The non-competitive NMDA antagonist MK-801 increases food intake in rats

A metabolic balance of GLP-1 and NMDA receptors in the brain

Glucagon-like peptide-1 (GLP-1) and N-methyl-D-aspartate (NMDA) receptors in the brain regulate metabolic homeostasis. In a paper published in Nature, Petersen et al. describe a bimodal molecule that conjugates a GLP-1 analog with MK-801 (NMDA receptor antagonist), which lowers feeding and body weight to a greater extent than the GLP-1R agonist alone.

GLP-1-directed NMDA receptor antagonism for obesity treatment

The N-methyl-D-aspartate (NMDA) receptor is a glutamate-activated cation channel that is critical to many processes in the brain. Genome-wide association studies suggest that glutamatergic neurotransmission and NMDA receptor-mediated synaptic plasticity are important for body weight homeostasis1. Here we report the engineering and preclinical development of a bimodal molecule that integrates NMDA receptor antagonism with glucagon-like peptide-1 (GLP-1) receptor agonism to effectively reverse obesity, hyperglycaemia and dyslipidaemia in rodent models of metabolic disease. GLP-1-directed delivery of the NMDA receptor antagonist MK-801 affects neuroplasticity in the hypothalamus and brainstem. Importantly, targeting of MK-801 to GLP-1 receptor-expressing brain regions circumvents adverse physiological and behavioural effects associated with MK-801 monotherapy. In summary, our approach demonstrates the feasibility of using peptide-mediated targeting to achieve cell-specific ionotropic receptor modulation and highlights the therapeutic potential of unimolecular mixed GLP-1 receptor agonism and NMDA receptor antagonism for safe and effective obesity treatment.

Pharmacologically induced N-methyl-D-aspartate receptor hypofunction impairs goal-directed food seeking in rats

AIM

Acute N-methyl-D-aspartate (NMDA) receptor antagonism is an important pharmacological animal model of schizophrenia. In previous studies, schizophrenia patients show impaired goal-directed behavior in an outcome-specific devaluation procedure. In this study, we investigated whether the rat model of the NMDA receptor blockade also showed altered goal-directed behavior in a satiety-induced outcome devaluation paradigm.

METHODS

In experiments 1 and 2, we aimed to establish the satiety-induced outcome devaluation test using sucrose and lipid rewards in operant conditioning and free consumption paradigms. In experiment 3, we tested the effect of MK-801 (0.1 mg/kg, i.p.) on outcome-specific devaluation.

RESULTS

Experiments 1 and 2 demonstrated that 1-h ad libitum food consumption is sufficient to induce outcome-specific devaluation in both lever-press and free consumption tests in rats. Experiment 3 showed that the administration of MK-801 impaired satiety-induced devaluation in the lever-press test but not in the subsequent free consumption test.

CONCLUSIONS

Our results suggest that acute pharmacological NMDA receptor antagonism in rats is a useful animal model for impaired goal-directed behavior in schizophrenia.

Glutamate-dependent regulation of food intake is altered with age through changes in NMDA receptor phenotypes on vagal afferent neurons

Compared to younger individuals, older human subjects have significantly lower food intakes and an increased satiety response. N-methyl-d-aspartate (NMDA) receptors expressed by vagal afferent neurons originating from nodose ganglia (NG) are involved in modulating the satiety response. The present study investigated how NMDA receptor subunit phenotypes in NG neurons change with age and how these age-related alterations in food intake are modulated by presynaptic NMDA receptors in the NG of male Sprague Dawley rats (six week-old and sixty week-old). Food intake was measured at 30-, 60-, and 120-min following intraperitoneal administration of cholecystokinin (CCK) or the non-competitive NMDA receptor antagonist MK-801. Immunofluorescence was used to determine NMDA receptor subunit expression (NR1, NR2B, NR2C, and NR2D) in the NG. The results showed that, CCK reduced food intake at 30-, 60-, and 120-min post injection in both young and the middle-age animals, with no statistical difference between the groups at 30- and 60-min. In contrast, MK-801 produced an increase in food intake that was significantly higher in middle-age rats compared to young animals at all time points studied. NR1 subunit was expressed by almost all NG neurons in both age groups. In young rats, NR2B, NR2C, and NR2D subunits were expressed in 56.1%, 49.3%, and 13.9% of NG neurons, respectively. In contrast, only 30.3% of the neuronal population in middle-aged rats expressed NR2B subunit immunoreactivity, NR2C was present in 34.1%, and only 10.6% of total neurons expressed the NR2D subunit. In conclusion, glutamate-dependent regulation of food intake is altered with age and one of the potential mechanisms through which this age-related changes in intake occur is changes in NMDA receptor phenotypes on vagal afferent neurons located in NG.

The strength of reward-related learning depends on the degree of activation of ventral tegmental area dopamine neurons

We tested whether (1) the capacity of a reward-associated conditioned stimulus (CS) to cause conditioned activation of ventral tegmental area (VTA) dopamine (DA) neurons is associated with its capacity to elicit conditioned approach responses and (2) whether the acquisition of these capacities by a CS requires N-methyl-D-aspartate (NMDA) and muscarinic acetylcholine (mACh) receptor stimulation. Rats were trained to emit a conditioned approach response to a light CS that was previously paired with food and were treated systemically with scopolamine (a mACh receptor antagonist) or MK-801 (an NMDA receptor antagonist) either prior to each conditioning session (during which animals experienced paired CS and food presentations) or prior to the conditioned approach (CS-only) test. Brains were harvested after the CS-only test and processed for tyrosine hydroxylase (TH) (DA cells) and c-fos in the VTA. When animals received scopolamine or MK-801 treatment prior to conditioning sessions we observed significantly fewer TH-labeled (i.e., DA) cells in the VTA that expressed c-fos and significantly less conditioned approach responding during the CS-only test. Further analysis showed a correlation between the number of VTA DA cells activated and the number of conditioned approach responses. Treatments made prior to the CS-only test did not affect responding. Altogether these results suggest that the degree to which a CS elicits conditioned approach depends partially on the degree to which the CS activates VTA DA cells and that the acquisition of both of these capacities by a CS requires mACh and NMDA receptor stimulation.

Fasted/fed states regulate postsynaptic hub protein DYNLL2 and glutamatergic transmission in oxytocin neurons in the hypothalamic paraventricular nucleus

The neurons in the hypothalamus regulate food intake and energy metabolism on reception of systemic energy states. Accumulating evidences have indicated that synaptic transmission on the hypothalamic neurons is modulated by the metabolic condition related to fasted/fed states, and that this modulation of synaptic plasticity plays a role in regulation of feeding. It has been shown that oxytocin (Oxt) neurons in the paraventricular nucleus (PVN) of the hypothalamus sense and integrate various peripheral and central signals and thereby induce satiety. However, whether metabolic conditions regulate the synaptic transmission on Oxt neurons in PVN remains unclear. The present study examined whether the fasted/fed states regulate synaptic transmission on Oxt neurons in PVN. The miniature excitatory postsynaptic currents (mEPSCs) onto Oxt neurons in PVN were increased under ad lib fed condition compared to 24h fasted condition. Furthermore, the NMDA receptor-mediated EPSC on Oxt neurons was increased under fed, compared to fasted, condition. In Oxt neurons, dynein light chain 2 (DYNLL2), a protein suggested to be implicated in the NMDA receptor trafficking to the postsynaptic site, was increased under fed, compared to fasted, condition. The present results suggest that feeding increases excitatory synaptic input on PVN Oxt neurons via mechanisms involving DYNLL2 upregulation and NMDA receptor-mediated synaptic reorganization.

