Why can't males get "ready for another" right away?

A new study carried out in mice, by scientists at the Champalimaud Center in Portugal, refutes previously made ideas

If you type in a search engine - "why do men have to wait before being able to have sex again?" – you will certainly find prolactin in the results.

This tiny hormone is believed to be involved in hundreds of physiological processes in our body, including the male “post-ejaculatory refractory period”. This period begins when the man ejaculates and ends when he regains his sexual capacity.

And if you continue to research, you will see that this theory has even led the development of “Treatments” that promise to shorten the refractory period of man by reducing the levels of prolactin in the body.

Well, this is where the bad news starts for those who have already bought this type of “Treatments“. A new study carried out in mice by scientists at the Champalimaud Center in Portugal demonstrates that prolactin may not be responsible after all. The results were published on January 4, 2020 in scientific magazine Communications Biology.

The theory

It may sound ironic, but the truth is that the research project that is now opposing this dominant theory never had that purpose.

“When we started working on this project, we really wanted to explore this idea because we were fascinated by the possibility that a hormone that is normally associated with milk production in women is involved in a male reproductive process. "Our goal was to investigate, in more detail, the biological mechanisms by which prolactin is causing the refraction period."

What is the basis of this deeply rooted theory? According to Lima, its origin is the result of different lines of evidence.

On the one hand, some studies have shown that prolactin, in both humans and mice, is released at the time of ejaculation. As the refractory period starts right after ejaculation, prolactin presented itself as a good candidate to be responsible for it. Furthermore, abnormally high levels of prolactin in the body – hyperprolactinemia – are associated with decreased sexual response, anorgasmia and ejaculatory dysfunction. Finally, treatment with drugs that inhibit prolactin release, in situations of hyperprolactinemia, reverses sexual dysfunction.

“All these different results pointed to a central role of prolactin in the suppression of male sexual behavior”, says Lima. “However, this direct link between prolactin and the post-ejaculatory male refractory period has never been demonstrated. Still, this theory has asserted itself in such a way that it appears in textbooks as well as in the press.”

So why is prolactin no longer the explanation?

How did the team come to the conclusion that this theory was wrong?

To study the role of prolactin in the male refractory period, Susana Lima and her team carried out a series of experiments in mice.

“We chose the mouse as an animal model because the sequence of sexual behavior of this animal is very similar to that of man”, explains Susana Valente, first author of the study. “Furthermore, with mice, we can test several strains that have different sexual performance, which makes the data richer. In this case, we use two different strains: one that has a short refractory period and another that has a period that lasts several days.”

The team began by looking at whether prolactin levels also increased during sexual activity in male mice. “Through blood samples, we measure levels during different phases of sexual behavior. And, in fact, these increased significantly during sexual interaction,” says Valente.

Once this premise was confirmed, the researchers proceeded to study the relationship between prolactin and the length of the animals' refractory period.

“Our first manipulation was to increase prolactin levels, before the animals became sexually aroused. Specifically, we made sure that the artificial levels matched those we measured during natural sexual behavior. If prolactin were really responsible for the refractory period, the animals' sexual activity should decrease”, explains Valente.

To the team's surprise, this manipulation had no effect on the mice's sexual behavior. “Despite the increase in prolactin levels, both strains showed normal sexual behavior”, recalls the researcher.

Next, the investigators tested whether inhibiting prolactin release would have the opposite effect on the refractory period. In other words, they sought to assess whether animals, without prolactin, would become more sexually active. Again, the answer was no.

“If prolactin were really necessary for the refractory period, then males without prolactin, after ejaculation, should recover their sexual capacity more quickly compared to the control group”, highlights Valente. "But that didn't happen."

back to beginning

Taken together, the results of the two investigators provide a strong counter-evidence to the theory that prolactin triggers the male refractory period. Still, prolactin undoubtedly plays a role in male sexual behavior. But what could this role be?

“The possibilities are many”, says Lima. “For example, there are studies that point to a role for prolactin in the development of parental behavior. Furthermore, it is important to emphasize that the prolactin dynamics are quite different in male mice and in men. In mice, prolactin levels increase during mating. However, in men, prolactin appears to be released only at the time of ejaculation, and only when it happens. Therefore, there may be some differences in the role it plays in different species.”

So why do males have to wait and can't get “ready for another” right away?

“Our results indicate that it is very unlikely that prolactin is the cause,” says Lima. “We are now in a better position to advance in this line of investigation and discover what is really at the basis of this physiological process”, he concludes.

 

Author: Champalimaud Foundation – Science Communication
© 2021 – Science in the Regional Press / Ciência Viva

 

 

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