(Press-News.org) If you type into a search engine - "why do men have to wait before having sex again?" - you will very quickly come across Prolactin. This little hormone is thought to be involved in hundreds of physiological processes in the body. Among them is the male post-ejaculatory refractory period. This period begins when a male ejaculates and ends when he recovers his sexual capacity.
If you search a bit more, you'll see that this theory has even led to the development of so called "treatments". These promise to shorten the length of a person's refractory period by reducing their body's prolactin levels.
Well, here is some bad news for anyone who has bought any such merchandise. A new study in mice by scientists at the Champalimaud Centre for the Unknown in Portugal reveals that prolactin may actually not be the culprit after all. These results were published today (January 4th) in the journal Communications Biology.
The Theory
Ironically, the research project that ended up refuting the theory, never aimed to do so.
"When we started working on this project, we actually set off to explore the theory", recalls Susana Lima, the principal investigator who led the study. "Our goal was to investigate in more detail the biological mechanisms by which prolactin might generate the refractory period."
What is the basis of the theory? According to Lima, it emerged through several lines of evidence.
For one, some studies have shown that prolactin is released around the time of ejaculation in humans and rats. And since the refractory period starts right after ejaculation, prolactin seemed like a good candidate. Also, chronic abnormally high levels of prolactin are associated with decreased sexual drive, anorgasmia and ejaculatory dysfunction. Finally, treatment with drugs that inhibit prolactin release in situations of chronically high prolactin, reverse sexual dysfunction.
"These different results all point towards a central role for prolactin in suppressing male sexual behaviour", says Lima. "However, a direct link between prolactin and the male post-ejaculatory refractory period was never directly demonstrated. Still, this theory has become so widespread that it now appears in textbooks as well as in the popular press."
Why Not Prolactin?
How did the team end up discovering that the theory was wrong?
To study the role of prolactin in the male refractory period, Lima and her team performed a series of experiments in mice.
"We chose mice as our model animal because the sequence of sexual behaviour 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 behaviour using blood samples. And sure enough, they significantly increased during sexual interaction", says Valente
Once this aspect was confirmed, the researchers moved forward to investigate the relation between prolactin and the length of the animals' refractory period.
"Our first manipulation was to artificially increase prolactin levels before the animals became sexually aroused. We specifically made sure that the artificial levels matched those we measured during natural sexual behaviour. If prolactin was indeed the cause of the refractory period, the animals' sexual activity should have decreased", Valente explains.
To their surprise, this manipulation had no effect on the sexual behaviour of the mice. "Despite the elevation in prolactin levels, both strains of mice engaged in sexual behaviour normally", she recalls.
Next, the researchers turned to see if blocking prolactin would have the opposite effect on the refractory period. In other words, if animals without prolactin would be more sexually active. Again, the answer was "No".
"If prolactin was indeed necessary for the refectory period, males without prolactin should have regained sexual activity after ejaculation faster than controls", Valente points out. "But they did not."
Back To The Drawing Board
Together, Valente and Lima's results provide strong counter evidence to the theory claiming prolactin triggers the male refractory period. Still, prolactin is undoubtedly a part of male sexual behaviour. What could be its role?
"There are many possibilities", says Lima. "For instance, there are studies that point towards a role for prolactin in the establishment of parental behaviour. Also, it's important to note that prolactin dynamics are quite different in male mice and men. In mice, prolactin levels rise during mating. However, in men, prolactin seems to only be released around the time of ejaculation, and only when ejaculation is achieved. So there may be some differences in its role across species."
So what is the reason males have to wait 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.
INFORMATION:
A research study at the University of Chicago has found that in pregnancy, while the T cell response to a fetus becomes tolerant to allow for successful pregnancy, the part of the immune system that produces antibodies (known as the humoral response) becomes sensitized, creating memory B cells that can later contribute to the rejection of a transplanted organ.
The results help to clarify why it is that the immune system can tolerate a fetus during pregnancy, but later may be more likely to become sensitized to and reject an organ transplant. The study was published on January 4, 2021 in the END ...
Polarons are fleeting distortions in a material's atomic lattice that form around a moving electron in a few trillionths of a second, then quickly disappear. As ephemeral as they are, they affect a material's behavior, and may even be the reason that solar cells made with lead hybrid perovskites achieve extraordinarily high efficiencies in the lab.
