Type in a search engine "why do men have to wait before having sex again," and you will most likely be greeted by the word 'prolactin.'
The hormone is thought to be involved in the male refractory period — the time between orgasms in which a male has no sexual capacity.
A new study in mice by scientists at the Champalimaud Centre for the Unknown in Portugal reveals that prolactin may actually have nothing to do with the post-ejaculatory refractory period.
The study suggests that hormone treatments claiming to reduce this period may be completely ineffective.
Studying the male refractory period
Published this week in the journal Communications Biology, the research project originally set out to study the link between prolactin and the refractory period — the intention was not to refute the correlation.
"When we started working on this project, we actually set off to explore the theory", Susana Lima, the principal investigator who led the study, explained in a press statement.
"Our goal was to investigate in more detail the biological mechanisms by which prolactin might generate the refractory period."
In order to study the role of prolactin in the male refractory period, Lima and her team performed several experiments using mice.
"We chose mice as our model animal because the sequence of sexual behavior in mice is very similar to that of humans", explains Susana Valente, the first author of the study.
"Also, with mice, we can test different strains that exhibit different sexual performance, which makes the data richer. In this case, we used two different strains. One that has a short refractory period, and another that has a long one, lasting several days."
The team began by checking if prolactin levels also increase during sexual activity in male mice: "we measured the levels during the different stages of sexual behavior using blood samples. And sure enough, they significantly increased during sexual interaction", says Valente
Artificially manipulating prolactin levels
The researchers artificially increased prolactin levels before the animals were sexually aroused. "If prolactin was indeed the cause of the refractory period, the animals' sexual activity should have decreased", Valente explained.
To the researchers' surprise, the manipulation had no effect on the sexual behavior of the mice. "Despite the elevation in prolactin levels, both strains of mice engaged in sexual behavior normally", she recalls.
The researchers also tried blocking prolactin to see if it would have the opposite effect on the refractory period.
"If prolactin was indeed necessary for the refectory period, males without prolactin should have regained sexual activity after ejaculation faster than controls", Valente said. "But they did not."
Though prolactin undoubtedly plays a vital role in male sexual behavior, the researchers say, their observations strongly suggest that the hormone does not trigger the male refractory period, as previously believed.
So, ultimately, what is the chemical reason that men have to wait longer before round two?
"Our results indicate that prolactin is very unlikely to be the cause", says Lima. "Now we can move on and try to find out what's really happening", she concludes.