The happiness molecule makes the brain grow
For the first time, researchers have revealed that serotonin, the molecule of happiness, is essential for the multiplication of neurons during development and thus would have allowed us to have a “big” brain.
Humans have high-level brain skills, not least because they have a “big” brain whose outer layers, called the neocortex, are highly developed and folded. This is because they contain a multitude of neurons. But how did we come to this? Many external and internal factors intervened during evolution, but Lei Xing of the Max-Planck Institute in Germany and his colleagues have just identified a new one, and not the least: The serotonin, “Neurotransmitter of happiness”, is largely responsible for the size of the brain …
The mammalian neocortex, corresponding to the outer layers of the cerebral hemispheres, encompasses the cortex and other areas, for example belonging to the limbic system (which plays a role in emotions). It developed recently in the evolutionary history of vertebrates and explains why mammals have larger and more developed brains than other animals; it is involved in high-level cognitive functions, such as sensory perceptions, motor control, reasoning, awareness, and language. It is thanks to him that we speak, thinks, and dream …
If it is so developed, it is because it contains so many neurons. During embryonic and fetal development, in utero, neurons appear gradually because particular stem cells, neuronal progenitor cells, located in the subventricular area of the brain, multiply intensely, migrate, and then differentiate into neurons — that is, is neurogenesis. The more the progenitors divide, the more neurons there will be, and the bigger the brain will be.
The aim of the researchers was therefore to determine which factors promote the proliferation of neuronal progenitors. To do this, they were interested in serotonin because, from the early stages of development, it is produced by the placenta, thus bathing the neocortex of mammals, and some studies have already revealed that it is involved in the survival of stem cells. . Serotonin is also the neurotransmitter of happiness, which is involved in satisfaction, self-confidence, and optimism. Xing and his colleagues, therefore, performed a whole battery of complex experiments with three types of embryos and stem cells: from mice, ferrets, and … humans.
Thus, they have shown, for the first time, that serotonin is indeed a growth factor in neuronal progenitors in ferrets and humans, but not in mice. Why? Because the effects of serotonin on the multiplication of stem cells go through its attachment to one of its specific receptors, the serotonin 2A receptor. However, the embryonic neocortex of mice does not yet present serotoninergic receptors, unlike that of ferrets and humans. So that by adding 2A receptors to rodent embryos, in vivo, the researchers showed that the mice then grow up with a brain larger than usual, their neuronal progenitors multiplying more! Conversely, the fact of inhibiting the serotoninergic 2A receptors in the developing neocortex of ferrets, also in vivo, causes a significant decrease in the quantity of its neurons and its size … Confirming these results, Xing and his colleagues have also proved that the activation of 2A receptors of the human neocortex, ex vivo (with tissues resulting from interrupted pregnancies), promotes the proliferation of its progenitors and increases the quantity of neurons …
Here then is a new role for serotonin: it promotes the multiplication of neuronal progenitors and has contributed, during evolution, to the increase in the size of the neocortex of mammals and in particular of humans. But this is not to stop there: various neurodevelopmental or psychiatric diseases, such as Down syndrome, attention disorder with or without hyperactivity, and autism, are in part due to signaling abnormalities. serotonin or its 2A receptors during in utero development: researchers are therefore considering new therapeutic approaches for these pathologies. Based on the molecule of happiness.
