Induction of Fos protein in the rat hypothalamus elicited by insulin-induced hypoglycemia

Endogenous Androgens Diminish Food Intake and Activation of Orexin A Neurons in Response to Reduced Glucose Availability in Male Rats

Sex steroids modify feeding behavior and body weight regulation, and androgen reportedly augments food intake and body weight gain. To elucidate the role of endogenous androgens in the feeding regulation induced by reduced glucose availability, we examined the effect of gonadectomy (orchiectomy) on food intake and orexin A neuron’s activity in the lateral hypothalamic/perifornical area (LH/PFA) in response to reduced glucose availability (glucoprivation) induced by 2-deoxy-d-glucose (2DG) administration in male rats. Rats (7W) were bilaterally orchiectomized (ORX group) or sham operated (Sham group). Seventeen days after the surgery, food intake response to 2DG (400 mg/kg, i.v.) was measured for 4 h after the infusion. The same experiment was performed for the immunohistochemical examination of c-Fos-expressing orexin A neurons in the LH/PFA and c-Fos expression in the arcuate nucleus (Arc). Food intake induced by glucoprivation was greater in the ORX group than the Sham group, and the glucoprivation-induced food intake was inversely correlated with plasma testosterone concentration. Glucoprivation stimulated c-Fos expression of the orexin A neurons at the LH/PFA and c-Fos expression in the dorsomedial Arc. The number and percentage of c-Fos-expressing orexin A neurons in the LH/PFA and c-Fos expression in the dorsomedial Arc were significantly higher in the ORX group than the Sham group. This indicates that endogenous androgen, possibly testosterone, diminishes the food intake induced by reduced glucose availability, possibly via the attenuated activity of orexin A neuron in the LH/PFA and neurons in the dorsomedial Arc.

Central Mechanisms of Glucose Sensing and Counterregulation in Defense of Hypoglycemia

Glucose homeostasis requires an organism to rapidly respond to changes in plasma glucose concentrations. Iatrogenic hypoglycemia as a result of treatment with insulin or sulfonylureas is the most common cause of hypoglycemia in humans and is generally only seen in patients with diabetes who take these medications. The first response to a fall in glucose is the detection of impending hypoglycemia by hypoglycemia-detecting sensors, including glucose-sensing neurons in the hypothalamus and other regions. This detection is then linked to a series of neural and hormonal responses that serve to prevent the fall in blood glucose and restore euglycemia. In this review, we discuss the current state of knowledge about central glucose sensing and how detection of a fall in glucose leads to the stimulation of counterregulatory hormone and behavior responses. We also review how diabetes and recurrent hypoglycemia impact glucose sensing and counterregulation, leading to development of impaired awareness of hypoglycemia in diabetes.

Sensing Glucose in the Central Melanocortin Circuits of Rainbow Trout: A Morphological Study

In mammals, glucosensing markers reside in brain areas known to play an important role in the control of food intake. The best characterized glucosensing mechanism is that dependent on glucokinase (GK) whose activation by increased levels of glucose leads in specific hypothalamic neurons to decreased or increased activity, ultimately leading to decreased food intake. In fish, evidence obtained in recent years suggested the presence of GK-like immunoreactive cells in different brain areas related to food intake control. However, it has not been established yet whether or not those neuronal populations having glucosensing capacity are the same that express the neuropeptides involved in the metabolic control of food intake. Therefore, we assessed through dual fluorescent in situ hybridization the possible expression of GK in the melanocortinergic neurons expressing proopiomelanocortin (POMC) or agouti-related protein (AGRP). POMC and AGRP expression localized exclusively in the rostral hypothalamus, in the ventral pole of the lateral tuberal nucleus, the homolog of the mammalian arcuate nucleus. Hypothalamic GK expression confined to the ependymal cells coating the ventral pole of the third ventricle but some expression level occurred in the AGRP neurons. GK expression seems to be absent in the hypothalamic POMC neurons. These results suggest that AGRP neurons might sense glucose directly through a mechanism involving GK. In contrast, POMC neurons would not directly respond to glucose through GK and would require presynaptic inputs to sense glucose. Ependymal cells could play a critical role relying glucose metabolic information to the central circuitry regulating food intake in fish, especially in POMC neurons.

Distinct Neuronal Projections From the Hypothalamic Ventromedial Nucleus Mediate Glycemic and Behavioral Effects

The hypothalamic ventromedial nucleus (VMN) is implicated both in autonomic control of blood glucose and in behaviors including fear and aggression, but whether these divergent effects involve the same or distinct neuronal subsets and their projections is unknown. To address this question, we used an optogenetic approach to selectively activate the subset of VMN neurons that express neuronal nitric oxide synthase 1 (VMN^NOS1 neurons) implicated in glucose counterregulation. We found that photoactivation of these neurons elicits 1) robust hyperglycemia achieved by activation of counterregulatory responses usually reserved for the physiological response to hypoglycemia and 2) defensive immobility behavior. Moreover, we show that the glucagon, but not corticosterone, response to insulin-induced hypoglycemia is blunted by photoinhibition of the same neurons. To investigate the neurocircuitry by which VMN^NOS1 neurons mediate these effects, and to determine whether these diverse effects are dissociable from one another, we activated downstream VMN^NOS1 projections in either the anterior bed nucleus of the stria terminalis (aBNST) or the periaqueductal gray (PAG). Whereas glycemic responses are fully recapitulated by activation of VMN^NOS1 projections to the aBNST, freezing immobility occurred only upon activation of VMN^NOS1 terminals in the PAG. These findings support previous evidence of a VMN→aBNST neurocircuit involved in glucose counterregulation and demonstrate that activation of VMN^NOS1 neuronal projections supplying the PAG robustly elicits defensive behaviors.

Hypothalamic POMC or MC4R deficiency impairs counterregulatory responses to hypoglycemia in mice

Objective

Life-threatening hypoglycemia is a major limiting factor in the management of diabetes. While it is known that counterregulatory responses to hypoglycemia are impaired in diabetes, molecular mechanisms underlying the reduced responses remain unclear. Given the established roles of the hypothalamic proopiomelanocortin (POMC)/melanocortin 4 receptor (MC4R) circuit in regulating sympathetic nervous system (SNS) activity and the SNS in stimulating counterregulatory responses to hypoglycemia, we hypothesized that hypothalamic POMC as well as MC4R, a receptor for POMC derived melanocyte stimulating hormones, is required for normal hypoglycemia counterregulation.

Methods

To test the hypothesis, we induced hypoglycemia or glucopenia in separate cohorts of mice deficient in either POMC or MC4R in the arcuate nucleus (ARC) or the paraventricular nucleus of the hypothalamus (PVH), respectively, and measured their circulating counterregulatory hormones. In addition, we performed a hyperinsulinemic-hypoglycemic clamp study to further validate the function of MC4R in hypoglycemia counterregulation. We also measured Pomc and Mc4r mRNA levels in the ARC and PVH, respectively, in the streptozotocin-induced type 1 diabetes mouse model and non-obese diabetic (NOD) mice to delineate molecular mechanisms by which diabetes deteriorates the defense systems against hypoglycemia. Finally, we treated diabetic mice with the MC4R agonist MTII, administered stereotaxically into the PVH, to determine its potential for restoring the counterregulatory response to hypoglycemia in diabetes.

