© 2000 The American Society for Nutritional Sciences
Supplement
Second International Conference on Glutamate: Conference Summary1
Departments of Psychiatry, Pharmacology and Neuroscience and UPMC Center for Nutrition, University of Pittsburgh School of Medicine, Pittsburgh, PA 15213
 TOP INTRODUCTION REFERENCES |
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The Second International Conference on Glutamate focused broadly on the following four areas: 1) the function of glutamate in taste and as a flavoring agent in foods [as a salt, usually monosodium glutamate (MSG)]; 2) the participation of glutamate in carbohydrate and amino acid metabolism; 3) the role of glutamate as a neurotransmitter; and 4) the safety of glutamate in the food supply (when it is added as MSG).
The taste properties of glutamate included an examination of the
glutamate sensors in the oral cavity that are thought to participate in
taste perception. Glutamate occurs naturally in many foods (e.g.,
tomatoes, cheeses) and in prepared stocks and soups (in meat and fish
stocks and in soups, the glutamate released by protein hydrolysis
becomes a key flavoring agent); in each case, glutamate adds to the
food’s taste. It is also added to prepared foods in crystalline form
to enhance flavor. The flavor attributed to MSG becomes more complex,
and the tongue’s sensitivity to it is enhanced markedly in the
presence of certain nucleotides. This taste is readily identified in
Asian cultures as being distinct from the four basic tastes (sweet,
sour, salty, bitter), and has been named "umami" by the Japanese
(Yamaguchi and Ninomiya 2000
). Western cultures have had
difficulty in describing this taste, and thus heretofore have not
identified it as unique (e.g., "savory" and "meaty" are
sometimes used to describe this flavor in Western cultures)
(Löliger 2000
, Yamaguchi and Ninomiya 2000
). However, as discussed in several reports at the
conference, research on "umami" over the past 25 years, which has
included both behavioral and neurophysiologic approaches, appears to
have succeeded in establishing it as a fifth basic taste, alongside
sweet, sour, salty and bitter. The importance attached by the body to
the perception of this taste appears underscored by the identification
in the hypothalamus (a brain area important in appetite control) and
the orbital prefrontal cortex (a brain area important in the perception
of taste and smell) of neurons that respond selectively to the
application of MSG to the tongue (Nishijo et al. 2000
,
Rolls 2000
). Currently, it is not known why animals
developed an ability to taste umami or why it might be perceived as
pleasant. One speculation has been that it may provide a signal to the
animal that a protein-containing food is being ingested
(Kondoh et al. 2000
).
The past two decades have witnessed an explosion of information on the
biochemical and molecular features of neuronal glutamate receptors
(Nakanishi et al. 1998
,Ozawa et al. 1998
). Because the umami taste is linked inextricably to
glutamate sensing, it is thus not surprising to find receptor
methodologies now being applied to the search for specific glutamate
receptors in the oral cavity. The working hypothesis is that glutamate
taste sensors may be glutamate receptors bearing structural and
pharmacologic similarities to those characterized in brain (in which
glutamate is a neurotransmitter) (Brand 2000
). Using
behavioral and neurophysiologic paradigms (e.g., examining the
depolarization rates of taste fibers after the application of glutamate
agonists and antagonists to the tongue), together with biochemical and
molecular tools, investigators have begun to define some of the
molecular characteristics of this taste transduction process. As noted
in several reports at the conference, glutamate taste transduction may
involve one or more receptors that are similar, but probably not
identical to brain glutamate receptors (Brand 2000
,
Kurihara and Kashiwayanagi 2000
, Ninomiya et al. 2000
). If taste physiologists and pharmacologists find that
biochemical and molecular tools developed to examine brain glutamate
receptors are adaptable to their study in the oral cavity, it is likely
that great strides will be made over the next 20 years in defining the
molecular basis of the "umami" taste.
In addition to glutamate receptors in the oral cavity, the occurrence
of glutamate sensors (presumably receptors) elsewhere in the digestive
system was discussed at the conference. Most notably, neurophysiologic
evidence was presented regarding the ability of glutamate, applied
locally within the small intestine, to stimulate sensory afferents of
the vagus nerve. This stimulation was found to induce a reflex
activation of efferent fibers from the brain to the pancreas and
elsewhere, which conceivably might function to facilitate digestion,
and nutrient absorption and distribution (Niijima 2000
).
