Reverse microdialysis of N-methyl-D-aspartic acid into the lateral hypothalamus of rats: effects on feeding and other behaviors

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.

The Neurobiology of Binge-eating Disorder Compared with Obesity: Implications for Differential Therapeutics

PURPOSE

Emerging work indicates divergence in the neurobiologies of binge-eating disorder (BED) and obesity despite their frequent co-occurrence. This review highlights specific distinguishing aspects of BED, including elevated impulsivity and compulsivity possibly involving the mesocorticolimbic dopamine system, and discusses implications for differential therapeutics for BED.

METHODS

This narrative review describes epidemiologic, clinical, genetic, and preclinical differences between BED and obesity. Subsequently, this review discusses human neuroimaging work reporting differences in executive functioning, reward processing, and emotion reactivity in BED compared with obesity. Finally, on the basis of the neurobiology of BED, this review identifies existing and new therapeutic agents that may be most promising given their specific targets based on putative mechanisms of action relevant specifically to BED.

FINDINGS

BED is characterized by elevated impulsivity and compulsivity compared with obesity, which is reflected in divergent neurobiological characteristics and effective pharmacotherapies. Therapeutic agents that influence both reward and executive function systems may be especially effective for BED.

IMPLICATIONS

Greater attention to impulsivity/compulsivity-related, reward-related, and emotion reactivity-related processes may enhance conceptualization and treatment approaches for patients with BED. Consideration of these distinguishing characteristics and processes could have implications for more targeted pharmacologic treatment research and interventions.

Controlling feeding behavior by chemical or gene-directed targeting in the brain: what's so spatial about our methods?

Intracranial chemical injection (ICI) methods have been used to identify the locations in the brain where feeding behavior can be controlled acutely. Scientists conducting ICI studies often document their injection site locations, thereby leaving kernels of valuable location data for others to use to further characterize feeding control circuits. Unfortunately, this rich dataset has not yet been formally contextualized with other published neuroanatomical data. In particular, axonal tracing studies have delineated several neural circuits originating in the same areas where ICI injection feeding-control sites have been documented, but it remains unclear whether these circuits participate in feeding control. Comparing injection sites with other types of location data would require careful anatomical registration between the datasets. Here, a conceptual framework is presented for how such anatomical registration efforts can be performed. For example, by using a simple atlas alignment tool, a hypothalamic locus sensitive to the orexigenic effects of neuropeptide Y (NPY) can be aligned accurately with the locations of neurons labeled by anterograde tracers or those known to express NPY receptors or feeding-related peptides. This approach can also be applied to those intracranial "gene-directed" injection (IGI) methods (e.g., site-specific recombinase methods, RNA expression or interference, optogenetics, and pharmacosynthetics) that involve viral injections to targeted neuronal populations. Spatial alignment efforts can be accelerated if location data from ICI/IGI methods are mapped to stereotaxic brain atlases to allow powerful neuroinformatics tools to overlay different types of data in the same reference space. Atlas-based mapping will be critical for community-based sharing of location data for feeding control circuits, and will accelerate our understanding of structure-function relationships in the brain for mammalian models of obesity and metabolic disorders.

Behaviorally specific versus non-specific suppression of accumbens shell-mediated feeding by ipsilateral versus bilateral inhibition of the lateral hypothalamus

