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Why does the desire to continue eating dominate the signal that we are full? New research reveals that this is a struggle between two neighboring groups of brain cells in which the opioid system of the brain also plays a role.
He also found that the drug naloxone, by blocking the opioid system, stopped overeating. The study, which features in the Proceedings of the National Academy of Sciences, was performed on mice, but scientists believe the findings will help us better understand similar mechanisms in humans.
"Our work," says lead author of the study, Professor Huda Akil, neuroscientist at the Department of Psychiatry at the Institute of Molecular and Behavioral Neuroscience at the University of Michigan at Ann Arbor "Shows that satiety signals powerful enough to work against the desire to eat, which has a strong evolutionary value. "
Overweight or obesity increases the risk of developing several long-term conditions, such as cardiovascular disease and type 2 diabetes, as well as cancer. Overweight is a global public health problem that affects low- and middle-income countries as well as high-income countries.
Estimates from the World Health Organization (WHO) 2016 suggest that 39% of adults in the world are overweight and 13% are obese. The pressure to better understand the factors of obesity, such as the role of the brain in the regulation of food, has never been so great. Among these, let us note the authors of the study, "the mechanisms that modulate both initiation and cessation of food".
Professor Akil and his colleagues focused on two small groups of adjacent nerve cells, or neurons, in the hypothalamus, a small region of the brain that mediates multiple functions, such as controlling "motivated behaviors." called pro-opiomelanocortin (POMC) cells and agouti-associated peptide (AgRP). They reside in an area of the hypothalamus known as the arcuate nucleus (Arc).
Scientists already knew that both groups and the Arc were somehow involved in "food control". Indeed, in previous work, part of the team had previously revealed that on receiving certain signals, the POMC eating neurons and AgRP neurons act as the accelerator pedal, particularly when a lot of time has passed since the last food.
However, what remained uncertain was how these two groups interacted. A tool called optogenetics helped investigators map the mechanism signals using laser light to activate and deactivate selected cells in mice that ate too much.
They revealed that when they activated POMC cells, it also activated nearby AgRP cells. This meant that the accelerator pedal and the ignition brake were engaged at the same time, and that the result was that the acceleration pedal was won. "When both are stimulated at the same time, AgRP steals the show," says Professor Akil.
With a different optogenetic method, scientists saw that they could trigger POMC cells without activating nearby AgRP cells. This led to a rapid and significant decrease in mouse consumption.
With the help of a visualization tool, they also drew detailed maps of the involved routes. They produced a 3D map of tracks that start in POMC and AgRP. Once active, these signaling pathways trigger either the sensation of satiety or a desire to eat.
In later tests, the researchers explored the signals that are "downstream" from POMC and AgRP cell activation, revealing that their influence extends widely into the brain, even encompassing regions of the cortex that control perception, memory and attention.
In a final series of experiments, the team revealed that activation of AgRP also lights the opioid system of the brain. Naloxone, an opioid receptor inhibitor, has allowed rodents to stop eating behavior.
"This suggests that the endogenous opioid system of the brain may play a role in the desire to eat beyond what is needed," notes Professor Akil.
In general, studies on the metabolic factors of diet and overeating tend to focus on hormones such as ghrelin and leptin. These results suggest, however, that brain circuits, or "neural systems," also seem to play an important role. These could react to emotional, social and perceptual signals. Professor Akil is asking for further research into this aspect of overfeeding.
Posted in Daily Times, September 25e 2018.
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