Dose-related stimulation of feeding by systemic injections of monosodium glutamate

Effects of "Bioactive" amino acids leucine, glutamate, arginine and tryptophan on feed intake and mRNA expression of relative neuropeptides in broiler chicks

Feed intake control is vital to ensuring optimal nutrition and achieving full potential for growth and development in poultry. The aim of the present study was to investigate the effects of L-leucine, L-glutamate, L-tryptophan and L-arginine on feed intake and the mRNA expression levels of hypothalamic Neuropeptide involved in feed intake regulation in broiler chicks. Leucine, glutamate, tryptophan or arginine was intra-cerebroventricularly (ICV) administrated to 4d-old broiler chicks respectively and the feed intake were recorded at various time points. Quantitative PCR was performed to determine the hypothalamic mRNA expression levels of Neuropeptide Y (NPY), agouti related protein (AgRP), pro-opiomelanocortin (POMC), melanocortin receptor 4 (MC4R) and corticotrophin releasing factor (CRF). Our results showed that ICV administration of L-leucine (0.15 or 1.5  μmol) significantly (P < 0.05) increased feed intake up to 2 h post-administration period and elevated both hypothalamic NPY and AgRP mRNA expression levels. In contrast, ICV administration of L-glutamate (1.6  μmol) significantly (P < 0.05) decreased feed intake 0.25, 0.5 and 2 h post-injection, and increased hypothalamic CRF and MC4R mRNA expression levels. Meanwhile, both L-tryptophan (10 or 100  μg) and L-arginine (20 or 200  μg) had no significant effect on feed intake. These findings suggested that L-leucine and L-glutamate could act within the hypothalamus to influence food intake, and that both orexigenic and anorexigenic Neuropeptide genes might contribute directly to these effects.

Circadian integration of sleep-wake and feeding requires NPY receptor-expressing neurons in the mediobasal hypothalamus

Sleep and feeding rhythms are highly coordinated across the circadian cycle, but the brain sites responsible for this coordination are unknown. We examined the role of neuropeptide Y (NPY) receptor-expressing neurons in the mediobasal hypothalamus (MBH) in this process by injecting the targeted toxin, NPY-saporin (NPY-SAP), into the arcuate nucleus (Arc). NPY-SAP-lesioned rats were initially hyperphagic, became obese, exhibited sustained disruption of circadian feeding patterns, and had abnormal circadian distribution of sleep-wake patterns. Total amounts of rapid eye movement sleep (REMS) and non-REMS (NREMS) were not altered by NPY-SAP lesions, but a peak amount of REMS was permanently displaced to the dark period, and circadian variation in NREMS was eliminated. The phase reversal of REMS to the dark period by the lesion suggests that REMS timing is independently linked to the function of MBH NPY receptor-expressing neurons and is not dependent on NREMS pattern, which was altered but not phase reversed by the lesion. Sleep-wake patterns were altered in controls by restricting feeding to the light period, but were not altered in NPY-SAP rats by restricting feeding to either the light or dark period, indicating that disturbed sleep-wake patterns in lesioned rats were not secondary to changes in food intake. Sleep abnormalities persisted even after hyperphagia abated during the static phase of the lesion. Results suggest that the MBH is required for the essential task of integrating sleep-wake and feeding rhythms, a function that allows animals to accommodate changeable patterns of food availability. NPY receptor-expressing neurons are key components of this integrative function.

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.

Glutamatergic control of food intake in pigeons: effects of central injections of glutamate, NMDA, and AMPA receptor agonists and antagonists

The possible involvement of glutamatergic mechanisms in the control of food intake was studied in free-feeding and in 24-h food-deprived (FD24) pigeons for 1 h after intracerebroventricular (i.c.v.) treatment with glutamate (Glu, 0, 50, 150, 300, and 600 nmol). Glu injections dose dependently induced decreases (30-65%) in food intake (FI) and feeding duration (FD), and increases in latency to start feeding (LSF) in FD24 animals, but not in free-feeding ones. None of these treatments affected noningestive behaviors (locomotion, sleep, and preening). In FD24 pigeons, i.c.v. treatments with N-methyl-D-aspartic acid (NMDA, 0.1, 1, 4, 8, or 16 nmol) or D,L-alpha-amino-3-hydroxy-isoxazole proprionic acid (AMPA, 0.1, 1, 4, or 8 nmol) decreased FI and FD, but left LSF unchanged compared to vehicle-treated FD24 controls. Kainic acid (0.1, 0.5, and 1 nmol), or [trans-(1S,3R)-ACPD-(5NH4OH)] (ACPD, 0.1, 1, 4, 8, and 16 nmol) left unchanged the ingestive profile of FD24 pigeons. Pretreatment with the NMDA receptor antagonist MK-801 (15 nmol) and the AMPA-kainate receptor antagonist CNQX (390 nmol), 20 min before an i.c.v. injection of Glu (300 nmol) induced a partial blockade of the Glu-induced decreases in FI and FD and completely inhibited the Glu-induced increase in LSF in FD24 pigeons. I.c.v. injections of MK-801 (30 nmol) and of CNQX (780 nmol) increased FI and FD and reduced LSF in free-feeding pigeons. A lower dose of MK-801 (15 nmol) increased FI and FD, but not LSF. Conversely, a lower dose of CNQX (390 nmol) reduced LSF without changing FI or FD. These findings indicate the involvement of Glu as a chemical mediator in the regulation of food intake in the pigeon, possibly acting on multiple central mechanisms in this species through NMDA- and AMPA-sensitive Glu receptors.

