Consensus meeting: monosodium glutamate - an update.

OBJECTIVE: Update of the Hohenheim consensus on monosodium glutamate from 1997: Summary and evaluation of recent knowledge with respect to physiology and safety of monosodium glutamate. DESIGN: Experts from a range of relevant disciplines received and considered a series of questions related to aspects of the topic. SETTING: University of Hohenheim, Stuttgart, Germany. METHOD: The experts met and discussed the questions and arrived at a consensus. CONCLUSION: Total intake of glutamate from food in European countries is generally stable and ranged from 5 to 12 g/day (free: ca. 1 g, protein-bound: ca. 10 g, added as flavor: ca. 0.4 g). L-Glutamate (GLU) from all sources is mainly used as energy fuel in enterocytes. A maximum intake of 6.000 [corrected] mg/kg body weight is regarded as safe. The general use of glutamate salts (monosodium-L-glutamate and others) as food additive can, thus, be regarded as harmless for the whole population. Even in unphysiologically high doses GLU will not trespass into fetal circulation. Further research work should, however, be done concerning the effects of high doses of a bolus supply at presence of an impaired blood brain barrier function. In situations with decreased appetite (e.g., elderly persons) palatability can be improved by low dose use of monosodium-L-glutamate.

The new role of pharmacotherapy for weight reduction in obesity.

Obesity is associated with an increased risk for a wide variety of chronic health conditions. Despite this fact, less than half of obese patients are advised by healthcare professionals to lose weight. Creating a viable plan for losing weight and maintaining weight loss is difficult. Lifestyle change is always the cornerstone of treatment, but two new therapeutic agents approved for long-term use, sibutramine and orlistat, can help maximise success. Increased weight loss can lead to reductions in the risk of obesity-related co-morbidities. Sibutramine and orlistat offer new weight reduction opportunities for obese patients.

5,7-Dihydroxytryptamine: regional brain concentrations following intraventricular administration to rats.

The concentrations of 5,7-dihydroxytryptamine (5,7-DHT) and serotonin (5-HT) were measured in brainstem, hypothalamus and cerebral cortex 0, 2, 6, 12, and 24 hours following the bilateral, lateral ventricular injection of 5,7-DHT (100 microg/each ventricle) into adult male rats. At 6 hours, 5,7-DHT levels had decreased 99% from 0 hr values in all brain regions. Thereafter, 5,7-DHT levels continued to decline in cortex, but not in hypothalamus or brainstem; at 24 hr, but not 48 hr, 5,7-DHT peaks were still measurable in each brain region examined. Serotonin levels in all three regions also fell markedly by 2-6 hours after 5,7-DHT administration. At 24 hours, hypothalamus and brainstem 5HT levels had declined >70% and cerebral cortex approximately 50% below control values. The relevance of these findings to the protective action of monoamine reuptake blockers is discussed.

Diet, monoamine neurotransmitters and appetite control.

This article has attempted to point out some of the relationships between 5-HT and catecholamine (NE, DA) neurons in brain and the control of appetite and food intake. At least two bodies of evidence support this connection. The first is pharmacologic, and demonstrates that drugs that stimulate transmission across 5-HT and/or catecholamine synapses suppress hunger and food intake. The second is physiologic and metabolic, and reveals that the ingestion of foods, on either an acute (single meal) or chronic basis, can reliably modify the uptake of TRP and TYR into brain (and hypothalamus), and directly alter the synthesis of their transmitters (5-HT and the catecholamines, respectively). The synthesis of these two bodies of information has led to models by which (1) changes in dietary carbohydrate ingestion, by modifying brain TRP uptake and 5-HT production, may cause like changes in 5-HT release, and in the stimulation of 5-HT receptors in brain circuits that control carbohydrate appetite, and (2) dietary protein intake, by altering brain TYR uptake, directly influences DA and NE synthesis (notably in hypothalamus), perhaps providing a signal to brain circuits monitoring dietary protein adequacy regarding protein intake. In this case, one might imagine that stimulating DA and/or NE receptors in such circuits might suppress protein intake, a possibility we are now examining in rats. As indicated in the Introduction, the broader issue being touched upon in this article concerns the body's need to acquire and maintain an optimal (or adequate) nutritional balance (for growth and ultimately, reproductive success). Rats and humans evolved in an environment that does not provide continuous access to all essential nutrients, and one that presents nutrients in a complex matrix (other animals, plants) that can also include toxic compounds. Together with the fact that animals and humans do not carry a guidebook to healthy eating, we must presume that the brain mechanisms that have evolved to optimize the acquisition of essential nutrients are 'automatic' (i.e., not conscious) and quite complex. In this context, the relationships described here must be viewed as rudimentary, touching only a small portion of this complex regulatory mechanism. The hope is, as further insights develop, that we will gain a better understanding of the workings of these mechanisms, and also be able to apply this knowledge to the development of better pharmacologic (and other) aids for controlling appetite and obesity in our modern, man-made environment.

