Nature reveals the neural loops of the intestinal axis that eat sweets.

Learn about the latest progress in neuroscience ● click the blue letter to pay attention to us ● carbohydrate is the main energy source of almost all animals, so in the process of evolution, there are specific neural circuits in the brain for identifying, searching and consuming carbohydrate substances.the basic taste is sour, sweet, bitter and spicy. These four basic tastes are felt by four different taste cells, and their distribution on the tongue surface is uneven.the sweet taste cells are mainly concentrated in the tip of the tongue, so this part is more sensitive to sweet taste, while the sensitive part to bitter taste is in the root of tongue.this is a snack. You can know whether it is sweet or not by licking it, and you will know the bitter when you drink the medicine until you swallow it.so how does sweetness get to the brain? In the presence of taste buds on the tongue, specific taste receptor cells can sense sweet compounds, and this activity can be captured by ganglion neurons located behind the ear.these neurons send taste information from the tongue to the brain, which of course is only one of the brain's ways of perceiving sweetness.however, in recent years, studies have found that even in the absence of receptor cells, animals will have a strong preference for sugar, indicating that the mechanism is not related to taste.then there may be another mechanism for animals' preference for sugars.April 15, 2020, Charles S. Zuker, Department of Biochemistry and molecular biophysics, Columbia University, USA, published an article in nature, a top-level journal. They found that the gut brain axis signal vagus nerve (perception of intestinal signal) - solitary tract caudate nucleus (brain response) controlled glucose preference.the researchers designed an interesting experiment: one bottle contained glucose solution and the other contained artificial sweetener (saccharin) solution. The mice drank the two solutions at the same frequency in the first 16 hours, but after 16 hours, the mice preferred to drink glucose solution.in the animal model of depression, the loss of pleasure is the use of sugar water consumption rate index, but most of them use sucrose, while glucose and saccharin are used in this experiment.what is the difference between them? Sucrose, the main component of sugar, is sweet and odorless, and is an important food and sweet seasoning; glucose is colorless crystal, sweet, but not as sweet as sucrose, is an important energy source; saccharin, chemical name is o-benzoyl sulfonimide, is a kind of sweetener without heat.it is a white crystalline powder with a sweetness of 300-500 times that of sucrose, and it is a food additive.is it because glucose has both sweetness and calories that makes mice like it more? This is not the case. The researchers further found that mice also had a preference for non Metabolizable Glucose analogues, suggesting that the brain signaling system recognizes the sugar molecule itself rather than its calories or metabolites.what was even more surprising was that the mice still showed a preference for sugar after the sweet receptor cells were knocked out.that is to say, although the mice can't taste the sweetness, other feedback signals can still promote the consumption of sugars in mice.then this thing becomes very interesting. first of all, mice are naturally fond of sweeteners and sugars, and sweeteners are sweeter than sugars, so why do they prefer sugars over sweeteners. secondly, mice still like sugars even when there are no sweet receptors. one possibility is that there are specific groups of neurons in the brain that can effectively identify the difference between the two substances and make a preference choice. the nucleus tractus solitarius (CNST) perceives peripheral sensory information and transmits it to the brain, which may be involved in the above signal transmission. immunofluorescence showed that in glucose solution and non Metabolizable Glucose analogue solution, CNST neurons were greatly activated, but not in saccharin solution and aqueous solution. in addition, the above-mentioned carbohydrate preference behavior did not exist after the CNST neurons were shut down by AAV virus, which indicated that the brain region was involved in carbohydrate preference in mice. (recently published articles in the journal Nature revealed that the nucleus of the solitary tract (NTS) controls feeding behavior, which also belongs to the gut brain axis. For details, you can click: nature reveals the latest neural loop controlling food intake, or it is helpful to lose weight). Recent studies have shown that gut brain axis is the key mechanism of information transmission from intestine to brain through vagus nerve. therefore, the researchers believe that glucose preference may be regulated by gut brain axis signal rather than by sweet receptor. therefore, they buried a catheter in the duodenal bulb and delivered sugar through the catheter, which directly stimulated the intestinal tract. After bilateral vagotomy, the mice did not show preference for glucose solution and non Metabolizable Glucose analogue solution, indicating that gut brain axis signal participated in the above process. the researchers used a catheter to transport sugar and saccharin solution into the intestine, and dynamically observed the changes of calcium ion in vagal sensory neurons by using single photon miniscope. It was found that the vagus nerve ganglion could sense the glucose related signals in the gut. the researchers further used the CRE enzyme dependent trap technique to label the carbohydrate activated neurons in the CNTs, and injected retrograde tracing virus into the CNTs. It was found that the neurons received a large number of inputs from vagal ganglion neurons, while in drinking water and saccharin solution, the number of activated neurons in CNTs was less, and the inputs from vagus ganglion neurons were also less. this indicates that the vagus nerve transmits signals to CNST to regulate glucose preference. pre enkephalin gene (PENK) is enriched in CNST, then whether the neurons expressing PENK in this brain region participate in glucose preference. the researchers found that about 85% of the CNST neurons activated by glucose solution were PENK positive neurons. Further, chronic activation of PENK positive neurons in CNST by chemical genetics technology can promote the uptake of glucose solution in mice. in conclusion, we found that there is a gut brain axis signaling pathway for carbohydrate preference: vagus nerve senses gut related signals and transmits them to PENK expressing neurons in brain CNST, which continuously enriches the brain's regulation mechanism of taste. References: the gut – brain axis mediators sugar preference