Hunger States Control the Directions of Synaptic Plasticity via Switching Cell Type-Specific Subunits of NMDA Receptors

It remains largely unknown whether and how hunger states control activity-dependent synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD). We here report that both LTP and LTD of excitatory synaptic strength within the appetite control circuits residing in hypothalamic arcuate nucleus (ARC) behave in a manner of hunger states dependence and cell type specificity. For instance, we find that tetanic stimulation induces LTP at orexigenic agouti-related protein (AgRP) neurons in ad libitum fed mice, whereas it induces LTD in food-deprived mice. In an opposite direction, the same induction protocol induces LTD at anorexigenic pro-opiomelanocortin (POMC) neurons in fed mice but weak LTP in deprived mice. Mechanistically, we also find that food deprivation increases the expressions of NR2C/NR2D/NR3-containing NMDA receptors (NMDARs) at AgRP neurons that contribute to the inductions of LTD, whereas it decreases their expressions at POMC neurons. Collectively, our data reveal that hunger states control the directions of activity-dependent synaptic plasticity by switching NMDA receptor subpopulations in a cell type-specific manner, providing insights into NMDAR-mediated interactions between energy states and associative memory. Significance statement: Based on the experiments performed in this study, we demonstrate that activity-dependent synaptic plasticity is also under the control of energy states by regulating NMDAR subpopulations in a cell type-specific manner. We thus propose a reversible memory configuration constructed from energy states-dependent cell type-specific bidirectional conversions of LTP and LTD. Together with the distinct functional roles played by NMDAR signaling in the control of food intake and energy states, these findings reveal a new reciprocal interaction between energy states and associative memory, one that might serve as a target for therapeutic treatments of the energy-related memory disorders or vice versa.

Role of ionotropic glutamate receptors in delay and probability discounting in the rat

RATIONALE

Discounting of delayed and probabilistic reinforcement is linked to increased drug use and pathological gambling. Understanding the neurobiology of discounting is important for designing treatments for these disorders. Glutamate is considered to be involved in addiction-like behaviors; however, the role of ionotropic glutamate receptors (iGluRs) in discounting remains unclear.

OBJECTIVES

The current study examined the effects of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor blockade on performance in delay and probability discounting tasks.

METHODS

Following training in either delay or probability discounting, rats (n = 12, each task) received pretreatments of the NMDA receptor antagonists MK-801 (0, 0.01, 0.03, 0.1, or 0.3 mg/kg, s.c.) or ketamine (0, 1.0, 5.0, or 10.0 mg/kg, i.p.), as well as the AMPA receptor antagonist CNQX (0, 1.0, 3.0, or 5.6 mg/kg, i.p.). Hyperbolic discounting functions were used to estimate sensitivity to delayed/probabilistic reinforcement and sensitivity to reinforcer amount.

RESULTS

An intermediate dose of MK-801 (0.03 mg/kg) decreased sensitivity to both delayed and probabilistic reinforcement. In contrast, ketamine did not affect the rate of discounting in either task but decreased sensitivity to reinforcer amount. CNQX did not alter sensitivity to reinforcer amount or delayed/probabilistic reinforcement.

CONCLUSIONS

These results show that blockade of NMDA receptors, but not AMPA receptors, decreases sensitivity to delayed/probabilistic reinforcement (MK-801) and sensitivity to reinforcer amount (ketamine). The differential effects of MK-801 and ketamine demonstrate that sensitivities to delayed/probabilistic reinforcement and reinforcer amount are pharmacologically dissociable.

mGluR5 positive allosteric modulation and its effects on MK-801 induced set-shifting impairments in a rat operant delayed matching/non-matching-to-sample task

RATIONALE

Positive allosteric modulators (PAMs) of type 5 metabotropic glutamate receptors (mGluR5) exert pro-cognitive effects in animal models of various neuropsychiatric diseases. However, few studies to date have examined ability of mGluR5 PAMs to reverse cognitive deficits in operant delayed matching/non-matching-to-sample (DMS/DNMS) tasks.

OBJECTIVES

This study aims to determine the ability of the mGluR5 PAM 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) to reverse set-shifting deficits induced by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801.

METHODS

Male Sprague-Dawley rats were initially trained to lever press for sucrose reinforcement under either DMS or DNMS conditions. Following successful acquisition of the task, reinforcement conditions were reversed (DNMS → DMS or DMS → DNMS). In Experiment 1, rats were treated daily prior to each session with vehicle/vehicle, vehicle/MK-801 (0.06 mg/kg) simultaneously, CDPPB (20 mg/kg)/MK-801 simultaneously, or CDPPB 30 min prior to MK-801. In Experiment 2, rats were treated with either vehicle/vehicle, vehicle/MK-801, or CDPPB 30 min prior to MK-801 only prior to sessions that followed task reversal.

RESULTS

In Experiment 1, no group differences in initial task acquisition were observed. Rats treated with vehicle/MK-801 showed significant set-shifting impairments following task reversal, which were partially attenuated by simultaneous administration of CDPPB/MK-801 and completely precluded by administration of CDPPB 30 min prior to MK-801. In Experiment 2, MK-801 did not impair reversal learning, and no other group differences were observed.

CONCLUSIONS

MK-801-induced deficits in operant set-shifting ability were prevented by pretreatment with CDPPB. MK-801 did not produce deficits in task learning when treatment was initiated following task reversal.

NMDA receptors control vagal afferent excitability in the nucleus of the solitary tract

Previous behavioral studies have demonstrated that presynaptic N-methyl-d-aspartate (NMDA) receptors expressed on vagal afferent terminals are involved in food intake and satiety. Therefore, using in vitro live cell calcium imaging of prelabeled rat hindbrain slices, we characterized which NMDA receptor GluN2 subunits may regulate vagal afferent activity. The nonselective NMDA receptor antagonist d,l-2-amino-5-phosphonopentanoic acid (d,l-AP5) significantly inhibited vagal terminal calcium influx, while the excitatory amino acid reuptake inhibitor d,l-threo-β-benzyloxyaspartic acid (TBOA), significantly increased terminal calcium levels following pharmacological stimulation with ATP. Subunit-specific NMDA receptor antagonists and potentiators were used to identify which GluN2 subunits mediate the NMDA receptor response on the vagal afferent terminals. The GluN2B-selective antagonist, ifenprodil, selectively reduced vagal calcium influx with stimulation compared to the time control. The GluN2A-selective antagonist, 3-chloro-4-fluoro-N-[4-[[2-(phenylcarbonyl)hydrazino]carbonyl] benzyl]benzenesulfonamide (TCN 201) produced smaller but not statistically significant effects. Furthermore, the GluN2A/B-selective potentiator (pregnenolone sulfate) and the GluN2C/D-selective potentiator [(3-chlorophenyl)(6,7-dimethoxy-1-((4-methoxyphenoxy)methyl)-3,4-dihydroisoquinolin-2(1H)-yl)methanone; (CIQ)] enhanced vagal afferent calcium influx during stimulation. These data suggest that presynaptic NMDA receptors with GluN2B, GluN2C, and GluN2D subunits may predominantly control vagal afferent excitability in the nucleus of the solitary tract.

