NMDA channels control meal size via central vagal afferent terminals

Leptin Sensitizes NTS Neurons to Vagal Input by Increasing Postsynaptic NMDA Receptor Currents

Leptin signaling within the nucleus of the solitary tract (NTS) contributes to the control of food intake, and injections of leptin into the NTS reduce meal size and increase the efficacy of vagus-mediated satiation signals. Leptin receptors (LepRs) are expressed by vagal afferents as well as by a population of NTS neurons. However, the electrophysiological properties of LepR-expressing NTS neurons have not been well characterized, and it is unclear how leptin might act on these neurons to reduce food intake. To address this question, we recorded from LepR-expressing neurons in horizontal brain slices containing the NTS from male and female LepR-Cre X Rosa-tdTomato mice. We found that the vast majority of NTS LepR neurons received monosynaptic innervation from vagal afferent fibers and LepR neurons exhibited large synaptic NMDA receptor (NMDAR)-mediated currents compared with non-LepR neurons. During high-frequency stimulation of vagal afferents, leptin increased the size of NMDAR-mediated currents, but not AMPAR-mediated currents. Leptin also increased the size of evoked EPSPs and the ability of low-intensity solitary tract stimulation to evoke action potentials in LepR neurons. These effects of leptin were blocked by bath applying a competitive NMDAR antagonist (DCPP-ene) or by an NMDAR channel blocker applied through the recording pipette (MK-801). Last, feeding studies using male rats demonstrate that intra-NTS injections of DCPP-ene attenuate reduction of overnight food intake following intra-NTS leptin injection. Our results suggest that leptin acts in the NTS to reduce food intake by increasing NMDAR-mediated currents, thus enhancing NTS sensitivity to vagal inputs.SIGNIFICANCE STATEMENT Leptin is a hormone that critically impacts food intake and energy homeostasis. The nucleus of the solitary tract (NTS) is activated by vagal afferents from the gastrointestinal tract, which promotes termination of a meal. Injection of leptin into the NTS inhibits food intake, while knockdown of leptin receptors (LepRs) in NTS neurons increases food intake. However, little was known about how leptin acts in the NTS neurons to inhibit food intake. We found that leptin increases the sensitivity of LepR-expressing neurons to vagal inputs by increasing NMDA receptor-mediated synaptic currents and that NTS NMDAR activation contributes to leptin-induced reduction of food intake. These findings suggest a novel mechanism by which leptin, acting in the NTS, could potentiate gastrointestinal satiation signals.

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.

Plasticity of vagal afferent signaling in the gut

Vagal sensory neurons mediate the vago-vagal reflex which, in turn, regulates a wide array of gastrointestinal functions including esophageal motility, gastric accommodation and pancreatic enzyme secretion. These neurons also transmit sensory information from the gut to the central nervous system, which then mediates the sensations of nausea, fullness and satiety. Recent research indicates that vagal afferent neurons process non-uniform properties and a significant degree of plasticity. These properties are important to ensure that vagally regulated gastrointestinal functions respond rapidly and appropriately to various intrinsic and extrinsic factors. Similar plastic changes in the vagus also occur in pathophysiological conditions, such as obesity and diabetes, resulting in abnormal gastrointestinal functions. A clear understanding of the mechanisms which mediate these events may provide novel therapeutic targets for the treatment of gastrointestinal disorders due to vago-vagal pathway malfunctions.

Inhibition of glycine transporter-1 in the dorsal vagal complex improves metabolic homeostasis in diabetes and obesity

Impaired glucose homeostasis and energy balance are integral to the pathophysiology of diabetes and obesity. Here we show that administration of a glycine transporter 1 (GlyT1) inhibitor, or molecular GlyT1 knockdown, in the dorsal vagal complex (DVC) suppresses glucose production, increases glucose tolerance and reduces food intake and body weight gain in healthy, obese and diabetic rats. These findings provide proof of concept that GlyT1 inhibition in the brain improves glucose and energy homeostasis. Considering the clinical safety and efficacy of GlyT1 inhibitors in raising glycine levels in clinical trials for schizophrenia, we propose that GlyT1 inhibitors have the potential to be repurposed as a treatment of both obesity and diabetes.

