Male fruit flies seem to enjoy ejaculation as much as men do. Their “orgasms” seem to be satisfying enough to reduce their craving for other rewards such as alcohol.

The experiment resembles the “orgasmatron”, a fictional machine for giving people instant orgasms featured in the 1973 film Sleeper . Galit Shohat-Ophir of Bar-Ilan University in Ramat Gan, Israel and her colleagues engineered male fruit flies so that they could make them ejaculate at will.

First, they genetically engineered neurons in the fruit flies’ abdomens so that they could be activated by exposure to red light. Once activated, the neurons produced corazonin, a chemical that makes the flies ejaculate.

Corazonin also boosted the flies’ production of neuropeptide F (NPF), a brain transmitter that is a handy barometer for levels of pleasure and reward . In humans, it has a counterpart called neuropeptide Y, levels of which are reduced in people with depression or post-traumatic stress disorder.

Pleasures of the fly

The first question the team addressed was whether ejaculation itself triggered pleasure, as it does in human males. In theory, many other components of the mating ritual , such as visual cues or making songs to attract mates, might be the thing that causes pleasure. “Male fruit flies produce very specific ‘love songs’ by vibrating one of their wings,” says Shohat-Ophir. So she devised a study that separated ejaculation from all the other elements of courtship.

The team placed engineered and normal male flies in a labyrinthine chamber. Half of the zones could be bathed in red light, which was intense enough to trigger ejaculation in the engineered flies – but had no effect on the normal flies.

When all the zones were unlit, the flies distributed themselves randomly. But when the red lights were turned on, half the engineered flies congregated in the red-light zone. This suggests they enjoyed it there because of the automatic climaxing.

“The team has shown that of all the behaviours involved in courtship and copulation, ejaculation – the final step – is rewarding to the fruit fly,” says Troy Zars of the University of Missouri in Columbia. “They’ve also shown ejaculation changes levels of NPF, a signal of reward, in the fly brain.”

Feeling good

“This sexual reward system is very ancient machinery, conserved from simple organisms all the way to us,” says Shohat-Ophir.

The team also found that flies that had ejaculated were less keen on food laced with alcohol, compared with non-climaxing control flies. “If the reward system is saturated, ethanol is no longer perceived as a reward,” says Shohat-Ophir.

In earlier experiments, the same group showed that jilted flies that drank alcohol boosted their NPF levels to match or exceed those of males that had mated . This shows, says Shohat-Ophir, that flies can use one reward to compensate for a lack of others, in this case offsetting a lack of sexual pleasure with the artificial pleasures of alcohol. Shohat-Ophir hopes these insights may be useful for treating addiction and depression in people.

But one big question remains unanswered: do female fruit flies also get pleasure from sex ? “We don’t know enough about that yet, but we’re studying it in the lab,” says Shohat-Ophir.

Journal reference: Current Biology , DOI: 10.1016/j.cub.2018.03.039