The nucleus accumbens shell (AcbSh) and lateral hypothalamus (LH) are linked in the control of food intake. Pharmacological inhibition of the LH may block AcbSh-elicited feeding, but the behavioral phenotype associated with this feeding suppression is unknown. To examine this phenotype, adult male Sprague-Dawley rats were implanted with three cannulas - one unilaterally in the AcbSh and two bilaterally in the LH - to allow for central drug injections. The AcbSh received injections of the AMPA receptor antagonist DNQX or the GABAA receptor agonist muscimol, while the LH received injections of the NMDA receptor antagonist D-AP5 or muscimol. Eating, drinking, grooming, locomotion, quiescence, and sleeping behaviors were measured every minute for 60 min post-injection. From these observational data, feeding bout durations, feeding frequency, and latency to feed were determined. AcbSh muscimol or DNQX increased food intake by increasing feeding bout durations and frequency and decreasing latency to feed. D-AP5 or muscimol, injected into the LH bilaterally or ipsilateral to the AcbSh injection, reversed these AcbSh-mediated effects. Though bilateral LH D-AP5 or muscimol injections blocked feeding responses, they also hastened onset of sleep. In contrast, ipsilateral LH D-AP5 or muscimol injections suppressed AcbSh-mediated feeding behaviors without substantially altering sleeping or other behaviors. These results suggest bilateral LH inhibition via NMDA receptor blockade or GABAA receptor activation produces behavioral effects that might indirectly suppress feeding, but ipsilateral LH inhibition through these receptors suppresses AcbSh AMPA and GABAA receptor-mediated feeding specifically. This evidence strengthens the concept of a feeding-specific association between these regions.

The role of NMDA receptors in human eating behavior: evidence from a case of anti-NMDA receptor encephalitis

Research in animal models has implicated N-methyl-D-aspartate (NMDA) receptors (NMDARs) in the control of food intake. Until now, these findings have been not replicated in humans. Here we describe a 22-year-old woman with anti-NMDAR encephalitis and no prior neurological or psychiatric history. Her clinical course was marked by successive eating disorders: anorexia followed by hyperphagia. We propose that, much as they do in other animals, NMDARs in humans interact with the neuroendocrine, homeostatic, and reward systems controlling food intake in the central and peripheral nervous system structures related to feeding and satiety.

Opposing synaptic regulation of amyloid-β metabolism by NMDA receptors in vivo

The concentration of amyloid-β (Aβ) within the brain extracellular space is one determinant of whether the peptide will aggregate into toxic species that are important in Alzheimer's disease (AD) pathogenesis. Some types of synaptic activity can regulate Aβ levels. Here we demonstrate two distinct mechanisms that are simultaneously activated by NMDA receptors and regulate brain interstitial fluid (ISF) Aβ levels in opposite directions in the living mouse. Depending on the dose of NMDA administered locally to the brain, ISF Aβ levels either increase or decrease. Low doses of NMDA increase action potentials and synaptic transmission which leads to an elevation in synaptic Aβ generation. In contrast, high doses of NMDA activate signaling pathways that lead to ERK (extracellular-regulated kinase) activation, which reduces processing of APP into Aβ. This depression in Aβ via APP processing occurs despite dramatically elevated synaptic activity. Both of these synaptic mechanisms are simultaneously active, with the balance between them determining whether ISF Aβ levels will increase or decrease. NMDA receptor antagonists increase ISF Aβ levels, suggesting that basal activity at these receptors normally suppresses Aβ levels in vivo. This has implications for understanding normal Aβ metabolism as well as AD pathogenesis.

Glutamate and GABA in lateral hypothalamic mechanisms controlling food intake

By the 1990s a convergence of evidence had accumulated to suggest that neurons within the lateral hypothalamus (LH) play important roles in the stimulation of feeding behavior. However, there was little direct evidence demonstrating that neurotransmitters in the LH could, like electrical stimulation, elicit feeding in satiated animals. The present paper is a brief review in honor of Bartley Hoebel's scientific contributions, emphasizing the evidence from my lab that the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter gamma aminobutyric acid (GABA) in the LH mediate feeding stimulation and feeding inhibition respectively. Specifically, we summarize evidence that LH injection of glutamate, or agonists of its N-methyl-D-aspartate (NMDA) and non-NMDA receptors, elicits feeding in satiated rats, that NMDA receptor antagonists block the eating elicited by NMDA and, more importantly, that NMDA blockade suppresses natural feeding and can reduce body weight. Conversely, GABA(A) agonists injected into the LH suppress feeding and can also reduce body weight, while GABA(A) receptor antagonists actually elicit eating when injected into the LH of satiated rats. It is suggested that natural feeding may reflect the moment-to-moment balance in the activity of glutamate and GABA within the LH.

