Hormonal and metabolic effects of neuroglucopenia

Intra-islet α-cell Gs signaling promotes glucagon release

Glucagon, a hormone released from pancreatic α-cells, is critical for maintaining euglycemia and plays a key role in the pathophysiology of diabetes. To stimulate the development of new classes of therapeutic agents targeting glucagon release, key α-cell signaling pathways that regulate glucagon secretion need to be identified. Here, we focused on the potential importance of α-cell Gs signaling on modulating α-cell function. Studies with α-cell-specific mouse models showed that activation of α-cell Gs signaling causes a marked increase in glucagon secretion. We also found that intra-islet adenosine plays an unexpected autocrine/paracrine role in promoting glucagon release via activation of α-cell Gs-coupled A2A adenosine receptors. Studies with α-cell-specific Gαs knockout mice showed that α-cell Gs also plays an essential role in stimulating the activity of the Gcg gene, thus ensuring proper islet glucagon content. Our data suggest that α-cell enriched Gs-coupled receptors represent potential targets for modulating α-cell function for therapeutic purposes.

α-cell Gq signaling is critical for maintaining euglycemia

Glucagon, a hormone released from pancreatic α cells, plays a key role in maintaining euglycemia. New insights into the signaling pathways that control glucagon secretion may stimulate the development of novel therapeutic agents. In this study, we investigated the potential regulation of α cell function by G proteins of the G_q family. The use of a chemogenetic strategy allowed us to selectively activate G_q signaling in mouse α cells in vitro and in vivo. Acute stimulation of α cell G_q signaling led to elevated plasma glucagon levels, accompanied by increased insulin release and improved glucose tolerance. Moreover, chronic activation of this pathway greatly improved glucose tolerance in obese mice. We also identified an endogenous G_q-coupled receptor (vasopressin 1b receptor; V1bR) that was enriched in mouse and human α cells. Agonist-induced activation of the V1bR strongly stimulated glucagon release in a G_q-dependent fashion. In vivo studies indicated that V1bR-mediated glucagon release played a key role in the counterregulatory hyperglucagonemia under hypoglycemic and glucopenic conditions. These data indicate that α cell G_q signaling represents an important regulator of glucagon secretion, resulting in multiple beneficial metabolic effects. Thus, drugs that target α cell–enriched G_q-coupled receptors may prove useful to restore euglycemia in various pathophysiological conditions.

Central NUCB2/Nesfatin-1-expressing neurones belong to the hypothalamic-brainstem circuitry activated by hypoglycaemia

Nesfatin-1 is a recently identified 82 amino acid peptide shown to have an anorexigenic effect on rodents when administrered centrally and peripherally. Nesfatin-1 is expressed not only in neurones of various brain areas, including the hypothalamic and brainstem nuclei, but also in peripheral organs, such as the stomach and the pancreas. Nesfatinergic neurones were reported to participate in the regulation of satiety signals and in the responses to other stimuli, including restraint stress, abdominal surgery, and lipopolysaccharide-induced inflammation. The present study aimed to investigate whether NUCB2/nesfatin-1 expressing neurones also take part in the central signalling activated in response to hypoglycaemia and therefore are involved in central glucose sensing. Using immunolabelling methods based on the detection of the neuronal activation marker c-Fos and of nesfatin-1, we showed that peripheral injection of insulin induced a strong activation of nesfatin-1-expressing neurones in the brain vagal-regulatory nuclei, including the arcuate nucleus, paraventricular nucleus, lateral hypothalamic area, dorsal motor nucleus of the vagus (DMNX) and nucleus of the tractus solitarius. In response to intracellular glucopaenia induced by i.p. or i.c.v. 2-deoxyglucose injection, the c-Fos/nesfatin-1 colocalisations observed at the hypothalamic and brainstem levels were similar to those observed after insulin-induced hypoglycaemia. Moreover, using Fluorogold as a retrograde tracer, we showed that nesfatinergic preganglionic DMNX neurones activated by hypoglycaemia target the stomach and the pancreas. Taken together, these results suggest that a subpopulation of nesfatinergic neurones belongs to the central network activated by hypoglycaemia, and that nesfatin-1 participates in the triggering of physiological and hormonal counter-regulations observed in response to hypoglycaemia.

