2-Mercaptoacetate and 2-deoxy-D-glucose induce Fos-like immunoreactivity in rat brain

Regulation of Energy and Glucose Homeostasis by the Nucleus of the Solitary Tract and the Area Postrema

The central nervous system regulates feeding, weight and glucose homeostasis in rodents and humans, but the site-specific mechanisms remain unclear. The dorsal vagal complex in the brainstem that contains the nucleus of the solitary tract (NTS) and area postrema (AP) emerges as a regulatory center that impacts energy and glucose balance by monitoring hormonal and nutrient changes. However, the specific mechanistic metabolic roles of the NTS and AP remain elusive. This mini-review highlights methods to study their distinct roles and recent findings on their metabolic differences and similarities of growth differentiation factor 15 (GDF15) action and glucose sensing in the NTS and AP. In summary, future research aims to characterize hormonal and glucose sensing mechanisms in the AP and/or NTS carries potential to unveil novel targets that lower weight and glucose levels in obesity and diabetes.

Glycemic Challenge Is Associated with the Rapid Cellular Activation of the Locus Ceruleus and Nucleus of Solitary Tract: Circumscribed Spatial Analysis of Phosphorylated MAP Kinase Immunoreactivity

Rodent studies indicate that impaired glucose utilization or hypoglycemia is associated with the cellular activation of neurons in the medulla (Winslow, 1733) (MY), believed to control feeding behavior and glucose counterregulation. However, such activation has been tracked primarily within hours of the challenge, rather than sooner, and has been poorly mapped within standardized brain atlases. Here, we report that, within 15 min of receiving 2-deoxy-d-glucose (2-DG; 250 mg/kg, i.v.), which can trigger glucoprivic feeding behavior, marked elevations were observed in the numbers of rhombic brain (His, 1893) (RB) neuronal cell profiles immunoreactive for the cellular activation marker(s), phosphorylated p44/42 MAP kinases (phospho-ERK1/2), and that some of these profiles were also catecholaminergic. We mapped their distributions within an open-access rat brain atlas and found that 2-DG-treated rats (compared to their saline-treated controls) displayed greater numbers of phospho-ERK1/2+ neurons in the locus ceruleus (Wenzel and Wenzel, 1812) (LC) and the nucleus of solitary tract (>1840) (NTS). Thus, the 2-DG-activation of certain RB neurons is more rapid than perhaps previously realized, engaging neurons that serve multiple functional systems and which are of varying cellular phenotypes. Mapping these populations within standardized brain atlas maps streamlines their targeting and/or comparable mapping in preclinical rodent models of disease.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Neural substrates of fear-induced hypophagia in male and female rats

Cessation of eating under fear is an adaptive response that aids survival by prioritizing the expression of defensive behaviors over feeding behavior. However, this response can become maladaptive when persistent. Thus, accurate mediation of the competition between fear and feeding is important in health and disease; yet, the underlying neural substrates are largely unknown. The current study identified brain regions that were recruited when a fear cue inhibited feeding in male and female rats. We used a previously established behavioral paradigm to elicit hypophagia with a conditioned cue for footshocks, and Fos imaging to map activation patterns during this behavior. We found that distinct patterns of recruitment were associated with feeding and fear expression, and that these patterns were similar in males and females except within the medial prefrontal cortex (mPFC). In both sexes, food consumption was associated with activation of cell groups in the central amygdalar nucleus, hypothalamus, and dorsal vagal complex, and exposure to food cues was associated with activation of the anterior basolateral amygdalar nucleus. In contrast, fear expression was associated with activation of the lateral and posterior basomedial amygdalar nuclei. Interestingly, selective recruitment of the mPFC in females, but not in males, was associated with both feeding and freezing behavior, suggesting sex differences in the neuronal processing underlying the competition between feeding and fear. This study provided the first evidence of the neural network mediating fear-induced hypophagia, and important functional activation maps for future interrogation of the underlying neural substrates.

Effect of LPS treatment on tyrosine hydroxylase expression and Parkinson-like behaviors

Puberty is a critical period of development during which the brain undergoes reorganizing and remodeling. Exposure to stress during this period is thought to interfere with normal brain development and increase susceptibility to mental illnesses. In female mice, pubertal exposure to lipopolysaccharide (LPS), a bacterial endotoxin, has been shown to alter sexual, anxiety-like, and depression-like behaviors and cognition in an enduring manner. However, the mechanisms underlying these effects remain unknown. The present study examined age and sex difference in tyrosine hydroxylase (TH) expression and dopamine-dependent and Parkinson-like behaviors following LPS treatment. The results show that LPS treatment during adulthood causes an enduring increase in TH expression in many of the brain regions examined. In contrast, there is no change in TH expression following LPS treatment during puberty. However, pubertal LPS treatment induces enduring behavioral deficits in tests of Parkinson-like behaviors, more so in male than in female mice. These results suggest that the low levels of TH following exposure to pubertal immune challenge may predispose mice to Parkinson-like behavior. These findings add to our understanding of stress and immune responses during puberty and their impact on mental health later in life.

Hypothermia induced by inhibition of fatty acid metabolism in anesthetized rats: contributions of the forebrain and vagal afferents

2-Mercaptoacetate (MA) is an antimetabolic drug that inhibits the utilization of fatty acids as an energy source. The intravenous injection of MA (1.2 mmol·kg^-1) elicited an increase in tail skin temperature and a decrease in body core temperature in urethane-chloralose-anesthetized, neuromuscularly blocked, artificially ventilated rats, although administration of the same amount of NaCl did not. The respiratory exchange ratio was significantly higher after administration of MA than that after the saline treatment. On the other hand, heat production was increased by either the MA- or NaCl-injection, suggesting a nonspecific effect caused by the hyperosmolality of the solutions. These results indicate that the MA-induced hypothermia was caused by an increase in heat loss but not by a decrease in heat production. The amplitudes of heat loss responses to MA in rats fasted overnight were significantly smaller than those in fed ones, suggesting a mechanism for suppression of heat loss in the fasted state. Rats pretreated with vagotomy, capsaicin-induced desensitization of sensory nerve fibers or decerebration did not exhibit the MA-induced hypothermic responses. It is possible that the MA-induced heat loss and hypothermia were mediated by the vagal afferents and required the forebrain for the full expression of the responses.

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.

Deletion of GPR40 fatty acid receptor gene in mice blocks mercaptoacetate-induced feeding

Both increased and decreased fatty acid (FA) availability contribute to control of food intake. For example, it is well documented that intestinal FA reduces feeding by triggering enterondocrine secretion of satietogenic peptides, such as cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1). In contrast, mechanisms by which decreased FA availability increase feeding are not well understood. Over the past three decades substantial research related to FA availability and increased feeding has involved use of the orexigenic compound mercaptoacetate (MA). Because MA reportedly inhibits FA oxidation, it has been assumed that reduced FA oxidation accounts for the orexigenic action of MA. Recently, however, we demonstrated that MA antagonizes G protein-coupled receptor 40 (GPR40), a membrane receptor for long and medium chain FA. We also demonstrated that, by antagonizing GPR40, MA inhibits GLP-1 secretion and attenuates vagal afferent activation by FA. Because both vagal afferent activation and GLP-1 inhibit food intake, we postulated that inhibition of GPR40 by MA might underlie the orexigenic action of MA. We tested this hypothesis using male and female GPR40 knockout (KO) and wild-type (WT) mice. Using several testing protocols, we found that MA increased feeding in WT, but not GPR40 KO mice, and that GPR40 KO mice gained more weight than WT on a high-fat diet. Metabolic monitoring after MA or saline injection in the absence of food did not reveal significant differences in respiratory quotient or energy expenditure between treatment groups or genotypes. These results support the hypothesis that MA stimulates food intake by blocking FA effects on GPR40.

