Central glucoprivation evoked by administration of 2-deoxy-D-glucose induces expression of the c-fos gene in a subpopulation of neuropeptide Y neurons in the rat hypothalamus

NTS Catecholamine Neurons Mediate Hypoglycemic Hunger via Medial Hypothalamic Feeding Pathways

Glucose is the essential energy source for the brain, whose deficit, triggered by energy deprivation or therapeutic agents, can be fatal. Increased appetite is the key behavioral defense against hypoglycemia; however, the central pathways involved are not well understood. Here, we describe a glucoprivic feeding pathway by tyrosine hydroxylase (TH)-expressing neurons from nucleus of solitary tract (NTS), which project densely to the hypothalamus and elicit feeding through bidirectional adrenergic modulation of agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons. Acute chemogenetic inhibition of arcuate nucleus (ARC)-projecting NTS^TH neurons or their target, AgRP neurons, impaired glucoprivic feeding induced by 2-Deoxy-D-glucose (2DG) injection. Neuroanatomical tracing results suggested that ARC-projecting orexigenic NTS^TH neurons are largely distinct from neighboring catecholamine neurons projecting to parabrachial nucleus (PBN) that promotes satiety. Collectively, we describe a circuit organization in which an ascending pathway from brainstem stimulates appetite through key hunger neurons in the hypothalamus in response to hypoglycemia.

Remote control of glucose-sensing neurons to analyze glucose metabolism

The central nervous system relies on a continual supply of glucose, and must be able to detect glucose levels and regulate peripheral organ functions to ensure that its energy requirements are met. Specialized glucose-sensing neurons, first described half a century ago, use glucose as a signal and modulate their firing rates as glucose levels change. Glucose-excited neurons are activated by increasing glucose concentrations, while glucose-inhibited neurons increase their firing rate as glucose concentrations fall and decrease their firing rate as glucose concentrations rise. Glucose-sensing neurons are present in multiple brain regions and are highly expressed in hypothalamic regions, where they are involved in functions related to glucose homeostasis. However, the roles of glucose-sensing neurons in healthy and disease states remain poorly understood. Technologies that can rapidly and reversibly activate or inhibit defined neural populations provide invaluable tools to investigate how specific neural populations regulate metabolism and other physiological roles. Optogenetics has high temporal and spatial resolutions, requires implants for neural stimulation, and is suitable for modulating local neural populations. Chemogenetics, which requires injection of a synthetic ligand, can target both local and widespread populations. Radio- and magnetogenetics offer rapid neural activation in localized or widespread neural populations without the need for implants or injections. These tools will allow us to better understand glucose-sensing neurons and their metabolism-regulating circuits.

Chronic inhibition of brain glycolysis initiates epileptogenesis

Metabolic abnormalities found in epileptogenic tissue provide considerable evidence of brain hypometabolism, while major risk factors for acquired epilepsy all share brain hypometabolism as one common outcome, suggesting that a breakdown of brain energy homeostasis may actually precede epileptogenesis. However, a causal link between deficient brain energy metabolism and epilepsy initiation has not been yet established. To address this issue we developed an in vivo model of chronic energy hypometabolism by daily intracerebroventricular (i.c.v.) injection of the nonmetabolizable glucose analog 2-deoxy-D-glucose (2-DG) and also investigated acute effects of 2-DG on the cellular level. In hippocampal slices, acute glycolysis inhibition by 2-DG (by about 35%) led to contrasting effects on the network: a downregulation of excitatory synaptic transmission together with a depolarization of neuronal resting potential and a decreased drive of inhibitory transmission. Therefore, the potential acute effect of 2-DG on network excitability depends on the balance between these opposing pre- and postsynaptic changes. In vivo, we found that chronic 2-DG i.c.v. application (estimated transient inhibition of brain glycolysis under 14%) for a period of 4 weeks induced epileptiform activity in initially healthy male rats. Our results suggest that chronic inhibition of brain energy metabolism, characteristics of the well-established risk factors of acquired epilepsy, and specifically a reduction in glucose utilization (typically observed in epileptic patients) can initiate epileptogenesis. © 2017 Wiley Periodicals, Inc.

The Suprachiasmatic Nucleus Modulates the Sensitivity of Arcuate Nucleus to Hypoglycemia in the Male Rat

The suprachiasmatic nucleus (SCN) and arcuate nucleus (ARC) have reciprocal connections; catabolic metabolic information activates the ARC and inhibits SCN neuronal activity. Little is known about the influence of the SCN on the ARC. Here, we investigated whether the SCN modulated the sensitivity of the ARC to catabolic metabolic conditions. ARC neuronal activity, as determined by c-Fos immunoreactivity, was increased after a hypoglycemic stimulus by 2-deoxyglucose (2DG). The highest ARC neuronal activity after 2DG was found at the end of the light period (zeitgeber 11, ZT11) with a lower activity in the beginning of the light period (zeitgeber 2, ZT2), suggesting the involvement of the SCN. The higher activation of ARC neurons after 2DG at ZT11 was associated with higher 2DG induced blood glucose levels as compared with ZT2. Unilateral SCN-lesioned animals, gave a mainly ipsilateral activation of ARC neurons at the lesioned side, suggesting an inhibitory role of the SCN on ARC neurons. The 2DG-induced counterregulatory glucose response correlated with increased ARC neuronal activity and was significantly higher in unilateral SCN-lesioned animals. Finally, the ARC as site where 2DG may, at least partly, induce a counterregulatory response was confirmed by local microdialysis of 2DG. 2DG administration in the ARC produced a higher increase in circulating glucose compared with 2DG administration in surrounding areas such as the ventromedial nucleus of the hypothalamus (VMH). We conclude that the SCN uses neuronal pathways to the ARC to gate sensory metabolic information to the brain, regulating ARC glucose sensitivity and counterregulatory responses to hypoglycemic conditions.

Actions of NPY, and its Y1 and Y2 receptors on pulsatile growth hormone secretion during the fed and fasted state

The hypothalamic NPY system plays an important role in regulating food intake and energy expenditure. Different biological actions of NPY are assigned to NPY receptor subtypes. Recent studies demonstrated a close relationship between food intake and growth hormone (GH) secretion; however, the mechanism through which endogenous NPY modulates GH release remains unknown. Moreover, conclusive evidence demonstrating a role for NPY and Y-receptors in regulating the endogenous pulsatile release of GH does not exist. We used genetically modified mice (germline Npy, Y1, and Y2 receptor knock-out mice) to assess pulsatile GH secretion under both fed and fasting conditions. Deletion of NPY did not impact fed GH release; however, it reversed the fasting-induced suppression of pulsatile GH secretion. The recovery of GH secretion was associated with a reduction in hypothalamic somatotropin release inhibiting factor (Srif; somatostatin) mRNA expression. Moreover, observations revealed a differential role for Y1 and Y2 receptors, wherein the postsynaptic Y1 receptor suppresses GH secretion in fasting. In contrast, the presynaptic Y2 receptor maintains normal GH output under long-term ad libitum-fed conditions. These data demonstrate an integrated neural circuit that modulates GH release relative to food intake, and provide essential information to address the differential roles of Y1 and Y2 receptors in regulating the release of GH under fed and fasting states.