Impaired hypothalamic Fto expression in response to fasting and glucose in obese mice

Objective:

Recent genome-wide association studies have identified a strong association between obesity and common variants in the fat mass and obesity associated (FTO) gene. FTO has been detected in the hypothalamus, but little is known about its regulation in that particular brain structure. The present study addressed the hypothesis that hypothalamic FTO expression is regulated by nutrients, specifically by glucose, and that its regulation by nutrients is impaired in obesity.

Research design and methods:

The effect of intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) administration of glucose on hypothalamic Fto mRNA levels was examined in fasted mice. Additionally, the effect of glucose on Fto mRNA levels was also investigated ex vivo using mouse hypothalamic explants. Lastly, the effect of i.p. glucose injection on hypothalamic Fto immunoreactivity and food intake was compared between lean wild-type and obese ob/ob mice.

Results:

In wild-type mice, fasting reduced both Fto mRNA levels and the number of Fto-immunoreactive cells in the hypothalamus, whereas i.p. glucose treatment reversed this effect of fasting. Furthermore, i.c.v. glucose treatment also increased hypothalamic Fto mRNA levels in fasted mice. Incubation of hypothalamic explants at high glucose concentration increased Fto mRNA levels. In ob/ob mice, both fasting and i.p. glucose treatment failed to alter the number of Fto-immunoreactive cells in the hypothalamus. Glucose-induced feeding suppression was abolished in ob/ob mice.

Conclusion:

Reduction in hypothalamic Fto expression after fasting likely arises at least partly from reduced circulating glucose levels and/or reduced central action of glucose. Obesity is associated with impairments in glucose-mediated regulation of hypothalamic Fto expression and anorexia. Hypothalamic Fto-expressing neurons may have a role in the regulation of metabolism by monitoring metabolic states of the body.

Obesity-associated improvements in metabolic profile through expansion of adipose tissue

Excess caloric intake can lead to insulin resistance. The underlying reasons are complex but likely related to ectopic lipid deposition in nonadipose tissue. We hypothesized that the inability to appropriately expand subcutaneous adipose tissue may be an underlying reason for insulin resistance and beta cell failure. Mice lacking leptin while overexpressing adiponectin showed normalized glucose and insulin levels and dramatically improved glucose as well as positively affected serum triglyceride levels. Therefore, modestly increasing the levels of circulating full-length adiponectin completely rescued the diabetic phenotype in ob/ob mice. They displayed increased expression of PPARgamma target genes and a reduction in macrophage infiltration in adipose tissue and systemic inflammation. As a result, the transgenic mice were morbidly obese, with significantly higher levels of adipose tissue than their ob/ob littermates, leading to an interesting dichotomy of increased fat mass associated with improvement in insulin sensitivity. Based on these data, we propose that adiponectin acts as a peripheral "starvation" signal promoting the storage of triglycerides preferentially in adipose tissue. As a consequence, reduced triglyceride levels in the liver and muscle convey improved systemic insulin sensitivity. These mice therefore represent what we believe is a novel model of morbid obesity associated with an improved metabolic profile.

[Metabolic syndrome. Origin within the central nervous system?]

All efforts based on current concepts of obesity have failed to stop the epidemic. Hitherto, the question of body mass regulation focused on regulatory principles centered on the hypothalamus. We present the novel view that the brain (cerebral hemispheres, hypothalamus) requests energy in an active manner from the body (allocation) or the environment (food intake). Disruption of one of the cerebral energy request pathways is highly relevant to the development of obesity, metabolic syndrome and diabetes type 2. We have reviewed the literature from this new perspective, putting the brain as the focal midpoint of all metabolic activity.

Brain glucose and insulin: effects on food intake and brain biogenic amines of rainbow trout