Now scientists at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University have used the lab's X-ray laser to watch and directly measure the formation of polarons for the first time. They reported their findings in Nature ...
Using a new combination of emitter molecules, researchers in Japan have demonstrated the promise of a novel approach to finally overcome a major challenge facing displays using organic light-emitting diodes: a blue light source matching the excellent performance of the red and green ones.
By splitting energy conversion and emission processes between two molecules, the researchers achieved devices that produce pure-blue emission with high efficiency, maintain brightness for relatively long times, and lack any expensive metal atoms--a set of properties that has so far been difficult ...
Babies who suffer oxygen deficiencies during birth are at risk of brain damage that can lead to developmental delays, cerebral palsy and even death. To prevent this, most women in labor undergo continuous monitoring of the baby's heart rate and receive supplemental oxygen if the heart rate is abnormal, with the thought that this common practice increases oxygen delivery to the baby. However, there is conflicting evidence about whether the long-recommended practice improves infant health.
Now, a comprehensive analysis - led by Washington University School ...
New Columbia Engineering study--first to investigate the long-term effect of soil moisture-atmosphere feedbacks in drylands--finds that soil moisture exerts a negative feedback on surface water availability in drylands, offsetting some of the expected decline
New York, NY--January 4, 2021--Scientists have thought that global warming will increase the availability of surface water--freshwater resources generated by precipitation minus evapotranspiration--in wet regions, and decrease water availability in dry regions. This expectation is based primarily on atmospheric thermodynamic processes. As air temperatures rise, more water evaporates into the air from the ocean and land. Because warmer air can hold more water vapor than dry air, a more humid atmosphere ...
ANN ARBOR, Michigan -- Michael Green, M.D., Ph.D., noticed that when his patients had cancer that spread to the liver, they fared poorly - more so than when cancer spread to other parts of the body. Not only that, but transformative immunotherapy treatments had little impact for these patient.
Uncovering the reason and a possible solution, a new study, published in Nature Medicine, finds that tumors in the liver siphon off critical immune cells, rendering immunotherapy ineffective. But coupling immunotherapy with radiotherapy to the liver in mice restored the immune cell function and led to better outcomes.
"Patients with liver metastases receive little benefit from immunotherapy, a treatment that has been a game-changer for many cancers. Our research suggests ...
MADISON, Wis. -- Deforestation dropped by 18 percent in two years in African countries where organizations subscribed to receive warnings from a new service using satellites to detect decreases in forest cover in the tropics.
The carbon emissions avoided by reducing deforestation were worth between $149 million and $696 million, based on the ability of lower emissions to reduce the detrimental economic consequences of climate change.
Those findings come from new research into the effect of GLAD, the Global Land Analysis and Discovery system, available on the free and interactive ...
COLUMBUS, Ohio - Scientists have developed a machine-learning method that crunches massive amounts of data to help determine which existing medications could improve outcomes in diseases for which they are not prescribed.
The intent of this work is to speed up drug repurposing, which is not a new concept - think Botox injections, first approved to treat crossed eyes and now a migraine treatment and top cosmetic strategy to reduce the appearance of wrinkles.
But getting to those new uses typically involves a mix of serendipity and time-consuming and expensive randomized clinical trials to ensure that a drug deemed effective for one disorder will be useful as a treatment for something else.
The Ohio State University researchers ...
The healing process that follows a brain injury could spur tumour growth when new cells generated to replace those lost to the injury are derailed by mutations, Toronto scientists have found. A brain injury can be anything from trauma to infection or stroke.
The findings were made by an interdisciplinary team of researchers from the University of Toronto, The Hospital for Sick Children (SickKids) and the Princess Margaret Cancer Centre who are also on the pan-Canadian Stand Up To Cancer Canada Dream Team that focuses on a common brain cancer known as glioblastoma.
"Our data suggest that the right mutational change in particular cells in the brain could be modified by injury to give rise to a tumour," says Dr. Peter Dirks, Dream ...
Spintronics refers to a suite of physical systems which may one day replace many electronic systems. To realize this generational leap, material components that confine electrons in one dimension are highly sought after. For the first time, researchers created such a material in the form of a special bismuth-based crystal known as a high-order topological insulator.
To create spintronic devices, new materials need to be designed that take advantage of quantum behaviors not seen in everyday life. You are probably familiar with conductors and insulators, which permit and restrict the flow of electrons, respectively. ...