Putative roles of neuropeptides in vagal afferent signaling

The vagus nerve is a major pathway by which information is communicated between the brain and peripheral organs. Sensory neurons of the vagus are located in the nodose ganglia. These vagal afferent neurons innervate the heart, the lung and the gastrointestinal tract, and convey information about peripheral signals to the brain important in the control of cardiovascular tone, respiratory tone, and satiation, respectively. Glutamate is thought to be the primary neurotransmitter involved in conveying all of this information to the brain. It remains unclear how a single neurotransmitter can regulate such an extensive list of physiological functions from a wide range of visceral sites. Many neurotransmitters have been identified in vagal afferent neurons and have been suggested to modulate the physiological functions of glutamate. Specifically, the anorectic peptide transmitters, cocaine and amphetamine regulated transcript (CART) and the orexigenic peptide transmitters, melanin concentrating hormone (MCH) are differentially regulated in vagal afferent neurons and have opposing effects on food intake. Using these two peptides as a model, this review will discuss the potential role of peptide transmitters in providing a more precise and refined modulatory control of the broad physiological functions of glutamate, especially in relation to the control of feeding.

The food intake-suppressive effects of glucagon-like peptide-1 receptor signaling in the ventral tegmental area are mediated by AMPA/kainate receptors

Glucagon-like peptide-1 receptor (GLP-1R) activation in the ventral tegmental area (VTA) is physiologically relevant for the control of palatable food intake. Here, we tested whether the food intake-suppressive effects of VTA GLP-1R activation are mediated by glutamatergic signaling within the VTA. Intra-VTA injections of the GLP-1R agonist exendin-4 (Ex-4) reduced palatable high-fat food intake in rats primarily by reducing meal size; these effects were mediated in part via glutamatergic AMPA/kainate but not NMDA receptor signaling. Additional behavioral data indicated that GLP-1R expressed specifically within the VTA can partially mediate the intake- and body weight-suppressive effects of systemically administered Ex-4, offering the intriguing possibility that this receptor population may be clinically relevant for food intake control. Intra-VTA Ex-4 rapidly increased tyrosine hydroxylase levels within the VTA, suggesting that GLP-1R activation modulates VTA dopaminergic signaling. Further evidence for this hypothesis was provided by electrophysiological data showing that Ex-4 increased the frequency of AMPA-mediated currents and reduced the paired/pulse ratio in VTA dopamine neurons. Together, these data provide novel mechanisms by which GLP-1R agonists in the mesolimbic reward system control for palatable food intake.

NR2B subunit of the NMDA glutamate receptor regulates appetite in the parabrachial nucleus

Diphtheria toxin-mediated, acute ablation of hypothalamic neurons expressing agouti-related protein (AgRP) in adult mice leads to anorexia and starvation within 7 d that is caused by hyperactivity of neurons within the parabrachial nucleus (PBN). Because NMDA glutamate receptors are involved in various synaptic plasticity-based behavioral modifications, we hypothesized that modulation of the NR2A and NR2B subunits of the NMDA receptor in PBN neurons could contribute to the anorexia phenotype. We observed by Western blot analyses that ablation of AgRP neurons results in enhanced expression of NR2B along with a modest suppression of NR2A. Interestingly, systemic administration of LiCl in a critical time window before AgRP neuron ablation abolished the anorectic response. LiCl treatment suppressed NR2B levels in the PBN and ameliorated the local Fos induction that is associated with anorexia. This protective role of LiCl on feeding was blunted in vagotomized mice. Chronic infusion of RO25-6981, a selective NR2B inhibitor, into the PBN recapitulated the role of LiCl in maintaining feeding after AgRP neuron ablation. We suggest that the accumulation of NR2B subunits in the PBN contributes to aphagia in response to AgRP neuron ablation and may be involved in other forms of anorexia.

Morphine and MK-801 administration leads to alternative N-methyl-D-aspartate receptor 1 splicing and associated changes in reward seeking behavior and nociception on an operant orofacial assay

The NMDA receptor plays a large role in opioid-induced plastic changes in the nervous system. The expression levels of its NR1 subunit are altered dramatically by morphine but no changes in its alternative splicing have been reported. Changes in the splicing of the N1, C1, C2, and C2' cassettes can alter the pharmacology and regulation of this receptor. Western Blots run on brain tissue from rats made tolerant to morphine revealed altered splicing of the N1 cassettes in the accumbens and amygdala (AMY), and the C1 cassette in the AMY and the dorsal hippocampus (HIPP). After 3days of withdrawal C2'-containing NR1 subunits were down-regulated in each of these areas. These were not due to acute doses of morphine and may represent long-term alterations in drug-induced neuroplasticity. We also examined the effects of morphine tolerance on an operant orofacial nociception assay which forces an animal to endure an aversive heat stimulus in order to receive a sweet milk reward. Morphine decreased pain sensitivity as expected but also increased motivational reward seeking in this task. NMDAR antagonism potentiated this reward seeking behavior suggesting that instead of attenuating tolerance, MK-801 may actually alter the rewarding and/or motivational properties of morphine. When combined, MK-801 and morphine had an additive effect which led to altered splicing in the accumbens, AMY, and the HIPP. In conclusion, NR1 splicing may play a major role in the cognitive behavioral aspects especially in motivational reward-seeking behaviors.

Activation of nucleus accumbens NMDA receptors differentially affects appetitive or aversive taste learning and memory

Taste memory depends on motivational and post-ingestional consequences; thus, it can be aversive (e.g., conditioned taste aversion, CTA) if a novel, palatable taste is paired with visceral malaise, or it can be appetitive if no intoxication appears after novel taste consumption, and a taste preference is developed.The nucleus accumbens (NAc) plays a role in hedonic reactivity to taste stimuli, and recent findings suggest that reward and aversion are differentially encoded by the activity of NAc neurons. The present study examined whether the requirement for N-methyl-D-aspartate (NMDA) receptors in the NAc core during rewarding appetitive taste learning differs from that during aversive taste conditioning, as well as during retrieval of appetitive vs. aversive taste memory, using the taste preference or CTA model, respectively. Bilateral infusions of NMDA (1 μg/μl, 0.5 μl) into the NAc core were performed before acquisition or before retrieval of taste preference or CTA. Activation of NMDA receptors before taste preference training or CTA acquisition did not alter memory formation. Furthermore, NMDA injections before aversive taste retrieval had no effect on taste memory; however, 24 h later, CTA extinction was significantly delayed. Also, NMDA injections, made before familiar appetitive memory retrieval, interrupted the development of taste preference and produced a preference delay 24 h later. These results suggest that memory formation for a novel taste produces neurochemical changes in the NAc core that have differential requirements for NMDA receptors during retrieval of appetitive or aversive memory.

Differential behavioral profiling of stimulant substances in the rat using the LABORAS™ system

Preclinical testing requires rapid and reliable evaluation of the main in vivo effects of novel test substances usually in rodents. Nevertheless, the techniques primarily used up to now involve either automated measurement of motor activity or direct observation of behavioral effects by extensively trained investigators. The advantages of these approaches are respectively high-throughput and comprehensive behavioral assessment. Nevertheless, motor activity is only one aspect of animal behavior and it cannot predict the full neurobehavioral profile of a substance, whereas direct observation is time-consuming. There is thus a need for novel approaches that combine the advantages of both automatic detection and comprehensive behavioral analysis. In the present study, we used the LABORAS™ system to analyze motor and non-motor behavior in rats administered various stimulant substances with or without known psychotomimetic properties or abuse liability (amphetamine, cocaine dizocilpine (MK-801), ketamine, modafinil and nicotine). The data show that LABORAS™ clearly detects the stimulating effects on motor behaviors of amphetamine, cocaine, dizocilpine and ketamine in a dose- and time-dependent manner. Differential effects of these test substances on non-motor behaviors, such as grooming, eating and drinking could also be detected. Nicotine displayed only slight stimulating effects on locomotion, whereas modafinil was virtually without effect on the behaviors evaluated by the system. These data with different stimulant substances suggest that LABORAS™ presents an advantage over classical methods performing automated measurements restricted to locomotion. Furthermore, the procedure is considerably more rapid than behavioral observation procedures. Characterization of the behavioral profile of test substances using LABORAS™ should therefore accelerate preclinical studies. In addition, the multi-faceted parameters measured by LABORAS™ permit a more detailed comparison of the behavioral profiles of novel substances with standard reference substances, thereby providing important indicators for orienting further substance evaluation and supporting drug development.