Results

Stimulation of epinephrine and glucagon release in response to hypoglycemia or glucopenia was diminished in both POMC- and MC4R-deficient mice, relative to their littermate controls. Similarly, the counterregulatory response was impaired in association with decreased hypothalamic Pomc and Mc4r expression in the diabetic mice, a phenotype that was not reversed by insulin treatment which normalized glycemia. In contrast, infusion of an MC4R agonist in the PVH restored the counterregulatory response in diabetic mice.

Conclusion

In conclusion, hypothalamic Pomc as well as Mc4r, both of which are reduced in type 1 diabetic mice, are required for normal counterregulatory responses to hypoglycemia. Therefore, enhancing MC4R function may improve hypoglycemia counterregulation in diabetes.

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Hypothalamic POMC as well as MC4R is necessary to counteract hypoglycemia.

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Type 1 diabetic mice exhibit a reduced Pomc and Mc4r expression in the hypothalamus.

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Insulin treatment does not restore Pomc and Mc4r expression in diabetic mice.

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MC4R agonist improves hypoglycemia counterregulation in diabetic mice.

Brain GLUT4 Knockout Mice Have Impaired Glucose Tolerance, Decreased Insulin Sensitivity, and Impaired Hypoglycemic Counterregulation

GLUT4 in muscle and adipose tissue is important in maintaining glucose homeostasis. However, the role of insulin-responsive GLUT4 in the central nervous system has not been well characterized. To assess its importance, a selective knockout of brain GLUT4 (BG4KO) was generated by crossing Nestin-Cre mice with GLUT4-floxed mice. BG4KO mice had a 99% reduction in GLUT4 protein expression throughout the brain. Despite normal feeding and fasting glycemia, BG4KO mice were glucose intolerant, demonstrated hepatic insulin resistance, and had reduced glucose uptake in the brain. In response to hypoglycemia, BG4KO mice had impaired glucose sensing, noted by impaired epinephrine and glucagon responses and impaired c-fos activation in the hypothalamic paraventricular nucleus. Moreover, in vitro glucose sensing of glucose-inhibitory neurons from the ventromedial hypothalamus was impaired in BG4KO mice. In summary, BG4KO mice are glucose intolerant, insulin resistant, and have impaired glucose sensing, indicating a critical role for brain GLUT4 in sensing and responding to changes in blood glucose.

Knockdown of Neuropeptide Y in the Dorsomedial Hypothalamus Promotes Hepatic Insulin Sensitivity in Male Rats

Recent evidence has shown that alterations in dorsomedial hypothalamic (DMH) neuropeptide Y (NPY) signaling influence glucose homeostasis, but the mechanism through which DMH NPY acts to affect glucose homeostasis remains unclear. Here we report that DMH NPY descending signals to the dorsal motor nucleus of the vagus (DMV) modulate hepatic insulin sensitivity to control hepatic glucose production in rats. Using the hyperinsulinemic-euglycemic clamp, we revealed that knockdown of NPY in the DMH by adeno-associated virus-mediated NPY-specific RNAi promoted insulin's action on suppression of hepatic glucose production. This knockdown silenced DMH NPY descending signals to the DMV, leading to an elevation of hepatic vagal innervation. Hepatic vagotomy abolished the inhibitory effect of DMH NPY knockdown on hepatic glucose production, but this glycemic effect was not affected by vagal deafferentation. Together, these results demonstrate a distinct role for DMH NPY in the regulation of glucose homeostasis through the hepatic vagal efferents and insulin action on hepatic glucose production.

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.

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.

Hypothalamic glucose sensing: making ends meet

The neuroendocrine system governs essential survival and homeostatic functions. For example, growth is needed for development, thermoregulation maintains optimal core temperature in a changing environment, and reproduction ensures species survival. Stress and immune responses enable an organism to overcome external and internal threats while the circadian system regulates arousal and sleep such that vegetative and active functions do not overlap. All of these functions require a significant portion of the body's energy. As the integrator of the neuroendocrine system, the hypothalamus carefully assesses the energy status of the body in order to appropriately partition resources to provide for each system without compromising the others. While doing so the hypothalamus must ensure that adequate glucose levels are preserved for brain function since glucose is the primary fuel of the brain. To this end, the hypothalamus contains specialized glucose sensing neurons which are scattered throughout the nuclei controlling distinct neuroendocrine functions. We hypothesize that these neurons play a key role in enabling the hypothalamus to partition energy to meet these peripheral survival needs without endangering the brain's glucose supply. This review will first describe the varied mechanisms underlying glucose sensing in neurons within discrete hypothalamic nuclei. We will then evaluate the way in which peripheral energy status regulates glucose sensitivity. For example, during energy deficit such as fasting specific hypothalamic glucose sensing neurons become sensitized to decreased glucose. This increases the gain of the information relay when glucose availability is a greater concern for the brain. Finally, changes in glucose sensitivity under pathological conditions (e.g., recurrent insulin-hypoglycemia, diabetes) will be addressed. The overall goal of this review is to place glucose sensing neurons within the context of hypothalamic control of neuroendocrine function.

Glucokinase inhibitor glucosamine stimulates feeding and activates hypothalamic neuropeptide Y and orexin neurons

Maintaining glucose levels within the appropriate physiological range is necessary for survival. The identification of specific neuronal populations, within discreet brain regions, sensitive to changes in glucose concentration has led to the hypothesis of a central glucose-sensing system capable of directly modulating feeding behaviour. Glucokinase (GK) has been identified as a glucose-sensor responsible for detecting such changes both within the brain and the periphery. We previously reported that antagonism of centrally expressed GK by administration of glucosamine (GSN) was sufficient to induce protective glucoprivic feeding in rats. Here we examine a neurochemical mechanism underlying this effect and report that GSN stimulated food intake is highly correlated with the induction of the neuronal activation marker cFOS within two nuclei with a demonstrated role in central glucose sensing and appetite, the arcuate nucleus of the hypothalamus (ARC) and lateral hypothalamic area (LHA). Furthermore, GSN stimulated cFOS within the ARC was observed in orexigenic neurons expressing the endogenous melanocortin receptor antagonist agouti-related peptide (AgRP) and neuropeptide Y (NPY), but not those expressing the anorectic endogenous melanocortin receptor agonist alpha-melanocyte stimulating hormone (α-MSH). In the LHA, GSN stimulated cFOS was found within arousal and feeding associated orexin/hypocretin (ORX), but not orexigenic melanin-concentrating hormone (MCH) expressing neurons. Our data suggest that GK within these specific feeding and arousal related populations of AgRP/NPY and ORX neurons may play a modulatory role in the sensing of and appetitive response to hypoglycaemia.