Peripheral and central glucose sensing in hypoglycemic detection

Hypoglycemia poses a serious threat to the integrity of the brain, owing to its reliance on blood glucose as a fuel. Protecting against hypoglycemia is an extended network of glucose sensors located within the brain and in the periphery that serve to mediate responses restoring euglycemia, i.e., counterregulatory responses. This review examines the various glucose sensory loci involved in hypoglycemic detection, with a particular emphasis on peripheral glucose sensory loci and their contribution to hypoglycemic counterregulation.

A parabrachial-hypothalamic cholecystokinin neurocircuit controls counterregulatory responses to hypoglycemia

Hypoglycemia engenders an autonomically mediated counterregulatory (CR)-response that stimulates endogenous glucose production to maintain concentrations within an appropriate physiological range. Although the involvement of the brain in preserving normoglycemia has been established, the neurocircuitry underlying centrally mediated CR-responses remains unclear. Here we demonstrate that lateral parabrachial nucleus cholecystokinin (CCK(LPBN)) neurons are a population of glucose-sensing cells (glucose inhibited) with counterregulatory capacity. Furthermore, we reveal that steroidogenic-factor 1 (SF1)-expressing neurons of the ventromedial nucleus of the hypothalamus (SF1(VMH)) are the specific target of CCK(LPBN) glucoregulatory neurons. This discrete CCK(LPBN)→SF1(VMH) neurocircuit is both necessary and sufficient for the induction of CR-responses. Together, these data identify CCK(LPBN) neurons, and specifically CCK neuropeptide, as glucoregulatory and provide significant insight into the homeostatic mechanisms controlling CR-responses to hypoglycemia.

Regulation of gonadotropin secretion by monitoring energy availability

Nutrition is a principal environmental factor influencing fertility in animals. Energy deficit causes amenorrhea, delayed puberty, and suppression of copulatory behaviors by inhibiting gonadal activity. When gonadal activity is impaired by malnutrition, the signals originating from an undernourished state are ultimately conveyed to the gonadotropin-releasing hormone (GnRH) pulse generator, leading to suppressed secretion of GnRH and luteinizing hormone (LH). The mechanism responsible for energetic control of gonadotropin release is believed to involve metabolic signals, sensing mechanisms, and neuroendocrine pathways. The availabilities of blood-borne energy substrates such as glucose, fatty acids, and ketone bodies, which fluctuate in parallel with changes in nutritional status, act as metabolic signals that regulate the GnRH pulse generator activity and GnRH/LH release. As components of the specific sensing system, the ependymocytes lining the cerebroventricular wall in the lower brainstem integrate the information derived from metabolic signals to control gonadotropin release. One of the pathways responsible for the energetic control of gonadal activity consists of noradrenergic neurons from the solitary tract nucleus in the lower brainstem, projecting to the paraventricular nucleus of the hypothalamus. Further studies are needed to elucidate the mechanisms underlying energetic control of reproductive function.

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

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

Tyrosine hydroxylase phosphorylation in catecholaminergic brain regions: a marker of activation following acute hypotension and glucoprivation

The expression of c-Fos defines brain regions activated by the stressors hypotension and glucoprivation however, whether this identifies all brain sites involved is unknown. Furthermore, the neurochemicals that delineate these regions, or are utilized in them when responding to these stressors remain undefined. Conscious rats were subjected to hypotension, glucoprivation or vehicle for 30, 60 or 120 min and changes in the phosphorylation of serine residues 19, 31 and 40 in the biosynthetic enzyme, tyrosine hydroxylase (TH), the activity of TH and/or, the expression of c-Fos were determined, in up to ten brain regions simultaneously that contain catecholaminergic cell bodies and/or terminals: A1, A2, caudal C1, rostral C1, A6, A8/9, A10, nucleus accumbens, dorsal striatum and medial prefrontal cortex. Glucoprivation evoked phosphorylation changes in A1, caudal C1, rostral C1 and nucleus accumbens whereas hypotension evoked changes A1, caudal C1, rostral C1, A6, A8/9, A10 and medial prefrontal cortex 30 min post stimulus whereas few changes were evident at 60 min. Although increases in pSer19, indicative of depolarization, were seen in sites where c-Fos was evoked, phosphorylation changes were a sensitive measure of activation in A8/9 and A10 regions that did not express c-Fos and in the prefrontal cortex that contains only catecholaminergic terminals. Specific patterns of serine residue phosphorylation were detected, dependent upon the stimulus and brain region, suggesting activation of distinct signaling cascades. Hypotension evoked a reduction in phosphorylation in A1 suggestive of reduced kinase activity. TH activity was increased, indicating synthesis of TH, in regions where pSer31 alone was increased (prefrontal cortex) or in conjunction with pSer40 (caudal C1). Thus, changes in phosphorylation of serine residues in TH provide a highly sensitive measure of activity, cellular signaling and catecholamine utilization in catecholaminergic brain regions, in the short term, in response to hypotension and glucoprivation.

Molecules affecting hypothalamic control of core body temperature in response to calorie intake

Core body temperature (CBT) and calorie intake are main components of energy homeostasis and two important regulators of health, longevity, and aging. In homeotherms, CBT can be influenced by calorie intake as food deprivation or calorie restriction (CR) lowers CBT whereas feeding has hyperthermic effects. The finding that in mice CBT prolonged lifespan independently of CR, suggested that the mechanisms modulating CBT may represent important regulators of aging. Here we summarize the current knowledge on the signaling molecules and their receptors that participate in the regulation of CBT responses to calorie intake. These include hypothalamic neuropeptides regulating feeding but also energy expenditure via modulation of thermogenesis. We also report studies indicating that nutrient signals can contribute to regulation of CBT by direct action on hypothalamic preoptic warm-sensitive neurons that in turn regulate adaptive thermogenesis and hence CBT. Finally, we show the role played by two orphans G protein-coupled receptor: GPR50 and GPR83, that were recently demonstrated to regulate temperature-dependent energy expenditure.