The effects of central (intracerebroventricular, 9 microg fish(-1)) and peripheral (intraperitoneal, 4 mg kg(-1)) administration of bovine insulin, as well as the effect of hyperglycemia (oral administration of 1 g glucose fish(-1)) and brain glucodeprivation (intracerebroventricular administration of 2-deoxy-D-glucose) on food intake and levels of brain (telencephalon, preoptic area, and hypothalamus) biogenic amines (serotonin, dopamine, noradrenaline and their metabolites 5-hydroxyindoleacetic acid, and dihydroxyphenylacetic acid) were assessed on rainbow trout ( Oncorhynchus mykiss). Treatment with insulin inhibited food intake after 26 or 52 h of administration, central or peripheral, respectively. This effect was still apparent after 74 h of central treatment. When assessing changes in the levels of biogenic amines after 26 h of central insulin administration, there was a significant increase in the levels of 5-hydroxyindoleacetic acid, and in the ratio of dihydroxyphenylacetic acid/dopamine of insulin-treated fish, in telencephalon and hypothalamus, respectively. These results suggest that peripherally administered insulin is involved in a feedback regulatory loop with food intake and body weight. Moreover, at least part of the effects of insulin could be mediated by hypothalamic dopaminergic activity. The strong hyperglycemia induced by oral administration of glucose did not induce significant changes either on food intake (control versus treated), or in brain levels of biogenic amines. The intracerebroventricular administration of 2-deoxy-D-glucose induced an increase in food intake without altering plasma glucose levels, suggesting that fish brain possesses a control system for detecting hypoglycemia in plasma and therefore keep brain glucose levels high enough for brain function.

Endogenous opioid analgesia in hemorrhagic shock

BACKGROUND

Hemorrhagic shock produces an immediate activation of the autonomic nervous system and endogenous opioid pathways. Our studies have demonstrated that endogenous opioid activation aggravates the hemodynamic and inflammatory responses to shock. However, it is unclear whether endogenous opioid activation is triggered by noxious stimuli and furthermore whether it produces analgesia.

METHODS

Experiments were conducted in chronically catheterized, conscious, unrestrained, nonheparinized, male, Sprague-Dawley rats subjected to fixed pressure hemorrhage. Blood samples were obtained for determinations of circulating beta-endorphin and substance P. Analgesia was measured using the tail-flick response to a noxious stimulus before and during hemorrhage. The contribution of sensory neurons to eliciting the neuroendocrine, opioid, and inflammatory responses to hemorrhage was investigated in capsaicin-treated animals.

RESULTS

Hemorrhagic shock produced marked naltrexone-sensitive analgesia without significant modulation of substance P. Peripheral sensory denervation did not alter the hemodynamic, neuroendocrine, or inflammatory responses to shock.

CONCLUSION

Endogenous opioid activation during shock produces analgesia. Sensory neuron activation appears to have limited effect on shock-induced hemodynamic and proinflammatory responses. Furthermore, these results suggest that the activation of neuroendocrine and opioid pathways during shock is not likely to be a response to noxious stimuli.

Stress-specific opioid modulation of haemodynamic counter-regulation

1. The haemodynamic and cardiovascular responses to stress, in addition to being under control of the autonomic nervous system, are also under opiate modulation. Our studies have provided evidence for activation of the endogenous opioid system in haemorrhagic shock, sepsis and trauma. Furthermore, we have demonstrated that both central and systemic opiate administration to naïve rats result in marked alterations in haemodynamic responses, which are associated with activation of the sympathetic nervous system. 2. Because of the ubiquitous presence of opiate receptors in both the central nervous system and peripheral tissues, as well as their production and release centrally and peripherally, this facilitates an endocrine as well as a paracrine contribution to modulating vascular responses to stress, either directly or indirectly. Results from previous studies suggest that endogenous opioids are not involved in mediating the lipopolysacharide-induced hypotensive response. 3. In more recent studies, we have examined the role of opiate receptor activation in modulating the haemodynamic and neuroendocrine responses to fixed pressure haemorrhagic shock in conscious unrestrained rats. Using systemic opiate blockade (naltrexone, 15 mg/kg, i.p.) prior to haemorrhage, we have observed that blood loss required to achieve mean arterial blood pressure of 40 mmHg was higher in naltrexone-treated animals than in time-matched saline controls. Interestingly, the haemodynamic modulation exerted by naltrexone cannot be attributed to differences in circulating catecholamine levels. Haemorrhage produced an immediate and progressive increase in circulating adrenaline and noradrenaline levels, reaching values that were 50- and 20-fold higher than basal, respectively. Naltrexone pretreatment did not alter the time-course or magnitude of the rise in circulating levels of catecholamines. 4. These results indicate that endogenous opioid activation contributes to the haemodynamic dishomeostasis associated with blood loss. Our findings suggest stress-specific roles for opiate-sensitive haemodynamic counter-regulatory responses.