The ketone body β-hydroxybutyric acid influences agouti-related peptide expression via AMP-activated protein kinase in hypothalamic GT1-7 cells

β-Hydroxybutyric acid (BHBA) acts in the brain to influence feeding behaviour, but the underlying molecular mechanisms are unclear. GT1-7 hypothalamic cells expressing orexigenic agouti-related peptide (AGRP) were used to study the AMP-activated protein kinase (AMPK) pathway known to integrate dietary and hormonal signals for food intake regulation. In a 25 mM glucose culture medium, BHBA increased intracellular calcium concentrations and the expression of monocarboxylate transporter 1 (MCT1 (SLC16A1)). Phosphorylation of AMPK-α (PRKAA1 and PRKAA2) at Thr(172) was diminished after 2 h but increased after 4 h. Its downstream target, the mammalian target of rapamycin, was increasingly phosphorylated on Ser(2448) after 2 h but not changed after 4 h of BHBA treatment. After 4 h, BHBA treatment also increased Agrp mRNA expression. This increase was prevented by preincubation with the AMPK inhibitor Compound C. The inhibition of MCT1 activity by p-hydroxymercuribenzoate suppressed BHBA-stimulated AMPK phosphorylation but did not prevent BHBA-induced Agrp mRNA expression. This finding demonstrates that BHBA triggers the AMPK pathway resulting in orexigenic signalling under 25 mM glucose culture conditions. Under conditions of 5.5 mM glucose, however, BHBA marginally increased intracellular calcium but significantly decreased AMPK phosphorylation and Agrp mRNA expression, demonstrating that under physiological conditions BHBA reduces central orexigenic signalling.

Molecular analysis of central feeding regulation by neuropeptide Y (NPY) neurons with NPY receptor small interfering RNAs (siRNAs)

Hypothalamic neuropeptides play important roles in central feeding behavior. Among them, neuropeptide Y (NPY) has the strongest orexigenic action. It is synthesized in NPY-expressing neurons in the arcuate nucleus (ARC), which projects to other nuclei, mainly to the paraventricular nucleus (PVN). PVN, which possesses NPY-Y1, -Y2 and -Y4, -Y5 receptors, is considered as feeding center for central feeding behavior. Herein I review recent results on feeding behavior obtained by gene knockdown technologies. The small interfering RNA (siRNA) plasmid-based vectors, which drive transcription of siRNA by U6 RNA polymerase III promoter to produce knockdown of the NPY and its receptor (Y1, Y2, Y4 and Y5) genes, were stereotaxically injected into mouse ARC and PVN. Feeding behaviors were measured for 6days after siRNA vector injection. NPY and its receptor mRNA levels were decreased, which were measured by RT-PCR and in situ hybridization, and simultaneous decrease in their proteins was also detected in separate nuclei by immunohistochemistry. In the NPY system, decrease in NPY, Y1 and Y5 expressions in specialized nuclei diminished central feeding behavior, whereas decrease in Y2 or Y4 expression in both ARC or PVN did not affect feeding behavior. Thus, specialized change in expressions of NPY and its receptors (especially Y1 and Y5) are important for regulation of endogenous feeding behavior in central regulation. Further analysis of NPY receptors may provide better understanding of feeding behavior and of potential therapeutic targets.

The role of neuropeptide Y in energy homeostasis

When administered into the brain, NPY acts at Y1 and Y5 receptors to increase food intake. The response occurs with a short latency and is quite robust, such that exogenous NPY is generally considered to be the most potent of a growing list of orexigenic compounds that act in the brain. The role of endogenous NPY is not so straightforward, however. Evidence from diverse types of experiments suggests that rather than initiating behavioral eating per se, endogenous NPY elicits autonomic responses that prepare the individual to better cope with consuming a calorically large meal.

Cocaine- and amphetamine-regulated transcript peptide (CART) in the telencephalon of the catfish, Clarias gariepinus: distribution and response to fasting, 2-deoxy-D-glucose, glucose, insulin, and leptin treatments

The cocaine- and amphetamine-regulated transcript peptide (CART)-containing system in the forebrain of Clarias gariepinus was studied with immunocytochemistry. While the immunoreactivity was prominently seen in the neurons of the entopeduncular nucleus (EN) located in the ventral telencephalon, CART-immunoreactive fibers were widely distributed in the dorsal and ventral telencephalon. In view of the established role of CART in energy metabolism, we investigated the response of the CART immunoreactive system to positive and negative nutritional conditions. Neurons of the EN and fibers in the different areas of the telencephalon showed significant reduction in CART immunoreactivity following 48 hours food deprivation, or 2 hours following intracranial administration of 2-deoxy-D-glucose (2DG, 100 ng/g body weight, a metabolic antagonist of glucose). However, intracranial injection of glucose (100 ng/g body weight) resulted in a distinct increase in CART immunoreactivity in these components. In mammals, insulin and leptin have been recognized as adiposity agents that convey peripheral energy status-related information to brain. Intracranial administration of insulin (3 mU/fish) and leptin (10 ng/g body weight) significantly increased CART immunoreactivity in the EN neurons and in the fiber network within 2 hours. Superfusion of the EN-containing tissue fragments in the medium enriched in glucose, insulin, or leptin evoked a significant increase in CART immunoreactivity in the EN neurons, but 2DG reduced the immunoreactivity. We suggest that CART-containing neurons of the EN, and fibers in the telencephalon, may process the energy status-related information and contribute to satiety.

The effect of 2-deoxy-D-glucose on Werner syndrome RecQ helicase gene

Caloric restriction (CR) is known to effectively elongate mammalian life-spans. The compound 2-deoxy-D-glucose (2DG), which is often used as an inhibitor of glucose utilization, is a mimetic agent of CR. In this study, we examined the changes of telomerase and Werner's syndrome RecQ (WRN) helicase after treatment with 2DG, because of the involvement of recQ helicase in the regulation of telomeres. Interestingly, 2DG treatment increased the expression of WRN protein in accordance with induction of its promoter activity and gene expression. Furthermore, the activation of telomerase was observed after 2DG treatment, whereas it resulted in the reduction of cell proliferation. These results suggest that 2DG could up-regulate telomere maintenance factors accompanied with suppression of proliferation.