Glucoprivation in the ventrolateral medulla decreases brown adipose tissue sympathetic nerve activity by decreasing the activity of neurons in raphe pallidus

In urethane/α-chloralose anesthetized rats, cold exposure increased brown adipose tissue sympathetic nerve activity (BAT SNA: +699 ± 104% control). Intravenous administration of 2-deoxy-D-glucose (2-DG; 200 mg·ml(-1)·kg(-1)) reversed the cold-evoked activation of BAT SNA (nadir: 139 ± 36% of control) and decreased BAT temperature (-1.1 ± 0.2°C), expired CO(2) (-0.4 ± 0.1%), and core temperature (-0.5 ± 0.0). Similarly, unilateral nanoinjection of the glucoprivic agent 5-thioglucose (5-TG; 12 μg/100 nl) in the ventrolateral medulla (VLM) completely reversed the cold-evoked increase in BAT SNA (nadir: 104 ± 7% of control), and decreased T(BAT) (-1.4 ± 0.3°C), expired CO(2) (-0.2 ± 0.0%), and heart rate (-35 ± 10 beats/min). The percentage of rostral raphé pallidus (RPa)-projecting neurons in the dorsal hypothalamic area/dorsomedial hypothalamus that expressed Fos in response to cold exposure (ambient temperature: 4-10°C) did not differ between saline (28 ± 6%) and 2-DG (30 ± 5%) pretreated rats, whereas the percentage of spinally projecting neurons in the RPa/raphé magnus that expressed Fos in response to cold exposure was lower in 2-DG- compared with saline-pretreated rats (22 ± 6% vs. 42 ± 5%, respectively). The increases in BAT SNA evoked by nanoinjection of bicuculline in the RPa or by transection of the neuraxis at the pontomedullary border were resistant to inhibition by glucoprivation. These results suggest that neurons within the VLM play a role in the glucoprivic inhibition of BAT SNA and metabolism, that this inhibition requires neural structures rostral to the pontomedullary border, and that this inhibition is mediated by a GABAergic input to the RPa.

Minireview: The value of looking backward: the essential role of the hindbrain in counterregulatory responses to glucose deficit

This review focuses on evidence indicating a key role for the hindbrain in mobilizing behavioral, autonomic and endocrine counterregulatory responses to acute and profound glucose deficit, and identifies hindbrain norepinephrine (NE) and epinephrine (E) neurons as essential mediators of some of these responses. It has become clear that hindbrain NE/E neurons are functionally diverse. However, considerable progress has been made in identifying the particular NE/E neurons important for particular glucoregulatory responses. Although it is not yet known whether NE/E neurons are directly activated by glucose deficit, compelling evidence indicates that if they are not, the primary glucoreceptor cells must be located in the immediate vicinity these neurons. Hindbrain studies identifying cellular markers associated with glucose-sensing functions in other brain regions are discussed, as are studies examining the relationship of these markers to counterregulatory responses of NE/E neurons. Further investigations to identify glucose-sensing cells (neurons, ependymocytes, or glia) controlling counterregulatory responses are crucial, as are studies to determine the specific functions of glucose-sensing cells throughout the brain. Likewise, examination of the roles (if any) of hindbrain counterregulatory systems in managing glucose homeostasis under basal, nonglucoprivic conditions will also be important for a full understanding of energy homeostasis. Nevertheless, the accumulated evidence demonstrates that hindbrain glucose sensors and NE/E neurons are essential players in triggering counterregulatory responses to emergencies of glucose deficit.

Ascending caudal medullary catecholamine pathways drive sickness-induced deficits in exploratory behavior: brain substrates for fatigue?

Immune challenges can lead to marked behavioral changes, including fatigue, reduced social interest, anorexia, and somnolence, but the precise neuronal mechanisms that underlie sickness behavior remain elusive. Part of the neurocircuitry influencing behavior associated with illness likely includes viscerosensory nuclei located in the caudal brainstem, based on findings that inactivation of the dorsal vagal complex (DVC) can prevent social withdrawal. These brainstem nuclei contribute multiple neuronal projections that target different components of autonomic and stress-related neurocircuitry. In particular, catecholaminergic neurons in the ventrolateral medulla (VLM) and DVC target the hypothalamus and drive neuroendocrine responses to immune challenge, but their particular role in sickness behavior is not known. To test whether this catecholamine pathway also mediates sickness behavior, we compared effects of DVC inactivation with targeted lesion of the catecholamine pathway on exploratory behavior, which provides an index of motivation and fatigue, and associated patterns of brain activation assessed by immunohistochemical detection of c-Fos protein. LPS treatment dramatically reduced exploratory behavior, and produced a pattern of increased c-Fos expression in brain regions associated with stress and autonomic adjustments paraventricular hypothalamus (PVN), bed nucleus of the stria terminalis (BST), central amygdala (CEA), whereas activation was reduced in regions involved in exploratory behavior (hippocampus, dorsal striatum, ventral tuberomammillary nucleus, and ventral tegmental area). Both DVC inactivation and catecholamine lesion prevented reductions in exploratory behavior and completely blocked the inhibitory LPS effects on c-Fos expression in the behavior-associated regions. In contrast, LPS-induced activation in the CEA and BST was inhibited by DVC inactivation but not by catecholamine lesion. The findings support the idea that parallel pathways from immune-sensory caudal brainstem sources target distinct populations of forebrain neurons that likely mediate different aspects of sickness. The caudal medullary catecholaminergic projections to the hypothalamus may significantly contribute to brain mechanisms that induce behavioral "fatigue" in the context of physiological stressors.

Dietary fat sensing via fatty acid oxidation in enterocytes: possible role in the control of eating

Various mechanisms detect the presence of dietary triacylglycerols (TAG) in the digestive tract and link TAG ingestion to the regulation of energy homeostasis. We here propose a novel sensing mechanism with the potential to encode dietary TAG-derived energy by translating enterocyte fatty acid oxidation (FAO) into vagal afferent signals controlling eating. Peripheral FAO has long been implicated in the control of eating ( 141 ). The prevailing view was that mercaptoacetate (MA) and other FAO inhibitors stimulate eating by modulating vagal afferent signaling from the liver. This concept has been challenged because hepatic parenchymal vagal afferent innervation is scarce and because experimentally induced changes in hepatic FAO often fail to affect eating. Nevertheless, intraperitoneally administered MA acts in the abdomen to stimulate eating because this effect was blocked by subdiaphragmatic vagal deafferentation ( 21 ), a surgical technique that eliminates all vagal afferents from the upper gut. These and other data support a role of the small intestine rather than the liver as a FAO sensor that can influence eating. After intrajejunal infusions, MA also stimulated eating in rats through vagal afferent signaling, and after infusion into the superior mesenteric artery, MA increased the activity of celiac vagal afferent fibers originating in the proximal small intestine. Also, pharmacological interference with TAG synthesis targeting the small intestine induced a metabolic profile indicative of increased FAO and inhibited eating in rats on a high-fat diet but not on chow. Finally, cell culture studies indicate that enterocytes oxidize fatty acids, which can be modified pharmacologically. Thus enterocytes may sense dietary TAG-derived fatty acids via FAO and influence eating through changes in intestinal vagal afferent activity. Further studies are necessary to identify the link between enterocyte FAO and vagal afferents and to examine the specificity and potential physiological relevance of such a mechanism.

