microdialysis samples obtained from food deprived rats compared to freely feeding rats. After re-feeding, a significant rise in glucose levels by 45% was observed in the formerly food deprived rats. In freely feeding rats, 8-OH-DPAT (0.3 mg/kg, i.p.) reduced glucose level in the LH significantly. The effect of 8-OH-DPAT on brain glucose was antagonized by pre-treatment with the 5-HT1A antagonist WAY 100635 (3 mg/kg i.p.) which had no effect on its own. The data indicate, therefore, that the effect of 8-OH-DPAT on hypothalamic glucose is mediated by 5-HT1A receptors. In contrast, the same dose of 8-OH-DPAT proven effective in the brain had no effect on peripheral glucose. Only a very high dose of the 5-HT1A agonist (1.8 mg/kg i.p.) had a hyperglycaemic effect in the periphery. In conclusion, the present results show for the first time, that glucose in the lateral hypothalamus increases with a meal. The data demonstrate furthermore 8-OH-DPAT-induced changes of hypothalamic glucose level, implicating 5-HT1A receptors being involved not only in the control of hypothalamic 5-HT as shown before, but also in the control of hypothalamic glucose.

Hypothalamic glucose sensor: similarities to and differences from pancreatic beta-cell mechanisms.

Glucose-responsive neurons in the ventromedial hypothalamus (VMH) are stimulated when glucose increases from 5 to 20 mmol/l and are thought to play an essential role in regulating metabolism. The present studies examined the role of glucose metabolism in the mechanism by which glucose-responsive neurons sense glucose. The pancreatic, but not hepatic, form of glucokinase was expressed in the VMH, but not cerebral cortex, of adult rats. In brain slice preparations, the transition from 5 to 20 mmol/l glucose stimulated approximately 17% of the neurons (as determined by single-cell extracellular recording) from VMH but none in cortex. In contrast, most cells in both VMH and cortex were silent below 1 mmol/l and active at 5 mmol/l glucose. Glucosamine, 2-deoxyglucose, phloridzin, and iodoacetic acid blocked the activation of glucose-responsive neurons by the transition from 5 to 20 mmol/l glucose. Adding 15 mmol/l mannose, galactose, glyceraldehyde, glycerol, and lactate to 5 mmol/l glucose stimulated glucose-responsive neurons. In contrast, adding 15 mmol/l pyruvate to 5 mmol/l glucose failed to activate glucose-responsive neurons, although pyruvate added to 0 mmol/l glucose permitted neurons to maintain activity. Tolbutamide activated glucose-responsive neurons; however, diazoxide only blocked the effect of glucose in a minority of neurons. These data suggest that glucose-responsive neurons sense glucose through glycolysis using a mechanism similar to the mechanism of pancreatic beta-cells, except that glucose-responsive neurons are stimulated by glycerol and lactate, and diazoxide does not generally block the effect of glucose.

Stimulation of rat hypothalamus by microdialysis with K+: increase of ACh release elevates plasma glucose.

The effects of stimulation of the ventromedial hypothalamus (VMH) or lateral hypothalamus (LH) with potassium chloride through a microdialysis probe were studied. The concentrations of ACh and norepinephrine (NE) in the dialysate obtained from the hypothalamic nuclei and plasma glucose concentration were measured. Stimulation of the hypothalamic nuclei, VMH and LH, with potassium increased the plasma glucose level as well as the extracellular concentrations of ACh and choline. Addition of atropine, a muscarinic ACh receptor antagonist, into the potassium solution reduced the increase in the level of plasma glucose. Cholinergic stimulation of these nuclei with neostigmine increased the extracellular concentrations of ACh and plasma glucose. Stimulation of the nuclei with potassium also increased the release of NE. However, stimulation of the VMH or LH with NE and/or pargyline, a monoamine oxidase inhibitor, through the dialysis probe membrane did not significantly increase the plasma glucose concentration. These results suggest that activation of the muscarinic cholinergic or ACh-receptive neurons in the hypothalamic nuclei, VMH and LH, contribute to the elevation of plasma glucose level.

Suppression of central noradrenergic neuronal activity inhibits hyperglycemia.