Feeding behavior as seen through the prism of brain microdialysis

The knowledge of feeding behavior mechanisms gained through brain microdialysis is reviewed. Most of the chemical changes so far reported concern to the limbic system in rodents. A picture showing increases and decreases of extracellular neurotransmitters correlating to different aspects of feeding behavior is gradually emerging. Depending on the region, the same neurotransmitter may signal opposite aspects of feeding. Dopamine (DA) in the nucleus accumbens (NAC) correlates with food reward, stimulus saliency, and goal directed hyperlocomotion but in the ventromedial hypothalamus DA correlates with satiety and hypolocomotion. The findings accumulated in the last 25 years suggest that the control of a particular function relies on the interaction of several neurotransmitters rather than on a single neurotransmitter. The poor sensitivity of most analytical techniques hinders time and spatial resolution of microdialysis. Therefore, neurochemical correlates of short lasting behaviors are hard to figure out. As new and more sensitive analytical techniques are applied, new neurochemical correlates of feeding show up. Sometimes the proper analytical techniques are simply not available. As a consequence, critical signals such as neuropeptides are not yet completely placed in the puzzle. Despite such limitations, brain microdialysis has yielded a great deal of knowledge on the neurochemical basis of feeding.

Amygdalar glutamatergic neuronal systems play a key role on the hibernating state of hamsters

BACKGROUND

Excitatory transmitting mechanisms are proving to play a critical role on neuronal homeostasis conditions of facultative hibernators such as the Syrian golden hamster. Indeed works have shown that the glutamatergic system of the main olfactory brain station (amygdala) is capable of controlling thermoregulatory responses, which are considered vital for the different hibernating states. In the present study the role of amygdalar glutamatergic circuits on non-hibernating (NHIB) and hibernating (HIB) hamsters were assessed on drinking stimuli and subsequently compared to expression variations of some glutamatergic subtype mRNA levels in limbic areas. For this study the two major glutamatergic antagonists and namely that of N-methyl-D-aspartate receptor (NMDAR), 3-(+)-2-carboxypiperazin-4-yl-propyl-1-phosphonate (CPP) plus that of the acid α-amine-3-hydroxy-5-methyl-4-isoxazol-propionic receptor (AMPAR) site, cyano-7-nitro-quinoxaline-2,3-dione (CNQX) were infused into the basolateral amygdala nucleus. Attempts were made to establish the type of effects evoked by amygdalar glutamatergic cross-talking processes during drinking stimuli, a response that may corroborate their major role at least during some stages of this physiological activity in hibernators.

RESULTS

From the behavioral results it appears that the two glutamatergic compounds exerted distinct effects. In the first case local infusion of basolateral complexes (BLA) with NMDAR antagonist caused very great (p < 0.001) drinking rhythms while moderately increased feeding (p < 0.05) responses during arousal with respect to moderately increased drinking levels in euthermics. Conversely, treatment with CNQX did not modify drinking rhythms and so animals spent more time executing exploratory behaviors. These same antagonists accounted for altered glutamatergic transcription activities as displayed by greatly reduced GluR1, NR1 and GluR2 levels in hippocampus, ventromedial hypothalamic nucleus (VMN) and amygdala, respectively, plus a great (p < 0.01) up-regulation of GluR2 in VMN of hibernators.

CONCLUSION

We conclude that predominant drinking events evoked by glutamatergic mechanisms, in the presence of prevalently down regulated levels of NR1/2A of some telencephalic and hypothalamic areas appear to constitute an important neuronal switch at least during arousal stage of hibernation. The establishment of the type of glutamatergic subtypes that are linked to successful hibernating states, via drinking stimuli, may have useful bearings toward sleeping disorders.