N-methyl-d-aspartate receptor coagonist d-serine suppresses intake of high-preference food

d-Serine is abundant in the forebrain and physiologically important for modulating excitatory glutamatergic neurotransmission as a coagonist of synaptic N-methyl-d-aspartate (NMDA) receptor. NMDA signaling has been implicated in the control of food intake. However, the role of d-serine on appetite regulation is unknown. To clarify the effects of d-serine on appetite, we investigated the effect of oral d-serine ingestion on food intake in three different feeding paradigms (one-food access, two-food choice, and refeeding after 24-h fasting) using three different strains of male mice (C57Bl/6J, BKS, and ICR). The effect of d-serine was also tested in leptin signaling-deficient db/db mice and sensory-deafferented (capsaicin-treated) mice. The expression of orexigenic neuropeptides [neuropeptide Y (Npy) and agouti-related protein (Agrp)] in the hypothalamus was compared in fast/refed experiments. Conditioned taste aversion for high-fat diet (HFD) was tested in the d-serine-treated mice. Under the one-food-access paradigm, some of the d-serine-treated mice showed starvation, but not when fed normal chow. HFD feeding with d-serine ingestion did not cause aversion. Under the two-food-choice paradigm, d-serine suppressed the intake of high-preference food but not normal chow. d-Serine also effectively suppressed HFD intake but not normal chow in db/db mice and sensory-deafferented mice. In addition, d-serine suppressed normal chow intake after 24-h fasting despite higher orexigenic gene expression in the hypothalamus. d-Serine failed to suppress HFD intake in the presence of L-701,324, the selective and full antagonist at the glycine-binding site of the NMDA receptor. Therefore, d-serine suppresses the intake of high-preference food through coagonism toward NMDA receptors.

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.

Vagal afferent NMDA receptors modulate CCK-induced reduction of food intake through synapsin I phosphorylation in adult male rats

Vagal afferent nerve fibers transmit gastrointestinal satiation signals to the brain via synapses in the nucleus of the solitary tract (NTS). Despite their pivotal role in energy homeostasis, little is known about the cellular mechanisms enabling fleeting synaptic events at vagal sensory endings to sustain behavioral changes lasting minutes to hours. Previous reports suggest that the reduction of food intake by the satiation peptide, cholecystokinin (CCK), requires activation of N-methyl-D-aspartate-type glutamate receptors (NMDAR) in the NTS, with subsequent phosphorylation of ERK1/2 (pERK1/2) in NTS vagal afferent terminals. The synaptic vesicle protein synapsin I is phosphorylated by pERK1/2 at serines 62 and 67. This pERK1/2-catalyzed phosphorylation increases synaptic strength by increasing the readily releasable pool of the neurotransmitter. Conversely, dephosphorylation of serines 62 and 67 by calcineurin reduces the size of the readily releasable transmitter pool. Hence, the balance of synapsin I phosphorylation and dephosphorylation can modulate synaptic strength. We postulated that CCK-evoked activation of vagal afferent NMDARs results in pERK1/2-catalyzed phosphorylation of synapsin I in vagal afferent terminals, leading to the suppression of food intake. We found that CCK injection increased the phosphorylation of synapsin I in the NTS and that this increase is abolished after surgical or chemical ablation of vagal afferent fibers. Furthermore, fourth ventricle injection of an NMDAR antagonist or the mitogen-activated ERK kinase inhibitor blocked CCK-induced synapsin I phosphorylation, indicating that synapsin phosphorylation in vagal afferent terminals depends on NMDAR activation and ERK1/2 phosphorylation. Finally, hindbrain inhibition of calcineurin enhanced and prolonged synapsin I phosphorylation and potentiated reduction of food intake by CCK. Our findings are consistent with a mechanism in which NMDAR-dependent phosphorylation of ERK1/2 modulates satiation signals via synapsin I phosphorylation in vagal afferent endings.

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.

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.