Inhibition of fatty acid oxidation activates transforming growth factor-beta in cerebrospinal fluid and decreases spontaneous motor activity

We have previously reported that transforming growth factor (TGF)-beta in the cerebrospinal fluid (CSF) is involved in the mechanism underlying the regulation of spontaneous motor activity (SMA) by the central nervous system after exercise. However, it remained unclear what physiological condition triggers the activation of TGF-beta. We hypothesized that the shortage of energy derived from fatty acid (FA) oxidation observed in the early phase of exercise activated TGF-beta in the CSF. To test this hypothesis, we investigated whether mercaptoacetate (MA), an inhibitor of FA oxidation, could induce an activation of TGF-beta in the CSF and a decrease in SMA. Intraperitoneal (i.p.) administration of MA activated TGF-beta in CSF in rats and depressed SMA; 2-deoxyglucose, an inhibitor of carbohydrate oxidation, on the other hand, depressed SMA but failed to activate CSF TGF-beta. Intracisternal administration of anti-TGF-beta antibody abolished the depressive effect of MA on SMA. We also found that the depression of SMA and the activation of TGF-beta in the CSF by i.p. MA administration were eliminated by vagotomy. Our data suggest that TGF-beta in the CSF is activated by the inhibition of FA oxidation via the vagus nerve and that this subsequently induces depression of SMA.

Hindbrain catecholamine neurons modulate the growth hormone but not the feeding response to ghrelin

The gastrointestinal peptide, ghrelin, elicits feeding and secretion when administered systemically or centrally. Previous studies have suggested that hypothalamic projections of hindbrain catecholamine neurons are involved in both of these actions of ghrelin. The purpose of this study was to further assess the role of hindbrain catecholamine neurons in ghrelin-induced feeding and GH secretion and to determine the anatomical distribution of the catecholamine neurons involved. We lesioned noradrenergic and adrenergic neurons that innervate the medial hypothalamus by microinjecting the retrogradely transported immunotoxin, saporin (SAP) conjugated to antidopamine-beta-hydroxylase (DSAP) into the paraventricular nucleus of the hypothalamus. Controls were injected with unconjugated SAP. We found that the DSAP lesion did not impair the feeding response to central or peripheral ghrelin administration, indicating that these neurons are not required for ghrelin's orexigenic effect. However, the GH response to ghrelin was prolonged significantly in DSAP-lesioned rats. We also found that expression of Fos, an indicator of neuronal activation, was significantly enhanced over baseline levels in A1, A1/C1, C1, and A5 cell groups after ghrelin treatment and in A1, A1/C1, and A5 cell groups after GH treatment. The similar pattern of Fos expression in catecholamine cell groups after GH and ghrelin and the prolonged GH secretion in response to ghrelin in DSAP rats together suggest that activation of hindbrain catecholamine neurons by ghrelin or GH could be a component of a negative feedback response controlling GH levels.

Glut2-dependent glucose-sensing controls thermoregulation by enhancing the leptin sensitivity of NPY and POMC neurons

The physiological contribution of glucose in thermoregulation is not completely established nor whether this control may involve a regulation of the melanocortin pathway. Here, we assessed thermoregulation and leptin sensitivity of hypothalamic arcuate neurons in mice with inactivation of glucose transporter type 2 (Glut2)-dependent glucose sensing. Mice with inactivation of Glut2-dependent glucose sensors are cold intolerant and show increased susceptibility to food deprivation-induced torpor and abnormal hypothermic response to intracerebroventricular administration of 2-deoxy-d-glucose compared to control mice. This is associated with a defect in regulated expression of brown adipose tissue uncoupling protein I and iodothyronine deiodinase II and with a decreased leptin sensitivity of neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons, as observed during the unfed-to-refed transition or following i.p. leptin injection. Sites of central Glut-2 expression were identified by a genetic tagging approach and revealed that glucose-sensitive neurons were present in the lateral hypothalamus, the dorsal vagal complex, and the basal medulla but not in the arcuate nucleus. NPY and POMC neurons were, however, connected to nerve terminals from Glut2-expressing neurons. Thus, our data suggest that glucose controls thermoregulation and the leptin sensitivity of NPY and POMC neurons through activation of Glut2-dependent glucose-sensing neurons located outside of the arcuate nucleus.

Functional magnetic resonance imaging and c-Fos mapping in rats following a glucoprivic dose of 2-deoxy-D-glucose

The glucose analogue, 2-deoxy-D-glucose (2-DG) is an inhibitor of glycolysis and, when administered systemically or centrally, induces glucoprivation leading to counter-regulatory responses, including increased feeding behaviour. Investigations into how the brain responds to glucoprivation could have important therapeutic potential, as disruptions or defects in the defence of the brain's 'glucostatic' circuitry may be partly responsible for pathological conditions resulting from diabetes and obesity. To define the 'glucostat' brain circuitry further we have combined blood-oxygen-level-dependent pharmacological-challenge magnetic resonance imaging (phMRI) with whole-brain c-Fos functional activity mapping to characterise brain regions responsive to an orexigenic dose of 2-DG [200 mg/kg; subcutaneous (s.c.)]. For phMRI, rats were imaged using a T(2)*-weighted gradient echo in a 7T magnet for 60 min under alpha-chloralose anaesthesia, whereas animals for immunohistochemistry were unanaesthetised and freely behaving. These complementary methods demonstrated functional brain activity in a number of previously characterised glucose-sensing brain regions such as those in the hypothalamus and brainstem following administration of 2-DG compared with vehicle. As the study mapped whole-brain functional responses, it also identified the orbitofrontal cortex and striatum (nucleus accumbens and ventral pallidum) as novel 2-DG-responsive brain regions. These regions make up a corticostriatal connection with the hypothalamus, by which aspects of motivation, salience and reward can impinge on the hypothalamic control of feeding behaviour. This study, therefore, provides further evidence for a common integrated circuit involved in the induction of feeding behaviour, and illustrates the valuable potential of phMRI in investigating central pharmacological actions.

Leptin and the systems neuroscience of meal size control

The development of effective pharmacotherapy for obesity will benefit from a more complete understanding of the neural pathways and the neurochemical signals whose actions result in the reduction of the size of meals. This review examines the neural control of meal size and the integration of two principal sources of that control--satiation signals arising from the gastrointestinal tract and CNS leptin signaling. Four types of integrations that are central to the control of meal size are described and each involves the neurons of the nucleus tractus solitarius (NTS) in the dorsal hindbrain. Data discussed show that NTS neurons integrate information arising from: (1) ascending GI-derived vagal afferent projections, (2) descending neuropeptidergic projections from leptin-activated arcuate and paraventricular nucleus neurons, (3) leptin signaling in NTS neurons themselves and (4) melanocortinergic projections from NTS and hypothalamic POMC neurons to NTS neurons and melanocortinergic modulation of vagal afferent nerve terminals that are presynaptic to NTS neurons.