Contribution of excitatory amino acids to hypoglycemic counter-regulation

We determined the contribution of central N-methyl-D-aspartate (NMDA) receptor activation to the neuro-endocrine counter-regulatory response to insulin-induced hypoglycemia. Glucose kinetics, gluconeogenic substrate balance and counter-regulatory hormonal responses were determined in two groups of conscious dogs fitted with chronic vascular catheters and intracerebroventricular (i.c.v.) cannula. Peripheral insulin infusion (5 mU/kg per min for 3 h) decreased plasma glucose levels 40% and increased the rate of glucose appearance (R(a)) 2-fold. This was associated with significant increases in net hepatic uptake of glycerol and lactate, without any change in the net hepatic uptake of alanine. i.c.v. pretreatment with MK-801, an NMDA receptor antagonist, blunted (50%) the rise in glucose R(a) as well as the increase in the net hepatic uptake of glycerol and lactate. Hypoglycemia increased plasma cortisol (3-fold to 14.3+/-1 mg/dl) and epinephrine levels (14-fold to 3811+/-172 pg/ml), and this stress response was attenuated (30% and 60%, respectively) by MK-801 pretreatment. In controls, MK-801 did not alter the increase in norepinephrine or glucagon elicited by hypoglycemia. These results indicate that during hypoglycemia, central excitatory amino acids contribute to the modulation of the glucoregulatory response through activation of NMDA receptors, resulting in stimulation of the sympathoadrenal and hypothalamic-pituitary adrenal axis. This mechanism appears to play an important role in the sustained elevation in hepatic glucose production during hypoglycemia.

Gut-derived proteolysis during insulin-induced hypoglycemia: the pain that breaks down the gut

The metabolic events associated with early response to injury have received little attention because of the confounding effects of the hemodynamic alterations that normally occur during this early phase. We have used a well established and reproducible model of insulin-induced hypoglycemia in the conscious dog to define the glucose and amino acid kinetic alterations as well as the hormonal and interorgan amino acid and gluconeogenic precursor flux characteristics of the "ebb" phase of postinjury metabolism. The results from our whole-body response have demonstrated on enhanced rate of whole body proteolysis and amino acid oxidation. The site of the majority of the proteolytic response has been demonstrated to be the extra-hepatic splanchnic tissues or gut. These findings have been supported by studies focusing on the specific organ changes, which have demonstrated alterations compatible with impaired proliferation at the level of the gut mucosa. Furthermore, the regulation of this gut-derived proteolysis has been demonstrated to be mediated by the glucopenia at the level of the central nervous system. The specific site of this response is still elusive; however, the mediators seem to involve not only the traditional hormonal and neurotransmitter pathways but also the release of endogenous opioids and opiates. Although a cause-effect relationship has not yet been demonstrated for the control of gut-derived proteolysis by opioids and opiates, we present evidence that leads us to hypothesize that relationship as a possible regulatory mechanism.

Ventromedial hypothalamic lesions in rats suppress counterregulatory responses to hypoglycemia

The central nervous system has been implicated in the activation of counterregulatory hormone release during hypoglycemia. However, the precise loci involved are not established. To determine the role of the ventromedial hypoglycemia, we performed hypoglycemic clamp studies in conscious Sprague-Dawley rats with bilateral VMH lesions produced by local ibotenic acid injection 2 wk earlier. Rats with lesions in the lateral hypothalamic area, frontal lobe, sham operated (stereotaxic needle placement into hypothalamus without injection), and naive animals served as control groups. The clamp study had two phases. For the first hour plasma glucose was fixed by a variable glucose infusion at euglycemia (approximately 5.9 mM). Thereafter, for an additional 90 min, glucose was either allowed to fall to (a) mild hypoglycemia (approximately 3.0 mM) or (b) more severe hypoglycemia (approximately 2.5 mM). Glucagon and catecholamine responses of lateral hypothalamic area-, frontal lobe-lesioned, sham operated, and naive animals were virtually identical at each hypoglycemic plateau. In contrast, glucagon, epinephrine, and norepinephrine responses in the VMH-lesioned rats were markedly inhibited; hormones were diminished by 50-60% during mild and by 75-80% during severe hypoglycemia as compared with the other groups. We conclude that the VMH plays a crucial role in triggering the release of glucagon and catecholamines during hypoglycemia.