Monoamines and protein intake: are control mechanisms designed to monitor a threshold intake or a set point?

The concentration of TYR in brain changes directly with dietary protein content in the 0-10% PE range, but not higher. The effect is large: TYR concentrations rise as much as two- to threefold between 0% and 10% dietary protein content. This increase produces a clear stimulation of the rate of catecholamine synthesis, observed both for DA and NE, and notably in the hypothalamus, a brain area involved in appetite regulation. A similar relationship to chronic dietary protein intake may also exist for tryptophan and its neurotransmitter product, 5HT. Because the natural diet of rats, the animal model most commonly used in such studies, typically contains between 6% and 14% protein, and may contain less under unfavorable environmental circumstances, rats in the wild may frequently operate on the portion of the protein intake curve producing maximal changes in brain TYR (and perhaps TRP) concentrations. If so, then the production of catecholamines and 5HT may be similarly affected. By such a scenario, the brain might receive information regarding the animal's success in acquiring adequate amounts of protein in its diet. A similar argument can also be made for monkeys in the wild, based on their dietary habits, and thus possibly for humans. From this perspective, animals are hypothesized to monitor/regulate their intake of protein based on a threshold, rather than a set-point model. This notion is not new or unique to amino acids. For example, one current notion of leptin action is that it serves as a signal for energy intake important during periods of deficiency, but not excess. More generally, given the primacy in nature of the need to acquire adequate amounts of food in order to survive and reproduce, and the difficulty in achieving this nutritional goal, it may be that appetite control mechanisms have evolved in nature to center more on attaining and exceeding adequacy than on maintaining intake around a set-point well in excess of adequacy.

Can nutrient supplements modify brain function?

Over the past 40 y, several lines of investigation have shown that the chemistry and function of both the developing and the mature brain are influenced by diet. Examples are the effect of folate deficiency on neural tube development during early gestation, the influence of essential fatty acid deficiency during gestation and postnatal life on the development of visual function in infants, and the effects of tryptophan or tyrosine intake (alone or as a constituent of dietary protein) on the production of the brain neurotransmitters derived from them (serotonin and the catecholamines, respectively). Sometimes the functional effects are clear and the underlying biochemical mechanisms are not (as with folate and essential fatty acids); in other cases (such as the amino acids tyrosine and tryptophan), the biochemical effects are well understood, whereas the effect on brain function is not. Despite the incomplete knowledge base on the effects of such nutrients, investigators, physicians, and regulatory bodies have promoted the use of these nutrients in the treatment of disease. Typically, these nutrients have been given in doses above those believed to be required for normal health; after they have been given in pure form, unanticipated adverse effects have occasionally occurred. If this pharmacologic practice is to continue, it is important from a public safety standpoint that each nutrient be examined for potential toxicities so that appropriate purity standards can be developed and the risks weighed against the benefits when considering their use.

Cerebrospinal fluid concentrations of tryptophan and 5-hydroxyindoleacetic acid in Macaca mulatta: diurnal variations and response to chronic changes in dietary protein intake.

