Study: Men smell and that will stress you out

ferret-specific neurons

A study in Nature Methods has kicked up a bit of a fuss:

In 2007, his lab observed that mice spend less time licking a painful injection—a sign that they’re hurting—when a person is nearby, even if that “person” is a cardboard cutout of Paris Hilton. Other scientists began to wonder if their own data were biased by the same effect. “There were whisperings at meetings that this was confounding research results,” Mogil says.

—–

Male, but not female, experimenters induce intense stress in rodents that can dampen pain responses, according to a paper published today in Nature Methods. Such reactions affect the rodents’ behaviour and potentially confound the results of animal studies, the study suggests.

Yup, the paper says that the stench of men is just plain stressful to rodents. And it’s just human males, but males from many (most?) species.

It is pretty well-established that many animals have neurons that have an innate response to the odor of other animal species. Look at the percent of neurons in the vomeronasal organ (VNO) of the mouse that detect the scent of specific other animals:

Conspecific cells

 

I suppose that means it shouldn’t be surprising that there would be a way to detect males across species. And the data from this paper kinda-sorta points to that: bedding from male guinea pigs, rats, cats, and dogs induced stress-related behaviors but not when the bedding came from castrated males (poor guys). Overall, the affect of the stress was stronger on the female than the male mice.

There are three interesting take-aways from this paper. First, obviously, is that males stress out mice they handle more than females do – and they stress out female mice more than male mice. Second, certain male-specific effects seem to require both an odor and the physical presence of the (male/female) handler. Third, the odor isn’t likely to be a pheromone but rather a complex mix of odors. Three of the stress-inducing odors that they identify are 300 μM (E)-3-methyl-2-hexenoic acid (3M2H), 0.75–3 mM androstenone and 0.75–3 mM androstadienone (4,16-androstadien-3-one), which I’m sure everyone is familiar with! The first is a fatty acid that contributes to “Caucasian male underarm odor” (yum), the second is a steroid found in sweat and urine (and celery), and the third is a metabolite of testosterone.

Also, let’s take a moment to pity the poor grad students who had to take “repeated rectal measurement of core [mouse] body temperature”.

References

Isogai Y, Si S, Pont-Lezica L, Tan T, Kapoor V, Murthy VN, & Dulac C (2011). Molecular organization of vomeronasal chemoreception. Nature, 478 (7368), 241-5 PMID: 21937988

Sorge, R., Martin, L., Isbester, K., Sotocinal, S., Rosen, S., Tuttle, A., Wieskopf, J., Acland, E., Dokova, A., Kadoura, B., Leger, P., Mapplebeck, J., McPhail, M., Delaney, A., Wigerblad, G., Schumann, A., Quinn, T., Frasnelli, J., Svensson, C., Sternberg, W., & Mogil, J. (2014). Olfactory exposure to males, including men, causes stress and related analgesia in rodents Nature Methods DOI: 10.1038/nmeth.2935

Why you’ll become an alcoholic unless you get more sex

One very social behavior involves a man and a woman who love each other very much (hint: I’m talking about sex).  Flies who love each other very much obviously also mate, although you may not know that they undergo a courtship ritual first – not just any ol’ fly is getting to home plate.  That’s a behavior I’ll talk more about in a future post.  What I want to talk about here is instead what happens to that unlucky guy who, know matter how hard he tries, isn’t getting any.

A recent paper looked at this very question by taking a bunch of flies, and either having one group that either had a lot of sex or were rejected.  And can I say how awesome this sounds?  Listen to the protocol: one group of male flies experienced 1-hour sessions of sexual rejection three times a day for four days.  Another group experienced six-hour sessions of mating with multiple receptive virgin females for four days.  Let’s just say that you probably couldn’t do this kind of science in people.

The flies were then given the choice between food with alcohol and food without alcohol.  When the flies were sexually satisfied, they went without the alcohol; the flies were rejected needed that extra beer.  It turns out even virgin flies choose the alcohol – though they like it less than the rejected flies – which means that it is the lack of sex that mainly influences how much they need to drink.  If these same flies are allowed to mate?  Then they don’t need the alcohol anymore!

Desire, motivation and addiction in the brain are normally associated with the neural chemical dopamine.  But in this paper they looked at a neural peptide instead.  In humans, the neural peptide Y regulates alcohol consumption, as does all kinds of stress like PTSD and early maternal separation.  The equivalent peptide in fly is neural peptide F (NPF).  When they measured the amount of NPF in these flies, they found that it matched the desire for alcohol: the sexually rejected males had the lowest amount of NPF, the virgins had a little more, and the mated males had the most.  By decreasing the amount of NPF with siRNA or artificially activating it, they were able to control how much the flies wanted the alcohol.

So what is happening in the brain in response to sex?  Sex releases this neuropeptide – NPF in flies, NPY in humans – and the peptide is rewarding!  You love it (no surprise there)!  The peptide probably sets in motion changes in the larger reward system, modifying dopamine transmission over the course of many days.  This reveals the importance of investigating how we interact with our environment and fellow creatures in order to understand how our brain really works.

Reference

Shohat-Ophir, Kaun, Azanchi, Huberlein.  Sexual deprivation increases ethanol intake in Drosophila.  Science (335) 1351-1355.  DOI: 10.1126/science.1215932

See also the perspective.