The effects of single and repeated exposure to 2.45 GHz radiofrequency fields on c-Fos protein expression in the paraventricular nucleus of rat hypothalamus

This study investigated the effects of microwave radiation on the PVN of the hypothalamus, extracted from rat brains. Expression of c-Fos was used to study the pattern of cellular activation in rats exposed once or repeatedly (ten times in 2 weeks) to 2.45 GHz radiation in a GTEM cell. The power intensities used were 3 and 12 W and the Finite Difference Time Domain calculation was used to determine the specific absorption rate (SAR). High SAR triggered an increase of the c-Fos marker 90 min or 24 h after radiation, and low SAR resulted in c-Fos counts higher than in control rats after 24 h. Repeated irradiation at 3 W increased cellular activation of PVN by more than 100% compared to animals subjected to acute irradiation and to repeated non-radiated repeated session control animals. The results suggest that PVN is sensitive to 2.45 GHz microwave radiation at non-thermal SAR levels.

The role of the bed nucleus of the stria terminalis in stress-induced inhibition of pulsatile luteinising hormone secretion in the female rat

The bed nucleus of the stria terminalis (BNST) occupies a central position in the neural circuitry regulating the hypothalamic-pituitary-adrenocortical axis response to stress. The potential role of the BNST in stress-induced suppression of the gondotrophin-releasing hormone (GnRH) pulse generator, the central regulator of the reproductive system, was assessed by examining the effects of micro-infusion of corticotrophin-releasing factor (CRF) or its antagonist into the BNST on pulsatile luteinising hormone (LH) secretion or stress-induced inhibition of LH pulses, respectively. Ovariectomised oestrogen-treated rats were implanted chronically with bilateral cannulae in the dorsolateral BNST and i.v. catheters. CRF (25, 50 or 100 pmol in 200 nl of artificial cerebrospinal fluid) administered bilaterally into the BNST resulted in a dose-dependent decrease in LH pulse frequency, and induced Fos expression in glutamic acid decarboxylase immunostained neurones in the medial preoptic area. These results suggest that the activation of hypothalamic GABAergic neurones in response to intra-BNST administration of CRF may be involved in the suppression of LH pulses. Furthermore, administration of CRF antagonist (280 pmol astressin-B, three times at 20-min intervals) into the BNST effectively blocked the suppression of pulsatile LH secretion in response to restraint (1 h) but not hypoglycaemic (0.25 U insulin/kg, i.v.) stress. These data suggest that CRF innervation of the dorsolateral BNST plays a key, but differential, role in stress-induced suppression of the GnRH pulse generator.

Brain insulin action regulates hypothalamic glucose sensing and the counterregulatory response to hypoglycemia

OBJECTIVE

An impaired ability to sense and appropriately respond to insulin-induced hypoglycemia is a common and serious complication faced by insulin-treated diabetic patients. This study tests the hypothesis that insulin acts directly in the brain to regulate critical glucose-sensing neurons in the hypothalamus to mediate the counterregulatory response to hypoglycemia.

RESEARCH DESIGN AND METHODS

To delineate insulin actions in the brain, neuron-specific insulin receptor knockout (NIRKO) mice and littermate controls were subjected to graded hypoglycemic (100, 70, 50, and 30 mg/dl) hyperinsulinemic (20 mU/kg/min) clamps and nonhypoglycemic stressors (e.g., restraint, heat). Subsequently, counterregulatory responses, hypothalamic neuronal activation (with transcriptional marker c-fos), and regional brain glucose uptake (via (14)C-2deoxyglucose autoradiography) were measured. Additionally, electrophysiological activity of individual glucose-inhibited neurons and hypothalamic glucose sensing protein expression (GLUTs, glucokinase) were measured.

RESULTS

NIRKO mice revealed a glycemia-dependent impairment in the sympathoadrenal response to hypoglycemia and demonstrated markedly reduced (3-fold) hypothalamic c-fos activation in response to hypoglycemia but not other stressors. Glucose-inhibited neurons in the ventromedial hypothalamus of NIRKO mice displayed significantly blunted glucose responsiveness (membrane potential and input resistance responses were blunted 66 and 80%, respectively). Further, hypothalamic expression of the insulin-responsive GLUT 4, but not glucokinase, was reduced by 30% in NIRKO mice while regional brain glucose uptake remained unaltered.

CONCLUSIONS

Chronically, insulin acts in the brain to regulate the counterregulatory response to hypoglycemia by directly altering glucose sensing in hypothalamic neurons and shifting the glycemic levels necessary to elicit a normal sympathoadrenal response.

Recurrent hypoglycemia alters hypothalamic expression of the regulatory proteins FosB and synaptophysin

A limiting factor to the clinical management of diabetes is iatrogenic hypoglycemia. With multiple hypoglycemic episodes, the collective neuroendocrine response that restores euglycemia is impaired. In our animal model of recurrent hypoglycemia (RH), neuroendocrine deficits are accompanied by a decrease in medial hypothalamic activation. Here we tested the hypothesis that the medial hypothalamus may exhibit unique changes in the expression of regulatory proteins in response to RH. We report that expression of the immediate early gene FosB is increased in medial hypothalamic nuclei, anterior hypothalamus, and posterior paraventricular nucleus of the thalamus (THPVN) of the thalamus following RH. We identified the hypothalamic PVN, a key autonomic output site, among the regions expressing FosB. To identify the subtype(s) of neuronal populations that express FosB, we screened candidate neuropeptides of the PVN for coexpression using dual fluorescence immunohistochemistry. Among the neuropeptides analyzed [including oxytocin, vasopressin, thyrotropin-releasing hormone, and corticotropin-releasing factor (CRF)], FosB was only identified in CRF-positive neurons. Inhibitory gamma-aminobutyric acid-positive processes appear to impinge on these FosB-expressing neurons. Finally, we observed a significant decrease in the presynaptic marker synaptophysin within the PVN of RH-treated vs. saline-treated rats, suggesting that rapid alterations of synaptic morphology may occur in association with RH. Collectively, these data suggest that RH stress triggers cellular changes that support synaptic plasticity, in specific neuroanatomical sites, which may contribute to the development of hypoglycemia-associated autonomic failure.

Central resistin induces hepatic insulin resistance via neuropeptide Y

Sensing of peripheral hormones and nutrients by the hypothalamus plays an important role in maintaining peripheral glucose homeostasis. The hormone resistin impairs the response to insulin in liver and other peripheral tissues. Here we demonstrate that in normal mice resistin delivered in the lateral cerebral ventricle increased endogenous glucose production during hyperinsulinemic-euglycemic clamp, consistent with induction of hepatic insulin resistance. In agreement, central resistin inhibited Akt phosphorylation and increased the expression of glucose-6-phosphatase, the enzyme regulating glucose output in the liver. Central resistin induced expression of proinflammatory cytokines as well as suppressor of cytokine signaling-3, a negative regulator of insulin action in liver. Central infusion of resistin was associated with neuronal activation in the arcuate, paraventricular and dorsomedial nuclei, and increased neuropeptide Y (NPY) expression in the hypothalamus. The effects of central resistin on glucose production as well as hepatic expression of proinflammatory cytokines were abrogated in mice lacking NPY. Pretreatment of wild-type mice with antagonists of the NPY Y1 receptor, but not the Y5 receptor, also prevented the effects of central resistin. Together, these results suggest that resistin action on NPY neurons is an important regulator of hepatic insulin sensitivity.