Orexin-induced feeding requires NMDA receptor activation in the perifornical region of the lateral hypothalamus

Food intake is stimulated following administration of orexin-A into the perifornical region of the lateral hypothalamus (LH/PFA). Orexin neurons originating in the LH/PFA interact with a number of hypothalamic systems known to influence food intake, including glutamatergic neurons. Glutamatergic systems in the LH/PFA were demonstrated to initiate feeding through N-methyl-d-aspartic acid (NMDA) receptors. Male Sprague-Dawley rats fitted with brain guide cannulas to the LH/PFA were used in two experiments. In the first experiment, a combination microdialysis/microinjection probe was used to deliver artificial cerebrospinal fluid (aCSF) or 500 pmol of orexin-A into the LH/PFA. Orexin-A increased interstitial glutamate to 143 ± 12% of baseline ( P < 0.05), which remained elevated over the 120-min collection period. In the second experiment, the NMDA receptor antagonist d-2-amino-5-phosphonopentanoic acid (d-AP5; 10 nmol) was administered before orexin-A. The orexin-induced increase in food intake (from 1.1 ± 0.4 to 3.2 ± 0.5 g, P < 0.05) during the first hour was absent in rats receiving d-AP5 + orexin-A (1.2 ± 0.5 g). There was no effect of d-AP5 alone on food intake. These data support glutamatergic systems in the LH/PFA mediating the feeding response to orexin-A through NMDA receptors.

Applicability of reverse microdialysis in pharmacological and toxicological studies

A recent application of microdialysis is the introduction of a substance into the extracellular space via the microdialysis probe. The inclusion of a higher amount of a drug in the perfusate allows the drug to diffuse through the microdialysis membrane to the tissue. This technique, actually called as reverse microdialysis, not only allows the local administration of a substance but also permits the simultaneous sampling of the extracellular levels of endogenous compounds. Local effects of exogenous compounds have been studied in the central nervous system, hepatic tissue, dermis, heart and corpora luteae of experimental animals by means of reverse microdialysis. In central nervous studies, reverse microdialysis has been extensively used for the study of the effects on neurotransmission at different central nuclei of diverse pharmacological and toxicological agents, such as antidepressants, antipsychotics, antiparkinsonians, hallucinogens, drugs of abuse and experimental drugs. In the clinical setting, reverse microdialysis has been used for the study of local effects of drugs in the adipose tissue, skeletal muscle and dermis. The aim of this review is to describe the principles of the reverse microdialysis, to compare the technique with other available methods and finally to describe the applicability of reverse microdialysis in the study of drugs properties both in basic and clinical research.

A neural systems analysis of the potentiation of feeding by conditioned stimuli

Associative learning processes play many important roles in the control of food consumption. Although these processes can complement regulatory mechanisms in the control of eating by providing opportunities for the anticipation of upcoming needs, they may also contribute to inappropriate or pathological consumption patterns by overriding internal regulatory signals. In this article, we first review some of the ways in which associative learning can contribute to the control of feeding, and then describe a neural systems analysis of a simple animal model of the control of feeding by Pavlovian-conditioned stimuli (CSs). Food-sated rats increase their food consumption after presentation of CSs that were previously paired with food while the rats were food-deprived. This cue-potentiated feeding is independent of conditioned approach responses, and is at least somewhat specific to the foods associated with those CSs. A series of studies that used neuroanatomical tract tracing, immediate early gene expression, and neurotoxic disconnection lesion techniques implicated circuitry that includes the basolateral complex of the amygdala, the lateral hypothalamus, and the medial prefrontal cortex, but not the amygdala central nucleus, nucleus accumbens, or lateral orbitofrontal cortex, in cue-potentiated feeding. These studies also showed dissociations between cue-potentiated feeding and other learned motivational phenomena that are known to depend on function of amygdala systems. The data suggest that cue-potentiated feeding is uniquely mediated by cortical and amygdalar neurons that directly target the lateral hypothalamus, and thus gain access to hypothalamic neuropeptide and other systems involved in the promotion and suppression of eating.