Immune challenge and satiety-related activation of both distinct and overlapping neuronal populations in the brainstem indicate parallel pathways for viscerosensory signaling

Caudal brainstem viscerosensory nuclei convey information about the body's internal state to forebrain regions implicated in feeding behavior and responses to immune challenge, and may modulate ingestive behavior following immune activation. Illness-induced appetite loss might be attributed to accentuated "satiety" pathways, activation of a distinct "danger channel" separate from satiety pathways, or both. To evaluate neural substrates that could mediate the effects of illness on ingestive behavior, we analyzed the pattern and phenotypes of medullary neurons responsive to consumption of a preferred food, sweetened milk, and to intraperitoneal lipopolysaccharide challenge that reduced sweetened milk intake. Brainstem sections were stained for c-Fos, dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, and glucagon-like peptide-1 (GLP-1) immunoreactivity. Sweetened milk intake activated many neurons throughout the nucleus of the solitary tract (NTS), including A2 noradrenergic neurons in the caudal half of the NTS. LPS challenge activated a similar population of neurons in the NTS, in addition to rostral C2 adrenergic and mid-level A2 noradrenergic neurons in the NTS, many C1 and A1 neurons in the ventrolateral medulla, and in GLP-1 neurons in the dorsal medullary reticular nucleus. Increased numbers of activated GLP-1 neurons in the NTS were only associated with sweetened milk ingestion. Evidence for parallel processing was reflected in the parabrachial nucleus, where sweetened milk intake resulted in activation of the inner external lateral, ventrolateral and central medial portions, whereas LPS challenge induced c-Fos expression in the outer external lateral portions. Thus, signals generated in response to potentially dangerous physiological conditions seem to be propagated via specific populations of catecholaminergic neurons in the NTS and VLM, and likely include a pathway through the external lateral PBN. The data indicate that immune challenge engages multiple ascending neural pathways including both a distinct catecholaminergic "danger" pathway, and a possibly multimodal pathway derived from the NTS.

2-Deoxy-D-glucose, but not mercaptoacetate, increases food intake in decerebrate rats

We examined food intake in chronically maintained decerebrate rats in response to two antimetabolic drugs known to stimulate food intake, 2-mercaptoacetate (MA) and 2-deoxy-D-glucose (2DG). MA reduces fatty acid oxidation, and 2DG reduces glucose utilization. Because previous work has shown that insulin-induced hypoglycemia increases food intake in decerebrate rats, we predicted that 2DG would have this same effect. MA-induced feeding requires vagal sensory neurons that terminate in the hindbrain. Cholecystokinin-induced suppression of feeding, which likewise requires vagal sensory neurons, has been shown to suppress food intake in decerebrate rats. Therefore, we predicted that MA's effects on feeding would also persist in decerebrate rats. In our experiments, the test diet (40% milk, diluted with water) was infused intraorally through a chronic cheek fistula. We found that sham controls consumed 258% and 230% of their baseline milk intake in response to 2DG and MA, respectively. Decerebrates consumed 239% of their baseline milk intake in response to 2DG, but did not increase their intake in response to MA. Because decerebration separates the hindbrain from the forebrain, these results indicate that 2DG-induced glucoprivation is capable of acting within the hindbrain to activate fundamental reflex circuitry for consummatory feeding responses, as shown previously for hypoglycemia. In contrast, MA affects food consumption only after forebrain processing of MA-induced vagal afferent signals and in the presence of intact ascending and descending neural pathways.

Dorsal hindbrain 5'-adenosine monophosphate-activated protein kinase as an intracellular mediator of energy balance

The fuel-sensing enzyme AMP-activated protein kinase (AMPK) has been implicated in central nervous system control of energy balance. Hypothalamic AMPK activity is increased by food deprivation, and this elevation is inhibited by refeeding or by leptin treatment. The contribution of extrahypothalamic AMPK activity in energy balance control has not been addressed. Here, we investigate the effects of physiological state on the AMPK activity in hindbrain nucleus tractus solitarius (NTS) neurons because treatments that reduce energy availability in these neurons trigger behavioral, endocrine, and autonomic responses to restore energy balance. Food-deprived rats showed significantly increased AMPK activity in both NTS- and hypothalamus-enriched lysates compared with those that were ad libitum fed. Pharmacological inhibition of AMPK activity in medial NTS neurons, by intraparenchymal injection of compound C, suppressed food intake and body weight gain compared with vehicle. Fourth ventricle (4th i.c.v.) compound C delivery increased heart rate and spontaneous activity in free-moving rats. Suppression of AMPK activity has been implicated in leptin's anorectic action in the hypothalamus. Given the role of leptin signaling in food intake inhibition within the medial NTS, we also examined whether stimulation of hindbrain AMPK by 4th i.c.v. administration of 5-aminoimidazole-4-carboxamide-riboside (AICAR), an AMP-mimicking promoter of AMPK activity, could attenuate the inhibition of food intake by 4th i.c.v. leptin. The intake-suppressive effects of leptin (at 2 and 4 h) were completely reversed by AICAR. We conclude that 1) hindbrain AMPK activity contributes to energy balance control through regulation of food intake and energy expenditure, 2) leptin's intake-reducing effects in the NTS are mediated by AMPK, and 3) central nervous system AMPK controls whole-body homeostasis at anatomically distributed sites across the neuraxis.

Neurons in the paraventricular nucleus of the hypothalamus inhibit sympathetic outflow to brown adipose tissue

The paraventricular nucleus of the hypothalamus (PVH) plays an important role in energy homeostasis, regulating neuroendocrine, behavioral, and autonomic functions. However, the role of the PVH in regulating thermogenesis and energy expenditure in brown adipose tissue (BAT) is unclear. The present study investigated the effect of activating neurons within the PVH on BAT thermogenesis. In urethane- and chloralose-anesthetized, artificially ventilated rats maintained at a core body temperature of 37.0-38.0 degrees C, microinjection of N-methyl-d-aspartate (NMDA, 12 pmol in 60 nl) in the PVH did not increase BAT sympathetic nerve activity (SNA) or BAT thermogenesis. In contrast, the increase in BAT SNA evoked by body cooling was completely reversed by microinjection of NMDA in the PVH. Additionally, the increases in BAT SNA evoked by body cooling, by microinjection of prostaglandin E(2) (170 pmol in 60 nl) in the medial preoptic area or by microinjection of bicuculline (30 pmol in 60 nl) in the dorsomedial hypothalamus were completely reversed by microinjection of bicuculline (30 pmol in 60 nl) in the PVH. Although the increases in BAT SNA and thermogenesis evoked by microinjection of NMDA (12 pmol in 60 nl) in the raphe pallidus (RPa) was markedly attenuated following microinjection of bicuculline (30 pmol) in the PVH, the increases in BAT SNA and thermogenesis evoked by microinjection of bicuculline (30 pmol in 60 nl) in the RPa were unaffected by microinjection of bicuculline in the PVH. These results demonstrate that disinhibition of neurons in the PVH inhibits BAT SNA likely via activation of a GABAergic input to BAT sympathetic premotor neurons in the RPa.

