Descartes, glutamate and the fifth taste
Imagine the following scenario:
Physiologists claim that there are only four basic colors: violet, blue, green and yellow. We see these colors and nothing but these colors. The light sensitive cone has been discovered and the basis of color vision has been clarified. We, lay persons as regards to process of seeing, believe in science and see exactly these colors. However, sometimes things look somehow more colorful and in fact really beautiful, as for example some flowers and some landscapes during sunset. At other times, things don't look as colorful and beautiful. Because we know from physiology that there are only the colors mentioned above, we can account for the differences in color vision by making the assumption that sometimes there is some color amplifier in the light, and sometimes there isn't. The assumption of a color amplifier that is more or less present together with the colors that we see accounts for all the phenomena of color vision.
Some Japanese scientists had claimed for almost 100 years that there is one more color - Seki (the Japanese word for red) - but the impressive results of physiology (we only have receptors for the above mentioned colors) and our daily experiences - we see four basic colors and sometimes a color amplifier is also present - was not taken seriously. Only when cones for red light were discovered recently was it dismantled as a scientifically wrong hypothesis.
This scenario may appear hardly plausible. One might argue that it is simply impossible that scientific theories about the workings of our senses would render our most basic sensory intuitions invalid. As regards colors, we can hardly imaging that our experience of color does not include red, in that any theory of color perception would distort our immediate experience of colors in the way described above. We simply have a hard time to admit that a theory would influence our immediate experience of sensory qualities in such a strong way, i.e. bad sensory qualities are dependent on empirical findings of the natural sciences. Since the times of Descartes it has been clear that I may be deceived in that there is something red over there; however, I cannot be deceived in that I have the sensation of something red here and now. Every person has immediate access to sensory qualities and this access cannot be impaired or even completely abolished by the natural sciences. Empirical findings can never, so it appears detach us from experiencing the world as it is.
Given the argument just made, it is surprising that the scenario just described has in fact happened been repeated few months ago. It did not regard the sense of vision but rather the sense of taste; in any other respect, what has happened was exactly analogous to the apparently completely absurd situation described above. The facts are in brief as follows:
Generations of physiologists have told generations of physicians, that the surface of our tongue has four receptors for taste which signal sweet, sour, salty and bitter. Any other taste, so we learned, is represented by a combination of these four basic tastes and about 1000 different smells that can be distinguished by humans and that are blended together with the tastes to give a unified experience. „If you caught a cold and you have a blocked nose, your steak will only taste salty”, as a famous physiologist from Freiburg University used to remark on the point that the sense of taste does not deliver much in terms of different flavors.
It is surprising that this view had been proven wrong about 100 years ago by the Japanese scientist Kikunae Ikeda who had discovered a fifth taste. He had called this taste umami which may be literally translated as “good tasting” and which represents the taste of meats, cheeses and mushrooms. Many foods rich in protein have this taste, which result from their content of their amino acid-L-glutamate.
In the western world this taste remained, possibly because of its strange unknown name although we all taste it on a daily basis. This led to the labeling of many foods containing this taste as a so called “taste amplifier” which in fact is nothing but L-glutamate. It was as if the food industry did not have the guts to refer to this taste as a taste because whatever did not appear in the textbooks did not exist – the fifth taste of umami.
This state of affairs reaches deep into philosophical discuss: in many philosophical arguments immediate sensory experiences (so called qualia) are used to justify certain experiential claims. Qualia often serve the purpose of justifying specific knowledge. The scenario just mentioned about the fifth taste weakens philosophical arguments that use qualia drastically. We may simply be wrong, even when referring to seemingly immediately experienced quality of sensation.
The scenario lends credit to the somewhat recently discredited ideas of the linguist Sapir and Whorff, who claimed decades ago that language has a strong influence upon our perceptual experiences. Every college student knows that eskimos have more words for white and people from the rainforest have more words for green etc. Such claims have been replicated for example by mentioning that people who live in the alps also have about 20 words for snow (5). As a matter of fact, however recent evidence provides considerable support for the Sapir and Wolff hypothesis: in the language of the Setswana as well in the language of the tribe of the Berinmo on Papua New Guinea there is only one word for blue and green. Accordingly the people who speak this language can hardly discriminate blue and green colors, but as colors they do not have a special name (3, 4). These studies provide independent evidence for the importance of language for perception.
Back to the recently discovered glutamate receptor on the tongue. The main new finding is that for the first time, a taste receptor has been characterized with molecular biological means (2), although there have been molecular biology papers on the receptor for hot (we may ask, whether hot is a sixth sense of taste; (c.f. 1). Most interestingly the molecular structure of the receptor is similar to the glutamate receptor in our brain that guarantees fast neurotransmission by the neurotransmitter L-glutamate. This amino acid is not only part of our food but also quite likely the most important facilitatory neurotransmitter of the central nervous system that is used by cortical pyramidal cells for the purpose of rapid, precise and plastic transmission of information. The glutamate receptor in the brain (called mGluR4) has a 1000-fold higher affinity to glutamate than the glutamate receptor on the taste buds on the tongue (this receptor is called taste-mGluR4). The amino acid sequence of the taste glutamate receptor is a shortened variant of the brain glutamate receptor. As glutamate concentrations in the food are far higher than the glutamate concentration at synapses in the brain, it makes sense that the brain receptor has a higher affinity. If both receptors, the one in the brain and the one on the tongue, were identical, either our brain would work inefficiently or we would have a very strong taste of glutamate all the time, even if we only ate traces of meat (c.f. 2).
The finding that a tasty substance is identical with an excitatory neurotransmitter and that there are huge homologies between the brain and tongue glutamate receptors fits nicely within the framework of the tight relations between the gut and the brain. We know that cholezystocinin, the protein that moves the gall bladder is also used as a neurotransmitter; we know that the vegetative nervous system has mental as well as digestive function and we know for a long time that the brain and the guts are both derived from the ectoderm.
To sum up, a scientific discovery which almost went unnoticed in the press has many facets: it doesn’t matter whether you are a neurobiologist or a linguist, a psychologist or a nutritional scientist, a philosopher or a physiologist – the discovery of the receptor for umami has much to offer for everybody’s taste.
Original title: Spitzer M (2000) Descartes, Glutamat und der fünfte Geschmack. (Geist & Gehirn). Nervenheilkunde 19(3):163-164