Comparison of the MK-801-induced appetitive extinction deficit with pressing for reward and associated pERK1/2 staining in prefrontal cortex and nucleus accumbens

Administration of the noncompetitive N-methyl-d-aspartate (NMDA)-receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) has been shown to produce extinction deficits on appetitive operant tasks. The present study sought to further explore this by comparing extinction pressing to pressing for the primary reward and examining associated neural correlates to determine if the MK-801 extinction profile resembled the behavioral and neural profile associated with pressing for primary reward. Immunohistochemical labeling of phosphorylated extracellular signal-regulated kinase-1 and -2(pERK1/2) in the prelimbic (PrL) and infralimbic (IL) cortices and nucleus accumbens shell (AcbSh) and core (AcbC) was examined after rewarded or extinction lever pressing conditions. A dose-response curve revealed a within-day extinction deficit following administration of 0.05 mg/kg MK-801. All doses of MK-801 were associated with reduced IL pERK1/2 staining but only the 0.05 mg/kg dose was associated with elevated AcbSh pERK1/2 labeling. Extinction pressing under the influence of MK-801 was elevated compared to that seen during rewarded pressing-whether on MK-801 or saline. Rewarded pressing following saline or MK-801 was associated with elevated pERK1/2 in the PrL with no similar patterns in the MK-801/extinction group. There was more pERK1/2 labeling in the AcbSh of the MK-801 extinction group than any other condition. These data suggest that the MK-801-induced extinction deficit may be due to the combination of an underactive cortical behavioral inhibition system and an overactive AcbSh reward system.

A tale of two endings: modulation of satiation by NMDA receptors on or near central and peripheral vagal afferent terminals

Glutamate is the neurotransmitter responsible for fast excitatory transmission from vagal afferents to second order neurons in the NTS. Antagonism of NMDA-type glutamate receptors in the NTS increases food intake and attenuates reduction of food intake by vagally mediated satiation signals, such as cholecystokinin. Although, the cellular location(s) of NMDA receptors that participate in satiation is uncertain, recent findings suggest that attenuation of satiation by NMDA receptor antagonists is due, at least in part, to their action on primary vagal afferents themselves. While evidence is accumulating that NMDA receptors located on vagal afferent endings in the hindbrain are involved in control of food intake, there also is preliminary evidence that peripheral NMDA receptors also may influence vagal control of food intake. Hence, NMDA receptor expression on central and perhaps peripheral vagal afferent endings could provide a parsimonious mechanism for modulation of satiation signals by endogenously released glutamate.

Intra-accumbens injection of a dopamine aptamer abates MK-801-induced cognitive dysfunction in a model of schizophrenia

Systemic administration of the noncompetitive NMDA-receptor antagonist, MK-801, has been proposed to model cognitive deficits similar to those seen in patients with schizophrenia. The present work investigated the ability of a dopamine-binding DNA aptamer to regulate these MK-801-induced cognitive deficits when injected into the nucleus accumbens. Rats were trained to bar press for chocolate pellet rewards then randomly assigned to receive an intra-accumbens injection of a DNA aptamer (200 nM; n = 7), tris buffer (n = 6) or a randomized DNA oligonucleotide (n = 7). Animals were then treated systemically with MK-801 (0.1 mg/kg) and tested for their ability to extinguish their bar pressing response. Two control groups were also included that did not receive MK-801. Data revealed that injection of Tris buffer or the random oligonucleotide sequence into the nucleus accumbens prior to treatment with MK-801 did not reduce the MK-801-induced extinction deficit. Animals continued to press at a high rate over the entire course of the extinction session. Injection of the dopamine aptamer reversed this MK-801-induced elevation in lever pressing to levels as seen in rats not treated with MK-801. Tests for activity showed that the aptamer did not impair locomotor activity. Results demonstrate the in vivo utility of DNA aptamers as tools to investigate neurobiological processes in preclinical animal models of mental health disease.

Reduction of food intake by cholecystokinin requires activation of hindbrain NMDA-type glutamate receptors

Intraperitoneal injection of CCK reduces food intake and triggers a behavioral pattern similar to natural satiation. Reduction of food intake by CCK is mediated by vagal afferents that innervate the stomach and small intestine. These afferents synapse in the hindbrain nucleus of the solitary tract (NTS) where gastrointestinal satiation signals are processed. Previously, we demonstrated that intraperitoneal (IP) administration of either competitive or noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists attenuates reduction of food intake by CCK. However, because vagal afferents themselves express NMDA receptors at both central and peripheral endings, our results did not speak to the question of whether NMDA receptors in the brain play an essential role in reduction of feeding by CCK. We hypothesized that activation of NMDA receptors in the NTS is necessary for reduction of food intake by CCK. To test this hypothesis, we measured food intake following IP CCK, subsequent to NMDA receptor antagonist injections into the fourth ventricle, directly into the NTS or subcutaneously. We found that either fourth-ventricle or NTS injection of the noncompetitive NMDA receptor antagonist MK-801 was sufficient to inhibit CCK-induced reduction of feeding, while the same antagonist doses injected subcutaneously did not. Similarly fourth ventricle injection of d-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (d-CPPene), a competitive NMDA receptor antagonist, also blocked reduction of food intake following IP CCK. Finally, d-CPPene injected into the fourth ventricle attenuated CCK-induced expression of nuclear c-Fos immunoreactivity in the dorsal vagal complex. We conclude that activation of NMDA receptors in the hindbrain is necessary for the reduction of food intake by CCK. Hindbrain NMDA receptors could comprise a critical avenue for control and modulation of satiation signals to influence food intake and energy balance.

Blockade of hindbrain NMDA receptors containing NR2 subunits increases sucrose intake

We have previously shown that blockade of N-methyl-d-aspartate (NMDA) receptors in the caudal brain stem delays satiation and increases food intake. NMDA receptors are heterodimers made up of distinct, but different, ion channel subunits. The NR2 subunits of the NMDA receptor contain the binding site for glutamate. About half of vagal afferents express immunoreactivity for NMDA NR2B subunit and about half of the NR2B expressing afferents also express NMDA NR2C or NR2D subunits. This suggests that increased food intake may be evoked by interference with glutamate binding to NMDA channels containing the NR2B subunit. To test this, we measured deprivation-induced intake of 15% sucrose solution following fourth ventricle and intra-nucleus of the solitary tract (intra-NTS) injections of Conantokin G (Con G; NR2B blocker), d-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (d-CPPene; NR2B/2A blocker), and (+/-)-cis-1-(phenanthren-2yl-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA; NR2D/C blocker). Fourth ventricular administration of Con G (5, 20, 40, 80 ng), d-CPPene (3.0, 6.25, 12.5, 25, 50, 100 ng), and PPDA (300, 400 ng) increased sucrose intake significantly compared with control. Likewise, injections of Con G (10 ng), d-CPPene (5 ng, 10 ng), and PPDA (0.5, 1.0, 2.5, 5.0 ng) directly into the NTS significantly increased sucrose intake. These results show that hindbrain injection of competitive NMDA antagonists with selectivity or preference for the NMDA receptor NR2B or NR2C subunits increases food intake.

