Gal Science: The Virgin Brain -The Toast

As The Toast searches for its one true Gal Scientist, we will be running a ton of wonderful one-off pieces by female scientists of all shapes and sizes and fields and education levels, which we are sure you will enjoy. They’ll live here, so you can always find them. Most recently: Stop Being Terrified of Chemicals.

Some people consider losing one’s V-card to be a huge life-changing deal, while others find it anticlimactic (and a social construct.) Regardless of your experience, however, you have probably not stopped to wonder whether losing your virginity changes the genetic expression in your brain. Lucky for you, some scientists do wonder about these things.

It would be pretty difficult and probably unethical to conduct human studies on virginity (I mean would you be willing to lose your virginity in a lab, FOR SCIENCE?), so scientists have turned towards our adorable friends from the prairie.

The prairie vole is one of the few mammals that mates for life. This has been known for a while, but they have recently exploded in scientific popularity. The promise of finally being able to scientifically study LOVE has intrigued both scientists and the press. A recent study by Wang et al. (yep.) investigates what happens to the DNA in the brains of prairie voles when they undergo “mating based pair bonding.”

What is mating based pair bonding? Well, basically, the scientists take virgin female prairie voles and put them in a cage with a sexually experienced male prairie vole for 6 hours. The voles either have sex during that time or they don’t (there is video evidence.) If they have sex, the female starts to bond to the male and prefers him over a stranger (this is measured by essentially testing how much time she spends cuddling him at a later time.) But if they don’t have sex, the female shows no preference for him. (Side note: If the female vole spends 24 hours with the male without sex, a preference will ultimately develop as shown by Williams et al. 1992.) So, basically, either sex or time seals the deal.

Wang and others wanted to know what happens in the brain during the sexual 6-hour bonding process. Having sex releases two important neurotransmitters in the brain, oxytocin and dopamine. These neurotransmitters only work when they activate the oxytocin or dopamine receptors. The cell makes these receptors from its own DNA. Dopamine receptors are diverse and complicated, so we’ll focus on the oxytocin receptors for now. In humans, oxytocin is generally thought to increase “trust.” In the prairie vole, oxytocin receptors are particularly important in the development of the pair bond. The monogamous prairie voles have very dense oxytocin receptors, while their close (but non-monogamous) cousins, the montane voles, have fewer oxytocin receptors. It’s even been shown that artificially increasing the oxytocin receptors in female voles makes it easier for her to develop a preference for a male partner. In the study published in Nature Neuroscience, Wang et al. manipulate the density of oxytocin receptors in the brains of the female prairie voles by messing with their DNA. Specifically they epigenetically modulated which pieces of DNA were available for turning into useful things (like, say, oxytocin receptors), and which were not available.

You probably know from biology class that each cell in your body contains a complete copy of your DNA. But a skin cell is not the same as a heart cell or a bone cell or a brain cell. And brain cells are not all alike either. Epigenetics is basically a way to control which sections of DNA each cell has access to by regulating how loosely the sections of DNA are wound up. If you loosen up a section of DNA (acetylation) your cell has better access to it and you get a lot of that gene expressed (and that receptor produced). If you tighten up a section of DNA (methylation), you prevent that section of DNA from being expressed.

Wang et al. used a histone de-acetylase (HDAC) inhibitor to keep the section of DNA that codes for the oxytocin receptor acetylated (loosened), so more oxytocin receptors would be created. The virgin prairie voles who got the HDAC inhibitor indeed had more oxytocin receptors in certain parts of their brains. But the amazing thing is they didn’t need to have sex with their 6 hour male buddy to develop a preference for him! The DNA manipulation was enough to replace the important sex component of the experiment.

This result is interesting and everything, but it doesn’t actually tell us that sex changes the brain. So far Wang et al. have just shown that you can eliminate the need for sex by epigenetically increasing oxytocin receptors. But, of course what we all really want to know is: does sex change the brain?

Wang et al. wanted to know the same thing, so they did one final experiment. They measured the amount of acetylation on the oxytocin receptor part of the DNA in virgin voles and paired, sexed voles. And not too surprisingly, the female voles who had had sex had more acetylation on the oxytocin receptor part of their DNA and consequently more oxytocin receptors. Having sex changed their brains.

Like most experiments, this one leaves us with many questions. What exactly is it about the sex that causes the brain change? If you could pop that prairie vole cherry by artificial means, would you get the same oxytocin receptor changes? My guess is no, but we won’t know unless someone does that experiment. Similarly, is the virginity of these voles important? Would these experiments have the same result if the female voles were sexually experienced?

Alternatively, is sex sufficient to cause partner preference and the reported brain changes? The 6 hours that the voles spent together might be critical for the increase in oxytocin receptors. One experiment that Wang et al. did not do was put the voles together for a short time (just an hour or so) and allow sex to occur. If these ‘sex-only’ voles had the same brain changes, this would mean that the increase in oxytocin receptors was due to sex specifically. If they did not have the same brain changes, it would mean that the increase in oxytocin receptors was due to the bonding process as a whole.

Of course this has limited applications to human relationships. We don’t get put in a room with a potential mate for 6 hours and then get tested on how much we cuddle hir later. We can’t look at the data and say “Yep, I’m totally bonded to that person.” Even something that seems straight forward like ‘sex’ is not well defined for us. Does any type of sexual experience count here?

But let’s speculate wildly about what this means for us anyway. If we ignore the above caveats and assume that ‘sex’ causes more oxytocin receptors to be created in the human brain, does this change what sex means? Should we protect our virginity like Jane Austen characters, lest we accidentally ‘fall in love’ with a sex-buddy when we don’t want to? That’s obviously extreme, and only your own experience will tell you how much ‘preference’ you have for someone after sex. If anything, the study simply suggests that sex can change the brain in a way that helps build a partner bond.

Wang, H., Duclot, F., Liu, Y., Wang, Z., Kabbaj, M., 2013. Histone deacetylase inhibitors facilitate partner preference formation in female prairie voles. Nat. Neurosci. 16, 919–924.

Rebekah Evans has a Ph.D. in neuroscience and is a research fellow at the National Institutes of Health. She is currently conducting experiments to figure out how dopamine neurons integrate information. She blogs about how amazing neurons and other cells are at Cellularscale.blogspot.com and tweets @cellularscale.