Arginine-vasopressin mediates central and peripheral glucose regulation in response to carotid body receptor stimulation with Na-cyanide

Hypoxic stimulation of the carotid body receptors (CBR) results in a rapid hyperglycemia with an increase in brain glucose retention. Previous work indicates that neurohypophysectomy inhibits this hyperglycemic response. Here, we show that systemic arginine vasopressin (AVP) induced a transient, but significant, increase in blood glucose levels and increased brain glucose retention, a response similar to that observed after CBR stimulation. Comparable results were obtained after intracerebral infusion of AVP. Systemic AVP-induced changes were maintained in hypophysectomized rats but were not observed after adrenalectomy. Glycemic changes after CBR stimulation were inhibited by pharmacological blockage of AVP V1a receptors with a V1a-selective receptor antagonist ([β-Mercapto-β,β-cyclopentamethylenepropionyl1,O-me-Tyr2, Arg8]-vasopressin). Importantly, local application of micro-doses of this antagonist to the liver was sufficient to abolish the hyperglycemic response after CBR stimulation. These results suggest that AVP is a mediator of the hyperglycemic reflex and cerebral glucose retention following CBR stimulation. We propose that hepatic activation of AVP V1a receptors is essential for this hyperglycemic response.

Habituation of insulin-induced hypoglycemic transcription activation of lateral hypothalamic orexin-A-containing neurons to recurring exposure

A CNS component of glucose counterregulatory collapse is supported by evidence for nonuniform genomic responsiveness of neurons in characterized central autonomic loci during recurring insulin-induced hypoglycemia (IIH). We have reported that exacerbated hypoglycemia and attenuated patterns of glucagon and epinephrine secretion in rats treated by daily sc injection of the intermediate-acting insulin formulation, Humulin NPH (NPH), are correlated with diminished immunodemonstrability of the AP-1 transcription factor, Fos, in several components of the central metabolic regulatory circuitry, including the lateral hypothalamic area (LHA). Neurons that synthesize the potent orexigenic peptide neurotransmitter, orexin-A, are restricted to the LHA and adjacent hypothalamic loci, and project throughout the central neuroaxis to structures that govern autonomic and behavioral motor output. Dual-label immunocytochemical and real-time RT-PCR techniques were utilized here to evaluate the functional status of this LHA phenotype during a single versus repetitive exposure to prolonged IIH. Tissue sections were collected at predetermined rostrocaudal levels of the LHA after acute or repeated NPH administration, and processed for nuclear Fos- and cytoplasmic orexin-A-immunoreactivity (-ir). Mean numbers of orexin-A-ir neurons were not different between treatment groups. Colabeling of these cells for Fos was increased relative to controls following a single injection of insulin, but numbers of Fos-ir-positive orexin-A neurons were significantly reduced after treatment with four versus one dose of insulin. Prepro-orexin mRNA levels in microdissected LHA tissue were upregulated during acute hypoglycemia, but were returned to control levels by repeated IIH. These data corroborate previous evidence that IIH is an activational stimulus for orexin-A-synthesizing neurons in the LHA, and further demonstrate that induction of cfos and prepro-orexin gene expression by acute hypoglycemia is attenuated by precedent exposure to hypoglycemia. The current results thus provide unique evidence for neurotransmitter-specific habituation of LHA neuronal sensitivity to IIH.

Hypothalamic AMP-activated protein kinase mediates counter-regulatory responses to hypoglycaemia in rats

AIMS/HYPOTHESIS

Appropriate counter-regulatory hormonal responses are essential for recovery from hypoglycaemia. Although the hypothalamus is known to be involved in these responses, the molecular mechanisms have not been fully elucidated. AMP-activated protein kinase (AMPK) functions as a cellular energy sensor, being activated during energy depletion. As AMPK is expressed in the hypothalamus, an important site of neuroendocrine regulation, the present study was undertaken to determine whether hypothalamic AMPK mediates counter-regulatory responses to hypoglycaemia.

MATERIALS AND METHODS

Hypoglycaemia was induced by i.p. injection of regular insulin (6 U/kg) in Sprague-Dawley rats. Hypothalamic AMPK phosphorylation and activities were determined 1 h after i.p. insulin injection. To investigate the role of hypothalamic AMPK activation in mediating counter-regulatory responses, an AMPK inhibitor, compound C, was pre-administered intracerebroventricularly (i.c.v.) or dominant-negative (DN)-AMPK was overexpressed in the hypothalamus before induction of hypoglycaemia.

RESULTS

Insulin-induced hypoglycaemia increased hypothalamic AMPK phosphorylation and alpha2-AMPK activities in rats. The change was significant in the arcuate nucleus/ventromedial hypothalamus (ARC/VMH) and paraventricular nuclei (PVN). Prior i.c.v. administration of compound C attenuated hypoglycaemia-induced increases in plasma concentrations of corticosterone, glucagon and catecholamines, resulting in severe and prolonged hypoglycaemia. ARC/VMH DN-AMPK overexpression impaired early counter-regulation, as evidenced by reduced glucagon and catecholamine responses. In contrast, PVN DN-AMPK overexpression attenuated late counter-regulation and corticosterone responses.

CONCLUSIONS/INTERPRETATION

Systemic hypoglycaemia causes hypothalamic AMPK activation, which is important for counter-regulatory hormonal responses. Our data indicate that hypothalamic AMPK acts as a fuel gauge, sensing the whole-body energy state and regulating not only energy homeostasis but also neuroendocrine functions.

Activation of feeding-related neural circuitry after unilateral injections of muscimol into the nucleus accumbens shell

Chemical inhibition of neurons in the nucleus accumbens shell (AcbSh) elicits intense, behaviorally specific, feeding in satiated rats. We have demonstrated previously that this treatment activates a number of brain regions, most significantly the lateral hypothalamus (LH). This activation could be elicited through a direct neural connection with the AcbSh or secondarily through changes in autonomic activity, stress, or circulating levels of orexigenic or satiety factors. In the present study, we used the immunohistochemical localization of Fos protein to map neuronal activation after unilateral muscimol injections into the AcbSh to determine whether AcbSh-mediated Fos expression remains lateralized in the circuit and whether secondary systemic changes in the rat can be excluded as primary factors in the activation of downstream component nuclei. Rats receiving only saline injections exhibited very little Fos immunoreactivity. In contrast, unilateral injections of muscimol into the AcbSh consistently increased Fos expression in several brain regions. Three distinct patterns of expression were observed. Fos synthesis in the LH was increased only on the side of the brain ipsilateral to the muscimol injection. Fos expression remained primarily ipsilateral to the injection site in the septohypothalamic, paraventricular hypothalamic (PVN), paratenial thalamic, and lateral habenular nuclei, and medial substantia nigra, but was increased bilaterally in the piriform cortex, supraoptic nucleus, central nucleus of the amygdala, and nucleus of the solitary tract. Smaller numbers of Fos-immunoreactive cells were seen unilaterally in the bed nucleus of the stria terminalis, medial ventral pallidum, arcuate nucleus, and ventral tegmental area and bilaterally in the supraoptic and tuberomammillary nuclei. The labeling in the LH, PVN, and other unilaterally labeled structures provides evidence that these brain regions are components of an AcbSh-mediated neural circuit and suggests that they may be involved in the expression of AcbSh-mediated feeding behavior.