Antagonism of glutamatergic NMDA and mGluR5 receptors decreases consumption of food in baboon model of binge-eating disorder

Excessive consumption of highly palatable foods may contribute to the development of weight gain. Therefore medications that selectively suppress eating of such foods would be useful in clinical practice. We compared the effects of the glutamatergic antagonists memantine and MTEP to dexfenfluramine in baboons given periodic access to highly palatable food and ad libitum access to a standard chow diet. Three days a week baboons received a sugar-coated candy during the first meal and standard standard-diet chow pellets were available in subsequent meals. All baboons derived a greater amount of energy from the single single-candy meal than from the standard diet across an entire day. Pre-treatment with dexfenfluramine, memantine, and MTEP produced decreases in candy consumption without altering candy-seeking behaviour. At the same time, dexfenfluramine and memantine, but not MTEP, produced a decrease in seeking and consumption of standard chow pellets. Both memantine and MTEP are promising agents for the treatment of obesity.

Vagotomy upregulates expression of the N-methyl-D-aspartate receptor NR2D subunit in the stomach

BACKGROUND

N-methyl-D-aspartate (NMDA) receptor is the major excitatory neurotransmitter receptor in the central nervous system. Recent evidence has pointed to expression of NMDA receptor in peripheral non-neuronal tissues and organs; however, little is known about the expression of the receptor in the stomach. The present study identified what types of NMDA receptor subunits are expressed in the rat stomach and examined whether vagotomy affects their expression.

METHODS

To see the expression and distribution of the mRNAs and proteins for the NMDA receptor subunits, real-time reverse transcription-polymerase chain reaction, Western blotting, in situ hybridization, and immunohistochemistry were carried out on rat stomach with and without vagotomy.

RESULTS

Of the NMDA receptor subunits, NR1, NR2 (2A, 2B, 2C, 2D), and NR3 (3A, 3B), all NR subunit mRNAs except for the NR2B subunit mRNA were expressed in the intact rat stomach, with huge expression of NR2D mRNA. Expression of NR2D subunit mRNA and protein significantly increased after vagotomy. Increased expression was found in mucosal cells, the myenteric plexus, and the outer membrane of the stomach with vagotomy.

CONCLUSIONS

Vagotomy upregulates expression of the NR2D subunit in the stomach and therefore, the NR2D subunit may be implicated in gastric motility.

NMDA receptor in conditioned flavor-taste preference learning: blockade by MK-801 and enhancement by D-cycloserine

Conditioned flavor-taste preference (CFTP) is a robust form of learning in which animals acquire a preference for a flavor (e.g. Kool-Aid) previously mixed with a highly preferred tastant (e.g. fructose) over a flavor previously mixed with a less-preferred tastant (e.g. saccharin). Here, the role of the N-methyl-D-aspartate (NMDA) glutamate-glycine receptor (NR) was probed using systemic MK-801, a non-competitive antagonist, and D-cycloserine (DCS), a glycine agonist. Rats were injected with MK-801 (100 microg/kg) or vehicle 30 min prior to a daily 2-h conditioning session with 1-bottle access to a Kool-Aid flavor (grape or cherry) mixed with either 8% fructose (CS+/F) or 0.2% saccharin (CS-/S). CFTP expression was measured in 2-bottle preference tests between the Kool-Aid flavors mixed with 0.2% saccharin (CS+/S vs. CS-/S). While vehicle-treated rats acquired a preference for CS+/S over CS-/S, MK-801 prior to conditioning completely blocked CFTP learning. The effect of MK-801 was specific to CFTP acquisition, because follow-up experiments demonstrated that MK-801 did not induce a conditioned taste aversion, cause state-dependent learning, or affect CFTP expression. In a second approach, rats were injected with DCS (15 mg/kg) 60 min prior to daily conditioning. In contrast to MK-801, administration of DCS prior to conditioning enhanced CFTP learning (but not reversal conditioning). These results demonstrate that NR neurotransmission is critical for CFTP learning. Furthermore, enhancement of CFTP learning by DCS suggests that endogenous levels of glycine or D-serine may be a limiting factor in CFTP learning.

Ingestive effects of NMDA and AMPA-kainate receptor antagonists microinjections into the lateral hypothalamus of the pigeon (Columba livia)

This study examined the ingestive and behavioral effects of NMDA- and AMPA/kainate glutamatergic receptor blockade in the lateral hypothalamic area (LHy) of free-feeding pigeons (Columba livia). Injections of MK-801 (NMDA receptor antagonist; 6 nmol) or CNQX (AMPA/kainate receptor antagonist; 25.8 nmol) into the LHy of free-feeding pigeons induced significant increases in food intake and in feeding duration, as well as reductions in the latency to start feeding. Duration, latency and volume of water intake, as well as duration of sleep-like behavior, alert immobility, locomotion and preening were not changed by these treatments in the LHy. These results indicate that glutamatergic inputs to cells containing NMDA and/or AMPA receptors located in the LHy could modify both the beginning of a feeding bout (or the end of a period of satiety) and its duration (satiation). Our data also suggest that these inhibitory glutamatergic influences on feeding behavior are tonically active in the LHy.

NMDA antagonist MK-801 impairs acquisition of place strategies, but not their use

Evidence that NMDA receptors contribute to synaptic plasticity in the hippocampus has stimulated research on their role in behavioral learning and memory. Recent studies indicate that NMDA antagonists decrease use of place strategies by rats in a T-maze task that can be solved using either a "place" or "response" strategy. In the present study, rats were given MK-801 before maze exposure and/or training on this redundant strategy T-maze task. MK-801 did not impair rats' ability to learn the task, but did change the strategies they used on a probe trial administered after learning. MK-801 decreased use of place strategies only when administered before both maze exposure and training; rats given MK-801 only before maze exposure or only before training tended to use place strategies on the probe trial. These results show that MK-801 does not prevent rats from utilizing previously acquired spatial information, but does appear to impair the acquisition of spatial information needed for place strategies.

Opposite effects of MK-801 on the expression of food and morphine-induced conditioned place preference in rats

Behavioural studies have provided strong evidence for common substrates in the rewards of natural and addictive substances, but it is still unclear whether there is a common glutamatergic NMDA receptor mechanism involved in the processing of reward for both. The present study was designed to investigate the effects of MK-801 (0.1mg/kg) on the expression of place preference conditioned with food and morphine (5.0mg/kg) in rats. The data indicates that MK-801 potentiates the expression of food-induced conditioned place preference (CPP) but retards that of morphine CPP. It also demonstrates that the opposite effects of MK-801 on food and morphine CPP expression were caused neither by hyperactivity nor by the impairment of memory retrieval. These results suggest that MK-801 enhances food craving and inhibits morphine craving in rats, and that the roles of glutamatergic NMDA receptor mechanisms in the reward processing of natural reinforcers and addictive drugs may be dissociable.