Neural proliferation and restoration of neurochemical phenotypes and compromised functions following capsaicin-induced neuronal damage in the nodose ganglion of the adult rat

We previously reported that neuronal numbers within adult nodose ganglia (NG) were restored to normal levels 60 days following the capsaicin-induced destruction of nearly half of the neuronal population. However, the nature of this neuronal replacement is not known. Therefore, we aimed to characterize neural proliferation, neurochemical phenotypes, and functional recovery within adult rat NG neurons following capsaicin-induced damage. Sprague-Dawley rats received intraperitoneal injections of capsaicin or vehicle solution, followed by 5-bromo-2-deoxyuridine (BrdU) injections to reveal cellular proliferation. NG were collected at multiple times post-treatment (up to 300 days) and processed for immunofluorescence, RT-PCR, and dispersed cell cultures. Capsaicin-induced cellular proliferation, indicated by BrdU/Ki-67-labeled cells, suggests that lost neurons were replaced through cell division. NG cells expressed the stem cell marker, nestin, indicating that these ganglia have the capacity to generate new neurons. BrdU-incorporation within β-III tubulin-positive neuronal profiles following capsaicin suggests that proliferating cells matured to become neurons. NG neurons displayed decreased NMDAR expression up to 180-days post-capsaicin. However, both NMDAR expression within the NG and synaptophysin expression within the central target of NG neurons, the NTS, were restored to pre-injury levels by 300 days. NG cultures from capsaicin-treated rats contained bipolar neurons, normally found only during development. To test the functional recovery of NG neurons, we injected the satiety molecule, CCK. The effect of CCK on food intake was restored by 300-days post-capsaicin. This restoration may be due to the regeneration of damaged NG neurons or generation of functional neurons that replaced lost connections.

Gut-brain signalling: how lipids can trigger the gut

The gut plays a unique role in the metabolic defence against energy excess and glucose imbalance. Nutrients, such as lipids, enter the small intestine and activate sensing mechanisms to maintain energy and glucose homeostasis. It is clear that a lipid-induced gut-brain axis exists and that cholecystokinin and a neuronal network are involved, yet the underlying mechanisms in gut lipid sensing that regulate homeostasis remain largely unknown. In parallel, studies underscore the importance of enzymes involved in lipid metabolism within the brain, such as adenosine monophosphate -activated protein kinase, to maintain homeostasis. In this review, we will first examine what is known regarding the mechanisms involved in this lipid-induced gut-brain neuronal axis that regulate food intake and hepatic glucose production. We will also discuss how enzymes that govern brain lipid metabolism could potentially reveal how lipids trigger the gut, and that both the gut and brain may share common biochemical pathways to sense lipids.

Transient receptor potential A1 increase glutamate release on brain stem neurons

Low temperature and noxious chemicals activate transient receptor potential (TRP) A1 channel in spinal nociceptive neurons. TRPA1 ligands are found as common ingredients of pungent spices and can trigger autonomic reflexes and alter blood pressure. Recent discovery of TRPA1 on visceral afferent pathway suggests such nonnociceptive responses may be closely related to TRPA1. To understand potential contributions of TRPA1 on autonomic response, we have tested TRPA1 agonist allyl isothiocyanate (AITC) effects on mechanically dispersed nucleus tractus solitarii neurons. AITC of 200 microM increased glutamate release in 58% of tested neurons and TRP channel antagonist ruthenium red (50 microM) blocked the AITC actions. tetrodotoxin and Cd did not eliminate AITC-evoked glutamate release increase. TRPA1 expressed in capsaicin-sensitive and insensitive neurons.

