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.
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.