In rats, dietary protein is known to influence brain tryptophan (TRP) concentrations and serotonin (5HT) synthesis. However, few studies have examined this relationship in primates (including humans). We therefore studied the effect in monkeys of changes in chronic protein intake on plasma and cerebrospinal fluid (CSF) concentrations of TRP and 5-hydroxyindoleacetic acid (5HIAA), the principal 5HT metabolite. Juvenile male monkeys (Macacca mulatta) consumed for sequential 4-week periods diets differing in protein content (approximately 23%-->approximately 16%--> approximately 10%-->approximately 6% protein [%-energy/day]). Each day, food was presented as a morning meal of fruit, and an afternoon meal consisting of a pelleted, commercial diet and fruit. During week 4 on each diet, blood and CSF were sampled diurnally via indwelling catheters. Plasma and CSF TRP varied diurnally and with dietary protein content. On all diets, CSF TRP declined modestly in the morning, and increased in the afternoon; the magnitude of the increments varied directly with dietary protein content. Diurnal variations were absent for CSF 5HIAA; however, CSF 5HIAA varied directly with chronic dietary protein content. We conclude that dietary protein content can chronically influence CSF TRP concentrations in monkeys. The variation in CSF 5HIAA suggests chronic protein intake may influence serotonin synthesis and turnover, perhaps via changes in TRP concentrations.

Pituitary hormone secretion in normal male humans: acute responses to a large, oral dose of monosodium glutamate.

Numerous studies have shown that the administration of a glutamate receptor agonist or a high dose of glutamate stimulates pituitary hormone secretion in animals. However, only a single human study has reported that an oral load of glutamic acid induced the secretion of prolactin and probably adrenocorticotropic hormone (ACTH) (but not other pituitary hormones). Because of glutamate's use in foods as monosodium glutamate (MSG), a flavoring agent, and the limited amount of human data, we studied the effect of a large oral dose of MSG in humans on the secretion of prolactin and other pituitary hormones. Fasting male subjects bearing venous catheters received on separate days each of the following four treatments: a vehicle, MSG (12.7 g), a high protein meal (a physiologic stimulus of prolactin secretion) by mouth, or an intravenous infusion of thyrotropin-releasing hormone (TRH, a pharmacologic stimulus of prolactin secretion). Plasma hormone responses were quantitated by RIA at 20-min intervals for 4 h. The protein meal induced a modest increase and TRH infusion a substantial increase in plasma prolactin, whereas MSG ingestion did not. MSG ingestion also did not raise the plasma concentrations of any of the other pituitary hormones measured (luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone, growth hormone) or of cortisol. Ingestion of MSG raised plasma glutamate concentrations 11-fold; the protein meal did not raise plasma glutamate. The results demonstrate that MSG ingestion in humans does not modify anterior pituitary hormone secretion. One implication is that diet-derived glutamate may not penetrate into hypothalamic regions controlling anterior pituitary function.

Effects of acute tryptophan depletion on mood in bulimia nervosa.

BACKGROUND: The present study investigated the role of serotonin in the pathophysiology of bulimia nervosa (BN) by studying the affective and appetitive responses of women ill with BN to an acute tryptophan depletion (ATD) paradigm. METHODS: Twenty-two women with BN and 16 healthy control women (CW) were studied on 2 separate days during the follicular stage of the menstrual cycle. Participants drank a control mix of essential amino acids (100 g + 4.6 g tryptophan) on one day and a tryptophan deficient (100 g - 4.6 g tryptophan) mixture (ATD) on the other in a double-blind fashion. Mood/appetite ratings and blood samples were taken at baseline and at intervals up to 420 minutes. Participants were then presented with an array of foods and were allowed to binge and vomit if they desired. RESULTS: CW and BN women had a similar and significant reduction in plasma tryptophan levels and the tryptophan: LNAA ratio after ATD. After ATD, the BN women had a significantly greater increase in peak (minus baseline) depression, mood lability, sadness and desire to binge compared to the CW. BN subjects and CW had similar peak changes in mood after the control amino acid mixture. BN subjects and CW consumed similar amounts of food after the two amino acid treatments. CONCLUSIONS: Women with BN seem more vulnerable to the mood lowering effects of ATD, suggesting they have altered modulation of central 5-HT neuronal systems.

