After finding “the one” and getting “in the mood” comes the grand finale, “a happy ending”. In the third and last part of this series on the Neurobiology of Sex, Susana Lima, principal investigator at Champalimaud Centre for the Unknown, tells us about her work on one of the quickest behavioural transitions in male nature: from passionate lover to snoring sack of potatoes – or, in biological terms, the male post-ejaculation behaviour.
“When we observed this phenomenon in mice, the change was shocking, one moment the female is the most desired object in the world and then, after ejaculation, the male unceremoniously pushes her aside.” says Susana. “I became fascinated by how the physical act of ejaculation is communicated to the brain to bring about this remarkable behavioural switch.”
According to Susana, the physiological mechanism underlying this transition is not known, but there is one main candidate that is thought to play an important role: the hormone prolactin. “When people hear ‘prolactin’ they immediately think, ‘wait, isn’t that the milk-production hormone?’ It is, but it actually does much more than that; it’s involved in more than 300 physiological processes. From water balance to immune response, to development, sperm production, a bunch of different things. For decades, scientists have hypothesised that prolactin is one of the main hormones in charge of establishing the sexual refractory period, which begins with ejeculation and ends when the male is able to engage in sexual behaviour once again, but it still needs to be proved.”
Support for this hypothesis comes from several sources. “For one, a surge in the level of prolactin in the blood has been observed in both males and females after orgasm and ejaculation. Also, males who suffer from overproduction of prolactin due to tumors in the hypophysis have a suppressed sexual drive, which can be recovered after treatment to reduce prolactin levels. Finally, lactating women also exhibit a reduced sex drive.”
So will prolactin inhibitors be the next generation’s Viagra? “It is too early to tell.” Susana laughs. “Together, these lines of evidence suggest prolactin as a serious candidate, but to be sure that it’s actually the cause we are now attacking this difficult question on different fronts. First, we wanted to confirm that a prolactin surge also happens after ejaculation in mice. To do that, we measured blood prolactin levels during different points of sexual behaviour and found that it does indeed. Now, we are tackling this question by trying to manipulate the length of the refractory period. It is already known that the length of the refractory period is not necessarily fixed. For example, it can be shortened by the presentation of a new female.”
“It is actually a funny story. The shortening of the refractory period is called ‘the Coolidge effect’ after the 30th President of the United States. It is said that once the president and his wife were visiting a chicken farm, when the first lady remarked to the farm owner, gesturing at a rooster that was jumping from hen to hen, ‘can you please show my husband how this rooster is able to have sex many time in a row?’ To which the president replied, ‘yes, I see that, but please tell my wife that if she payed attention she would notice that the rooster is actually having sex with a different female every time.’”
“Is prolactin involved in that?”, asks Susana. “To find out, we are applying or blocking prolactin in brain areas that we know are important for sexual behaviour and see which of them ‘light up’. What I like about prolactin is that it’s a nice evolutionary story. It is a very old hormone that might be mediating the refractory period on multiple levels, from sperm production, to ejaculation, to social behaviour. Will this turn out to be true? We hope to know the answer soon enough”, she concludes with a smile.
Liad Hollender works as a Science Writer at the Science Communication Office at Champalimaud Research
Edited by: Catarina Ramos and Ana Gerschenfeld (Science Communication office)
Image Credit: Francisco Esteves and Ana Rita Mendes