12 January - 20 June 2016

Brain cells sensing amino acids lead to feelings of satiety

A study by researchers from the University of Warwick, published in the journal Molecular Metabolism has found that a type of cell in the brain can detect certain amino acids and release signals to other parts of the brain that deal with feelings of satiety.

A study by researchers from the University of Warwick, published in the journal Molecular Metabolism has found that a type of cell in the brain can detect certain amino acids and release signals to other parts of the brain that deal with feelings of satiety.

The study indicates that amino acids have long been known to be important for satisfying hunger. More recently, studies have shown that the brain plays a key role in satiety as direct injections of amino acids in to the hypothalamus have been shown to supress hunger in rats. This area of the brain receives many signals related to food and is thought to integrate information to "provide an appropriate response to maintain energy homeostasis". Dale et al. note that one type of hypothalamic cell, the tanycytes, have been thought to play an important role in energy homeostasis and to be sensitive to glucose by dependencies on sweet taste receptors. As the umami taste receptors, which contribute to the "savoury taste of glutamate in humans and many non-aromatic amino acids in rodents" share a common part with sweet receptors, Dale et al. hypothesised that tanycytes might be able to detect amino acids through the umami receptor.

Dale et al. used brains from rats, sliced to expose the tanycyte layer and exposed these to the essential amino acids l-arginine, and l-lysine and the non-essential amino acids l-alanine, l-serine and l-proline. Calcium fluorescence was used to determine the response of tanycytes cells while direct bio-sensing for ATP releases from tanycyte cells was performed using custom-made electrodes. The researchers discovered that the tanycytes responded to the amino acids by producing a "wave" of Ca2+ and a subsequent wave of ATP. Dale et al. note that as ATP signals require a particular type of receptor to spread, and that as these receptor types are known to be expressed by hypothalamic neurons, this shows that tanycytes can inform the hypothalamus about amino acid availability. The scientists then wanted to see if the tanycytes’ ability to detect amino acids was related to the umami taste receptors. Dale et al. used brain slices from mice with part of the umami receptors knocked out to see how this altered amino acid response. The researchers found various reductions in responses for different amino acids and different umami receptors which led Dale et el.  to conclude that there are at least two mechanisms of tanycyte amino acid sensing via umami taste receptors.

In discussion, Dale et al. reiterate that they have shown tanycytes to detect amino acids through at least two umami receptors and state that their work has shown the "first known non-neuronal mechanism of amino acid sensing in the brain". In conclusion, the study states that the work warrants further investigation in to whether tanycytes may be “physiological mediators of satiety signals and act to reduce food intake”. A University of Warwick Press Release notes that foods high in the essential amino acids l-arginine, and l-lysine such as pork shoulder, sirloin steak, mackerel, plums, avocadoes, lentils and almonds, activate tanycytes and provide quicker feelings of satiety. Professor Nicholas Dale is quoted as saying that "Amino acid levels in blood and brain following a meal are a very important signal that imparts the sensation of feeling full. Finding that tanycytes, located at the centre of the brain region that controls body weight, directly sense amino acids has very significant implications for coming up with new ways to help people to control their body weight within healthy bounds". 

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