Have American scientists found a way to develop aphrodisiacs? Remember: no drug, no plant, no food has ever been scientifically proven to boost or increase libido. Not even Viagra: the famous blue pill acts mechanically, by increasing blood flow to the penis to make an erection possible, but it does not directly affect the desire, or not, to make love. However, researchers at Stanford University in the United States have just uncovered the complex brain circuit that controls male sexual desire. Their discovery, published this Friday, August 11 in the prestigious journal Cellcould – eventually – change the situation, making it possible to treat the lack, but also the excess of sexual appetite, and even to reduce the refractory period, this interval after an ejaculation during which a new report is not possible .
In the long term, because for the moment, their work concerns… mice. There are still many steps before arriving at the love potions that alchemists have been looking for for centuries. It will be necessary to demonstrate, first, that the same cerebral circuit also exists in humans (the researchers say they are almost certain that this is the case). Then, find the right molecules (which looks more complex). Finally, as with any drug, test them (we’re not there yet). Their work remains no less fascinating, because until now we did not know much about the libido, except that it was innate, evolution having taken care to ensure the survival of the body. ‘species.
So what did they discover? What happens in the brain of a male mouse meeting a female mouse. “The sensory information arrives in a part of the amygdala called BNST, activated specifically by the presence of a female. Thanks to a neurotransmitter, these neurons go to stimulate a few nerve cells in the pre-optic hypothalamus. will then send impulses to two other very deep areas, one which is thought to control movement, and the other which is the seat of dopaminergic neurons.Dopamine will then go to the nuclei accumbens, which play an important role in the reward system,” explains Nirao Shah, professor of psychiatry and neurobiology at Stanford, and last author of the study. Results obtained thanks to state-of-the-art techniques combining imaging, molecular genetics and optogenetics (a technique that activates cells using light).
Save endangered species
One and the same circuit therefore controls both sexual recognition, libido, coupling and pleasure. A very powerful circuit: by activating it artificially, the researchers were able to push the male mice to mate again immediately after a first ejaculation, whereas in the species studied, the refractory period normally lasts five days. On the contrary, by deactivating these same neurons, the rodents no longer seek to copulate. “This is an important discovery, because until now it was not known why mating was attractive. In nature, it is a dangerous act for animals: during intercourse, they are exposed to their predators. But our work shows that they do it anyway because it activates their reward circuitry,” says Nirao Shah.
For the researcher, this mechanism of pleasure is probably the same in humans. At this stage, this has not been formally demonstrated, but there are many clues to this effect. “First, all the areas involved are found in mice, rats, but also birds, monkeys and humans. This is an important point”, underlines the neurobiologist. Moreover, whatever the animals, the pre-optic nucleus of the hypothalamus has more neurons in the male than in the female. Finally, previous experiments have shown that by stimulating this area in monkeys, the animals were more inclined to mate. “The fact that these areas are conserved between different species, from an anatomical and functional point of view, shows that a similar circuit very probably exists in humans as well,” notes Nirao Shah. However, proving it is tricky: the resolution of the functional imaging does not exceed one millimeter, while the size of the groups of neurons concerned turns out to be lower. It will therefore require further technological progress to achieve this.
What about women?
In the meantime, this work should first make it possible to save species threatened with extinction. “These animals in captivity are often not very inclined to mate, and biologists have difficulty encouraging them to do so. By acting on this circuit, we could solve this problem”, imagines Nirao Shah. The development of drugs for human use, on the other hand, seems a more distant prospect, but not totally unrealistic. “Human sexuality is more complex than that of animals. Added to this are ethical considerations, which also complicate the development and marketing of treatments that improve or slow down the libido”, he underlines. A first use, perhaps less controversial, would be for patients on antidepressants: “Their libido is often affected by the treatment, to the point of pushing them to stop it”, recalls the researcher.
The other big question remains: what about women – or rather, in this case, rats, since this research only concerns animals? Nirao Shah’s team was the first, ten years ago, to show that a part of the hypothalamus, different from that found in males, probably played a role in mating. They are now continuing their investigations to find out more.