"This means, it was just correlation."

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Ejaculation study debunks a longstanding theory about the refractory period

One hormone was thought to explain why men have to take time off after an orgasm ( called the male refractory period). Now scientists aren't so sure.

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Scientists are still lacking hard data on how long it takes men to recharge after sex. But for years, they thought they at least had an explanation for why the downtime was needed.

Now, it may be time to retire the existing explanation for why back-to-back ejaculations are a biological struggle.

The prevailing explanation for the “post-ejaculation refractory period” (the idea that post-orgasm, men have to wait before becoming aroused again) centered around a hormone called prolactin. Prolactin is perhaps most famous for sparking milk production in females during pregnancy, but it also has deep ties to orgasms (usually accompanied by ejaculation, but not always).

High levels of prolactin have been linked to low sexual desire. Studies have also shown that prolactin spikes during ejaculation and orgasm, suggesting it’s that hormone causing the body to demand a break afterward. One notable exception stems from a case study covering a man who was able to orgasm repeatedly. In one trial, he only needed two minutes between orgasms — he also didn’t have a typical post-release prolactin surge seen in the control group.

But past research may have sent scientists in the wrong direction, explains Susana Lima, the first author of a new study published this week in Communications Biology. Lima is a researcher at the Champalimaud Research Center for the Unknown, a private biomedical research institution.

In a study on two types of mice, she found that messing with prolactin levels showed no effect on the male refractory period.

“This means, it was just correlation," Lima tells Inverse. "Causation was never tested."

How the study worked – This study was based on the sexual experiences of two types of mice: one type with a short refractory period and one with a long recovery period lasting several days. The male and female mice were first kept separated and then introduced to one another.

The team took blood samples from the male mice on four occasions: Before meeting their female counterparts, after the mice initially mounted their sexual partners (signaling intent), after a certain number of mounts (five for one type of mouse and three for the other, if you must know), and then after the mice ejaculated.

The scientists could tell the mice had ejaculated as they displayed “stereotypical shivering” and fell over.

The male refractory period isn't well understood, but the prevailing idea is that it gives the body time to replenish seminal fluid and sperm post-ejaculation, says Lima.

The detailed blood samples revealed that prolactin did increase during sexual activity. But subsequent trials illuminated that it didn’t affect the refractory period.

In follow-up experiments, the team artificially manipulated the level of prolactin in each male mouse’s blood before they had sex. These higher levels didn’t change their sexual behavior — if prolactin did inhibit sex, the mice would have had less sexual desire, the authors explain.

The authors also blocked the release of prolactin post-orgasm, to see if that might inspire more sexual activity by shortening the refractory period. They found no significant increases.

A mouse study may not totally spell the end for the prolactin theory, but because prolactin has similar functions in humans and mice, Lima says she expects these results to hold up in people.

“I think we can take our results as strong evidence against prolactin and the refractory period in men,” she says.

What’s the new explanation? – If this study doesn’t entirely discount the idea that prolactin contributes to the refractory period, it does suggest that scientists should also be entertaining other ideas. Those ideas likely combine multiple systems and hormones in the body that rise and fall in tandem, not just one smoking gun, Lima says.

"Most likely, both central systems and peripheral systems are working in coordination..."

She explains, in turn, that there are two main systems that likely underly the refractory period: a central brain mechanism and a "peripheral one" — a mechanism that deals with the physical elements like the penis, glands, and other organs that "support erection, emission, and ejaculation." The central brain mechanism has more to do with behavioral elements, like sexual desire.

In short, there’s a lot going on in the brain and body during ejaculation, and understanding why it takes time for the body time to recover may come down to learning the intricacies of each piece of the action.

“Most likely, both central systems and peripheral systems are working in coordination, such that the male doesn’t have an erection with no desire or desire without being able to be erect,” Lima says.

For now, the true cause of the male refractory period is still a mystery — much like the female orgasm.

Abstract: In many species, ejaculation is followed by a state of decreased sexual activity, the post-ejaculatory refractory period. Several lines of evidence have suggested prolactin, a pituitary hormone released around the time of ejaculation in humans and other animals, to be a decisive player in the establishment of the refractory period. However, data supporting this hypothesis is controversial. We took advantage of two different strains of house mouse, a wild derived and a classical laboratory strain that differ substantially in their sexual performance, to investigate prolactin’s involvement in sexual activity and the refractory period. First, we show that there is prolactin release during sexual behavior in male mice. Second, using a pharmacological approach, we show that acute manipulations of prolactin levels, either mimicking the natural release during sexual behavior or inhibiting its occurrence, do not affect sexual activity or shorten the refractory period, respectively. Therefore, we show compelling evidence refuting the idea that prolactin released during copulation is involved in the establishment of the refractory period, a long-standing hypothesis in the field of behavioral endocrinology.

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