Effect of CCK-8 on insulin-induced hyperphagia and hypothalamic orexigenic neuropeptide expression in the rat

The influence of cholecystokinin octapeptide (CCK-8) on normal and insulin-induced feeding and expression of orexigenic hypothalamic neuropeptides was investigated in male rats. CCK-8, administered during meals (4 microg/kg) or continuously (32 microg/kg over 60 min), blunted the stimulating effect of insulin (50 IU/kg) on feeding by reducing meal size (-60%; P<0.05 or -86%; P<0.0001, respectively). Rats without access to food and injected with IP insulin (50 IU/kg) showed increased hypothalamic mRNA levels of orexin (+30%; P<0.05) and melanin-concentrating hormone (+52%; P<0.05), as compared with ad libitum-fed and saline-injected control rats. Continuous IP infusion of CCK-8 (32 microg/kg) blunted these increases. Our results suggest that both orexin and melanin-concentrating hormone participate in the response to insulin hypoglycemia without food being present; these neurons may be involved in mechanisms related to insulin-induced hyperphagia. Signals triggered by peripheral CCK-8 act to decrease the expression of orexin and melanin-concentrating hormone. This may be associated with a reduction in hyperphagia.

Induction of Fos immunoreactivity labeling in rat forebrain metabolic loci by caudal fourth ventricular infusion of the monocarboxylate transporter inhibitor, alpha-cyano-4-hydroxycinnamic acid

Caudal fourth ventricular (CV4) infusion of the monocarboxylate transporter inhibitor, alpha-cyano-4-hydroxycinnamic acid (4CIN), causes hyperglycemia coincident with Fos expression in the hindbrain nucleus tractus solitarius, a rare central source of metabolic deficit signaling. The present studies examined the hypothesis that hindbrain lactoprivic signaling activates central autonomic pathways that regulate systemic glucostasis by examining the effects of this drug treatment paradigm on patterns of Fos expression in forebrain structures that integrate sensory input from metabolic sensors and coordinate motor responses to energy shortages. Two hours after CV4 infusion of graded doses of 4CIN or vehicle alone, adult female rats were sacrificed by transcardial perfusion and sections through the telencephalic and diencephalic metabolic loci were processed for Fos immunoreactivity (-ir). Fos labeling of the hypothalamic paraventricular (PVH), dorsomedial (DMH), and ventromedial (VMH) nuclei was significantly elevated, relative to the vehicle-treated controls, in response to the lowest dose of 4CIN, e.g. 10 microg/animal. Treatment with higher doses of 4CIN (25 or 50 microg) further augmented numbers of Fos-ir-positive neurons in these structures, and also elicited staining of the bed nuclei of the stria terminalis (BST), medial preoptic (MPN), arcuate (ARH), supraoptic (SO), and anterior hypothalamic nuclei (AHN), and lateral hypothalamic area (LHA). Mean numbers of Fos-immunolabeled neurons in the ARH, DMH, LHA, AHN, MPN, and SO were not different between animals infused with 25 versus 50 microg 4CIN, whereas neuronal labeling in the VMH, BST, and PVH was significantly greater in the high- versus the middle-dose groups. The present data show that pharmacological inhibition of lactate uptake within the caudal hindbrain results in dose-dependent neuronal Fos immunoexpression within characterized forebrain components of the central metabolic circuitry, and that these patterns of neuronal transcriptional activation parallel observed drug effects on blood glucose levels. These results suggest that lactoprivic signaling by metabolic 'sensing' neurons in the caudal hindbrain initiates central neural mechanisms that control systemic energy availability, and that local lactate-'sensitive' neurons are connected neuroanatomically with principal higher-order autonomic metabolic loci that regulate glucostasis.

Characterization of the role of endogenous cholecystokinin on the activity of the paraventricular nucleus of the hypothalamus in rats

Activation of the hypothalamic-pituitary-adrenal axis by fasting seems to involve cholecystokinin (CCK) receptors. This work aims to characterize the role of endogenous CCK in the activity of the paraventricular nucleus (PVN) of the hypothalamus during food withdrawal. We investigated, by c-Fos immunohistochemistry, the effect of CCK1 and CCK2 receptor antagonists (SR-27,897 and L-365,260, respectively) on c-Fos levels expression induced by food deprivation. Under our conditions, the number of cells expressing c-Fos was reduced both by SR-27,897 and L-365,260 in food-deprived rats. To investigate the importance of glucose availability, we studied the effect of CCK receptor antagonists on c-Fos synthesis induced by the glucose antimetabolite 2-deoxyglucose. In these animals, only SR-27,897 decreased c-Fos expression in the PVN. Our results indicate that the effect of CCK antagonists is mainly perceptible when glucose availability decreases, and suggest that CCK-ergic inputs could drive the activity of the PVN under fasting/low glucose conditions.

Cortisol acts through central mechanisms to blunt counterregulatory responses to hypoglycemia in conscious rats

Physiological levels of cortisol have been found to blunt neuroendocrine and metabolic responses to subsequent hypoglycemia in humans. The aim of this study was to determine whether cortisol acts directly on the brain to elicit this effect. A total of 41 conscious unrestrained Sprague-Dawley rats were studied during 2-day experiments. Day 1 consisted of two episodes of clamped 2-h hyperinsulinemic (30 pmol. kg(-1) x min(-1)) hypoglycemia (2.8 +/- 0.1 mmol/l; n = 12; ANTE HYPO), euglycemia (6.2 +/- 0.1 mmol/l; n = 12; ANTE EUG), or euglycemia (6.2 +/- 0.1 mmol/l) plus simultaneous intracerebroventricular (ICV) infusion of cortisol (25 microg/h; n = 9; ANTE EUG+Cort) or saline (24 microl/h; n = 8; ANTE EUG+Sal). For all groups, day 2 consisted of a 2-h hyperinsulinemic (30 pmol x kg(-1) x min(-1)) hypoglycemic (2.9 +/- 0.2 mmol/l) clamp. Plasma epinephrine and glucagon incremental area under the curve (Delta AUC) responses were significantly less in ANTE EUG+Cort and ANTE HYPO versus both ANTE EUG and ANTE EUG+Sal (P < 0.05). The Delta AUC responses of plasma norepinephrine were significantly lower in ANTE EUG+Cort versus both ANTE EUG and ANTE EUG+Sal (P < 0.05). Endogenous glucose production was significantly less in ANTE HYPO and ANTE EUG+Cort versus the other groups (P < 0.05). Lastly, the glucose infusion rate to maintain the desired hypoglycemia was significantly greater in ANTE EUG+Cort and ANTE HYPO versus the other two groups (P < 0.05). In summary, ICV infusion of cortisol significantly blunted norepinephrine, epinephrine, glucagon, and endogenous glucose production responses to next-day hypoglycemia. We conclude that cortisol can act directly on the central nervous system to blunt counterregulatory responses to subsequent hypoglycemia in the conscious rat.

