A desire switch in mice opens the door to modulating male sexual desire

Stanford University scientists have detected the neural network that controls sexual drive in rodents and manage to turn it on and off at will

Enrique Alpañés
A male mouse courts a female in a laboratory.
A male mouse courts a female in a laboratory.Bettina Wernisch/VetMed Vienna

It’s often said that the brain is the most important sexual organ of the human being. Now, a group of neurologists from Stanford University has extended this maxim to mice. Their study, published in the journal Cell, claims to have detected the brain circuit that is activated in male mice when they detect the presence of a female, awakening sexual desire and inducing mating and the satisfaction derived from it. It is the neural route that a stimulus follows until it becomes an instinct.

Until recently, the brain was perceived as an impossible tangle of neurons, but in the last decade, scientists have begun to unravel this web, precisely locating the neural circuits that drive different incentives. That is what has happened now with the sexual drive of male mice. It was known that this drive was approximately located in the preoptic hypothalamus, but this experiment honed in even further. The experiment detected the particular neural circuit of desire, and more importantly, offered scientists the ability to plug or unplug it at will.

“[If we unplug it] the males simply won’t mate, even if they recognize the female,” Nirao Shah, professor of psychiatry and neurobiology and lead author of the study, explains in a telephone conversation. “But, if we activate those cells, they will start to mate again, even after ejaculation.” This detail is especially significant. Most male mammals after ejaculation enter a period in which they have no sexual desire. This is known as the refractory period. In the case of mice, it lasts about five days, but when this neural circuit is stimulated, it practically disappears and lasts just one second. “That’s a reduction of more than 400,000,” says Shah, “which tells us that these neurons direct the sexual drive in a very powerful way.”

Viagra for the mind

The Stanford team’s findings may open the door to creating drugs that work as a male desire switch in humans. There are reasons to think that the mice findings could be translated to humans. “This part of the mouse brain has an anatomical analogue in our brain,” says Shah, “so it is very likely that there is a similar set of cells in this region of the human brain that regulate male sexual behavior.”

This hypothetical drug, which would still require years of research, would be different from Viagra. “[Viagra] acts on the blood vessels in the penis and this is how it causes an erection, but it doesn’t seem to affect libido per se,” explains Shah. A possible neural drug would not affect the physical mechanisms of erection, but rather the cerebral ones. It would be a Viagra for the mind, a pill capable of mitigating a hyperactive sex drive or enhancing a low one. Desire in a pill.

One of the advantages of having fine-tuned the route of male desire so precisely was that the researchers were able to activate the circuit without causing side effects. “In most of the previous experiments, if male sexual behavior was regulated, this ultimately ended up affecting aggression,” Shah explains. “But the brain cells that we have identified in this study do not.”

The study also identified the mechanisms of sexual pleasure. “We know that the act of mating must be rewarding for mice, as they engage in it repeatedly, even when exposed to danger. And we believe that we have identified at least one of these mechanisms in the study,” says the neurobiologist.

This neural circuit triggers a reward system, which makes the mice want more. “It was always thought that there was a pleasure center for sex somewhere in the brain. But defining or characterizing these cells has been very difficult,” explains Shah. During the experiment, the mice were given the chance to switch on the sex neural circuit at will: by going to a specific area of the cage, it was automatically activated. When the mice figured out the mechanism, they returned to that spot over and over again. “Up to 200 times in a few minutes,” explains Shah.

These discoveries open the door to better understanding the mechanisms of male sexual pleasure. But it’s just a first step in a broader investigation. Shah’s group is trying to figure out the equivalent circuitry in females, which he acknowledges could take a few years. In addition to possible pharmacological applications, these findings could help to better understand the innate differences between males and females and the forces that drive human sexuality.

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