Feeding behavior and gene expression of appetite-related neuropeptides in mice lacking for neuropeptide Y Y5 receptor subclass

Neuropeptide Y (NPY) is a potent neurotransmitter for feeding. Besides NPY, orexigenic neuropeptides such as agouti-related protein (AgRP), and anorexigenic neuropeptides such as α-melatonin stimulating hormone (MSH) and cocaine-amphetamine-regulated transcript (CART) are also involved in central feeding regulation. During fasting, NPY and AgRP gene expressions are up-regulated and POMC and CART gene expressions are down-regulated in hypothalamus. Based on the network of peptidergic neurons, the former are involved in positive feeding regulation, and the latter are involved in negative feeding, which exert these feeding-regulated peptides especially in paraventricular nucleus (PVN). To clarify the compensatory mechanism of knock-out of NPY system on feeding, change in gene expressions of appetite-related neuropeptides and the feeding behavior was studied in NPY Y5-KO mice. Food intake was increased in Y5-KO mice. Fasting increased the amounts of food and water intake in the KO mice more profoundly. These data indicated the compensatory phenomenon of feeding behavior in Y5-KO mice. RT-PCR and ISH suggested that the compensation of feeding is due to change in gene expressions of AgRP, CART and POMC in hypothalamus. Thus, these findings indicated that the compensatory mechanism involves change in POMC/CART gene expression in arcuate nucleus (ARC). The POMC/CART gene expression is important for central compensatory regulation in feeding behavior.

The role of hypothalamic ingestive behavior controllers in generating dehydration anorexia: a Fos mapping study

Giving rats 2.5% saline to drink for 3-5 days simply and reliably generates anorexia. Despite having the neurochemical and hormonal markers of negative energy balance, dehydrated anorexic rats show a marked suppression of spontaneous food intake, as well as the feeding that is usually stimulated by overnight starvation or a 2-deoxy-d-glucose (2DG) challenge. These observations are consistent with a dehydration-dependent inhibition of the core circuitry that controls feeding. We hypothesize that this inhibition is directed at those neurons in the paraventricular nucleus and lateral hypothalamic area that constitute the hypothalamic "behavior controller" for feeding rather than their afferent inputs from the arcuate nucleus or hindbrain that convey critical feeding-related sensory information. To test this hypothesis, we mapped and quantified the Fos-immunoreactive response to 2DG in control and dehydrated rats drinking 2.5% saline. Our rationale was that regions showing an attenuated Fos response to 2DG in dehydrated animals would be strong candidates as the targets of dehydration-induced suppression of 2DG feeding. We found that the Fos response to combined dehydration and 2DG was attenuated only in the lateral hypothalamic area, with dehydration alone increasing Fos in the lateral part of the paraventricular nucleus. In the arcuate nucleus and those regions of the hindbrain that provide afferent inputs critical for the feeding response to 2DG, the Fos response to 2DG was unaffected by dehydration. Therefore, dehydration appears to target the lateral hypothalamic area and possibly the lateral part of the paraventricular nucleus to suppress the feeding response to 2DG.

The melanocortinergic pathway is rapidly recruited by emotional stress and contributes to stress-induced anorexia and anxiety-like behavior

Neurons producing melanocortin receptor agonist, alpha-MSH derived from proopiomelanocortin, and antagonist, agouti-related protein, are known to be sensitive to metabolic stress such as food deprivation and glucoprivation. However, how these neurons respond to emotional/psychological stress remained to be elucidated. We report here that acute emotional stressors, i.e. restraint and forced swim, evoked mRNA expression of c-fos, a neuronal activation marker, in a high percentage of proopiomelanocortin neurons (up to 53% for restraint stress and 62% for forced swim), with marked variations along the rostro-caudal axis of the arcuate nucleus. In contrast, only a small population of agouti-related protein neurons in this brain region was activated. These neuronal activation patterns were correlated with behavioral reactions. Both stressors suppressed feeding and induced anxiety-like behavior in the elevated plus-maze test, as reflected by a reduction in the percentage of entries and time spent in the open arms. Central pretreatment with SHU9119, a melanocortin receptor antagonist, dose dependently attenuated the anorectic and anxiogenic effects elicited by acute restraint or forced swim. These results indicate that the melancortinergic pathway can be rapidly recruited by acute emotional stress, and that activation of melanocortin signaling is involved in mediating stress-induced anorexia and anxiety.

Neuropeptide Y in normal eating and in genetic and dietary-induced obesity

Neuropeptide Y (NPY) is one the most potent orexigenic peptides found in the brain. It stimulates food intake with a preferential effect on carbohydrate intake. It decreases latency to eat, increases motivation to eat and delays satiety by augmenting meal size. The effects on feeding are mediated through at least two receptors, the Y1 and Y5 receptors. The NPY system for feeding regulation is mostly located in the hypothalamus. It is formed of the arcuate nucleus (ARC), where the peptide is synthesized, and the paraventricular (PVN), dorsomedial (DMN) and ventromedial (VMN) nuclei and perifornical area where it is active. This activity is modulated by the hindbrain and limbic structures. It is dependent on energy availability, e.g. upregulation with food deprivation or restriction, and return to baseline with refeeding. It is also sensitive to diet composition with variable effects of carbohydrates and fats. Leptin signalling and glucose sensing which are directly linked to diet type are the most important factors involved in its regulation. Absence of leptin signalling in obesity models due to gene mutation either at the receptor level, as in the Zucker rat, the Koletsky rat or the db/db mouse, or at the peptide level, as in ob/ob mouse, is associated with increased mRNA abundance, peptide content and/or release in the ARC or PVN. Other genetic obesity models, such as the Otsuka-Long-Evans-Tokushima Fatty rat, the agouti mouse or the tubby mouse, are characterized by a diminution in NPY expression in the ARC nucleus and by a significant increase in the DMN. Further studies are necessary to determine the exact role of NPY in these latter models. Long-term exposure to high-fat or high-energy palatable diets leads to the development of adiposity and is associated with a decrease in hypothalamic NPY content or expression, consistent with the existence of a counter-regulatory mechanism to diminish energy intake and limit obesity development. On the other hand, an overactive NPY system (increased mRNA expression in the ARC associated with an upregulation of the receptors) is characteristic of rats or rodent strains sensitive to dietary-induced obesity. Finally, NPY appears to play an important role in body weight and feeding regulation, and while it does not constitute the only target for drug treatment of obesity, it may nevertheless provide a useful target in conjunction with others.

Feeding and neuroendocrine responses after recurrent insulin-induced hypoglycemia

Prior exposure to hypoglycemia impairs neuroendocrine counterregulatory responses (CRR) during subsequent hypoglycemia. Defective CRR to hypoglycemia is a component of the clinical syndrome hypoglycemia-associated autonomic failure (HAAF). Hypoglycemia also potently stimulates food intake, an important behavioral CRR. Because the increased feeding response to hypoglycemia is behavioral and not hormonal, we hypothesized that it may be regulated differently with recurrent bouts of hypoglycemia. To test this hypothesis, we simultaneously evaluated neuroendocrine CRR and food intake in rats experiencing one or three episodes of insulin-induced hypoglycemia. As expected, recurrent hypoglycemia significantly reduced neuroendocrine hypoglycemic CRR. Epinephrine (E), norepinephrine (NE) and glucagon responses 120 min after insulin injection were significantly reduced in recurrent hypoglycemic rats, relative to rats experiencing hypoglycemia for the first time. Despite these neuroendocrine impairments, food intake was significantly elevated above baseline saline intake whether rats were experiencing a first (hypoglycemia: 3.4+/-0.4 g vs. saline: 0.94+/-0.3 g, P<0.05) or third hypoglycemic episode (hypoglycemia: 3.8+/-0.3 g vs. saline: 1.2+/-0.3 g, P<0.05). These findings demonstrate that food intake elicited in response to hypoglycemia is not impaired as a result of recurrent hypoglycemia. Thus, neuroendocrine and behavioral (stimulation of food intake) CRR are differentially regulated by recurrent hypoglycemia experience.