Hindbrain administration of NMDA receptor antagonist AP-5 increases food intake in the rat

Hindbrain administration of MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) channel blocker, increases meal size, suggesting NMDA receptors in this location participate in control of food intake. However, dizocilpine (MK-801) reportedly antagonizes some non-NMDA ion channels. Therefore, to further assess hindbrain NMDA receptor participation in food intake control, we measured deprivation-induced intakes of 15% sucrose solution or rat chow after intraperitoneal injection of either saline vehicle or D(-)-2-amino-5-phosphonopentanoic acid (AP5), a competitive NMDA receptor antagonist, to the fourth ventricular, or nucleus of the solitary tract (NTS). Intraperitoneal injection of AP5 (0.05, 0.1, 1.0, 3.0, and 5.0 mg/kg) did not alter 30-min sucrose intake at any dose (10.7 +/- 0.4 ml, saline control) (11.0 +/- 0.8, 11.2 +/- 1.0, 11.2 +/- 1.0, 13.1 +/- 2.2, and 11.0 +/- 1.9 ml, AP5 doses, respectively). Fourth ventricular administration of both 0.2 mug (16.7 +/- 0.6 ml) and 0.4 mug (14.9 +/- 0.5 ml) but not 0.1 and 0.6 mug of AP5 significantly increased 60-min sucrose intake compared with saline (11.2 +/- 0.4 ml). Twenty-four hour chow intake also was increased compared with saline (AP5: 31.5 +/- 0.1 g vs. saline: 27.1 +/- 0.6 g). Furthermore, rats did not increase intake of 0.2% saccharin after fourth ventricular AP5 administration (AP5: 9.8 +/- 0.7 ml, vs. saline: 10.5 +/- 0.5 ml). Finally, NTS AP5 (20 ng/30 nl) significantly increased 30- (AP5: 17.2 +/- 0.7 ml vs. saline: 14.6 +/- 1.7 ml), and 60-min (AP5: 19.4 +/- 0.6 ml vs. saline: 15.5 +/- 1.4 ml) sucrose intake, as well as 24-h chow intake (AP5: 31.6 +/- 0.3 g vs. saline: 26.1 +/- 1.2 g). These results support the hypothesis that hindbrain NMDA receptors participate in control of food intake and suggest that this participation also may contribute to control of body weight over a 24-h period.

NMDA channels control meal size via central vagal afferent terminals

The N-methyl-D-aspartate (NMDA) ion channel blocker MK-801 administered systemically or as a nanoliter injection into the nucleus of the solitary tract (NTS), increases meal size. Furthermore, we have observed that ablation of the NTS abolishes increased meal size following systemic injection of dizocilpine (MK-801) and that MK-801-induced increases in intake are attenuated in rats pretreated with capsaicin to destroy small, unmyelinated, primary afferent neurons. These findings led us to hypothesize that NMDA receptors on central vagal afferent terminals or on higher-order NTS neurons innervated by these vagal afferents might mediate increased food intake. To evaluate this hypothesis, we examined 15% sucrose intake after 50-nl MK-801 injections ipsilateral or contralateral to unilateral nodose ganglion removal (ganglionectomy). On the side contralateral to ganglionectomy, vagal afferent terminals would be intact and functional, whereas ipsilateral to ganglionectomy vagal afferent terminals would be absent. Three additional control preparations also were included: 1) sham ganglionectomy and 2) subnodose vagotomy either contralateral or ipsilateral to NTS cannula placement. We found that rats with subnodose vagotomies increased their sucrose intake after injections of MK-801 compared with saline, regardless of whether injections were made contralateral (12.6 +/- 0.2 vs. 9.6 +/- 0.3 ml) or ipsilateral (14.2 +/- 0.6 vs. 9.7 +/- 0.4 ml) to vagotomy. Rats with NTS cannula placements contralateral to nodose ganglionectomy also increased their intake after MK-801 (12.2 +/- 0.9 and 9.2 +/- 1.1 ml for MK-801 and saline, respectively). However, rats with placements ipsilateral to ganglionectomy did not respond to MK-801 (8.0 +/- 0.5 ml) compared with saline (8.3 +/- 0.4 ml). We conclude that central vagal afferent terminals are necessary for increased food intake in response to NMDA ion channel blockade. The function of central vagal afferent processes or the activity of higher-order NTS neurons driven by vagal afferents may be modulated by NMDA receptors to control meal size.

Extracellular signal-regulated kinase 1/2 signaling pathway in solitary nucleus mediates cholecystokinin-induced suppression of food intake in rats

Increased food intake is a major factor in the development of obesity, and the control of meal size is a valid approach to reduce food intake in humans. Meal termination, or satiety, is thought to be organized within the caudal brainstem where direct signals from the food handling alimentary canal and long-term signals from the forebrain converge in the solitary nucleus. Cholecystokinin (CCK) released from the gut after ingestion of food has been strongly implicated in nucleus tractus solitarius (NTS)-mediated satiation, but the exact cellular and intracellular signaling events are not understood. Using Western blotting and immunohistochemistry with phosphospecific antibodies, we demonstrate here that peripheral administration of CCK in rats leads to rapid activation of the extracellular signal-regulated kinase (ERK) signaling cascade in NTS neurons and that blockade of ERK signaling with microinfusion of a selective mitogen-activated ERK kinase inhibitor into the fourth ventricle attenuates the capacity of CCK to suppress food intake. In addition, we show that CCK-induced activation of ERK results in phosphorylation of the voltage-dependent potassium channel Kv4.2 and the nuclear transcription factor CREB (cAMP response element-binding protein). The results demonstrate that ERK signaling is necessary for exogenous CCK to suppress food intake in deprived rats and suggest that this pathway may also be involved in natural satiation and the period of satiety between meals through coupling of ERK activation to both cytosolic and nuclear effector mechanisms that have the potential to confer acute and long-term changes in neuronal functioning.

Function of GABAergic and glutamatergic neurons in the stomach

Gamma-aminobutyric acid (GABA) and L-glutamic acid (L-Glu) are transmitters of GABAergic and glutamatergic neurons in the enteric interneurons, targeting excitatory or inhibitory GABA receptors or glutamate receptors that modulate gastric motility and mucosal function. GABAergic and glutamatergic neuron immunoreactivity have been found in cholinergic enteric neurons in the stomach. GABA and L-Glu may also subserve hormonal and paracrine signaling. Disruption in gastrointestinal function following perturbation of enteric GABA receptors and glutamate receptors presents potential new target sites for drug development.

Intracerebroventricular administration of MK-801 increases food intake through mechanisms independent of gastric emptying

Systemic or hindbrain administration of MK-801, a noncompetitive N-methyl-D-aspartate receptor antagonist, increases meal size. To examine whether MK-801 enhances intake by increasing gastric emptying, we administered MK-801 (2.0 microg/3.0 microl) into the fourth ventricle [intracerebroventricular (ICV)] and measured feeding and gastric emptying of 5-ml NaCl or 15% sucrose loads. In a parallel experiment, we examined food intake and gastric emptying following intraperitoneal (IP) injection of MK-801 (100 microg/kg). MK-801, either IP or ICV, increased 30-min sucrose intake compared with control (12.3 +/- 0.7 vs. 9.8 +/- 0.5 and 16.6 +/- 2.0 vs. 10.7 +/- 0.7 ml, for IP and ICV administration, respectively). Also, IP MK-801 increased 5-min gastric emptying of NaCl (4.13 +/- 0.1 ml emptied) and sucrose (3.11 +/- 0.1 ml emptied) compared with control (3.75 +/- 0.2 and 2.28 +/- 0.1 ml emptied for NaCl and sucrose loads, respectively). In contrast, ICV MK-801 did not alter NaCl emptying (3.82 +/- 0.1 ml emptied) compared with control (3.82 +/- 0.3 ml emptied) and actually reduced gastric emptying of sucrose (2.1 +/- 0.2 and 2.94 +/- 0.1 ml emptied, for MK and vehicle, respectively). These data confirm previous results that systemic as well as hindbrain injection of MK-801 increases food intake. However, because ICV MK-801 failed to increase gastric emptying, these results indicate that MK-801 increases food intake through mechanisms independent of altered gastric emptying.