Glutamate and the UMAMI taste: sensory, metabolic, nutritional and behavioural considerations. A review of the literature published in the last 10 years

Monosodium glutamate (MSG) is used increasingly often in processed foods and in home cooking in the Western world. This substance is responsible for a pleasurable taste sensation, the Umami taste. This review covers recent developments in sensory studies of glutamate effects, and traces the Umami taste from sensory receptors on the tongue to the brain. The metabolism of glutamic acid, as revealed from recent literature, is described. A specific section is devoted to safety issues. In addition, effects of glutamic salts on nutrition and ingestive behaviours are shown to be potent. Animal and human works are treated separately, with special attention to the specific methods used in both cases. Future areas of research include further investigation of sensory physiology, role of glutamate as an excitatory substance in the brain, acquisition of food likes and impact on long-term food selection, food intake, and body weight control.

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

A role for excitatory amino acids in the control of feeding behavior has not been extensively investigated. Nevertheless, there is direct and circumstantial evidence to indicate that some circuits involved with feeding behavior include glutamatergic elements. To test the hypothesis that endogenous glutamate participates in the control of food intake, we performed experiments to determine whether MK-801, a non-competitive N-methyl-D-aspartate (NMDA) ion channel antagonist, is capable of altering intake of liquid and solid foods in hungry or satiated rats. Following a 16 h fast, intake of 15% sucrose was significantly enhanced by systemic treatment with MK-801. Water intake was not altered by the NMDA antagonist. Rats did not ingest more rat chow after MK-801, unless they had been fasted. When a more palatable food (cookies) was offered, MK-801 did increase intake. Thus MK-801 enhanced food intake only when feeding was initiated by food-deprivation or increased palatability. In conclusion, our results support the hypothesis that endogenous glutamate plays a role in the control of food intake. Blockade of NMDA receptor function by MK-801 may diminish or delay satiety signals, rather than initiate feeding behavior per se.

The lateral hypothalamus: a primary site mediating excitatory amino acid-elicited eating

Lateral hypothalamic (LH) injections of the excitatory neurotransmitter glutamate, or its excitatory amino acid (EAA) agonists, kainic acid (KA), D,L-alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA), or N-methyl-D-aspartic acid (NMDA), can rapidly elicit an intense feeding response in satiated rats. To determine whether the LH is the actual locus of this effect, we compared these compounds' ability to stimulate feeding when injected into the LH, versus when injected into sites bracketing this region. Food intake in groups of adult male rats was measured 1 h after injection of glutamate (30-900 nmol), KA (0.1-1.0 nmol), AMPA (0.33-3.3 nmol), NMDA (0.33-33.3 nmol) or vehicle, through chronically implanted guide cannulas, into one of seven brain sites. These sites were: the LH, the anterior and posterior tips of the LH, the thalamus immediately dorsal to the LH, the amygdala just lateral to the LH, or the paraventricular and perifornical areas medial to the LH. The results show that across doses and agonists the eating-stimulatory effects were largest with injections into the LH. In the LH, glutamate between 300 and 900 nmol elicited a dose-dependent eating response of up to 5 g within 1 h (P < 0.01). Each of the other agonists at doses of 3.3 nmol or less elicited eating responses of at least 10 g with injections into this site. Injections into the other brain sites produced either no eating, or occasionally smaller and less consistent eating responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Lateral hypothalamic injections of glutamate, kainic acid, D,L-alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid or N-methyl-D-aspartic acid rapidly elicit intense transient eating in rats