NMDA receptors mediate feeding elicited by neuropeptide Y in the lateral and perifornical hypothalamus

Neuropeptide Y (NPY) and N-methyl-d-aspartate (NMDA) receptors in the lateral (LH) and perifornical hypothalamus (PFH) are believed to be involved in the stimulation of feeding behavior. To investigate the possibility that neurons with these receptors interact to stimulate eating, the NMDA receptor antagonists d-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) or 7-chlorokynurenic acid (7-CK) were injected into the LH or PFH of satiated rats 5 min prior to NPY in the same site and subsequent food intake was measured 1, 2, and 4 h postinjection. The injection of NPY (78 pmol/0.3 microl aCSF) in the PFH produced an average food intake of 9.7 g in 4 h, compared to the intake of 1.3 g after the artificial cerebrospinal fluid (aCSF) vehicle. D-AP5 (1, 10, or 20 nmol/0.3 microl aCSF) pretreatment suppressed NPY-induced eating, with the 20 nmol dose of D-AP5 producing up to an 80% suppression of elicited food intake down to 1.9 g in 4 h. Similar effects were produced with the LH as the injection site. Illustrating the specificity of the NMDA receptor antagonist's suppression of NPY-elicited feeding, D-AP5 suppressed NMDA-elicited feeding but did not affect the eating response induced by kainic acid. Consistent with the effects of D-AP5, the NMDA receptor antagonist 7-CK (40 nmol/0.3 microl dimethyl sulfoxide, DMSO) suppressed feeding elicited by NPY in the LH by 78%. Collectively, the findings suggest that the feeding elicited by NPY is dependent upon the activation of the NMDA receptors in the LH and PFH.

Lateral hypothalamic signaling mechanisms underlying feeding stimulation: differential contributions of Src family tyrosine kinases to feeding triggered either by NMDA injection or by food deprivation

In rats, feeding can be triggered experimentally using many approaches. Included among these are (1) food deprivation and (2) acute microinjection of the neurotransmitter l-glutamate (Glu) or its receptor agonist NMDA into the lateral hypothalamic area (LHA). Under both paradigms, the NMDA receptor (NMDA-R) within the LHA appears critically involved in transferring signals encoded by Glu to stimulate feeding. However, the intracellular mechanisms underlying this signal transfer are unknown. Because protein-tyrosine kinases (PTKs) participate in NMDA-R signaling mechanisms, we determined PTK involvement in LHA mechanisms underlying both types of feeding stimulation through food intake and biochemical measurements. LHA injections of PTK inhibitors significantly suppressed feeding elicited by LHA NMDA injection (up to 69%) but only mildly suppressed deprivation feeding (24%), suggesting that PTKs may be less critical for signals underlying this feeding behavior. Conversely, food deprivation but not NMDA injection produced marked increases in apparent activity for Src PTKs and in the expression of Pyk2, an Src-activating PTK. When considered together, the behavioral and biochemical results demonstrate that, although it is easier to suppress NMDA-elicited feeding by PTK inhibitors, food deprivation readily drives PTK activity in vivo. The latter result may reflect greater PTK recruitment by neurotransmitter receptors, distinct from the NMDA-R, that are activated during deprivation-elicited but not NMDA-elicited feeding. These results also demonstrate how the use of only one feeding stimulation paradigm may fail to reveal the true contributions of signaling molecules to pathways underlying feeding behavior in vivo.