Gastrointestinal mechanisms of satiation for food

Satiation for food comprises the physiological processes that result in the termination of eating. Satiation is evoked by physical and chemical qualities of ingested food, which trigger afferent signals to the brain from multiple sites in the GI tract, including the stomach, the proximal small intestine, the distal small intestine and the colon. The physiological nature of each signal's contribution to satiation and overall control of food intake is likely to vary, depending on the level of the GI tract from which the signal arises. This article is a critical, though non-exhaustive, review of our current understanding of the mechanisms and adaptive value of satiation signals from the stomach and intestine.

Response properties of antral mechanosensitive afferent fibers and effects of ionotropic glutamate receptor antagonists

The ionotropic glutamate receptors N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are present peripherally in the primary sensory afferent neurons innervating the viscera. Multiple studies have reported roles of glutamate receptors in gastric functions. However, no study has previously shown the direct influence of ionotropic glutamate receptor antagonist on vagal sensory neurons. The objective of this study was to investigate the effects of NMDA and AMPA receptor antagonists on mechanotransduction properties of vagal afferent fibers innervating the rat stomach. Action potentials were recorded from the hyponodal vagus nerve innervating the antrum of the Long-Evans rats. For antral distension (AD), a small latex balloon was inserted into the stomach and positioned in the antrum. The antral contractions were recorded with solid-state probe inserted into the water-filled balloon. Antral units were identified to isovolumic (0.2-1 ml) or isobaric AD (5-60 mm Hg). NMDA and AMPA receptor antagonists were injected in a cumulative fashion (1-100 micromol/kg, i.v.). After the conclusion of experiment, the abdomen was opened and receptive field was mapped by probing the serosa of the stomach. Thirty-two fibers were identified to AD. The receptive fields of 26 fibers were located in the posterior part of the antrum. All fibers exhibited spontaneous firing (mean: 7.00+/-0.97 impulses/s). Twenty fibers exhibited a rhythmic firing that was in phase with antral contractions, whereas 12 fibers exhibited non-rhythmic spontaneous firing unrelated to spontaneous antral contraction. Both groups of fibers exhibited a linear increase in responses to graded isovolumic or isobaric distensions. NMDA (memantine HCl and dizocilpine (MK-801)) and AMPA/kainate (6-cyano-7-nitroquinoxaline 2,3-dione; CNQX) receptor antagonists dose-dependently attenuated the mechanotransduction properties of these fibers to AD. However, competitive NMDA antagonist dl-2-amino-5 phosphopentanoic acid (AP-5) had no effect. The study documents that glutamate receptor antagonists can attenuate responses of gastric vagal sensory afferent fibers innervating the distal stomach, offering insight to potential pharmacological agents in the treatment of gastric disorders.

NMDA receptor blockade attenuates CCK-induced reduction of real feeding but not sham feeding

Systemic injection of MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor ion channels, increases meal size and delays satiation. We examined whether MK-801 increases food intake by directly interfering with actions of cholecystokinin (CCK). Prior administration of MK-801 (100 microg/kg ip) reversed the inhibitory effects of CCK-8 (2 and 4 microg/kg ip) on real feeding of both liquid and solid foods. MK-801 alone did not alter 30-min sham intake of 15% sucrose compared with intake after saline. Furthermore, while CCK-8 (2 or 4 microg/kg ip) reduced sham intake, this reduction was not attenuated by MK-801 pretreatment. To ascertain whether MK-801 attenuation of CCK-induced reduction of real feeding was associated with attenuated inhibition of gastric emptying, we tested the effect of MK-801 pretreatment on CCK-induced inhibition of gastric emptying of 5-ml saline loads. Ten-minute gastric emptying was accelerated after MK-801 (3.9 +/- 0.2 ml) compared with saline vehicle (2.72 +/- 0.2 ml). CCK-8 (0.5 microg/kg ip) reduced 10-min emptying to 1.36 +/- 0.3 ml. Pretreatment with MK-801 did not significantly attenuate CCK-8-induced reduction of gastric emptying (0.9 +/- 0.4 ml). This series of experiments demonstrates that blockade of NMDA ion channels reverses inhibition of real feeding by CCK. However, neither inhibition of sham feeding nor inhibition of gastric emptying by CCK is attenuated by MK-801. Therefore, increased food intake after NMDA receptor blockade is not caused by a direct interference with CCK-induced satiation. Rather, increased real feeding, either in the presence or absence of CCK, depends on blockade of NMDA receptor participation in other post-oral feedback signals such as gastric sensation or gastric tone.

Cholinergic neurotransmission participates in increased food intake induced by NMDA receptor blockade

MK-801, a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, enhances gastric emptying while increasing food intake. Although our previously reported results implicate the vagus in MK-801's effect on feeding, it is not clear whether vagal motor fibers participate in the feeding response. Control of gastric emptying is exerted, in part, by cholinergic vagal motor neurons. Therefore, we examined the ability of MK-801 to increase meal size in the presence or absence of the muscarinic receptor antagonist atropine methyl nitrate. Both central and systemic administration of MK-801 significantly increased intake of 15% sucrose. Intraperitoneal injection of atropine abolished MK-801-induced increase in sucrose intake, whereas administration into the fourth ventricle had no effect. To determine whether augmentation of cholinergic tone produces an enhancement of food intake in the absence of MK-801, we tested the ability of cisapride, a gastric prokinetic agent that promotes acetylcholine release through an action on presynaptic serotonin (5-HT4) receptors, to increase sucrose consumption. Cisapride (500 microg/kg ip) induced a small but significant increase in 15% sucrose intake (15.5 +/- 0.5 ml) compared with NaCl (13.0 +/- 0.6 ml). Furthermore, when MK-801 (100 microg/kg ip) was given in combination with cisapride, intake was significantly higher (19.8 +/- 0.9 ml) than following either agent given alone. Pretreatment with atropine abolished the cisapride-induced increase in intake (12.1 +/- 0.9 ml) as well as the increased intake induced by combining MK-801 and cisapride. These results suggest that blockade of NMDA-gated ion channels in the hindbrain increases food intake, in part, via a peripheral muscarinic cholinergic mechanism.

Feeding induced by microinjections of NMDA and AMPA-kainate receptor antagonists into ventral striatal and ventral pallidal areas of the pigeon

The participation of glutamatergic circuits of the ventral basal ganglia in feeding-related regulatory mechanisms has been extensively indicated in primate and rodent species. In avian species, it has been shown that ICV injections of MK-801 or of CNQX increase food intake and reduce the latency of feeding initiation in free-feeding pigeons. In the present study, the effects of local injections of MK-801 (6 nmol), CNQX (160 nmol) or vehicle (0.2 microl) into a number of ventral striatopallidal nuclei on feeding, drinking and non-ingestive (sleep, preening) spontaneous behaviors were investigated in free-feeding pigeons (Columba livia). Intense feeding responses associated with an increased duration of feeding behavior were consistently recorded after injections of MK-801 or CNQX into the medial two-thirds of the tuberculum olfactorium (TO), the ventral aspect of lobus parolfactorium (LPOv), or the ventral pallidum (VP). In contrast, the latency of feeding initiation was unaffected by these treatments. No changes in drinking, preening or sleep responses were observed after drug injections into the TO/LPOv/VP area. These data indicate that glutamate-mediated circuits in the TO/LPOv/VP area can play an inhibitory role in feeding behavior in this species, contributing to the conclusion of a feeding bout, thus delaying satiation processes, and that these effects may be mediated by AMPA and NMDA receptors. Additionally, our data support the notion that a region functionally and anatomically comparable to the mammalian accumbens shell may be present in the TO/LPOv/VP region of the pigeon, and that the existence of a glutamatergic circuit in the ventral striatum controlling feeding-related phenomena may represent a highly conserved attribute throughout the amniote's evolution.