Influence of ovarian hormones on development of ingestive responding to alterations in fatty acid oxidation in female rats

Adult male rats have been demonstrated to increase food intake in response to administration of drugs that interfere with oxidation of fatty acids (e.g. methyl palmoxirate and mercaptoacetate [MA]), effects that are larger in animals maintained on a high-fat diet. In contrast, while administration of MA has been reported to stimulate food intake in pre-pubertal female rats, food intake is not stimulated by MA in adult female rats. Instead, administration of MA to adult females results in changes in reproductive behavior and physiology. The present experiments were designed to examine the effects of administration of MA on food intake in adult female rats. The results demonstrated that, as previously reported, food intake was stimulated by MA in adult male rats on low-fat and high-fat diets, but food intake was not stimulated by MA in gonadally-intact adult female rats on either low-fat or high-fat diet. Further, MA did not stimulate food intake in female rats ovariectomized as adults. However, when females were ovariectomized prior to the onset of puberty (postnatal day 25-28), food intake was stimulated by administration of MA in adulthood. Finally, cyclic injections of 17-beta-estradiol benzoate given to females ovariectomized prior to the onset of puberty abolished the stimulatory effects of MA on food intake in adult females. Taken together, the data suggest that exposure to estrogens during the time of puberty in female rats can persistently alter adult ingestive responding to signals related to changes in energy utilization.

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.

Central lipoprivation-induced suppression of luteinizing hormone pulses is mediated by paraventricular catecholaminergic inputs in female rats

The present study aims to clarify the role of fatty acids in regulating pulsatile LH secretion in rats. To produce an acute central lipoprivic condition, mercaptoacetate (MA), an inhibitor of fatty acids oxidation, was administered into the fourth cerebroventricle (4V) in ad libitum fed ovariectomized (OVX) rats (0.4, 2, and 10 micromol/rat) with or without an estradiol (E2) implant producing diestrus plasma E2 levels. Pulsatile LH secretion was suppressed by 4V MA administration in a dose-dependent manner in both OVX and OVX plus E2 rats. Mean LH levels and LH pulse frequency and amplitude were significantly reduced by the highest dose of MA in OVX rats, and by the middle and highest dose of MA in E2-treated rats, suggesting that estrogen enhanced LH suppression. Blood glucose levels increased immediately after the highest dose of MA in both groups. Fourth ventricular injection of trimetazidine (2 and 3 micromol/rat), another inhibitor of fatty acids oxidation, also inhibited pulsatile LH release, resulting in significant and dose-dependent suppression of LH pulse frequency and an increase in blood glucose levels in OVX plus E2 rats. In contrast, peripheral injection of the highest 4V dose of MA (10 micromol/rat) did not alter LH release or blood glucose levels. Microdialysis of the hypothalamic paraventricular nucleus (PVN) revealed that norepinephrine release in the region was increased by 4V MA administration. Preinjection of alpha-methyl-p-tyrosine, a catecholamine synthesis inhibitor, into the PVN completely blocked the lipoprivic inhibition of LH and the counter-regulatory increase in blood glucose levels in OVX plus E2 rats. Together, these studies indicate that fatty acid availability may be sensed by a central detector, located in the lower brainstem to maintain reproduction, and that noradrenergic inputs to the PVN mediate this lipoprivic-induced suppression of LH release.

Paraventricular alpha1- and alpha2-adrenergic receptors mediate hindbrain lipoprivation-induced suppression of luteinizing hormone pulses in female rats

Acute central lipoprivation suppresses pulsatile luteinizing hormone (LH) release and increases blood glucose levels through noradrenergic input to the hypothalamic paraventricular nucleus (PVN) in female rats. The present study was conducted to identify adrenergic receptor subtypes involved in central lipoprivation-induced suppression of pulsatile LH secretion and increases in plasma glucose levels in female rats. Acute hindbrain lipoprivation was produced by injection into the fourth cerebroventricle (4V) of 2-mercaptoacetate (MA), an inhibitor of fatty acid oxidation, in estradiol-implanted ovariectomized rats. Two min before MA injection, alpha1-, alpha2- or beta-adrenergic receptor antagonist was injected into the PVN. Injection of MA into the 4V suppresses pulsatile LH release in PVN vehicle-treated rats, whereas pretreatment of animals with injection of alpha1- or alpha2-adrenergic antagonist into the PVN blocked the effect of the 4V MA injection on LH pulses. beta-Adrenergic antagonist did not affect MA-induced suppression of LH pulses. The counter-regulatory increase in plasma glucose levels after 4V MA injection was also partially blocked by pretreatment with alpha1- and alpha2-adrenergic receptor antagonists. These results suggest that alpha1- and alpha2-adrenergic receptors in the PVN mediate hindbrain lipoprivation-induced suppression of LH release and counter-regulatory increases in plasma glucose levels in female rats.

Brain glucose sensing, counterregulation, and energy homeostasis

Neuronal circuits in the central nervous system play a critical role in orchestrating the control of glucose and energy homeostasis. Glucose, beside being a nutrient, is also a signal detected by several glucose-sensing units that are located at different anatomical sites and converge to the hypothalamus to cooperate with leptin and insulin in controlling the melanocortin pathway.

Fos-like immunoreactivity in Siberian hamster brain during initiation of torpor-like hypothermia induced by 2DG

Systemic 2-deoxy-d-glucose (2DG) produces pronounced torpor-like hypothermia (not< approximately 15 degrees C) in the Siberian hamster. Siberian hamsters are heterothermic, naturally undergoing photoperiod-dependent torpor during winter-like photoperiods. Fos was used to identify neural structures activated during the initiation of torpor-like hypothermia induced by 2DG treatment. The Fos-like immunoreactivity (Fos-li) in the area postrema and nucleus of the solitary tract that predominantly characterizes other 2DG-induced responses was absent during 2DG-induced torpor in the present experiment. Fos-li was seen in a number of forebrain and hindbrain sites during entry into hypothermia, but the densest Fos-li was found in the parvocellular portion of the paraventricular nucleus. Fos-li in the medial nucleus of the amygdala and the dorsal lateral septum also distinguished 2DG-induced torpor from other 2DG-induced behaviors. The possible involvement of neuropeptide Y pathways during 2DG-induced expression of reversible hypothermia is discussed.

Ontogeny of hypothalamic-hindbrain feeding control circuits

Ingestive behavior is a complex product of distributed central control systems that respond to a diverse array of internal and external sensory stimuli. Relatively little is known regarding the pathways and mechanisms by which relevant signals are conveyed to the neural circuits that ultimately control ingestive motor output. This report summarizes findings regarding the postnatal development of descending hypothalamic inputs to the hindbrain dorsal vagal complex (DVC). Evidence accumulated primarily in rats indicates that descending neural projections from the hypothalamus to the DVC are both structurally and functionally immature at birth. The progressive postnatal maturation of these projections occurs in parallel with newly emerging physiological and behavioral responsiveness to treatments and stimuli that affect food intake in adults. Thus, the postnatal emergence of new feeding controls may reflect the emerging access of these controls to DVC neural circuits.