A convergence of evidence suggests that stimulation of lateral hypothalamic (LH) neurons can elicit eating, but the neurotransmitters that mediate this effect are unknown. To determine whether glutamate might be involved, it was injected through chronic guide cannulas directly into the LH of satiated adult male rats and consequent food intake was measured. Glutamate produced a dose-dependent eating response (mean intakes of 3.7 g at 300 nmol and 5.2 g at 900 nmol) only within the first hour after injection. As a first step in determining the receptor types mediating this response, agonists for specific excitatory amino acid (EAA) receptors were similarly tested. Kainic acid (KA), D,L-alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) or N-methyl-D-aspartic acid (NMDA) injected into the LH each elicited eating in a dose-dependent fashion beginning at 0.33 to 1.0 nmol. At maximally effective doses (1.0-33 nmol), each agonist elicited food intakes of approximately nine grams within 1 h. Finally, analysis of meal and behavioral patterns produced by LH injection of glutamate (600 nmol) and KA (1.0 nmol) revealed that the elicited eating usually began 2-3 min postinjection and consisted of a single normal to large size meal. There were no other behavioral effects during this initial postinjection period and no effects on other oral behaviors, like drinking or gnawing, at any time. Collectively, these findings suggest that glutamate may act through several subtypes of its receptors on some LH neurons to elicit eating.

Central effects of monosodium glutamate on feeding behavior in adult Long-Evans rats

Monosodium glutamate (MSG) is known as a neurotoxic molecule when injected neonatally in rats, where it produces a marked decrease in food intake and an increase in adipose tissue mass. But, in adult rats subcutaneous injections of MSG produce a small, dose-dependent increase in food intake. It is not known if this action is centrally or systemically mediated. Therefore, the feeding pattern of adult rats injected intracerebroventricularly with MSG was measured. Seven days after installation of a cannula in the right lateral ventricle, rats were injected either with artificial cerebrospinal fluid or twice with 3 mg/brain MSG within a 3-day interval. The feeding pattern was recorded via a complete computerized system during 24 h. Feeding behavior was significantly modified by MSG treatments. These effects were observed immediately after drug injections, that is, upon the first meal, as well as during the 24 h that followed. For the first meal, modifications in meal size (+285%; p = 0.0001), meal duration (x10; p = 0.0005), postmeal interval (x4; p = 0.0005), and the satiety ratio (-50%; p = 0.01) were observed. During the 24-h postinjection period, modifications in meal number (-3; p = 0.0007), total amount of food eaten (+21%,; p = 0.007), time spent eating (+40%; p = 0.007), meal duration (+53%; p = 0.005), and meal size (+44%; p = 0.01) were noted. When the two MSG injections were compared, differences were also noted. For the first meal, postmeal interval (-50%; p < 0.005) and satiety ratio (-50%; p < 0.005) were decreased after the second injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of male sexual behavior by serotonin application in the medial preoptic area

The study investigated the possible involvement of serotonin in the medial preoptic area in the regulation of sexual behavior of male rats. Injection of serotonin in the medial preoptic area resulted in an inhibition, whereas cyproheptadine (a serotonin antagonist) produced a slight facilitation, of male sexual behavior.

Dissociation of opioid and nonopioid analgesic responses following adult monosodium glutamate pretreatment

Neonatal administration of monosodium glutamate (MSG: 2-4 mg/g, SC) selectively destroys circumventricular organs, especially the arcuate nucleus and median eminence of the hypothalamus, and also attenuates both nonopioid (continuous cold-water swim: CCWS) and opioid (morphine) analgesia when rats are tested as adults. The present study evaluated whether administration of MSG (1-6 g/kg, SC) or its equiosmotic control (2.37 M NaCl) to adult rats altered either basal nociception on the tail-flick and jump tests or analgesia following morphine (5 mg/kg, SC) or CCWS (2 degrees C for 3.5 min). MSG treatment dose-dependently produced small but significant increases in basal nociceptive thresholds in adult rats. Morphine analgesia was significantly reduced on both tests following pretreatment with MSG (30-32%) and hypertonic NaCl (17-25%). In contrast, MSG (55-247%), but not NaCl pretreatment potentiated both nonopioid CCWS analgesia on both tests and CCWS hypothermia. These data are discussed in terms of differential neonatal and adult MSG effects, dissociations between opioid and nonopioid pain-inhibition, and the role of MSG in altering adaptive mechanisms to environmental stressors.

Area postrema lesions block feeding induced by systemic injections of monosodium glutamate

Glutamate is an amino acid neurotransmitter capable of producing widespread receptor-mediated neuronal excitation. Recently we reported that high doses of monosodium glutamate (MSG) given systemically stimulate food intake in a dose-related fashion. Since glutamate does not cross the blood-brain barrier, it seems possible that feeding was stimulated by an action of glutamate on neurons within circumventricular organs (CVOs), areas of the brain in which the blood-brain barrier is deficient. In this experiment, we tested the hypothesis that systemic MSG stimulates feeding by an action on the area postrema (AP), a CVO in the caudal hindbrain. AP-lesioned rats (APLs) and sham-operated controls (shams) were injected with saline or MSG (2 and 6 g/kg, SC, one dose per week). Food intake was measured for 3 hr immediately following the injection. Shams increased their food intake significantly in response to both doses of MSG, but APLs did not. This result suggests that systemic glutamate may stimulate feeding by an action on the AP.