Central but not peripheral glucoprivation is impaired in monosodium glutamate-treated rats

In the present study, newborn male Wistar rats were injected, subcutaneously, five times, every other day, with monosodium glutamate (MSG, 4 g/kg bw) or saline (as control, C), during the neonatal period. MSG animals developed destruction of the arcuate nuclei (ARC) with absence of NPY-immunoreactive cell bodies, which impaired both the food intake (baseline) and the 2-deoxy-D-glucose (2DG) glucoprivic feeding response. Increases in the immunoreactivity of corticotropin-releasing hormone-cell bodies in the paraventricular nuclei might have developed to compensate for the atrophy of the pituitary in MSG-treated rats. After systemic 2DG injection, neither the C nor the MSG rats increased their food intake, but they showed similar hyperglycemic responses, whereas plasma free fatty acids (FFA) increased only in the C group. In other groups, 2DG, norepinephrine (NE), neostigmine (NEO) and saline were intracerebroventricularly (i.c.v.) administered. In this condition, impairment of the hyperglycemic and food intake responses, associated to a lower increase in plasma FFA levels, were observed. As opposed to this, the MSG treatment gives support to NE effects, enhancing food intake, as well as plasma glucose and FFA levels. After NEO, plasma glucose increased only in the MSG group, while plasma FFA levels were elevated in the C rats. Taken together, the results obtained after MSG treatment point to a separate neural control of the hyperglycemic response and of the lipid mobilization when stimulated by central 2DG, NE or NEO administration. It seems likely that the excitatory neural pathway that controls lipid metabolism and is present in C rats was destroyed by the MSG treatment.

The therapeutic potential of neuropeptide Y in central nervous system disorders with special reference to pain and sympathetically maintained pain

Neuropeptide Y (NPY), a widely distributed peptide, has been shown to have numerous effects in both the central and peripheral nervous systems. In particular, NPY has an important role in mediating analgesia and hyperalgesia by distinct central and peripheral mechanisms. At least six NPY receptor subtypes are known to exist and the development of subtype-specific ligands targeted at NPY receptors will offer novel therapeutic agents. This article will review the involvement of NPY in diverse pathologies of the nervous system, including pain, and will propose a role for NPY in the maintenance of sympathetically maintained pain.

Persistent alterations in neuropeptide Y release in the paraventricular nucleus of rats subjected to dietary manipulation during early life

The aim of the present experiment was to determine the influence of nutritional manipulations during early life on feeding regulatory mechanisms. For this purpose, neuropeptide Y (NPY) release in the hypothalamic paraventricular nucleus was measured in male offspring born to dams fed either on a control (C) diet, on a high-carbohydrate (HC) diet or on a high-fat (HF) diet during gestation and lactation periods. In addition, we examined the development of dietary preferences in these rats. NPY release was measured in vivo through the push-pull technique after a stimulation with 2-deoxy-glucose (2DG), a blocker of carbohydrate metabolism. NPY release was significantly enhanced in the HC rats after 2DG (+89% vs. control and +54% vs. HF rats; P < 0.01). In a two-bottle choice test, a clear preference for carbohydrate (62% vs. 38%; P < 0.01) was present as early as 30 days of age in control rats. The establishment of this preference in HC and HF rats was delayed by 2 and 3 months, respectively. Therefore, each type of dietary manipulation during early life has left a specific imprint in the offspring. The change in reactivity of the NPY system to glucopenia persisted in adulthood. When combined with the early changes in the dietary preferences, this can lead to adverse effects on body weight when abundant and palatable food is offered. These data support the hypothesis of an intrauterine and perinatal programming of the central regulatory mechanisms and reinforce the necessity of a preventive approach for the treatment of obesity and related metabolic disorders.

Basomedial hypothalamic injections of neuropeptide Y conjugated to saporin selectively disrupt hypothalamic controls of food intake

Neuropeptide Y (NPY) conjugated to saporin (NPY-SAP), a ribosomal inactivating toxin, is a newly developed compound designed to selectively target and lesion NPY receptor-expressing cells. We injected NPY-SAP into the basomedial hypothalamus (BMH), just dorsal to the arcuate nucleus (ARC), to investigate its neurotoxicity and to determine whether ARC NPY neurons are required for glucoprivic feeding. We found that NPY-SAP profoundly reduced NPY Y1 receptor and alpha MSH immunoreactivity, as well as NPY, Agouti gene-related protein (AGRP), and cocaine and amphetamine-related transcript mRNA expression in the BMH. NPY-SAP lesions were localized to the injection site with no evidence of retrograde transport by hindbrain NPY neurons with BMH terminals. These lesions impaired responses to intracerebroventricular (icv) leptin (5 microg/5 microl x d) and ghrelin (2 microg/5 microl), which are thought to alter feeding primarily by actions on ARC NPY/AGRP and proopiomelanocortin/cocaine and amphetamine-related transcript neurons. However, the hypothesis that NPY/AGRP neurons are required downstream mediators of glucoprivic feeding was not supported. Although NPY/AGRP neurons were destroyed by NPY-SAP, the lesion did not impair either the feeding or the hyperglycemic response to 2-deoxy-D-glucose-induced blockade of glycolysis use. Similarly, responses to glucagon-like peptide-1 (GLP-1, 5 microg/3 microl icv), NPY (5 microg/3 microl icv), cholecystokinin octapeptide (4 microg/kg ip), and beta-mercaptoacetate (68 mg/kg ip) were not altered by the NPY-SAP lesion. Thus, NPY-SAP destroyed NPY receptor-expressing neurons in the ARC and selectively disrupted controls of feeding dependent on those neurons but did not disrupt peptidergic or metabolic controls dependent upon circuitry outside the BMH.