Once absorbed from the gut, glutamate quickly becomes an important
participant in key metabolic activities throughout the body. As
discussed at the conference, recent evidence using stable isotopes
shows that dietary glutamate is a major energy source for the
intestines, accounting for half of the energy consumed during digestion
(Reeds et al. 2000
). The role of glutamate as a
cosubstrate in the transamination and deamination of several other
amino acids was also reviewed. These reactions provide carbon skeletons
for gluconeogenesis or ATP generation (Brosnan 2000
). A
discussion of nitrogen elimination, a necessary corollary of
gluconeogenesis, focused on the roles of both glutamate and glutamine
in hepatic nitrogen elimination via urea synthesis
(Watford 2000
). Overall, as succinctly noted by Brosnan,
"no other amino acid displays such remarkable metabolic
versatility."
In considering nitrogen movement in the body, both Brosnan (2000)
and Watford (2000)
noted that glutamate
concentrations are high intracellularly (not extracellularly),
consistent with the idea that glutamate is important in intracellular
nitrogen transfer reactions, whereas the opposite is true for
glutamine, i.e., this amino acid appears to have as a focus the
shuttling of nitrogen among cells and organs. This important
distinction reappears when glutamate and glutamine compartmentation in
neurons and glia is discussed.
Finally, Battaglia (2000)
reviewed recent data regarding
glutamate shuttling in the fetus. The placenta (much like the
intestines in adults) utilizes glutamate as an important source of
energy. Indeed, the placenta is said to account for >60% of the total
fetal glutamate disposal rate. The fetal liver has been identified as
the key provider of glutamate, although the placenta is fully capable
of utilizing maternal-derived glutamate as well.
Currently, it is not known why the intestines and the placenta consume
large quantities of glutamate for energy generation. As noted at the
conference, however, this phenomenon may explain why glutamate
concentrations in plasma and blood rise relatively modestly after large
MSG or glutamate doses have been ingested by adults (either as MSG
added to food or as glutamate contained in food proteins) (Tsai and Huang 2000
), and why fetal plasma and blood glutamate
concentrations do not rise in response to marked elevations in maternal
plasma and blood glutamate concentrations (Stegink et al. 1975
).
Generally, the view was expressed that more is known about the
metabolism of essential than nonessential amino acids (such as
glutamate). The relative paucity of data on nonessential amino acids
was thought to follow from the technical difficulty in quantitating the
rapid turnover and complex metabolism of these amino acids. Stable
isotopic methods now appear capable of handling such difficulties, as
demonstrated by the studies discussed by Reeds (2000)
.
They offer a positive indication that a great deal more information
will be forthcoming regarding the details of the metabolism of
glutamate and other nonessential amino acids.
The discussion then shifted to brain, in which endogenous glutamate
functions as an excitatory neurotransmitter (i.e., it causes
depolarization of neurons). The depolarizing property of glutamate has
been known for half a century (Fonnum 1984
). But details
on glutamate’s role as a transmitter have emerged only recently,
particularly with the advent of biochemical and molecular methodologies
for identifying and characterizing glutamate receptors and
transporters. Because glutamate excites neurons, it was noted that it
has the potential, when neuronal release is uncontrolled, to overexcite
them and cause their death (Meldrum and Garthwaite 1990
). This notion led to the concept of glutamate as an
excitotoxin, along with the suggestion that dietary
glutamate (e.g., in the form of MSG) might also be excitotoxic to brain
neurons (Meldrum 1993
, Olney 1994
).
However, because animals ingest large quantities of glutamate daily
(both as the free amino acid and as a constituent of protein) and use
glutamate in a variety of metabolic roles in the body, it was
recognized early on that the brain is well protected from the rest of
the body with respect to these large glutamate fluxes (Pardridge 1979
). This view has only been reinforced by work conducted
over the past two decades (Smith 2000
). Indeed, even
work examining the neurotoxicity of certain dietary glutamate agonists
such as ß-N-methylaminoalanine and domoic acid
(Meldrum 1993
) failed to support the concept of dietary
glutamate toxicity because these agents gain access to brain
via transport mechanisms different from the
glutamate transporter (i.e., they are not effectively excluded from
brain, like glutamate, because they do not use the same transporter)
(Smith 2000
).
Given the large concentrations of glutamate that are present normally
in brain, it was further recognized that potent mechanisms must exist
within brain that strictly compartmentalize the amino acid locally.