Involvement of the entorhinal cortex in the stress response to immobilization, but not to insulin-induced hypoglycaemia

Although the involvement of the limbic system in the neuroendocrine responses to some stressors has been documented, the specific role of the entorhinal cortex has not been elucidated. In this study, we investigated the involvement of the entorhinal cortex in stress responses. Fos immunoreactivity, a widely used marker for neuronal activation, was detected in the entorhinal cortex of rats subjected to immobilization stress, whereas no marked staining was observed in the entorhinal cortex of the control and insulin-induced hypoglycaemia groups. Lesion of the entorhinal cortex produced by ibotenic acid significantly attenuated the adrenocorticotropic hormone (ACTH) release evoked by immobilization; however, no significant change in ACTH release was observed in insulin-induced hypoglycaemia. No significant difference between entorhinal-lesioned rats and control rats was observed in blood glucose concentrations when subjected to either immobilization or to insulin-induced hypoglycaemia. Together, these results indicate that the entorhinal cortex is closely involved in the stress response to immobilization but not to insulin-induced hypoglycaemia.

Neuronal activation of brain vagal-regulatory pathways and upper gut enteric plexuses by insulin hypoglycemia

Neuronal activation of brain vagal-regulatory nuclei and gastric/duodenal enteric plexuses in response to insulin (2 U/kg, 2 h) hypoglycemia was studied in rats. Insulin hypoglycemia significantly induced Fos expression in the paraventricular nucleus of the hypothalamus, locus coeruleus, dorsal motor nucleus of the vagus (DMN), and nucleus tractus solitarii (NTS), as well as in the gastric/duodenal myenteric/submucosal plexuses. A substantial number of insulin hypoglycemia-activated DMN and NTS neurons were choline acetyltransferase and tyrosine hydroxylase positive, respectively, whereas the activated enteric neurons included NADPH- and vasoactive intestinal peptide neurons. The numbers of Fos-positive cells in each above-named brain nucleus or in the gastric/duodenal myenteric plexus of insulin-treated rats were negatively correlated with serum glucose levels and significantly increased when glucose levels were lower than 80 mg/dl. Acute bilateral cervical vagotomy did not influence insulin hypoglycemia-induced Fos induction in the brain vagal-regulatory nuclei but completely and partially prevented this response in the gastric and duodenal enteric plexuses, respectively. These results revealed that brain-gut neurons regulating vagal outflow to the stomach/duodenum are sensitively responsive to insulin hypoglycemia.

Glucokinase is the likely mediator of glucosensing in both glucose-excited and glucose-inhibited central neurons

Specialized neurons utilize glucose as a signaling molecule to alter their firing rate. Glucose-excited (GE) neurons increase and glucose-inhibited (GI) neurons reduce activity as ambient glucose levels rise. Glucose-induced changes in the ATP-to-ADP ratio in GE neurons modulate the activity of the ATP-sensitive K(+) channel, which determines the rate of cell firing. The GI glucosensing mechanism is unknown. We postulated that glucokinase (GK), a high-Michaelis constant (K(m)) hexokinase expressed in brain areas containing populations of GE and GI neurons, is the controlling step in glucosensing. Double-label in situ hybridization demonstrated neuron-specific GK mRNA expression in locus ceruleus norepinephrine and in hypothalamic neuropeptide Y, pro-opiomelanocortin, and gamma-aminobutyric acid neurons, but it did not demonstrate this expression in orexin neurons. GK mRNA was also found in the area postrema/nucleus tractus solitarius region by RT-PCR. Intracarotid glucose infusions stimulated c-fos expression in the same areas that expressed GK. At 2.5 mmol/l glucose, fura-2 Ca(2+) imaging of dissociated ventromedial hypothalamic nucleus neurons demonstrated GE neurons whose intracellular Ca(2+) oscillations were inhibited and GI neurons whose Ca(2+) oscillations were stimulated by four selective GK inhibitors. Finally, GK expression was increased in rats with impaired central glucosensing (posthypoglycemia and diet-induced obesity) but was unaffected by a 48-h fast. These data suggest a critical role for GK as a regulator of glucosensing in both GE and GI neurons in the brain.

PVN activation is suppressed by repeated hypoglycemia but not antecedent corticosterone in the rat

The mechanism(s) underlying hypoglycemia-associated autonomic failure (HAAF) are unknown. To test the hypothesis that the activation of brain regions involved in the counterregulatory response to hypoglycemia is blunted with HAAF, rats were studied in a 2-day protocol. Neuroendocrine responses and brain activation (c-Fos immunoreactivity) were measured during day 2 insulin-induced hypoglycemia (0.5 U insulin x 100 g body x wt(-1) x h(-1) iv for 2 h) after day 1 hypoglycemia (Hypo-Hypo) or vehicle. Hypo-Hypo animals demonstrated HAAF with blunted epinephrine, glucagon, and corticosterone (Cort) responses and decreased activation of the medial hypothalamus [the paraventricular (PVN), dorsomedial (DMH), and arcuate (Arc) nuclei]. To evaluate whether increases in day 1 Cort were responsible for the decreased hypothalamic activation, Cort was infused intracerebroventricularly (72 microg) on day 1 and the response to day 2 hypoglycemia was measured. Intracerebroventricular Cort infusion failed to alter the neuroendocrine response to day 2 hypoglycemia, despite elevating both central nervous system and peripheral Cort levels. However, day 1 Cort blunted responses in two of the same hypothalamic regions as Hypo-Hypo (the DMH and Arc) but not in the PVN. These results suggest that decreased activation of the PVN may be important in the development of HAAF and that antecedent exposure to elevated levels of Cort is not always sufficient to produce HAAF.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Co-distribution of Fos- and mu opioid receptor immunoreactivity within the rat septopreoptic area and hypothalamus during acute glucose deprivation: effects of the mu receptor antagonist CTOP

Mu opioid receptors occur throughout the brain, but central sites where ligand neuromodulatory effects occur during glucopenia have not been identified. The present studies investigated whether septal, preoptic, and hypothalamic neurons that express immunoreactivity for this receptor are transcriptionally activated in response to the glucose antimetabolite, 2-deoxy-D-glucose (2DG), and if intracerebroventricular (icv) administration of the selective mu receptor antagonist, CTOP, modifies this functional response to glucose substrate imbalance. Neurons labeled for mu receptor-immunoreactivity (-ir) were observed in the lateral septal nucleus (LS), medial septum (MS), anterior division of the stria terminalis (BSTa), median preoptic nucleus (MEPO), medial preoptic nucleus (MPN), parastrial nucleus (PS), anterior hypothalamic periventricular nucleus (PVa), and lateral hypothalamic area (LPO). 2DG injection (400 mg/kg i.p.) resulted in co-labeling of mu receptor-positive neurons in the LS, MS, BSTa, MEPO, PVa, and LPO for nuclear Fos-ir. Icv delivery of CTOP decreased mean numbers of co-labeled neurons in the LS, MS, BSTa, and MEPO. These results provide evidence for transactivational effects of glucopenia on mu opioid receptor-expressing neurons within the septum, preoptic area, and hypothalamus, and suggest that the functional status of these receptors within discrete septopreoptic sites may be critical for maximal glucoprivic induction of the Fos stimulus-transcription cascade within local cells. These results thus support the view that the neural loci described above may serve as substrates for regulatory effects of mu opioid receptor ligands on central compensatory activities during acute glucose deprivation.