Neuropeptide Y is required for hyperphagic feeding in response to neuroglucopenia

To investigate the role played by the orexigenic peptide, neuropeptide Y (NPY), in adaptive responses to insulin-induced hypoglycemia, we measured hypothalamic, feeding, and hormonal responses to this stimulus in both wild-type (Npy+/+) and NPY-deficient (Npy-/-) mice. After administration of insulin at a dose (60 mU ip) sufficient to cause moderate hypoglycemia (plasma glucose levels, 40 +/- 3 and 37 +/- 2 mg/dl for Npy+/+ and Npy-/- mice, respectively; P = not significant), 4-h food intake was increased 2.5-fold in Npy+/+ mice relative to saline-injected controls. By comparison, the increase of intake in Npy-/- mice was far smaller (45%) and did not achieve statistical significance (P = 0.08). Hyperphagic feeding in response to insulin-induced hypoglycemia was therefore markedly attenuated in mice lacking NPY, and a similar feeding deficit was detected in these animals after neuroglucopenia induced by 2-deoxyglucose (500 mg/kg ip). A role for NPY in glucoprivic feeding is further supported by our finding that Npy mRNA content (measured by real-time PCR) increased 2.4-fold in the hypothalamus of Npy+/+ mice by 7 h after insulin injection. Unlike the feeding deficits observed in mice lacking NPY, the effect of hypoglycemia to increase plasma glucagon and corticosterone levels was fully intact in these animals, as were both the nadir glucose value and time to recovery of euglycemia after insulin injection (P = not significant). We conclude that NPY signaling is required for hyperphagic feeding, but not neuroendocrine responses to moderate hypoglycemia.

Third ventricular alloxan reversibly impairs glucose counterregulatory responses

Glucokinase (GK) is hypothesized to be the critical glucosensor of pancreatic beta-cells and hypothalamic glucosensing neurons. To understand the role of GK in glucoprivic counterregulatory responses, we injected alloxan, a GK inhibitor and toxin, into the third ventricle (3v) to target nearby GK-expressing neurons. Four and 6 days after 3v, but not 4v, alloxan injection, alloxan-treated rats ate only 30% and their blood glucose area under the curve was only 28% of saline controls' after systemic 2-deoxy-D-glucose. In addition, their hyperglycemic response to hindbrain glucoprivation induced with 5-thio-glucose was impaired, whereas fasting blood glucose levels and food intake after an overnight fast were elevated. These impaired responses were associated with the destruction of 3v tanycytes, reduced glial fibrillary acidic protein-immunoreactivity surrounding the 3v, neuronal swelling, and decreased arcuate nucleus neuropeptide Y (NPY) mRNA. Nevertheless, hypothalamic GK mRNA was significantly elevated. Two weeks after alloxan injection, 3v tanycyte destruction was reversed along with restoration of feeding and hyperglycemic responses to both systemic and hindbrain glucoprivation. At this time there were significant decreases in GK, NPY, and proopiomelanocortin mRNA. Thus, neural substrates near and around the 3v affected by alloxan may be critically involved in the expression of these glucoprivic responses.

Correlations between carbohydrate metabolism and corticotrop axis parameters in anorexia nervosa

The hypercortisolism is one of the hormonal features of anorexia nervosa (AN) in the undernutrition phase. This abnormality seems to be related to the nutritional factors because the weight restoration leads to the normalization of cortisol. We have investigated glycemia, plasma insulin and C-peptide like markers of carbohydrate metabolism, and, also, adrenocorticotrope hormone (ACTH), beta-endorphins and cortisol in basal and dynamics conditions in 142 patients with AN. Insulin negatively correlated with the values of cortisol at 16:00 h (r=-0.28, P<0.05, N=45); 04:00 h (r=-0.29, P<0.01, N=38). C-peptide negatively correlated with the values of cortisol at 08:00 h (r=-0.36, P<0.05, N=36); 12:00 h (r=-0.49, P<0.01, N=36); 16:00 h (r=-0.38, P<0.02, N=36); 20:00 h (r=-0.39, P<0.02, N=36); 04:00 h (r=-0.51, P<0.01, N=31); urinary free cortisol (r=-0.42, P<0.01, N=35); dexamethasone suppression test (DST; r=-0.52, P<0.01, N=30). C-peptide negatively correlated with the values of ACTH at 08:00 h (r=-0.33, P<0.05, N=37); 24:00 h (r=-0.38, P<0.05, N=32); 04:00 h (r=-0.49, P<0.01, N=31); DST (r=-0.45, P<0.02, N=25). We did not identify correlations between carbohydrate metabolism indices and beta-endorphins. These findings suggest that the nutritional factors may cause or maintain the hormonal changes of corticotrop axis in the undernutrition states.

Hypothalamus-brain stem circuitry responsible for vagal efferent signaling to the pancreas evoked by hypoglycemia in rat

Circulating glucose levels significantly affect vagal neural activity, which is important in the regulation of pancreatic functions. Little is known about the mechanisms involved. This study investigates the neural pathways responsible for hypoglycemia-induced vagal efferent signaling to the pancreas and identifies the neurotransmitters involved. Vagal pancreatic efferent nerve activities were recorded in anesthetized rats. Insulin-induced hypoglycemia, a decrease of blood glucose levels from 114 +/- 5 to 74 +/- 6 mg dl(-1), stimulated an increase in pancreatic efferent nerve firing from a basal rate of 1.1 +/- 0.3 to 19 +/- 3 impulses 30 s(-1). In contrast, vagal primary afferent neuronal discharges recorded in the nodose ganglia were unaltered by systemic hypoglycemia. Vagal afferent rootlet section plus splanchnicotomy had no effect on hypoglycemia-induced vagal efferent firing, suggesting a central site of action. Decerebration reduced the increase in nerve firing stimulated by hypoglycemia from 21 +/- 4 to 9.6 +/- 2 impulses 30 s(-1). Chemical ablation of the lateral hypothalamic area, but not the arcuate nucleus, inhibited pancreatic nerve firing evoked by hypoglycemia. Microinjection of the orexin-A receptor antagonist SB-334867 into the dorsal motor nucleus of the vagus (DMV) inhibited pancreatic nerve firing evoked by insulin-induced hypoglycemia by 56%. In contrast, injection of orexin-A (20 pmol) into the DMV elicited a 30-fold increase in pancreatic nerve firing. We concluded that systemic hypoglycemia stimulates pancreatic efferent nerve firing through a central mechanism. Full expression of pancreatic nerve activities during hypoglycemia requires both the forebrain and the brain stem. In addition to activating neurons in the brain stem, central neuroglucopenia activates subpopulations of neurons in the lateral hypothalamic area that contain orexin. The released orexin acts on DMV neurons to stimulate pancreatic efferent nerve activities and thus regulate pancreatic functions.

Differential suppression of hyperglycemic, feeding, and neuroendocrine responses in anorexia

We have used the anorexia shown by rats given hypertonic saline to drink to investigate central mechanisms that can inhibit feeding. Rats dehydrated in this manner for 3 or 5 days showed a severe attenuation of the compensatory feeding observed after an overnight fast compared with control euhydrated rats or rats whose food was restricted to match the intake of anorexic rats. Food intake after injections of 2-deoxy-d-glucose (2-DG) was also significantly decreased in dehydrated animals compared with that after a 2-DG injection given before dehydration. However, all the dehydrated animals demonstrated a robust eating response after water was returned whether they had received injection of 2-DG or vehicle. Despite a profound reduction in 2-DG-induced feeding, other glucoregulatory responses to 2-DG remained intact in dehydrated animals. After 2-DG injection, corticosterone secretion and blood glucose were significantly elevated from preinjection values whether or not animals were dehydrated. Thus the mechanisms responsible for anorexia in dehydrated animals specifically target stimulatory feeding pathways but leave intact other counterregulatory glucometabolic motor events.