Excitatory amino acid receptor subtype agonists induce feeding in the nucleus accumbens shell in rats: opioid antagonist actions and interactions with mu-opioid agonists

Administration of mu-opioid receptor subtype agonists into the nucleus accumbens shell elicits feeding which is dependent upon the normal function of mu-, delta- and kappa-opioid receptors, D(1) dopamine receptors and GABA(B) receptors in the nucleus accumbens shell for its full expression. Whereas the AMPA antagonist, DNQX administered into the nucleus accumbens shell elicits a transient, though intense feeding response, feeding is elicited by excitatory amino acid agonists administered into the lateral hypothalamus. The present study examined whether excitatory amino acid agonists elicited feeding following administration into the nucleus accumbens shell of rats, whether such feeding responses were altered by opioid antagonist pretreatment, and whether such feeding responses interacted with feeding elicited by mu-opioid agonists. Both AMPA (0.25-0.5 microg) and NMDA (1 microg) in the nucleus accumbens shell significantly and dose-dependently increased food intake over 4 h. Both feeding responses were blocked by naltrexone pretreatment in the nucleus accumbens shell. The mu-opioid agonist, [D-Ala(2),NMe-Phe(4),Gly-ol(5)]-enkephalin in the nucleus accumbens shell significantly increased food intake which was significantly enhanced by AMPA cotreatment. This enhanced feeding response was in turn blocked by pretreatment with either general or mu-selective opioid antagonists. In contrast, cotreatment of NMDA and the mu-opioid agonist in the nucleus accumbens shell elicited feeding which was significantly less than that elicited by either treatment alone. These data indicate the presence of important interactions between excitatory amino acid receptors and mu-opioid receptors in the nucleus accumbens shell in mediating feeding responses in nondeprived, ad libitum-fed rats.

Food-related gastrointestinal signals activate caudal brainstem neurons expressing both NMDA and AMPA receptors

Vagal mechano- and chemosensors in the gastrointestinal tract and the portal-hepatic axis signaling the arrival of nutrients are major determinants of the satiation process. Although glutamate and its various receptor subtypes have been shown to transmit gustatory and cardiovascular sensory information at the level of the solitary nucleus (nucleus tractus solitarius; NTS), their involvement in the transmission of gastrointestinal satiety signals is not clear. Gastrointestinal sensors were stimulated by gastric balloon distension or by intraduodenal infusion of either linoleic acid or glucose in chronically catheterized, non-anesthetized rats, leading to activation of second order neurons in the NTS as detected by c-Fos immunohistochemistry. Subsequent (double)-immunohistochemistry for either NMDA or AMPA glutamate receptors was used to determine the percentage of activated neurons expressing a particular receptor subtype. Gastric distension and duodenal nutrient stimuli produced slightly, but significantly different patterns of c-Fos induction in the dorsal vagal complex. Expression of NMDA receptors, as detected by a NR2ab subunit-specific antibody, was abundant throughout the dorsal medulla. The percentage of neurons in the NTS activated by gastric distension (63.9+/-2.9%), linoleic acid (62.8+/-1.4%), and glucose (64.1+/-1.4%), expressing NMDA receptor was similar. Expression of AMPA receptors, as detected by a GLUR2/3 subunit-specific antibody, was equally abundant throughout the dorsal medulla. Again, the percentage of activated neurons expressing GLUR2/3 was similar for the gastric distension (59.8-65.6%) and duodenal linoleic acid (60.6-67.0%) stimuli, and for the various subnuclei of the NTS. Finally, GLUR1-specific immunoreactivity was much less abundant, with only a small percentage of distension-activated (4.4+/-0.4%) and linoleic acid-activated (5.1+/-0.4%) neurons expressing this receptor subunit. The results suggest a widespread, general involvement of both NMDA and AMPA receptors in primary afferent signal transmission at the level of the NTS, with no differential recruitment of the examined receptor subtypes by the different gastrointestinal sensory stimuli. This may indicate a high degree of convergence among sensory signals, or alternatively, the presence of other transmission systems such as peptides referring sensory specificity to second order neurons.

Locus coeruleus noradrenergic lesions attenuate intraoral intake

I.p. injections of DSP-4 in male rats decreased norepinephrine (NE) levels to varying degrees throughout the brain with 66.7% reductions in the hypothalamic paraventricular nucleus. Intake of intraorally infused sucrose was reduced for 14 days but daily pellet intake recovered within 5 days post-injection. Intraventricular NE restored intraoral sucrose intake in DSP-4-lesioned rats without affecting controls. Intraventricular infusion of neuropeptide Y (NPY) reduced intraoral intake in controls but had no effect in DSP-4-lesioned rats. NPY markedly inhibited intraoral intake in DSP-4-treated rats that also received NE. These data confirm studies showing that NPY decreases consummatory ingestive behavior and suggest that this inhibition involves ascending noradrenergic projections from locus coeruleus.

MK801 increases feeding and decreases drinking in nondeprived, freely feeding rats

The noncompetitive NMDA receptor antagonist MK801 has been reported to increase food intake in rats during scheduled test meals of palatable foods or after food deprivation, but not in nondeprived rats given rodent chow. To determine if MK801 has an effect on spontaneous meals, MK801 (100 microg/kg) was administered 15 min prior to dark onset to nondeprived rats maintained on powdered rodent chow, and spontaneous food and water access was measured. MK801 increased the length of the first meal and the amount of time spent feeding within the meal. Conversely, MK801 decreased the length and size of the first drinking bout. MK801 did not alter the latency to the first meal or drinking bout, nor the intervals between successive meals or bouts. The effects of MK801 on feeding and drinking bouts were partially confirmed by measuring total chow and water intake over the first 2 h of the dark period. Thus, acute MK801 can significantly alter spontaneous chow feeding and drinking in nondeprived rats when administered prior to dark onset.

Additive satiety-delaying effects of capsaicin-induced visceral deafferentation and NMDA receptor blockade suggest separate pathways

Both ablation of visceral afferents and blockade of NMDA receptor-mediated glutamatergic transmission by MK-801 result in overconsumption of sucrose solution and other food, apparently by interrupting visceral signals and thus delaying satiation. If these two manipulations act on the same pathway, namely, the propagation of vagal afferent signals to NTS neurons, their effects would be expected to be non-additive. To test this hypothesis, two groups of rats - one with prior systemic capsaicin (n=11) and one with vehicle treatment (n=10) - were trained to drink 15% sucrose solution after 15 h food deprivation every 3-4 days, and then injected with MK-801 (100 microg/kg, i.p.) or saline. Both capsaicin and MK-801 produced the expected significant (P<.001) increase in 30 and 60 min sucrose intake if compared to their respective controls. Administration of MK-801 to capsaicin-treated rats further increased 60 min sucrose intake significantly (P<.001) in a fully additive fashion. These results suggest that the two treatments do not impinge on the same neural pathway to delay satiation. MK-801 may interfere with signals from capsaicin-resistant vagal afferents, or alternatively may act on other areas in the brain or periphery.