Insulin and glucose administration stimulates Fos expression in neurones of the paraventricular nucleus that project to autonomic preganglionic structures

Insulin and glucose play a key role in the control of body energy homeostasis. However, the anatomical organization of the network of central insulin and glucose sensitive areas is still unclear. In the present study, we used a multiple-labelling technique combining retrograde tracing and Fos-like immunohistochemistry, to analyse the anatomical projections from hypothalamic neurones activated by the combined stimulus of insulin and glucose. After intraperitoneal injections of a bolus of insulin plus glucose, Fos-like immunoreactive neurones were observed in the paraventricular nucleus (PVN), ventromedial and arcuate nuclei, as well as the lateral hypothalamic area. In addition, neurones projecting to the autonomic preganglionic levels in the brainstem and spinal cord potentially involved in the control of glucose metabolism were identified by injections of fluorochrome tracers. Thus, Fluorogold was injected into the intermediolateral cell column of the lower spinal cord and Fast Blue was injected into the dorsal motor nucleus of the vagus. Perikarya of descending neurones were detected chiefly in the dorsal, medial and lateral parvocellular subnuclei and also in the posterior magnocellular subnucleus of the PVN. In contrast, insulin-glucose activated neurones in the PVN were observed mainly in the medial parvocellular and posterior magnocellular subnuclei. Fluorogold/Fos double-labelled neurones were only observed in the ventral zone of the medial parvocellular subnucleus. These data indicate that, within the PVN, there could be neurones responding to insulin-glucose administration, which are involved in the sympathetic control of the classical regulatory structures of body energy homeostasis, such as the liver and pancreas, and which could play a role in the output of the neuronal circuitry controlling food intake.

The elevation of plasma adrenocorticotrophic hormone and expression of c-Fos in hypothalamic paraventricular nucleus by microinjection of neostigmine into the hippocampus in rats: comparison with acute stress responses

We have reported that the microinjection of neostigmine into the hippocampus of rats induced responses similar to stress responses in terms of catecholamines and glucose in plasma. In order to test the hypothesis that hippocampal neostigmine injection is a possible animal model of acute stress responses, we investigated c-Fos expression in the hypothalamic paraventricular nucleus (PVN) and plasma levels of adrenocorticotrophic hormone (ACTH) after hippocampal neostigmine injection and compared these levels with those resulting from stressful conditions such as immobilization and insulin-induced hypoglycemia. The patterns of expression of Fos-ir in the PVN after microinjection of neostigmine into the hippocampus were not different from those seen in the two stressful situations. After microinjection of neostigmine, plasma ACTH levels significantly increased. Taken together, the results of this study indicate that microinjection of neostigmine into the hippocampus is a potential experimental model for acute stress responses.

Hypoglycemia activates orexin neurons and selectively increases hypothalamic orexin-B levels: responses inhibited by feeding and possibly mediated by the nucleus of the solitary tract

Orexins are novel appetite-stimulating peptides expressed in the lateral hypothalamic area (LHA), and their expression is stimulated by hypoglycemia in fasted rats. We investigated activation of orexin and other neurons during insulin-induced hypoglycemia using the immediate early gene product Fos. Insulin (50 U/kg) lowered plasma glucose by >50% after 5 h and stimulated feeding sixfold compared with saline-injected controls. Hypoglycemic rats allowed to feed and normoglycemic controls both showed sparse Fos-positive (Fos+) neurons in the LHA and the paraventricular nucleus (PVN) and arcuate nucleus (ARC) and showed none in the nucleus of the solitary tract (NTS), which relays visceral feeding signals to the LHA. In the LHA, total numbers of Fos+ neurons were comparable in fed hypoglycemic and control groups (60 +/- 6 vs. 52 +/- 4 cells/mm2, P > 0.05), as were Fos+ neurons immunoreactive for orexin (1.4 +/- 0.4 vs. 0.6 +/- 0.4 cells/mm2, P > 0.05). By contrast, hypoglycemic rats that were fasted showed significantly more Fos+ nuclei in the LHA (96 +/- 10 cells/mm2, P < 0.05, vs. both other groups) and Fos+ orexin neurons (8.4 +/- 3.3 cells/mm2, P < 0.001, vs. both other groups). They also showed two- to threefold more Fos+ nuclei (P < 0.001) in the PVN and ARC than both fed hypoglycemic rats and controls and showed strikingly abundant Fos+ neurons in the NTS and dorsal motor nucleus of the vagus. In parallel studies, whole hypothalamic orexin-A levels were not changed in hypoglycemic rats, whether fasted or freely fed, whereas orexin-B levels were 10-fold higher in hypoglycemic fasted rats than in control and hypoglycemic fed groups. These data support our hypothesis that orexin neurons are stimulated by falling glucose levels but are readily inhibited by signals related to nutrient ingestion and suggest that they may functionally link with neuronal activity in the NTS. Orexin-A and -B may play specific roles in behavioral or neuroendocrine responses to hypoglycemia.

Intraventricular lactate infusion attenuates the transactivational effects of the glucose antimetabolite, 2-deoxy-D-glucose, on hypothalamic vasopressinergic neurons

Glucopenia stimulates neurohypophyseal arginine vasopressin (AVP) secretion and expression of the transcription factor, Fos, by paraventricular (PVN) and supraoptic (SON) magnocellular neurons. Recent studies suggest that central compensatory responses to glucose substrate imbalance are initiated by regulatory signals of periventricular origin. Since the glycolytic endproduct, lactate, is a preferred substrate for central neuronal respiration, we investigated whether intracerebroventricular (i.c.v.) infusion of this monocarboxylate fuel attenuates transactivational effects of glucoprivation on PVN and SON AVP neurons. Continuous intraventricular infusion of sodium lactate (1.0 or 10.0 microM/h) or vehicle was initiated before intraperitoneal (i.p.) injection of the glucose antimetabolite, 2-deoxy-D-glucose (2DG), or saline. Anterior hypothalamic tissue obtained 2 h after systemic injections was processed for colocalization of cytoplasmic AVP- and nuclear Fos-immunoreactivity (Fos-ir). Fos-ir was absent from the PVN and SON of rats treated by i.c.v. infusion of vehicle or either dose of lactate. Intraventricular administration of 10.0 microM lactate/h, but not the lower dose, significantly decreased mean numbers of colabeled AVP neurons in each structure in glucoprivic animals. These data suggest that Fos stimulus-transcription cascade is activated in these cells by decreased central availability of this monocarboxylate fuel, and that cellular sources of regulatory signaling of lactate utilization exist within the periventricular CNS.