Origins of lateral hypothalamic afferents associated with N-methyl-d-aspartic acid-elicited eating studied using reverse microdialysis of NMDA and Fluorogold

Afferent projections to the tuberal lateral hypothalamus (tLH), where excitatory amino acid application is most effective in eliciting feeding, and to the anterior, posterior and medial regions of the hypothalamus were studied using reverse microdialysis of N-methyl-D-aspartic acid (NMDA) and Fluorogold (FG). NMDA at 660 microM delivered for 10 min was effective in stimulating food intake only when administered into the tLH, causing a mean intake of 9.3 g compared to less than 1 g in any other site. Subsequent administration of FG through the dialysis probe retrogradely in labeled neurons in brain structures associated with the feeding response including the frontal cortex, amygdala, nucleus accumbens (NA), preoptic areas, substantia nigra, ventral tegmental area (VTA), parabrachial nucleus, and the nucleus of the solitary tract (NST). Labeling after anterior and posterior LH infusion of FG was similar to that seen after tLH delivery with some apparent differences, whereas FG administration into the medial hypothalamus produced a distinctly different pattern of labeling compared to the other groups. Some of the observed labeling appeared to be almost exclusively associated with the tLH where NMDA elicits feeding. In particular, amygdala, preoptic area and shell of the accumbens labeling was noticeably denser in tLH eaters than in all other groups. These findings are consistent with the role of LH glutamate and NMDA receptors in the regulation of food intake and identify afferents to the region which possibly mediate endogenous LH glutamate's effects on feeding.

NMDA receptor function within the anterior piriform cortex and lateral hypothalamus in rats on the control of intake of amino acid-deficient diets

Animals decrease intake of an indispensable amino acid (AA)-deficient or devoid diet, due in part to decreased dietary limiting AA (DLAA) concentrations within the anterior piriform cortex (APC), and to a recognition process that occurs as early as 20 min following exposure to AA deficiencies. Glutamate levels within the APC change in response to AA deficiencies. The APC projects to the lateral hypothalamus (LH), where glutamate acts to stimulate food intake. We hypothesize that the APC, through glutamatergic projections to the LH, inhibits the LH, which signals to reject the AA-deficient or devoid diet, and trigger aversions to the AA-deficient or devoid diet via an ascending pathway to the APC. We examined the effects of (1) bilateral APC and LH blockade of glutamate's NMDA receptors with the antagonist, D-AP5, (2) APC blockade of AMPA receptors with the antagonist, NBQX, to block glutamate transmission from the APC, and (3) direct injection of the agonist, NMDA, into the LH on intake of the AA-deficient, devoid, or corrected diet. Administration of D-AP5 into the APC increased intake of AA-deficient diet by 6 h, but D-AP5 in the LH decreased AA-devoid diet preferentially over AA corrected intake sooner. NBQX in the APC increased AA-deficient diet intake, also at 6 h. NMDA injection into the LH-stimulated intake of the AA corrected diet by 3 h, but did not affect AA-devoid diet intake. Thus, the glutamate receptors in the APC and LH are involved in the feeding responses to AA-deficient diet, albeit with regional differences. We suggest that glutamate mediates the anorectic responses to AA-deficient diets through recognition of AA-devoid diet with the glutamatergic output cells of the APC sending glutamate-based signals for changes in food intake within the LH and through learned avoidance of AA-deficient diet within the APC, as indicated through the more immediate and prolonged periods of activation within the LH and APC, respectively.

Effects of inflammation produced by chronic lipopolysaccharide administration on the survival of hypocretin neurons and sleep

