Charles Zuker, PhD, an investigator at Columbia's Mortimer B. Zuckerman Mind Brain Behavior Institute - Photo © Columbia University
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While our sense of smell allows us to discern the palette of aromas emanating from our food, taste remains essential to be able to assess its edible and nutritional qualities. Forget the illustration from your school biology textbooks however, in which the tongue was divided into four parts, each responding to a specific taste: salty, sweet, bitter and sour. It is false! The receptors located in the taste buds are distributed randomly over the tongue and elsewhere in the mouth. Furthermore, for more than a century, the idea of a fifth taste has been circulating - umami.
At the start of the 20th century, Kikunae Ikeda, a chemist at the Imperial University of Tokyo had an intuition: the four tastes, then considered exhaustive, did not explain what he felt when tasting some seaweed broth from Japanese cuisine. He managed to extract from it a substance from which he removed all components perceived as salty, sweet, bitter or sour. The remaining component left him with a “delicious” taste, or umai in Japanese, leading him to baptise this fifth taste as “umami”. He shared the discovery with his peers and in 1909, published a study that was not unanimously received; many doubted the result. Also, it was not translated into English until 2002, after the development of molecular biology identified the umami receptors in 2000. Ikeda was right! But why are there only five tastes? Each one provides information about the essential content of the food - sweet indicates the energy content of foods and bitterness is characteristic of tainted food. For umami, the receptors are sensitive to glutamate, a basic component of proteins, essential in our diet. So, according to the needs of our body, each taste supposedly specifically directs our dietary behaviour.
The issue no longer falls within the field of molecular biology (the nature of the receptors) and becomes a question of neuroscience. Last June, Charles Zuker and his team at Columbia University in the United States, showed that in the gustatory cortex, which deals with information about taste, the neurons that respond preferentially to sweetness and bitterness “propel” their axons towards the amygdala, the region of the brain that plays a key role in the emotions. The neurons that respond to sweetness propel towards a sub-region of the amygdala associated with feelings of pleasure; those of bitterness towards a sub-region associated with aversion.
Better still, these neuronal networks determine behaviour! Using optogenetics, a technique used to activate neurons using light, scientists showed that in animals, activating the neuronal network associated with the taste of sweetness caused an increase in the consumption of quinine-treated water, usually a very bitter and repellent substance. Inversely, activating neuronal inputs associated with bitterness leads to a considerable drop in the consumption of sugared water, usually very attractive. The animals were always able to tell each taste apart, only their behavioural response changed. By acting on the circuits connecting the neurons in the gustatory cortex to the amygdala, the expected behaviour was short-circuited. Taste therefore, does determine a specific behaviour. Such circuits will undoubtedly soon be identified for the other three tastes.
This study is a superb illustration of systems neuroscience, a growing discipline that aims at understanding how neuronal activity and the circuits connecting neurons in the brain deal with sensory information and determine behaviour. Studying the simple systems associated with reproducible responses (like taste) with leading-edge techniques is the best way of understanding the fundamental principles behind the functioning of the brain as a whole.
Adrien Peyrache heads at McGill University in Canada a research laboratory devoted to studying the neuronal processes involved in spatial navigation and memory.