Brain glucose sensing and body energy homeostasis: role in obesity and diabetes

The brain has evolved mechanisms for sensing and regulating glucose metabolism. It receives neural inputs from glucosensors in the periphery but also contains neurons that directly sense changes in glucose levels by using glucose as a signal to alter their firing rate. Glucose-responsive (GR) neurons increase and glucose-sensitive (GS) decrease their firing rate when brain glucose levels rise. GR neurons use an ATP-sensitive K+ channel to regulate their firing. The mechanism regulating GS firing is less certain. Both GR and GS neurons respond to, and participate in, the changes in food intake, sympathoadrenal activity, and energy expenditure produced by extremes of hyper- and hypoglycemia. It is less certain that they respond to the small swings in plasma glucose required for the more physiological regulation of energy homeostasis. Both obesity and diabetes are associated with several alterations in brain glucose sensing. In rats with diet-induced obesity and hyperinsulinemia, GR neurons are hyporesponsive to glucose. Insulin-dependent diabetic rats also have abnormalities of GR neurons and neurotransmitter systems potentially involved in glucose sensing. Thus the challenge for the future is to define the role of brain glucose sensing in the physiological regulation of energy balance and in the pathophysiology of obesity and diabetes.

Neurons containing orexin in the lateral hypothalamic area of the adult rat brain are activated by insulin-induced acute hypoglycemia

Orexin-A and -B (also known as hypocretin-1 and -2) are neuropeptides which stimulate food intake when administered intracerebroventricularly. Orexins are specifically localized in neurons within and around the lateral hypothalamic area (LHA). Previous electrophysiological studies have demonstrated that some neurons in the LHA are activated by hypoglycemia, and are therefore termed 'glucose-sensitive neurons'. In the present study, we examined whether orexin-containing neurons are activated in the hypoglycemic states, using Fos-like immunoreactivity (FLI) as a marker of neuronal activation. We observed that FLI was induced in the LHA by acute insulin treatment. Double staining with anti-Fos and anti-orexin antibodies revealed that up to 33% of the orexin-containing neurons in the LHA also expressed FLI under the hypoglycemic condition. These results suggest that some populations of neurons which contain orexins are activated under hypoglycemic conditions.

Neural substrate for an integrated metabolic control of feeding behavior

Evidence indicates that feeding behavior in rats is controlled by a mechanism that integrates information about different aspects of fuel metabolism. We investigated the neural substrate for this integrated control by measuring the effect of metabolic inhibitors given alone and in combination on food intake and neuronal activity as reflected by the expression of c-Fos protein. Combined administration of methyl palmoxirate (5 mg/kg po), an inhibitor of fatty acid oxidation, and 2,5-anhydro-D-mannitol (150 mg/kg ip), which decreases liver ATP content, increased feeding in rats more than expected on the basis of eating responses after treatment with either inhibitor given alone. Combined treatment also produced a synergistic increase in Fos-like immunoreactivity in several brain areas, including the nucleus of the solitary tract, area postrema, and parvocellular portion of the hypothalamic paraventricular nucleus. These findings provide strong evidence for the involvement of selected brain regions in the metabolic control of food intake and suggest that metabolic information used to control feeding behavior is integrated in the periphery or at the level of the brain stem.

Inhibition of luteinizing hormone secretion and expression of c-fos and corticotrophin-releasing factor genes in the paraventricular nucleus during insulin-induced hypoglycaemia in sheep

Insulin can act within the brain to stimulate ovine luteinizing hormone (LH) secretion, but insulin-induced hypoglycaemia inhibits LH via unknown brain sites, possibly involving corticotrophin-releasing factor (CRF). Castrate male sheep, with (E+) or without (E-) subcutaneous oestradiol implants, were blood sampled every 12 min for 8 h. Insulin (0.25 or 0.5 IU/kg) was injected at 4 h via the carotid artery or jugular vein. All treatments reduced LH output with no differences between dose rate nor route of administration, but sensitivity was greater in E+ than E-sheep. There was no evidence for an effect of insulin on LH 0-1 h postinjection; however, 1-3 h after insulin, when hypoglycaemia was established, LH pulses were inhibited in both E+ and E- sheep (P<0.001). Additional intravenous (i.v.) glucose injections given 1 h (20 mmol) and 2 h (10 mmol) after insulin (0.5 IU/kg) were each followed by an LH pulse within 30 min (75% response in both E+ and E-sheep). In a separate experiment, sheep were killed 2 h after i.v. insulin (0.5 IU/kg) or saline. In-situ hybridization revealed c-fos mRNA in the paraventricular nucleus (PVN), but not in any other hypothalamic nuclei nor in the hindbrain; and this was linked with increased CRF gene expression in the PVN. Similar c-fos and CRF gene expression was seen in insulin-treated sheep given additional i.v. glucose (20 and 10 mmol, respectively, 40 and 20 min ante mortem), but not in saline-treated controls. Therefore, insulin-induced hypoglycaemia inhibited LH secretion, with oestradiol potentiating the effect, and was associated with gonadal steroid-independent c-fos gene expression and increased CRF gene expression in the PVN. The ovine PVN may be involved in mediating insulin-induced hypoglycaemic inhibition of LH by a mechanism which might involve CRF.

Insulin selectively downregulates alpha2-adrenoceptors in the arcuate and dorsomedial nucleus

Intracerebroventricular infusion of insulin (2 mU/ day) produced selective downregulation of 3H-paraminoclonidine binding to alpha2-adrenoceptors in the hypothalamic arcuate (14%) and dorsomedial (19%) nuclei out of 16 forebrain areas in Wistar rats. Binding of 3H-prazosin to alpha1-adrenoceptors was unaffected. This is in keeping with the known effect of insulin on catecholamine and neuropeptide Y metabolism in these brain regions that play an important role in energy homeostasis.

CNS sites involved in sympathetic and parasympathetic control of the pancreas: a viral tracing study

The viral transneuronal tracing method was used to identify the CNS cell groups that regulate the parasympathetic and sympathetic outflow systems of the pancreas. Pseudorabies virus (PRV) was injected into the pancreas of vagotomized rats and after 6 days survival, the pattern of transneuronal labeling in the CNS sympathetic regulatory regions was determined. The converse experiment was performed in order to elucidate the central parasympathetic cell groups that regulate the pancreas. Immunohistochemical methods were used to identify putative neuropeptide- and catecholamine-containing CNS neurons involved in these regulatory circuits. The major finding of this study indicates that five brain regions, viz., paraventricular hypothalamic nucleus, perifornical hypothalamic region, A5 catecholamine cell group, rostral ventrolateral medulla, and lateral paragigantocellular reticular nucleus, contain a considerable amount of overlap in cell body labeling. In addition, the ventrolateral part of the periaqueductal gray matter and gigantocellular reticular nucleus, ventral part also showed a similar overlap, but the numbers of neurons found in these areas were considerably lower than the five major regions. These data suggest that these brain regions may provide parallel and possibly redundant, autonomic pathways affecting glucagon and adrenaline release.