Serotonin-type 3 receptors mediate intestinal lipid-induced satiation and Fos-like immunoreactivity in the dorsal hindbrain

Several gastrointestinal stimuli, including some intestinal nutrients, have been shown to exert their satiating effect via activation of serotonin type-3 (5-HT(3)) receptors. The presence of lipids in the small intestine potently suppresses food intake; however, whether 5-HT(3) receptors play a role in this response has not been directly examined. Therefore, using the selective 5-HT(3) receptor antagonist ondansetron, we tested the hypothesis that duodenal infusion of lipid suppresses intake of both sucrose solution and chow through 5-HT(3) receptor activation. Rats duodenally infused with 72 and 130 mM Intralipid suppressed 1-h 15% sucrose intake by 33 and 67%, respectively. Suppression of sucrose intake by 72 mM Intralipid was significantly attenuated by ondansetron at all doses tested (0.5, 1.0, 2.0, and 5.0 mg/kg ip), whereas the lowest effective dose of ondansetron to attenuate suppression of intake by 130 mM Intralipid was 1.0 mg/kg. Furthermore, infusion of 130 mM Intralipid suppressed 1- and 4-h chow intake by 35 and 20%, respectively. Ondansetron administered as low as 0.5 mg/kg significantly attenuated 1-h Intralipid-induced suppression of chow intake and completely reversed the suppression by 4 h. Administration of ondansetron alone did not alter sucrose or chow intake compared with vehicle injection at any time. Finally, to test whether Intralipid-induced neuronal activation of the dorsal vagal complex is mediated by 5-HT(3) receptors, Fos-like immunoreactivity (Fos-LI) was quantified in ondansetron-pretreated rats following intestinal lipid infusion. Ondansetron (1 mg/kg) significantly attenuated duodenal intralipid-induced Fos-LI in the dorsal hindbrain. These data support the hypothesis that 5-HT(3) receptors mediate both satiation, as well as hindbrain neuronal responses evoked by intestinal lipids.

Acute lipoprivation suppresses pulsatile luteinizing hormone secretion without affecting food intake in female rats

The present study examined the effect of acute lipoprivation on pulsatile luteinizing hormone (LH) secretion in both normal-fat diet, ad libitum-fed and fasted female rats. To produce an acute lipoprivic condition, mercaptoacetate (MA), an inhibitor of fatty acid oxidation, was administered intraperitoneally to ad libitum-fed or 24-h fasted ovariectomized (OVX) rats with or without an estradiol (E2) implant, that produces a negative feedback effect on LH pulses. The steroid treatment was performed to determine the effect of estrogen on lipoprivic changes in LH release, because estrogen enhances fasting- or glucoprivation-induced suppression of LH pulses. Pulsatile LH secretion was suppressed by MA administration in a dose-dependent manner in the ad libitum-fed OVX and OVX+E2 rats. LH pulses were more severely suppressed in the 24-h-fasted OVX and OVX+E2 rats compared to the ad libitum-fed rats. Estrogen slightly enhanced lipoprivic suppression but the effect was not significant. In the present study, increased plasma glucose and free-fatty acid concentrations may indicate a blockade of fatty acid metabolism by the MA treatment, but food intake was not affected by any of the MA doses. Acute vagotomy did not block lipoprivic suppression of LH pulses. Thus, the present study indicates that lipid metabolism is important for maintenance of normal reproductive function even in rats fed a normal-fat diet and lipoprivation may be more critical in fasted animals that probably rely more heavily on fatty acid oxidation to maintain appropriate metabolic fuel levels. In addition, failure of blockade of lipoprivic LH inhibition by vagotomy suggests that lipoprivic information resulting in LH suppression is not transmitted to the brain via the vagus nerve.

Role of brainstem TRH/TRH-R1 receptors in the vagal gastric cholinergic response to various stimuli including sham-feeding

Pavlov's pioneering work established that sham-feeding induced by sight or smell of food or feeding in dogs with permanent esophagostomy stimulates gastric acid secretion through vagal pathways. Brain circuitries and transmitters involved in the central vagal regulation of gastric function have recently been unraveled. Neurons in the dorsal vagal complex including the dorsal motor nucleus of the vagus (DMN) express thyrotropin-releasing hormone (TRH) receptor and are innervated by TRH fibers originating from TRH synthesizing neurons in the raphe pallidus, raphe obscurus and the parapyramidal regions. TRH injected into the DMN or cisterna magna increases the firing of DMN neurons and gastric vagal efferent discharge, activates cholinergic neurons in gastric submucosal and myenteric plexuses, and induces a vagal-dependent, atropine-sensitive stimulation of gastric secretory (acid, pepsin) and motor functions. TRH antibody or TRH-R1 receptor oligodeoxynucleotide antisense pretreatment in the cisterna magna or DMN abolished vagal-dependent gastric secretory and motor responses to sham-feeding, 2-deoxy-D-glucose, cold exposure and chemical activation of cell bodies in medullary raphe nuclei. TRH excitatory action in the DMN is potentiated by co-released prepro-TRH-(160-169) flanking peptide, Ps4 and 5-HT, and inhibited by a number of peptides involved in the stress/immune response and inhibition of food-intake. These neuroanatomical, electrophysiological and neuropharmacological data are consistent with a physiological role of brainstem TRH in the central vagal stimulation of gastric myenteric cholinergic neurons in response to several vagal dependent stimuli including sham-feeding.

GABA receptor subtype antagonists in the nucleus accumbens shell and ventral tegmental area differentially alter feeding responses induced by deprivation, glucoprivation and lipoprivation in rats

GABA(A) and GABA(B) receptor agonists stimulate feeding following microinjection into the nucleus accumbens shell and ventral tegmental area, effects blocked selectively and respectively by GABA(A) and GABA(B) receptor antagonists. GABA antagonists also differentially alter opioid-induced feeding responses elicited from these sites. Although GABA agonists and antagonists have been shown to modulate feeding elicited by deprivation or glucoprivation, there has been no systematic examination of feeding elicited by homeostatic challenges following GABA antagonists in these sites. Therefore, the present study examined the dose-dependent ability of GABA(A) (bicuculline, 75-150 ng) and GABA(B) (saclofen, 1.5-3 microg) antagonists administered into the nucleus accumbens shell or ventral tegmental area upon feeding responses elicited by food deprivation (24 h), 2-deoxy-D-glucose-induced glucoprivation (500 mg/kg) or mercaptoacetate-induced lipoprivation (70 mg/kg). A site-specific effect of GABA receptor antagonism was observed for deprivation-induced feeding in that both bicuculline and saclofen administered into the nucleus accumbens shell, but not the ventral tegmental area, produced short-term (1-4 h), but not long-term (24-48 h) effects upon deprivation-induced intake without meaningfully altering body weight recovery. In contrast to the relative inability of GABA receptor antagonism in both sites to alter 2-deoxy-D-glucose-induced intake, mercaptoacetate-induced intake was eliminated by saclofen and significantly reduced by bicuculline in the nucleus accumbens shell and eliminated by both bicuculline and saclofen in the ventral tegmental area. These data reinforce the findings that GABA(A) and GABA(B) receptors in the nucleus accumbens shell and ventral tegmental area are not only important in the modulation of pharmacologically induced feeding responses, but also participate in differentially mediating the short-term feeding response to food deprivation in the nucleus accumbens shell as well strongly modulating lipoprivic, but not glucoprivic feeding responses in both sites.