Indeed, the glutamate present in and used by the brain as a
neurotransmitter has been found to be synthesized within neurons, and
actively removed from the synapse (once released) to be recycled to the
neuron in nonneurotransmitter form (glutamine; it is converted back to
glutamate in the neuron). Glial cells are primary participants in the
process of synaptic glutamate removal and recycling (Daikhin and Yudkoff 2000
, Yudkoff et al. 1993
). These latter
functions of glial cells may also help to explain why neural elements
in a portion of the brain lying outside the blood-brain
barrier (the median eminence, a part of the hypothalamus) are not
destroyed when blood glutamate concentrations are made artificially
high. Tanycytes, a specialized glial cell present throughout the median
eminence, may effectively maintain low intracellular glutamate
concentrations in this region, even in the face of large glutamate
influxes secondary to greatly elevated blood glutamate concentrations.
Only when glutamate influx becomes unusually high may the glutamate
concentrating power of these cells fail (e.g., when neonatal mice are
given extremely high parenteral doses of glutamate), leading
to local glutamate increases and neuronal toxicity (Goldsmith 2000
).
Overall, the low rate of glutamate penetration into brain, together
with the occurrence in brain of the metabolic machinery for
compartmentalizing the actions of glutamate, evidently affords great
protection to brain neurons from accidental or purposeful vagaries in
systemic and local brain glutamate concentrations. Questions remain for
the future, however, and one is particularly relevant to the present
discussion, i.e., if glutamate is carefully modulated in its access to
glutamate receptors within the brain, what constraints are there on
this interaction outside the brain (particularly, outside of the
blood-brain barrier and away from potent glutamate uptake
transporters on glia)? Glutamate receptors likely occur on the tongue,
on vagal afferent fibers and also on a variety of other cells types in
the periphery, where they may subserve signaling functions (Erdo 1991
, Dingledine and Conn 2000
). Are these
glutamate receptors protected from the pools of glutamate around them
that subserve metabolic roles, and if so, how? And as a corollary, are
the metabolic pools of glutamate the source of the glutamate molecules
that stimulate these receptors, or do neurons exist that provide the
glutamate stimuli (no such neurons have yet been identified)? Perhaps
these questions will be resolved in time for the next symposium.
Finally, because of glutamate’s ubiquity in the food supply, both as a
natural constituent and as an added, flavor-enhancing agent (MSG),
the issue of adverse reactions to MSG was considered at the conference.
Over the past 30 years, dietary MSG has been reputed to induce a
variety of unwanted effects in humans, including sweating, muscle pain
and fatigue, headache, skin reactions and asthma. Are these effects
real, and if so, are they attributable to actions of dietary glutamate
or MSG? Although the occurrence of one or more adverse responses to MSG
has been reported repeatedly in the published literature, study design
has invariably left questions regarding outcome bias, either on the
part of the subject or the experimenter. As discussed by several
clinical investigators at the conference, by today’s scientific
standards, such effects rarely occur in studies in which both
experimenter and experimentee are carefully blinded to the treatment,
in which the subjects’ medical conditions are adequately controlled (a
particularly important consideration in the design of asthma studies)
and in which the criteria for identifying MSG responders include their
showing both a reproducible, positive response to MSG and a
reproducible nonresponse to the placebo (Geha et al. 2000
, Simon 2000
, Stevenson 2000
).
Few molecules of biological importance appear to have as many roles in body function as glutamate. Among these roles, it functions as a tastant on the tongue (a flavoring agent in food), a metabolic fuel in the gastrointestinal tract, an amino acid constituent of proteins, a carbon skeleton that shuttles amino groups among amino acids, a participant in hepatic ammonia detoxification, an energy substrate for the placenta, a neurotransmitter in brain and also a signaling molecule for cells outside the brain. This symposium provided an assessment of our knowledge regarding these (and other) glutamate functions since the glutamate symposium in 1978. Remarkable advances have been made, and the consensus at the meeting was that there appears to be every likelihood that the discovery process will continue unabated over the next 20 years.
1 Presented at the International Symposium on Glutamate, October 12–14, 1998 at the Clinical Center for Rare Diseases Aldo e Cele Daccó, Mario Negri Institute for Pharmacological Research, Bergamo, Italy. The symposium was sponsored jointly by the Baylor College of Medicine, the Center for Nutrition at the University of Pittsburgh School of Medicine, the Monell Chemical Senses Center, the International Union of Food Science and Technology, and the Center for Human Nutrition; financial support was provided by the International Glutamate Technical Committee. The proceedings of the symposium are published as a supplement to The Journal of Nutrition. Editors for the symposium publication were John D. Fernstrom, the University of Pittsburgh School of Medicine, and Silvio Garattini, the Mario Negri Institute for Pharmacological Research.
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