Hypothalamic noradrenergic neuronal activity (NNA) and hepatic glucose output are stimulated by stress. The aim of the present investigation was to examine whether the blockade of noradrenergic responses to stress might suppress the associated hyperglycemia. Mass spectrometry was used for analysis of norepinephrine (NE) and its neuronal metabolite 3,4-dihydroxyphenylglycol (DHPG) in rat hypothalamus, and the ratio DHPG/NE was used as an index of NNA. Treatment of rats with 2-deoxy-D-glucose (500 mg/kg ip, -30 min), yohimbine (10 mg/kg ip, -20 min), or neostigmine (2 micrograms icv, -60 min) increased both NNA and serum glucose (P < 0.05). When rats were additionally pretreated with pentobarbital (60 mg/kg ip; -60 min), the NNA responses were blocked (P < 0.01). At the same time the hyperglycemic responses were also inhibited (P < 0.01). In rats that had reduced NNA due to 7 days "gentling," serum glucose levels were also significantly reduced (P < 0.001) compared with naive controls. The data demonstrate that inhibition of central noradrenergic activity is also associated with an inhibition of hyperglycemia, raising the concept that therapies aimed at reducing central NNA may have a role in the management of diseases with excessive hepatic glucose output such as non-insulin-dependent diabetes mellitus.

Site of action of putative lipostatic factor: hypothalamic metabolism of parabiotic rats

Studies were conducted to determine whether metabolic adaptation occurred in the hypothalamus of overfed parabiotic rats and their partners to distinguish between the adaptations caused by increased caloric intake and those caused by the production of a "lipostatic factor." The induction of overfed obesity in one parabiotic partner was employed to test the hypothesis that a putative lipostatic factor produced in the obese parabiotic elicited the hypophagic-lipid-mobilizing effect observed in the lean parabiotic via alterations in hypothalamic fatty acid and glucose metabolism. Fatty acid oxidation in the ventrolateral hypothalamus (VLH) of overfed parabiotic rats and their partners was lower than in ad libitum parabiotic rats. Net flux of glucose through the VLH gamma-aminobutyric acid (GABA) shunt was elevated in overfed parabiotic rats compared with the net flux observed in their partners and ad libitum parabiotic rats, the levels being similar in these last two groups. Net flux of glucose through the ventromedial hypothalamic (VMH) pentose shunt in overfed parabiotic rats and their partners was elevated relative to ad libitum parabiotic rats. The putative lipostatic factor may act to regulate energy balance through modification of VLH fatty acid oxidation and/or glucose flux via the VMH pentose shunt.

Impaired glucose tolerance coincides with abnormal release of growth hormone following a glucose load as well as in response to TRH in acromegaly.

It is known that some acromegalic patients exhibited a paradoxical release of growth hormone (GH) after glucose administration. We have attempted to investigate a relationship between the paradoxical GH secretion with the abnormal glucose tolerance test present in some cases of acromegaly. We also studied the inappropriate increase in GH levels following thyrotropin releasing hormone (TRH) injection which is present in some acromegalics. We found that only those patients who had an abnormal glucose tolerance test exhibited simultaneously, the paradoxical release of GH, moreover, the same patients showed GH release following TRH administration. This observation suggests that some acromegalics have an abnormality in their hypothalamic glucose receptor and such abnormality is associated with abnormal GH secretion when TRH is administered. On basis of these findings it is suggested that the hypothalamus may play an important role in the pathogenesis of acromegaly in these cases.

Uptake and utilization of metabolites in specific brain sites relative to feeding status

Two experiments were conducted to assess potential alterations in fatty acid and glucose metabolism within specific brain sites in relation to the feeding status of the rat. An in vivo serial analysis of brain palmitate and glucose uptake demonstrated that hypothalamic uptake of these substrates was reciprocally altered with respect to satiety. Hypothalamic uptake of palmitate was increased by 300% and uptake of glucose was decreased by 30% in fasted compared with fed rats. Other regional differences were observed and discussed in the text. An in vitro analysis showed that hypothalamic fatty acid oxidation was affected by feeding status. The ventrolateral hypothalamus (VLH) of fasted rats had 45% greater rates of fatty acid oxidation than VLH of fed rats. No alterations were observed for VLH glucose oxidation and ventromedial hypothalamic glucose and fatty acid oxidation when comparing fed and fasted rats. Other brain sites did not show variance for glucose and fatty acid metabolism relative to feeding status. Fatty acid uptake and subsequent metabolism in the hypothalamus and other brain sites may be one component of food intake control and energy balance regulation.