The relative roles of phenylalanine and tyrosine as substrates for DOPA synthesis in PC12 cells.

The relative contributions of tyrosine (TYR) and phenylalanine (PHE) to the synthesis of dihydroxyphenylalanine (DOPA) were studied in PC12 cells following inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine (NSD-1015). Cells were incubated with varying concentrations of unlabeled L-TYR and L-PHE, and either L-(3)H-TYR or L-(3)H-PHE. Following incubation, labeled and unlabeled TYR, PHE, and DOPA were quantitated following HPLC separation. PC12 cells synthesized DOPA from both TYR and PHE. Raising the concentration of one amino acid relative to that of the other increased the proportion of DOPA synthesized from that amino acid. TYR suppressed DOPA synthesis from (3)H-PHE at concentrations lower than that observed for a similar inhibition by PHE of DOPA synthesis from (3)H-TYR. Inhibition of total DOPA synthesis occurred only at high concentrations of either amino acid. The results suggest that in the PC12 cell, TYR and PHE can be used interchangeably as substrates for TYR hydroxylation, and that the proportion of catecholamine synthesized will depend on the relative proportions of each substrate available to the cell. However, TYR is clearly the preferred substrate for tyrosine hydroxylase.

Effects of dietary polyunsaturated fatty acids on neuronal function.

Diets deficient in linoleic acid (18:2n-6), or that have unusual ratios of linoleic acid to alpha-linolenic acid (18:3n-3) induce changes in the polyunsaturated fatty acid (PUFA) composition of neuronal and glial membranes. Such changes have been linked to alterations in retina and brain function. These functional effects are presumed to follow from the biochemical consequences of modifying membrane PUFA content; known effects include modifications in membrane fluidity, in the activities of membrane-associated, functional proteins (transporters, receptors, enzymes), and in the production of important signaling molecules from oxygenated linoleic and alpha-linolenic acid derivatives. However, despite the demonstration that central nervous system function changes when dietary PUFA intake is altered, and that in general, membrane PUFA content influences membrane functions, little work has focused specifically on brain and retina to reveal the underlying biochemical bases for such effects. This review examines this issue, looking at known effects of dietary PUFA on neurons in both the central and peripheral nervous systems, and attempts to identify some approaches that might promote productive investigation into the underlying mechanisms relating changes in dietary PUFA intake to alterations in neuronal and overall nervous system functioning.

Simultaneous glutamate and perfusion fMRI responses to regional brain stimulation.

Functional magnetic resonance imaging (fMRI) rests on the assumption that regional brain activity is closely coupled to regional cerebral blood flow (rCBF) in vivo. To test the degree of coupling, cortical brain activity was locally stimulated in rats by reversed microdialysis infusion of picrotoxinin, alphagamma-aminobutyric acid-A antagonist. Before and during the first 30 minutes of infusion, simultaneous fMRI (rCBF) and neurochemical (interstitial glutamate concentration) measures of brain activity were highly correlated (r = 0.83). After 30 minutes of picrotoxinin-induced stimulation, glutamate levels decreased but rCBF remained elevated, suggesting that additional factors modulate the relationship between neuronal neurotransmitters and hemodynamics at these later stages.

The effect of phenylalanine on DOPA synthesis in PC12 cells.