The number of hypocretin-containing neurons is markedly decreased in most patients with the sleep disorder narcolepsy. It is presently not known why the loss of hypocretin neurons occurs in these patients. In the present study, we tested the role of inflammation in the degeneration of hypocretin neurons. The proinflammagen lipopolysaccharide (LPS) was infused chronically for 30 days (flow rate=0.22 microg/h) into the lateral hypothalamus in rats. Compared with chronic infusions of phosphate-buffered saline (PBS), LPS infusions produced a decline in the number of hypocretin (29.7% reduction), melanin concentrating hormone (MCH; 24.7% reduction), and neuronal nuclear antigen (NeuN)-immunoreactive neurons, as well as a dense distribution of reactive astrocytes and microglia within the lateral hypothalamus. LPS infusions also produced a large increase in the amounts of wakefulness 6 days after the onset of infusion (72.5+/-8.7% of wakefulness during lights-on period compared with 45.3+/-1.8% in PBS-treated rats). Amounts of wakefulness returned to control levels in all LPS-treated rats 30 days after the onset of infusion. A single injection of LPS (1, 5, or 10 microg) did not produce a significant decline in the number of hypocretin, MCH, or NeuN-positive neurons. The loss of hypocretin neurons produced by chronic LPS administration suggests that inflammation may play a role in the loss of hypocretin neurons in narcolepsy.

Activation of a subpopulation of orexin/hypocretin-containing hypothalamic neurons by GABAA receptor-mediated inhibition of the nucleus accumbens shell, but not by exposure to a novel environment

Gamma-amino butyric acid (GABA)A receptor stimulation in the nucleus accumbens shell produces intense hyperphagia in rats and increases Fos expression in the lateral hypothalamus. To explore the involvement of hypothalamic orexin/hypocretin- or melanin concentrating hormone-immunoreactive neurons in this effect, the GABAA agonist, muscimol (0, 50 ng), was infused directly into the nucleus accumbens shell of rats; 90 min later, their brains were collected and subsequently processed for immunohistochemistry. A group exposed to a novel environment was included to evaluate the specificity of Fos expression changes with regard to general arousal. Alternating sections through the hypothalamus were double-stained for orexin/hypocretin-Fos or melanin concentrating hormone-Fos combinations. Intra-accumbens shell muscimol treatment significantly increased the percentage of orexin/hypocretin-containing neurons expressing Fos in the lateral, but not medial, portion of the perifornical/lateral hypothalamic area. Regardless of treatment condition, greater percentages of orexin/hypocretin-containing neurons in the medial portion of the hypothalamus expressed Fos relative to cells located more laterally. None of the manipulations increased Fos expression in melanin concentrating hormone-immunoreactive neurons. Muscimol treatment also markedly increased Fos expression in the arcuate nucleus, which connects reciprocally to the lateral/perifornical hypothalamic area. Thus, orexin/hypocretin-containing neurons in lateral sectors of the hypothalamus, along with cells in the arcuate nucleus, display phasic increases in Fos expression after an orexigenic pharmacological manipulation of the nucleus accumbens shell, but to a lesser degree after the heightened arousal associated with exposure to a novel environment.

Dual roles in feeding for AMPA/kainate receptors: receptor activation or inactivation within distinct hypothalamic regions elicits feeding behavior

We have previously shown that hypothalamic injections of glutamate, or agonists of its ionotropic receptors (iGluRs), elicit intense feeding responses in satiated rats [Brain Res. 613 (1993) 88, Brain Res. 630 (1993) 41]. While attempting to clarify the role of the AMPA and kainate (KA) receptor subtypes in glutamatergic feeding systems, we discovered that lateral hypothalamic (LH) injection of high doses of the competitive AMPA/KA receptor antagonist, NBQX (10 and 30 nmol), elicited a pronounced feeding response. We questioned whether this effect was due to inactivation of AMPA or possibly KA receptors. To determine whether other AMPA/KA antagonists can also elicit feeding, we tested whether injection of CNQX, another AMPA/KA receptor antagonist, also stimulates eating and whether these feeding stimulatory effects were due to antagonists' actions in the LH or in other hypothalamic sites. Here we report that NBQX and CNQX elicit feeding in a dose dependent manner and are most effective when injected into the perifornical hypothalamus (PFH), or into the paraventricular nucleus (PVN) and, to a lesser extent, into the LH of satiated rats. In contrast, AMPA was most effective in stimulating feeding when injected into the LH, confirming previous reports. These data suggest that either activation or inactivation of AMPA/KA receptors in distinct but overlapping hypothalamic sites may be sufficient to induce feeding behavior, indicating a broadened role for glutamate in hypothalamic feeding mechanisms.