Metabolic sensors: viewing glucosensing neurons from a broader perspective

Glucose is a critical substrate for brain and organ function. Specialized glucosensing neurons, which are involved in the control of energy homeostasis and neuroendocrine function, are located in specific anatomic locations in the brain. Glucose-excited neurons increase their firing rate when ambient glucose levels rise. This glucosensing capacity appears to be regulated by a combination of glucokinase and an ATP-sensitive K(+) (K(ATP)) channel whose activity is regulated by ATP derived from glucose metabolism. Glucose inhibited neurons decrease their firing rate when glucose levels rise, although it is unclear what mechanism is used to control this function. Neuropeptide Y and proopiomelanocortin neurons in the hypothalamic arcuate nucleus are examples of neurons that are capable of sensing both glucose and a host of other peripheral metabolic signals, possibly by their actions on the K(ATP) channel. These metabolic sensing neurons are intimately involved in energy homeostasis, and it is postulated that glucose is only one of several peripheral metabolic signals involved in this process under physiologic conditions. However, when glucose supply is severely limited, glucose appears to assume primacy as a stimulant of glucosensing in order to activate the counterregulatory and ingestive processes necessary to restore the vital supply of glucose. Thus, the role of glucosensing is postulated to be a relative one that is dependent upon the supply of peripheral glucose.

Neurochemical phenotype of hypothalamic neurons showing Fos expression 23 h after intracranial AgRP

Agouti-related protein (AgRP) is coexpressed with neuropeptide Y (NPY) in a population of neurons in the arcuate nucleus (ARC) of the hypothalamus and stimulates food intake for up to 7 days if injected intracerebroventricularly. The prolonged food intake stimulation does not seem to depend on continued competition at the melanocortin-4 receptor (MC4R), because the relatively specific MC4R agonist MTII regains its ability to suppress food intake 24 h after AgRP injection. Intracerebroventricular AgRP also stimulates c-Fos expression 24 h after injection in several brain areas, so the neurons exhibiting delayed Fos expression might be particularly important in feeding behavior. Thus we aimed to identify the neurochemical phenotype of some of these neurons in select hypothalamic areas, using double-label immunohistochemistry. AgRP-injected rats ingested significantly more chow (10.2 +/- 0.6 g) vs. saline controls (3.4 +/- 0.7 g) in the first 9 h (light phase) after injection. In the lateral hypothalamus (particularly the perifornical area) 23 h after injection, AgRP induced significantly more Fos vs. saline in orexin-A (OXA) neurons (25.6 +/- 4.9 vs. 4.8 +/- 3.1%), but not in melanin-concentrating hormone (MCH) or cocaine- and amphetamine-regulated transcript (CART) neurons. In the ARC, AgRP induced significantly more Fos in CART (40.6 +/- 5.9 vs. 13.4 +/- 1.8%) but not NPY neurons. In the paraventricular nucleus, there was no significant difference in Fos expression induced by AgRP vs. saline in oxytocin and CART neurons. We conclude that the long-lasting hyperphagia induced by AgRP is correlated with and possibly partially mediated by hyperactive OXA neurons in the lateral hypothalamus and CART neurons in the ARC, but not by NPY and MCH neurons. The substantial increase in light-phase food intake by AgRP supports a role for the arousing effects of OXA. Activation of CART neurons in the ARC (which likely coexpress proopiomelanocortin) could indicate attempts to activate counterregulatory decreases in food intake.

Immunotoxic catecholamine lesions attenuate 2DG-induced increase of AGRP mRNA

Injections of the immunotoxin, saporin conjugated to anti-dopamine-beta-hydroxylase (DSAP), into the hypothalamic paraventricular nucleus (PVH) selectively destroy norepinephrine (NE) and epinephrine (E) terminals in the medial hypothalamus and abolish glucoprivic feeding. We utilized PVH DSAP injections to examine the role of NE/E neurons in the previously reported 2-deoxy-D-glucose (2DG)-induced increases in mRNA levels for the orexigenic peptides, AGRP and NPY. Northern blot analysis revealed that DSAP lesions elevated basal but blocked 2DG-induced increases in AGRP mRNA levels. Changes in NPY mRNA were not detectable. AGRP neurons may contribute to circuitry activated by NE/E neurons for elicitation of glucoregulatory responses.

Zinc-deficient rats are insensitive to glucoprivation caused by 2-deoxy-D-glucose

Three-choice macronutrient intake studies indicate that zinc-deficient (Zn-) rats selectively decrease intake of carbohydrate. Because glucoprivic stimuli increase food intake and selection for carbohydrate, the ability of Zn- rats to respond to glucoprivation induced by 2-deoxy-D-glucose (2-DG) was tested. Rats were fed a Zn-adequate (Zn+) or Zn- diet. In part 1, rats were challenged with 0, 250, or 400 mg 2-DG/kg BW (i.p.) after zinc deficiency was established. In part 2, rats received saline or 2-DG while zinc deficiency was being induced and then after deficiency was established. Food intake was increased after injection of 2-DG to Zn+ rats; however, food intake was not higher after 2-DG administration to Zn- rats. A dose-response test for 2-DG further confirmed these results. In part 2, it was found that Zn- rats lose the response to 2-DG administration when zinc deficiency-induced anorexia begins, after 3 days of consuming a zinc-deficient diet. It appears that the ability to sense blood glucose concentrations may be impaired during zinc deficiency, and this impairment could be a part of the anorexia that develops during zinc deficiency in the rat.

Brain glucose-sensing mechanisms: ubiquitous silencing by aglycemia vs. hypothalamic neuroendocrine responses

Interest in brain glucose-sensing mechanisms is motivated by two distinct neuronal responses to changes in glucose concentrations. One mechanism is global and ubiquitous in response to profound hypoglycemia, whereas the other mechanism is largely confined to specific hypothalamic neurons that respond to changes in glucose concentrations in the physiological range. Although both mechanisms use intracellular metabolism as an indicator of extracellular glucose concentration, the two mechanisms differ in key respects. Global hyperpolarization (inhibition) in response to 0 mM glucose can be reversed by pyruvate, implying that the reduction in ATP levels acting through ATP-dependent potassium (K-ATP) channels is the key metabolic signal for the global silencing in response to 0 mM glucose. In contrast, neuroendocrine hypothalamic responses in glucoresponsive and glucose-sensitive neurons (either excitation or inhibition, respectively) to physiological changes in glucose concentration appear to depend on glucokinase; neuroendocrine responses also depend on K-ATP channels, although the role of ATP itself is less clear. Lactate can substitute for glucose to produce these neuroendocrine effects, but pyruvate cannot, implying that NADH (possibly leading to anaplerotic production of malonyl-CoA) is a key metabolic signal for effects of glucose on glucoresponsive and glucose-sensitive hypothalamic neurons.