Paradoxical release of growth hormone during oral glucose tolerance test in patients with abnormal glucose tolerance

Oral glucose tolerance tests were performed on 24 patients characterized as having abnormal glucose tolerance (AGT) and on 27 control subjects. Serums for glucose, growth hormone and insulin determinations were serially obtained for 4 hr after glucose administration. As serum glucose declined 2 hr or more after glucose ingestion a rise in growth hormone, as has been previously described, was observed in 40% of control subjects and 12% of AGT patients. However, of interest was a paradoxical early increase in growth hormone levels noted in 44% of lean AGT subjects occurring during the first 2 hr of the test with glucose levels rising. This response was seen in only one of 8 obese patients with AGT and in none of the control subjects. An abnormality in the hypothalamic glucose receptors in the ventromedial nucleus is a possible explanation for the changes observed. It is possible that this early inappropriate increase in growth hormone release may in some nonobese subjects with AGT contribute to the abnormal oral glucose tolerance tests observed.

Hypothalamic obesity in female rats in absence of vagally mediated hyperinsulinemia.

In order to assess the role of vagally mediated hyperinsulinemia in hypothalamic obesity, plasma insulin and glucose levels were assayed in vagotomized and sham-vagotomized female rats after a 6-h fast and after a measured glucose meal both before and 10-14 days after ventromedial hypothalamic (VMH) lesions. Both groups displayed similar gains in body weight in the first 10 days after VMH lesions, but only the sham-vagotomized VMH-lesioned animals displayed elevated fasting insulin levels. Fasting glucose levels did not differ either before or after the lesion. The insulin response to oral glucose was increased in VMH rats, both in vagotomized and sham-vagotomized animals, and it is concluded that the hyperresponsiveness to oral glucose is independent of vagal mediation. Vagotomy markedly exaggerated the glucose and insulin response to oral glucose loading in both intact rats and rats with VMH lesions, probably as a result of more rapid absorption of glucose from the intestine. It is concluded that the fasting hyperinsulinemia that is characteristic of VMH animals is under vagal control and that its elimination does not prevent the development of obesity.

Increased feeding in response to decreased glucose utilization in the rat and monkey.

SMITH, GERARD P., AND ALAN N. EPSTEIN. Increased feeding in response to decreased glucose utili<ation ilz the rat and monkey. Am. J Physiol. 217(4) : 1083-1087. 1969.-The glucostatic hypothesis for the control of food intake predicts that decreased glucose utilization leads to increased food intake. Since the glucose analogue, 2-deoxy-D-glucose (Z-DG), produces decreased intracellular glucose utilization of most tissues and particularly of brain, the injection of 2-DG into 5 monkeys and 28 rats provided an explicit test of the predicted relationship. Both monkeys and rats ate more after 2-DG. The most effective systemic doses of Z-DG were 300 mg/kg (iv) in monkeys and 750 mg/kg (ip) in rats. Larger doses ( < 1,000 mg/kg) produced drowsiness, stupor, ataxia, or retching, but not convulsions. When separate groups of rats were tested with either insulin (6 U Iletin) or 2-DG (750 mg/kg), they increased their mean food intake to approximately the same amount, but the rats treated with Z-DG ate sooner. Shorter latency of feeding with 2-DG is interpreted as resulting from the fact that 2-DG produces decreased glucose utilization of brain directly, whereas insulin reduces cerebral glucose utilization indirectly as a consequence of hypoglycemia. Since Z-DG produced marked hyperglycemia during the period of increased feeding, these results show that the abrupt onset of decreased glucose utilization, not hypoglycemia, is a sufficient condition for feeding in mammals.