Repeated binge access to a palatable food alters feeding behavior, hormone profile, and hindbrain c-Fos responses to a test meal in adult male rats

Repetitive cycles of palatable food access and chronic calorie restriction alter feeding behaviors and forebrain neural systems. The purpose of this study was to determine the behavioral, endocrine, and meal-related hindbrain neural activation in adult male Sprague-Dawley rats exposed to a binge-access feeding schedule. The binge-access schedule consisted of repeated twice-per-week episodes of acute calorie restriction (to one-third of the previous day's intake) followed by 2 h of concurrent access to high-calorie palatable food (sweetened fat: 90% vegetable shortening-10% sucrose) and chow. The binge-access rats consumed more calories during the "binge" period than rats with continuous access to sweetened fat (continuous-access group) or subjected to repeated acute calorie restriction only (chow-restricted group). The binge-access group also exhibited a approximately 25% increase in sweetened fat intake from week 1 to week 6. Persistence of the binge phenotype in the binge-access animals was demonstrated 2 wk, but not 4 wk, after ad libitum chow. The binge-access and chow-restricted groups maintained a similar normal body composition and hormonal profiles, whereas the continuous-access animals developed an obese phenotype. Terminal ghrelin levels were significantly higher in the binge-access group than in the continuous-access group. Consumption of a standardized meal resulted in more c-Fos-positive cells along the anterior-posterior nucleus of the solitary tract regions in the binge-access group than in naive controls. These results suggest that repeated cycles of acute calorie restriction followed by palatable food produce physiological alterations that may facilitate overconsumption of a highly palatable food during limited-access periods.

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.

A novel procedure for assessing the effects of drugs on satiation in baboons: effects of memantine and dexfenfluramine

RATIONALE

Procedures for studying the effects of medications on satiation will assist the development of obesity medications.

OBJECTIVES

Develop a procedure for measuring satiation during consumption of bland and highly palatable food and determine the effect of acute intramuscular administration of dexfenfluramine (DFEN), which increases serotonin levels, and memantine (MEM), which blocks N-methyl-D: -aspartate receptors.

MATERIALS AND METHODS

A modified progressive ratio (PR) procedure was used to track changes in reinforcing strength when a food was consumed. The response requirement increased after each reinforcement, and reinforcing strength was estimated using the breakpoint (BP), which was the last completed response cost. There was one preferred food (sweet candy) and one chow pellet PR session per week. During each session, four male and four female adult baboons experienced three 1-h PR trials, separated by 30 min. Chow pellets were available at all other times. We examined the BP for one to 20 candies or chow pellets. Drug effects were examined when baboons had access to one and ten candies or chow pellets.

RESULTS

BPs for candy were greater than for pellets. Varying the pellet/candy pieces per delivery produced an inverted U-shaped function on the first trial, i.e., maximal BP was observed for three items, and the BP for multiple items, but not a single item, decreased across trials, i.e., BP decreased with food intake and satiation. DFEN and MEM decreased responding with the greatest effects at ten deliveries, suggesting that DFEN and MEM enhanced satiation.

CONCLUSION

Drugs that enhance satiation for several types of food may be particularly effective for decreasing food intake.

Presynaptic melanocortin-4 receptors on vagal afferent fibers modulate the excitability of rat nucleus tractus solitarius neurons

The nucleus tractus solitarius (NTS) integrates visceral sensory signals with information from the forebrain to control homeostatic functions, including food intake. Melanocortin 3/4 receptor (MC3/4R) ligands administered directly to the caudal brainstem powerfully modulate meal size but not frequency, suggesting the enhancement of visceral satiety signals. Using whole-cell recordings from rat brainstem slices, we examined the effects of melanocortin ligands, alpha-melanocyte-stimulating hormone (alphaMSH) and melanotan II (MTII), on EPSC in NTS neurons. Thirty-two percent of NTS neurons responded to perfusion with MTII or alphaMSH with either an increase (24%) or a decrease (8%) in the frequency, but not amplitude, of spontaneous EPSCs; the effects of MTII were abolished by pretreatment with SHU9119. After surgical vagal deafferentation, only four of 34 (9%) NTS neurons responded to MTII with an increase in EPSC frequency. When EPSCs were evoked by electrical stimulation of the tractus solitarius in Krebs' solution with 2.4 mm Ca(2+)(e), alphaMSH and MTII increased the amplitude in six of the 28 neurons tested, decreased amplitude in 14 with no effect in the remaining eight neurons. In four of six neurons unresponsive to MTII, decreasing Ca(2+)(e) levels to 1.5 mM uncovered an excitatory effect of MTII on EPSC amplitude. Reverse transcription-PCR analysis revealed the presence of MC4R, but not MC3R, in nodose ganglia. These results show that MC4R signaling leads mainly to presynaptic modulation of glutamatergic synaptic transmission and suggest that melanocortinergic-induced decrease of food intake may occur via enhancement of vagal afferent satiation signals from the gastrointestinal tract.