Molecular integration of hypothalamo-pituitary-adrenal axis-related neurohormones on the GnRH neuron

Gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus is pivotal to the regulation of reproductive physiology in vertebrates. GnRH and the reproductive axis, in general, can be inhibited during periods of stress or injury. Stress, in the form of mechanical, psychological or immunological insult to an organism results in the activation of the hypothalamo-pituitary-adrenal (HPA) axis initiated by the hypothalamic release of corticotropin-releasing factor (CRF). Recent studies indicate that CRF may act either directly on the GnRH neuron to down-regulate GnRH synthesis, or indirectly via a β-endorphin-mediated pathway. Moreover, in vitro studies suggest that CRF-related peptides can increase the sensitivity of the GnRH neuron to prolactin by increasing the synthesis of the prolactin receptor.

Rapid changes in hypothalamic neuropeptide Y produced by carbohydrate-rich meals that enhance corticosterone and glucose levels

Prior studies have demonstrated that chronic consumption over several weeks of a high-carbohydrate (65%) diet, compared to a moderate-carbohydrate (45%) or low-carbohydrate (15%) diet, potentiates the expression, synthesis and release of hypothalamic NPY. This effect occurs specifically in neurons of the arcuate nucleus (ARC) which project to the paraventricular nucleus (PVN). In the present experiments, tests involving acute manipulations were conducted to determine whether such diet-induced changes in NPY can occur rapidly, perhaps within 1-2 h, and whether these effects can be linked to specific changes in circulating glucoregulatory hormones or glucose itself., In adult, albino rats maintained on lab chow, the acute manipulations included the presentation of either a high-carbohydrate, moderate-carbohydrate or high-fat diet for 90 min at the onset of the natural feeding cycle. They also involved manipulations of glucose itself, either through the ingestion of a glucose (20%) solution in a drinking tube or intraperitoneal injection of a glucose solution (10%). After a high-carbohydrate meal compared to a moderate-carbohydrate or high-fat meal, NPY gene expression examined via in situ hybridization is found to be significantly enhanced in the ARC. The high-carbohydrate meal also potentiates NPY immunoreactivity in the ARC and PVN but has little effect on NPY in other hypothalamic areas examined and actually causes a reduction in the feeding-stimulatory peptide, galanin, specifically in the PVN. The meal-induced increase in NPY is associated with specific endocrine patterns, as revealed by measurements in serum collected from trunk blood or from rats implanted with a chronic jugular catheter. After a high-carbohydrate meal, levels of glucose, together with corticosterone and insulin, are significantly elevated, while non-esterified fatty acids are reduced. A possible effect of circulating glucose on hypothalamic NPY is further suggested by the finding that the consumption or a single injection of a glucose solution at the onset of the feeding cycle similarly elevates NPY mRNA and peptide immunoreactivity in the ARC and PVN. These results demonstrate that hypothalamic NPY can change rapidly in response to dietary carbohydrate. They also suggest that this effect may be related to changes in circulating CORT as well as to the availability or utilization of glucose.

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.

Differential functions of hypothalamic galanin cell grows in the regulation of eating and body weight

Evidence suggests that hypothalamic galanin (GAL) has a variety of functions related to energy and nutrient balance, reproduction, water balance, and neuroendocrine regulation. The focus of this chapter is the role of GAL in eating and body weight regulation. Findings described herein demonstrate that GAL, in a cell group of the anterior region of the paraventricular nucleus (aPVN) that projects to the median eminence, has a role in the control of fat intake, fat metabolism, and body fat. This function of aPVN GAL neurons is carried out in close relation to circulating insulin and glucose. Galanin-expressing perikarya in the medial preoptic area (MPOA) have a similar function, although GAL here operates in association with the female steroids estrogen and progesterone. These GAL cell groups of the aPVN and MPOA contrast with those in the arcuate nucleus as well as the magnocellular vasopressin-containing neurons of the PVN and supraoptic nucleus, which show no relation to fat balance. This evidence reveals differential functions for the distinct GAL neuronal cell groups of the hypothalamus.

Is there appetite after GLP-1 and PACAP?

Anitobesity drugs must increase the sensitivity of the hypothalamic satiety center towards leptin and antagonize the synthesis and action of NPY. The array of pharmacologic tools available is vast and presently ineffective. Among peptide analogs considered for evaluation [NPY-5 antagonists and CCK-A, bombesin, amylin and melanocyte-stimulating hormone-4 (or melanin-concentrating hormone?) agonists], is there a place for GLP-1 and PACAP? GLP-1 receptors present in ARC, PVN, VMN, and SON are the target for both central and blood-borne GLP-1 in those hypothalamic neurons endowed with GLUT-2 and glucokinase. GLP-1, hypersecreted by L-cells after a meal, is a potent insulinotropic agent and, together with glucose, reduces food intake and induces c-fos in the ARC. PACAP is present in the ARC, PVN, and SCH, and its hypothalamic type I receptor elevates cAMP and inositol triphosphate in the PVN, where it may perhaps antagonize NPY-induced food intake and hyperinsulinemia. However, irrelevant neuroendocrine, autonomic, and circadian functions are also activated by this peptide, making it a less than ideal base on which to build an obesity treatment.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

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.

Hypothalamic galanin: control by signals of fat metabolism

The peptide, galanin (GAL), is known to stimulate eating behavior, reduce energy expenditure and affect the release of metabolic hormones. Further, the activity of this peptide in the hypothalamus is modulated, in turn, by these hormones as well as by the ingestion of nutrients. The focus of this investigation is on signals related to nutrient metabolism that may also affect GAL production and, through these neurochemical events, control the ingestion of specific nutrients. Three experiments were performed in normal-weight male, Sprague-Dawley rats. In Experiment 1, the impact of food deprivation (24 and 48 h) was examined. Experiment 2 tested the effects of the compound, 2-deoxy-D-glucose (2-DG, 200 and 400 mg/kg), which blocks glucose utilization, whereas Experiment 3 studied mercaptoacetate (MA, 200 and 600 micromol/kg), which blocks fatty acid oxidation. Eating behavior was examined in some rats, whereas hypothalamic GAL activity was measured in others using radioimmunoassay, immunohistochemistry and in situ hybridization. Both food deprivation and MA (600 micromol/kg), but not 2-DG, affected GAL in the hypothalamus, in one specific area. This is the anterior parvocellular region of the paraventricular nucleus (aPVN), which has a dense concentration of GAL-containing neurons and terminals. GAL gene expression and peptide immunoreactivity in this area is enhanced by food deprivation; in contrast, it is reduced by injection of MA. Other hypothalamic sites with dense concentrations of GAL-containing neurons or fibers are unaffected by food deprivation or MA, and the antimetabolite 2-DG has no impact on GAL in any area. Behavioral measurements indicate that these shifts in GAL activity are accompanied by specific changes in eating behavior. Food deprivation which enhances aPVN GAL produces a marked increase in fat ingestion, whereas MA which reduces aPVN GAL causes a specific reduction in fat ingestion along with a stimulation of protein intake. In contrast, 2-DG preferentially enhances ingestion of carbohydrate. These findings suggest a possible relationship between GAL activity in the aPVN and the metabolic and behavioral processes of fat metabolism and ingestion.