Hyperphagia induced by GABAA receptor-mediated inhibition of the nucleus accumbens shell: dependence on intact neural output from the central amygdaloid region

To investigate the role of corticolimbic input in modulating feeding-related nucleus accumbens (Acb) circuitry, researchers temporarily deactivated sites within the basolateral amygdaloid complex (BLA) or central amygdaloid region (CeA) via GABA(A) agonist (muscimol) infusions and measured feeding responses following muscimol infusions into the Acb shell. Hyperphagia elicited by intra-Acb shell muscimol was not altered by coinfusions of intra-BLA muscimol. In contrast, muscimol infusions into the CeA dose-dependently reduced feeding elicited either by intra-Acb shell GABA(A) receptor stimulation or by food deprivation and produced a syndrome of forepaw treading. Intra-CeA tetrodotoxin infusions also blocked intra-Acb shell muscimol-induced hyperphagia. Hence, feeding elicited by intra-Acb shell GABA(A) receptor stimulation requires intact neural output from the CeA but not the BLA.

Immunolesions of glucoresponsive projections to the arcuate nucleus alter glucoprivic-induced alterations in food intake, luteinizing hormone secretion, and GALP mRNA, but not sex behavior in adult male rats

Metabolic signals such as insulin, leptin and glucose are known to alter hypothalamic function. Although insulin and leptin are known to directly alter hypothalamic areas that regulate reproduction, the mechanisms by which glucose alters reproductive function are not as clear. Catecholaminergic neurons in the A1/C1 region in the hindbrain are glucose-responsive and project to the arcuate nucleus. To determine if this pathway is involved in the regulation of sex behavior and luteinizing hormone (LH) secretion, this catecholaminergic pathway was lesioned with injections of saporin conjugated with anti-dopamine-beta-hydroxylase (DSAP) or unconjugated saporin (SAP) in adult male rats. Rats were given glucoprivic challenges and feeding and sex behavior was observed. As was expected, the DSAP-treated rats showed decreased feeding during glucoprivation (250 mg/kg 2-deoxy-D-glucose, 2DG) compared to SAP controls. Glucoprivation caused a significant reduction in sex behavior in both SAP and DSAP animals equally, compared to saline treatments (p < 0.05). At the end of the experiment, animals were given a final challenge with 2DG or saline, euthanized by decapitation and trunk blood was assayed for plasma LH levels. In situ hybridization analysis revealed that 2DG treatment caused a significant reduction in GALP mRNA in SAP controls compared to saline treatment. This reduction in GALP mRNA was prevented with DSAP treatment. In SAP animals, 2DG elicited a significant decrease in plasma LH levels (p < 0.05); this reduction in plasma LH was absent in the DSAP-treated male rats. These data indicate that the A1/C1 efferents to the ventromedial hypothalamus are involved in the glucostatic regulation of GALP mRNA, feeding behavior and LH secretion, but not sex behavior in the adult male rat.

New developments in tracing neural circuits with herpesviruses

Certain neurotropic viruses can invade the nervous system of their hosts and spread in chains of synaptically connected neurons. Consequently, it is possible to identify entire hierarchically connected circuits within an animal. In this review, we discuss the use of neurotropic herpesviruses as neuronal tract tracers. Although a variety of tract tracing viruses are available, each with its own unique infection characteristics, we focus on the widespread use of attenuated strains of pseudorabies virus (PRV), a swine herpesvirus with a broad host range. In particular, we focus on new applications of PRV for tract tracing including use of multiple infections by PRV reporter viruses to test for circuit convergence/divergence within the same animal. We provide examples of these combined application techniques within the context of an animal model to study the naturally occurring reversal of seasonal obesity in Siberian hamsters.

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.

Does perinatal omega-3 polyunsaturated fatty acid deficiency increase appetite signaling?

OBJECTIVE

To investigate the effect of maternal dietary omega-3 polyunsaturated fatty acid (PUFA) deficiency and repletion on food appetite signaling.

RESEARCH METHODS AND PROCEDURES

Sprague-Dawley rat dams were maintained on diets either supplemented with (CON) or deficient in (DEF) omega-3 PUFA. All offspring were raised on the maternal diet until weaning. After weaning, two groups remained on the respective maternal diet (CON and DEF groups), whereas a third group, born of dams fed the DEF diet, were switched to the CON diet (REC). Experiments on food intake began when the male rats reached 16 weeks of age. Food intake was stimulated either by a period of food restriction, by blocking glucose utilization (by 2-deoxyglucose injection), or by blocking beta-oxidation of fatty acids (by beta-mercaptoacetate injection).

RESULTS

DEF animals consumed more than CON animals in response to all stimuli, with the greatest difference (1.9-fold) demonstrated following administration of 2-deoxyglucose. REC animals also consumed more than CON animals in response to food restriction and 2-deoxyglucose but not to beta-mercaptoacetate.

DISCUSSION

These findings indicate that supply of omega-3 PUFA, particularly during the perinatal period, plays a role in the normal development of mechanisms controlling food intake, especially glucoprivic (i.e. reduced glucose availability) appetite signaling. Dietary repletion of omega-3 PUFA from 3 weeks of age restored intake responses to fatty acid metabolite signaling but did not reverse those in response to food restriction or glucoprivic stimuli.

Fatty acid oxidation and control of food intake

Fatty acid oxidation is thought to play a role in the control of food intake, and a low postprandial oxidation of ingested fat may contribute to the overeating on a high-fat diet. Evidence for a role of fatty acid oxidation in control of food intake is mainly derived from the stimulation of feeding in response to administration of the acyl-CoA-dehydrogenase inhibitor mercaptoacetate (MA) and other inhibitors of fatty acid oxidation in different species (rat, mouse, man). Denervation studies suggest that this "lipoprivic feeding" is related to changes in hepatic fatty acid oxidation. In contrast to the strong case for a feeding stimulatory effect of an inhibition of fatty acid oxidation, the evidence for a feeding suppressive effect of a stimulation of fatty acid oxidation is inconsistent and comparatively weak. Ingestion of medium-chain fatty acids (MCFA) and peripheral administration of substances known to increase fatty acid oxidation, such as the fatty acid synthase inhibitor C75 and beta3-adrenergic agonists, decrease feeding. Yet, these substances also reduce the rats' usual preference for saccharin solution, indicating that the feeding suppressive effect is not only due to a stimulation of fatty acid oxidation. A possible approach to answer the question of whether a stimulation of hepatic fatty acid oxidation enhances satiety is to selectively increase expression and activity of the enzyme CPT 1alpha in the liver. CPT 1alpha transfers long-chain fatty acids in the cytosol from CoA to carnitine, which is the precondition for the entry of long-chain fatty acids into mitochondria and the rate-controlling step in mitochondrial fatty acid oxidation. The generation of rats with inducible, liver-specific overexpression of CPT 1alpha should permit to critically examine the putative contribution of hepatic fatty acid oxidation to the control of food intake.

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.

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.