Do serotonin1–7 receptors modulate short and long-term memory?

Evidence from invertebrates to human studies indicates that serotonin (5-hydroxytryptamine; 5-HT) system modulates short- (STM) and long-term memory (LTM). This work is primarily focused on analyzing the contribution of 5-HT, cholinergic and glutamatergic receptors as well as protein synthesis to STM and LTM of an autoshaping learning task. It was observed that the inhibition of hippocampal protein synthesis or new mRNA did not produce a significant effect on autoshaping STM performance but it did impair LTM. Both non-contingent protein inhibition and 5-HT depletion showed no effects. It was basically the non-selective 5-HT receptor antagonist cyproheptadine, which facilitated STM. However, the blockade of glutamatergic and cholinergic transmission impaired STM. In contrast, the selective 5-HT(1B) receptor antagonist SB-224289 facilitated both STM and LTM. Selective receptor antagonists for the 5-HT(1A) (WAY100635), 5-HT(1D) (GR127935), 5-HT(2A) (MDL100907), 5-HT(2C/2B) (SB-200646), 5-HT(3) (ondansetron) or 5-HT(4) (GR125487), 5-HT(6) (Ro 04-6790, SB-399885 and SB-35713) or 5-HT(7) (SB-269970) did not impact STM. Nevertheless, WAY100635, MDL100907, SB-200646, GR125487, Ro 04-6790, SB-399885 or SB-357134 facilitated LTM. Notably, some of these changes shown to be independent of food-intake. Concomitantly, these data indicate that '5-HT tone via 5-HT(1B) receptors' might function in a serial manner from STM to LTM, whereas working in parallel using 5-HT(1A), 5-HT(2A), 5-HT(2B/2C), 5-HT(4), or 5-HT(6) receptors.

N-methyl-D-aspartate receptor subunit phenotypes of vagal afferent neurons in nodose ganglia of the rat

Most vagal afferent neurons in rat nodose ganglia express mRNA coding for the NR1 subunit of the heteromeric N-methyl-D-aspartate (NMDA) receptor ion channel. NMDA receptor subunit immunoreactivity has been detected on axon terminals of vagal afferents in the dorsal hindbrain, suggesting a role for presynaptic NMDA receptors in viscerosensory function. Although NMDA receptor subunits (NR1, NR2B, NR2C, and NR2D) have been linked to distinct neuronal populations in the brain, the NMDA receptor subunit phenotype of vagal afferent neurons has not been determined. Therefore, we examined NMDA receptor subunit (NR1, NR2B, NR2C, and NR2D) immunoreactivity in vagal afferent neurons. We found that, although the left nodose contained significantly more neurons (7,603), than the right (5,978), the proportions of NMDA subunits expressed in the left and right nodose ganglia were not significantly different. Immunoreactivity for NMDA NR1 subunit was present in 92.3% of all nodose neurons. NR2B immunoreactivity was present in 56.7% of neurons; NR2C-expressing nodose neurons made up 49.4% of the total population; NR2D subunit immunoreactivity was observed in just 13.5% of all nodose neurons. Double labeling revealed that 30.2% of nodose neurons expressed immunoreactivity to both NR2B and NR2C, whereas NR2B and NR2D immunoreactivities were colocalized in 11.5% of nodose neurons. NR2C immunoreactivity colocalized with NR2D in 13.1% of nodose neurons. Our results indicate that most vagal afferent neurons express NMDA receptor ion channels composed of NR1, NR2B, and NR2C subunits and that a minority phenotype that expresses NR2D also expresses NR1, NR2B, and NR2C.