Neuropeptide Y in relation to carbohydrate intake, corticosterone and dietary obesity

Neuropeptide Y (NPY) is known to stimulate eating behavior and to be related to behavioral patterns of carbohydrate ingestion. The present report investigates this relationship further to: (1) characterize the specific NPY projection activated in different dietary paradigms; (2) understand associated changes in circulating hormones that may mediate dietary effects on NPY neurons; and (3) determine whether endogenous NPY in conditions with macronutrient diets can be linked to body fat. Male albino Sprague-Dawley rats were tested in two feeding paradigms, one in which the rats were given a choice of the macronutrients, carbohydrate, fat or protein, or the other involving a single diet varying in carbohydrate of fat content. These studies consistently demonstrated a close association between the ingestion of carbohydrate and NPY levels, specifically in the arcuate nucleus (ARC) and medial portion of the paraventricular nucleus (PVN) of the hypothalamus. In addition to revealing increased NPY activity in animals that naturally select high carbohydrate when given a choice of macronutrients, a single diet with 65% carbohydrate (10% fat), compared to a control diet with 45% carbohydrate (30% fat), significantly potentiates NPY gene expression and NPY-immunoreactivity, as determined by in situ hybridization and immunohistochemistry. A further lowering of carbohydrate to 15% has little effect on NPY. Studies of medial hypothalamic fragments in vitro also reveal enhanced NPY release from hypothalamic tissue taken from rats maintained on high-carbohydrate diet. Together with NPY, circulating corticosterone (CORT) levels are also highest in a high-carbohydrate condition and positively correlated with NPY in the ARC. An association between NPY and adiposity in these dietary conditions is indicated by significantly higher levels of NPY in the medial PVN in rats with high body fat, whether consuming a high-carbohydrate of high-fat diet. This evidence, linking NPY to carbohydrate intake and circulating CORT, suggests a role for this peptide in glucose homeostasis that is normally exhibited under conditions when carbohydrate stores are low. Disturbances in this homeostatic process, associated with hyperinsulinemia and higher levels of NPY, become evident with only a moderate rise in body fat on a high-carbohydrate as well as high-fat diet.

Hypothalamic and hypophyseal regulation of growth hormone secretion

1. Regulation of pulsatile secretion of growth hormone (GH) relies on hypothalamic neuronal loops, major transmitters involved in their operation are growth hormone releasing hormone (GHRH) synthetized mostly in arcuate nucleus (ARC) neurons, and somatostatin (SRIH), synthetized both in hypothalamus periventricular (PVe) and ARC neurons. 2. Neurons synthetizing both peptides can inhibit each other in a reciprocal manner. Other neuropeptides synthetized in ARC neurons, such as galanin, or in ARC interneurons, such as neuropeptide Y (NPY), are able to modulate synthesis and release of GHRH and SRIH into the hypothalamohypophyseal portal system. 3. In addition, the hitherto uncharacterized endogenous ligand of the recently cloned growth hormone releasing peptide receptor, expressed mostly in the ARC, triggers GH release, presumably by actions on ARC interneurons. 4. Thyroid, gonadal, and adrenal steroid hormones also affect the GHRH-SRIH balance; a differential distribution of sex steroid receptors in the ARC and the PVe is likely to account for the different pattern of GH secretion in male and female animals. 5. Growth hormone itself is able to inhibit the amplitude of GH secretory episodes and to increase their frequency, by entering the brain (presumably by receptor-mediated internalization at the level of the choroid plexus) and acting subsequently on ARC neurons. 6. At the pituitary level, major neurotransmitters regulating GH cells act on receptors of the VIP/PACAP/GHRH family and of the somatostatin family, in particular, sst2 and sst3. Those are coupled to accumulation of cAMP as a second messenger. 7. In addition, patch-clamp experiments and measurement of intracellular Ca2+ indicate that GH cells present characteristic, GHRH-dependent, but self-maintained Ca2+ spikes and [Ca2+]i transients, which reflect adaptive mechanisms to constraints of episodic release. 8. Recent data on transcription factors affecting GH gene expression and somatotrope differentiation are also summarized. 9. Regulation and differentiation of somatotropes also depend upon paracrine processes within the pituitary itself and involve growth factors and several neuropeptides, for instance, vasoactive intestinal peptide, angiotensin 2, endothelin, and activin. 10. Finally, characteristic changes occur in the GH secretory pattern under discrete, pathological conditions, such as abnormal growth and dwarfism, diabetes, and acromegaly, as well as during inflammatory processes.

Neuropeptide Y antibody attenuates 2-deoxy-D-glucose induced feeding in rats

2-deoxy-D-glucose (2-DG) has been shown to induce increased feeding responses in animals. Recent studies suggest the possible involvement of neuropeptide Y (NPY) in 2-DG-induced feeding. The present study examined the effect of immunoneutralization of endogenous NPY on 2-DG-induced feeding. NPY antibody injected into the paraventricular nucleus of the rats significantly attenuated 2-DG-induced feeding, suggesting that hypothalamic NPY may mediate, at least partly, the effect of 2-DG on food intake.

The growth hormone-releasing peptide KP-102 induces c-fos expression in the arcuate nucleus

Growth hormone-releasing hexapeptide (GHRP) stimulates GH secretion by acting on both the pituitary and the hypothalamus through a poorly understood mechanism. To reveal the hypothalamic action of GHRP, rat brains were processed for in situ hybridization for c-fos mRNA as a marker of neuronal activity after systemic administration of a newly developed GHRP, KP-102. Hypophysectomized adult male Wistar rats were administered KP-102 through an indwelling right atrial cannula. KP-102 treatment was accompanied by transient expression of the c-fos gene selectively in the ventromedial and ventrolateral regions of the arcuate nucleus (ARC). The distribution of c-fos gene-expressing cells overlapped that of GRF mRNA-containing neurons in the ventrolateral region on adjacent sections, whereas few c-fos mRNA signals were detected in the dorsomedial region where somatostatin mRNA signals were localized. To confirm this observations, hypothalamic sections were subjected to double-label in situ hybridization. Twenty-three percent of c-fos mRNA-containing cells were GRF neurons, comprising 20% of the GRF neurons in the ARC. The remaining c-fos mRNA containing cells were unidentified. KP-102 thus appears to act on a subpopulation of GRF neurons and unidentified cells in the ARC to stimulate GH secretion.