Adipose tissues store excessive energy. The body can burn fat to supply energy during a calorie deficit. These adipose tissues produce various signaling molecules with wide-ranging effects on metabolism and well-being. However, in recent years researchers have found that fatty tissues are also well-innervated and continually communicating with the brain. Thus, fat tissues are connected to the sympathetic nervous system, and higher sympathetic activity results in fat burning. Adipose tissues also have sensory neurons, and the higher activity of these neurons acts as brakes and prevents fat burning.
What does fat say to the brain? And how does it talk to the brain? It is essential to understand for many reasons. By understanding this communication’s intricacies, science can better understand obesity and metabolic disorders. Most people imagine adipose tissues as something inert. However, that is changing fast. Now science knows that adipose tissues talk in many languages. Fat cells can talk to various other body parts.
In recent years, researchers have developed a better understanding of the endocrinal function of adipose tissues. For example, they now know that adipose tissues secrete many hormone-like compounds and thus influence various body functions, appetite, metabolic rate, immune system, and much more. However, fat tissues appear to talk to the body and brain in more than one way. Therefore, it is wrong to imagine them as tissues that produce bioactive compounds and hormones and thus send various messages to the body and brain. Instead, adipose tissues appear to have dual-sided communication with the brain, which is happening continually.
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A new study confirms that adipose tissues produce hormones responsible for sending various signals, and fat cells are also rich in nerve cells. In the new study published in the journal Nature, Scripps Researcher scientists focused on the role of sensory neurons in fat cells, as they carry vital information from adipose tissues to the brain1. The confirmation that fat cells have sensory neurons participating in the active fat and brain communication is mind-blowing. It is the first study that could confirm that fat cells and the brain are continually talking using neuronal pathways. Scientists at Scripps Institute say that the implications of these findings are immense.
This is not the first study to show that adipose tissues are innervated. Researchers have long known that fat cells are connected to the brain with nerve cells. However, they did not know that these nerve cells also have a sensory function. In recent years, science is increasingly realizing the importance of sensory neurons in well-being. In mammals, adipose tissues mainly serve as an energy source. However, in recent years, science has developed a better understanding of various hormones and signaling molecules released by fat cells. These signaling molecules and their dysfunction play an important role in metabolic disorders like obesity, diabetes, atherosclerosis, fatty liver disease, and more.
To date, researchers believe that most nerve cells in the adipose tissues belong to the sympathetic nervous system. They thought a one-way conversation was happening between the brain and fat tissues. When the brain senses that more energy output is needed, it sends signals to the fat tissues to start converting white-fat cells to brown fat cells and switch on the fat-burning pathways. In the new study, researchers used a special method called HYBRID to visualize better where and how the signals between adipose tissues and the brain are traveling. They found that many nerve fibers leaving the fat tissues did not connect to the sympathetic nervous system. Instead, they are connected to the dorsal root ganglia, a brain area where sensory neurons originate.
Further, researchers used another technique to confirm their findings. They used a special virus to destroy sensory neurons in the fat cells. This helped them understand the changes that occur in the body if these sensory neurons are absent or damaged. It also helps understand the result of the overexpression of sensory neurons. The study found that if the brain did not receive these sensory signals, it resulted in a higher transformation of white-fat cells to brown-fat cells, greater fat breakdown, and higher thermogenesis. In addition, it resulted in a higher activity of the sympathetic nervous system.
Therefore, researchers concluded that sensory and sympathetic neurons have opposing actions. For example, sympathetic neurons are needed for fat burning. However, sensory signals act as brakes and reduce sympathetic neurons’ activity. It means that this two-way communication between adipose tissues and the brain by using sensory neurons and sympathetic neurons ensures proper maintenance of fat cells. It appears that sympathetic neurons act as the gas pedal, while sensory neurons as a brake.