DOPA synthesis from phenylalanine was studied in PC12 cells incubated with m-hydroxybenzylhydrazine, to inhibit aromatic L-amino acid decarboxylase. DOPA synthesis rose with increasing concentrations of either phenylalanine or tyrosine; maximal rates (approximately 55 pmol/min/mg protein for tyrosine; approximately 40 pmol/min/mg protein for phenylalanine) occurred at a medium concentration of approximately 10 microM for either amino acid. The Km for either amino acid was about 1 microM (medium concentration). At tyrosine concentrations above 30 microM, DOPA synthesis declined; inhibition was observed at higher concentrations for phenylalanine (> or =300 microM). These effects were most notable in the presence of 56 mM potassium. Measurements of intracellular phenylalanine and tyrosine suggested the Km for either amino acid is 20-30 microM; maximal synthesis occurred at 120-140 microM. In the presence of both phenylalanine and tyrosine, DOPA synthesis was inhibited by phenylalanine only at a high medium concentration (1000 microM), regardless of medium tyrosine concentration. The inhibition of DOPA synthesis by high medium tyrosine concentrations was antagonized by high medium phenylalanine concentrations (100, 1000 microM). Together, the findings indicate that for PC12 cells, phenylalanine can be a significant substrate for tyrosine hydroxylase, is a relatively weak inhibitor of the enzyme, and at high concentrations can antagonize substrate inhibition by tyrosine.

Exogenous glutamate enhances glutamate receptor subunit expression during selective neuronal injury in the ventral arcuate nucleus of postnatal mice.

Administration of high doses of glutamate (Glu) leads to selective neurodegeneration in discrete brain regions near circumventriclular organs of the early postnatal mouse. The arcuate nucleus-median eminence complex (ARC-ME) appears to be the most Glu-sensitive of these brain regions, perhaps because of the intimate relationships between its neurons and specialized astroglial tanycytes. To investigate the mechanism of Glu-induced neuronal loss, we administered graded doses of the sodium salt of glutamate (MSG) to postnatal mice, measured their plasma Glu concentrations, and performed microscopic analyses of the ARC-ME region 5 h after treatment. Nursing, 7-day-old mouse pups (CD1, Charles River, Hollister, Calif.) were injected subcutaneously with single doses of 0.1-0.5 or 1.0-4.0 mg of MSG per g BW, or with water vehicle alone. Mice were decapitated 5 h later and the brains immediately fixed by immersion in buffered aldehydes. Frontal vibratome tissue sections at comparable levels of the ARC-ME were examined by light microscopy. A dose of 4.0 mg MSG/g BW caused neurodegeneration throughout the ARC region, while 1.0 mg/g MSG resulted in less extensive damage. Injection of 0.2 mg MSG/g BW, which raised plasma Glu concentrations 17-fold after 15 min, was the minimum dose tested at which nuclear and cytoplasmic changes were observed in a small group of subependymal neurons near the lateral recesses of the third ventricle. Higher doses of 0.3-0.5 mg MSG caused injury to additional neurons situated farther laterally, but damage remained confined to the ventral region of the ARC nucleus. Ultrastructural examination showed some subependymal neurons with pyknotic nuclei, reduced cytoplasmic volume, and swollen subcellular organelles, while others had fragmented and condensed nuclear material. Immunostaining for tyrosine hydroxylase indicated that dopamine neurons were spared at the threshold dose, but suffered damage after higher doses of MSG. Immunostaining for Glu receptor subtypes revealed that 0.2 mg MSG/g BW enhanced neuronal expression of NMDAR1 and of GluR2/4, and that higher doses of MSG preferentially increased NMDAR1 expression in injured neurons. These results extend previous reports of Glu sensitivity in the ARC-ME region of 7-day postnatal mice. A dose of 0.2 mg MSG/g BW s.c. causes clear but discrete injury to specific subependymal neurons of undetermined phenotype near the base of the third ventricle. Slightly higher doses of MSG evoke damage of additional neurons confined to the ventral region of the ARC traversed by tanycytes. These same greater amounts of MSG promote dose-related increase in the expression of NMDAR1 more than of GluR2/4 in injured ARC neurons, suggesting that elevated Glu receptor levels may contribute to or be related to neuronal cell death. Taken together with previous findings, the data suggest that Glu responsitivity in the ARC-ME of the postnatal mouse may result from transient developmental conditions involving the numerical ratios and juxtaposition between tanycytes and neurons, expression of Glu receptors, and perhaps other ontogenetic factors which may not persist in the mature adult.