Extracellular glutamate increases in the lateral hypothalamus during meal initiation, and GABA peaks during satiation: microdialysis measurements every 30 s

Glutamate injected into the lateral hypothalamus can initiate eating, and gamma-aminobutyric acid (GABA) can stop it. This leads to the hypothesis that glutamate inputs are active at the beginning of a meal, and GABA is released at the end. To test this theory, the authors used microdialysis to sample glutamate and GABA simultaneously before, during, and after a meal. Food-deprived rats ate a meal of chow. Glutamate increased during the first third of the meal, then decreased to below baseline while the rats were still eating. GABA also increased at the start of the meal but continued rising and peaked during the last third of the meal. Glutamate may drive a hypothalamic system for eating, and GABA may oppose it.

Effects of calcium and glutamate receptor agonists on leaf consumption by lepidopteran neonates

Calcium and glutamate receptor (GluR) agonists affect apple leaf consumption by neonates of the apple pest, the codling moth, Cydia pomonella (L.) Initial apple leaf consumption was advanced by the presence of trans-1-amino-(1S,3R)-cyclopentanedicarboxylic acid (trans-ACPD), but not by calcium chloride or N-methyl-D-aspartate (NMDA). However, during the 3 h following hatch, CaCl(2) and NMDA increased the quantity of apple leaf tissue consumed, but trans-ACPD had no such effects. Stimulatory effects of CaCl(2) and NMDA on leaf consumption were abolished if codling moth larvae were concurrently exposed to calcium chelator EDTA. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA) and kainic acid had no effects either on commencement or intensity of leaf consumption. We hypothesize that in codling moth larvae, apple leaf consumption is induced via metabotropic GluR, and sustained feeding is regulated via NMDA GluRs. Practical aspects of this finding are discussed.

Amygdalo-hypothalamic circuit allows learned cues to override satiety and promote eating

Organisms eat not only in a response to signals related to energy balance. Eating also occurs in response to "extrinsic," or environmental, signals, including learned cues. Such cues can modify feeding based on motivational value acquired through association with either rewarding or aversive events. We provide evidence that a specific brain system, involving connections between basolateral amygdala and the lateral hypothalamus, is crucial for allowing learned cues (signals that were paired with food delivery when the animal was hungry) to override satiety and promote eating in sated rats. In an assessment of second-order conditioning, we also found that disconnection of this circuitry had no effect on the ability of a conditioned cue to support new learning. Knowledge about neural systems through which food-associated cues specifically control feeding behavior provides a defined model for the study of learning that may be informative for understanding mechanisms that contribute to eating disorders and more moderate forms of overeating.

Regional differences in feeding and other behaviors elicited by N-methyl-D-aspartic acid in the rodent hypothalamus: a reverse microdialysis mapping study

Regional differences in the feeding stimulatory actions of hypothalamically delivered N-methyl-D-aspartate (NMDA) were investigated. NMDA (660 microM intraprobe) delivered by reverse microdialysis into the tuberal lateral hypothalamus (tLH) reliably elicited feeding in satiated rats. The average food intake was 8.6 g in 50 min, and during the infusion rats spent 26% of the time eating, compared to less than 1% before NMDA treatment. In contrast, NMDA did not affect feeding when reverse dialyzed into the anterior LH (aLH), posterior LH (pLH) or the medial hypothalamus (MH). NMDA had no apparent behavioral effect in the aLH; in contrast, it significantly decreased the time spent resting/sleeping when infused into each of the other three areas tested. Additionally, in the medial hypothalamus, NMDA infusions increased time spent grooming; while in the pLH only alertness was significantly increased. These data underscore the functional and anatomical heterogeneity of the hypothalamus, and implicate glutamate and NMDA receptors in different portions of the hypothalamus in the control of eating, grooming and arousal.